Running Clearance
It has already been said that this is a "plain" bearing, that is, it is formed from a series of cylindrical surfaces running on one another. The unique selection of materials and pre-pack lubricant, when used, minimise the friction between surfaces and the wear rate experienced through rubbing. Great care is taken with the surface finish on all contacting surfaces and all possible steps are taken in the design of the bearing to minimise heat build-up. Unique tribological conditions are generated within the bearing and highly specialised surface treatments are utilised to protect and enhance performance.
Plain bearings require space within the bearing that is not necessary in rolling element types. This space results in a bearing with more play than a ball or roller race, and if this is not understood, it can result in both poor selection and inappropriate installation of the bearing.
Plastics, being resistant, absorb and damp vibration, without generating any noise. Surface finishes are more controllable and as well as generally providing lower coefficients of friction, the wear products are far less abrasive than those in metallic bearings. These advantages result in plastic bearings being more wear tolerant than the more common metallic plain bearings.
Loading Characteristics - The PV Factor
PV is the product of Surface Velocity and Pressure per unit area, between the two surfaces in contact. A high PV indicates a high bearing performance capability and a low PV indicates a lesser performance.
Considerable confusion can occur in comparing PV figures, as several different measurement methods are used to determine the PV value. Sheedy Bearings apply standard metric units to define the PV value, that is, Newton per Millimetre Second (N/mm Sec) and all PV figures quoted in this material are in these units.
As PV is the product of Pressure and Velocity, it follows that increasing the speed of a plastic bearing will reduce its load bearing capability. Conversely, decreasing the speed of the bearing increases the load bearing capability, the practical load limit being the maximum load the bearing can carry at rest, the Static Load, without deforming to a point where it cannot function.
The load bearing capability of all Sheedy Bearings is therefore a mathematical relationship. This relationship depends upon the internal geometry of the bearing, the strength of the various bearing components and peripheral factors such as lubrication, temperature, the characteristics of the load. The relationship is unique for each bearing type and is represented as a series of Speed/Load curves or tables. More generalised methods are being formulated as a natural development from our ongoing research and quality testing programme.
