Rubber O-ring Drive Belts

[Brian Rupnow]

Many of the small machines that I build to be powered by the engines I make are driven by o-ring drive belts. They work quite well, and they are cheap. I never put much thought into it before, as both engine and machine could be positioned far enough apart to use some O-rings I have on hand. Just make sure the o-ring is stretched enough to transmit power and not slip. And therein lies the question of "How much do I stretch them?"  My current build, the sawmill edger is going to require some drive belts where the pulleys involved can not be moved apart. They are fixed in place by the design of the machine. So---first step will be to take an o-ring I already have, calculate the diameter of the o-ring and put it between two pulleys of the correct size and check the center to center distance with the o-ring not stretched at all.

[Firebird]

Hi Brian

I have used the O rings supplied with oil filters. They range from 2 1/2'' dia to 4'' approx.

Cheers

Rich

[Brian Rupnow]

Here we have an old 1/8" nominal cross section o-ring which I've had for a few years. (It actually measures at about .138" cross sectional diameter). When you lay the two sides close to one and other and roughly measure the length out to out (which is difficult with only two hands) and measure it, the length is about 10.75". Take double that dimension, and you find that the total length is 21.5". Divide that number by 3.14" and it shows that the actual diameter is 6.847". We round that off to 7" and that is the diameter of the o-ring. O-ring diameter is calculated on the outside diameter of the o-ring, not the centerline nor the inside.

[crueby]

The Unimat lathe I used to have before the Sherlines used o-ring 'belts', they were fairly thick, something like 3/16" or so. Held up very well, quite strong, needed to be stretched over the pulleys to keep from slipping. I think they were urethane.

[Brian Rupnow]

This picture shows my mill kitted up with two 2" diameter pulley blanks. The o-ring is slack with only enough distance between the two "pulleys" to keep the belt from falling off. The pulleys are currently 7 3/4" apart center to center.

[Mcgyver]

On a round or V belt I think not slipping is predominately about having it properly grabing in the V groove (not bottoming out).   A soft material properly seated in a V will really resist slipping with minimal tension, otoh if it bottoms out its likely to slip even with high tension...at least on a round belt.

You can get O rings of just about any dimensions, and lots of different materials.   For machines I like a soft urethane belting....but a model would look much better with black belts vs the safety colours urethane belting comes in.  Another nice part about the O rings is you can get sets the same size for that multiple belt look...it would be tough to make urethane ones that identical in length

[Brian Rupnow]

The center to center distance of the pulleys had to be increased to 10 1/4" before I felt that the tension was adequate to transmit torque without slipping. That is a total of 2 1/2" more. That works out to a factor of  1.32% of stretch required to attain good o-ring grip.
1.32 x 7.75"=10.25". Lets call it 1 1/3.  The inverse of that should work as well. If the center to center distance on two of my pulleys is 5.288", I should be able to buy an o-ring of 33% less than 5.288", which would work out to 3.49" center to center. If I have 2" diameter pulleys, the true length of the o-ring would be 2 x 3.14=6.28" +3.49 +3.49=13.26". Divide the 13.26 by 3.14 and I get 4.229" diameter o-ring. So--I would buy 4 1/8" diameter o-ring.  This is all pretty subjective stuff, but it gives me a point in the right direction for buying O-rings.

[Mcgyver]

The center to center distance of the pulleys had to be increased to 10 1/4" before I felt that the tension was adequate to transmit torque without slipping. That is a total of 2 1/2" more. That works out to a factor of  1.32% of stretch required to attain good o-ring grip.

They're not going to work that well on a flat surface - try it with a V cut that the belt wedges in.  I bet you'll need minimal tension which will reduce the force on the bearings, friction etc

[Dan Rowe]

Brian instead of O rings you might consider watchmakers lathe belts.
https://www.ebay.com/itm/3M-2-8mm-Transmission-Belt-Watchmaker-Lathe-Jewelers-Belt-Watchmakers-PU/281732790200?hash=item41989537b8:m:mvQMR9GoCJaea-6VXJ2fFaQ

Dan

[ddmckee54]

My old CNC router was converted from a New Hermes engraver.   The spindle on that sucker was initially driven by an o-ring that was over 80" long.  It took a really serpentine path that allowed it to maintain an even tension on the belt no matter where the spindle was located in the work envelope.  The belt was polyurethane but it used both flat pulleys and vee pulleys.  The vee pulleys kept the belt tracking properly, and the flat pulley was used to allow the spindle to raise and lower without affecting belt alignment. This was about a 1/8" diameter belt and it had a 60 watt motor driving it.  It must have given somebody nightmares when they were designing it.

The raw poly-urethane belt stock is dirt cheap and all you need to make a custom size o-ring is a razor blade and a soldering  iron.

Don

[Brian Rupnow]

Dan Rowe--Thanks for the information. I didn't know about that stuff. I will consider it.---Brian

[Rustkolector]

I have used standard O-rings for water pump, generator drive, and polishing tumbler drives. Like the old steam engine rope drives, I groove my pulleys to match the cross section of the O-ring belts. The half round pulley groove gives maximum O-ring/pulley contact decreasing the tension required. For greater torque loads just use larger diameter and/or multiple O-rings. Tension needed is only slight, but I have never measured it. Visit this site for round and O-ring drive belt info:

 https://www.globalspec.com/MyGlobalSpec/NewProfile?forcetok=1&GoToUrl=%2Fsearch%2Fproducts%3Fpage%3Dms%23sqid%3D34184354%26comp%3D319%26show%3Dproducts&areaId=319