GB2196406A

GB2196406A - Crown wheel cam gearbox

Info

Publication number: GB2196406A
Application number: GB08723047A
Authority: GB
Inventors: Mark Trufitt
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1986-10-20
Filing date: 1987-10-01
Publication date: 1988-04-27
Also published as: GB8625046D0; GB8723047D0

Abstract

A gearbox comprises a casing 40, a first rotatable shaft 45, a second coaxial rotatable shaft 44, a crown wheel 46 mounted on the first shaft 45 for rotation about an axis angled (48) with respect to the axis of rotation of the first shaft 45 and operatively connected to the second shaft 44. The crown wheel 46 is provided with a set of circumferentially spaced cam followers 49,50 on each face. A first set of cam profiles 51 is circumferentially spaced about the axis of the gearbox and located on the inner surface of the gearbox casing 40 to engage cam followers 50 and a second set of cam profiles 52 is also circumferentially spaced about the axis of the gearbox and located around the second shaft 44 to engage cam followers 49 from the second of the sets of cam followers. <IMAGE>

Description

SPECIFICATION A reduction gearbox The invention relates to reduction gearboxes and in particular to gearboxes wherein the drive is by cam followers engaging cam surfaces.

UK Patent GB2 11 7474A describes a crown wheel cam gearbox having a crown wheel eccentrically mounted on an input drive shaft. A first set of cam profiles on one side of the cam wheel is engaged by a set of roller cam followers located in the gearbox housing and a second set of cam profiles on the other side of the crown wheel is engaged by a second set of roller cam followers located around the periphery of the output shaft of the gearbox.

The reduction gearing of this gearbox is determined by the numbers of cam profiles on the two sides of the crown wheel and the numbers of cam followers. The eccentricity of the crown wheel, determining the swash motion, is an important factor in power transfer from input shaft to output shaft vis the cam surfaces and corresponding cam followers. For optimum conditions, it was shown that the cam profiles were pointed placing constraints on the cam material strength and power transfer from the cam followers in the pointed regions.

The object of the present invention is to provide a reduction gearbox wherein power transmission takes place by the action of cam followers on cam surfaces and wherein the prior art problem concerning the cam profile is alleviated.

The invention provides a gearbox comprising: a gearbox casing; a first rotatable shaft; a second coaxial rotatable shaft; a crown wheel mounted on the first shaft for rotation about an axis angled with respect to the axis of rotation of the first shaft and operatively connected to the second shaft; the crown wheel being provided with a set of circumferentially spaced cam followers on each face of the crown wheel; a first set of cam profiles circumferentially spaced about the axis of the gearbox and located on the inner surface of the gearbox casing to engage cam followers from a first of the sets of cam followers; and a second set of cam profiles circumferentially spaced about the axis of the gearbox and located around the second shaft to engage cam followers from the second of the sets of cam followers.

The invention will now be described by way of example only with reference to the accompanying Drawings of which: Figure 1 shows a perspective view of a prior art crown wheel cam gearbox; partly cut away; Figure 2A-2F respectively show the effect on the cam profile of increasing the throw angle of the crown wheel; Figure 3 is a cross sectional view through a reduction gearbox according to the invention; Figure 4 illustrates the cam profile of the Figure 3 gearbox; and Figure 5 show a modified form of the Figure 3 gearbox.

Figure 1 shows a known reduction gearbox described in UK patent number GB21 17474A connecting an input drive shaft 2 to an output shaft 3. A crown wheel 4 is mounted on bearings on the input shaft 2 such that its axis 7 is at an angle to the rotational axis if the input shaft 2. Saw tooth cam profiles 5 and 6 are provided respectively on the input and output sides of the crown wheel 4. The cam profiles 5 are engaged by a set of tapered input rollers 13 located in the casing 18 of the gearbox and the cam profiles 6 are engaged by an output set of tapered rollers 14 located at an end 24 of the output shaft 3. As the input shaft 2 is rotated the cam wheel rotates relative to the input shaft and executes a swash plate motion.Movement of the cam wheel through the interaction of cam profiles and rollers causes the output shaft 3 to rotate with a reduction gearing dependent on the numbers of rollers 13,14 and cam profiles 5,6.

The throw angle T is the dominant influence on the amplitude or height of the cam profile: the greater the throw angle the greater the height.

