Press Fits

Press fitting is a simple, low-cost method for assembling parts or components.  A common example is the forced insertion of a metal pin or shaft into a plastic hub or boss.  The radial forces hold the shaft in place, although the interference fit can create very high tensile or hoop stresses in the plastic part.  This type of assembly may be unreliable if the plastic part is already weakened by a weld or knit-line.  The part may fail immediately, fail later under load, or stress-relieve to the point that it becomes loose.

Immediate failure from hoop stress creates a crack parallel to the axis of the hub, thus relieving the stress.  If there is no immediate failure, subsequent cracking can occur when an applied load increases the hoop stress to the failure point.  This is a greater concern with low-elongation materials.

The plastic material may also creep, or undergo stress relaxation such that the stress can be maintained at some lower level.  In this case, the radial force will also decrease, possibly to a point where it, in combination with the metal-to-plastic friction, is no longer enough to prevent the shaft from slipping

The stress generated on insertion of a shaft or pin depends on many factors, including the materials of boss and shaft, the molded shape and the operating environment.  There is a greater possibility of fracture if the plastic material has low elongation, as is the case with many filled or reinforced plastics.  If the molded part is out of round, assembly stress will be increased, leading to a greater possibility of failure.  Higher temperature, both during and after assembly can increase stress relaxation and the presence of oil, water or other fluid can also affect the press-fit.  

Important factors to take into consideration when designing a press-fit include:

• Interference: The interference force depends on the joint diameter and the polymer used.
• Coefficient of friction between shaft and boss: With the interference force, this establishes capacity to transmit torque or load.
• Relaxation modulus: All thermoplastic materials undergo stress relaxation over time and more so at high temperature.  When allowing for this factor, the initial fit must not overstress the plastic.
• Overall tolerance “stack-up”: Be sure to take this factor into account when estimating the press-fit stress.
• End-use testing: To insure product reliability, this should be done under actual operating conditions