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PLAIN BEARING FAILURE
6.0 Chemical effects
- Chemical effects Index
6.5 Sulphur Attack of White Metal
| Click on photograph to enlarge | |
 | Copper sulphide deposits on white metal thrust pads |
 | Copper sulphide deposit on white metal journal bearing |
| Main Characteristics | Black deposit on the trailing edge of thrust pad, Photograph 'a' and in the load area in the bottom of a journal bearing, Photograph 'b' (right). The location of the deposits suggests chemical attack. Diagnosis depends on the chemical identification of the deposit. |
| Cause | The deposit was identified by x-ray diffraction as copper sulphide. Attack of the copper present in tin-rich white metal by reaction with active sulphur compounds in the process gas contaminating the lubricant forming deposit of copper sulphides (Cu2S, CuS) on the bearing surface. |
| Note | The reaction is a temperature dependent one. The fact that there is hardly any deposit present on the second pad from the right in Photograph 'a' indicates that this pad has been carrying little of the load, presumably as it is thinner than the others. Copper-free lead-rich white metal is more resistant to attack by sulphur, though under severe conditions (high temperature, high concentration of active sulphur compounds) a deposit of lead sulphide (PbS) may be formed. |
| 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 | Sulphur attack is more of a problem on floating-bush shaft seals lined with tin-rich white metal, where it can take up the clearance and cause seal failure. It can also cause breakdown of the lubricant film in thrust bearings, leading to wiping and loss of the evidence. Failure of journal bearings is much less likely, so that, in the case of wiping failures with thrust bearings, it always pays to look at the associated journal bearings to see if there is any evidence of deposits. |