The present invention relates generally to speed variations suitable for use in the controlled driving of any device with the aid of any motor, such as for example in a motor vehicle for driving, with the aid of the engine of the vehicle, an accessory such as an alternator, water pump, compressor or fan. 
The invention relates more particularly to speed variators which consist of two pulleys, one being the driving pulley and the other the driven pulley, and of a belt passing in the form of an endless loop over the two pulleys, each of the pulleys comprising a hub and, disposed facing one another and around the hub, two annular plates at least one of which--here for the sake of convenience referred to as the movable plate--is mounted for axial movement relative to the hub under the control of resilient return means consisting of an annular member, referred to as the diaphragm, which comprises on the one hand a peripheral portion forming a dished plate spring and on the other hand a central portion divided by slots into radial fingers. 
A speed variator of this kind is described in U.S. Pat.  No.  4,132,120 filed on the Mar. 31, 1977 and issued on the Jan. 2, 1979. 
As described in this U.S. patent, the diaphragm of each of the pulleys bears axially, by its peripheral portion forming a dished plate spring, against the movable plate associated with it, while by its central portion divided by slots into radial fingers, it bears axially against a member, referred to as the support member, axially fastened to the corresponding hub. 
Both the axial support of the diaphragm against the movable plate and the axial support of the diaphragm against the support member take the form of axial rocking actions. 
During operation, the movable plate is displaced between two end positions--namely an advanced and a retracted position--relative to the other plate, which here for the sake of convenience will be called the fixed plate, and in one of these positions the diaphragm has a first generally frustoconical configuration, while in the other position it has a second generally frustoconical configuration which is the inverse of the first configuration. 
From one of these frustoconical configurations to the other the diaphragm therefore acts like a hinge, both in contact with the movable plate and in contact with the support member. 
The rocking axial support thus achieved between the diaphragm and the movable plate is inevitably a source of not inconsiderable wear on the part of one and/or the other of these members, and this is aggravated by the fact that in practice these members are made of metal. 
This rocking axial support may also be the cause of scratching and/or peeling of the surface of the diaphragm, to the detriment of its working life, particularly when the diaphragm has been shot-blasted. 
In addition, it is necessary to ensure suitable centring of the diaphragm relative to the movable plate against which it bears, failing which during the rotation of the unit, in the course of operation, the diaphragm would be subject to a relative reciprocating movement in relation to the respective plate, which would inevitably lead to additional wear apart from that already due to the rocking nature of the support, as indicated above. 
In addition to the fact that such accumulated wear will unavoidably result in a reduction of the resilient loading of the movable plate applied axially by the diaphragm, this wear is also detrimental to the life of the diaphragm and may even give rise to rupturing of the diaphragm. 
In order to achieve the desirable centring of the diaphragm relative to the movable plate against which it bears, it is proposed in the abovementioned U.S. Pat.  No.  4,132,120 to attach to the diaphragm, in the peripheral portion of the latter which forms a dished spring plate, a certain number of engagement blocks by which it is in engagement with the movable plate, for which purpose cutouts complementary to the engagement blocks are provided on the latter. 
Although from the point of view of the desired centring an arrangement of this kind has given and still does give satisfaction, and despite the fact that in addition such an arrangement advantageously makes it possible, with the aid of the same engagement blocks, to achieve rotational connection of the movable plate to the diaphragm, and through the latter and the support member to the corresponding hub, it has the disadvantage of not eliminating the unavoidable occurrence of wear between the diaphragm and the movable plate, because of the rocking axial support between them. 
Furthermore, through the punctiform presence of engagement blocks at the various points on the peripheral portion of the diaphragm which forms a dished plate spring, this arrangement gives rise to discontinuous stressing in the diaphragm, since each of these engagement blocks inevitably gives rise to a concentration of stressing in its fastening zone, and such discontinuity of stressing is unavoidably detrimental to the operating conditions of the diaphragm and to its life. 
Finally, in the case of a driving pulley, such an arrangement still makes it possible for the diaphragm to be turned inside out in the retracted position of the movable plate, so that it is necessary to make employ means preventing this from happening. 
The present invention relates generally to an arrangement which enables these disadvantages to be avoided and which in addition leads to other advantages.