Note: Descriptions are shown in the official language in which they were submitted.
CA 02611672 2007-12-10
1
SURFACE-DENSIFIED TOOTHED SECTION FROM A SINTERED MATERIAL
AND HAVING SPECIAL TOLERANCES
The invention relates to a method and an associated device for producing an at
least par-
tially surface-densified metallic toothed element, which comprises a densified
sintering
material.
Sintered toothed elements such as for example powder-metallurgically produced
gear-
wheels are widely used. Sintered materials generally have a lower density
compared with
conventionally forged materials for example comprising steel. Therefore,
surface densifi-
cation of a sintered workpiece is desirable.
It is an object of the present invention to enable an improvement in regard to
possible
fields of application of a metallic toothed element comprising a sintering
material.
This object is achieved by a method according to claim 1 for producing an at
least partially
surface-densified metallic toothed element, by a toothing according to claim
5, by a device
according to claim 8 and by a computer program product according to claim 9.
Advanta-
geous embodiments and further developments are indicated in the respective
dependent
claims.
According to a feature of the invention, which may be used independently and
also tog-
hether with the further features of the disclosure, a method is suggested for
producing a
tooth system from densified sintering material, whereby a predensified tooth
preform is
densified by at least 0.05 mm from its surface at least in one area by means
of iteratively
determined data to yield its final form, and a final form quality in a range
of at least fHa = 4,
Fa = 7 and Ffa = 7 being achieved. Here fHa means the deviation relative to
the tooth sys-
tem, Fa the total deviation and ffa the profile shape deviation of the flanks.
The stated val-
ues correspond to the DIN classes relating to deviation.
According to one further development, provision is made for iteration to take
account of
parameters which relate to material behaviour during surface densification of
the tooth
shape. In one embodiment, iteration for determining a preform is based on
input data,
which are taken from a final form set value. Preferably, at least one rolling
die is used,
which is of the same quality as the subsequently produced final form.
Iterative determina-
CA 02611672 2007-12-10
2
tion and the resultant extremely precise treatment during surface
densification allow the
quality of the die to be transferred to the preform. In particular, the
extremely precise sur-
face densification makes it possible for the tooth system to gain this final
form quality after
surface densification without a further material-removing postmachining step.
For exam-
ple, a toothed workpiece is produced with a core density of at least 7.4 g/cm3
and with a
surface density which is at its maximum in at least one area of a tooth flank,
the maximum
surface density in the area extending to a depth of at least 0.02 pm.
In a further development of themethod for producing an at least partially
surface-hardened
metallic toothed element, which comprises a densified sintering material, a
preform of the
toothed element is produced with a locally selective oversize relative to a
final size of the
toothed element and rolled to the final size by means of at least one rolling
die, the
toothed element being densified in locally varied manner at least in the area
of at least
one flank and/or one root of a tooth of the toothed element to generate a
densified outer
layer at one surface.
A toothed element is in this case for example a gearwheel, a toothed rack, a
cam, a P
rotor, a toothed ring, a sprocket or the like. The densified sintering
material is produced in
particular using powder-metallurgical methods. For example, a metal powder is
sintered
under pressure in conjunction with heat treatment. Moreover, metal powder is
for example
injection-moulded together with plastics and is sintered in particular under
pressure pref-
erably with heat treatment. To shape a sintered workpiece, use is made in
particular of a
sintering mould, which exhibits at least virtually the final size of the
toothed element to be
produced. The workpiece resulting directly from the sintering process is
preferably used
as a preform. In another variant, however, it is also possible for at least
one further sur-
face treatment step to be arranged downstream. In this case, the preform has
an oversize
which should be regarded as a difference from the final size, the difference
preferably
being defined point-by-point perpendicularly to the surface.
The rolling die used is for example a roller which is equipped with a tooth
system which
may be brought into engagement with the tooth system of the toothed element.
Such a
rolling die is rolled over a surface of the toothed element in particular
under pressure.
Preferably, two or more such rolling dies are used in particular at the same
time. For ex-
ample, a gearwheel to be produced may be arranged centrally between two
rolling dies.
By advancing the two rolling dies, surface densification of the sintering
material of which
CA 02611672 2007-12-10
3
the tooth system is made may then be brought about. In general, such a
production proc-
ess is revealed for example by Takeya et al, "Surface rolling of sintered
gears", SAE 1982
World Congress, Technical Paper 820234. DE 33 250 37, US 4,059,879, EP 0 552
272
Al, EP 1 268 102 Al, US 5,729,822, US 5,711,187, US 5,884,527, US 5,754,937,
US
6,193,927, EP 0 600 421 Al, GB 2,250,227 also each reveal different production
meth-
ods, sintering materials, dies, densification sequences and devices for
sintered tooth sys-
tems which may likewise be adapted for use with the invention. Reference is
made to the
above documents in the context of this disclosure.
For example, a first rolling die may also be used under a first pressure
substantially for
rough contour rolling and then a second rolling die under a second pressure
may be used
to achieve specifically adjusted surface densification.
The locally selective oversize should in particular be so dimensioned that the
toothed ele-
ment is densified in locally varied manner at least in the region of at least
one flank and/or
one root of a tooth of the toothed element in an outer layer at a surface.
Preferably, full
density is achieved within the densified outer layer, the full density
preferably being un-
derstood in relation to the density of a comparable powder-forged tooth. For
example, at
the core a preform of a sintering material preferably has a density of at
least 6.8 g/cm3,
preferably at least 7.1 g/cm3 and in particular at least 7.3 g/cm3. In the
densified outer
layer the preform has, for example, a density of at least 7.7 g/cm3,
preferably at least 7.8
g/cm3, which corresponds to the density of a powder-forged preform of the same
material.
Particularly advantageously, a stress-appropriate strength profile is then
achieved. More-
over, a highly stressable sintered tooth system with a locally variable and
stress-
appropriate density profile is preferably provided. In more highly stressed
areas in particu-
lar, the density profile may display a greater density level over a relatively
large area com-
pared with directly adjacent areas exposed to lower load. By determining an
optimised
oversize, a tooth system produced in this way may also be economically
produced in a
small number of operations.
According to one embodiment, the in each case differently densified outer
layers are pro-
duced together via different oversizes along a flank and/or tooth root of the
preform. For
example, the depth of the densified outer layer, in each case taken
perpendicularly to the
surface, exhibits a maximum density for instance at the site of maximum
stress. This may
be the case half-way up the tooth, for example, and reduce in each case
steadily to zero
CA 02611672 2007-12-10
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towards the tooth tip and the tooth root. In particular to avoid pitting,
provision is made, for
example, for particularly high densification to be established in the
sintering material in an
area between 20% and 30%, in particular between 23% and 25%, below the working
cir-
cle. However, other profiles may also be provided. When designing a
densification profile,
a force profile on a tooth flank of the toothed element is taken into account
in particular
depending on the purpose for which it is to be used. For example, the forces
arising at the
teeth of a gearwheel in a transmission are used for this purpose, and the
resultant equiva-
lent stress profiles below the surface are used. This procedure is also
possible with other
tooth systems.
It is particularly advantageous if an oversize on a first flank of the tooth
is selected to be
different from that on a second flank of the tooth. In this respect, a force
transmission di-
rection is taken into account depending on the purpose for which a toothed
element is to
be used. In the case of a gearwheel in a transmission, account is taken, for
example, of
the fact that, depending on the direction of rotation, different forces arise
at the tooth
flanks in the direction of rotation than contrary to the direction of
rotation. Moreover, differ-
ent densification may be compensated due to the direction of rotation of a
rolling die.
Preferably, the oversizes are so selected that, after a densification process,
an identical
compaction profile results along the first and second tooth flanks.
For example, to prevent stress cracking in a tooth root or tooth base area, a
locally densi-
fied surface layer is also sought in these areas. It is particularly
convenient for an asym-
metrical oversize to be selected at a tooth base. For example, a left-hand
tooth root area
has a different densification depth from a right-hand tooth root. In
particular, a preferably
constant variation of the outer layer depth may be provided in each case
between two
teeth through corresponding variation of the oversize.
Preferably, when designing a tooth system a different, in particular
asymmetrical oversize
is provided not relative to just one flank, but rather preferably relative to
two mutually fac-
ing flanks. In addition, a different oversize is provided in the tooth root,
which is preferably
asymmetrical. Tooth flanks and tooth roots of a tooth system may in each case
be asym-
metrical. An oversize should here be understood not only to mean the provision
of extra
material, but also to include an undersize. This means that less sintering
material is pro-
vided in an area than needs to be provided for the final contour after a
machining step.
The undersize established ensures, for example, that upon displacement of
sintering ma-
CA 02611672 2007-12-10
terial no undesired raised portions arise. The undersize therefore constitutes
an area of a
toothed preform to be filled in particular by displacement of sintering
material.
There is additionally the possibility of providing different angles of
pressure on a tooth
5 system tooth. For instance, the angle of pressure of the one flank of the
tooth may differ
by at least 15% from the angle of pressure of the other flank of the tooth.
