Reinforcing Press Fits
Press fits between plastic parts and metal shaft are considerably more effective if the metal surface is modified in an appropriate manner. In extensive trials, chequered knurling has been found to be very well-suited as an aid to a positive connection.
Press fits are based on the principle of converting into a mechanical stress the interference between two joined surfaces. These normal stresses then allow axial forces and torque to be transmitted. In the case of press fits involving plastic parts, however, the forces and moments that can be transmitted are limited, because stress relaxation takes place due to the viscoelastic behaviour. Suitable geometric modifications to the metal shaft can result in a positive fit that aids, or reinforces, the friction fit. This measure can increase the amount of force that can be transmitted, while simultaneously slowing the relaxation process.
At the Institute for Materials Engineering and Polymer Processing (Institut für Werkstofftechnik und Kunststoffverarbeitung / IWK) the axial and azimuthal reinforcing effectiveness of different left-right knurling was investigated. As a rule, knurling combines both an active and a passive component: One the one hand, the peaks of the knurling displace the polymer; however, the polymer deforms with time to fill in the intermediate depressions.
The test specimens were POM copolymer hubs in the form of a sleeve and a steel shaft with a nominal diameter of 15 mm. A dimension of 0.3 mm was chosen as the interference between the nominal diameter of the shaft and the diameter of the bore in the hub, corresponding to a strain of 2%.
Shafts with three different types of left-right knurling were employed, namely, two cross knurls and one chequered knurl. The axial reinforcing action of the positive fit was tested on a tensile testing machine. To measure the azimuthal reinforcing action, a testing device was designed and built in house.
The investigations confirmed that, in press fits involving plastic hubs, knurling on metal shafts is very well-suited to reinforce a positive fit. Chequered knurling was found to be the most effective. With regard to the reinforcing action in the axial direction, the two cross knurls were almost equivalent. The chequered knurling was less effective in terms of increasing the transmitted force, but slowed the relaxation quite noticeably. Interestingly, after 1000 hours, the chequered knurling also provided the greatest reinforcing action overall in the axial direction. As expected, the chequered knurling was superior regarding azimuthal reinforcing action. The two other geometries were considerably less effective.