GB2121120A - Rolling bearings - Google Patents

Abstract

The invention relates to rolling bearings, but more specifically to a tapered roller bearing of the type used in applications where, in service, the inner and outer race members rotate together without relative movement for much of the time. Fretting corrosion known as "false brinelling" which can occur in these circumstances is prevented by modifying the surface coefficient of friction of one or more of the inner and outer raceways and the rollers in such a way as to ensure that creep occurs while in service. The surface treatment takes the form of a phosphate coating applied by a chemical treatment process to produce a layer of manganese phosphate or zinc phosphate on the surface of the component concerned. Advantageously a small amount of misalignment between the inner and outer race members may be deliberately introduced to enhance the effect of the surface treatment. <IMAGE>

Description

SPECIFICATION Improvements relating to rolling bearings This invention relates to rolling bearings for use in applications in which the inner and outer races of the bearing are required to rotate in the same direction at the same speed for at least part of the time in operation. The invention is particularly, although not exclusively, concerned with tapered roller bearings and the remaining discussion assumes this type of bearing.

A typical example of the above-described condition is found in the pocket bearing of an automotive gearbox. Such a bearing is shown in section in Figure 1 of the accompanying drawings.

The bearing comprises coaxial input and output shafts 1, 2 respectively and layshaft 3. The pocket bearing in this case lies between the coaxial input and output shafts and is identified by the reference 4. When the gearbox operates in direct drive the shafts 1, 2 rotate together and the bearing 4 rotates as an assembly with them.

Under such operating conditions bearings sometimes suffer a form of damage known as "false brinelling" which is characterised by local wear at the lines of contact formed between the rolling elements and the raceways. The wear originates from fretting corrosion caused by minute relative vibratory motions.

The present invention seeks to provide a tapered roller bearing which will survive the operating conditions described without sustaining false brinelling damage.

Experimental and theoretical studies have furnished evidence that when the bearing is subjected to more than a certain minimum amount of angular misalignment the roller/cage assembly tends to creep circumferentially relative to the two races which rotate together in a fixed circumferential relationship. This creeping motion is evidently induced by cyclic friction forces between the rollers and the races, resulting from the slight but finite angular misalignment of the axes of the races. When such creeping of the roller/cage assembly occurs, the rollers are constantly changing their positions within the races and no localised wear damage develops. In these circumstances the bearing can operate satisfactorily over long periods of operation without producing evidence of false brinelling damage.

It should be pointed out that most applications of the type illustrated in Figure 1 work perfectly satisfactorily with standard bearings and it would only be in extreme and uncommon circumstances, prolonged operation in direct drive accompanied by misalignment and axial load, that false brinelling damage might be caused.

The present invention seeks to provide a bearing in which creep is deliberately encouraged in order to reduce or eliminate false brinelling damage.

According to the invention there is provided a rolling bearing comprising inner and outer race members having respective inner and outer raceways formed thereon, a plurality of rolling members located between the inner and outer raceways and wherein one or more of the inner and outer raceways and the rolling members are surface treated to modify their coefficient of friction in such a way that, in operation, the rolling members are induced to creep when the inner and outer raceways are rotating at the same speed.

For example the modification of any one of the inner or outer raceways or the rolling members to reduce their surface friction will have the effect of inducing creep. This can be achieved by applying to the chosen component a friction modifying surface treatment. The effect of this is to cause a large imbalance in the creep-inducing forces acting at the inner and outer raceway contacts.

Because of this large imbalance, the roller/cage assembly tends to creep over a wide range of angular misalignments, even down to the very small amounts of misalignment which arise unavoidably as a result of manufacturing inaccuracies and deflections under load. Such a bearing will experience circumferential creep of the roller/cage assembly without the need to impose deliberately large and undesirable amounts of angular misalignment. However, it has been found that the deliberate misalignment of the two race members may still be beneficial in certain circumstances where even the bearing treated in accordance with the invention fails to creep.With the treated bearing, however, the amount of misalignment needed is very much smaller than that required to induce creep in a standard bearing and amounts of angular misalignment in the range 0.3 to 0.7 milli-radian, with a preferred figure in the region of 0.5 milliradians, have been found satisfactory in ensuring creep in all conditions. Such misalignment should not be sufficient to impair the performance of the bearing in other respects.

In one embodiment of the present invention a selected component as aforsaid is phosphate coated by a known chemical treatment process to produce a shallow layer of manganese phosphate or zinc phosphate on the rolling surface of the component, while the remaining components are left untreated.

In another embodiment, the rolling members and one of the inner or outer raceways are treated to modify their friction properties while the other raceway is left untreated.

The friction modifying treatment may be any suitable process which produces the desired effect while at the same time does not impair the performance of the bearing in other respects.

Test experience reveals that when an untreated bearing is accurately aligned, the degree of fretting damage is negligible, but under increasing amounts of angular misalignment the contracting surfaces of the rollers, and the inner and outer raceways may suffer increasing amounts of damage. However, if the angular misalignment imposed on the bearing exceeds a certain limiting value, the bearing does not thereafter sustain fretting damage. There is therefore evidence that the bearing experiences a change in its mode of operation as the angular misalignment crosses this limiting value.

