GB2060123A

GB2060123A - Motion Transmitting Apparatus

Info

Publication number: GB2060123A
Application number: GB8026342A
Authority: GB
Original assignee: USM Corp
Current assignee: USM Corp
Priority date: 1979-10-10
Filing date: 1980-08-13
Publication date: 1981-04-29
Also published as: DK426780A; IT8025209A0; SE8006972L; IL61230A; AU538778B2; GB2060123B; FR2467326B1; IL61230A0; AU6311680A; JPS5655742A; CH649139A5; FR2467326A1; BE885610A; DE3037758A1; IT1133833B; CA1126540A; NL8005555A

Abstract

In motion transmitting apparatus comprising a ring gear 42, a tubular flexible gear 43 and wave generating means 35 operable to progressively force the flexible gear 43 into engagement with the ring gear 42, the tubular flexible gear comprises a diaphragm 51 attached to a shaft 45 and in order to reduce axial stress when the apparatus is assembled, which may produce residual stresses causing failure of the diaphragm, the thickness of the diaphragm 51 is between 1 and 2 per cent of the bore diameter of the tubular portion of the flexible gear 43. <IMAGE>

Description

SPECIFICATION Motion Transmitting Apparatus This invention is concerned with motion transmitting apparatus and in particular with such apparatus in which relative motion occurs between elliptical wave generating means, a tubular flexible gear (flexspline), and a fixed ring gear. The motion occurs by inserting the wave generating means into the flexible gear and rotating it to progressively force the flexible gear into engagement with the ring gear at one or more points. In such apparatuses, the tubular flexible gear has a tubular portion with external teeth adjacent one end thereof arranged to mesh with teeth of the ring gear and a diaphragm adjacent the other end thereof.The diaphragm serves two functions; it attenuates the axial excursion of the tubular portion by restricting it to a circular shape at the diaphragm and provides means whereby the flexible gear can be attached to an output shaft of the apparatus.

Up to the present, tubular flexible gears of such apparatus have been designed to cope only with operating stresses. Such operating stresses include the radial deflection stress in the tubular portion and the axial deflection stress in the diaphragm. The axial deflection stress is caused by the diaphragm end of the tubular portion not remaining in the same plane but instead scalloping. Along the major axis of the elliptical wave generating means, the scalloping is towards the diaphragm while along the minor axis the scalloping is away from the diaphragm. As the wave generating means rotates, the scalloping pattern also rotates with the maximum excursion being between the major and minor axes. To minimise the axial deflection stress in the diaphragm, its thickness has been kept to the minimum consistent with being thick enough to transmit the rnaximum torque required.Typically, the thickness of the diaphragm has been 0.8 per cent of the internal bore diameter of the tubular portion.

It has been found that, in apparatuses used up to the present tubular flexible gears sometimes fail at the diaphragm end during operation particularly at the edge of the portion of diaphragm which is attached to the output shaft.

Because these failures occurred during operation, it was believed that the axial deflection stress caused the failures and that this could be avoided by reducing the thickness of the diaphragm.

However, we have found that the assembler of the apparatus frequently inadvertently applies excessive axial force to fit the wave generating means into the tubular portion of the flexible gear.

Since this axial force is transmitted to the diaphragm, the diaphragm is caused to "dish" and thus residual stresses may be retained in the diaphram which can cause it to fail during subsequent operation.

It is an object of the present invention to provide an improved motion transmitting apparatus having a tubular flexible gear in which the possibility of residual stresses occurring in its diaphram are reduced.

The invention provides motion transmitting apparatus comprising a ring gear, a tubular flexible gear comprising a diaphragm adjacent one end thereof, the diaphragm being attached to a shaft, and wave generating means operable to progressively force the flexible gear into engagement with the ring gear, the thickness of the diaphram being between 1 and 2 per cent of the bore diameter of the tubular flexible gear.

There now follows a detailed decription, to be read with reference to the accompanying drawings, of a motion transmitting apparatus which is illustrative of the invention. It is to be understood that the illustrative apparaus has been selected for description by way of example and not of limitation of the invention.

In the drawings: Figure 1 is a side elevational view, partly in cross-section, of the illustrative motion transmitting apparatus; Figure 2 is a cross-sectional view of the flexible gear of the illustrative apparatus showing it in an unstrained condition and with two deflections indicated by broken lines; Figure 3 is a cross-sectional view of the wave generator and flexible gear of the illustrative apparatus being fitted together; Figure 3A is a fragmentary view in crosssection of an alternative flexible gear; and Figure 4 is a graph showing curves for deflection stress and axial force stresses involved as the wave generator and flexible gear are fitted together.