In one embodiment provision is made for 2 % to at least 15 % higher density to
be pro-
duced at least 20 pm below a surface of a first tooth flank than at the same
level on a sec-
ond tooth flank. Preferably, a density is achieved on the first flank of the
tooth which cor-
responds at least roughly to the density which is achieved for a powder-forged
toothed
element, whereas the second flank has a lower density. For example, a density
in the
range between 7.2 g/cm3 and 7.7g/cm3 is established on the one flank, while in
the corre-
sponding area of the second flank a density of between 7.5 g/cm3 and 7.82
g/cm3 is es-
tablished. In particular, this again takes account, for example, of different
loads on the two
tooth flanks as a function of direction of rotation. Preferably, an elasticity
and hardness
profile is then achieved which is appropriate to requirements. It is
additionally preferable
that noise development is thereby reduced, for example in a transmission.
Provision is additionally made for a local oversize to be selected to be at
least 10 %
greater on a first flank of the tooth than the oversize on a second flank of
the tooth at the
same level. In a first variant, this makes it possible, for example, for an
identical densifica-
tion profile to be achieved on the first and second tooth flanks due to
exposure to different
pressures during densification as a function of direction of rotation. In a
further variant, a
different densification profile is achieved, for example, on the first and
second tooth flanks.
Different maximum densities may then in particular be achieved, whose depths
as well as
their location are adjusted specifically in relation to the height of the
tooth system.
It is particularly convenient for a maximum local oversize to amount to at
least 15 pm,
preferably at least 100 pm and particularly preferably at least 400 pm. If the
density of the
preform lies in a range of between 7.2 g/cm3 and 7.5 g/cm3, a maximum oversize
of be-
tween 20 and 150 pm is preferably provided. If the density of the preform lies
between 6.7
g/cm3 and 7.2 g/cm3, a maximum oversize of between 50 pm and 500 pm is
preferably
used. An oversize may locally also be negative, lateral redistribution of
material thereby
being taken into account, for example. Lateral redistribution may take place
due to mate-
CA 02611672 2007-12-10
6
rial flow resulting from a rolling process. In particular, an at least locally
negative oversize
may be provided which is locally below the final size. The negative oversize
amounts pref-
erably to at most 100 pm. According to one embodiment, the negative oversize
amounts
at most to less than 50 pm and in particular to less than 20 pm. In
particular, the maxi-
mum negative oversize lies in a range of between 100 pm and 20 pm.
Densification is preferably achieved which reaches a depth of between 1 mm and
1.5 mm
at least in one area of a tooth system tooth flank. Densification at the tooth
root may be
less, on the other hand. According to one embodiment, the maximum
densification depth
of a tooth flank is greater by at least the factor 6 than a maximum
densification depth in an
area of the associated tooth root. This makes it possible for the tooth system
on the one
hand to be sufficiently strong, while on the other hand also retaining a
degree of ductility.
Tooth breakage is thereby prevented.
In one embodiment of the method, provision is made for the preform and the
rolling die to
roll towards one another until a final shaping movement is generated between
the toothed
element produced thereby and the rolling die. This is used for example to
produce mutu-
ally meshing gearwheels. Preferably, during the rolling process with the
rolling die the
distance between rolling die and preform is reduced. To this end, in
particular a rolling
pressure is accordingly established or adjusted. In addition to the
possibility of force con-
trol, the machine may also be provided with path control. Furthermore, it is
possible to
provide a combination of force and path control when producing the tooth
system. Pure
path control may then take place in one part of production while pure force
control takes
place in another part of production. They may also alternate repeatedly.
In a further embodiment, a cycloid-shaped and/or involute tooth system may
arise as a
result of the rolling movement between the preform and the rolling die.
In addition to toothed elements in the form of gearwheels, other toothed
elements may
also be produced. For example, the toothed element takes the form of a cam, in
particular
as is used, for example, for mechanical actuation of an adjusting device, for
example for
adjusting a valve or the like. Preferably, an improved strength profile with
lower suscepti-
bility to wear is provided by locally varied densification of an outer layer
on a flank of a
cam.
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Further improvement of surface hardening may in particular be achieved in that
the
method for producing an at least partially surface-densified metallic toothed
element in-
cludes a thermal and/or chemical surface hardening process.
In a first variant, case hardening is used for example as the thermal and/or
chemical hard-
ening process. Preferably, in addition to an increase in hardness a reduction
in distortion
is here achieved. In a further variant, a carbonitriding process is used, for
example. More-
over, a nitriding or nitrocarburizing process and a boronizing process may be
used. With
these processes in particular, together with heat treatment, a reduction in
distortion is like-
wise achieved. By adjusting the prevailing pressure, hardening may likewise be
influ-
enced. For example, a vacuum may be established, in particular if case
hardening is un-
dertaken. There is also the possibility of undertaking induction hardening.
According to one embodiment, hardening is only partially performed, for
example only in
the area of the tooth system.
In a preferred variant, provision is made for a method for producing an at
least partially
surface-hardened metallic toothed element, which comprises a densified
sintering mate-
rial, to include the steps of "cold or warm pressing, sintering, sizing and
surface densifica-
tion rolling and case hardening". For example, first of all cold pressing of a
metal powder
takes place in a mould which exhibits at least roughly the final size of the
toothed element
to be produced. In a second step, the sintering process takes place for
example with ex-
posure to heat and with or without exposure to pressure. Preferably, sizing
and surface
densification then proceed by means of rolling. As has already been mentioned
above,
sizing and surface densification rolling preferably take place simultaneously
using at least
two rolling dies. Then hardening, in particular case hardening, may finally
take place, this
enabling further hardening of the surface.
Further possible method steps or procedures and also closer details of
workpieces are
indicated hereinafter by way of example. However, the method steps may also be
per-
formed using other materials and achieved density values. The usable sintering
materials
are generally usable as follows for the purposes of the invention, materials
which may be
used being stated by way of example:
- mixed powders (admixed powders): for example iron powder is mixed with other
preferably elemental powders. For example:
CA 02611672 2007-12-10
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Ancorsteel 1000+1.5-3.5 w/o Cu + 0.6-1.1 w/o graphite + 0.5-1.2 w/o lubricant
Ancorsteel 1000B+1.5-2.2 w/o Ni + 0.4-0.9 w/o graphite + 0.6-1.1 w/o lubricant
- partially alloyed powders (diffusion alloyed powders): a powder in which the
alloy
constituent(s) are bound metallurgically to elemental powder or pre-alloyed
pow-
der. For example: Distaloy AB, Distaloy 4600A, Distaloy AE, Distaloy 4800A
- pre-alloyed powders: powders of two or more elements which are alloyed
during
powder production, the powder particles being evenly distributed. For example:
Ancorsteel 4600V, Ancorsteel 2000, Ancorsteel 86, Ancorsteel 150HP
- hybrid alloy: prealloyed or partially alloyed powder with elemental or iron-
alloyed
additions, which are mixed together to achieve the desired material
composition.
For example:
Ancorsteel 85P+1.5-2.5 w/o Ni + 0.4-0.8 w/o graphite + 0.55-1.1 w/o lubricant
ad-
dition
Distaloy AE + 1.5-2.5 w/o Ni + 0.4-0.8 w/o graphite + 0.55-0.95 lubricant
addition
Ancorsteel 85HP + 1.1-1.6 w/o FeMn + 0.35-0.65 w/o graphite + 0.6-0.95
lubricant
addition
1. The workpiece has a core density of between 6.5 and 7.5 g/cm3. The surface
density
amounts to more than 7.5 g/cm3. A maximum density is produced to a depth of
0.1 mm.
Starting materials for the preform are metallic sintering powders, in
particular pre-alloyed
materials, partially alloyed materials or hybrid alloys.
With a pre-alloyed material, cold pressing, sintering in a temperature range
between
1100 C and 1150 C, surface-densification, case hardening and then grinding are
per-
formed, in order to achieve a final workpiece shape with tooth system.
With a partially alloyed metallic sintering material, warm pressing is
performed at a press
temperature in a range of between 50 C and 80 C, followed by high temperature
sintering
in a range preferably of between 1250 C and 1280 C, surface densification and
then vac-
uum case hardening and honing, in order to achieve the final shape of a
workpiece with
tooth system.
With a hybrid alloy comprising a sintering material, warm pressing is carried
out, in which
preferably the powder and the die are heated. Preferably, they are heated to a
range of
between 120 C and 150 . This is followed by a sintering step, for example in
the form of
CA 02611672 2007-12-10
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high-temperature sintering, surface densification and then induction
hardening. Post-
treatment may be dispensed with, for example.
2. The preform is powder-forged. This preform is surface-densified at least
partially in the
area of the tooth flanks and/or of the tooth root. A core density of the
workpiece amounts
to between 5.7 g/cm3 and 7.7 g/cm3. A surface density in the area of the [sic]
amounts to
more than 7.8 g/cm3, all pores remaining in this area preferably being closed
off at the
surface. However, a maximum density may also be produced up to a depth of 1.5
mm.
