Note: Descriptions are shown in the official language in which they were submitted.
CA 02583207 2013-05-10
ROLLER BEARING WITH A CERAMIC ROLLING PART
FIELD OF THE INVENTION
The invention relates to a rolling bearing, having at least one first rolling
part made from
a ceramic material and having at least one second rolling part made from a
steel.
BACKGROUND OF THE INVENTION
Accordingly, the invention relates to hybrid bearings. Hybrid bearings are
rolling
bearings which have bearing rings, or at least the raceways of the bearing
rings, made from steel
and which are provided with rolling bodies made from ceramic, or in which at
least one bearing
ring is made from ceramic and the rolling bodies are made from steel.
The materials used for the ceramic rolling bearing element are all industrial
ceramics, in
particular silicon nitride (Si3N4), but also silicon carbides and also
aluminum and zinc oxides.
The use, construction and advantages and disadvantages of using hybrid
bearings are
extensively described in the publication by FAG Kugelfischer Georg Schafer
KGaA, Publ. No.
WL 40 204 DA 80/11 from 1990 "Hochleistungskeramik in FAG Walzlagern"
[High-performance ceramics in FAG roller bearings].
Excellent properties of ceramic for use as roller bearing material for roller
bearing parts
which are subject to high and extremely high loads include properties such as
low weight, low
thermal expansion, high hardness and heat resistance, good dimensional
stability at extremely
high temperatures, high chemical resistance and high corrosion resistance,
high modulus of
elasticity, lower frictional torque at high rotational speeds and lower heat
production, lack of
magnetism and insulator properties.
The centrifugal force of the rolling bodies should also be taken into account
when
determining the loads on fast-moving roller bearings. At very high rotational
speeds, such as for
example for applications in driving mechanisms, the centrifugal forces are
generally even
dominant over the bearing loads. On account of the low weight of ceramic, at
high rotational
speeds the centrifugal forces generated are lower at rolling bodies made from
ceramic. The
density of ceramic, for example silicon nitride, is only approx. 40% of the
density of steel.
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Ceramic materials have a significantly higher modulus of elasticity, for
example 1.5
times, than steel. Consequently, for the same loading, the specific stress in
the rolling contact is
higher than with contact parts made from steel, since the pressure ellipse
produced by the
ceramic rolling bodies in the rolling contact is smaller. In general,
therefore, rolling pairings in
which both components are made from ceramic, and in particular rolling
pairings in which one
rolling part is made from steel and the other is made from ceramic, are not
able to withstand as
high a load as rolling pairings in which both contact parts are made from
steel.
Components made from ceramic have a fundamentally different failure mechanism
than
components made from roller bearing steel. On account of the brittleness of
the ceramic
material, in the event of overloading, ceramic fractures without any
significant plastic
deformation. This property has caused the person skilled in the art to
consider that in the event
of damage to components made from ceramic, such as small pieces of the surface
breaking off,
in a roller bearing, the ceramic components will be the first to suffer
extensive damage, and the
steel components will only undergo such damage at a later stage, as a
secondary phenomenon.
However, as has emerged and as has also been described in the prior art cited
above, under
certain circumstances ceramic components which have suffered preliminary
damage as a result
of impurities and inclusions and pores, cracking nuclei and microcracks and
overloading and/or
as a result of foreign particles in the rolling contact, have initially proven
to still have a long
service life. The abrasion of the ceramic and/or particles which have broken
out of the surface of
the ceramic component in some cases act as an abrasive on the raceways made
from steel or
pass into the rolling contact, where they first of all damage outer layers of
the rolling parts made
from steel.
The sensitivity of the surfaces of the roller bearing parts made from steel is
dependent on
the nature and magnitude of the stresses prevailing at and below the surface
(in the outer layer).
In this context, the term nature is to be understood as meaning residual
stresses (tensile or
compressive stresses) in the outer layer of the component or stresses acting
on the component as
a result of external action. Tensile stresses at the surface and in the outer
layer below the surface
increase the sensitivity of the component. Tensile stresses of this type are
caused on the one
hand by the pretreatment (heat treatment and hard machining) of the component
and by
operating conditions to which the component is exposed. For example, at the
outer raceway of
an inner ring and at the edges, the residual tensile stresses are further
intensified by tensile
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stresses from the required press fit of the inner ring, for example on a
shaft. If this inner ring
additionally rotates at high speeds, the tensile stresses may reach a level at
which the
susceptibility of the inner ring to failure is significantly increased.
