Note: Descriptions are shown in the official language in which they were submitted.
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BT/86038
CAMSHAFT AND METHOD FOR ITS PRODUCTION
This invention relates to a tubular fabricated
camshaft for a reciprocating piston machine, such as an
internal combustion engine or a compressor, wherein a
plurality of camshaft elements are secured to a central
S tubular member passing through such elements. The
elements comprise cams and may also comprise bearings,
gears etc. all of which are secured to and axially spaced
along the central tubular member.
Camshafts have conventionally been produced as
either castings or forgings, usually of steel, and have
comprised a solid central member having a plurality of
axially spaced integral cams and bearings thereon.
However, it has been recognised that advantages are to be
gained in manufacturing camshafts as fabrications of
separate elements wherein the cams and bearings are
initially pre~formed to shape and then assembled on and
secured at predetermined positions along the length of a
central tubular shaft. Such fabricated tubular camshafts
offer the advantages, when compared with solid cast or
forged camshafts, of weight reduction; facilitation of
lubricant supply to the cam and bearing surfaces from the
hollow interior of the shaft and the possibility of
selecting different materials for the cam, bearing and
shaft.
For example, GB-A-275842 lYassenoff) teaches the
welding of preformed cam rings and bearing rings to a
central tubular shaft wherein the cams, apart from a
central aperture, may be solid or may be formed from
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drawn tubing or from strips of metal bent to a required
cam profile. GB A-1115093 (~KN Screws & Fasteners
Limited) teaches the mechanical locking of preformed cam
rings to a central tubular shaft, the cam rings being of
substantially constant wall thickness and formed by
drawing a tube to the desired cam profile and then
cutting portions off the drawn tube to provide the
individual cam rings. In this specification each cam
ring is mechanically locked to the shaft at a desired
axial position and angular orientation by a key engaged
within the hollow cam ring nose and within an aperture in
the shaft wall; each cam rinq optionally being further
secured to the shaft by an adhesive, or by soldering,
brazing or welding.
GB-A-1117816 (GKN Screws & Fasteners Limited)
teaches the securing of centrally apertured preformed
solid cam and bearing elements to a central tubular shaft
by radially outwardly deforming the shaft into gripping
engagement within the cam and bearing apertures. Such
radially outward deformation of the shaft is taught as
being acco~plished by drawing an oversize mandrel through
the shaft or by expanding the shaft by fluid pressure or
explosive forming techniques. The central aperture of
each solid cam or bearing is recessed radially outwardly,
effectively to provide a spline arrangement, to enhance a
secure gripping engagement between the radially outwardly
expanded part of the shaft and the cams and bearings.
JP-A-7644/1971 (Nakamura et al) teaches the securing
of preformed cam rings to a central tubular shaft by
fluid pressure radially outward expansion of the shaft
into the hollow interior of each cam ringO The fluid
pressure is applied to one end of the tubular shaft (the
other end being sealed) whilst the preformed cam rings
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are held in predetermined axial spacing and angular
orientation relative to the shaft in a closed die having
cam recesses corresponding to and accommodating the cam
rings in their desired positions.
A similar teaching to that of JP-A-7644/1971 is
contained in GB-A-1530519 (Klockner Humboldt Deutz AG)
whexein a central tubular shaft is radially outwardly
expanded either by hydraulic, electro-hydraulic or
mechanical means into the hollow interiors of preformed
cam rings of substantially constant wall thickness.
It is an object of the present invention to provide
a method of manufacturing a fabricated camshaft, and a
camshaft produced by such method, wherein cam elements in
the form of hollow cam rings are sec~red to a central
tubular shaft in a manner which offers various advantages
in comparison with the known methods of securing cam
rings to a central tubular shaft.
In accordance with a first aspect of the invention
there is provided a method of producing a camshaft having
a central tubular shaft and a plurality of cam rings
spaced axially therealong characterised in that the cam
rings are of initially circular cross sectional form and
are assembled in axially spaced relation around the
tubular shaft in a forming tool and that the forming tool
is closed to press each of the initially circular cross
section cam rings to a desired cam profile whilst
deforming the tubular shaft relative to the cam rings
whereby the cam rings are retained on the shaft against
axial and angular displacement relative thereto.
