Groove for round belt pulleys

Anyone know if there is a specific shape I should machine a pulley to accept polyutethan round belts?

"Our" Tubal Cain reckons 30-35 degree V up to 1/4" / 6mm, depth 1.25 x cord diameter.

HTH

Rod

That's interesting, Rod

GHT [for the Universal PillarTool] started at 50° included angle, and revised that to 40°

Looks like it's not too critical.

MichaelG.

I use 60* for the 3 mm fused belts that I use on my hot air engines, I make the top of the groove about 4 mm.

Ian S C

Returning to the original question ... I think we are all agreed that [except for idler pulleys] the best basic shape is vee groove, truncated if you wish, with an included angle somewhere in the range 30-60 degrees.

Any advance on that ?

MichaelG.

I have seen a round profile at the bottom of the V on older stuff but the modern belts are pretty compressible so it may be tricky to get it right. I think there is info in older copies of Machinery's.

As pressure on the belt needs to increase under load I think Tubal Cain's figures make sense. A more acute angle will help with that.

One thing I do know is that round belt into a old L1 (light 1) profile as on my Pultra is pretty useless for anything other than very light work on rather small diameters - what ever diameter goes into it.

John

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I'm happy to be corrected, but: I think that 'round profile' is just a tidy alternative to truncating the vee [i.e. I believe that the belt is intended to ride on the flanks of the vee, and not touch the profiled bottom]. Idler pulleys, such as those on the BCA are, of course, a different matter; in those, the radius of the belt is smaller than that in the pulley groove, to accomodate some mis-alignment.

MichaelG.

Old round belt pulleys were round bottomed, I think that was because they used round leather belts, a higher co. of friction. Modern plastic belting works better with a V groove, I don't know that the angle makes a lot of difference. Last I made was using a threading tool 60 deg. works well, but not high power.

I think I'll just leave people to figure out what happens when these systems are used under load.

I have seen something similar for leather belting but can't remember where. These are for manilla rope.

Anyway it would be tricky to work out on belts with different rates of compression with load but going on the above I would try the English one with no chance of it bottoming. On the other hand the belt coming out of the V may be "sticky"  of the angle is too acute.

John

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Edited By John W1 on 20/10/2015 12:19:41

Useful illustration, John

Thanks for posting it.

MichaelG.

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P.S.  I was [slightly] amused to see that both drawings dimension the Radius at the bottom of the groove as a decimal fraction of the Diameter of the rope ... Practical, I suppose, but it does seem odd draughting.

P.P.S. ... More importantly, I note that both varieties have curved faces to the vee.

Edited By Michael Gilligan on 20/10/2015 13:12:08

I seem to remember from my old leather belt driven Hobbies fretsaw that the belt tended to mould itself into a V shape. Presumably the same happens with a rope drive. Plastic belts are much more elastic, which may account for some differences in recommended groove profiles.

Rod

I know what you mean, Rod ... but isn't the English language frustrating sometimes !!

Plastic belts are much more elastic [and therefore don't exhibit plastic deformation]

MichaelG.

I'd guess TC obtained his views from the English version. Neither are supposed to bottom but I find that hard to believe on the American one.

It also mentions that some think curved faces are undesirable.

Personally I feel for anything home made that could use a V belt the smaller width ones that SimplyBearings sell are preferable. There are also sewing machine belts of a similar design to these in polyurethane but the drive doesn't seem so strong and the top of the belts are noticeably wider than the top of the V.

John

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Edited By John W1 on 20/10/2015 15:27:50

Touché. The decisions about mixing technical English with everyday usage can be sometimes be quite tough.

Cheers,

Rod

I recall that TC specified the reduced angle because the more own vees were prone to slipping with synthetic belts. It's a problem I anticipate with the Adept when turning large-diameter titanium and tough stainless.

Neil

You can run then really tight Neal. That's why I think it's best to give them a miss, cause unless they are getting towards that they slip with very little load on them.

V belts tend to tighten with load but I do wonder if that happens on the miniature types.

John

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I knew I had seen round grooved pulleys some where - HPC catalogue. Specially for polyurethane belting.

John

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Thanks, John ... I didn't know they did those

Shame about the prices though.

MichaelG.

Went out to the workshop and had a quick look at the Super Adept, the angle of the Vs on the pulleys ranges from about 90* on the lathe, to about 75* on the jack shaft, I know the motor pulley is 60*, because I made that one, I run 5 mm polyurethane belts.

Ian S C

BBC website today **LINK**

Rod