A production process may proceed as follows: selection of the powder material,
cold
pressing of the powder material, sintering preferably at a temperature of
around 1120 C,
then forging, preferably at a temperature of around 1000 C, possible removal
of an oxida-
tion layer, surface densification in particular by rolling, surface hardening,
in particular
case hardening, and then possible partial grinding to a final contour. The
method may
proceed wholly or partially on a production line.
In a further embodiment, the surface hardening takes the form of vacuum case
hardening,
which is followed by a honing step for partial areas of the tooth system.
3. In particular for the production of rotors and oil pump wheels, a preform
made from an
aluminium-containing material is surface-densified in the area of the tooth
flanks and/or
the tooth roots. The surface densification in particular results in a final
shape of the tooth
system. The core density of the workpiece amounts preferably to between 2.6
g/cm3 and
2.8 g/cm3.
The sintering material is warm pressed for example, for example at a
temperature of be-
tween 40 C and 65 C, then dewaxed, for example at a temperature of more than
400 C,
in particular in a temperature range of between 420 C and 440 C, then
sintered, for ex-
ample in a temperature range of above 550 , in particular in a temperature
range of be-
tween 600 C and 630 C, then homogenised and cooled, for example to a
temperature of
between 480 C and 535 C, surface densification then taking place, in
particular by rolling.
Then, hardening can take place, for example in a temperature range of between
120 C
and 185 C for a period of between 6 h and 24 h.
CA 02611672 2007-12-10
4. The preform is preferably densified along the tooth flanks and the tooth
root, in particu-
lar two rolling dies being used, the preform being arranged rotatably in the
middle thereof.
A core density of the workpiece amounts, depending on the material, to
preferably be-
tween 7.2 g/cm3 and 7.5 g/cm3, the surface density being greater than 7.8
g/cm3 at least in
5 places depending on the material. A maximum density is present in particular
to a depth of
1 mm, possibly even therebeyond.
According to one embodiment of the production steps, it is proposed to cold
press pre-
alloyed material, then to sinter it, in particular in a temperature range of
between 1100 C
10 and 1150 C, to perform surface densification and hardening and optionally
partially to
grind the surface.
In a further embodiment, a partially alloyed sintering material is warm
pressed, in particu-
lar in a temperature range of between 50 C and 90 C, high-temperature
sintering is per-
formed, in particular in a temperature range of between 1240 C and 1290 C,
surface den-
sification is performed, followed by vacuum case hardening and optionally then
honing.
Another embodiment provides for hot pressing of a hybrid alloy, the powder and
the
pressing die preferably being heated to a temperature range of between 120 C
and
160 C. A sintering step is followed by surface densification, which is
followed by harden-
ing, preferably induction hardening.
5. It is also possible for pre-sintering to be followed by surface
densification and then in
turn resintering is provided as a method step in the production of a toothed
workpiece.
Pre-sintering may take place for example in a temperature range of between 650
C and
950 C. Resintering may take place for example at a sintering temperature
conventional for
the material, for example between 1050 C and 1180 C. There is also a
possibility of high
temperature sintering, for example in the range of between 1250 C and 1280 C.
Harden-
ing and/or remachining may then optionally follow, for example honing or even
grinding.
The preceding pressing may take place under cold, warm or hot conditions, the
pressing
die and the powder preferably being heated up in the last case. Hot pressing
takes place
in a temperature range of between 120 C and 160 C, for example.
CA 02611672 2007-12-10
11
6. In a further development, sinter hardening follows a resintering step. This
may option-
ally be followed by grinding or honing.
7. In a further production process, the preform is densified at a temperature
of above
150 C, in particular above 500 C. For example, the preform may be guided
directly from a
sintering furnace into a machine for surface densification. The preform may
then be at a
temperature which is for example above 600 C, in particular even above 800 C.
Prefera-
bly, the die(s) for surface densification is/are heated, for example to a
temperature of
around 150 C. According to another embodiment, the surface densification die
is cooled,
preferably by cooling proceeding inside the die.
8. In a further production process, surface densification takes place while
the preform is
being at least partially heated. In particular, heating proceeds to a
temperature which
makes surface densification easier. Preferably, induction heating is used for
this purpose.
This is followed by rapid cooling, in order to achieve a martensitic
structure. In this way an
ausforming process may, for example, be combined with surface densification.
A further development of the invention provides for surface densification to
be performed
using the widest possible range of methods. In one embodiment in particular,
surface den-
sification is performed in a first area using a different method from in a
second, different
area. Methods which can be used here are shot peening, shot blasting,
densification by
means of a ball, a roller or by means of another rotatable body, by means of
tooth-shaped
dies, in particular rolling dies and the like. These methods are also suitable
in each case
separately from one another for enabling the necessary surface densification.
For example, the tooth root is not densified at all or only slightly with a
die with which the
tooth flank is also densified. It is possible to densify the surface in one
portion to such an
extent that only the pores at the surface are closed. Then the tooth root can
be treated
with another die or surface densification method. In this way, in particular,
a different sur-
face densification can be achieved along the tooth flank than at the tooth
root. Different
surface qualities, for example relating to roughness, can be established in
this way, for
example. Even the maximum surface densification can be different due to the
various
techniques. It is also possible for the entire toothed workpiece to achieve
surface densifi-
cation, for example using surface blasting. In particular, even aluminium-
containing sinter-
CA 02611672 2007-12-10
12
ing material or other oxide-forming sintering materials can be treated in this
way, since
surface densification can additionally also enable removal of an oxide layer.
The invention further relates to a preform for a method for producing an at
least partially
surface-hardened metallic toothed element, which comprises a densified
sintering mate-
rial, a first and a second flank of a tooth each having different asymmetrical
oversizes.
Provision is additionally made for a first and a second root area of a tooth
to have differ-
ent, in particular asymmetrical, oversizes.
The invention further relates to a toothed element comprising a metallic
sintering material,
the toothed element exhibiting locally varied densification at least in the
area of at least
one flank of a tooth of the toothed element. Preferably, this results in
elasticity of the pow-
der-metallurgical material appropriate for many applications together with
surface harden-
ing. Particularly preferably, noise reduction is enabled for example in the
case of gear-
wheels during power transmission while at the same time good wear resistance
is pro-
vided.
In a first variant, the toothed element is a spur-toothed gear.
For improved power transmission in particular, as well as for noise reduction
between
gearwheels, in a further variant the toothed element is a helical gear.
Moreover, in another
variant a bevel gear may be provided. In accordance with the description given
above, it is
expedient for mutually facing tooth flanks of a toothed element to exhibit
asymmetrical
densification.
Furthermore, it is expedient for asymmetrical densification to be present in a
root area.
This densification is then adapted in particular to forces arising when the
toothed element
is used as intended. To prevent stress fracture, provision is made in
particular for the
depth of the locally densified outer layer to be only such that sufficient
tooth elasticity or
rigidity is still ensured. Particularly preferably, the depth of the densified
outer layer is less
in the root area than on a tooth flank.
One special form which the toothed element may take is that of a cam. The
above expla-
nations may be applied accordingly thereto, cam flanks taking the place of
tooth flanks, for
example.
CA 02611672 2007-12-10
13
Various compositions may be provided as the material for a toothed element. In
a first
variant, an iron material is selected as the main constituent of the sintering
material and at
least one alloy constituent is selected from the group comprising carbon,
molybdenum,
nickel, copper, manganese, chromium and vanadium. One iron alloy is for
example Fe -
1.0 Cr -0.3 V +0.2 relative to a reference alloy 15CrNiMo6. A further iron
alloy is for exam-
ple Fe -1.5 Mo +0.2 C relative to 20MnCr5. Another example of an iron-
containing alloy is
Fe -3.5 Mo relative to 16MnCr5. Likewise, for example, the alloy C 0.2% Cr
0.5% Mn
0.5% Mo 0.5% may be used, the remainder being iron and impurities. Further
composi-
tions may also be provided.
Preferably, to reduce the weight of a toothed element, provision is made for
aluminium or
magnesium to be selected as the main constituent of the sintering material.
According to
one aspect of the invention, a surface-densified tooth system of sintering
material com-
prises at least 80% aluminium and at least copper and magnesium as further
sintering
materials. In a first embodiment, silicon is additionally used as a sintering
material, for
example in a range of from approximately 0.45% to approximately 0.8%,
preferably of
between 0.6% and 0.75 %. However, silicon may also be present in a higher
range, for
example of between 13% and 17%, in particular between 14.5% and 15.5%. If the
silicon
content is higher, the copper content in the sintering material is reduced.
