Machinery's Handbook, 31st Edition
Thrust Bearings 2433 10) Minimum film thickness, h min . By using the operating number, the value of α = dimensionless film thickness is found from Fig. 13 . Then the actual minimum film thickness is calculated from the formula: h b min α = In general, this value should be 0.001 inch for small bearings and 0.002 inch for larger and high-speed bearings. 11) Coefficient of friction, f. Found from Fig. 14. 12) Friction power loss, P f . This horsepower loss now is calculated by the formula , P fWU 33 000 f = 13) Actual oil flow, Q. This flow over the pad in gallons per minute is calculated from the formula . Q iabU 00591 α = 14) Temperature rise, Δ t. Found from the formula . t c fp 00217 α ∆ = where c = specific heat of oil in Btu/gal/ ° 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 tilting pad thrust bearing for 70,000 pounds thrust at 3600 rpm. The shaft diameter is 6.5 inches and a maximum OD of 15 inches is available. The oil inlet temperature is 110 ° F and the supply pressure is 20 pounds per square inch. A maximum temperature rise of 50 ° F is acceptable and results in a viscosity of 18 centipoises. Use a value of 3.5 Btu/gal/ ° F for c .
1) Inside diameter. Assume D 1 = 7 inches to clear shaft. 2) Outside diameter. Given maximum D 2 = 15 inches. 3) Radial pad width. a 2 = 15 7 4 inches − = 4) Pitch-line circumference. . B 2 7 15 34 6 inches π = + = a k 5) Number of pads. . . . i 4 346 08 69 # = = Select 6 pads: i = 6. 6) Length of pad. . . . b 6 34 6 0 8 4 61 inches # = =
Make b = 4.75 inches. 7) Pitch-line velocity.
. 12 34 6 3600 10,400 # = = ft/min
U
8) Bearing unit load.
, 70 000 614 psi
# # =
=
p
. 6 4 475
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