Fhp

63,000

where N = rotating speed, rpm

Also

A = surface area of bearing = pDL, in2 (m2) u = surface speed = p^Q^ in/s (m/s)

Substituting, we obtain m L DN

In obtaining approximate losses for estimation purposes, the concentric clearance c is substituted for the local film thickness h.

If we consider the previous example where D = 5 in (127 mm), L = 5 in (127 mm), c = 0.0025 in (0.064 mm), N = 5000 rpm, and m = 2 X 10-6 lb • s/in2 (14 X 10-3 Pa • s), the horsepower loss is

FHP = (766,000)(0.0025) = 16.32 hp Note that for thrust bearings, the frictional horsepower loss is mN2 OD4 - ID4

where OD = outside diameter ID = inside diameter

A general rule of thumb is that the frictional horsepower in a thrust bearing is approximately twice that in a journal bearing.

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