GB2175668A - Gear box - Google Patents

Abstract

A gear box which provides a high velocity ratio comprising first and second coaxial toothed members (4,6) which have different radii, one member (4) of which is fixed and one member (6) of which is rotatable, third and fourth toothed members (3,5) which mesh respectively with the first and second toothed members (4,6) and which rotate in unison about an axis (9) which is offset to the axis (12) of the first and second members (4,6). There are input drive means (1) arranged to drive the third and fourth members (3,5) so that they execute an epicyclic motion with respect to the first and second members (4,6) and output drive means (8) drivable by the rotatable one (6) of the first and second members (4,6). <IMAGE>

Description

SPECIFICATION Gear box This invention relates to a gear box, and has particular application to a gear box having a high velocity ratio.

Gear boxes having a high velocity ratio are required in a number of applications. For example, in powering a helicopter it is conventional to use a turbine having a high rotational velocity which must be coupled via a gear box to the rotor which rotates at a relatively low speed. A similar problem arises in powering various types of marine vessel, in which a high velocity turbine is used as the source of power, and this must be coupled via a gear box to a propeller rotating at low speed. One conventional way of providing the gear box needed in such applications is to stack a number of epicyclic gear boxes one on another, but the result, as one would expect, is a gear box which is bulky, heavy, complicated and expensive.

According to the present invention there is provided a gear box comprising first and second coaxial toothed members which have differing radii from one another, one of which members is fixed and one of which is rotatable, third and fourth toothed members which mesh respectively with the first and second toothed members and are rotatable in unison about an axis offset with respect to the axis of the first and second members, input drive means arranged to drive the third and fourth members so that they execute an epicyclic motion with respect to the first and second members, and output drive means drivable by the rotatable one of the first and second members.

Preferably the difference between the radii of the first and second members is relatively small compared to the radius which each of those members has. This provides for a large velocity ratio.

The accompanying drawing shows diagrammatically one form of gear box according to the present invention. This comprises an input shaft 1 which drives a sun gear 2 of the epicyclic gear system. Meshing with the sun gear 2 are a plurality of planetary gears 3. For simplicity of illustration two such gears are shown, though in practice three or more such gears might be provided, or even only a single such gear. The planetary gears 3 also mesh with the inside of a fixed annular gear ring 4. Fixed to each of the planetary gears 3, for rotation in unison therewith, is a further planetary gear 5. The gears 3 should be stronger than the gears 5 as they carry a greater load. The gears 5 mesh with the inside of a second annular ring 6 which is arranged to be rotatable with respect to the ring 4. A bearing 7 is provided to permit this relative rotation.

An input shaft 8 is connected to the ring 6 for rotation thereby. Each of the planetary gears 5 has a stub shaft 9 which is received in a plate 10 common to all the stub shafts. kelative rotation between the stub shaft 9 and the plate 10 is permitted by bearings 11. The plate 10 3150 has a central aperture which receives a stub shaft 12 connected to the sun gear 2, and relative rotation between the stub shafts 12 and the plate 10 is permitted by a bearing 13.

In use, drive provided by the input shaft 1 causes the sun gear 2 to rotate and this in turn causes the planetary gears 3 and 5 to execute an epicyclic movement while meshing with their respective ring gears 4 and 6. Because the ring gears 4 and 6 having differing radii this causes the ring gear 6 to rotate, the ring gear 4 being fixed. Because the difference in the radii is small compared to the radius which each ring gear possesses the rotational speed of the ring gear 6 is small compared to the rotational speed at which the gears 3 and 5 rotate about the axis of the input shaft.The fact that drive to the gears 3 and 5 is provided by a sun gear 2, rather than having these gears driven directly, means that the ratio of the rotational speed tJOUT of the ring gear 6 (and hence of the output shaft 8) to the rotational speed sIN of the input shaft 1 is even lower. These rotational speeds can be expressed by the following relationship:

where r2=radius of gear 2 r3=radius of gear 3 radius of gear 5 If the additional gear reduction provided by the sun gear 2 is not required the gears 3 can be driven directly by an input shaft, and this possibility is indicated diagrammatically by broken lines in the drawing.

In this case the relationship between the rotational speed "'IN of the input shaft to the rotational speed OUT is:

wherein r3 and r5 have the same meanings as before and r2 is the radius of the gap between the gears 3, where the gear 2 was in the previous arrangement.

In the illustrated embodiment in which the ring gear 6 has a greater radius than the ring gear 4, the shaft 8 rotates in the opposite sense to the shaft 1. If the position were reversed, and the ring gear 4 had a greater radius than the ring gear 6, the shafts 1 and 8 would rotate in the same direction.

Various modifications can be made to the embodiment described above. For example, it may be appropriate for the connection between each gear 3 and its respective gear 5 to be made not wholly rigid, so as to allow for some flexing between the two gears. This may be desirable to allow for machining tolerances in the manufacture of the gear box. Also the plate 10 and the stub shafts 9 may be eliminated if instead a free running gear is mounted between the shaft 12 and the gears 5.

Another possible modification is to have the ring gear 4 rotatable and the ring gear 6 fixed, with the output shaft fixed to the ring gear 4. In the construction of this type the input and output shafts are concentric and enter the gear box from the same side.

Also, it would be possible to operate the gear box in such a way as to interchange the functions of the input and output shafts, thus creating, for example, a gear box in which a high velocity increase is achieved. However, there are disadvantages in this, notatably the high friction which results.

Claims (6)

1. A gear box comprising first and second coaxial toothed members which have differing radii from one another, one of which members is fixed and one of which is rotatable, third and fourth toothed members which mesh respectively with the first and second toothed members and are rotatable in unison about an axis offset with respect to the axis of the first and second members, input drive means arranged to drive the third and fourth members so that they execute an epicyclic motion with respect to the first and second members, and output drive means drivable by the rotatable one of the first and second members.

2. A gear box as claimed in claim 1 wherein of the first and second coaxial members the fixed coaxial member is of smaller radius than the rotatable coaxial member.

3. A gear box as claimed in claim 1 wherein of the first and second coaxial members the fixed coaxial member is of larger radius than the rotatable coaxial member.

4. A gear box as claimed in any of claims 1, 2 or 3, wherein the difference between the radii of first and second coaxial members is small in comparison to the radius which each of those members has.

5. A gear box as claimed in any preceding claim wherein the connection between third and fourth toothed members is not wholly rigid.

6. A gear box substantially as herein described and as illustrated in the accompanying drawing.