GB2272955A - Roller traction drive assembly - Google Patents

Abstract

A roller drive assembly comprises a drive spindle (1), at least two planet rollers (4) each in driving contact with said spindle and an outer annulus (5) with which each planet roller has two distinct engagement zones. Each engagement zone has a bevel surface (5c, 5d) engaging a similarly configured surface on each planet roller (4). As shown both engaging surfaces are part-conical. Alternatively the surfaces on the planet rollers may be part-spherical (see Fig 3 not shown). The portion of the planet rollers engaging the spindle (1) may be cylindrical or part-spherical (Fig 3). Axial Loading is applied to the outer annulus by means of a conical washer (17). Resilient bearing mounting means 15, 16 allow limited movement of each planet roller within its carrier (3). <IMAGE>

Description

ROLLING TRACTION DRIVE ASSEMBLIES The present invention relates to rolling traction drive assemblies wherein a drive spindle provides rotary drive to an output member by way of planet rollers engaging the spindle and the output member and particularly to a drive assembly wherein there is a central drive spindle rotating at high speed, and there is a speed reduction between the central drive spindle and the output member.

In this form of transmission, the weakest element in terms of fatigue is usually the central drive spindle which may be rotating at a speed of the order of 20,000 revolutions per minute and has during each revolution a multiplicity of stress cycles, arising from the contact with the planet rollers.

It is the general object of the invention to provide an improved drive assembly in which alignment movement of the planet rollers equalises the loads and torques between multiple driving contacts, thus reducing the stresses at each contact and permitting much increased life expectancy of the assembly.

There follows a description by way of example only of several embodiments of the invention, reference being made hereinafter to the accompanying drawings, in which: Figure 1 is a sectional view of a rolling traction drive assembly according to one embodiment of the invention; Figure 2 is a sectional view taken at right angles to the view shown in Figure 1; Figure 3 illustrates a different embodiment of the invention; Figure 4 illustrates a further embodiment of the invention; Figure 5 illustrates a modified form of the embodiment shown in Figure 1; and Figure 6A and Figure 6B illustrate a detailed modification of part of the embodiments described in the previous figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 of the accompanying drawings illustrates a rolling traction drive assembly. An input, high speed drive member is constituted by a spindle 1. A roller bearing 2 supports one end of a rotary carrier 3 which supports a plurality of planet rollers 4 which are preferably equally spaced about the periphery of the drive spindle and each engage an end portion of the drive spindle and also each engage an outer annulus 5. The spindle 1, the planet rollers with their carrier 3 and the annulus 5 constitute an epicyclic drive for which in this embodiment of the invention the carrier is the output member and the annulus is the reaction member.

In different configurations the annulus could be the output member. The outer annulus is supported in a casing 6 relative to which the carier 3 is journalled by means of a bearing 7 and indirectly by roller bearing 2. The casing 6 is screwed and locked to the casing 8 of a motor. In this embodiment the spindle comprises the output shaft of the motor which includes a bearing 9 for the spindle 1.

For reasons which will be apparent, the ring 5 provides two engagement surfaces, the normal to which is oblique to a plane normal to the axis of the bearing spindle. Preferably, the bearing surfaces face inwardly and obliquely towards the zone or, as will be seen, a respective zone of contact of the planet rollers with the central spindle 1. In this embodiment of the invention the annulus 5 is constituted by two torsionally connected but axially free bevelled rings 5a and 5b of which the respective bevelled surfaces Sc and 5d effectively comprise a double conical annulus. The bearing surfaces may be arcuate instead of conical.

In this embodiment of the invention, each planet roller is provided with two opposite stub axle portions 11, 12 each of which is supported in a respective bearing 13, 14 mounted for limited movement relative to the carrier 3. For this purpose, each of the bearings is mounted within a resilient ring 15 or 16 within the carrier 3.

In Figure 1 the outer annuli 5a and 5b are subject to an axial load by means of a conical washer 17 which is axially compressed on assembly against its seating 18 in casing member 8 and which is prevented from rotation by spring pins 19 extending through the washer to engage in blind holes 20 in member 8. Annuli 5a, 5b and washer 17 are thus all held against rotation in the casing. Annulus 5a engages an abutment in the form of annular shoulder 21 in casing 6 which supports the loads coming from washer 17 by way of annulus contacts 5d and 5c.

The speed reduction ratio of output shaft 3 to input spindle 1 in this kinematic arrangement is 1/E+1 where E is the ratio of annulus contact diameter to spindle diameter - in this example about 6:1, thus giving an overall reduction ratio of 7:1.

Figure 3 of the drawings illustrates to a smaller scale an embodiment similar to that of Figure 1. Parts common to the figures have the same reference numbers and will not be further described.

