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PLAIN BEARING FAILURE
6.0 Chemical effects
- Chemical effects Index
6.7 Sulphur Attack of Silver Bearing
| Click on photograph to enlarge | |
 | Silver sulphide deposit on thrust pads |
| Main Characteristics | Black deposit on trailing edge of thrust pads, suggesting chemical attack. Diagnosis depends on chemical identification of the deposit. |
| Cause | The deposit was identified by x-ray diffraction as silver sulphide. Silver is even more prone to attack than copper. These pads come from a heavily-loaded, high speed thrust bearing. Silver was chosen for the bearing material because of the expected high bearing temperature. (Note the oil feed groove at the inlet edge of the pad, giving individual pad lubrication to assist cooling.} In this case the attack has come from the presence of the sulphur-containing the load-carrying additive zinc dialkyl dithiophosphate commonly used in High Duty (HD) Hydraulic Oils. |
| Note | This example comes from a high speed centrifugal pump with an integral gear. An HD Hydraulic Oil was used to provide additional protection for the gears. These were turbine quality gears that can operate perfectly satisfactorily with oils without load-carrying additives. A change to a turbine oil solved the problem. (This was the same machine that provided the previous example Sulphur Attack of Copper-Based Bearing Alloy that involved the journal bearing. Note that the pads are showing signs of wiping at the trailing edge where deposit has effectively reduced the load-carrying capacity. |
| 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 | Extreme pressure (EP) and High Duty hydraulic oils that contain active sulphur additives should be avoided in systems where there are silver bearing alloys. Silver can be used for the bearing surface on the swash plate of multiple piston hydraulic pumps. |