In accordance with a further aspect of the invention
there is provided a fabricated camshaft comprising a
plurality of cam rings axially spaced along a central
tubular shaft wherein each cam ring is retained on the
shaft against axial and angular displacement relative to
an associated portion of the shaft which is deformed
5 relative to the cam ring, said portion of the shaft
having been mechanically deformed relative to the cam
ring during pressing of the cam ring to its desired cam
profile from an initially circular cross section cam ring
located about the initially undeformed shaft.
It will be appreciated that the fabricated camshaft,
and method for its production, as described in the two
preceding paragraphs differs from all of the prior art
teachings referred to above in that the cam rings are
formed from an initially circular cross section to a
desired cam profile solely by being pressed within the
forming tool in a predetermined axially spaced
relationship about the central tubular shaft whilst,
during the same pressing operation, the shaft is deformed
relative to the cam rings whereby the cam rings are
retained on the shaft against axial and angular
displacement relative thereto. ~articular advantages
offered pursuant to the invention are:-
1. the circular cam rings are provided initially as
portions of predetermined axial dimension cut from a
relatively inexpensive circular cross section tube;
and
2. the circular cam rings can be assembled on the
central tubular shaft and easily axially located in
cam forming dies of the forming tool whereas, when
the cam rings were preformed to a desired cam
profile as taught by the prior art before being
assembled on the central tubular shaft, it was then
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comparatively difficult to locate the cam rings
accurately within cam profile cavities of a jig or
the like prior to the securing of the rings to the
shaft whether by welding, mechanical radially
outward deformation of the shaft or fluid pressure
deformation of the shaft.
The pressing of the cam rings and deformation of the
shaft may be carried out with the cam rings and the shaft
at ambient temperature. Alternatively, the initially
circular cam rings may be heated prior to their assembly
around the shaft and then cooled to achieve an
interference fit relative to the shaft either before,
during or after the pressing of the cam rings and
deformation of the shaft in the forming tool.
lS The method according to the invention may include
the additional step of further deforming the tubular
shaft relative to the cam rings by fluid pressure applied
internally of the shaft~ Preferably, such further
deformation of the shaft is carried out in a further
forming tool having an axially extending central cavity
and a plurality of cam shaped cavities into which fit the
shaft and the cam rings respectively; the interior
dimensions of the cam shaped cavities being in excess of
the exterior dimensions of the cam rings whereby, upon
the application of fluid pressure internally of the
shaft, the cam rings are elastically deformed and the
shaft is plastically deformed~
The method of the invention conveniently also
includes the additional step of subjecting the camshaft
to a heat treatment process to harden the cam rings.
. .
The method may also include the additional step of
further attaching the cam rings to the tubular shaft by a
method employing brazing, soldering, hot welding (gas,
electric arc, laser or electron beam), cold welding,
dipping, gluing, pinning, mechanical interlocking or any
other suitable attachment method. Such a further
attachment method may conveniently be carried out during
the said heat treatment process, such method comprisiny
the melting of a braze or solder metal prepositioned
between the shaft and the cam ring~
Other features of the invention will become apparent
from the following description given herein solely by way
of example with reference to the accompanying drawings
wherein:
Figure 1 is a somewhat diagrammatic isometric view
of a camshaft constructed in accordance with the
invention;
Figure 2 is an isometric view of one end of the
camshaft shown in Figure l showing in more detail the
deformation of the central tubular shaft relative to the
cam rings;
Figure 3 is a longitudinal cross sectional view
through that part of the camshaft shown in Figure 2;
Figure 4 is a diagrammatic transverse cross
sectional view of a circular cam ring assembled around a
central tubular shaft at a cam forming die position of a
forming tool before the tool is closed;
Yigure S is a cross sectional view similar to that
of Figure 4 but showing the tool in its closed position
whereby the initially circular cam ring has been pressed
to a desired cam profile and the shaft has been deformed
relative to the cam ring:
Figure 6 is a similar transverse cross sectional
view of the formed cam ring on the deformed shaft (as
shown in Figure 5) but located within a cooperating die
cavity in a further forming tool for the application of
fluid pressure to the interior of the shaft;
Figures 7 and 8 are diagrammatic cross secticnal
views of a formed cam ring on a deformed shaft located
within a further forming tool similar to that shown in
Figure 6 but wherein the cooperating die cavity is
modified thereby to modify the effect of the fluid
pressure applied internally of the shaft.
lS Referring firstly to Figures 1 to 3 of the drawings,
there is shown a camshaft produced in accordance with the
invention and comprising, in this example, a central
tubular shaft 10 upon which are mounted eight cam rings
12 of predetermined cam profile and five bearing journals
14 of circular profile; the tubular shaft being radially
deformed relative to the cam rings whereby the rings are
retained on the shaft against axial and angular
displacement relative thereto.