Locally high contact
pressures caused by particles which have broken off from the ceramic
components and entered
the rolling contact may damage the outer layer of the raceways in such a way
that microcracks
are formed. These microcracks then propagate to produce pitting and further,
extensive damage.
The hard and sharp edges of the possible break-out locations on the rolling
part made
from ceramic have similar effects on the rolling parts made from steel. These
break-out
locations cause the steel surface to be highly stressed as a result of
abrasive wear. As a result, at
high rotational speeds and loads, the rate at which damage progresses may be
drastically
increased.
The use of ceramic components having the materials problems described above
has as
far as possible been avoided by suitable testing, enabling defective parts of
this type to be
scrapped. However, the rolling contact is endangered, in particular in
bearings which are greatly
influenced by the surroundings, by foreign particles which pass into the
rolling contact, for
example together with the lubricant. Foreign particles of this type then
likewise first of all cause
destruction of rolling parts made from steel, in accordance with the
mechanisms outlined above.
SUMMARY OF THE INVENTION
Therefore, the object of the invention is to provide a roller bearing which
avoids the
above-mentioned drawbacks.
According to the invention, this object is achieved by virtue of the fact that
at the roller
bearing, in which at least one first rolling part is made from a ceramic
material and at least one
second rolling part is made from a steel with a martensitic microstructure, at
least one portion of
the surface of the second rolling part, in a load-free state, has residual
compressive stresses
beneath the surface. The portion of the surface or, as in the case of balls,
the entire surface is
intended for rolling contact with the first, ceramic rolling part.
The residual compressive stresses, at least in the outer layer of the portion,
are preferably
produced by a thermochemical process in which the lower regions of the outer
layer, at between
100 p.m and 700 t.tm below the surface, may have residual compressive
stresses, but even in the
vicinity of the surface the residual compressive stresses are sufficiently
high for it to be virtually
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,
impossible for them to be converted into the detrimental tensile stresses even
under extreme
operating conditions.
Residual compressive stresses of this type can be deliberately produced by
mechanical or
thermochemical means. Using the mechanical route, compressive stresses of this
type are
produced, for example, by shot peening. Shot peening compacts the
microstructure at the
surface of the machine component and thereby produces residual compressive
stresses.
Therefore, on account of the values which have been set, the surface of the
component
according to the invention, even close to the surface, is relatively
insensitive to high surface
pressures and/or to notch effects and is therefore more suitable for a rolling
pairing with
components made from ceramic.
A further configuration of the invention provides for the outer layer to have
a surface
hardness of at least 850 HVO.3, but preferably a surface hardness of 900
HVO.3, at least at the
portion, at a depth of 0.05 mm below the surface. The Vickers hardness is
tested with a 0.3 kg
load on the test pyramid (made from diamond).
According to one configuration of the invention, the level of the residual
compressive
stresses, down to a depth of 40 pm, is at least -300 MPa or below, i.e. for
example -1000 MPa
or less. Accordingly, the compressive stress as an absolute value (in
accordance with the
mathematical definition of the term value), at a depth down to 40 pm,
corresponds to at least
300 MPa and above, for example 1200 MPa. This state is preferably formed by a
thermochemical modification to at least the outer layer and at least in the
portion for the rolling
contact.
The roller bearing part = second rolling part is load-free if, produced as a
finished
component, it is, at least at the portion with the compressive stresses below
the surface, still not
exposed to any loads which alter or are superimposed on the nature and
magnitude of the
residual compressive stresses in accordance with the invention. The loads may
be of kinematic
or mechanical nature. Loads of this type result, for example, from the roller
bearing part being
installed in the surrounding structure, for example loads from a press fit on
an inner ring of the
roller bearing. Other loads are those to which the component is exposed in
operation, such as for
example centrifugal forces from rotation or pressures resulting from
punctiform, linear or areal
touching in sliding or rolling contact with other elements. Further examples
of such loads
include notch effects, bending, tension and compression.
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,=
The residual compressive stresses are formed at least in a portion below the
surface of
the bearing part which is intended for rolling pairing with at least one
further rolling part. These
residual compressive stresses in the microstructure of the steel are
preferably, as has already
been mentioned above, produced by means of a thermochemical heat treatment
process in the
outer layer. The entire surface or just a portion of the roller bearing part
according to the
invention are therefore relatively insensitive to high surface pressures
and/or notch effects even
close to the surface, on account of the values which are set. Examples of
portions at surfaces
include the edge zones of the raceways at bearing rings of a roller bearing or
the edge zones at
the bearing rings which are intended for radial guidance of a cage. One
example of a machine
element which has the residual compressive stresses according to the invention
formed under its
entire surface is the ball as a rolling body in a roller bearing.