Thus, a first mix-
ture may comprise for example copper in a proportion of 4% to 5%, silicon in a
proportion
of 0.45% to approximately 0.8%, magnesium in a proportion of approximately
0.35% to
0.7%, the remainder being at least mainly aluminium. In addition, a pressing
aid is pref-
erably added. This may be in a proportion of between 0.8 and 1.8%. For
example, a wax,
in particular amide wax, may be used for this purpose. A second mixture may
comprise for
example copper in a proportion of 2.2% to 3%, silicon in a proportion of 13%
to approxi-
mately 17%, magnesium in a proportion of approximately 0.4% to 0.9%, the
remainder
being at least mainly aluminium. A pressing aid may again be used, as stated
above by
way of example. After surface densification, at least one area of the tooth
system has a
density of for example more than 2.5g/cm3, preferably up to maximum density.
Preferably,
a toothed workpiece produced in this way has a tensile strength of at least
240 N/mm2 and
a hardness of at least HB90 auf. If the silicon content is higher, the density
may in particu-
lar amount to even more then 2.6 g/cm3.
CA 02611672 2007-12-10
14
In a second embodiment, additionally at least zinc is used as a sintering
material in addi-
tion to copper and magnesium as additives and aluminium. Preferably, the
copper content
is in a range of between 1.2% and 2.1%, in particular between 1.5% and 1.65%,
that of
magnesium between 1.9% and 3.1 %, preferably between 2.45% and 2.65%, and that
of
zinc between 4.7% and 6.1 %, in particular between 2.3% and 5.55%. The
remainder is at
least mainly aluminium. In addition, a pressing aid may here too be used as
described
above. After surface densification a toothed workpiece made from this mixture
preferably
comprises at least one area of the toothed system in which the density ranges
from at
least 2.58 g/cm3 to the maximum density. Preferably, a toothed workpiece
produced in this
way has a tensile strength of at least 280 N/mm2 and a hardness of at least
HB120.
It is particularly convenient for a toothed element to be sintered together
with a further
functional component, in particular a shaft or a further gearwheel. In
particular, this makes
it easier to maintain a precise working distance between a plurality of
toothed elements,
for example in a transmission.
In a further embodiment, the toothed element is a component of a pump. For
example, the
gearwheel is an involute gear, which is caused to mesh with a further involute
gear.
Moreover, the invention relates to a device for producing an at least
partially surface-
densified toothed element in particular for carrying out an above-described
method, with
die control adapted to a varying oversize. The device in particular comprises
at least one
rolling die, which, preferably by means of the adapted die control, may act on
the preform
in adapted engagement preferably under an adapted pressure and/or with a
controlled
path. In particular, the device comprises a rolling die with a toothed
surface, which may be
brought into engagement with the tooth system of the toothed element and
rolled there-
over.
The present invention further provides a device for producing an at least
partially surface-
hardened toothed element from a preform consisting at least in a surface
region of a sin-
tering material, the device comprising a die which provides compensation of
different
oversizes at the first and second flanks of a tooth of the preform to be
densified by means
of rolling motion. The rolling die may here have a contour necessary for
shaping, for ex-
ample an involute tooth system, on just one flank or on both flanks of a
tooth. In another
variant, however, mutually different oversizes are present on each of the
first and second
CA 02611672 2007-12-10
flanks of a tooth of the tooth system of the rolling die. This may be a
different involute
tooth system, for example.
The invention also relates to a method for designing an oversize to achieve
surface densi-
5 fication of a sintered metal toothed element in a rolling process, the
oversize being deter-
mined iteratively. In a first step, for example, a geometry and in particular
a torque and/or
a pressure distribution are predetermined. In a further step, for example,
rolling die design
is defined. Moreover, a preform is established with a locally defined
oversize. Selection
may for example proceed with reference to data libraries. Such a data library
contains
10 experimental density profiles determined with reference to various
parameters, for exam-
ple. Moreover, simulation of the densification or rolling process may take
place. To this
end, for example, the kinematics of the rolling process are simulated in
conjunction with
simulation of elastic and plastic properties of the preform and optionally of
the rolling die.
To simulate the elastic or plastic properties of the preform, reference is
made, for exam-
15 ple, to continuum mechanics models in conjunction with a discrete solution
by means of
for example finite element or finite volume methods.
In a preferred embodiment, a geometry of a rolling die is determined
iteratively taking ac-
count of the oversize. For example, an oversize of an involute tooth system of
the rolling
die may be determined. An oversize may be determined in corresponding manner
for a
tooth system other than an involute tooth system.
In a particularly preferred embodiment, in a first step an oversize of a
preform of the
toothed element, locally varied at least in one area of a flank of a tooth and
definable at
least point by point, is generated automatically using at least one design set
value, in a
second step the geometry of a rolling die is automatically generated, in a
third step a roll-
ing process and resultant local densification profile for at least one outer
layer of the
toothed element is simulated and in a fourth step an automatic evaluation of
the densifica-
tion profile generated is compared with a set value and optionally the method
is repeated
from the first step using at least one variation for the purpose of
optimisation, until a stop-
ping criterion is fulfilled. Variation proceeds for example with the aid of an
optimisation
method. A stopping criterion is for example a tolerance between the desired
density profile
and the density profile achieved in the simulation. Moreover, a stopping
criterion may also
consist of exceeding a predeterminable number of iterations.
CA 02611672 2007-12-10
16
It is particularly convenient for the design set value to be selected from the
group compris-
ing material density, geometry, torque and pressure distribution. Torque
should here be
understood as the torque arising depending on the purpose for which a toothed
element is
to be used.
In particular, to prevent material fractures it is convenient for material
stress to be simu-
lated at least in the area of densification and in particular to be used for
evaluation. This
preferably prevents a surface from being brittle as a result of stresses and
having a ten-
dency towards stress fracture despite the surface being sufficiently hardened.
Furthermore, it is advantageous for data stored in a database library to be
used for varia-
tion. In particular, methods may then be used for optimisation and for data
analysis for
example using neuronal networks. Moreover, features stored in the database are
used for
example for optimisation using a genetic algorithm.
In a further embodiment, at least one of the steps may be replaced by a set
value. Pref-
erably, rolling die geometry is firmly preset. In this way it is possible to
take account, for
example, of the fact that a rolling die is substantially more complex to
modify than for ex-
ample a preform. Another embodiment provides a reverse procedure. Preferably,
a pre-
form or the rolling die for producing a final form as well as the pressing die
for producing
the preform are calculated on the basis of the final form.
Finally, the present invention provides a computer program product with
program code
means, which are stored on a computer-readable medium, for carrying out at
least one of
the above-described methods when the program is run on a computer. A computer-
readable medium is for example a magnetic, a magneto-optical or an optical
storage me-
dium. Moreover, a memory chip is used, for example. In addition, a computer-
readable
medium may also be provided by means of a remote memory, for example by means
of a
computer network.
The computer program may be stored, for example, in a surface densification
machine.
Calculation may also take place separately from the surface densification
machine. How-
ever, the machine has a controller, in particular a position- and/or force-
controlled control-
ler, into which the coordinates and movement sequences may be input in order
to densify
the preform.
CA 02611672 2007-12-10
17
According to a further aspect the invention, a pressing die shape is provided
with which a
preform may be pressed from sintering material, this subsequently being
surface-densified
to yield the final form. This pressing die shape is calculated iteratively.
Preferably, this is
likewise undertaken on the basis of data from a final contour of the workpiece
with its
tooth system.
A contact rolling tester may also be provided, which offers the possibility of
being able to
undertake test rollings for the widest possible range of surface
densifications. Further-
more, data may in particular also be determined which may be included after
evaluation in
the calculation method. For example, characteristic values suitable therefor
may be built
up from a plurality of measurements. In the process, start values for
iterative calculation of
preform, die or pressing die may be obtained, for example. The contact rolling
tester may
also include automated measurement of surface-densified workpieces which have
a tooth
system.
Further concepts are proposed below, which may be combined with the previously
pro-
posed aspects or indeed performed independently thereof.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a method is
provided for produc-
ing a tooth system from densified sintering material, a predensified tooth
preform being
densified by at least 0.05 mm from its surface at least in one area by means
of iteratively
determined data to yield its final form, and a final form quality in a range
of at least fHa = 4,
Fa = 7 and Ffa = 7 being achieved. Here fHa means the deviation relative to
the tooth sys-
tem, Fa the total deviation and ffa the profile shape deviation of the flanks.
The stated val-
ues correspond to the DIN classes relating to deviation.
According to one further development, provision is made for iteration to take
account of
parameters which relate to material behaviour during surface densification of
the tooth
shape. In one embodiment, iteration for determining a preform is based on
input data,
which are taken from a final form set value. Preferably, at least one rolling
die is used,
which is of the same quality as the subsequently produced final form.
Iterative determina-
tion and the resultant extremely precise treatment during surface
densification allow the
quality of the die to be transferred to the preform. In particular, the
extremely precise sur-
CA 02611672 2007-12-10
18
face densification makes it possible for the tooth system to gain this final
form quality after
surface densification without a further material-removing postmachining step.
