PLAIN BEARING AND METHOD

Abstract

A plain bearing may comprise a bearing substrate, a bearing overlay, and an interlayer disposed between the bearing substrate and the bearing overlay. The interlayer may comprise hexagonal boron nitride.

Claims

1. A plain bearing comprising: a bearing substrate; a bearing overlay; and an interlayer disposed between the bearing substrate and the bearing overlay, the interlayer comprising hexagonal boron nitride

2. A plain bearing according to claim 1, wherein the hexagonal boron nitride is configured as a plurality of particles embedded in the interlayer.

3. A plain bearing according to claim 2, wherein the plurality of hexagonal boron nitride particles have a diameter less than a thickness of the interlayer.

4. A plain bearing according to claim 2, wherein the plurality of hexagonal boron nitride particles have a diameter less than 5 m, 3 m or 2 m.

5. A plain bearing according to claim 1, wherein the interlayer comprises Ni.

6. A plain bearing according to claim 1, wherein the interlayer is a thickness between 3 m and 7 m.

7. A plain bearing according to claim 1, wherein the interlayer is an electrolytically-deposited layer.

8. A plain bearing according to claim 1, wherein the bearing overlay is lead-free.

9. A plain bearing according to claim 1, wherein the bearing overlay comprises Sn, a SnCu alloy, a SnZn alloy, a SnNi alloy, Bi or a Bi alloy.

10. A plain bearing according to claim 1, wherein the bearing overlay comprises a plurality of hard particles.

11. A plain bearing according to claim 1, wherein the bearing overlay comprises a plurality of hard particles, the plurality of hard particles comprising boron carbide.

12. A plain bearing according to claim 10, wherein the bearing overlay comprises between 0.5 and 5 wt % of the plurality of hard particles.

13. A plain bearing according to claim 1, wherein the bearing overlay is a thickness between 8 m and 20 m.

14. A plain bearing according to claim 1, wherein the bearing substrate comprises a bearing lining supported by a bearing backing, and the interlayer is defined on the bearing lining.

15. A plain bearing according to claim 14, wherein the bearing lining comprises Cu.

16. A method for forming a bearing comprising hexagonal boron nitride, comprising: electrolytically depositing an interlayer from an electrolyte comprising a plurality of hexagonal boron nitride particles.

17. A method according to claim 16, wherein the interlayer comprises Ni and the electrolyte comprises a Ni salt.

18. (canceled)

19. (canceled)

20. A plain bearing according to claim 1, wherein the bearing overlay comprises a plurality of hard particles comprising one of carbide, nitride, or oxide.

21. A plain bearing comprising: a bearing substrate; a lead-free bearing overlay; an interlayer comprising Ni disposed between the bearing substrate and the bearing overlay; and a plurality of hexagonal boron nitride particles embedded in the interlayer.

22. A plain bearing according to claim 21, further comprising a plurality of one of carbide, nitride, or oxide particles incorporated into the lead-free bearing overlay.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, in which:

[0027] FIG. 1 is a cross section through a portion of a plain bearing embodying the present invention;

[0028] FIG. 2 is a metallurgical microsection of a bearing embodying the invention;

[0029] FIG. 3 is a graph of bearing temperature and applied load against time, during a test of a bearing embodying the invention; and

[0030] FIG. 4 is a graph of bearing temperature plotted against time, and linearly-increasing bearing load, during tests of three types of bearings.

DETAILED DESCRIPTION

[0031] FIG. 1 is a schematic sectional view of a bearing embodying the invention, on a section plane perpendicular to a running surface 2 of the bearing. The running surface is formed from a bearing overlay 4 separated from a bearing substrate 6 by an interlayer 8. The substrate comprises a bearing lining 10 bonded to a bearing backing 12.

[0032] The backing may be in the form of a semi-cylindrical bearing shell, fabricated from steel. The lining is of bronze and conforms to the shape of the backing. The interlayer is of Ni incorporating h-BN particles and is formed over the interlayer by electro-deposition as described below. The overlay is of tin, optionally containing 0.5 to 5 wt % of hard particles.

[0033] FIG. 2 is a metallurgical section of an embodiment of the invention, fabricated by the inventors for testing. In this embodiment, both the tin overlay 4 and the interlayer 8 are approximately 10 m thick. The interlayer is bonded to the bronze lining 10. The bearing backing does not appear in this metallurgical section.

[0034] In a preferred embodiment, the interlayer thickness is between 3 and 7 m and preferably about 5 m. The interlayer shown in FIG. 2 is therefore a little thicker than would be preferred, but the thickness of the interlayer can be controlled as described further below.

[0035] In embodiments of the invention, the interlayer comprises nickel incorporating h-BN particles. The interlayer can be formed by electrolytic deposition from a suitable electrolyte. In one embodiment, the electrolyte comprises:

500 g/l NiSO.sub.4*7H.sub.2O;
50 g/l boric acid;
1 g/l saccharin;
5-30 g/l h-BN;

Dispersants; and

Surfactants

[0036] The electrolyte has a pH of between 2 and 3 and a temperature of between 60 C and 70 C. Electrolytic deposition is carried out at a current density of 1-10 A/dm.sup.2.

[0037] The h-BN is added to the electrolyte in a dispersion of powder of the following particle size range:

diameter 10%: 0.36 m
diameter 50%: 1.65 m
diameter 90%: 7.50 m
In other words, 10% of the particles are of diameter 0.36 m or less, 50% are of diameter 1.65 m or less, and 90% are of diameter 7.50 m or less.

