GB2043834A - Constant Velocity Universal Drive Coupling - Google Patents

Abstract

A constant velocity universal drive coupling comprises several apertured discs 12 joined alternately at their outer and inner peripheries 20, 22. The inner periphery is segmented by means of slots 16 forming leaves 24 which are riveted and welded to opposed leaves on the adjacent disc. The outer peripheries are welded together. <IMAGE>

Description

SPECIFICATION Constant Velocity Universal Drive Coupling This invention relates to a constant velocity universal drive coupling.

A constant velocity universal coupling has to transmit torque between two abutting shafts which are arranged at an acute angle to each other. From the point of view of a coupling member mounted on the drive shaft, for example, the driven shaft axis itself rotates about the drive shaft axis to define the surface of a cone, and the coupling member therefore has to be able to transmit torque despite this angulation of the axes. The most common type of universal coupling is of the Hooke's type and has two forks pivoted at right angles to each other by an Xshaped member. This coupling has a number of disadvantages, which include the fact that the drive is not exactly constant velocity but varies over the course of a rotation.This can be overcome by using two couplings connected together in series, providing the input and output shafts are equally inclined to the intermediate shaft and the corresponding forks at each end lie in the sample plane. However, the use of two couplings causes further problems, since the angulation causes a slight fore-shortening to take place which can introduce axial thrust into the assembly. This if often countered by introducing a sliding splined joint in the intermediate shaft.

Unfortunately this introduces a third point of lateral freedom leading to high speed instability and whirl problems.

This invention however is concerned with a different type of coupling known as the diaphragm type. This consists of a series of diaphragms arranged such that driving torque can be transmitted through each diaphragm to its neighbour despite the angulation of the axis, by virtue of the ability of each diaphragm to deflect proportionately as it rotation about its axis. The resultant axial curvature between each pair of diaphragms in the nest of diaphragms produces a progressive and gradual change to the angle of the drive axis, until the full amount of the axis angulation is collectively achieved.

It will be appreciated that deflection of the diaphragm plates occurs as a form of elastic buckling which travels orbitally around the diaphragm plates during each rotation or cycle.

Each diaphragm tends therefore to rotate on the homokinetic plane of its immediate neighbours, and so conforms to the first law of constant velocity drive angulation. The degree of buckling must be within the elastic limit in order to minimise the effect of the work done in deforming the diaphragms, which mainly converts to unwanted heat if the energy is not reciaimed by the elastic responses of the material.

Therefore, as only small deflections can be tolerated by each diaphragm, it follows that a considerable number of diaphragms are necessary to obtain any appreciable degree of angulation. However, the greater number of diaphragms used, the more adversely is the lateral stability of the coupling affected. This unwanted effect then dictates the use of an internal knuckle to control the deviation of the axis to a fixed point of angulation, which unfortunately relates badly to the curving relationship of the axis through the nest of diaphragms. This incompatability of the straight articulating axis controlled by the knuckle with the curving bend of the multi-diaphragm arrangement does not aid the associated problem of dynamic balancing nor does it help to distribute the bending equally along the coupling.

Another problem with this type of coupling is that because it relies on complex buckling it it difficult to make safe design predictions.

This invention provides a constant velocity universal drive coupling as defined inthe appended claims.

The invention will now be described by way of example with reference to the drawings, in which: Fig. lisa diagrammatic side view of two adjacent diaphragms in a coupling embodying the invention; Fig. 2 is a front view of one of the diaphragms; and Fig. 3 is a diagrammatic side view illustrating the manner of use of the coupling.

Fig. 4 is a side view of a modified coupling partially in section, Fig. 5 is a view, corresponding to Fig. 2, of another modified coupling, and Fig. 6 is part of a cross section on the line VI VI shown in Fig. 5.

Referring now to Figs. 1 and 2, the high-speed coupling 10 illustrated includes a number of identical circular convex diaphragms 1 2 connected together in series. Each diaphragm has a large hole 14 cut in its centre, and a series of radial slots 1 6 extending from the central hole 14 towards and terminating in holes 17 a short distance from the periphery 1 8 of the disc. As shown, there are twelve such slots, but a larger or smaller number may be used as appropriate.

