GB2105817A - Gear transmission - Google Patents

Abstract

In a two-stage gear transmission, for improving load distribution, a plurality of countershafts (32) are disposed parallel to one another, each of them comprising a countershaft gear (34) meshing with a central pinion (16) and a countershaft pinion (36) meshing with a central gear-wheel (20). The toothings have helix angles ( beta 1, beta 2) at which in respect of each countershaft (30) the moments (MR and MA) of the radial forces (FR1) and (FR2) acting on their toothings, on the one hand, and of the axial forces (FA1) and (FA2), on the other hand, substantially cancel one another. Each of the countershafts (30) has an axial bore (32) for a bearing member (28) through which they are mounted on an axle member (26). <IMAGE>

Description

SPECIFICATION Gear transmission The invention relates to gear transmissions with two or more stages.

Such transmissions are known in which, between input and output gear-wheels formed by a central pinion and a central internally toothed gear-wheel, both rotatable on a common central axis, there is at least one countershaft carrying a countershaft gearwheel meshing with the central pinon and a countershaft pinion meshing with the internally toothed gear-wheel. With the drive input to the central pinion this, together with the countershaft gear-wheel of the or each counter-shaft, thus forms a first stage of the transmission and the countershaft pinion(s) together with the internally toothed gear-wheel forms a second stage. The construction of the output gear-wheel with internal toothing permits a second stage with a transmission ratio which is considerably higher than in similar transmissions which differ in that the output gear-wheel has external toothing.

The construction of the output gear-wheel as an internally toothed gear-wheel nevertheless inevitably restricts the space availabie for bearings of the countershafts in the region of the countershaft pinions. Precisely in this region, however, bearings of ample dimensions are particularly important, since the largest peripheral forces, and accordingly also the largest radial forces, occur on the teeth of the countershaft gear-wheels meshing therewith.

The radial forces on the teeth of the countershaft gear-wheels produces a moment which tends to tilt the countershaft about an axis which extends normal to the common plane of the axes of the countershaft and of the central gear-wheels. In the bearings of the or each countershaft this moment gives rise to loads which are additional to the loads from the peripheral forces. In order to take the resulting bearing loads, in known gear transmission of this general type, bearings are provided axially on each side of each countershaft, and these require considerable space with the consequence that the highest possible transmission ratio achievable for a given size of transmission casing is not as great as might be desired.

The object underlying the invention is therefore that of making it possible to avoid or at least reduce the restrictions in respect of the space required for the bearings of such countershafts.

According to the invention, there is provided a gear transmission with a plurality of stages and comprising at least one countershaft carrying a larger diameter gear-wheel that meshes with a smaller diameter externally toothed gear-wheel mounted on a central axis and a smaller diameter gear-wheel that meshes with an internally toothed gear-wheel mounted on said central axis, the or each said countershaft being provided with an axial bore by which it is mounted on bearing means, and the toothings of said gear-wheels having helix anlges at which, in operation, the moments of the radial forces acting on the teeth of the gear-wheels of the or each countershaft counteract at least a substantial part of the moments of the axial foreces so acting.

The bearing means disposed inside each countershaft itself has to transmit substantially only the resultant of the peripheral forces acting on the teeth of the countershaft gear-wheels. The member which takes this resultant does not itself rotate and can therefore be supported in a space-saving manner on each side of the countershaft, the principal requirement being that the support in the region of the internally toothed central gear-wheel should take up so little space that sufficient space is leftforthe construction of this central gear-wheel itself. The substantial, if not complete mutual cancellation of the abovementioned moments in respect of an axis normal to the axes of the central gear-wheel and of the countershaft has the consequence that the or each countershaft has only a slight tendency, if any tendency at all, to tilt in relation to the associated bearing means.Edge pressures on the bearing means, even if it is formed by a member with only a short axial length, can therefore be kept low or are practically eliminated.

In a preferred embodiment of the invention the centre point of the or each bearing means coincides with the point of intersection of the axis of the associated countershaft and the line connecting the pitch points of the countershaft gear-wheels, at least approximately. This ensures that any tendency of the countershaftto tilt about an axis normal to the axes of the countershaft and of the central gearwheels is likewise avoided or at least reduced to a practically negligible level, since the resultant of the peripheral forces acting on each countershaft extends through the point of intersection mentioned.

The balancing of forces and moments is further improved if each countershaft is tiltable to a limited extent on the respective bearing means, about the centre point of the latter. In this case the or each bearing means can form for the associated countershaft a self-aligning bearing which also takes axial forces.

If it should not be possible to achieve an at least approximate balancing of axial forces having regard to the desired substantially complete balancing of moments, a thrust bearing separate from the respective bearing means can be provided for the or each counter-shaft. This thrust bearing can also take slight moments which have not yet been cancelled out.

The helix angles of the toothings of the countershaft gear-wheels may be different; as a rule where the gear-wheels have different diameters it is expedient for the toothing of the larger gear-wheel to have a large helix angle that that of the smaller gear-wheel.

An illustrative embodiment of the invention is explained in greater detail below with the aid of the accompanying diagrammatic drawings, in which; Figure 1 shows an axial section through a twostage gear transmission with load distribution through three countershafts, Figure 2 shows a simplified front view in the direction of arrow II in Figure 1, showing the toothings of a countershaft and the toothings meshing therewith only in the form of respective pitch circles, Figure 3 shows the corresponding plan view in the direction of arrow III in Figure 2, and Figure 4 shows the toothed components of the gear transmission in an oblique view, partly cut away.

