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PLAIN BEARING FAILURE
6.0 Chemical effects
- Chemical effects Index
6.3 Copper Deposit on Thrust bearing
| Click on photograph to enlarge | |
 | Deposit of metallic copper on the surface of a fixed, inclined pad thrust bearing |
| Main Characteristics | Note the uniform appearance of the golden-brown deposit on the white metal bearing surface, in the oil grooves as well as on the bearing surface. This clearly has an electrolytic origin. Diagnosis depends on chemical identification of the deposit. |
| Cause | Deposit was identified by x-ray diffraction as pure copper. Copper deposits preferentiallyfrom solution on other metal surfaces that have a more negative Standard Electrode Potential (SEP). The SEP of copper = +0.345, tin = -0.136; hence copper will plate out from solution on to tin-rich white metal, just as it does on a steel penknife blade (SEP for iron = -0.441) dipped into a solution of copper sulphate. The lubrication system contained copper pipes and a copper alloy cooler, but the problem was to find a mechanism that would produce sufficient dissolved copper in the oil to plate out on the bearing. The bearing was from a centrifugal refrigeration compressor that used R12 as the refrigerant. R12 contains chlorine that could attack the copper, but R12 is stable to over 250°C well above any expected temperature in the system. The only hypothesis was that gaseous cavitation was taking place in the bearing (R12 is soluble in mineral oil) and that sufficiently high temperatures occurred on bubble collapse to decompose the R12, producing free chlorine that reacted with water in solution in the oil to give the required electrolyte. The bearing was changed to a tilting-pad one, where it was thought that pressure gradients in the oil in the bearing would be reduced. Although the hypothesis was never fully proved, the change in bearing type solved the problem. |
| Note | |
| Possible Confusion with Other Types of Damage | Other deposits described in this Section tend to look superficially the same. The final diagnosis requires chemical analysis |
| Comment | The identification of the deposit in this case was simple. Once that fact had been established it became possible to look for an explanation. The hypothesis suggested seemed improbable, but in the absence of any alternative suggestion, it seemed worth trying to modify the conditions in the bearing by changing the geometry. This was a standard package refrigeration unit and it subsequently turned out that, despite assurances from the manufacturer that it was limited to the company involved, in fact it was a common problem with these units. |