For exam-
ple, a toothed workpiece is produced with a core density of at least 7.4 g/cm3
and with a
surface density which is at its maximum in at least one area of a tooth flank,
the maximum
surface density in the area extending to a depth of at least 0.02 pm.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a method is
provided for produc-
ing a tooth system from densified sintering material, a predensified tooth
preform being
densified at least in one area by means of iteratively determined data to
yield its final form,
and roughness in the area being improved relative to the preform by at least
400 %, a
surface hardness of at least HB 130 being established. Preferably, a final
form core den-
sity is established which has a density of at least 7.3 g/cm3, and a surface
hardness is
impressed thereon which has a convex profile from the surface towards the
middle of the
final form.
The tooth system of predensified material has a roughness in a first surface-
densified
area which is at least 400 % less than the roughness in a second area, which
is surface-
densified less or not at all. The roughness R2 amounts for example in the
first area to less
than 1 pm. In a further embodiment, a surface hardness of at least HV 700
[0.3] is present
at the surface of the final form, while at a depth of 0.4 mm from the surface
a hardness of
at least HV 500 [0.3] is present. Another embodiment has a surface hardness of
at least
HV 700 [0.3] at the surface of a tooth flank and at a tooth base, a hardness
of at least HV
500 [0.3] being present at a depth of 0.6 mm from the surface at the tooth
base and a
hardness of at least HV 500 [0.3] being present at a depth of 0.8 mm from the
surface at
the tooth flank. Implementation of surface densification makes it possible
purposefully to
be able to establish precise densification and also hardening in accordance
with desired
set values.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a calculation
method is provided
for designing a preform of a tooth system of sintering material, data being
input into the
calculation process which are determined from a predetermined tooth system
final form,
one or more tooth system stress parameters being determined as a function of
at least
one use condition of the final form, a local preform oversize being
calculated, which corre-
CA 02611672 2007-12-10
19
lates with expected surface densification of the preform, and stress on the
sintering mate-
rial below the surface also being included in the calculation.
Preferably, the calculation is additionally based on penetration of the die
into the work-
piece to be produced, wherein it is possible in particular to take account of
the behaviour
of the sintering material on penetration and after penetration. For example,
the calculation
method provides for elastic deformation of the sintering material to be
densified to be
taken into account. The calculation method may also provide for
elastic/plastic deforma-
tion of the sintering material to be surface densified to be taken into
account. Preferably,
the depth of maximum stress under the surface for example when the workpiece
is used
as a power-transmitting gearwheel is included in the calculation method. The
calculation
method may additionally allow shrinkage of the sintering material on sintering
to be in-
cluded in the calculation. Empirically determined data may likewise be
included in the cal-
culation.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a calculation
method is proposed
for designing a surface densification die of a preform of a tooth system of in
particular
densified sintering material for establishing a predetermined tooth geometry,
data deter-
mined from the predetermined tooth geometry to be produced being iteratively
included
for calculation of die machine kinematics taking into account mutually
associated machine
axes of a workpiece, from which the die to be produced is formed, and of at
least one die
former, the coupled system coordinates thereof and the relative movement
thereof. This
makes it possible, instead of using repeated tests, measurement results and
adaptation of
the workpiece former ultimately to achieve a final form, to bring this about
by means of
iterative calculation. This takes significantly less time and allows the most
varied influenc-
ing parameters to be taken into account. Simulation of the design is in
particular also en-
abled, such that for example a mode of action of the die to be produced on a
designed
preform can be checked by simulation.
According to one embodiment, contact conditions between the workpiece to be
produced
and the die former between a tip and a root of the tooth system are included
in the calcu-
lation method. Preferably, maximum stress at the surface in the area of a
tooth system
root is here also included in the calculation. Moreover, it is possible for
maximum stress
below the surface in the area of a tooth system flank to be included in the
calculation. This
CA 02611672 2007-12-10
method is particularly suitable for sintering materials, but also for steel
workpieces or
workpieces of other materials.
According to a further concept of the invention, which may be used
independently and
5 also together with the further features of the disclosure, a pressing die
with a press ge-
ometry for producing a tooth system preform of sintering material is proposed,
the press
geometry having a profile, adapted to surface densification of the tooth
system, with at
least one raised portion, which generates an indentation at least in the area
of the preform
tooth system, which indentation may be filled with sintering material on
surface densifica-
10 tion.
Preferably, the raised portion forms an indentation in the area of a tooth tip
of the tooth
system on a face of the preform. It is possible, by iterative calculation for
example, to de-
termine the height of the raised portion or depth of the indentation as well
as further di-
15 mensions thereof. Instead of a raised portion on one side, in a further
embodiment a
raised portion is provided on both sides, in order to bring about an
indentation on each of
the two faces of the tooth. According to a further development, the raised
portion is ar-
ranged in an area of the geometry which brings about an indentation on a tooth
tip of the
preform, the raised portion bringing about a dimension such that the
indentation formed at
20 least partially reduces growth of the tooth tip as a result of forming of
the preform into the
final form by surface densification. In this way, for example, a preform may
be calculated
and in particular manufactured with at least one indentation on one face of a
tooth system
for counterbalancing the material piled up on surface densification of a
running surface of
the tooth system. It is also possible in this way to calculate and in
particular manufacture a
preform with at least one indentation on a tooth tip of a tooth system in
order to reduce
growth of the tooth tip in height on surface densification at least of the
flanks of the tooth
system. The calculation method for determining the geometry of a preform or of
a press-
ing die preferably provides for the geometry to be determined on the basis of
data from a
final form of the preform and for at least one indentation or raised portion
to be calculated,
which has the effect, at least partially, of compensating material
displacement during sur-
face densification.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a method for
surface densification
of a tooth system is proposed, wherein the number of repetitions of a
densification move-
CA 02611672 2007-12-10
21
ment by a shaping surface densification die of a face of the preform is
calculated itera-
tively. Preferably, overrolling is calculated iteratively until a
predetermined surface density
is achieved. In one further development, feed of the shaping die is calculated
iteratively.
According to one embodiment, overrolling of the preform takes place fewer than
20 times
to obtain the predetermined geometry of a final surface densification form.
Preferably,
overrolling takes place fewer than 10 times. In particular, overrolling of the
preform is per-
formed less than 6 times, until a predetermined geometry of a final surface
densification
form is achieved. It should here be taken into account that surface
densification is not yet
terminated upon achieving this. Instead, the die is then run over the surface
several more
times, in particular fewer than 25 times, preferably fewer than 15 times. This
ensures ac-
curacy of the surface shape.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a method is
proposed in which
reversing rolling is performed on a tooth system of sintering material, in
order to densify
the preform to yield the final surface densification form. Preferably, the
preform is briefly
unloaded by the shaping die prior to a change in direction. It has emerged
that reversing,
i.e. changing the direction of movement, enables the achievement of uniform
densifica-
tion. It was possible, furthermore, to minimise problems still further during
manufacture by
decreasing the pressure of the die on the workpiece before the change in
direction takes
place. The die can stay in contact with the workpiece at this point, or it may
be briefly de-
tached from the surface.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, surface
densification of a work-
piece with at least one tooth system of sintering material is proposed,
wherein a first sur-
face of the workpiece is densified using a different method from a second
surface of the
workpiece. Preferably, a first tooth system of the workpiece displays
different densification
from a second tooth system of the workpiece. In a further development, an
internal tooth
system of the workpiece undergoes different surface densification from an
external tooth
system of the workpiece. There is also the possibility of surface-densifying
the external
tooth system by means of a rolling method while a second face is a bore, which
is surface
densified using a different method. Preferably, a bore in the workpiece has a
hardened
surface after surface densification and is then brought into its final form.
This allows the
CA 02611672 2007-12-10
22
bore to be used for a shaft or an axle. Accuracy may be improved in that,
after hardening
of the tooth system, surface densification takes place.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a shaft is provided
with at least
one first and one second tooth system, the first tooth system being rolled
from sintering
material and surface densified. Below, features relating to the shaft or the
tooth systems
are stated. The further disclosure relating to the tooth system, the
materials, the produc-
tion steps etc. may be used in particular for further embodiments.
According to one embodiment, the shaft comprises a second tooth system, which
is pro-
duced by a different method from the first tooth system. This enables a
plurality of combi-
nations, which provide different material solutions for each instance of
stress. The second
tooth system forms a workpiece with the first tooth system, according to a
further em-
bodiment. For example, both tooth systems may have been produced together in a
press-
ing machine. Preferably, the first and second tooth systems have been
iteratively calcu-
lated and produced accordingly. According to one embodiment, production may
take
place successively, while it may take place simultaneously according to
another embodi-
ment. This also applies in particular to further forming steps such as for
example surface
densification.
In a further development, the second tooth system comprises a hardened surface
without
surface densification. For certain cases of stress, the density achieved by
sintering or the
strength inherent in the material used is sufficient. This applies for example
to pump appli-
cations.