The means by which each of the cam rings 12 is
retained on the shaft 10 is illustrated diagrammatically
in Figure 4. In Figure 4 there is shown a central
tubular shaft 10 of uniform circular cross sectional form
which is conveniently formed of a low carbon steel
although it could be formed of aluminium or any other
suitable plastically deformable material. Also shown is
a cam ring 12 of circular cross sectional form and
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constant wall thickness which is of a slightly greater
internal diameter than the external diameter of the
tubular shaft; the cam ring being provided as a "slice"
cut from a length of tubing of uniform cixcular cross
sectional form. The material of the tube from which the
cam ring is cut is conveniently a high strength
hardenable ductile steel or other steel suited to cam
performance.
Also shown in Figure 4 are two opposed parts 16 and
18 of a forming tool including respective opposed cam
forming die cavities 20 and 22 in the two parts of the
tool; the cavity 20 being to the desired cam profile and
the cavity 22 being semi-circular.
To produce the camshaft, the necessary number of
lS circular cam rings 12 and circular bearing journals 14
are assembled on the central tubular shaft 10 with each
cam ring and bearing journal located respectively in a
corresponding die cavity 22. The tool is then closed to
the position shown in Figure 5 whereby the initially
circular cam rings 12 are each pressed to the cam profile
defined by the shape of the cam forming die cavity 22-22.
At the same time the central tubular shaft 10 is deformed
radially within each hollow cam ring essentially to the
shape shown in Figure 5 whereby the cam rings are
retained on the shaft against axial and angular
displacement relative thereto. It will be appreciated
that each of the radially deformed portions 24 of the
tubular shaft 10 may not be deformed precisely to the
shape shown in Figure 6; for example, each deformed
portion may not entirely fill the hollow interior of its
associated cam ring. The parts 26 of the shaft located
axially between adjacent cam rings, or between a cam ring
and a bearing journal, are retained in the forming tool
. .
in a cooperating generally cylindrical cavity against
deformation during closure of the ~orming tool although,
as may be best seen in Figure 3, the tubular shaft may
become deformed radially outwardl~ around at least part
of its periphery to provide radially outwardly extending
bulges 28 on each axially outer side of a cam ring or
bearing journal to provide an additional retention of the
respective cam ring or journal against axial movement
relative to the shaft.
.
The mechanical pressing operation described above
may be carried out with all of the elements of the
camshaft maintained at ambient temperature i.e. the
production of the camshaft is carried out by a cold
pressing operation. Alternatively, each of the initially
circular cam rings, and the bearing journalsr may be
heated prior to their assembly on the central tubular
shaft and then cooled either before, during or after the
mechanical pressing operation to achieve an interference
fit of the cam rings and bearing journals relative to the
shaft. In such an areangement it will be appreciated
that, at ambient temperature, the internal diameters of
the initially circular cam rings and of the bearinq
journals are slightly less than the external diameter of
the central tubular shaft.
The invention also provides additional methods of
enhancing the retention of the cam rings and bearing
journals against axial and angular displacement relative
to the shaft. For example, although not illustrated in
the drawings, the cam rings and bearing journals may be
3~ further secured to the central shaft by brazing,
soldering, hot welding (e.g. gas, electric arc, laser or
electron beam), cold welding, dipping, gluing, pinning,
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mechanical interlocking or by any other suitable securing
method.
Furthermore, the retention of the cam rings in
particular may be enhanced by the application of fluid
pressure applied internally to the central tubular shaft
10 in a further forming tool. Referring to ~igure 6 of
the drawings, there is shown closed opposed parts 30 and
32 of a further forming tool having respective die
cavities complimentary to each pressed cam ring profile
but of slightly greater internal dimensions than the
external dimensions of the cam ring. As illustrated
diagrammatically in Figure 6, fluid pressure is applied
internally of the central tubular shaft 10, after the die
parts 30 and 32 have been closed and locked together,
whereby the shaft is radially outwardly expanded by the
application of the 1uid pressure. During such radially
outward expansion, the tubular shaft 10 is plastically
deformed whereas the cam rings 12 are elastically
deformed due to the fact that the die cavities 34-36 have
interior dimensions in excess of the exterior dimensions
of the cam rings thereby permitting limited radially
outward elastic expansion o the cam rings. When the
internal fluid pressure is released from within the
shaft, the elastically deformed cam rings will then
contract back onto the plastically deformed central
tubular shaft.
