GB2238359A - Toothed clutch or brake - Google Patents

Abstract

An engagement device comprising two relatively rotatable members (24, 29) each having a set (22, 23) of face teeth, the members being capable of relative axial movement to provide mutual engagement of the sets of teeth. The teeth are shaped to permit high speed slip between the members when the sets of teeth are disengaged but still in contact under axial load.

Description

TOOTHED COUPLING This invention relates to a toothed coupling such as a clutch or brake.

Standard automotive clutches have to be able to accommodate large differences in speeds between the two components to be coupled, and the consequent high speed slipping between the components which occurs until synchronisation is completed and coupling is effected. It is usually desirable, when shockless engagement of the components is required, for the friction torque transmitted during this high speed slipping to be at the same level as that which can be maintained after synchronisation has been completed. In order to achieve this, smooth contacting faces have to be used, and heavy axial loading provided on them, to allow the clutch to work efficiently despite the low coefficients of friction which result between the faces of the components which are to be coupled, particularly when they are running fully wetted with a lubricant.

However, there are applications where these high rates of slip are not normally required, and where intermittent overload of the clutch is prevented from being transmitted through the coupling, by allowing the clutch to react to the overload and hence transmitting greatly reduced transmitted torque until normality is restored. Examples of such applications are torque limiting clutch devices and certain types of transmission where only synchronous or near synchronous engagements are made. In both these cases, the torque transmitted is required to be at the full level until the overload limit is reached, and thereafter to fall until the cause of the overload has been removed.

The present invention is directed towards providing a toothed clutch ov brake having a high torque capacity at low levels of axial loading, with allowance for engagement of the components at relatively small speed differences.

Disengagement is intended to be achieved when the applied torque exceeds a pre-set torque limit. When thus disengaged, the torque still transmitted falls to a level which can be maintained at higher speed differences without damage.

Disengagement has also to occur when the axial load is removed.

According to the present invention there is provided a coupling between toothed transmission members having mating surfaces which accommodate high speed slip between the surfaces when the teeth are disengaged under axial load.

This invention furthermore provides an engagement device comprising two relatively rotatable members each having a set of face teeth, the members being capable of relative axial movement to provide mutual engagement of the sets of teeth, the teeth being shaped to permit high speed slip between the members when the sets of teeth are in first contact prior to engagement.

Preferably the profile of each tooth comprises a convexly shaped crest, and steep sides, there being smooth transition between the crest and the sides. For providing relative axial movement of the members there may be an actuator which is unable to cause full engagement of the teeth unless the relative rotation of the members is reduced from the high speed slip condition.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figures 1A and 1B show a known, standard face tooth coupling; Figure 1A being a plan view of the face, and Figure 1B being a side, circumferential view of one coupling face confronted by a similar, complementarily toothed face; Figure 2 shows in cross section the modified tooth shape of the coupling of the present invention; Figures 3A and 3B show the toothed coupling of Figure 2 in operation; and Figure 4 shows a typical clutch or brake application of the present invention.

The invention is based on a well known type of coupling, namely the face tooth coupling. Most standard toothed couplings have face teeth which are quite unsuited to fast overrunning of the mating faces; such action would result in damage to the coupling, and would produce heat and noise. A standard face tooth coupling 1, such as that shown in Figures 1A and lB, has mating teeth 2. These teeth are symmetrical and have flat, parallel crests 3 and troughs 4, and have sides 5 which taper substantially towards the crest of each tooth, such that the crest is substantially thinner in cross section than the base 6.

Additionally the corners of each tooth, between the sides and the crest, are often sharply angular.

Referring now to Figure 2 and Figures 3A and 3B, the modified coupling 9 is produced as a result of shaping the symmetrical tooth and choosing the material to accommodate high speed slip in certain circumstances, and is particularly designed so that, when out of full engagement but still under axial load, the surfaces can rotate relative to each other in either direction with film lubrication generated between them, thus giving a normal and damage free regime of operation for at least a specified period of time. The shape of the teeth has been significantly modified to achieve this. The tip or crest 10 is convexly shaped - for example arcuate, with a large radius of curvature. The crest of the modified tooth may have substantially the same width in cross section as its base 11, and the corners 12 of each tooth are smoothly curved.

The circumferential width of each tooth should be optimised for generation of the hydrodynamic lubricating action. The optimum width is dependent on operating speeds, size and axial engagement load, which is itself dependent on torque ratings and overload capacity.

The camber and shape of the tooth crest also affects the generation of the lubricant film and the contact stress.

