GB2149053A - Epicyclic gear assembly - Google Patents

Abstract

Two separate coaxial sun gears, one (33) of which is fixed and the other (34) of which is rotatable, mesh with a planet gear (31) which provides, integrally, teeth meshing with both sun gears, in order to avoid the need to conjoin two planet gear elements. With common planet teeth the two sun gears (33,34) have the same reference circle diameter. Preferably, the teeth of the two sun gears have the same pitch and the number of teeth of the respective two sun gears is unequal. This can be achieved by applying an addendum correction factor to the teeth of one or both sun gears. <IMAGE>

Description

SPECIFICATION Epicyclic gear assembly This invention relates to epicyclic gear assemblies.

It is conventional for epicyclic gear assemblies to comprise a pair of coaxial sun gears, one of which is fixed and the other of which is rotatable, and one or more planetary clusters each comprising a pair of coaxially arranged planet gears meshing with the respective sun gears. This type of gear assembly is known as a compound epicyclic gear.

In the known epicyclic gear assemblies of this type, the or each planetary cluster consists of a compound of two different planet gears secured coaxially together in a suitable manner. In other words, two individual planet gears are joined to form one rotatable body.

The securement may be effected by bonding, keying, dowels, collets or the like. However, each planet gear must have a pitch circle which is compatable with the meshing sun gear, and must also be located in perfect radial and angular relationship with the sun gear for the assembly to operate properly.

This means therefore that the two separate planet gears which are brought together to form a planetary cluster must be secured together so that they are accurately coaxial and with the correct angular alignment, or they must be accurately cut in precise radial relationship on a single blank. This is often difficult to achieve satisfactorily. The joining together of the two planet gears to form a compound planet gear has to be carried out at the final assembly stage of the gear assembly, and this again gives rise to disadvantages.

The space available for carrying out the bonding or other securing method is limited. The assembly time is lengthy and requires skilled personnel. The joining together of two planet gears to form a compound gear is also disadvantageous for other reasons. The bond or other securement between the two gears which make up the compound planet gear is subject to high shear forces in use. Additionally, the need to combine two separate gears into one compound gear gives rise to unnecessary expense in manufacture. Moreover, if the complete gear unit has to be dismantled after the bonding together or other securement of the planet gear components, then this can give rise to problems of non-alignment when the gear unit is reassembled.

It is an object of the present invention to provide an epicyclic gear assembly which overcomes, or at least substantially reduces, the aforesaid disadvantages associated with known epicyclic gear assemblies.

In accordance with the present invention there is provided an epicyclic gear assembly comprising a fixed sun gear, and a planet gear, wherein the planet gear provides, integrally, teeth meshing with the teeth of the fixed sun gear and teeth meshing with the teeth of the rotatable sun gear.

In a preferred embodiment of the invention the planet gear teeth meshing with the two sun gears are the same in number and configuration and the two sun gears have the same reference circle diameter.

The planet gear may have a single set of teeth extending axially substantially the length of the planet gear and meshing with both sun gears.

With such an arrangement, preferably, the respective teeth of the two sun gears have the same pitch, and the number of teeth of the respective two sun gears is unequal.

In addition to overcoming the disadvantages of known gear assemblies, the provision of a one-piece planet gear with integral teeth for both sun gears means that this component of the gear assembly can be manufactured by a single machining operation, thus reducing manufacturing time and cost.

In order that the invention may be fully understood, one preferred embodiment of epicyclic gear assembly in accordance with the invention will now be described by way of example and with reference to the accompanying drawing, in which: Figure 1 is a sectional view through a conventional planetary gear cluster for use in an epicyclic gear assembly; and, Figure 2 is a sectional view through a gear box which incorporates an epicyclic gear assembly in accordance with the invention.

The conventional planetary gear cluster or planetary gear assembly shown in Fig. 1 and indicated generally at 10 comprises a pair of planet gears 11 and 1 2 which are secured together coaxially. Planet gear 1 2 is rotatably mounted on a shaft 1 3 by means of a pair of bearings 14. Planet gear 11 is mounted upon a reduced diameter portion 1 5 of planet gear 1 2. A bonding pin 1 6 is used to secure the two planet gears 11, 1 2 in the proper axial and angular relationship to one another. Additional securing means, such as an adhesive, may be used between the contacting surfaces of the two planet gears 11 and 12.