The resuits of these tests clearly show that there is a range of misalignment which should be avoided if fretting damage is to be eliminated. In particular, misalignment less than the limiting value referred to above should be avoided.

However, this in itself is not an acceptable solution to the problem of fretting damage because the critical level of misalignment needed to prevent fretting is found to be rather more than is allowable from other aspects of bearing performance. A limiting value of 2.5 milli-radians is typical, and is known to be more than the acceptable level of misalignment from the point of view of bearing fatigue life.

Further tests carried out using bearings fitted with phosphate coated rollers have shown that the desired roller creep mechanism is induced at significantly smaller amounts of angular misalignment, typically of the order of 0.5 milliradians.

Figure 2 of the accompanying drawings is a diagrammatic cross-sectional view through a tapered roller bearing in which axes of the inner and outer race members have been deliberately misaligned. This drawing will be used to explain in more detail the mechanism of roller creep.

The bearing shown in Figure 2 comprises outer and inner race members 10 and 11 having respective raceways formed on their facing surfaces. A plurality of tapered rollers, three of which are shown under references 12, 13 and 14, are located between the raceways of the inner and outer race and are spaced from one another by a cage assembly 1 5. The axis 1 6 of the outer race member 10 is misaligned with respect to the axis 17 of the inner race member by an angle . The angle v has been deliberately exaggerated for clarity.

In considering the mechanism of roller creep, it is first helpful to consider what happens to an individual roller when operating in a misaligned bearing in which the inner and outer race members rotate together. At any position around the bearing, the roller experiences the effects of the external misalignment as relative angular movements of the raceways between which the roller is located. It is convenient to consider the plane of misalignment as passing through the roller positions identified as 0 and 1 800 in Figure 2-i.e. in the plane of the drawing.

A roller in the position referred to as 0 (roller 12) will experience the bearing misalignment as a concentration of load towards the large ends of the line contacts formed between the roller and the two raceways. It follows that a roller in the 1 800 position (roller 14) will experience the opposite effect -- i.e. a concentration of load towards the small end, as shown. At the two intermediate positions -- i.e. 900 (roller 13) and 2700 (roller not shown), the rollers will experience properly aligned contact with both raceways in the radial plane. In addition to these radial displacements, at certain positions the rollers experience the effects of external bearing misalignment as a relative angular displacement of the axes of the raceways in a tangential plane.

Thus, the roller 13 at the 900 position is loaded between raceways whose axes are misaligned in the plane of the diagram.

Research has indicated that roller alignment in the tangential plane is controlled primarily by the guiding influence of the contact between the large end face of the roller and the rib on the large end of the inner race member. Therefore, for preliminary considerations at least, it may be assumed that the roller at all times remains properly aligned on the inner race member. It then follows that, when the roller passes through the 900 position, its axis will remain aligned to the axis of the inner race member but will be skewed relative to the outer race member by an angle equal to the angle of misalignment improsed on the bearing -- i.e. the angle qw.

It is believed that the beneficial results obtained by using a surface treatment are the result of reduced sliding friction between the surfaces when one of them is so treated. Because friction may be reduced, slip due to skewing occurs at a smaller angle of misalignment and therefore the creep mechanism persists to a lower limiting value. The roller is allowed to pivot more freely in the cup under the cyclic influence of any external angular misalignment and this in consequence encourages the operation of the creep mechanism.

Claims (9)

1. A rolling bearing comprising inner and outer race members having respective inner and outer raceways formed thereon, a plurality of rolling members located between the inner and outer raceways and wherein one or more of the inner and outer raceways and the rolling members are surface treated to modify their coefficient of friction in such a way that, in operation, the rolling members are induced to creep when the inner and outer raceways are rotating at the same speed.

2. A rolling bearing as claimed in claim 1 wherein the outer and inner race members are axially misaiigned with respect to one another.

3. A rolling bearing as claimed in claim 2 wherein the axis of the outer and inner race members are misaligned by an angle a in the range 0.3 to 0.7 milli-radians.

4. A rolling bearing as claimed in claim 3 wherein the angle a is approximately 0.5 milli-radians.

5. A rolling bearing as claimed in any one of the preceding claims wherein said surface treatment takes the form of a phosphate coating applied by a chemical treatment process to produce a shallow layer of manganese phosphate or zinc phosphate on the surface of the component concerned.

6. A rolling bearing as claimed in any one of the preceding claims wherein the surface treatment is applied to just one of the inner or outer raceways or the rolling members, the remaining components remaining untreated.

7. A rolling bearing as claimed in any one of claims 1 to 5 wherein the surface treatment is applied to the rolling members and to one of the raceways, the other raceway remaining untreated.

8. A rolling bearing as claimed in any one of the preceding claims wherein the rolling elements take the form of tapered rollers.

9. A rolling bearing as claimed in claim 1, substantially as hereinbefore described.