Friction clutch

The invention relates to a friction clutch in which an axial play is provided between an inner disk and a hub of a clutch disk. The axial play can be influenced by a device during assembly of the friction clutch in an engine and transmission of a motor vehicle drivetrain such that the hub is displaced, within the axial play, into the position in the axial play which lies closest to a flywheel of the friction clutch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a friction clutch in accordance with the preamble 
of the main claim. 
2. Description of the Related Art 
A prior art clutch disk is disclosed by German Laid-Open Specification 43 
24 203 that has a friction clutch with an inner disk. The inner disk is 
connected to the hub in a rotationally fixed manner by a toothing and both 
components are fixed in the axial direction with respect to one another, 
with an axial play which results from production tolerances being provided 
between the two parts. 
U.S. Pat. No. 5,301,779 discloses a prior art device for guiding a 
plurality of inner disks of a multidisk clutch in a rotationally fixed 
manner on the hub with securing means which extend in the axial direction 
between two adjacent inner disks. The securing means are provided in the 
hub to axially secure the latter in the axial direction. In this case too, 
an axial play is provided between the securing means and the disks. This 
play is also required, for example, in view of the wear to the individual 
disks. 
It has now been found, particularly in the case of vehicles which are used 
in motor sports events, that uneven force transmission may arise during 
the start-up operation, which stems from the fact that at the beginning of 
torque transmission, i.e. during the clutch engaging operation, the hub 
may adopt a different axial position, specifically to the extent of the 
play between the securing means and the corresponding stops. If the hub 
adopts a position which is remote from the flywheel during the start-up 
operation, during the engagement operation the hub has to be displaced 
axially from the transmission shaft, specifically toward the flywheel. As 
the torque transmission increases, this axial displacement requires an 
increasingly high force and, under certain circumstances may be carried 
out jerkily. The force required to move the hub toward the flywheel has an 
adverse effect on the torque transmission from the engine to the 
transmission during the start-up operation. 
SUMMARY OF THE INVENTION 
The object of the present invention is to improve the start-up operation by 
influencing the axial position of the hub within the construction-related 
axial play. 
According to the invention, this object is achieved by a friction clutch 
for use in a drive train of a motor vehicle, comprising, a flywheel 
rotatable about an axis of rotation and connectable to a crank shaft of 
the motor vehicle, a clutch housing fixedly connected to said flywheel, a 
pressure plate arranged in said clutch housing such that said pressure 
plate is rotatably fixed and axially movable with respect to said clutch 
housing, a disk having an internal toothing and arranged between said 
pressure plate and said flywheel such that said disk is clampable between 
said pressure plate and said flywheel when said pressure plate is urged 
toward said flywheel, a hub having an external toothing and rotatable 
about said axis of rotation, said internal toothing of said disk and said 
external toothing of said hub creating a rotatably fixed and axially 
movable connection between said hub and said disk such that disk is 
axially movable with respect to said hub within an amount of axial play 
and a device operatively arranged for axially displacing said hub within 
said amount of axial play to a position within said amount of axial play 
that is closest to said flywheel during assembly of said friction clutch 
into the drive train of the motor vehicle. 
The present invention provides a device which displaces the hub, within the 
axial play, into the position which lies closest to the flywheel when the 
engine, friction clutch and transmission are being assembled. This ensures 
that there is no need for any further displacement of the hub toward the 
flywheel during the clutch engagement operation--even if wear to the disk 
or disks may occur as early as in this situation. Since the hub does not 
have to be moved to engage the clutch, there is no expectation of any 
resistance from axial movement of the hub which could have an adverse 
effect on the clutch engagement operation. 
In one embodiment of the present invention, an internal toothing of the hub 
is fitted onto external toothing of the transmission input shaft and the 
device comprises a plastic material element arranged between the internal 
toothing of the hub and the external toothing of the transmission shaft. 
During assembly of the engine, friction clutch and transmission, this 
plastic material inside the toothing ensures that a high pushing force is 
required to introduce the transmission shaft into the internal toothing of 
the hub. As a result, the hub is displaced toward the flywheel as far as 
its axial mobility allows. A precondition for this is that the friction 
clutch is in the closed state at least during the final part of this axial 
displacement during assembly. 
Furthermore, it is proposed that the plastic material is designed such that 
its melting point is reached or exceeded at the operating temperatures of 
the hub and/or transmission input shaft. Selecting the plastic material in 
this way allows the resistance of the plastic material to axial 
displacement to be influenced during operation of the friction clutch such 
that the axial mobility is fully restored during operation of the friction 
clutch. This ensures that during subsequent gear change operations both 
the engagement operation and the release operation are performed without 
hindrance. Preferably, the material is designed in such a way that the 
plastic material is made to flow even by the temperature produced in the 
area of the toothing after a start-up operation carried out under racing 
conditions. 