[0038] The duration and current density of the electrolytic deposition is controlled to achieve a predetermined interlayer thickness on the bronze bearing lining.

[0039] As an alternative, a NiCl.sub.2-based electrolyte may be used, in a Watts nickel bath, with the same h-BN powder, or particles, added to the electrolyte as in the NiSO.sub.4 electrolyte described above.

[0040] In the microsection in FIG. 2, the presence of h-BN particles within the nickel-based interlayer can be seen. The sizes of the h-BN particles in the layer are typically 1 m or less. The inventors have found that by controlling the electrolytic deposition process using the electro-deposition conditions described above, smaller particles of h-BN can be incorporated into the interlayer in preference to larger h-BN particles. The larger particles remain in the electrolyte. This enhances the performance of the interlayer and of the bearing.

[0041] In order to optimise the performance of the bearings, the inventors have carried out tests of different bearings embodying the invention. These experiments were carried out using a test rig in which a controlled load is applied to a plain bearing coupled to a rotating shaft. The load is applied perpendicular to the axis of rotation of the shaft, and the bearing is coupled to an eccentric portion of the shaft to generate dynamic loading. Oil is supplied to the plain bearing, in conventional manner, but the eccentrically-rotating journal is formed with a geometry imperfection (an axial groove) to continuously disrupt the elastohydrodynamic oil film and thus cause contact between the bearing and the shaft, leading to overloading of the bearing.

[0042] Bearings of diameter 53 mm, width 19 mm and bearing clearance 53 m, formed from two semi-cylindrical bearing shells, were used in the tests. Two thermocouples positioned in the loaded half on either side of the bearing monitored bearing temperature in order to record scuffing and seizure events.

[0043] FIG. 3 illustrates a typical test of a bearing embodying the invention. FIG. 3 is a graph showing traces for the bearing temperature, the load applied to the bearing, and the current drawn by the electric motor driving the shaft as measured during a test of a bearing. All of these parameters are plotted against time.

[0044] As the test progresses, a target load, or applied load, is applied to the bearing perpendicular to the eccentric shaft axis. The load is linearly increased with time until the bearing fails, to give a measure of bearing performance that allows different bearings to be compared.

[0045] The bearing temperature monitors degradation of the bearing. As shown in FIG. 3, after approximately 2 minutes 30 seconds, the bearing temperature rises (marked First Event in FIG. 3) as a scuffing event occurs, when a portion of the overlay has been worn through. Importantly, however, the temperature then falls after the scuffing event, so that the bearing can continue to operate even though a portion of the interlayer may be exposed. As the applied load continues to increase, after about 3 minutes and 36 seconds, the bearing seizes (marked Seizure onset in FIG. 3).

[0046] In a real application of the bearing, the delay between the initial scuffing event and the seizure of the bearing may advantageously allow a repair to be made before significant damage is caused.

[0047] The inventors have carried out tests to assess the performance advantage of bearings embodying the invention. These tests used the same testing procedure as described above with reference to FIG. 3, with an applied bearing load increasing with time until bearing failure. First, six bearings comprising a conventional nickel interlayer over a bronze lining were tested for reference. Six bearings according to a first embodiment of the invention were then tested. These were termed Variant A and comprise a nickel interlayer electro-deposited (over a bronze lining) from a NiSO.sub.4 electrolyte as described above, containing 10 g/l h-BN. Variant A achieved an average seizure load about 50% higher than the reference bearings. Six bearings according to a second embodiment, termed Variant B, were prepared in the same way as Variant A but the interlayer in Variant B was deposited from an electrolyte containing 20 g/l h-BN. Variant B achieved an average seizure load about 90% higher than the reference bearings.

[0048] In each of the bearings in these tests the thickness of the nickel-based layer were the same, as was the underlying bearing substrate.

[0049] FIG. 4 is a graph of bearing temperature against time for the bearing tests. The graph shows three traces. The solid line shows the average measured temperature for the six reference bearings. The dashed line shows the average measured temperature for the six Variant A bearings. The dotted line shows the average measured temperature for the six Variant B bearings. The applied bearing load increased linearly with time, as shown on the horizontal axis of the graph in FIG. 4.

[0050] As shown in FIG. 4, the Reference bearings failed after about 22 seconds, corresponding to an applied load of about 13 MPa, at a temperature of about 160 C. By contrast, the Variant A bearings failed after about 50 seconds, corresponding to an applied load of more than 20 MPa, surviving without bearing failure to a temperature of 195 C. The Variant B bearings failed after more than 1 minute, corresponding to an applied load of about 25 MPa, at a temperature of about 190 C.

[0051] Notably, the bearings of Variant A and Variant B not only achieved higher failure loads but also failed in a different way from the reference bearings. In all of the six tests of reference bearings, the tests were ended because an overload current was drawn by the drive motor driving the eccentric shaft. This implies an unacceptably high level of bearing friction, caused by bearing seizure.

[0052] By contrast, in the six tests of Variant A bearings and the six tests of Variant B bearings, all of the tests were ended because of an excessive rise in bearing temperature. Although this indicates bearing failure, it also indicates that the bearing friction did not reach the excessive level which occurred on failure of the reference bearings.

[0053] This probably indicates seizure caused by an increased level of microwelding on failure of the reference bearings, which is a more damaging failure mode than the increasing running temperature exhibited by the Variant A and Variant B bearings.