View Full Version : O ring drive belt.

12-26-2009, 05:37 PM
I have made an high speed cartrige spindle and wish to use an O ring drive belt to drive.

Something like 5:1 speed increase is the plan so the pulley at the spindle would be 40mm diameter and the drive pulley 200mm.

So the motor would run at 2800rpm and the spindle 14000rpm.(maximum)

I see these belts used on pantographs and tool grinders like THIS ONE (http://bbs.homeshopmachinist.net/showpost.php?p=488231&postcount=18) but am having trouble finding specs of this belting to calculate if it will work.

Will be used for 3mm carbide slot drills and I'd estimate 1/4 hp would be a max load at the belt.

Any experience or advice?

Thank you

12-26-2009, 05:51 PM
Iu have a little watchmakers lathe and wanted to drill 0.020" holes in jets, had a Viton ring as a belt, it got hot, i toched it some time after and burnt my fingertips with the acid that came out of it!, changed to Neoprene!
[it still works]

12-26-2009, 05:56 PM
Any help?


12-26-2009, 06:39 PM
mc master Carr . Urethane belting . Its round comes 1/8 ,3/16 and larger diameter . it is orange in color. You buy by the foot and put it to gether your self by melting the ends. Works real good and does not slip.

12-26-2009, 08:30 PM
Hmm. A 40mm diameter pulley at 14,000 rpm- that belt is going to stretch and swing wide. I see that on my tp grinder with neoprene o-ring, and it doesn't spin that fast. The driven pulley on that is about 30mm dia at the bottom of the groove.

Some kind of corded belt might be a better choice- just my opinion. You also have torque to consider- how much slippage is there going to be under load- belt will get hot, power will be wasted, pulleys will get hot and transfer that heat to the spindle-

I would personally try to use a flat belt like a power head on a vacuum cleaner uses. It can still be a non-corded one, or you could make your own corded belt like I have done. It's not that hard, and is an interesting project. I've done that in dacron and in polyester, using tool handle coating called Plasti Dip. You'd need a tp grinder to surface the layups.

12-26-2009, 09:09 PM
I am using a 1/4" o-ring with a 1/2 hp brushless motor on my mill right now. Maximum is 3000 rpm on the drive motor. The fact that the small pulley will be turning at high rpm isn't going to make the belt go wide Darryl, especially on a small pully. I use o-ring drive up to 35000 rpm on the driven pulley.

12-26-2009, 10:57 PM
Hmm. I'll have to check the rpm on my tp grinder- maybe it's higher than I think. I'm using a 1/4 inch o-ring on that, and it does blow out-

Just checked- I can get the driven spindle up to about 15k rpm and the belt is just about leaving the pulley. Maybe it's weak by now though.

12-26-2009, 11:18 PM
It the rim speed of the driving pulley that counts. The belt won't be going any faster than that regardless of ratios.

12-27-2009, 12:33 AM
http://www.glacern.net/free_photo_upload/belt%20stretch.jpgWell, my driving pulley is 3 inches diameter, with the driven one being 1 1/4 diameter. Davids spindle doing 14krpm with a 40mm pulley is going to have about the same belt speed as mine, so the o-ring will have to be stronger than mine currently is, or it will stretch almost to the point of coming off. My belt (o-ring) is weak now I'm sure, so the result is worse than it could be. I could just replace my ring and be able to run the spindle faster, but I don't usually care to run it any faster than I normally do, about 7-8 k. This is with the spindle going about 14k.

12-27-2009, 05:53 AM
A pic is worth 14k rpm...

Without internal supports, a round belt really expands.

Most Dumores and commercial grinders ran a thin flat belt... Wonder why ???

HP transferable, depends on width/thickness of belt (for flat belts) speed, and diameter of pulleys.

12-27-2009, 08:40 AM
Lots of helpful suggestions, thanks:)

sounds like Evan is doing what I want to do so I'll press ahead...

Evan, would that setup run for an hour with the belt not overheating?

Here is where I am at, this setup works but is nowhere near fast enough. The drive pulley is crowned and has no teeth, tensioner not shown. the ratio is about 2.3:1 giving about 6000rpm


A bigger drive pulley is not going to work driven off the spindle because there just isnt enough room so I need another motor mounted somewhere.

12-27-2009, 09:35 AM
I've run my mill just lately for several hours under heavy load facing off a new sub table and the belt is holding up just fine.

A.K. Boomer
12-27-2009, 11:06 AM
It the rim speed of the driving pulley that counts. The belt won't be going any faster than that regardless of ratios.

True but there are limitations within the ratio's and some can and will cause deflection and stretch due to the nature of the O-ring not having thread reinforcement ---- the smaller the driven pulley the greater the the return curve angle - this is fine if belt speed feet per second is kept within certain parameters but like so many problemo's this situation has two negatives working against it -
One is the belt creating its own internal "stir fry" (internal heat) as the smaller the radius is not just added to the FPS its more like multiplied,
Two - while the belt is getting all weak and mushy it also has another compound effect that's trying to destroy it - getting from the point A entry of the small pulley and changing its direction to exit point B, due to the belt having mass as darryl pointed out this can cause deflection --- actually seeing the deflection that is causing the belt to lose grip on the smaller pulley is as simple as looking at the trailing side of the belt leaving the pulley,

Due to the rubber not wanting to immediately conform it will try and keep the small pulleys radius so what you will end up with is actually an inboard hump instead of a straight line directly back to the larger pulley, The more you see this hump you can also count on one other thing, although you cannot witness it the belt that's leading into the smaller pulley just got done with a huge straight line - the last thing it wants to do is be immediately bent due to both its internal integrity AND the immediate change of direction --- so the first part of the smaller pulley that's engaging the belt is actually barely in contact with it - the higher the speed the more degree's of this critical contact area you give up...

This compounded "tug of war" between these forces are met with a slight countering force as the entire belt is wanting to go into a circular pattern - but this is no match for the forces that are going on directly at the radius of the small pulleys entry and exit.

loose nut
12-27-2009, 11:17 AM
Iu have a little watchmakers lathe and wanted to drill 0.020" holes in jets, had a Viton ring as a belt, it got hot, i toched it some time after and burnt my fingertips with the acid that came out of it!, changed to Neoprene!
[it still works]

Breaks down into Hydrofluoric acid under heat or flame, very nasty stuff.

12-27-2009, 11:53 AM

Try a smaller length "belt", so it has some loading.

12-27-2009, 12:56 PM
That's right, they need to be under a lot of stretch to work right.

12-27-2009, 04:18 PM
Agreed. When new, the o-ring was tighter. It hasn't been changed since I made the thing.

One factor that does come into play is that when the ring is blown out like that, it does become a little tighter, so the lesser contact area with the driven pulley is at least partially offset by a little higher contact force. I don't know if that logic holds up, but it sounds ok right now :)

My gut feeling says that even though the o-ring will work in this application, a flat belt would be better, and one with a fiber layer to resist stretching would be better still. I'd be inclined to go with a flat section on the driven pulley for best contact area with the belt, and rely on a humped drive pulley and maybe the idler too to keep the belt aligned. That's just my thought- I need to experiment more with this to see how well it would work in practice.