The only effective difference between Figure 3 and Figure 2 lies in the shape of the engagement surface of the planet rollers; the significance of this will now be described.

The shape of the planet roller in Figure 3 is part-spherical. A central zone of the surface of the roller engages the central spindle and the outer zones engage the two bevelled portions of the annulus. The projection line A in Figure 3 indicates that the normal to the engagement surface of each bevelled portion with the planet roller passes through the zone of engagement of the planet roller with the central spindle.

The shape of the planet roller shown in Figure 3 and the mounting of the roller permits the planet roller to align itself between the spindle and the outer rings.

In this embodiment a mutual convexity (actually a plano-convexity) of each engagement portion of the annulus and the respective part of the roller is provided by the part-spherical shape of the roller; alternatively the engagement portions of the annulus could be convex and the respective parts of the roller could be conical or curved in any suitable conformity.

The planet rollers in Figure 1 differ from those in Figure 3 in that there is a central generally right-cylindrical portion 4a, which extends the contact land relative to the spindle to lessen the contact stress thereon, and two part-spherical portions 4b and 4c engaging the bevelled or conical portions of the outer ring 5. In Figure 4 the planet rollers are further modified so that they comprise two axially spaced apart generally right-cylindrical portions 4a and curved outer margins 4b and 4c. In this arrangement there are two distinct zones of engagement and torque transmission of each planet roller with the central spindle: this further reduces the contact stress on the spindle. More engagement zones can be provided by further increase of roller width.

In Figure 4, the projection lines B and C denote acceptable variation in the normal to the engagement surface of a planet roller with the outer ring relative to the contact region between the planet roller and the central spindle.

These lines intersect the periphery of the spindle at the central plane of the planet roller and the central plane of the respective cylindrical portion 4a.

When planet rollers are used embodying either single or multiple cylindrical engagement zones it is necessary for the roller to be able to align itself to the spindle surface. This alignment movement of the planet rollers can be provided by the flexible bearing supports 15 or 16 already described or, alternatively, by bearing blocks 22 such as shown in Figure 6. In each bearing block the respective stub axle is mounted within the block by means of a roller bearing 23. This bearing permits axial movement of the planet rollers. The bearing block 22 itself is mounted in a radial slot 24 in the carrier so that radial movement of the roller axle and thereby of the planet roller is permitted. The bearing blocks are circumferentially stiff, that is to say they are not permitted movement in the direction circumferentially of the central spindle.

The axial loading of the ring may be provided by a constant end spring load as shown in Figures 1 to 4. Alternatively a load varying with transmitted torque may be provided. For example, as shown in Figure 5, which otherwise illustrates an embodiment similar to that of Figure 1, the annulus 5 is loaded by means of equi-spaced ball ramps 25 between the ring 5 and an annular member 26 which itself is provided with a static pre-load and axial flexibility as shown in Figure 1. At the other end of the annulus 5 are balls 27 between the annulus and an abutment ring 28 engaging the shoulder 21.

The torque reaction from the planet rollers on to the torsionally connected annuli 5a and 5b is taken by ball ramps 25. Ball thrust race 27 gives negligible torsional restraint. The axial load generated by the ball ramps compresses the planet rollers between contacts Sc and 5d, the load path being via balls 27, race 28, through casing 6 to motor casing 8 and back through washer 17.

Claims (8)

1. A roller drive assembly comprising a drive spindle, at least two planet rollers each in driving contact with said spindle and an outer annulus with which each planet roller has two distinct engagement zones, wherein for each zone a planet roller and the respective portion of said annulus are mutually convex; and the planet rollers can execute a rotation in a plane in which the axes of the roller and spindle lie, without loss of said driving contact.

2. A roller drive assembly according to claim 1 wherein the annulus has two annular portions each forming a respective one of said engagement zones with said planet rollers, the annular portions facing obliquely towards a zone of engagement of the planet rollers and said central spindle.

3. A roller drive assembly according to claim 2 wherein said annular portions are part-conical in form and are engaged by part-spherical portions of the planet rollers.

4. A roller drive assembly according to claim 3 wherein the portion of each planet roller engaging the spindle is part-spherical.

5. A roller drive assembly according to claim 3 wherein each planet roller has a generally right-cylindrical portion engaging the spindle.

6. A roller drive assembly according to claim 5 wherein each planet roller has at least two generally right-cylindrical portions engaging the spindle.

7. A roller drive assembly according to any of claims 2 to 6 wherein the said annular portions are torsionally connected and axially relatively free and means are provided for axially loading the annular portions.

8. A roller drive assembly according to any foregoing claim wherein each planet roller has a mounting which allows movement of the roller in directions axially and radially of the spindle.