Machinery's Handbook, 31st Edition
Thrust Bearings
2425
4) Pitch-line circumference, B. Found from the formula B D D 2 1 2 π = + ^ h
5) Number of pads,i. Assume an oil groove width, s (0.062 inch may be taken as a mini mum), and find the approximate number of pads, assuming the pad length is equal to a . Note that if a chamfer is found necessary to increase the oil flow (see Step 13), the oil groove width should be greater than the chamfer width. i a s B app = + Then i is taken as the nearest even number. 6) Length of pad, b. Readily determined from the number of pads and groove width. b i B s = − 7) Pitch-line velocity, U. Found in feet per minute from the formula U BN = 12 8) Film thickness, h. Found in inches from the formula . h W ia UZ 20910 9 3 = − 9) Depth of step, e. According to the general parameter . e h = 07 10) Friction power loss, P f . Found from the formula . P h ia U Z 73510 f 13 2 2 = − 11) Pad step length, b 2 . This distance, on the pitch line, from the leading edge of the pad to the step in inches is determined by the general parameters . . b b 22 12 2 = 12) Hydrodynamic oil flow, Q. Found in gallons per minute from the formula . Q iahU 66510 4 = − 13) Temperature rise, Δ t. Found in degrees F from the formula . t cQ P 424 f ∆ = If the flow is insufficient, as indicated by too high a temperature rise, chamfers can be added to provide adequate flow as in Steps 12–15 of the flat plate thrust bearing design. Example: Design a step thrust bearing for positioning a 7 ⁄ 8 -inch diameter shaft operating with a 25-pound thrust load and a speed of 5000 rpm. The lubricating oil has a viscosity of 25 centipoises at the operating temperature of 160°F and has a specific heat of 3.4 Btu per gal per °F. 1) Internal diameter. Assumed to be 1 inch to clear the shaft. 2) External diameter. Because the example is a positioning bearing with low total load, unit load will be negligible and the external diameter is not established by using the for mula given in Step 2 of the procedure, but a convenient size is taken to give the desired overall bearing proportions. D 3inches 2 =
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