2.4 Press Fits
A press fit is used when available space and torque to be transmitted
is limited. Tolerances of mating parts have to be closely controlled to assure a minimum
and avoid all excessive interference.
Formulas for press fit are:
where:
- p = unit pressure on the interfering surfaces (lb/in2)
- e = amount of interference (in)
- E = modulus of elasticity (psi)
- d = shaft diameter (in)
- m = coefficient of friction (assume 0.1-0.2)
- L = length of interference surfaces (in)
- S = combined stress resulting from circumferential tension and radial compression (psi)
- T = slip torque (to be divided by safety factor of 2) (lb in)
Graph gives value of p for different d/D ratios and different values of e.
Table 2-4
Oil impregnated sintered bearings are manufactured under strictly
controlled conditions, and are subjected to in-process inspection. They are tested for
radial crushing strength of magnitude:
where:
Should additional shaft to bearing clearance be required, a ball
burnishing operation should be used for the following reasons:
a) to maintain concentricity
b) to maintain surface finish of I.D.
c) to reduce contamination of surface
The required size of the bearing can be determined from equations:
where:
W = bearing load (lbs)
L = bearing length (in)
N = shaft speed (rpm)
d = bearing I.D. (in)
Above values are reasonable for the following conditions: continuous
rotation, oil impregnation without additional lubrication.
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