Each disc is attached to its two neighbouring discs as follows. The outer periphery of the disc (see disc 1 2a in Fig. 1) is welded at 20 around the periphery to one of the adjacent discs 1 2b. The inner periphery 22 of the disc 1 2a is joined to the inner periphery of the outer adjacent disc 1 2c.

The inner peripheries are joined together by joining each leaf 24 formed between two slots 1 6 to a corresponding leaf of the adjacent disc. This is achieved by the use of rivets 26 close to the inner ends of the rivets, and also by welding at 28 along the inner edge of the leaves. In this way each leaf 24 is capable of transmitting torque in shear to its opposite member in the adjacent disc.

Thus it can be seen that the rolling action of the discs will translate into a wave formation moving around the leaves in the disc. As the rolling action takes place leaves will close together on the closing side of the elements and correspondingly open out at the diametrally opposite side of the discs, as shown in Fig. 1. In reality, during actual rotation of the unit the internal leaf ends are orbiting in the homokinetic plane thus ensuring constant velocity transmission through the coupling.

The configuration of each pair of riveted leaves can be likened to a wishbone. Each pair of wishboned discs is attached to its neighbour by simple welding around the periphery. This peripheral rim is relatively stiff and does not experience bending loads, but only torsional shear loads, so that welding can be used satisfactoriiy.

Where diametral limitations are considered unimportant the disc could carry an external flange with a rim of bolts marrying one disc to the other; however welding is preferred.

The design of the dish-shaped discs is such that as the leaves pass through the opening phase of the cycle and each leaf 'unrolls' from its allied member, the pair of leaves forming the 'wishbone' cannot open of 'roll out' sufficiently to expose each rivet to undesirable tensile forces but leaves the rivet to perform in shear only, which is a relatively constant load and therefore nonfatigueing. The rivet heads in fact make ideal buffer stops to protect the discs from abusive over bending. For some applications, small dome headed bolts may be used instead of rivets. By this means the clamping effect on the convex configuration of the plate helps to pre-stress the leaves to offset the effects of the cycling stresses and so enhances the fatigue characteristics of the coupling.

It should be noted that for the torsional mode the inner edge of each leaf is welded. This is to prevent the cantilever leaf rotating around the rivet in the lateral (or shear) plane and so increases the stiffness of the leaf in the lateral (or shear) plane and increases the torsional capacity of each joint accordingly.

The welding is not affected by axial bending for the reasons just described, that is to say that as the rivet is shielded from tensile forces so must be the inner weld. After welding the material is heat treated to restore its properties.

When the outer weld 20 is made, the discs (e.g. 1 2a and 1 2b) being of convex shape abut at a slight angle as shown in Fig. 1. This facilitates flexing in use.

In Fig. 3 a complete coupling 10 is seen which has six rolling discs, the inner four 2) of which are as described above and the outer two (30) of which are blank. The coupling connects a drive shaft 32 to a driven shaft 34. These shafts are shown in line but can angulate to adopt an acute angle between them. The coupling 10 described will, by virtue of the capacity of the leaves 24 collectively to absorb small axial movement by flexing, not normally require special precautions to be taken to avoid axial thrusts. However a splined coupling 36 can be included if desired to provide greater axial freedom, when necessary, e.g. with an expanding engine case, as shown with the coupling 1 0a in Fig. 3. Here, the coupling permits the outer member of the slider to be turned inwards inside the coupling.This allows the slider to coincide with the centre of articulation and thus does not impair the essential two points of freedom which are necessary for stable high speed rotation.

It is seen from the above that the coupling is very short in length giving good lateral stability without the need for a constraining knuckle.

Nevertheless there is a high potential axial flexibility coupled with high diametral stiffness. As the coupling relies on simple bending rather than buckling stress computation is less difficult.