Referring now to Fig. 2 of the drawing, there is shown a gear box, indicated generally at 21, comprising a housing 22 in which is mounted a gear train, indicated generally at 23 and driven from a rotatable input shaft 24.

The final output gear 25 of the gear train 23 drives a carrier gear 26 which is secured by pins 27 to a planet cage 28. The carrier gear 26 constitutes the input drive for the cage 28.

The carrier gear-cage assembly is rotatable on bearings 29.

A planet pin 30 has its one end mounted in the planet cage 28 and its other end held by the carrier gear 26. A planet gear 31 is rotatably mounted on the pin 30 by means of a pair of spaced bearings 32. The planet gear 31, or planetary cluster, is provided with circumferential teeth which extend the full axial length of the planet gear. Meshing with these planet gear teeth is a first sun gear 33 which is mounted fixedly within the housing 22 and a second sun gear 34 which is mounted for rotation within the housing and which is positioned coaxially with respect to the first sun gear 33. The rotatable sun gear 34 is connected to an output shaft 35 of the gear box. In a gear box a number of such planetary gear clusters would be provided.

In operation, rotation of the carrier gear 26 and of the cage 28 by the gear train causes the planet gear 31 to orbit about the fixed sun gear 33 and to rotate about the planet pin 30. This motion of the planet gear 31 causes rotation of the second sun gear 34 which in turn rotates the output shaft 35.

It will be appreciated therefore that the planet gear 31, being elongate, replaces the conjoined planet gear components such as are shown in Fig. 1. The provision of an elongate planet gear 31 with just a single set of teeth meshing with both sun gears 33 and 34 means that the two sun gears 33 and 34 must have a common reference circle diameter. This is because, effectively, the planet gear 31 constitutes the merger of a pair of identical coaxially-arranged planet gears.

With an identity of the planetary tooth configuration which meshes with the two sun gears 33 and 34, the two sun gears must have identical reference circle diameters. Also, the two sun gears 33 and 34 must have a common pitch for their teeth, but have respective different numbers of teeth. This can be achieved by an addendum modification to one or both of the sun gears. The addendum of a tooth is the distance from its tip to the pitch circle. In other words, with the number of teeth on sun gear 33 being different from the number of teeth on sun gear 34, and with the two sun gears having the same tooth pitch, the reference circle diameter of each sun gear can be made to be the same by the introduction of an addendum correction factor to one or both of the two sun gears. A correction factor may also be applied to the mating planet gear 31, but this is optional.

Although in the preferred embodiment described above the single elongate planet gear 31 is provided with a single set of teeth common to both the sun gears, thus requiring the two sun gears to have the same reference circle diameter, one could alternatively achieve the object of avoiding the need to join two planet gear elements by forming two sets of teeth of respective different configuration integrally one at each end of a common planet gear element such as the gear 31. It would not then be necessary for the two sun gears to have the same reference circle diameter. Even with the provision of teeth of different configuration at the two ends of the common planet gear, the planet gear could still be manufactured as a unitary item with relatively simple machining operations, or be prelocated to achieve constant radial relationship of each planet assembly.

In yet a further alternative configuration of the one-piece planet gear, the teeth could be provided at each end only of the elongate planet gear 31, with a gap between the two sets, but with the tooth shape, number and configuration being the same for each set, whereby the same conditions would apply to the sun gears as described above with reference to Fig. 2.

Claims (7)

1. An epicyclic gear assembly comprising a fixed sun gear, a rotatable sun gear coaxial with respect to the fixed sun gear, and a planet gear, wherein the planet gear provides, integrally, teeth meshing with the teeth of the fixed sun gear and teeth meshing with the teeth of the rotatable sun gear.

2. A gear assembly as claimed in claim 1, in which the planet gear teeth meshing with the two sun gears are the same in number and configuration and the two sun gears have the same reference circle diameter.

3. A gear assembly as claimed in claim 2, in which the planet gear has a single set of teeth extending axially substantially the length of the planet gear and meshing with both sun gears.

4. A gear assembly as claimed in claim 2 or 3, in which the respective teeth of the two sun gears have the same pitch, and the number of teeth of the respective two sun gears is unequal.

5. A gear assembly as claimed in claim 4, in which the addendum of the teeth of one of said sun gears is different from the addendum of the teeth of the other sun gear.

6. A gear assembly as claimed in any preceding claim, in which the planet gear is mounted within a planet cage which is secured to and driven by a carrier gear coupled by way of a gear train to an input driving shaft.

7. An epicyclic gear assembly substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawing.