Other Bearing Materials
An extensively hard wood of great density, termed as lignum vitae, has been employed for bearing applications. Along with water as cooling and lubricant medium its antifriction properties and wear are comparable along with those of bearing metals. Lignum vitae has been employed with satisfactory results particularly in conditions of step brings of vertical water turbine; marine service, roll neck bearings of rolling mills and even paper mill machinery.
Now days, in such cases where employ of water as lubricant is essential, especially whether sand and grit are represent soft vulcanised rubber bearings have been employed. A soft, tough, resilient rubber serves as a yielding support, permitting grit to pass via the bearing without scoring the shaft or the rubber. Longitudinal grooves in the rubber lining allow free passage of the cooling water along with any foreign matter represent. With feathered edges these grooves are extremely effective also in forming passages in the front of that the supporting pressure is built up in the fluid film. Such bearings have coefficient of friction that compares well along with roller bearings and pressure of 4.0 to 5.5 MPa might be carried if journal is extremely 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 should all the time be below boiling point. In several cases rubber bearings have been found to offer as much as ten times the service as bearings of metals or lignum vitae.
Rubber bearings have been effectively used in centrifugal and deep well pumps, and employed in washers and various other applications where water must be employed as lubricant. The cushioning and resilience properties of rubber may be exploited in reducing vibrations of high speed shafts.
Neutral and Synthetic composite materials, plastic and reinforced plastic are also being employed as bearing materials now-a-days. Though, their characteristics are not well established as still. Powder metallurgy bushing allows oil to penetrate in the materials due to its porosity and is excellent for its antifriction properties.
Bearings are frequently ball-indented in order to offer small basins for the storage of lubricant whilst the journal is at rest. This supplies several lubricants throughout starting. The bearing walls may sometime be indented and filled along with graphite to offer lubricating effect at the start.