The geartransmission shown has a casing 10 in which are mounted a drive shaft 12 and an output shaft 14, with a common central geometrical axis A.

On the drive shaft 12 a helically toothed central pinion 16 is formed; on the output shaft 14 is fastened a dish-shaped resilient hub 18, on which is formed an internal helically toothed central gearwheel 20.

Three axle members 26, on each of which a bearing member 28 is formed, are secured to an outer wall 22 and an intermediate wall 24 of the casing 10 and are uniformly spaced at 120 intervals as shown in Figure 4. On each of the three bearing members 28 is mounted a countershaft 30, which for this purpose has an axial bore 32. The geometric axes of the axle members 26, which coincide at least approximately with the axes B of the countershafts 30, extend with the same spacing parallel to the common geometric axis A of the drive shaft 12 and output shaft 14.

On each countershaft 30 there are formed a countershaft gear-wheel 34, which meshes with the central pinion 16 and is thus likewise helically toothed and a countershaft pinion 36 which meshes with the central gear-wheel 20 and is therefore likewise helically toothed. The central pinion 16 and the countershaft gear-wheel 34 form a first gear stage, and the countershaft pinion 36 and central gear-wheel 20 on the other hand form a second gear stage. The centre point C of each bearing member 28 lies on the one hand on the axis B of the associated countershaft 30 and on the other hand on the connecting line D between the pitch points E1 and E2 of the two gear stages.

When mention is made here of the centre point C of the bearing member 28, this does not mean that it is necessary to have a one-element bearing means for each countershaft 30. Each bearing may consist of two elements, axially spaced apart, formed on the associated axle member 26, or the bore 32 in the countershaft 30 may comprise two elements axially spaced apart, for example in the form of blind holes, in which the countershaft 30 is mounted directly or indirectly on the bearing. However, for preference each bearing 28 is a single member 28 which is slightly cambered, so that the countershaft 30 mounted on it can make slight tilting movements about the centre point C. If the bearing member 28 is in the form of a self-aligning bearing, it can additionally take axial forces.The same effect can also be achieved by a resilient intermediate mounting between each bearing member 28 and the associated countershaft 30. In all cases C is the centre point of the entire bearing system formed inside the countershaft 30.

In the direction of rotation of the central pinion 16, the countershafts 30, and the central gear-wheel 20, which direction is indicated by the arrows G, H and J in Figures 1,2 and 4, a force FN1 directed normal to the intermeshing tooth flanks is transmitted at the pitch point E1 of each gear stage formed by the central pinion 16 and a countershaft gear-wheel 34.

This normal force FN1 can be split up in accordance with a rectangular spatial co-ordinate system into a peripheral force Ful a radial force FR1 and an axial force FAl. These forces are shown in Figures 1 to 3 in the manner in which they act on the countershaft 30.

The corresponding forces occurring at the pitch point E2 are likewise shown, namely the normal force FN2 and its components, the peripheral force FU2, the radial force FR2 and the axial force FA2.

The radial forces FR1 and FR2 produce, in respect of an axis which extends at right angles to the plane of the two axes A and B through the centre point C of the associated bearing member 28, a moment MR which, in the case of the senses of rotation indicated, is directed counterclockwise. At the same time, however, the axial forces FAl and FA2 occurring at the two pitch points E1 and E2 produce a moment MA in respect of the same axis extending through the centre point C, and this moment MA is oppositely directed to the moment MR. For preference the helix angle 1 and 2 of the intermeshing toothings are so selected that the moment MA is equal in magnitude to the moment MR, SO that the two moments cancel one another. As a rule it is for this purpose necessary to select the helix angle ss1 larger than ss2. if in this case a resultant axial force FA2 - FA1 remains on each of the individual countershafts30, and if the bearing member 28 is not in the form of a self-aligning bearing, thrust bearings 38 and 40 will be formed on the gear transmission casing 10, and on these bearings either the countershaft gear-wheels 34 or the countershaft pinions 36, depending on the direction of rotation of the drive shaft 12, will be supported.

Claims (8)

1. A gear transmission with a plurality of stages and comprising at least one countershaft carrying a larger diameter gear-wheel that meshes with a smaller diameter externally toothed gear-wheel mounted on a central axis and a smaller diameter gear-wheel that meshes with an internally toothed gear-wheel mounted on said central axis, the or each said countershaft being provided with an axial bore by which it is mounted on bearing means, and the toothings of said gear-wheels having helix angles at which, in operation, the moments of the radial forces acting on the teeth of the gear-wheels of the or each countershaft counteract at least a substantail part of the moments of the axial forces so acting.

2. A gear transmission according to claim 1, wherein the centre point of the bearing means for the or each said countershaft is at or near a point of intersection of the axis of the associated countershaft with a line connecting the pitch points of the countershaft gear-wheels.

3. A gear transmission according to claim 2, wherein the or each countershaft is tiltable to a limited extent on the respective bearing means about the central region of the bearing means.

4. A gear transmission according to claim 3, wherein the or each bearing means forms a self aligning bearing for its associated countershaft.

5. A transmission according to any one of the preceding claims wherein, discrete from said bearing means, at least one thrust bearing is provided for the or each countershaft.

6. Ageartransmission according to any one of the preceding claims, wherein the helix angles of the toothings of the countershaft gear-wheels are different from each other.

7. A gear transmission according to claim 6 wherein the toothing of the larger diameter gearwheel has a larger helix angle than that of the smaller diameter gear-wheel.

8. A gear transmission constructed and arranged for use and operation substantially as described herein with reference to the accompanying drawings.