Furthermore, it has proven advantageous for at least the first tooth system to
have in each
case different flank pitches on at least one tooth at the same level on the
tooth. This is
advantageous in applications where a main direction of rotation and in
particular only one
direction of rotation is predetermined for the shaft. The various flank
pitches may thereby
be designed to be wear- and noise-reducing.
In another embodiment, the second tooth system is forged. It may additionally
be surface
densified. This tooth system may absorb a greater amount of transmitted power
than the
first tooth system, for example.
CA 02611672 2007-12-10
23
Preferably, the second tooth system is made from a different material from the
first tooth
system. The second tooth system is made from steel, for example. However, the
second
tooth system may also consist of a different sintering material than the first
tooth system.
In addition, the shaft may likewise consist of sintering material. It may be
of the same ma-
terial, for example, as the first tooth system. The shaft may also be formed
at least to-
gether with the first tooth system, i.e. pressed from powder material,
preferably in a com-
mon pressing die.
In one exemplary method of producing the above-described shaft, at least the
first tooth
system is exposed to surface densification and a bore for receiving the shaft
is surface-
densified and then honed, before the shaft and the first tooth system are
connected to-
gether. To this end, iterative calculation of a preform of the first tooth
system preferably
takes place on the basis of a final form of the shaft with the first tooth
system.
Such a shaft is preferably used in automotive technology and in transmission
construction
and domestic appliances.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a preform is used
to produce a
tooth system of sintering material, the preform having a negative oversize.
Preferably, the
negative oversize is arranged at least on one flank of a tooth of the tooth
system. In par-
ticular, the negative oversize may extend asymmetrically along the flank.
In a further development, a negative oversize is provided on each flank of a
tooth. For
example, a tooth comprises a first negative oversize on a first flank and a
second negative
oversize on a second flank at the same level, the first and the second flank
extending
asymmetrically relative to one another.
Preferably, the negative oversize is arranged between a tip area of the tooth
and an over-
size on a flank of the tooth. Additionally or alternatively, the negative
oversize may be ar-
ranged in a corner area of the tooth root. It is additionally possible for the
flanks of a tooth
to have different pitches.
CA 02611672 2007-12-10
24
In addition to an external tooth system or other toothing type, surface
densification may
also be performed on a tooth system which comprises internal toothing. A
surface-
densified gearwheel is ultimately obtained from the preform.
A further development provides a method for producing a tooth system from a
sintering
material in which at least one negative oversize determined by means of
iterative calcula-
tion is assigned to a preform, which oversize is filled at least in part by
displacement of the
sintering material during surface densification of the tooth system.
Preferably, oversize
material adjacent the negative oversize is displaced into the negative
oversize. The pre-
form may be surface densified to yield the desired final form, hardening
and/or a surface
finish-machining being optionally performed. This may take place beforehand or
after sur-
face densification. Possible methods of finish-machining are honing and
grinding.
Preferably, the negative oversize is designed by means of iterative
calculation, in which a
simulation of the surface densification using the preform determines whether
the adjacent
oversize is so designed with regard to shape that the negative oversize may be
smoothed
to yield the desired final contour. To this end, a machine is made available
for calculating
and/or implementing surface densification of a tooth system, wherein a
calculated kine-
matics value may be input, by means of which a negative oversize on a flank of
the tooth
system may be smoothed to a desired final contour using surface densification.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a method for
producing surface
densification on a tooth system is proposed in which at least two preforms are
simultane-
ously provided with surface densification in one device.
According to one embodiment, the preforms are arranged on parallel shafts and
come
simultaneously into engagement with at least one surface densification die.
According to a second embodiment, at least two preforms are arranged on a
common
shaft and brought jointly into engagement with at least one die for surface
densification.
Moreover, a device for producing surface densification on a tooth system is
proposed in
which at least two preforms may be held in the device for surface
densification and simul-
taneously formed.
CA 02611672 2007-12-10
Provision is made, for example, for at least one shaft to move in such a way
that the two
preforms come into engagement with a surface densification die. In one further
develop-
ment, at least three shafts for at least two preforms and at least one die are
arranged par-
5 allel to one another and form a triangle, wherein at least one of the shafts
may be moved
towards the other two shafts. In a further embodiment, at least two preforms
may be
mounted on a common shaft, the die having a greater length than the length of
the at least
two preforms added together. Preferably, the preforms lie with their end faces
against one
another. In another embodiment, there is a distance between the preforms,
wherein the
10 die projects along the shaft beyond both outer end faces of the preforms.
According to a further concept of the invention, which may be used
independently and
also together with the further features of the disclosure, a component is
proposed with a
surface-densified tooth system of sintering material, the component, when
viewed over a
15 cross-section, having a gradient relative to the sintering materials used.
Preferably, the component exhibits a gradient which has a step function. The
sintering
materials are provided with a transition boundary at least in this area.
According to one
embodiment, this transition boundary is present along the entire face between
first and
20 second sintering materials. In another embodiment, such an area has no
fixed boundary
but rather a gradual transition. In particular, the component may comprise
different sinter-
ing materials, which extend into one another without a pronounced mixing zone
of in-
creasing or decreasing gradient.
25 In a first further development of the component, the sintering material of
the tooth system
has a lower core density than the sintering material of an area of the
component adjoining
the tooth system. In a second further development of the component, the
sintering mate-
rial of the tooth system has a higher core density than the sintering material
of an area of
the component adjoining the tooth system.
A further embodiment comprises a component which has a first tooth system of a
first
sintering material and a second tooth system of a second sintering material.
Preferably, a tooth system comprises different flank angles at the same level
on one tooth.
CA 02611672 2007-12-10
26
For example, a first sintering material may be arranged in an outer area of
the component
and form the tooth system, and a second sintering material is arranged in an
inner area of
the component and forms a bore.
Moreover, methods are proposed for producing a surface-densified tooth system
on a
component, wherein a first sintering material is admitted into a mould before
a second
sintering material is added, then pressing and sintering take place and only
one of the two
sintering materials is densified by means of surface densification of the
tooth system,
while the other sintering material is not modified in any way.
In a further development, a second surface densification is performed, which
only affects
the as yet un-surface-densified sintering material. Preferably, the first
sintering material
forms at least one surface of the tooth system flanks and the second material
an underlay
for the tooth system.
In a further proposed method for producing a surface-densified tooth system on
a compo-
nent, a first sintering material is admitted into a mould before a second
sintering material
is added, then pressing and sintering are performed and the first and second
sintering
materials are densified by means of surface densification of the tooth system.
To perform the method, it has proven advantageous for a movement sequence for
surface
densification to be determined iteratively by taking account of the material
behaviour of at
least one of the two sintering materials.
In a further development of both methods, relative rotation takes place
between the
mould, in particular a pressing die, and a sintering material to be
introduced, such that the
sintering material collects in an outer area of the mould as a function of the
speed of rela-
tive rotation.
Provision may also be made for the first and at least the second sintering
material to be
added to the mould with at least a period of time overlap.
Furthermore, reference is made to US 5,903,815. This reveals various sintering
materials,
sintering material conditions, moulds, principles relating to the processing
of two or more
sintering materials, applications and method steps. In this regard, reference
is made in the
CA 02611672 2007-12-10
27
context of the disclosure to the content of this publication, which belongs to
the disclosure
content of this invention.
According to a further concept of the invention, it is proposed also to
provide, as part of
the production method in addition to the tooth system surface densification
step, grinding
or honing of the densified tooth flanks and/or tooth roots in particular in
the case of a
forged gearwheel, chain wheel or toothed ring. Preferably, a density of at
least 7.6 g/cm3
is achieved as core density by forging. Surface densification can therefore
bring about full
densification and/or also tooth system shape precision. In a further
development, an over-
size within a range of 4 pm to 8 pm of material beyond the final size is
available for a ma-
terial-removing machining step after surface densification. If, instead of
forging, pressing,
sintering and hardening, in particular case hardening, is performed,
preferably 30 pm to
50Nm of oversize is available for honing and 50pm to 0.3 mm, preferably 0.1 mm
to 0.2
mm of oversize is available for grinding after surface densification.
Iterative calculation
makes it possible to determine the areas and oversizes beforehand and
subsequently
also to implement them in this way in the method. For a bore in the gearwheel,
chain
wheel or toothed ring, surface densification is preferably likewise provided,
followed by
hardening and then preferably honing. For this purpose, the bore may likewise
still have
an oversize of between 30 pm and 50 pm after surface densification.
A further advantage involves lubrication during surface densification. In
addition to using
emulsions, oils in particular may also be used as lubricants. This is
preferable in the case
of hot rolling, for example at temperatures of over 220 C. Furthermore it is
proposed to
perform hot rolling at a temperature of between 500 C and 600 C, wherein
preferably oil
cooling is used, in order on the one hand to provide lubrication and on the
other hand to
cool the die.