Once the number of cam profiles, and hence the cam's base length, has been established, the throw angle must be suitably chosen.

Figures 2A-2F show the effect of increasing the throw angle of the crown wheel 4 on the cam profile, for a cam of fixed base length L.

Theincrease of throw angle is represented by the increase of cam height a. In each case the locus 31 of a roller 32 is shown.

With the smaller throw angles as shown in Figures 2A and 2B the resulting shallow cam flank angle 33 causes the loads on the roller bearing to be large. By increasing the throw angle (ie the height a), the cam flank becomes steeper and, for the same power transmitted through the gearbox, the roller bearing loads are reduced. With further increase of the throw angle, the upper crest 34 of the cam profile becomes pointed and the roller begins to lose contact with the cam profile in the region of the upper crest 34 as shown by the portion 35 of the locus 31-of the roller centre in Figures 2E and 2F. The effect of losing cam contact is to reduce the power capacity of the gear box. In addition, the more pointed the cam profile is made the structurally weaker it becomes and the maximum loading must therefore be reduced.An optimum range of throw angle is thought to be as illustrated in Figures 2D and 2E. This gives a cam flank angle 33 of about 45".

Figure 3 shows a reduction gearbox according to the present invention. A gearbox casing 40 has an end cap 41 with bearings 42 and 43 provided respectively for an output shaft 44 and a coaxial input shaft 45. A further bearing 42A is arranged between the end portion and the input shaft. A crown wheel 46 is eccentrically mounted on the input shaft 45 on a rotational bearing 47 set at a throw angle 48. Circumferentially spaced around the respective faces of the crown wheel 46 are two sets of tapered roller cam followers 49 and 50. The input rollers 50 on the input shaft side of the crown wheel 46 engage a set of cam profiles provided circumferentially around the inner end 51 of the gearbox casing end cap 41.The output rollers 49, on the side of the crown wheel adjacent to the output shaft, engage a similar set of cam profiles provided circumferentially around an end drive face extention 52 to the output shaft 44.

The- arrangement is similar to that shown in GB2117474A. Here rotation of the input shaft imparts a relative rotation and a swashplate motion of thecrown wheel because of the interaction of the input rollers 50 and the cam profiled end 51. The movement of the crown wheel imparts a rotation to the output shaft because of the interaction of the output rollers 49 and the cam profiled face 52. This arrangement has an important advantage over the prior art arrangement described above.

The cam profiles are effectively complementary to those shown in Figure 2 and thus there are no peaks and consequently the cam surfaces are inherently stronger.

Figure 4 shows an approximate profile of the cam profiled surfaces 51 and 52 for a small throw angle 48.

An alternative gearbox arrangement is shown in Figure 5 where like reference numerals are used to indicate like integers. Here the tapered rollers are colinearly arranged with the input rollers 50 radially within the output rollers 49. By this means theweight and lateral extent of the crown wheel can be reduced.

Further details not given herein and modification of the invention will be -apparent to those skilled in the art.

Claims

1. A gearbox comprising: a gearbox casing; a first rotatable shaft; a second coaxial rotatable shaft; a crown wheel mounted on the first shaft for rotation about an axis angled with respect to the axis of rotation of the first shaft and operatively connected to the second shaft; the crown wheel being provided with a set of circumferentially spaced cam followers on each face of the crown wheel; a first set of cam profiles circumferentially spaced about the axis of the gearbox and located on the inner surface of the gearbox casing to engage cam followers from a first of the sets of cam followers; and a second set of cam profiles circumferentially spaced about the axis of the gearbox and located around the second shaft to engage cam followers from the second of the sets of cam followers.

2. A gearbox as claimed in claim 1 wherein the cam followers are rollers.

3. A gearbox as claimed in claim 2 wherein the rollers are tapered such that the extrapolations of their conical surfaces intersect at a point on the axis of the gearbox.

4. A gearbox as claimed in any one preceding claim wherein the crown wheel is provided with annular grooves in the opposed faces thereof in which arelocated the respective first and second sets of cam followers.

5. A gearbox as claimed in any one of claims 2 to 4 wherein the axes of rotation of the first and second sets of cam followers lie in a common plane.

6. A gearbox substantially as described and with reference to Figures 3 and 4 of the Drawings.

7. A gearbox substantially as described and with reference to Figures 4 and 5 of the Drawings.