Multi-disc coupling device, an automatic transmission equipped t

Planetary gear transmission systems or components – Condition responsive control – Speed responsive control adjusted or opposed by torque

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192 7014, 192107M, F16H 6102

Patent

active

058608907

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

The present invention relates to a multi-disc friction coupling device, in particular a clutch or a brake, of the type which can be used for example in automatic transmissions for motor vehicles for the selective coupling of two rotary members of the transmission and thus to control the transmission ratio between the input and the output of the transmission.
The present invention also relates to an automatic transmission equipped therewith.
The present invention also relates to a method of making a disc of the multi-disc coupling device.
Automatic transmissions currently use, for controlling the transmission ratio which they provide, multi-disc clutches comprising an alternation of plates- or discs-connected to one and respectively to the other of two rotary members which must be selectively coupled and uncoupled. The coefficient of friction of the material used for the discs and the number of discs and their diameter have an effect on the axial engagement force which it will be necessary to apply to the clutch in order that the latter is capable of transmitting to one of the rotary members the torque received from the other. These clutches work in an oil bath whose function is cooling, the reduction of wear and the elimination of any residual friction even if the clearance between the discs is very small when the clutch is in the disengaged state. Normally, the discs are made of hardened steel and their thickness is chosen to be as low as is reasonably possible. In fact it is sought to reduce the axial dimension of the multi-disc clutch.
In practice, these clutches do not always have the qualities expected of them. In particular, the change from the coupled state to the uncoupled state is sometimes sudden. Furthermore, the clutch is subject to wear phenomena and consequently to a deterioration of its functional qualities over the course of time.
These defects assume a particular importance in the case of certain types of automatic transmission, such as described in WO-A-92 07 206, in which the friction characteristics between the discs are one of the elements which determine the points of change from one ratio to another in the transmission. The term "point of change" refers to each pair of values, speed of rotation/torque to be transmitted, for which the transmission automatically changes from one ratio to another. There are an infinite number of possible points of change because each value of torque to be transmitted corresponds to a speed of rotation at which the change of ratio in question will be made.
In the transmissions according to WO-A-92 07 206, the change from a given transmission ratio to a lower transmission ratio is carried out when the clutch, submitted to a calibrated engagement force, is no longer capable of transmitting the torque and therefore begins to slip. If the coefficient of friction is uncertain, the point of change for such a change of ratio will itself be uncertain.
The change from a ratio to a higher ratio results from a different process according to which the calibrated engagement force progressively overcomes a contrary force generated by a gearing reaction. At the same time, the passage of an increasing proportion of the power through the clutch progressively causes this gearing reaction to disappear. This finally makes it possible for the engagement force to acquire total supremacy. This process necessitates high quality in the frictional relationship between the discs if it is desired to avoid jolts.
Such ratio change processes give rise, between the discs, to phases of friction under load which are longer than in the case of entirely conventional automatic transmissions where the clutches and the brakes are actuated by actuators which are themselves controlled by logic control circuits.
It has been shown according to the invention that the coefficient of friction between steel discs varied with temperature and that these variations were the source of most of the observed operational defects. For example, during a progressive engagement process of the clutc

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