In another embodiment the plastic material is applied to the internal 
toothing of the hub and/or the external toothing of the transmission input 
shaft. In this case, it is entirely sufficient to apply the plastic 
material to one of the two components (hub or transmission input shaft). 
The plastic material may advantageously be applied to the appropriate 
component by injection molding. However, it is also conceivable, by way of 
example, for the transmission shaft to be provided with an appropriate 
coating by being immersed in plastic material. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages, and specific objects attained by its use, reference 
should be had to the drawing and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Referring to FIG. 1, a friction clutch 1 according to an embodiment of the 
present invention includes a flywheel 2 arrangable on a crankshaft 29 of 
an internal combustion engine (not shown). A clutch housing 3 is fixedly 
connected on the flywheel 2 via fastening devices such, for example, as 
threaded bolts 31. A pressure plate 5 is mounted within the clutch housing 
3 such that it is rotationally fixed and axially displaceable with respect 
to the clutch housing 3. The guidance of the pressure plate 5 may, for 
example, be effected by tangential leaf springs or axially running bearing 
edges between the pressure plate 5 and the clutch housing 3. A diaphragm 
spring 4 is arranged axially between the pressure plate 5 and the clutch 
housing 3 and is supported on the clutch housing 3 via wire rings 7 and a 
plurality of spacer bolts 6 which are circumferentially distributed. An 
area of the diaphragm spring 4 arranged radially outside the spacer bolts 
6 exerts an axial force on the pressure plate 5 to press the friction disk 
of a clutch disk 25 against the flywheel 2 to transmit torque. Radially 
inside the spacer bolts 6, the diaphragm spring 4 is provided with spring 
tongues which interact with a release system (not shown). In the present 
case, the clutch disk 25 comprises a hub 14 connected in a rotationally 
fixed manner to an inner disk 8. The connection is via toothings 9, 11 and 
is designed in essentially a play-free rotationally fixed connection with 
a predetermined amount of axial play. An internal toothing 15 of the hub 
14 is attached in a rotationally fixed manner to a transmission shaft 17 
(see FIG. 3). All the components of the friction clutch 1 are arranged 
concentrically with respect to an axis of rotation 10 and are rotatable 
about this axis together with the crankshaft of the internal combustion 
engine. In the present case, the friction clutch 1 also includes two outer 
disks 12 connected to the clutch housing 3 in a rotationally fixed and 
axially displaceable manner. 
FIG. 2 shows the clutch disk 25 of the friction clutch in FIG. 1. This 
clutch disk 25 comprises the hub 14 with the internal toothing 15 for a 
rotatably fixed engagement with the transmission input shaft 17 (see FIG. 
3) and the external toothing 11 for rotatably fixed engagement with the 
internal toothing 9 of the inner disk 8. The hub 14 further includes a 
circumferential web which forms a first axial stop 20 for the inner disk 8 
in the direction of the flywheel 2. A second axial stop 19 is arranged on 
the opposing side of the inner disk 8 such that an axial play S is formed 
between the inner disk 8 and the hub 14. The first and second axial stops 
20 and 19 are required to prevent the hub 14 from axially migrating 
outside of an axial region having an axial play S during operation of the 
friction clutch. A connection between inner disk 8 and hub 14 without 
axial play is theoretically possible, but is not to be recommended for 
various reasons. Firstly, the accuracy required for the production process 
is inhibiting, and secondly the play has a beneficial effect on the 
ventilation of the clutch. 
It has been found that under certain operating conditions, the clutch with 
a clutch disk having axial play exhibits uneven torque transmission during 
the start-up operation of the vehicle. This is undesirable in any start-up 
operations and is especially so in start-up operation for racing sports 
cars. The reason for these uneven torque transmissions is that before the 
start-up operation, while the first gear is engaged and the clutch is 
being ventilated via the operating system, the axial position of the hub 
14 on the transmission input shaft 17 is not defined due to the axial play 
S of the position of the hub 14. The engine vibrations may cause the hub 
14 to move with respect to the inner disk 8, within the range of an axial 
play S, toward or away from the flywheel 2 (see FIG. 1). If the position 
of the hub 14 at the initiation of a start-up operation is such that the 
axial play S is situated between the inner disk 8 and the right-hand axial 
stop 19, as shown in FIG. 2, even a slight movement of the inner disk 8 
toward the flywheel 2 during the clutch engagement operation inevitably 
leads to the hub 14 also being forced toward the flywheel 2. The movement 
of the hub 14 is made more difficult by the fact that torque is already 
being transmitted via the toothing 15. Accordingly, the hub 14 is movable 
toward the flywheel 2 in an appropriate manner only with the aid of 
relatively great engagement forces, (i.e., the clutch engagement 
operation) when the axial play S is on the flywheel side between the first 
axial stop 20 and the inner disk 8 at the initiation of a start-up 
operation, the start-up operation occurs without uneven torque 
transmission because the engagement movement of the pressure plate 5 and 
the inner disk 8 toward the flywheel is performed without requiring axial 
displacement of the hub 14. In this connection, it should be noted that 
the axial displacement between the inner disk 8 with its internal toothing 
9 on the external toothing 11 of the hub 14 is subject to lower 
circumferential forces because the location of the toothings 9, 11 is a 
greater radial distance from the axis of rotation 10 than the toothing 15. 