The particular number of slots 16 and hence leaves 24 which are provided will depend on the particular application for which the coupling is required, as will the number of diaphragms 12 employed. Where less strength is required some of the leaves could simply be omitted. While there need only be three leaves, in practice there will normally be a minimum of six. The shape of the slot termination holes 1 7 can be changed from that shown. The diaphragm needs to have one periphery segmented so as to allow flexing to a non-circular shape during angulation. In principle it could be the outer periphery which is thus relieved, the inner periphery being continuous though the illustrated construction is to be preferred.

In certain cases, particularly at high speed, it will be found that to contain the flexing stress of the leaves within the acceptable fatigue strength of the material, to ensure effecting infinite life, a conflicting situation arises in the desire to ensure the critical vibrational frequency of the drive shaft is outside the working range of speed. To ensure a comparatively low spring rate (and hence stress levels) in the leaf whilst obtaining a higher spring rate to ensure higher critical frequencies it is necessary to supply the coupling with a centre axial spring (Fig. 4).

The centre spring 35 is situated on the axis of the coupling in the space within the annular discs 30 and is designed to increase the axial lateral spring rate of the coupling without affecting the torsional qualities of the diaphragm pack. The spring 35 is secured to each blank and plate 36 embracing the ends of the diaphragm pack. This permits the desired stiffness to be obtained without the need for the spring to support torsional loads and which will accommodate the required flexing movements readily. The torsional loads are diverted through the diaphragms and the flexing stresses in the diaphragms (which are a function of the angular movement of the coupling) are unaffected.

It is of course essential to provide a similar spring at each end of the drive shaft to ensure balanced reactions. Because of this it is not possible to incorporate an auxialiary sliding unit (as in Fig. 3). However the drive shafts usually obtain a high enough degree of axial freedom from the diaphragms collectively to suit most applications.

It is necessary to balance the springs to interact correctly with each other to obtain optimum performance and to ensure the springs are stiff enough not to whirl independently.

In some cases it is necessary to increase the torque capacity of the joint by increasing the spring rate of the diaphragms while obviating the need to provide individually tailored diaphragms for every application. This can be achieved (Figs. 5 and 6) by incorporating an annular clamp ring 41 on the pitch circle of rivets at the inner edge of the leaves. As mentioned elsewhere the inner diameter of the leaves rotate in a common plane for each diaphragm pair (viz the homokinetic plane). This makes it possible to ciamp the inner edge of the leaves by a pair of clamping rings 32 selected from a given range of outside diameters which accordingly adjust the effective working length of the leaf (and also the spring rate of the diaphragm) to meet a wide range of design requirements. The outer edges of the rings 32 are clamped or rounded at 42 where they meet the diaphragms.

The rings 41 also increase the torsional capacity of the diaphragm by increasing the twisting resistance of each individual leaf in the rotational mode.

By utilizing both the spring 35 (Fig. 4) and the clamping rings 41, it is possible to meet a wide variety of torque ratings, stress limitations and critical frequencies presented by requirements with a minimum number of diaphragm variants by appropriate choice of standardised parts.

Claims (10)

Claims

1. A drive coupling comprising a series of apertured discs joined alternately at their outer and inner peripheries, and at least some of which have a continuous annular portion at one radial position while being divided at one of their peripheries into leaves extending radially from the continuous portion to a junction with the corresponding leaves of an adjacent disc.

2. A coupling according to claim 1, in which the said continuous portion is the outer periphery and the said one periphery is the inner periphery.

3. A coupling according to claim 2, in which the joined outer peripheries are welded.

4. A coupling according to claim 2 or 3, in which the joined inner peripheries are riveted together.

5. A coupling according to claim 2, 3 or 4, in which the joined inner peripheries are welded.

6. A coupling according to any preceding claim in which the discs are convex when in the unstressed state.

7. A coupling according to any preceding claim, in which the said one periphery is divided into a minimum of six leaves.

8. A coupling according to any of the preceding claims, including a helical spring interconnecting the endmost discs and extending through the apertures in the intermediate discs.

9. A coupling according to any of the preceding claims, wherein the discontinuous peripheries of adjacent discs are clamped between clamping rings.

10. A drive coupling substantially as hereinbefore described with reference to Figs. 1 to 3 or Fig. 4 or Figs. 5 and 6 of the accompanying drawing.