It will be appreciated that the size o the cam ring
die cavities 34-35 in this further forming tool must not
be too large otherwise the radial expansion of the
central tubular shaft 10 may expand the cam rings 12
beyond their elastic limit; conversely the die cavities
must not be too small otherwise the cam rings will not
achieve the desired degree of elastic deformation.
The gripping of the cam rings 12 relative to the
shaft 10 may be further enhanced if each of the die
cavities 34-36 of the further forming tool is modifi~d as
shown in Figures 7 or 8 wherein, in Figure 7,
diametrically opposed shims 38 have been located across
~he cam width in each of the upper and lower die parts
30 and 32 whereas, in Figure 8, the shims have been
located across the parting line of the die parts. When
fluid pressure is applied internally of the central
tubular shaft 10 when the camshaft is located within a
forming tool of the type shown in Figures 7 or 8, the
initial die closure compresses all of the cam rings 12
across their width causing the cam rings to elongate
subsequent to which, during the application of fluid
pressure to the interior of the shaft 10, the shaft is
deformed into the distorted elongated cam profile. Upon
relaxation of the fluid pressure and opening of the dies,
each cam ring 12 effectively grips the associated
deformed portion 24 of the shaft in three areas, one on
each side of the cam nose and one at the base. An
advantage of this modified method of production is that
reliance is not placed entirely on the fluid pressure
applied internally of the shaft to generate the grip of
the cam rings relative to the shaft but only to fill the
~5 internal profile created by the sgueezing of the cam
across its width during die closure.
In any of the methods described above, each of the
cam rings 12 is pressed from its initially circular cross
section form substantially to its final desired cam
profile in the mechanical forming tool diagrammatically
illustrated in Figure 41 No further substantial
machining of the cam rings is required although a light
finish machining may be applied.
, . . . . . .
~ 3
12
It will generally be desirable to apply some form of
heat treatment either locally to the whole, or to
selected portions, of each cam ring or as a bulk heat
treatment to the complete camshaft. Such a bulk heat
treatment can harden the cam rings but not the central
tubular shaft as their chemical compositions, and thus
their reaction to hardening and tempering, will differ
although it will be appreciated that a bulk heat
treatment must be carried out before any radial expansion
step (such as that described with reference to Figures 6
to 8) otherwise such a heat treatment may relax the
assembled components to such an extent that the cam rings
12 become loosened from the central tubular shaft lO.
During a bulk heat treatment, the cam rings and
central tuhular shaft may be heated to a temperature
within the range 850 degrees C to 900 degrees C and it is
thus convenient, whilst the components are at this
elevated temperature, to carry out the optional
additional securing step referred to above by brazing or
soldering. For example, a brazing process could be
carried out by pre-positioning a foil or strip of braze
metal between the cam rings and the central tubular shaft
before the initial mechanical pressing operation; said
braze metal then melting during the bulk heat treatment
step to braze the cam rings (and the bearing journals if
desired) onto the central tubular shaft.
As mentioned above~ it is not essential that the
initial mechanical pressing step deforms those portions
of the central tubular shaft to the completely filled
position illustrated diagrammatically in Figure 5.
Similarly any subsequent fluid pressure expansion step,
such as that described with reference to Figures 6 to 8,
need not further deform those portions of the central
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tubular shaft fully into the hollow interior of each cam
ring. If desired, a piece of filler material (not
- illustrated) such as a steel bar may be initially located
within each cam ring below the cam nose position so as to
provide extra rigidity to the cam nose when formed.
It will be appreciated that, in production of a
camshaft in accordance with any of the above described
embodiments of the invention, a plurality of axially
spaced apart cam rings of initially circular cross
sectional form will be assembled onto a central tubular
shaft within the forming tool with a said cam ring
located at each of a plurality of corrresponding cam
forming die cavities within the forming tool. Assembly
of the initially circular cam rings (and the circular
bearing journals) on the central tubular shaft within the
forming tool is thus greatly facilitated in that it is
only necessary to locate the cam rings and bearing
journals at the respective die cavities, there being no
necessity accurately to locate any of the components
2G angularly relative to the central shaft before the
forming tool is closed. Thus one of the principle
advantages of the present invention is that the cam rings
themselves are susceptible of low production cost in that
they are produced simply as cut sections or "slices" of
predetermined axial dimension from a relatively
inexpensive circular cross section tube stock.
