Page 211 - Tribology in Machine Design

196 Tribology in machine design

sump, where it is cooled and filtered before being returned. The equation for
the heat removed by the oil per unit of time is

where c p is the specific heat of the oil, and y is the specific weight of the oil.
The flow Q in eqn (5.60) may consist of the hydrodynamic flow Q H, eqn
(5.57), film flow <2 f, and chamfer flow Q c as previously discussed, or any
others which may exist. The heat lost by radiation and convection may
often be neglected in well-flushed bearings.
The outlet temperature t 0 represents an average film temperature that
may be used to determine oil viscosity for bearing calculations, at least in
large bearings with oil grooves that promote mixing. The average film
temperature is limited to 70 °C or 80 °C in most industrial applications,
although it may be higher in internal combustion engines. Higher
temperatures occur beyond the place of minimum film thickness and
maximum shear. They may be estimated by an equation based on
experimental results. The maximum temperatures are usually limited by the
softening temperature of the bearing material or permissible lubricant
temperature.
In self-contained bearings, those lubricated internally as by drip, waste
packing, oil-ring feed or oil bath (immersion of journal), dissipation of heat
occurs only by radiation and convection from the bearing housing,
connected members and the shaft. Experimental studies have been directed
towards obtaining overall dissipation coefficients K for still air and for
moving air. These dissipation coefficients are used in an equation of the
form

where A is some housing or bearing surface area or projected area, r b is the
temperature of its surface, and t a is the ambient temperature.

5.5.4. Design for load bearing capacity
It is convenient to convert eqn (5.51) into a non-dimensional form. One
substitution is a commonly used measure of the intensity of bearing
loading, the unit load or nominal contact pressure, p, which is the load
divided by the projected bearing area (/ x d), thus

where / is the bearing length, d is the nominal bearing diameter, and p has
the same units as pressure.
The surface velocity sum, U=Ui + U 2,is replaced by

where n' = n l+n 2 is the sum of the rotational velocities. Also, c may be
expressed in terms of the more commonly reported diametral clearance, c d,