One configuration of the invention provides for the residual compressive
stresses to be
formed in a fully machined outer layer. The term fully machined outer layer is
to be understood
as meaning, for example, the finish-ground and if appropriate honed raceway of
the roller
bearing or a surface of a rolling body which has been machined in this manner,
for example the
surface of a ball or that of a roll, or such as the raceways of a ball screw
which has been
machined in this or some similar way. The residual compressive stresses are
formed, for
example, in a nitrogen-enriched outer layer. This outer layer is produced by
gas nitriding and
preferably by plasma nitriding.
In a further configuration of the invention, the starting point is for the
outer layer to be
produced by what is known as double (duplex) hardening, i.e. first of all the
steel is hardened,
and then it is subjected to a further thermochemical heat treatment, in which
the outer layer is
formed. The hardening process is the standard process known to the person
skilled in the art, in
which the steel is austenitized, quenched and tempered. Therefore, the first
heat treatment,
which is characterized by known hardening and includes a tempering operation,
is followed by a
second heat treatment, which produces the compressive stresses in accordance
with the
invention. It is provided that the steel is a high-temperature steel, the
microstructure of which
permits a tempering temperature in the first hardening process of at least 400
C. Accordingly,
the temperature used during the subsequent outer layer hardening is over 400
C, for example for
plasma nitriding is in a range from 400 C to 600 C. The tempering temperature
used in the first
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hardening process is above the temperature which is employed for the nitriding
of the outer
layer.
Further configurations of the invention provide for the invention to be used
for steels
having the designations and minimum compositions listed below:
a. having the designation M50 (AMS 6491), comprising
- 0.8 to 0.85% by weight of C
4 to 4.25% by weight of Cr
4 to 4.5% by weight of Mo
- 0.15 to 0.35% by weight of Mn
0.1 to 0.25% by weight of Si
- 0.9 to 1.1% by weight of V
- max. 0.015% by weight of P
- max. 0.008% by weight of S
and comprising further alloying constituents and iron, as well as standard
impurities.
b. having the designation M50NiL (AM6278), comprising:
- 0.11 to 0.15% by weight of C
4.0 to 4.25% by weight of Cr
- 4.0 to 4.5% by weight of Mo
- 1.1 to 1.3% by weight of V
3.2 to 3.6% by weight of Ni
0.15 to 0.35% by weight of Mn
0.1 to 0.25% by weight of Si
max. 0.015% by weight of P
max. 0.008% by weight of S
and comprising further alloying constituents and iron, as well as standard
impurities.
c. having the designation 32CD V13 (AMS6481), at least
comprising:
0.29 to 0.36% by weight of C
2.8 to 3.3% by weight of Cr
0.7 to 1.2% by weight of Mo
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,
- 0.15 to 0.35% by weight of V
- 0.4 to 0.7% by weight of Mn
- 0.1 to 0.4% by weight of Si
- max. 0.025% by weight of P
max. 0.02% by weight of S
and comprising further alloying constituents and iron, as well as standard
impurities.
d. having the designation Ti (S 18-0-1), comprising:
0.7 to 0.8% by weight of C
4 to 5% by weight of Cr
17.5 to 18.5% by weight of Wo
- 1 to 1.5% by weight of V
0 to 0.4% by weight of Mn
0.15 to 0.35% by weight of Si
max. 0.025% by weight of P
max. 0.008% by weight of S
and comprising further alloying constituents and iron, as well as standard
impurities.
e. having the designation RBD, comprising:
0.17 to 0.21% by weight of C
2.75 to 3.25% by weight of Cr
- 9.5 to 10.5% by weight of Wo
- 0.2 to 0.4% by weight of Mn
- 0 to 0.35% by weight of Si
- 0.35 to 0.5% by weight of V
max. 0.015% by weight of P
max. 0.015% by weight of S
and comprising further alloying constituents and iron, as well as standard
impurities.
f. having the designation Pyrowear 675 (AMS5930),
comprising:
0.06 to 0.08% by weight of C
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. = . ..=,
- 12.8 to 13.3% by weight of Cr
- 1.5 to 2.0% by weight of Mo
- 0.5 to 0.7% by weight of V
- 2.2 to 2.8% by weight of Ni
- 4.8 to 5.8% by weight of Co
- 0.5 to 1.0% by weight of Mn
- 0.2 to 0.6% by weight of Si
and comprising further alloying constituents and iron, as well as standard
impurities.
