F1 in schools wheel dimensions and bearing fitting

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poryongming
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Joined: Thu Feb 22, 2018 6:05 am

F1 in schools wheel dimensions and bearing fitting

Post by poryongming » Mon Mar 12, 2018 3:54 am

Hi all, I am involved in a project called F1 in schools-there have been previous threads on this forum. Basically we need to design and make using CAD/CAM a model car, which is powered by a CO2 cylinder. The time taken to cover a 20 meter track is recorded. I'm having some problems deciding what is the best fit for the bearings to fit the housing as well as the wheel shaft. The recommendation seems to be a 'press fit', but what would be the difference in dimensions to design in, with regard to the housing diameter versus the bearing outer diameter, and with regard to the shaft diameter versus the bearing inner diameter? I am thinking of using ABS/Acetal for the housing, and Acetal for the shaft. Since the car is powered by the discharge of CO2 rather than the wheels, the bearing loads are going to be low. Thanks in advance!

Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: F1 in schools wheel dimensions and bearing fitting

Post by Greg Locock » Mon Mar 12, 2018 12:06 pm

the normal liits and fits assume steel or aluminum shafts and housings. in your case the plastic will deform plastically long before a press fit really starts working.

however even though that sounds bad it isn't, you need minimum bearing preload for minimum rolling resistance, so just stick them in with loctite.

poryongming
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Joined: Thu Feb 22, 2018 6:05 am

Re: F1 in schools wheel dimensions and bearing fitting

Post by poryongming » Mon Mar 12, 2018 1:24 pm

Thanks Greg. I didn't realise those confusing tables assumed metal parts. OK then, will try not to make a mess of the loctite!:)

Mudflap
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Joined: Wed Feb 17, 2016 8:36 pm

Re: F1 in schools wheel dimensions and bearing fitting

Post by Mudflap » Thu Mar 15, 2018 12:21 am

There's two factors to account for - the interference should be low enough not to yield the plastic as Greg suggests but still high enough to prevent slip.
Lame's equations are typically used to determine interference stresses and contact pressures (from which friction forces/torques that oppose slip can be calculated).

In your case though - the practical thing is to do what he says and glue them in.
How much TQ does it make though?

fischele
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Joined: Tue May 15, 2018 4:24 pm

Re: F1 in schools wheel dimensions and bearing fitting

Post by fischele » Tue May 15, 2018 6:50 pm

Hello there. I think you wanted to use friction to keep the bearing where it's supposed to be.
We thought about mechanically locking the bearing in place with a nut and a thickening on the axis. (https://imgur.com/PSKmZ9u)
We also tried to fix the bearing with friction (POM - Al; surface approx. 30mm²), which worked fine as well (but won't be as reliable in the very long run).