The illustrative motion transmitting apparatus shown in Figure 1 comprises an input shaft 30 journalled at 31 in a housing portion 32. The housing portion 32 is secured by bolts 55 to a housing 54 after the apparatus has been assembled. The apparatus comprises wave generating means in the form of a generator 35 locked on the shaft 30 by a nut 33 received on a threaded portion 34. The generator 35 is elliptical in shape and has a flexible inner race 36 pressed on to it so that the inner race assumes the elliptical shape of the generator. The inner race 36 receives ball bearings 37 of uniform diameter which engage within an outer race 38 which is deflectable and is fitted on the inside of a flexible tubular portion 44 of a flexible tubular gear 43 of the apparatus. The ball bearings 37 are held in position by a ring 39 fitted against a small flange 40 of the inner race 36.

The tubular flexible gear 43 of the apparatus (also known as a flexspline gear) has exterior teeth 41 radially disposed about the periphery thereof. These teeth 41 are arranged to mesh with the interior teeth 42 of a ring gear of the apparatus which are disposed on an interior face of the housing 54. The number of teeth 41 differs from the number of teeth 42 by two or a multiple thereof so that operation of the generator 35 causes relative rotation. The gear 43 comprises the aforementioned flexible tubular portion 44 and a diaphragm 51 adjacent one end thereof.

The diaphragm 51 is attached to an output shaft 45 of the apparatus. The output shaft 45 is journalled in bearings 46 in a portion 47 of the housing 54. The interior end portion of the shaft 45 is attached to the diaphragm 51 by means of two collars 52a and 52b that are fixed to the shaft 45, engage the opposite sides of the diaphragm 51, and are bolted together by bolts 53. The peripheries of the collars 52a and 52b define an inflexible portion of the diaphragm 51 disposed about and attached to the shaft 45. The remainder of the diaphragm 51 forms a flexible portion disposed about the inflexible portion.

In the operation of the illustrative apparatus, as the input shaft 30 turns, the generator 35 is turned through the bearings 37 and the race 38.

As the generator 35 turns, the gear 44 is deflected into engagement with the ring gear.

This engagement takes place at two spaced points since the generator 35 has two lobes. In a modification of the illustrative apparatus, the generator may have three or more lobes in which case the engagement takes place at three or more points. At these points, the teeth 41 and 42 mesh and thus the motion of the input shaft 30 causes relative rotation of the output shaft 45 through the flexible gear 43.

When the tubular portion 44 is fitted around the outer race 38, with the elliptical shape imposed on it by the generator 35, a certain amount of flexing occurs in the flexible portion of the diaphragm 51. The space into which the flexible gear 43 must fit is only slightly larger than the gear 43 itself and, since the generator 35 has an elliptical shape, the tubular portion 44 is required to adopt the same elliptical shape. There is maximum radial deflection at the open end of the tubular portion 44 and this radial deflection gradually reduces along the length of the portion 44 until it has practically disappeared at the diaphragm 51. Because of the reduction in the deflection within the tubular portion 44, a scalloping condition occurs which is transmitted to the diaphragm 51 causing it to flex axially and produce an axial stress.

Figure 2 shows in exaggerated form the axial scalloping deflection X which occurs in the gear 43. The gear 43 is shown in full at a point A midway between the major and minor axes of the elliptical shape (at point A the gear is not deflected). In broken line, the gear 43 is shown at a point B on the major axis and at a point C on the minor axis. The gear 43 is deflected upwardly at point B and axially displaced towards the diaphragm 51. At point C, the deflection is downwards and the axial displacement is away from the diaphragm 51. As shown in Figure 2, the axial deflection X at the open end of the tubular portion 44 is equal to the axial deflection X at the diaphragm 51.

When the generator 35 is fitted into the gear 43 and turned, the deflection of the gear 43 also turns and thus the deflection wave in the diaphragm 51 turns as well. In previous motion transmitting apparatuses, the axial deflection stress in the diaphragm 51 was used to determine the diaphragm thickness and, in order to minimise that stress, the thickness of the diaphragm was held to a minimum still thick enough to deliver the required output torque. It has been found that the maximum stress which may occur is not necessarily associated with the operating stress in the apparatus but instead may be caused by high axial stress caused while the flexible gear and the generator are assembled.