The invention relates to roller bearing parts which are exposed to a rolling
load in at least
one pairing with a further roller bearing part or in contact with a plurality
of rolling parts. The
term rolling load also represents the generally undesirable sliding pairings,
such as slipping,
which may occur from time to time between the rolling parts. The rolling load
is generated
between the rolling parts as a result of the individual rolling parts rolling
along one another.
Rolling parts of this type are inner and outer rings and rolling bodies (balls
and rolls). In
particular for use in bearings for the aeronautical and aerospace industries,
there is provision for
one or more elements of the roller bearing to be combined with the compressive
stresses in
accordance with the invention.
The elements of the pairings have different materials from element to element
as
alternatives. For example, there is provision for an inner ring made from
steel which is provided
with the compressive stresses to be paired with rolling bodies (for example
balls) made from
suitable ceramic materials, such as Si3N4, or for one or two or more roller
bearing rings made
from ceramic to be paired with corresponding balls made from steel.
Alternatively, there is also provision for pairings of roller bearing parts
which, in
addition to the above-mentioned pairings, also form one or more pairings with
elements made
from the same material but of which at least one element includes the
compressive stresses in
accordance with the invention resulting from a thermochemical treatment, and
at least one of the
elements either does or does not include these compressive stresses. This is
the case, for
example, if rolling bodies made from steel and rolling bodies made from
ceramic are
simultaneously incorporated in a roller bearing. This also results in pairings
in which at least
two or more paired parts made from steel, of which, in addition, at least one
is paired with the
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". '=,
roller bearing part made from ceramic, have the compressive stresses in
accordance with the
invention and are made from identical or different steel grades. Therefore,
the invention also
applies to a roller bearing in which both at least one of the bearing rings
and at least some of the
rolling bodies made from steel have the residual compressive stresses
resulting from a
thermochemical treatment. In this case, it is possible for the bearing rings
to be made from a
steel of a composition which is identical to or different than that of the
rolling bodies made from
steel and for just one of the steel-on-steel parts to have the compressive
stresses.
Other features and advantages of the present invention will become apparent
from the
following description of the invention which refers to the accompanying
drawings.
The invention is explained in more detail below on the basis of an exemplary
embodiment and on the basis of in-house test and calculation results.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a diagrammatic and highly simplified illustration of a roller
bearing with
which the invention is realized.
Figure 2 shows, in diagram form, the region close to the surface in which the
residual
compressive stresses in accordance with the invention are formed.
Figure 3 illustrates measurement and calculation results in diagram form
relating to
stresses beneath the surface of single-hardened steel of grade M50 by
comparison with
double-hardened steel of grade M50.
Figure 4 shows, in diagram form, the hardness curve in the vicinity of the
surface for a
roller bearing component according to the invention made from double-hardened
steel M50.
Figure 5 illustrates the measurement and calculation results in diagram form
relating to
stresses beneath the surface of single-hardened steel of grade M50NiL by
comparison with
double-hardened steel of grade M50NiL.
Figure 6 shows the hardness curve in the vicinity of the surface for a roller
bearing
component according to the invention made from double-hardened steel M50NiL.
DESCRIPTION OF A PREFERRED EMBODIMENT
Figure 1 shows, as an exemplary embodiment of the invention, a roller bearing
1
comprising an inner ring 2 and rolling bodies 3 in the form of balls. The
balls are made from
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CA 02583207 2007-04-05
. . . ,
ceramic. The balls and the inner ring 2 form the rolling parts. The inner ring
2 is seated with a
press fit on a shaft 4. The shaft 4 rotates at high rotational speeds in the
clockwise or
counterclockwise direction as desired. The forces from the press fit and the
centrifugal forces
from the rotation generate tensile stresses 5 (circumferential stresses) in
the outer layer 6 of the
inner ring 2. At the unfitted ring, the microstructure of the outer layer 6
has residual
compressive stresses 7 which are superimposed in opposing fashion on the
tensile stresses 5 and
compensate for the latter on account of their greater magnitude and their
opposite direction of
action to the tensile stresses.
