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Pulley design

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Martin Dowing17/02/2021 08:24:18
355 forum posts
8 photos

I need to make rather large pulleys (140-200mm) for standard 10 mm V-belt.

Grove is to be 9.7mm wide on the top and 11mm deep as theory and published projects are saying.

What are correct grove angles?

What I have found that tor a small pulley (up to ~3 inches) correct grove angle is 32 deg and for larger pulleys it is 38 deg.

I understand it, as it seems logical that due to larger contact area of big pulley frictional force per unit of contact area does not need to be so high because total contact area is larger.

I am aware that many people are making pulleys with 30 deg angle, regardless of size.

So what is correct angle for large pulleys?

38 deg or perhaps less?

Phil P17/02/2021 08:45:33
802 forum posts
194 photos

Page 2 on this PDF should give you the dimensions you require.


gerry madden17/02/2021 09:33:29
250 forum posts
131 photos

I'm not sure the angle is to do with pulley contact area. I think its to do with the distortion of the cross-section of the belt as it bent. The tighter the bend the more the distortion.

I got so confused by data on the net so when I made some pulleys I bent the belt into the working radius of the pulley and measured the resulting 'v' angle on cross-section. My measurements seemed to agree with some of the internet guidance and after making them they seemed to work too!


Mike Poole17/02/2021 09:43:24
3305 forum posts
73 photos

It probably isn’t too critical in a hobby workshop application. Belt slip is unlikely and premature wear of the belt or pulley is also unlikely to be serious. If you want to transmit full load 24/7 then the belt manufacturers recommended specifications would be best adhered to but as long as you are in the right area it will be satisfactory.


Bo'sun17/02/2021 11:27:57
602 forum posts
2 photos

For hobby use, I'd go along with Gerry. Just make sure the belt doesn't bottom out in the groove.

Martin Dowing17/02/2021 15:03:21
355 forum posts
8 photos

Thanks for all your comments. I will go for 36 deg., not exactly what industrial art calls for larger diameters (38 deg) but more than usually modellers are doing (30 - 32deg).

Reasoning is that pulley will be used quite a lot but because it is going to be made of aluminium, then this angle will be a little bit steeper to accomodate for wear.


Howard Lewis17/02/2021 16:29:42
6021 forum posts
14 photos

According to Kempe's Engineers Year Book, section E9//5 Z, A , B and C section belts have angle beteen 34 and 38, so 36 looks about right.


oldvelo17/02/2021 19:30:11
294 forum posts
56 photos

Hi Martin

Have you considered using "Poly Vee Belts" for much better performance than "Vee Belts" as an alternative. Much easier to fabricate with the ability to use much smaller driver pulleys. Classic example is Wood working hand held power planers.


P.S. Poly Vee Belt Drive lives at my house

Martin Dowing17/02/2021 20:57:08
355 forum posts
8 photos


System will be used for motor speed reduction, so sizes of pulleys have to stay as designed.

Smaller pulleys are already rather small and further size reduction would only bring unnecessary strain..

But yes, I do use poly-v belts because they perform so much better.


Martin Dowing17/02/2021 23:02:28
355 forum posts
8 photos


I think, I misunderstood you.

Do you mean belts like those used for example in washing machines?

oldvelo18/02/2021 00:42:57
294 forum posts
56 photos

`Hi Martin

Yes washing machines, motor cars, and almost all modern machines use Poly Bee Belts

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