ELECTROPLATED COMPONENT OF A ROLLING ELEMENT BEARING

Abstract

A bearing component of a rolling element bearing, such as a rolling element, a bearing ring, and/or a cage for retaining rolling elements of a rolling element bearing. An outer surface of the bearing component is provided with a plating layer providing at least 97 wt. % tin. According to the invention, tin of the plating layer provides alpha and beta phases of tin in an alpha/beta phase ratio of less than 10%.

Claims

1. A bearing component of a rolling element bearing, such as a rolling element, comprising: a bearing ring, and/or a cage for retaining the rolling elements of a rolling element bearing, wherein an outer surface of the bearing component is provided with a ductile plating layer, the ductile plating layer comprises at least 97 wt. % tin, wherein the tin of the plating layer comprises alpha and beta phases of tin in an alpha/beta phase ratio of less than 10%, preferably no more than 4%, or the ductile plating layer comprises at least 97 wt. % electroless nickel, electrolytic nickel, or copper.

2. The bearing component of claim 1, wherein the bearing component is a cage for retaining the rolling elements of a rolling element bearing, wherein the cage is in sliding contact with an outer surface of the rolling elements or a surface of a bearing ring, and wherein the outer surface of the rolling elements or the surface of the bearing ring is provided with a layer of black oxide so that the ductile plating layer on the cage is in sliding contact with the layer of black oxide.

3. The bearing component of claim 2, wherein the black oxide layer has a thickness of between 0.2 to 1.5 m, preferably between 0.5 and 1 m.

4. The bearing component of claim 1, wherein the alpha/beta phase ratio of the tin plating layer is no more than 3%.

5. The bearing component of claim 1, wherein the ductile plating layer has a thickness of less than 5 m, preferably less than 3 m, and/or wherein the tin plating layer of a cage has a thickness of between 1 and 100 m, preferably between 5 and 30 m.

6. The bearing component of any preceding claim, wherein the tin plating layer consists of pure tin.

7. The bearing component of any one of claim 1, wherein the tin plating layer additionally comprises one or more elements selected from: antimony, bismuth, indium and silver.

8. The bearing component of claim 7, wherein the tin plating layer consists of 99.5-99.9 wt. % tin and 0.1-0.5 wt. % of antimony, bismuth, indium and/or silver.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows a cross-section of part of roller bearing comprising a cage provided with a surface layer of tin according to the invention;

[0021] FIG. 2 shows the effect of differing plating temperatures

DETAILED DESCRIPTION

[0022] A cross-sectional view of part of a roller bearing is shown in FIG. 1. The bearing 10 has an inner ring 12, an outer ring 15 and a row of cylindrical rollers 17 disposed therebetween on inner and outer raceways of the bearing rings. In the depicted example, the bearing outer ring 15 has flanges 15a at either axial side of the outer raceway, which extend in a radially inward direction and which guide the rollers during bearing operation. The rollers are held spaced apart by a cage 20, which in this example is centred and guided on the outer ring flanges 15a and is formed from a low-carbon steel material.

[0023] Let us assume that the bearing supports an engine crankshaft and is lubricated with engine oil. During bearing operation, a radially outer surface 22 of the cage is in sliding contact with radially inner surfaces of the outer ring flanges 15a. In the absence of a proper lubrication film, high friction and wear takes place. To mitigate this problem, the outer surfaces of the cage, the rings and/or the rollers are electroplated with a tin coating 25.

[0024] In accordance with the invention, the tin coating 25 comprises alpha-phase tin and beta-phase tin in an alpha/beta phase ratio of less than 10 mol % alpha-tin, preferably less than 5 mol %, and in case of the component being a cage of no more than 4%. A tin coating having a correspondingly high percentage of beta-phase tin has been found to generate low friction and exhibit excellent durability, as will be demonstrated by the following examples.

[0025] A number of samples were prepared and subjected to a pin-on-disc friction test, in which a disc sample is mounted on a rotating platform and a pin in the form of a ball is pressed against the rotating disc. The base material for the disc samples was a low-alloy carbon steel (ST24), which is a common material for bearing cages. The disc samples were turned and then electroplated with metal coatings as follows:

TABLE-US-00001 TABLE 1 material, thickness and hardness of coatings provided on disc samples Metal Thickness Hardness coating (m) (HV) Comment Bronze 10 200 Copper 0.8 160 Silver 25-50 40 Tin-A 10 60 Pure tin coating containing ~20 mol % alpha-phase tin, measured using X-ray diffraction Tin-B 8 60 Pure tin coating according to invention containing ~2 mol % alpha-phase tin, measured using X-ray diffraction

[0026] The coefficient of sliding friction was measured at the start of the test and at the end of the test under the following conditions:

[0027] Ball: DIN 100Cr6 steel, martensitically hardened (hardness 780-800 HV), diameter 12.7 mm, surface roughness Rq=0.01 m

[0028] Flat disc samples: ST24 cage steel, un-hardened (hardness 230 HV), surface roughness before coating Ra=1.6 m

[0029] Linear sliding speed: 1 m/s (400 rpm at 25 mm radius of the disc)

[0030] Lubrication oil: Engine oil, Fuchs TITAN EM 225.26 (HTHS 2.9), viscosity: 55.2 mm.sup.2/s at 40 C. and 10.1 mm2/s at 100 C.

[0031] Test oil temperature: 90 C.

