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Do interfere, but correctly Clive Maier explains the technique of press-fitting, what problems can occur and how to counteract them The PRW monthly Design Guides provide practical guidance for designers, toolmakers and moulders. Every month a different aspect of design technology is tackled and together these guides are becoming an indispensable reference point for those designing successful products. Having started with relatively basic design guidance, more sophisticated and detailed issues are now being addressed as the series progresses. When one object such as a shaft is assembled to another by forcing it into a hole that is slightly too small, the operation is known as press-fitting. Press-fits can be designed between similar plastics, dissimilar plastics, or more commonly between a plastic and a metal. A typical example occurs when a plastics hub in the form of a control knob or gear is pressed on a metal shaft. The position is reversed when a plastics sleeve or bearing is pressed into a metal bore. Press-fits are simple and inexpensive, but there are some problems to look out for. The degree of interference between the shaft and the hole is critical. If it is too small, the joint is insecure. If it is too great, the joint is difficult to assemble and the material will be overstressed. Unlike a snap-fit, the press-fit remains permanently stressed; it is the elastic deformation of the plastics part that supplies the force to hold the joint together. When plastics materials are exposed to permanent stress the result is creep. This means that as time goes by, the

Designers’ notes

Diagram 1: Press-fit parameters

● Consider the effect of part

tolerance and creep ● Consider the effect of

temperature changes between dissimilar materials ● If the press-fit will be used at elevated temperatures, verify the design by testing prototypes ● If you are still worried about creep, try knurling the metal shaft force exerted by the press-fit becomes less, although not necessarily to a significant extent. There are two other pitfalls for press-fits. Manufacturing tolerances on the shaft and hole must be taken into account to see whether the two extreme cases remain viable. And when the joint is made between dissimilar materials, an increase in temperature will change the degree of interference between the parts. Remember too, that at elevated temperatures the effect of creep will be greater. One way of countering the effect of creep in a shaft and hub press-fit is to provide a straight medium knurl on the metal shaft. The plastics hub material will tend to cold flow into the grooves of the knurl, giving a degree of mechanical interference between the parts. The frictional effect is also greater because the surface area of the joint has been increased by the knurl. When designing a press-fit, we need

to work out the correct amount of interference between the parts. Basing the calculation on classical theory for thickwalled cylinders, we can derive the following equation for the allowable diametric interference (Y) for a metal shaft in a plastics hub: Y=

Sd K

K+v E

Hub

Hub

where S = design stress, v = Poisson’s ratio, E = elastic modulus, K = geometry factor. The geometry factor K can be calculated from:

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() ()

1+ d D K= d 1D

2

2

The force (W) needed to press the parts together can be worked out from this equation: Sdlπµ W= K where µ = coefficient of friction and l = length of engaged surfaces. Values for Poisson’s ratio and coefficient of friction were given earlier in the Design Guide series. Clive Maier, Econology

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