How important is concentricity on motor pulley?

So I have an Alien Power Systems 6374 HEV which has a 10mm output shaft. I didn’t realize this and bought the DIY motor mount kit, which includes the 8mm motor pulley. So…no biggie, I took it down to the machine shop at school, he threw it on the lathe and presto, magic-o there’s a pulley with a 10mm bore now. It was a bit off, however. I put it on the motor, and spun by hand and the new hole is not centered completely. It is only off by a small amount, but enough that one can see the pulley move when the motor is spun. I don’t think the belt tension is so tight on these that it would matter, but…

Any ideas to whether or not this will cause problems? I don’t think it will but I would like to see if anyone has any experience with this? It maybe a millimeter off - haven’t gotten the calipers to it yet.

How did become off center on a lathe ,… …dont think it will work well now.

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It’s going to wobble. Your belts might not like that very well. They will get tight and loose during rotation.


Thanks for the input, @Randyc1! I’m not really sure. I was not allowed to machine it myself so one of the student workers had to do it for me. He grabbed it by the outer circle (where my fingers are holding it) and so he said maybe the outer and inner hubs were not concentric from the factory. I don’t believe this is the case, and more likely what happened is that he didn’t grab it completely straight (the pulley was not completely perpendicular to the clamp)…? He said looking back he should have flipped it around and grabbed it by the smaller inner hub to bore it out.

@SeanHacker, that’s what I figured as well…bummer. APS sells one made for their motors, but it would cost over $40 shipping included…Blah! I will just order another from DIY and have it lathed again, (I will keep an eye on him next time). I assumed he would grab it by the inner hub to begin with. Though I have had the course and can operate the machines, since I didn’t take the course AT THIS SCHOOL, I am not allowed to.

Yes !.. he probably grabbed it by the Flanges instead of the Hub.

I think you should just get a new one, to avoid other problems.

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Yeah you should go with a new one. Its really out of center not only a little! That way you stress your Motorbearings and will not find a good belt tension.

Its not that easy easy to machine longer pulleys by only grapping them at the hub. With my pulleys for 25mm Belt i got alot problems to do without a lathe, let them bore out by seller now. BTW 40€ for a ready machined Pulley with Keyway includet shipping is a good price IMO (-;


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I had the same issue, took my pulleys to a metal working shop to be drilled. I think the problem is that the keyway in the pulley drags it off centre slightly when being drilled. Its probably best to get new pulleys otherwise it would cause a vibration and maybe damage/wear to other components. I found that it caused the belt to slip more ofter aswell unless the tension was very high, which is not ideal either. In the end i switched to 2x 9mm belts and pulleys from alien drive as i was not able to find a 12mm pulley in the uk with a 10mm bore and correct teeth number. can Bore hole to desired Diameter 6,8,10mm with H7 precision.

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i have received one similar from enertion and i did use it although it was highly visible that the 8mm hole was not centered. But this will only affect the tension in the belt and it is not that of a big deal. I have been running it ever since and the belt is still in top shape and i see no problem with it.

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Thanks for the input, everyone! I ended up and ordered a 16tooth (I didn’t mean to order a 13 originally anyways, so I guess it was meant to be that I screwed it up). I think I read this somewhere but couldn’t find it again…Running a 15mm motor pulley with a 12mm belt is ok (12mm wheel pulley still)?

Hi DB. I see your last question was 21d ago but yes, a 15mm wide motor pulley is fine with a 12mm wide belt, especially seeing as you are using a 12mm wheel pulley.

Drilling pulleys is a job best done in a lathe if a key way is present. The key way will always spoil the job if using a simple drill press.

The outer belt guide lips on the pulleys are often crudely installed and cannot be relied on to be true with the toothed part. Always dial up the pulley using the toothed section so that when you bore out its perfectly true.