Other Bearing Materials

An extensively hard wood of great density, known as lignum vitae, has been used for bearing applications. With water as lubricant and cooling medium its antifriction properties and wear are comparable with those of bearing metals. Lignum vitae has been used with satisfactory results particularly in cases of step brings of vertical water turbine; paper mill machinery, marine service and even roll neck bearings of rolling mills.

More recently, in such cases where use of water as lubricant is necessary, especially if sand and grit are present soft vulcanised rubber bearings have been used. A soft, tough, resilient rubber acts as a yielding support, permitting grit to pass through the bearing without scoring the shaft or the rubber. Longitudinal grooves in the rubber lining allow free passage of the cooling water with any foreign matter present. With feathered edges these grooves are also very effective in forming passages in the front of which the supporting pressure is built up in the fluid film. These bearings have coefficient of friction which compares well with roller bearings and pressure of 4.0 to 5.5 MPa may be carried if journal is very smooth and load is applied after it has attained a peripheral speed of 150 m/min. The cooling water temperature in case of rubber bearings must always be below boiling point. In some cases rubber bearings have been found to give as much as ten times the service as bearings of lignum vitae or metals.

Rubber bearings have been successfully used in centrifugal and deep well pumps, and washers and several other applications where water must be used as lubricant. The resilience and cushioning properties of rubber may be exploited in reducing vibrations of high speed shafts.

Synthetic and neutral composite materials, plastic and reinforced plastic are also being used as bearing materials now-a-days. However, their characteristics are not well established as yet. Powder metallurgy bushing permits oil to penetrate into the materials because of its porosity and is good for its antifriction properties.

Bearings are frequently ball-indented in order to provide small basins for the storage of lubricant while the journal is at rest. This supplies some lubricant during starting. The bearing walls may some time be indented and filled with graphite to provide lubricating effect at the start.