[0032] Test duration: 4 hours

[0033] Furthermore, the surface roughness of the samples was measured before the test and after the test. The ball produces a wear track on the disc surface and it is the surface roughness of the wear track that is measured. The test results are given in Table 2.

TABLE-US-00002 TABLE 2 Friction coefficient measured with pin-on-disc test rig at the beginning and end of the tests, plus surface roughness before and after the tests. Surface Ra of Coeff. of Coeff. of roughness wear track friction in friction in Ra before after POD POD test, POD test, Coatings test (m) test (m) start of test end of test Un-coated 1.60 0.11 0.076 0.07 Bronze 1.7 1.42 0.085 0.08 Copper 0.8 0.286 0.05 0.046 Silver 1.11 0.084 0.02 0.018 Tin-A 1.68 0.54 0.048 0.044 Tin-B 1.98 0.795 0.022 0.02

[0034] As may be seen, the Tin-B coating in accordance with invention generates a low and stable coefficient of friction comparable to that of the much more expensive silver coating. It is also noteworthy that the Tin-B coating (with an alpha/beta phase ratio of 2%) generates lower sliding friction than the Tin-A coating (with an alpha/beta phase ratio of 20%) despite the fact that Tin-A coating has a lower surface roughness.

[0035] Without being bound by the theory, it is thought that this is attributable to the different crystalline structure of the two tin phases. The beta phase has a tetragonal structure and is highly ductile, since the crystal can be sheared along the tetragonal crystalline planes. The alpha phase has a cubic structure, which resists the shearing of the beta phase, making the metal more brittle. It is also thought that the high ductility and shearing behaviour of a tin coating according to the invention improves the durability of the coating.

[0036] A wear resistance test was performed on samples provided with the coatings as given in table 1.

[0037] Rings made of low-alloy carbon steel (ST24) were electroplated with the coatings and mounted on the shaft of a friction-torque test rig. The test conditions were as follows: [0038] Coated ring: ST 24, turned surface (Ra1.6 m before coating), ring external diameter 67.5 mm, inner diameter 60 mm and height 48 mm, hardness 230 HV [0039] Counter face: CRB 208ECP outer ring shoulders, diameter 67.5 mm, width 3 mm, DIN 100Cr6 hardened steel, hardness 780-800 HV, surface roughness Ra0.2 m [0040] Rotating speed: 500, 1000, 2000, 3000 rpm [0041] Load: 0-50 kg (on sample) [0042] Lubrication oil: Engine oil, Fuchs TITAN EM 225.26 (HTHS 2.9), viscosity: 55.2 mm.sup.2/s at 40 C. and 10.1 mm.sup.2/s at 100 C. [0043] Oil flow rate: 20 and 150 cc/min [0044] Oil temperature: ambient temperature, [0045] Test duration: 100 hours

[0046] Friction torque is measured during the test and wear life is determined by the following two measurements:

[0047] 1) When the friction torque exceeds 0.375 Nm

[0048] 2) When damage to the coating is observed under optical microscopy inspection.

[0049] The results of the test are shown in Table 3.

TABLE-US-00003 TABLE 3 Endurance life (hours) and friction torque measured at the end of the life. Test condition: load 300N, speed 3000 rpm, room temperature, oil flow 150 cc/min Friction torque Surface roughness Ra at end of Endurance Sample before test (m) the tests (Nm) life (hours) Uncoated 1.33-1.40 0.45-0.6 29-45 steel (ST24) Bronze 1.58-1.59 0.225 100 Copper 1.35-1.43 0.38 35 Silver 1.11-1.18 0.375-0.45 100 Tin-A 0.75-0.76 0.825 70 Tin-B 1.09-1.29 0.225 100

[0050] As may be seen from the results, the tin coating of the invention Tin-B reached the end of the 100-hr test and exhibited low frictional torque, while the comparative coating Tin-A only lasted 70 hours. The tin coating of the invention is more durable.

[0051] In order to produce a tin coating containing at least 90 mol. % beta-tin, the present inventors have found that it is necessary to carefully select and control the bath temperature during the electroplating process.

[0052] Experiments were conducted in which samples made of low-carbon steel were immersed in a stannous sulphate bath composition comprising 40 g/L of tin and 120 g/L of sulfuric acid. Each sample was connected as the cathode to an anode made of pure tin and electroplated at a specific temperature, which temperature was varied for the different samples, keeping all other process parameters constant. After completion of the electroplating process, the plating layer was examined using X-ray diffraction, to determine the relative content of alpha-tin to beta-tin. The results are shown in bar chart of FIG. 2.

[0053] As may be seen, a temperature of 35 C. leads to the production of a tin coating according to the invention, with an alpha/beta phase ratio of less than 3%, while a temperature of only 5 degrees higher leads to a significantly higher content of alpha-tin. A process temperature of 70 C. also resulted in a tin coating according to the invention.

[0054] Although the mechanism is not fully understood, it appears that narrow temperature ranges or windows exist in which the formation of alpha-tin is greatly suppressed. Such a window exists between a temperature of 25 C. and 35 C. Another window exists between 60 C. and 80 C., showing the same alpha/beta phase ratios as the first window for lower temperatures. A bearing cage according to the invention is electroplated with tin at a temperature that falls within an identified temperature window, to produce a tin coating with enhanced durability and low-friction performance.

[0055] A number of aspects/embodiments of the invention have been described. The invention is not restricted to the described embodiments, but may be varied within the scope of the accompanying patent claims.