GB2025153A

GB2025153A - Thermal expansion compensating bearings in electric machines

Info

Publication number: GB2025153A
Application number: GB7922743A
Authority: GB
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1978-07-01
Filing date: 1979-06-29
Publication date: 1980-01-16
Also published as: DE2829027A1; FR2430124A1; JPS558298A

Abstract

The inner raceway (32) of one ball bearing (17) for the shaft (15) of an electrical machine, such as a three-phase generator for vehicles, is of cylindrical shape to permit relative axial displacement between the shaft end (33) and the respective bearing plate (12), e.g. to compensate for differential expansion due to temperature fluctuations. <IMAGE>

Description

SPECIFICATION Electrical machine The invention relates to electrical machines, such as three phase generators for motor vehicles.

When electrical machines, particularly threephase generators in motor vehicles, are operated under considerable fluctuations of temperature, either as a result of self-heating or heating or cooling by the ambient conditions, special attention has to be paid to mechanical deformation caused by differing degrees of expansion as a result of the fluctuations in temperature. Considerable mechanical stresses occur in the bearings of the rotor shaft particularly when the rotor shaft is made from steel and the machine housing is made from aluminium, this being due to the fact that the coefficient of expansion of aluminium is approximately twice that of steel.The problem has been solved in the case of known machines by axially and fixedly clamping one bearing in the machine housing and compensating for the differing expansions of shaft and housing, caused by temperature, by making the second bearing of axially floating construction. When ball bearings are used, the inner race of the second bearing is rigidly drawn onto the rotor shaft, and the outer race is held in the bearing bush so as to be displaceable in an axial direction. A disadvantage of such machines is that, when they are mounted as generators on internal combustion engines, the bore of the second bearing frequently becomes enlarged as the vibratory forces increase. Furthermore, radial movements of the outer race have to be prevented and it must also be prevented from turning.

These problems do not arise in the abovementioned form in the case of machines having journal bearings, but journal bearings cannot be used owing to other operating conditions, such as frequent and considerable variation of the rotational speeds, and the possibility of satisfactory lubrication etc.

An electrical machine in accordance with the invention comprises a machine housing, a stator, a rotor and a rotor shaft and first and second bearings for journalling the rotor shaft in the machine housing, the first bearing being axially and fixedly clamped to.axially locate the shaft, and the second bearing being a ball bearing one of whose raceways is cylindrical to permit relative axial displacement between the respective shaft end and the housing.

Usually the inner raceway will be the cylindrical raceway. Thus, the reception bore for the outer race cannot become enlarged, since the second bearing can be regarded as a tightly fitting or force-fitted bearing over the entire operating range of temperature. A further advantage is that noise produced by the generator is affected to a lesser extent by misalignment of the rotor shaft and the machine housing.

Finally, there can be a saving on a metalclad bush particularly in the case of generators for commercial vehicles subjected to very high vibratory stress.

It is particularly advantageous that there is no longer any need to provide means for preventing turning of the outer race of the second bearing. Furthermore, there can be a saving on the inner race of the second bearing if the surface of the rotor shaft is hardened and ground to form the inner raceway.

One embodiment of the invention is described, by way of example, with reference to the drawing which is a longitudinal section of a three-phase generator for motor vehicles, in which a bearing in accordance with the invention is fitted.

The housing of the electrical machine is formed by a first bearing plate 11 and a second bearing plate 1 2. A stator assembly 1 3 having a stator winding 14 is clamped between the bearing plates 11 and 1 2. A generator shaft 1 5 is rotatably journalled in the machine housing 11, 12, namely in the first bearing plate 11 having a first ball bearing 1 6 and in the second bearing plate 1 2 having a second ball bearing 1 7. The rotor shaft carries a claw pole arrangement comprising a core 18, pole wheels 19 and 21 and an excitation winding 22. Furthermore, slip rings 23 and 24 are mounted on the bearing shaft 1 5 and are operatively connected to a voltage regulator and brush-holder unit 25.Furthermore, a rectifier arrangement 26 is located in the machine housing 11, 1 2.

The first ball bearing 1 6 is in the form of a conventional ball bearing and axially fixes the shaft 1 5. The outer race 27 of the second bearing 1 7 is rigidly fitted in a reception bore 28 in the second bearing plate 1 2. The outer race 27 has a guide groove 29 forming an outer raceway for the balls 31 of the second bearing 1 7. On the other hand, the inner race 32 of the second bearing 1 7 has a raceway of cylindrical form. In this manner, the end of the rotor shaft 1 5 can move in an axial direction relative to the second bearing plate 1 2 to compensate for differential expansion irrespective of whether the rotor shaft 1 5 is or is not rotating.

In a further embodiment, not shown in the drawing, that end 33 of the rotor shaft 1 5 which faces the second bearing plate 12, and which is located in the region of the second bearing 17, is of cylindrical shape and this itself constitutes the inner raceway of the bearing. In this case, the surface of the rotor shaft should be hardened and also ground.

Thus, there is no need to provide a separate inner race 32.

Claims

1. An electrical machine comprising a machine housing, a stator, a rotor and a rotor shaft and first and second bearings for journalling the rotor shaft in the machine housing the first bearing being axially and fixedly clamped to axially locate the shaft and the second bearing being a ball bearing one of whose raceways is cylindrical to permit relative axial displacement between the respective shaft end and the housing.

2. An electrical machine as claimed in claim 1 in which the cylindrical raceway of the second bearing is the inner raceway.

3. A electrical machine as claimed in claim 2 in which the outer race has a guide groove which forms the outer raceway for the balls of the second bearing.

4. An electrical machine as claimed in claim 2 or 3, in which the cylindrical inner raceway of the second bearing is formed on an inner race rigidly mounted on the rotor shaft and the second bearing has an outer race rigidly mounted in the machine housing.

5. An electrical machine as claimed in claim 2 or 3, in which the cylindrical inner raceway of the second bearing is on the rotor shaft itself.

6. A electrical machine as claimed in claim 5, in which the surface of the rotor shaft which is located in the region of the second bearing and forms the inner raceway is hardened and/or ground.

7. An electrical machine constructed substantially as herein described with reference to and as illustrated in the drawing.