Plastic Bearings: Press Fit and Running Clearance

Posted: November 15, 2017 in Basic Plastic Tips

When machining a bearing at home often “DIY” fabricators will not consider that plastics have different tolerances than metals. Even more importantly, consideration if an application requires a press fit or running clearance isn’t considered at all. But it really should be to have a properly functioning bearing! To provide complete assistance on this topic we need to first define “press fit” and “running clearance”. Press fit is most often on the outer diameter of the bearing and it is a small amount of extra material on top of the designed diameter of the bearing to allow it to be forced into its mating partner (perhaps a wheel) and “stick” there without rotating. Frankly, if that is the type of application your bearing is going into – you need to be concerned with a press fit.

Same goes for a running clearance. In most cases the shaft in the center of your bearing needs to spin freely right? Did you account of that in the design of your bearing or did you plan to push through a 2″ inner diameter bearing over a 2″ shaft? What you’re most likely going to have is a shaft with a quasi-press fit that sticks on the plastic. Instead you need to factor into your design some extra room on the inner diameter (ID) of the bushing.

At this point you’re probably wondering how much press fit or running clearance? Redwood Plastics offers a handy online machinist chart showing guidelines for just that. You can find the chart here:

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