Figure 2 shows the results of the invention in diagram form. The residual
compressive
stresses ores in MPa for a load-free outer layer are illustrated as a function
of the depth t of the
outer layer in pm. The residual compressive stresses 0
¨ res prevail at least beneath a portion of the
surface of a bearing ring or of a rolling body which is intended for rolling
loading. The abscissa
of the diagram stands for the outer layer depth tin micrometers. The ordinate
in the Figure
beneath the abscissa represents the compressive stresses, which are provided
with a negative
sign and result from tension and compression (in megapascals) in the outer
layer. The further
continuation of the ordinate in the Figure above the abscissa, in the
direction indicated by the
arrow, represents the tensile stresses, which are provided with a positive
sign and are inherently
disadvantageous. The line 8 marks the distance of 40 um from the surface, up
to which distance
the residual compressive stresses o
¨res have a maximum value of -300 and below. The positive
value of the compressive stresses resulting from the value of the residual
compressive stresses is
accordingly 300 MPa and more, e.g. even 1000 to 1200 Mpa.
Figure 3 shows the curve for residual compressive stresses 0
¨ res in MPa in an outer layer
as a function of the distance t (in pm) from the surface. The abscissa of the
diagram also
represents the layer depth t, starting from the outer surface, in micrometers,
and the ordinate
represents the resulting stresses in megapascals in the layer. The area in the
Figure above the
stresses a
¨ res = 0 represents the tensile stresses, which are provided with a positive
sign and are
inherently disadvantageous, while the area below Gres = 0 represents the
compressive stresses,
which are provided with a negative sign.
First of all, lines 10 and 11 compare the curves of the residual stresses in
load-free outer
layers for roller bearing components made from steel grade M50 without the
outer layer which
has been modified in accordance with the invention and for a component made
from steel grade
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CA 02583207 2007-04-05
". . = ,
M5ODH. The curves for the load-free states are marked by solid lines for both
steel grades. Line
shows the curve for the stresses for steel M50, and line 11 shows the curve
for steel M5ODH.
The letters DH at the end stand for double hardening and indicate a component
in which residual
compressive stresses have been produced by thermochemical means in the
vicinity of the
surface in accordance with the invention. This is then followed by a
comparison of the curves of
the stresses, illustrated by dashed lines 12 and 13, for the same outer layers
as those mentioned
above, but in this case under load.
In the load-free state, the outer zone of the component made from M50
initially, down to
a depth of approx. t = 0.08 - 0.01 mm below the surface, has residual
compressive stresses
which are approx. -500 MPa at the surface and then move toward zero at 0.08 -
0.01 mm. As the
depth t increases beyond 0.01, to a depth t of 0.12 mm, the line 10 moves in a
range in which,
apart from a somewhat negligible tendency toward tensile stresses, there are
virtually no
positive or negative residual stresses.
By contrast, the outer zone of the component made from M5ODH has the residual
compressive stresses, originating from the thermochemical treatment, in
accordance with the
invention. The curve of these stresses is illustrated from the value -1000 MPa
at the surface to
approx. -180 MPa at a depth t of approx. 0.12 mm below the surface, by line
11.
The stress curves for the same components or outer zones, but this time under
load, are
illustrated by dashed lines 12 and 13 in Figure 3. Loads on the components are
those which
cause tensile stresses at least in the outer zone region under consideration.
As is described
below, these tensile stresses can be superimposed on the residual compressive
stresses in such a
way that the residual compressive stresses are reduced or compensated for by
the operating
tensile stresses or are moderately exceeded by the tensile stresses. According
to the results
illustrated in Figure 3, the loads are characterized by a shift in the
stresses toward the positive
stress range of 150 MPa over the entire curve (circumferential stresses of 150
MPa resulting
from press fit and centrifugal force).
In the loaded state, the outer zone of the component made from M50 (line 12)
initially
still has residual compressive stresses of approx. -320 MPa at the surface,
but these have been
canceled out by tensile stresses at a depth of just 0.003 - 0.005 mm. At a
distance of greater than
0.003 to 0.005 mm from the surface and below, the outer layer has tensile
stresses. As the depth
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CA 02583207 2007-04-05
. .
t increases from 0.01 down to a depth t of 0.12 mm, the outer zone of the
component made from
M50 is under threat from virtually constant tensile stresses in the vicinity
of +200 Mpa.
In the loaded state, the outer zone of the component made from M5ODH with
compressive stresses in accordance with the invention (line 13) has residual
compressive
stresses of approx. -820 MPa at the surface, and these are only canceled out
by tensile stresses at
a depth of 0.12 mm. On account of the high levels of residual compressive
stresses produced by
thermochemical treatment at a relevant depth, the outer layer cannot see its
residual compressive
stresses exceeded by the tensile stresses resulting from loads, and is
therefore less at risk.
