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Plastics and thermal expansion/contraction

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WCraig

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Hi:

Can I, say, boil a plastic part in water so that it will fit tightly on another part when cool?

I want to make replacement idler wheels for a used 1X42 sander I picked up:

idler wheel parts.jpg


The plastic parts on both idler wheels are chewed up and broken. Apparently it used to have urethane (?) tires over a harder plastic hub. I want to machine a one-piece wheel out of some unidentified plastic. The bearings fit into a channel in the original hubs so that they couldn't slip off. That seems improbable for me to do at home but I thought that if I make the bore in the replacement wheels a few thous undersize, perhaps I could heat them enough to go over the bearings. (I would leave a shoulder on one side for the bearing to register against.) When cool, hopefully it would be tight enough that it wouldn't wander while I'm using it.

The outside diameter of the bearing is 40 mm. If this will work, how much undersize should I make the plastic wheel? The finished outside diameter of the wheel is just over 2 inches ( about* 51 mm).

Thanks,

Craig

* everything is pretty worn so it is hard to say what the original dimensions really were.
 

RJSakowski

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Generally, plastics have a higher coefficient of thermal linear expansion than metals. Glass and ceramics are lower than metals. You can find linear expansion coefficients for various materials on line. If you multiply the coefficient by the temperature difference and by the dimension in question, you can figure out what your room temperature dimension must be to allow a slip fit when assembling and still give you an interference fit when the temperature has returned to room temp.
 

whitmore

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Can I, say, boil a plastic part in water so that it will fit tightly on another part when cool?

I want to make replacement idler wheels for a used 1X42 sander...Apparently it used to have urethane (?) tires over a harder plastic hub.
Yes, that's possible, but probably not ideal. The plastic might have some internal strains already, and any stretching could
contribute to cracks as the polymer ages and gets brittle. Bedding a bearing in a plastic wheel can be done with any reasonably
gap-filling adhesive, like epoxy putty, or even hotmelt glue.

As for the urethane tires, maybe just sections of thick flexible hose <https://www.mcmaster.com/5233k77> would
be sufficient; it's inexpensive, and it's stretchy (or the tube wouldn't bend) so the heat-to-fit plan won't compromise
the strength.

Some plastics (depends on the processing) change shape when heated, so rough-machine, boiling-water soak,
fine-machine might be a useful plan.
 

WCraig

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You know, adhesive is probably the simple answer. I like the idea of hot melt glue since it would be pretty easy to undo.

The sander has a universal motor and some kind of gearing that is LOUD. Like REALLY LOUD. I may not keep the machine or I may butcher it to drive it with a normal quiet motor. I need to get it in basic working order to decide what I'm going to do next. I don't want to put a lot of $$$ and effort into this if I end up deciding to sell it.

Thanks,

Craig
 

Latinrascalrg1

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Hotmelt glue as in "hot glue gun" would more then likely melt in this particular application. Epoxy would be a better choice in comparison.
 

WCraig

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Hotmelt glue as in "hot glue gun" would more then likely melt in this particular application. Epoxy would be a better choice in comparison.
This is a ball-bearing idler wheel. Why do you think the glue would get hot enough to re-melt?

Craig
 

Latinrascalrg1

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This is a ball-bearing idler wheel. Why do you think the glue would get hot enough to re-melt?

Craig
Speed and friction. It may not be enough to completely melt out but it sure could produce enough heat to soften the glue enough to allow bearing movement... over time and you have a failure just dont know when.
 

-steve

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I would use Tygon tubing warmed and stretched over the drum. Adhesives may be a poor choice, but if you do it try Loctite 401 with the 770 primer.
-steve
 
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