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Plastic Post and Hub Press Fit Equations / Calculator

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Plastic Press fitting provides a simple, fast and economical means for parts assembly. Press fits can be used with similar or dissimilar materials and can eliminate screws, metal inserts, adhesives, etc. Since plastic materials will creep or stress relieve under continued loading, loosening of the press fit, at least to some extent can be expected. Testing under expected temperature cycles is obviously required.

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Geometry Factor:

Geometry Factor


Ep = Modulus of elasticity of considered plastic hub or boss: psi, (N/mm2)

vp = Poissons Ratio of Hub when different from shaft

σa = Allowable design stress for plastic: psi, (N/mm2)

σs = Allowable design stress for plastic: psi, (N/mm2)

σm = Allowable design stress for plastic: psi, (N/mm2)

σ = Hoop stress: psi, (N/mm2)

i = ds - di = Diametral interference: in, (mm)

ia = Allowable interference: in, (mm)

Γ = Geometry factor

Case #1

Shaft and Hub have identicle or very simular materials. Hoop stress given i is:

Hoop stress

and the allowable interference is:

interference equation

Case #2

Shaft is metal and hub is plastic, Hoop stress is given i is:

Hoop stress

and the interference is:


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