Page 57 - Engineering Plastics Handbook

Products and Design 31

■ Rectangular plate, all four edges fixed, load uniformly applied
■ Rectangular plate, all four edges simply supported, load uniformly
applied

Design for Assembly
Press-fitting
Press-fit assemblies are extensively used in high-performance applica-
tions. PAI press-fit suspension bushings are used in race cars [20]. Drill
bushings are press-fit assembled [21]; solderless connectors are press-
fit into printed circuitboards [22]; compliant press-fit connectors are
assembled with bi-spring press-fit contacts [23]; security magnet sen-
sors with crush-proof plastic casings are press-fit installed on doors,
windows, and cabinets [24]; and General Electric Home Security sen-
sors are press-fit assembled [25].
The maximum allowable diametral interference between a shaft and
a hub for press-fitting assembly is a function of part geometry, stress,
modulus of elasticity of shaft and hub, and Poisson’s ratio for shaft and
hub materials. When the shaft and hub are made of the same material,
the moduli of elasticity are the same and Poisson’s ratio is the same for
the shaft and hub, simplifying the equation for maximum diametral
interference. Calculating diametral interference is simplified when the
shaft is made of high-modulus material.
Temperature variations during operation cause total interference
changes with dimensional changes of the shaft and hub. Total interfer-
ence at a given temperature above room temperature is determined
with the following equation [6]:
T
T
i = 2(R s T − R )
i
T
where i = interference at a given temperature, mm (in)
R s T = inside radius of shaft at given temperature, mm (in)
T
R = radius of hub at given temperature, mm (in)
i
T = given temperature, °C (°F)
Optimum insertion force is a high-value property for press-fit assem-
bly. It is determined by the following equation [6]:

= 2πµhR p = F
F in s c out
where F = insertion force, N (lb)
in
µ= coefficient of friction
h = joint depth, mm (in)
R = radius of shaft, mm (in)
s