Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a method for
manufacturing an assembled camshaft or the like consisting
of a shaft tube and slid-on elements, by expanding the
shaft tube in the region of the elements by applying
internal pressure. The invention also relates to the
article of manufacture, namely an assembled camshaft or
the like consisting of a shaft tube and slid-on elements
produced by expanding the shaft tube in the region of the
elements. The above-mentioned slid-on elements may be
control cams, bearing rings and gear wheels or bevel gears
which have to be connected to the shaft tube in a
non-rotating way and with angular accuracy.
There are prior art camshafts in the case of which
finish-machined cams and bearing rings are shrunk on to a
shaft tube, using thermal processes (DE-OS 33 01 7~9).
With such shafts the selection of the material for the
elements is limited in that certain specific heat
expansion coefficients of the materials are required for
the shrinking process to be able to produce the necessary
fits. In order to ensure sufficient tension between the
shaft tube and the elements to achieve a non-rotating
connection, the shaft material, too, has to meet certain
more stringent requirements in respect of strength and
surface hardness. Because of the necessary thermal
treatments, joining the shaft tube and the elements is
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time-consuming and complica-ted. The increase in
temperature in the course of the joining operation
inevitably leads to a hardness loss of the elements.
There are other prior art camshafts in the case of
which individual cams are slid on to profiled bars in a
form fitting way and connected to these by, for instance,
shrinking, freezing, soldering, welding or glueing (DE-OS
23 36 241, DE-GM 79 20 957). This process does not permit
any weight advantages as compared to conventional
camshafts; modifying or adapting the angulax position of
the cams requires a complete redesign of the components.
It is also known to connect a camshaft consisting of
a hollow shaft and longitudinally and circumferentially
grooved bearing seats and cams by expanding the shaft tube
section by section by applying internal pressure and
pressing it into the grooves (DE-PS 25 46 802). In this
case, the cams having a uniform wall thickness are
tube-like so that a continuous fit on the shaft is
impossible, which means that the strength of the cams does
not appear to be ensured.
Finally it is known to fix internally round cams and
bearing seats on a shaft tube purely by force-locking
means by expanding the shaft tube along its entire length
by applying internal pressure, and in order to maintain
the pressure fit the tube, because of its thin walls, has
to be filled with a synthetic substance (DE-PS 32 27 693).
Without this additional measure it has so far not been
possible to produce a secure fit capable of being
torque-loaded. When expanding the shaft along its entire
length there is an inherent risk of bulging in the regions
between the cams, and there may be a notch effect at the
end faces of the cams which reduces the strength of the
shaft.
It is the object of the invention to provide an
assembled shaft which can be produced by a simple and
cheap production process permitting the transmission of
high torques and a greater degree of freedom in selecting
the materials for the shaft tube, cams and bearing seats.
The objective is achieved by providing a method
characterized by the fact that the materials of the
longitudinal portions of the shaft tube are subjected to
plastic deformation whereas the material of the elemen-ts
assumes the condition of a predominantly elastic
deformation. Without carrying out any thermal processes
it is possible in this way to produce fits permitting a
torque transmission which reaches up to 80~ of the
torsional strength of the shaft. The materials used for
the shaft may be relatively inferior materials such as St
35 to St 52, whereas for the cams and bearing seats high
strength materials may be used. The joining process does
not adversely affect the material values of either
material. As the connection is effected without any
external heat supply and as there is no need for heat
treatment for stress relieving purposes there are no
changes in the material structure adversely affecting the
connection nor are there any hardness losses. In contrast
to heat treatment processes, the dimensional changes which
occur can be calculated.
By having more freedom in selecting the material for
the elements, the cam material is more easily adapted to
different load conditions. It is possible to use cams and
bearing seats of different materials without putting the
required transmission of torque at risk. Suitable
processes and devices for hydraulically expanding
individual longitudinal portions can be realised easily
and cost-effectively as compared to expanding a shaft by
pressing in a plastics material.
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Another advantageous feature of the invention is that
the inner cross-section of the shaft tube remains open in
order to permit shaft cooling and inner lubrication of the
bearing seats. The shaft in accordance with the invention
S is easy to assemble and provided the joining processes are
taken into account accordingly it can be complete~ without
subsequently having to grind the cams or bearing seats.
The possibility of creating small distances between the
elements to be fixed and of selecting materials
specifically adapted to the functioning of the system
permit more freedom in designing the cylinder head.
Some details of the invention are explained with the
help of the enclosed drawing wherein:
Fig. la is a longitudinal section through a
connection prior to joining the cam and tube member;
Fig. lb shows a connection to Fig. la after joining;
Fig. lc shows a connection with two adjoining
elements after joining;
Fig. 2a is a longitudinal section and a cross-section
of an element provided with longitudinal recesses; and
Fig. 2b is a longitudinal section and a cross-section
of an element with a modified cross-section.
The figures show the tube 1 and the slid-on element
2, a cam 2a and a bearing bush 2b. The lengths and other
dimensions referred to in the claims and in the
description have been given identifying letters.
Fig. la shows that prior to joining, a
circumferential gap 3 is provided between the tube 1 and
the cam 2.
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Fig. lb shows a change in the cross-section of the
tube after joining. In particular, the limited length of
the expanded tube portion 4 is apparent.
Fig. lc shows an expanded tube portion ~ covering two
adjoining slid-on elements 2a and 2b, with a small free
distance 5 existing between them.
- Fig. 2a illustrates a slid-on element having three
circumferentially distributed indentations 6, whereas Fig.
2b shows an element with an aperture having conically
tapered end regions 7a, 7b.