Line 14 in Figure 4 illustrates the hardness curve in the outer layer of a DH-
treated roller
bearing component as described in Figure 3. The outer zone of the component
made from
M5ODH with compressive stresses in accordance with the invention (lines 11 and
13) has a
hardness of over 1000 HVO.3 at a depth of 0.05 mm below the surface and of 800
HVO.3 even at
a depth of 0.2 mm.
Figure 5 shows the curve for residual compressive stresses Gres in MPa in an
outer layer
as a function of the distance t (in lam) from the surface. The abscissa of the
diagram also
represents the layer depth t, starting from the surface, in micrometers, and
the ordinate
represents the resulting stresses in megapascals in the layer. The area below
a
- res 0 represents
the compressive stresses, which are provided with a negative sign.
Lines 15 and 16 compare the curves of the residual stresses of load-free outer
layers for
roller bearing components made from steel grade M50 NiL, without the outer
layer which has
been modified in accordance with the invention, and for a component made from
steel grade
M50 NiLDH. The curves for the load-free states are indicated by solid lines
for both steel
grades. Line 15 shows the curve for the stresses for steel M50 NiL, and line
16 shows the curve
for the steel M50 NiLDH. The letters DH at the end stand for double hardening.
This is
followed by a comparison of the curves, illustrated by dashed lines 17 and 18,
of the stresses in
the same outer layers, but under load. Line 17 shows the curve for the
stresses for steel M50
NiL, and line 18 shows the curve for steel M50 NiLDH with the outer layer
under load.
Figure 5 describes an exemplary embodiment of the invention in which a
material which
already had relatively good stress profiles and states in the outer layer was
nevertheless
improved still further by means of the invention.
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In the load-free state and in the loaded state, the outer zone of the
component made from
M50 NiL has residual compressive stresses beneath the surface. The same is
true of the
component made from M50 NiLDH. However, the magnitudes of the residual
compressive
stresses of the component made from M50 NiLDH are significantly higher - at a
depth of 0.02
mm below the surface, the difference between the residual compressive stress
for the component
made from M50 NiLDH (line 18) and the residual compressive stress for the
component made
from M50 NiL (line 17) is approx. 550 MPa. Therefore, in this case too, the
double-hardened
component can be subjected to high loads. As the depth of the outer layer
increases, this
difference drops, but is still over 200 MPa at a depth t of 0.12 mm. As is
also the case in the
results illustrated in Figure 3, the loads are characterized by a shift in the
stresses toward the
positive stress range of 150 MPa over the entire curve (circumferential
stresses of 150 MPa
resulting from press fit and centrifugal force).
Line 19 in Figure 6 illustrates the possible curve for the hardness in the
outer layer of a
roller bearing component according to the invention as has been described, for
example, in
Figure 5. The outer zone of the component made from M50 NiLDH with compressive
stresses
in accordance with the invention (lines 16 and 18) has a hardness of over 900
HVO.3 at a depth
of 0.05 mm below the surface, and a hardness of still over 800 HVO.3 at a
depth of 0.2 mm.
Figure 7 shows a diagram comparing the results 20 of service life tests for a
bearing
made from M50 and having rolling bodies made from steel, referred to below as
a standard
bearing, and the results 21 of a hybrid bearing with double-hardened running
rings made from
M5ODH and having rolling bodies made from ceramic. The measurement results 20
for the
standard bearings are illustrated in dot form, while the measurement results
21 for the hybrid
bearing are illustrated in the form of triangles. The horizontally directed
arrows leading from the
measurement results 20, 21 mark the specimens which had still not failed over
the course of the
running time T used in the diagram.
The test was carried out under the following conditions: axial load 4.1 kN as
loading at a
temperature of approx. 120 C in engine oil, which had been highly contaminated
through use,
for diesel engines at poiR of 2300 MPa for the steel balls and at po IR of
2600 MPa for the balls
made from ceramic. It can be seen that for a failure probability L of 10%
(L10), the test
specimens of the hybrid bearings according to the invention achieved a running
time T in hours
which was approximately 180 times or more that of the test specimens from
standard bearings.
13
CA 02583207 2007-04-05
. ' = = =
Although the present invention has been described in relation to a particular
embodiment
thereof, many other variations and modifications and other uses will become
apparent to those
skilled in the art. It is preferred, therefore, that the present invention be
limited not by the
specific disclosure herein, but only by the appended claims.
14