The invention is explained in detail below by way of example with reference to
the draw-
ings. However, these illustrated embodiments should not be regarded as
limiting the
scope and details of the invention. Instead, the features emerging from the
figures are not
limited to the respective individual embodiments. Rather, these features may
be combined
in each case with other features indicated in the drawings and/or in the
description, includ-
ing the description of the figures, in each case yielding further developments
which are not
shown.
CA 02611672 2007-12-10
28
In the drawings:
Fig. 1 shows a rolling arrangement,
Fig. 2 shows a first tooth,
Fig. 3 shows a second tooth,
Fig. 4 shows a third tooth,
Figs. 5 to 7 show various oversize profiles for various toothed elements,
Fig. 8 shows a first method diagram,
Fig. 9 shows a second method diagram,
Fig. 10 shows an oversize profile for a toothed element of a rolling die,
Fig. 11 is a schematic view of a calculated indentation on an end face,
Fig. 12 is a schematic view of calculated extreme die cases,
Fig. 13 is a schematic view of a procedure during iterative calculation and
associa-
tions during simulation,
Fig. 14 is a view of density profiles as a function of various starting
densities of the
preforms used,
Fig. 15 is an overview of the determined errors, which arise with different
surface
densification steps and co-characterize the material behaviour,
Fig. 16 shows an HV hardness profile over a tooth system flank for different
sur-
face densification steps,
Fig. 17 shows an HV hardness profile in a root area of a tooth system for
different
surface densification steps,
Fig. 18 is a schematic view of different calculated oversize profiles for
different
densities,
Fig. 19 is a schematic representation of parameters which may be involved in
the
iterative calculation.
Fig. 1 is a schematic view of an exemplary rolling arrangement. A first
rolling die 101 with
a first tooth system 102 is mounted so as to be rotatable about a first axis
103 in a direc-
tion of rotation 104. The first tooth system 102 is in engagement with a
second tooth sys-
tem 105 of a preform 106. The preform 106 is mounted so as to be rotatable
about a sec-
ond axis 107. This results accordingly in a second direction of rotation 108.
Moreover, the
second tooth system 105 is in engagement with a third tooth system 109 of a
second roll-
ing die 110. This second rolling die 110 is mounted so as to be rotatable
about a third axis
111 in a third direction of rotation 112. For example, the first axis 103 or
the second axis
CA 02611672 2007-12-10
29
107 may be fixed axes, while the other two axes may implement a feed movement.
For
example, the third axis 111 is displaceable in a direction of displacement 113
along a line
114 connecting the first 103, the second 107 and the third axis 111. For
example, a sizing
rolling process may be undertaken. In said process, the tooth flanks in
particular are
merely slightly compacted and the tooth bases in particular are not compacted.
This re-
sults in surface densification in a desired area. During surface
densification, on the other
hand, it is also possible just or additionally to surface-densify the tooth
base. For example,
for this purpose, during a rolling process progressive displacement takes
place in the di-
rection of displacement 113. In particular, by means of the first and of the
second rolling
die 101 , 110 it is also possible to densify an area of the tooth roots of the
preform 106. To
adjust the first and/or the second rolling die 110 and to apply a pressure
necessary for a
rolling process, an adjusting device, not shown, is preferably provided with a
transmission.
In this way, in particular very high pressures may also be applied.
Fig. 2 shows a first tooth 201 of an associated toothed element, not shown.
This toothed
element comprises a gearwheel. The geometry of the toothed element or of the
first tooth
201 is characterised by a first root circle 202, a first usable root circle
203, a first working
circle 204 and a first tip circle 205. On a first flank 206 the first tooth
201 comprises a first
oversize profile 207 prior to a rolling process. After completion of a rolling
process, a first
final size profile 208 is obtained, a first densified outer layer 209 being
accordingly ob-
tained. This is shown schematically by a first densification boundary line
210. This line
defines the area of the first tooth 201 within which full density has been
achieved. Full
density is preferably in relation to a density of a comparable powder-forged
tooth.
Fig. 3 shows a second tooth 301 of a toothed element, not shown. This toothed
element
likewise comprises a gearwheel. Second tooth 301 and gearwheel are
characterised by a
second tip circle 302, a second working circle 303, a second usable root
circle 304 and a
second root circle 305. To achieve an identical densification profile on a
second flank 306
and a third flank 307, a second oversize profile 308 and a third oversize
profile 309 are
provided. After a rolling process, a second final size profile 310 is obtained
on the second
flank 306 and a third final size profile 311 on the third flank 307. Moreover,
a second den-
sification boundary line 312 and a third densification boundary line 313 are
obtained. As a
result of the different forces acting in a direction of rotation due to
rolling movement on the
second flank 306 and the third flank 307, the second oversize profile 308 and
the third
oversize profile 309 differ. The different action of forces on the tooth
flanks 306, 307 dur-
CA 02611672 2007-12-10
ing a rolling process is clarified by the illustrated surface speed
directions. At the second
flank 306 a first surface speed direction 314 and a second surface speed
direction 315 are
obtained. These are directed away from the second working circle 303 in the
direction of
the second tip circle 302 or in the direction of second root circle 305. At
the third flank 307,
5 on the other hand, there are obtained a third surface speed direction 316
and a fourth
surface speed direction 317, which are directed towards one another.
Fig. 4 shows a third tooth 401 of a toothed element, not shown. This toothed
element like-
wise comprises a gearwheel. Gearwheel and third tooth 401 are again
characterised by a
10 third tip circle 402, a usable tip circle 403, a third working circle 404,
a third usable root
circle 405 and a third root circle 406. The illustrated third tooth 401 comes
from a tooth
system with a tip relief, preferably in the form of tip rounding. However,
other geometries
are also possible in this area. In this case, a tooth profile is narrowed in a
tooth tip area
401.1 between the third tip circle 402 and the usable tip circle 403. This
means that in this
15 area the tooth does not engage with an involute mating tooth system. In
this case, an ac-
tive tooth area is located solely in the area between the usable tip circle
403 and the us-
able root circle 405 or between the usable tip circle 403 and the third root
circle 406. A
fourth oversize profile 407 results, after a rolling process, in a fourth
densification bound-
ary line 408. Moreover, a fourth final size profile 410 is achieved on the
fourth flank 409.
Fig. 5 shows an oversize profile between two adjacent teeth of a toothed
element, not
shown. This toothed element again comprises a gearwheel. Gearwheel and teeth
are
characterised by a fourth root circle 502, a fourth usable root circle 503 of
the preform, a
fifth usable root circle 504 of the preform after a grinding process, a fourth
tip circle 505
after a milling process and a fifth tip circle 506 after a finishing process.
After a rolling
process, a fifth final size profile 507 is obtained. On the x-axis, a lateral
dimension is plot-
ted in millimetres. On the y-axis, the lateral dimension accordingly
perpendicular thereto is
likewise plotted in millimetres. The tooth system extends completely in the
plane of the
drawing.
Fig. 6 shows a composition of further oversize profiles. The x-axis shows the
standardised
pitch circle distance measured along a flank line of a toothed element. This
curve relates
in each case to the course from a tooth tip of a first tooth to a tooth tip of
a neighbouring
tooth. On the upper x-axis, the absolute pitch circle distance of the
appropriate flank line is
accordingly plotted in millimetres. The left y-axis indicates an oversize in
millimetres. The
CA 02611672 2007-12-10
31
right y-axis describes the corresponding radius of the associated tooth
system. A sixth
oversize profile 601, a seventh oversize profile 602 and an eighth oversize
profile 603 are
shown. Furthermore, an associated radius 604 of the corresponding tooth system
is
shown. The sixth oversize profile 601 and the eighth oversize profile 603 are
here sym-
metrical to a tooth base line of symmetry 605. In contrast, the seventh
oversize profile 607
is asymmetrical. In the vicinity of the tooth base line of symmetry 605, i.e.
in the tooth
base area, the oversizes in each case display a local minimum. This promotes a
reduction
in the risk of stress cracking.
Fig. 7 shows a further oversize profile, a ninth such oversize profile, which
extends asym-
metrically from a left tooth tip 702 to a right tooth tip 703. As has already
been shown in
Fig. 6, here too an oversize in the area of a tooth base 704 is smaller than
in the area of
the fifth 705 and the sixth flank 706. This serves in particular to prevent
stress cracking.
Fig. 8 shows a first method diagram. Starting from a target input 801, which
includes the
geometry, a gearwheel torque to be transmitted and pressure distribution, a
rolling die
geometry is generated with a first geometry generating module 802. In
addition, on the
basis of the target input 801 and on the basis of the geometry of the rolling
die, preform
geometry is generated in a second geometry generating module 803. In a first
simulation
module 804 a rolling process is simulated. Both the kinematics of the rolling
process and
the densification process, which is brought about during the rolling process,
are simulated.
In particular, a redistribution of material, as outlined for example in Fig.
3, is taken into
account. Simulation of plastic deformation takes place for example by means of
a finite-
element method. This may be coupled together with a CAD program. Optionally, a
second
simulation module 805 may be taken into account for simulating distortion.
Into this mod-
ule are input on the one hand both the target input 801 and the geometry of
the preform.