FIG. 3 shows a longitudinal section through the top half of a multidisk 
clutch 1' which includes a plurality of outer disks 12 and a plurality of 
inner disks 8. In operational terms, the customary structure of the 
multidisk clutch 1' is identical to the friction clutch 1 shown in FIG. 1. 
The inner disks 8 and the outer disks 12 of such multidisk clutches 
typically comprise carbon material when these clutches are designed for 
use in motor sports. The clutch 1' has a flywheel 2 to which the clutch 
housing 3 is attached, via fastening device such as the threaded bolts 31 
shown in FIG. 1. 
In the FIG. 3 embodiment, the diaphragm spring 4 is supported on the clutch 
housing 3 via a plurality of threaded bolts 13 which are circumferentially 
distributed. In the engaged state of the clutch 1', the prestressing force 
of the diaphragm spring 4 acts on the disks 8, 12 via the pressure plate 5 
and presses these disks 8, 12 together toward the flywheel 2. The inner 
disks 8 are connected to the hub 14 in a rotationally fixed connection but 
are axially displaceable relative to the hub in that they have internal 
toothing which engages in corresponding external toothing 11 on the hub 
14. The internal toothing 15 of the hub 14 is fitted onto an external 
toothing 16 of a transmission input shaft 17. A clutch operator 18 is 
diagramatically shown which acts on the radially inward facing tongues of 
the diaphragm spring 4. In the present embodiment, an axial stop 19' is 
arranged between the first and second inner disks 8 with respect to the 
diaphragm spring 4. The axial stop 19' is provided to axially secure the 
hub 14 with respect to the inner disks 8. In this case, a precondition for 
a successful start-up operation is that the axial play S is arranged 
between the axial stop 19' and the first inner disk 8 with respect to the 
diaphragm spring 4. The clutch 1' is ventilated by actuation of the clutch 
operator 18 in that the radially inner area of the diaphragm spring 4 is 
moved toward the flywheel 2. The actuation of the clutch operator results 
in a lifting of the radially outer area of the diaphragm spring 4 off of 
the pressure plate 5 such that all the disks 8, 12 can be ventilated. If a 
clutch engagement operation is now initiated and the hub 14 is held in the 
position shown in FIG. 3, a perfect start-up operation is attainable 
because the engagement movement of the disks 8, 12 toward the flywheel 2 
is not inhibited by a need to displace the hub 14 in the axial direction 
during the engagement operation. As mentioned above, the displacement in 
the toothing 11 with respect to the toothing 9 of the inner disk 8 is 
easier to execute because the location of the toothings 9, 11 is at a 
considerably greater radial distance from the axis of rotation 10. 
The present invention therefore consists of a plastic element 27 introduced 
between the toothing 16 of the transmission input shaft 17 and the 
internal toothing 15 of the hub 14. The plastic element 27, during 
assembly of engine, clutch and transmission, ensures that the first time 
the clutch is brought into service thereafter, the hub 14 is held in its 
position closest to the flywheel 2. When the transmission shaft 17 is 
introduced into the internal toothing 15 of the hub 14, the plastic 27 
exerts an appropriate axial force on the hub 14 such that during the first 
actuation or actuations of the friction clutch, the hub 14 is held in this 
position close to the flywheel 2. By selecting a material for the plastic 
element 27 such that the plastic becomes flowable at the operating 
temperature in the region of the hub 14 or of the transmission input shaft 
17, the flowability of this material of the plastic 27 may be utilized to 
reduce the axial force between the transmission input shaft 17 and the hub 
14, at the earliest after the first clutch engagement operation performed 
under racing conditions. The clutch 1 and/or 1' is then operable normally 
during subsequent gear change operations. The demands placed on the clutch 
engagement operation following a gear change operation are considerably 
lower than the demands placed on the first racing start. 
The plastic element 27 may comprise an axially extending strip as shown in 
FIG. 3. Alternatively the plastic element 27 may comprise a continuous 
annular piece 27a as shown in FIG. 3A of any cross-sectional shape or a 
cylindrical piece 27b as shown in FIG. 3B. In any embodiment, the plastic 
element 27, 27a, or 27b is arranged between the toothing 16 of the 
transmission input shaft 17 and the internal toothing 15 of the hub 14 for 
holding the hub 14 in its position closest to the flywheel 2. 
The invention is not limited by the embodiments described above which are 
presented as examples only but can be modified in various ways within the 
scope of protection defined by the appended patent claims.