The method in accordance with the invention is
particularly advantageous if, prior to expansion, the
difference Umin between the outer tube diameter da and the
inner element diameter Di as in Fig. la is at least 0.9
times the value of the outer diameter, multiplied by the
quotient of the 0.2% yield point Rp and the modulus of
elasticity E of the tube material, i.e. Umin > 0.9 da. P-
If the values are adjusted in this way it is possible to
obtain the required elastic pretension in the surface20 layer of the aperture of the element fixed by
force-locking.
The method in accordance with the invention has a
further optimum feature in that the expansion of the tube
takes place along an axial length portion which, at each
end, extends beyond the end face of the element by a
minimum of 50~ and a maximum of 150~ of the wall
thickness. That axial length portion is one-half the
ax R LaX E~ with reference to Fig. lb In
this way it is ensured in an advantageous way that the
tube wall joins the aperture of the element completely and
essentially in a stress-free way along the entire length,
which reduces the risk of micro-slipping and excludes the
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possibility of fit corrosion. At the same time, this
prevents bulging of the tube with notch effects at the end
faces of the elements.
Special conditions arise if several elements are
positioned closely together, which is the case in
particular with so-called three~ or four-valve engines,
i.e., if three or four valves are provided per cylinder.
If this is the case, as illustrated in Fig. lc, the method
in accordance with the invention has to be modified in
that the tube areas covering two or more elements have to
be expanded simultaneously. To the extent that this only
takes place up to a maximum free distance AaX between the
elements of up to 40~ of the wall thickness of the tube,
Lrad R~ there is no risk of tube bulging, and uniform
fixing of the different elements is ensured.
In an advantageous embodiment, illustrated in Fig.
lb, plastic radial surface deformation at the aperture of
the elements, if a ductile material such as steel is used,
reaches a material depth, A Lrad E~ which amounts to
approx. 10 to 15% of the smallest radial wall thickness of
the elements, Lrad E~ thereby achieving sufficiently high
connnecting forces without adversely affecting the
strength of the elements. With hard materials such as
cast materials, plastic surface deformation of this
magnitude is not possible nor is it necessary. In a
further advantageous embodiment, the elongation occurring
in the outer zone of the element in the tangential
direction after expansion should be of a magnitude of up
to 1% in order to avoid any surface damage if additional
loads occur. If a ductile material such as steel is used,
the preferred elongation values range between 0.1 and
0.4%, whereas with brittle materials such as cast
materials the elongation values should be between 0.01 and
0.2%.
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Usually and preferably, the method in accordance with
the invention is carried out in such a way that expansion
in the region of the cams takes place with in-ternal
hydraulic pressures of 2000 to 3500 bar whereas the
internal hydraulic pressures applied for expanding the
region of the more thin-walled beariny seats amount to
1000 to 2500 bar.
In a further advantageous embodiment of the method
illustrated in Fig. 2a it is possible, in addition to the
force locking effect, to achieve a form-fitting effect
between the shaft tube and the element. With this design
it is possible to allow the shaft material, in the course
of the forming operation, to flow into at least one, but
preferably several circumferentially distributed
lS longitudinal grooves or recesses 6, having a depth Ti
measured from the base circle (diameter Di) of the
cross-section in the range of 0.2~ to 1~ of the wall
thickness Lrad R of the tube in the aperture of the
element. A further possibility of achieving a
form-fitting connection, illustrated in Fig. 2b, consists
in providing a central element portion with an increased
cross-section which is followed by portions 7a, 7b whose
cross-section is tapered towards the end faces. The
embodiment of Fig. 2a primarily permits an improvement in
the permanent fit under torque loads whereas the
embodiment of Fig. 2b ensures a uniform area pressure
along the seat length and counteracts any inclination of
the tube towards bulging in front of the end faces of the
elements.
Apart from the above mentioned possibilities of
achieving a macro form fitting connection, it is also
possible to obtain a micro form fitting connection by
providing a hard particle coating on the surface of the
aperture of the elements which presses itself into the
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tube material in the course of connec-tlon. A further
method of achieving a micro form fitting connection
consists in producing machining -traces, especially
circumferential grooves in the aperture of the element,
which are intersected by the machining traces of the drawn
tube on the surface, thereby providing a close connection
when being pressed into each other. Machining traces in
the form of point-like indentations may also be achieved
particularly cheaply by sand b:Lasting or shot peening the
elements prior to joining.
By applying all of the above measures or a
combination of several of them it is possible to reduce
the expansion pressure required, which, in turn, permits a
reduction in the remaining tolerances of form of the shaft
lS after expansion. Furthermore, to increase service life,
it is possible to use hard and brittle materials for the
cams.
The invention furthermore relates to an assembled
camshaft or the like consisting of a shaft tube and
slid-on elements such as control cams, bearing rings, gear
wheels or bevel gears, produced by expanding the shaft
tube in the region of the elements by applying internal
pressure, especially in accordance with one of the
above-mentioned processes in the case of which the
material of the shaft tube in the longitudinal portions is
deformed plastically and the material of the elements
predominantly undergoes elastic deformation. As described
in detail, the deformation process is preferably carried
out by applying internal hydraulic pressure.
According to a preferred further embodiment, the
tensile strength of the shaft tube material should be 25
to 35~ lower than that of the material of the elements.
This favours the type of connection aimed at and, for
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reasons of costs, permits the selection of relatively soft
shaft materials.
In an advantageous embodiment, the cams may be cast,
steel or sintered elements finish-machined prior to
joining, and it is possible to select materials with high
hardness values such as ball bearing steel in order to
improve the service life of the camshaft in accordance
with the invention without adversely affecting the
strength of the connection.
The basic principle of the above-explained invention
may also be applied to hollow journals in boreholes, to
connecting two tubes by means of a sleeve or to connecting
two tube pieces inserted into each other.
