White Metal Bearings
Bearings are fitted in order to reduce both friction and wear between the running surfaces. This is achieved primarily by the lubricant, with a hydrodynamic oil film to keep the surfaces apart. The oil film can never completely prevent contact, particularly at times such as start up. Dirt particles larger than the oil film thickness may also enter the bearing and damage the journal, therefore a suitable bearing material must be used.
Shell bearings consist of a steel backing with a white metal lining bonded to it. These steel backing gives support to the bearings and improves fatigue life.
White metal bearings may be:
- Tin : 89 %
- Antimony: 7.5 %
- Copper : 3.5 %
- Lead : 83 %
- Antimony: 15 %
- Arsenic : 1 %
- Tin : 1 %
Other trace alloying elements are usually added to improve the grain structure. Tin based white metals are more commonly used as they have better fatigue strength and corrosive resistance.
There may be a bearing over-layer consisting of a galvanic coating which is typically 90 % lead and 10 % tin. This ensures good embeddability and conformity between surfaces. In addition there may be a flash layer of 100 % tin to prevent corrosion. This layer of tin is then removed during the running in process.
“Operation and Maintenance of Machinery in Motor Ships” by N.E. Chell