Corner radii between tooth flank and crest affect the way engagement of the coupling occurs.

Figure 3A illustrates two modified teeth of the present invention in full sliding engagement, where there is relative rotation between the two teeth, and the respective crests 10 of the two teeth slide over each other, a film of lubricant 13 being generated between them.

Figure 3B illustrates modified teeth in the locked position, where a crest 10 on one face is cooperatively engaged in a recess 14 on a second face. The side contact angle 15 for each tooth and recess in the locked position shows a predetermined divergence from the normal to the surface of the coupling at that point, as discussed hereinafter.

It is additionally intended for any given tooth shape to select the response time of the piston, or other actuator component applying the axial engaging force, such that when the teeth are out of engagement and riding on the lubricating oil film, re-engagement is not possible at more than a certain low speed difference. The resulting hydrodynamic lift between the teeth crests prevents re-engagement although the coupling is under axial load towards engagement. The hydrodynamically maintained separation allows overload overrun to occur without interruption of the axial engagement signal and with the output still fully connected. Full engagement may require an axial travel of, for example, 2 mm. The rate of movement of a hydraulic piston obviously depends on the rate of fluid filling to its cylinder, and this can be adjusted to be sufficiently slow that no appreciable engaging movement is possible in the time taken for a tooth to pass its corresponding cavity. Once the piston has been pushed back to allow the teeth to disengage, it will stay back until the relative rotation of the two faces has been sufficiently reduced to allow re-engagement. Thus, the speed of relative rotation above which re-engagement is not possible is determined by the width of the teeth and the piston fill time.

Additionally, it is required that the tooth side contact angles 15 be chosen such that while "cam-out" is adequate always to ensure separation when the engaging load is removed, only a relatively small axial force between the couplings is required to maintain engagement up to the desired torque limit. The side angles depend on the required axial load for a given torque; departure of the side angles from the vertical should be at least greater than the angle of static friction, but as the angle increases further, the axial load needed to sustain a certain torque rises.

For a typical tooth according to the present invention the side angles are likely to be in the range of 10 degrees to 30 degrees away from the normal to the coupling face.

Furthermore, for a tooth having a circumferential width of 5 mm, for example, the lengths of the sides are preferably in the range 4 to 8 mm with corner radii of 1 to 3 mm. The camber centre height (that is, the difference in the height of the tooth between the centre and the sides) is then preferably in the range 0.5 to 2 mm.

Referring now to the typical clutch or brake application shown in Figure 4, this invention has a number of significant advantages. Firstly, no friction materials are essential, although a facing 20 is an optional and practical addition where shown. Secondly, clutch operating pressures may be reduced by a factor 3 or 4 over prior art requirements, since only small axial forces are required between the couplings. Furthermore, whereas normal slipping friction clutches require packs of many plates, this device needs no more than a single pair of toothed faces 22 and 23: the axial space required is therefore much less than for normal friction clutches.Due to the positive engagement between the couplings, the whole clutch may be much smaller for a given torque capacity, whilst still retaining the ability to accept differential speeds after break-away under overload, or during engagements near but not precisely at synchronous speed. Finally, the need for only one pair of plates can ensure complete disengagement, and elimates the free-wheeling drag of normal plate clutch packs.

These advantages result in considerably reduced size and cost of clutches and brakes, and since most automatic transmissions need several of these components, the overall reduction in size and cost of such transmissions will be quite significant.

In addition, the operating efficiency of such a transmission is much influenced by the parasitic losses of items such as hydraulic pumps and seals. These losses will clearly be reduced, and operating efficiency improved, when the clutch and brake requirements can be satisified by the lower system operating pressures which are required by the couplings of the present invention.

Claims (4)

1. An engagement device comprising two relatively rotatable members each having a set of face teeth, the members being capable of relative axial movement to provide mutual engagement of the sets of teeth, the teeth being shaped to permit high speed slip between the members when the sets of teeth are disengaged but under axial load.

2. An engagement device according to claim 1 in which the profile of each tooth comprises a convexly shaped crest of which the radius or radii of curvature is or are large relative to the breadth of the crest and steep sides, there being smooth transitions between the crest and the sides.

3. An engagement device according to claim 2 in which for providing relative axial movement of the members there is an actuator which is unable to cause full engagement of the teeth unless the relative rotation of the members is reduced from the high speed slip condition.

4. An engagement device according to claim 2 or claim 3 wherein the sides of each tooth lie at an angle to the normal to the engaging face of the device, said angle being in the range of 10 degrees to 30 degrees.