Plain Bearing Classification, Properties and Usage

Plain bearings are bearings working in a zone of liquid, maximum, or dry friction. The zone of liquid friction is functionally the most advantageous because of low friction losses and minimum wear of sliding faces. Liquid lubrication can be realized either by the relative movement of sliding faces (hydrodynamic and aerodynamic lubrication) or by the lubrication inlet from an outer pressure supply (hydrostatic and aerostatics lubrication).

Friction losses in the maximum friction zone are roughly one degree higher, and in the dry friction zone two degrees higher, than for liquid friction. Wear of sliding faces differs by several degrees in the individual zones. These are the reasons for selecting lower operating parameters for bearings working permanently in the zone of maximum or dry friction. The limited durability must be taken into account as well.

Hydrostatic, aerostatics, and hybrid bearings work permanently in a liquid friction zone. Hydrodynamic and aerodynamic bearings work primarily in this zone. Hybrid bearings are lubricated hydrostatically or aerostatically during the start-up and run-out process and hydrodynamically or aerodynamically during operation.

Bearings with low lubrication demands, that is, the bearings with short-term lubrication, work in the zone of maximum friction. Hydrodynamic and aerodynamic bearings work in this zone during the start-up and run-out process. Bearings without lubrication work in the zone of dry friction.

Rough data for the selection and design solution of plain bearings are presented in the following two tables. The tables inform about the suitability of using the bearing types.

Demands	Bearings without lubrication	Bearings with maximum lubrication	Hydrodynamic bearings	Hydrostatic bearings	Aerodynamic bearings	Aerostatic bearings
Loading capacity	low	low to medium	medium to high	medium	very low	low
Small starting torque	normally not recommended	satisfactory	satisfactory	excellent	satisfactory	excellent
Small friction torque at steady state	normally not recommended	satisfactory	satisfactory	satisfactory	excellent	excellent
Precision of radial setting	bad	good	good	excellent	good	good
Lifetime	limited but predictable	limited but predictable	theoretically endless, but limited by starts and run-outs number	theoretically endless	theoretically endless, but limited by starts and run-outs number	theoretically endless
Mix of axial and radial loading capacity	axial supporting face must be done for absorbing axial load	axial supporting face must be done for absorbing axial load	axial supporting face must be done for absorbing axial load	axial supporting face must be done for absorbing axial load	axial supporting face must be done for absorbing axial load	axial supporting face must be done for absorbing axial load
Still running	good for stationery devices	excellent	excellent	excellent, apart the possible pump noise	excellent	excellent, but compressed noise is possible
Lubrication simplicity	excellent	excellent	separate system can be used with certain limitation of speed, loading, and diameter	additional high pressure pump necessary	excellent	supply of compressed, dry, and clean air necessary
Availability of standard parts	good to excellent	excellent	good	not suitable	not suitable	not suitable
Protection against pollution of product and environment	abrasion can be a limiting factor	normally satisfactory, but sealing is necessary, except when working liquid can be used for lubricant	normally satisfactory, but sealing is necessary, except when working liquid can be used for lubricant	normally satisfactory, but sealing is necessary, except when working liquid can be used for lubricant	excellent	excellent
Starts and run-outs number. Frequent rot. direction change	excellent	good, generally good	good, generally good	excellent	bad	excellent
Operating expenses	very low	very low	depends on the complexity of lubrication system	price of lubricant supply must be considered	none	price of gas supply must be considered

Ambient conditions	Bearings without lubrication	Bearings with limited lubrication	Hydrodynamic bearings	Hydrostatic bearings	Aerodynamic bearings	Aerostatic bearings
High temperature	satisfactory, depends on the material	beware of oxidation: lubrication resistance necessary	beware of oxidation: lubricant resistance necessary	beware of oxidation: lubricant resistance necessary	excellent	excellent
Low temperature	satisfactory, depends on the material	possible limitation from lubricant, respect to starting torque necessary	possible limitation from lubricant, respect to starting torque necessary	possible limitation from lubricant	excellent, ideally dried gas necessary	excellent, ideally dried gas necessary
Outside vibrations	normally satisfactory, except when impact loading peak exceeds loading capacity	normally satisfactory, except when impact loading peak exceeds loading capacity	satisfactory	excellent	normally satisfactory	excellent
Space demanded	small radial dimension	small radial dimension	small radial dimension, but the total space requirement depends on the lubrication system	small radial dimension, but the total space requirement depends on the lubrication system	small radial dimension	small radial dimension, but the total space requirement depends on the gas supply system
Dirt or dust	normally satisfactory, sealing is desirable	sealing is important with self-lubricated bearings	satisfactory, lubricant filtering is important	satisfactory, lubricant filtering is important	sealing is important	satisfactory
Vacuum	excellent	lubricant can be a limitation	lubricant can be a limitation	lubricant can be a limitation	normally cannot be used	cannot be used when vacuum is kept
Wet or humid environment	normally satisfactory, sealing is advantageous	normally satisfactory, sealing is advantageous	satisfactory	satisfactory	satisfactory	satisfactory
Radiation	satisfactory	lubrication can be a limitation	lubrication can be a limitation	lubrication can be a limitation	excellent	excellent