Machine Design

Bearing Heat

Heat is generated either by shearing of the oil film or by rubbing contact. In hydrostatic and hydrodynamic bearings, heat generation at running speeds is the result of oil shear, and the amount of temperature rise can be estimated if oil viscosity and shear rates are known. Temperature can be regulated by controlling the oil flow through the bearing or by using external cooling.

High-speed and close-clearance fluid-film bearings are difficult to cool. The flow rate through a journal bearing consists of a hydrodynamic portion and a hydrostatic portion. The hydrodynamic flow is proportional to RCw/2 multiplied by a constant which is a function of load or eccentricity e. Here, R = bearing radius, in.; C = clearance, in.; and w = journal speed, rpm. The hydrostatic flow is proportional to feed pressure and is also a function of feed groove shape, cube of oil-film thickness, and local viscosity.

Boundary-lubricated and self-lubricating bearings are more sensitive to sliding velocity than fluid-film types because the coefficient of friction is as much as ten times greater in the first two. Frictional heating is a function of bearing pressure, sliding velocity, and coefficient of friction. Therefore, if the coefficient of friction remains constant for a range of loads and speeds, a rough indication of heat load is provided by the PV factor.

Most plastic bearing materials are sensitive to PV because of their low thermal conductivities and high thermal-expansion rates.

TAGS: Bearings
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