On the other hand, the second simulation module 805 additionally allows
correction of the
determined geometry of the preform. In particular the first geometry
generating module
802, the second geometry generating module 803, the first simulation module
804 and
optionally the second simulation module 805 may be repeatedly performed in an
optimisa-
tion loop.
Fig. 9 shows a second method diagram. In a first step 901 a ninth oversize
profile 902 is
generated for a tooth profile 903. Then, in a second step 904, a second tooth
profile 905 is
generated for a third rolling die 906. Next, in a third step 907 a rolling
process is simu-
CA 02611672 2007-12-10
32
lated. In the process, the process of rolling the first tooth profile 903 on
the second tooth
profile of the rolling die 905 and the resultant densification are simulated.
Then, the first,
second and third steps 901, 904, 907 are optionally repeated in a variation
908.
Fig. 10 shows an oversize profile of a toothed element of a rolling die, i.e.
a tenth oversize
profile 1001 of a fifth tooth 1002 of a rolling die, not shown. On a seventh
flank 1003 and
an eighth flank 1004 of the fifth tooth 1002 there is provided a different
oversize. On the
seventh flank 1003 there is provided extra material, which is indicated by a
first arrow
1005. In contrast, on the eighth flank 1004 a tooth relief is provided, which
is indicated by
the second arrow 1006. In this example, the oversize relates to a regular
profile of an in-
volute tooth system. The asymmetrical embodiments of the two tooth flanks
1003, 1004
make it possible to take account in particular of asymmetrical material
loading of a toothed
element to be densified thereby. A symmetrical profile may also be achieved,
relative to
the final shape of the workpiece, for both flanks of a tooth by means of this
rolling die, for
which reason compensation in the range of from preferably less than 0.1 pm is
under-
taken.
Fig. 11 is a schematic view of a calculated indentation on an end face of a
tooth system.
The indentation serves at least to minimise, if not to compensate fully,
height- and/or
width-wise growth of the tooth caused by the displacement of the sintering
material
brought about by surface densification. The shape of the indentation is
dependent on the
oversize and on the dimensions of the tooth. The shape may be optimised
iteratively using
the calculation method. Simulation allows estimation of the subsequent actual
behaviour
of the preform.
Fig. 12 shows a schematic view of calculated extreme cases of dies for surface
densifica-
tion, which are calculable. The starting point of the calculation is the left
final geometry of
the tooth system. By taking account of rolling conditions, oversize parameters
and other
influencing factors, it is possible iteratively to determine die shapes
illustrated in each
case in the middle and to the right thereof.
Fig. 13 is a schematic view of a procedure during iterative calculation and
associations
during simulation. Starting from the predetermined final data of the workpiece
and its tooth
system, the machine kinematics may be modelled. Here, for example, the
mutually as-
signed machine axes are taken as the starting point. On the basis of the
kinematics and
CA 02611672 2007-12-10
33
functional associations, it is then possible to undertake optimisation of the
die to be de-
signed by means of the available degrees of freedom. In this regard, reference
is again
made to Fig. 12. The examples illustrated therein have corresponding
disadvantages, for
example excessively weak root region in the central illustration or
excessively pointed tip
shape in the right-hand illustration. Using additional influencing parameters
such as for
example strength considerations and/or stress profiles in the material, it is
then possible to
perform iteration towards a die contour suitable for the respective
requirement profile. For
the die for producing the preform, for example, the starting point may be the
determined
final geometry with the calculated oversizes.
Fig. 14 is a view of density profiles as a function of various starting
densities of the pre-
forms used. If the density of the preform is modified in its core and as it
proceeds out-
wards, the surface densification profile is influenced. This is clear from the
right-hand part
of Fig. 14. By modifying the respective preform, the density profile may
likewise be signifi-
cantly influenced after surface densification. Therefore, the starting core
density and the
shape of the preform constitute important parameters for iteration and
calculation.
Fig. 15 is an exemplary overview of the determined errors, which arise with
different sur-
face densification steps and co-characterize the material behaviour. The error
is indicated
in error classes according to DIN 3972 or DIN 3970. An important point when
determining
a suitable surface densification to be achieved by rolling is the change in
profile of the roll-
forming die. Using the above calculation method for the preform and the
rolling die, it is
possible to modify the rolling die on the basis of the determined results.
This is illustrated
in Fig. 15 using a preform with a core density of 7.3 g/cm3, which was engaged
with an
unmodified set of rolling dies and was surface-densified. As a function of a
feed move-
ment of the rolling die, the geometry of the gearwheel changes. The aim is
achieve the
desired final contour, as has been preset. The illustrations in Fig. 15 show
various situa-
tions relating to feed movements of various distances. By way of example, on
the left a
profile angle error is shown, in the middle a complete profile shape error and
on the right a
shape error. These were measured using the gearwheel produced in each case.
Thus, for
example, a tooth thickness reduction of 0.27 mm leads to a profile angle
deviation corre-
sponding to DIN class 7. In order to achieve the necessary final form of the
tooth thick-
ness reduction, however, 0.4 mm feed is necessary. However, this leads to an
increase in
the respective errors. This means that the final contour manufactured lies
outside the
necessary quality classes with regard to the other values. Therefore, it is
necessary to
CA 02611672 2007-12-10
34
modify the geometry of the die. Taking the values found as input values, it is
then possible
to determine a new die, perform the tests again and in this way iteratively
determine an
optimised die geometry. This calculation makes it possible to determine a
final die contour
with for example two or even just one iteration.
Fig. 16 shows an HV hardness profile for a flank of a tooth system plotted
over the dis-
tance from the surface on the x-axis in [mm]. With different surface
densification steps, the
hardness profile can be influenced by selecting a suitable oversize and feed
movement.
For example, the profile may be at least in part convex or indeed concave. As
indicated,
the preform designated AVA7-1 has a larger oversize than the preform
designated AVA4-
2. The two have contrary hardness profiles: while in the first portion AVA7-1
has a more
convex form until HV 550 is reached, AVA4-2 has a more concave profile. This
changes
below HV 550.
Fig. 17 shows an HV hardness profile in a root area of a tooth system for
different surface
densification steps. Due to the smaller oversize at this point compared with
the flank
oversize and due to the geometry, a different hardness profile is obtained.
The hardness
starts off more steeply, but then develops into an approximately straight
profile with just a
slight slope.
Fig. 18 is a schematic view of different calculated oversize profiles for
different densities
on the basis of a final tooth thickness. The diameter is plotted on the y-
axis. The oversize
is indicated on the x-axis. D_a or d_a indicates the usable tip circle
diameter or the tip
circle diameter, 0 is a set value for an oversize preset for example by a
value at the refer-
ence circle, d_b is the base circle diameter. A indicates the range of
preferred values for
the working circle area. B represents a critical area, since die failure may
occur there dur-
ing rolling.
Fig. 19 is a schematic representation of parameters which may be involved in
the iterative
calculation. In particular, these may be sites of maximum stress. As the left-
hand photo-
graph shows, pitting damage may occur on the flank. Therefore, an equivalent
stress pro-
file is preferably used, in which the following applies: maximum stress arises
under the
surface, in particular in an area of negative slip, therefore preferably under
the indicated
working circle diameter d w1. The right-hand photograph indicates tooth
breakage due to
excessive bending load. The consequence for the calculation model is that a
site of maxi-
CA 02611672 2007-12-10
mum tooth root stress is determined and taken into account. This may be
determined, for
example, using the 300 tangent according to DIN or using the Lewis parabola
according to
AGMA. For the equivalent stress it is preferably assumed that maximum stress
arises at
the surface.
5
Fig. 20 is a schematic view of a further possibility, in which, for example,
at least two pre-
forms can be densified simultaneously. In addition to movement of the die,
according to
one embodiment the preforms may also move in the direction of die.
Furthermore, it is
possible for two or more preforms to be arranged on one preform axis.
The invention may be used, for example, for camshaft gears, planetary gears,
sun wheels,
drive gears, differential gears, transmission gears, clutch gears, pump gears,
spur toothed
gears, helical gears, electric motors, internal combustion engines, adjustable
mecha-
nisms, external or internal tooth systems, external or internal spur- or
helical-toothed cy-
lindrical gears, spur-, helical- or spiral-toothed bevel gears, spiral gears
or worm gears
and for quick-acting screw thread shaft and quick-acting screw thread hub
joints. In a fur-
ther embodiment, one gearwheel is made of sintered metal. The other may be of
plastics
or another material, for example. There is also the possibility of at least
one of the two
gearwheels having a coating which has the effect in particular of minimising
noise. Pref-
erably, a skew bevel gearing may also be produced, in order thereby to form a
hypoid
transmission. In particular, the toothed workpieces may be used in automobile
technology,
engine technology, transmission technology, control mechanisms, force-
transmitting de-
vices, toys, precise mechanical devices, domestic appliances, in particular
mobile domes-
tic appliances, and other fields.