Figure 3 shows the generator 35 being fitted into the flexible gear 43. The arrow Fl represents the force used to insert the generator 35 and this force is transmitted through the gear 43 to the flexible portion of the diaphragm 51.The arrow F2 represents an opposing reaction force at the inflexible portion of the diaphragm 51 which is transmitted through the diaphragm 51 causing it to "dish" producing a peak deflection stress at point 10.

The problem of high deflection stress becomes even more critical during assembly if the generator is slightly askew or if thermal gradients exist across parts which are an interference fit or if there is dirt in the tubular portion 44 of the gear 43 or on the outer race 38. In some cases, the axial force applied during assembly produces excessive stresses in the diaphragm 51 at point 10 thereby producing a high residual stress in the diaphragm. Subsequently, the cyclic stresses applied in use can combine with the residual stress to cause the diaphragm to fail at point 10.

Figure 3A shows a modification of the illustrative apparatus in which the collars 52a and 52b are replaced by a thickened portion 56 which provides an inflexible hub but the problem of stress during assembly is still present.

Figure 4 shows, in the form of a graph, for various diaphragm thicknesses the relationship between deflection stress Y due to scalloping and the axial force stress Z caused by axial assembly forces. In the apparatuses to which the graph relates, the ratio between the input and the output is eighty to one and the proportions of flexible gear length and diaphragm clamping diameter are as known in the prior art. As can be seen from the graph, a very thin diaphragm produces a low deflection stress Y and prior art apparatuses have generally had diaphragms having thicknesses which are approximately 0.8 per cent of the bore diameter of the flexible gear 43. While a thin diaphragm produces a low deflection stress, we have found that an extremely high axial force stress Z may be produced in assembly. The axial force stress Z is approximately 90,000 pounds per square inch (6.21 x103 kilo newtons/sq.metre) or more. When the diaphragm thickness is approximately 0.8 per cent of the bore diameter of the flexible gear 43, the deflection stress Y is approximately 12,500 pounds per square inch (0.86x103 kilo newtons/sq. metre). The axial force used to compute these stress values is given by the formula Force=(8)D2 where D is the bore diameter of the flexible gear; this force is the upper limit of the force used to seat the generator in the gear 43.

In the illustrative apparatus, the thickness of the diaphragm 51 is increased to between 1 and 2 per cent of the bore diameter of the tubular flexible gear 43. This reduces the axial force stress dramatically to between 15,000 and 55,000 pounds per square inch (1.73 to 3.79x103 kilo newtons/sq. metre) and the deflection stress is only increased to between 15,000 and 30,000 pounds per square inch (1.73 to 2.07 x 103 kilo newtons/sq. metre).

With the diaphragm proportions of the illustrative apparatus, the deflection stress is slightly higher than in the prior art but this will still be below the fatigue endurance limit of the material. We have found that the deflection stress varies in proportion to the diaphragm thickness, but the axial force stress varies inversely as the square of the thickness and thus, a small increase in the axial diaphragm thickness increases the deflection stress by only a small amount but significantly lowers the axial force stress.

Claims

1. Motion transmitting apparatus comprising a ring gear, a tubular flexible gear comprising a diaphragm adjacent one end thereof, the diaphragm being attached to a shaft, and wave generating means operable to progressively force the flexible gear into engagement with the ring gear, the thickness of the diaphragm being between 1 and 2 per cent of the bore diameter of the tubular flexible gear.

2. Apparatus according to claim 1 wherein the diaphragm comprises an inflexible portion disposed about and attached to the shaft and a flexible portion disposed about the inflexible portion.

3. Apparatus according to claim 2 wherein the inflexible portion of the diaphragm comprises a collar fixed to the shaft.

4. Apparatus according to any one of claims 1, 2 and 3 wherein the wave generating means is elliptical.

5. Motion transmitting apparatus subsantially as hereinbefore described with reference to and as shown in the accompanying drawings.

6. A gear suitable for use in apparatus according to any one of claims 1, 2, 3, 4 and 5 comprising a flexible tubular portion with teeth radially disposed about the periphery thereof and a diaphragm portion disposed adjacent one end of the tubular portion, the thickness of the diaphragm portion being between 1 and 2 per cent of the bore diameter of the tubular portion.

7. A gear according to claim 6 wherein the diaphragm portion comprises an inflexible axial portion and a flexible portion disposed between the inflexible axial portion and the tubular portion.