Design of press fits

The following aspects play a role when designing press fits:
Stress relaxation
Coefficient of friction
Poissons ratio
Surface pressure between hub and shaft
Stresses
Axial force
Torque
Assembling

Stress relaxation

Stress relaxation in the plastic may lead to loosening of the connection. Isochronous stress-strain curves can supply information about the stress relaxation that will take place, see figure below. The relaxation modulus

should be used in the calculations to determine if the connection will still function during the design life time t. The isochronous stress - strain curves of the material under consideration can be found from the material database and selecting the "fct" tab under properties.

The stress level immediately after assembling is σο in this example, at a strain level εο. After a certain time the stress reduces to σ(t), while the strain remains constant.

Coefficient of friction

The external force or torque that can be transmitted by the connection depends on the coefficient of friction μ.

Poisson's ratio

Poisson’s ratio must be known to calculate the surface pressure between the shaft and the hub, and the material stresses in the shaft and the hub.

Surface pressure between hub and shaft

The theory for thick-walled cylinders is outlined below, neglecting edge effects.

General case

Various cases can be distinguished where the calculations change slightly:

Plastic hub – metal shaft
Plastic shaft – metal hub
Hub and shaft of same plastic

Stresses

Axial force

The axial force F required to press the two parts together and the axial bearing capacity of the connection can be approximated by the equation:

Torque

The maximum torque M that can be transmitted by the connection is approximately:

Assembling

Assembling can be made easier by warming up the hub and/or cooling down the shaft, thus reducing the interference. The change in diameter Δd can be calculated as follows.

Δd = ΔT . α . d

where

ΔT = temperature change
α = coefficient of thermal expansion
d = initial diameter