Nitrile O-ring belt drive - suggested stretch percentage..?

Hello all,

I'm currently building a small parts tumbler in which I intend to tumble aluminium parts amongst 3mm porcelain beads.

Some pics below:

Attachment 12364 Attachment 12363 Attachment 12362 Attachment 12361

Although the 12v 350mA 20RPM motor spins the tub reasonably well (provided that the static friction remains low) I'm thinking of adding an identical motor below it and connecting it via a nitrile o-ring belt so that it's 'helping out' the motor that's directly driving the tub, like so:

Attachment 12365

Now, let's say that an O-ring with an inside diameter of 100mm fits perfectly (un-stretched) around the two couplers:

Attachment 12366
I'm guessing that this wouldn't be ideal as there'd be very little traction or transfer of torque - so my question is: how much stretch percentage should I introduce to the o-ring for it to do its job as a conduit between the two motors? 10%? 20%?

Cheers!

Wal.

Why not just introduce a third pulley as an idler/tensioner and have a play? G.

Would this be a better arrangement ? two motors at the back.

Attachment 12372

With any of these systems, do you not think the motors need synchronising?

Geoffrey - agreed that experimenting is good, but rather than add a pulley I may just buy a selection of o-rings at varying inside ø's...

Eddy - yep, I had a look at a driving roller and a second idling roller design, trouble is that the motors I have run at 20rpm - so the tub would rotate at about 2rpm in that config...

I didn't want to get into bigger motor territory - 12v small and quiet(ish) was the aim!

At 60p/o-ring I may just buy a handful and report back on what works, just thought there might be a 'rule of thumb' for these things.

Wal.

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Originally Posted by GEOFFREY

With any of these systems, do you not think the motors need synchronising?

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Nah... They'll take their power off the same source - there may be a smidgen of discrepancy when they start spinning, but as long as they're spinning at the same rate they should sort themselves out...

Wal.

Use a flat belt with a spring loaded idling tension pulley between the two motors.

Experimentation will be the key.

I'd say try running the tumbler with just the 'helper' motor connected to see how much effect different o-rings have, as that way the motor/o-ring will get put under a good bit load, and let you know if it's going to work or not.

may I be so bolt as to ask why you wouldn't simply use a washing machine motor and drum

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Originally Posted by nobby

may I be so bolt as to ask why you wouldn't simply use a washing machine motor and drum

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Because:

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Originally Posted by Wal

I didn't want to get into bigger motor territory - 12v small and quiet(ish) was the aim!

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:)

Hi Wal, I cant help but think an o ring may have the tendency to slip ?

Could you not swap the couplers or machine them into pullys and then go with toothed belts?

.Me

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- Nick

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Originally Posted by Wal

I'm guessing that this wouldn't be ideal as there'd be very little traction or transfer of torque - so my question is: how much stretch percentage should I introduce to the o-ring for it to do its job as a conduit between the two motors? 10%? 20%?

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Motor power rating is 12V*0.35A=4.2W at 20RPM so available torque=4.2/(20/30*pi)=2Nm. Roughly.

The torque is transferred due to friction between the belt and 'couplers'. So the way to work it out is to calculate the friction force and multiply by the coupling radius to get the torque transmission capability. The force depends on the spring constant of the o-ring, which you could measure by handing weights from it and measuring the deflection, or use the young's modulus to calculate it ... but that gets involved as it's an elastomer so doesn't care too much about Hooke's law (the modulus is only constant for small deflections).

So use the co-efficient of friction between rubber and aluminium (about 0.5 - citation needed!) to find the required force - I'm guessing the couplings are 25mm diameter, so F=2Nm/25mm=80N which means tension is 80/0.5=160N. That's about the weight of four domestic cats ... according to Wikipedia the tensile strength of nitrite rubber is (at least) 10Nmm^-2, so 0.25cats/mm^2 which means you need at least 4/0.25=16mm^2. That's a (16/pi)^0.5=2.3mm radius, so 4.6mm diameter ring. Roughly. Add a bit more to be safe etc...

Feel free to experiment, but I think you'd be better off with something more substantial.

Quote:

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Originally Posted by Jonathan

Motor power rating is 12V*0.35A=4.2W at 20RPM so available torque=4.2/(20/30*pi)=2Nm. Roughly.

The torque is transferred due to friction between the belt and 'couplers'. So the way to work it out is to calculate the friction force and multiply by the coupling radius to get the torque transmission capability. The force depends on the spring constant of the o-ring, which you could measure by handing weights from it and measuring the deflection, or use the young's modulus to calculate it ... but that gets involved as it's an elastomer so doesn't care too much about Hooke's law (the modulus is only constant for small deflections).

So use the co-efficient of friction between rubber and aluminium (about 0.5 - citation needed!) to find the required force - I'm guessing the couplings are 25mm diameter, so F=2Nm/25mm=80N which means tension is 80/0.5=160N. That's about the weight of four domestic cats ... according to Wikipedia the tensile strength of nitrite rubber is (at least) 10Nmm^-2, so 0.25cats/mm^2 which means you need at least 4/0.25=16mm^2. That's a (16/pi)^0.5=2.3mm radius, so 4.6mm diameter ring. Roughly. Add a bit more to be safe etc...

Feel free to experiment, but I think you'd be better off with something more substantial.

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Foooking hell all that just to say get a fatter O-ring.!!! . .:hysterical:

Use an old tumble dryer, they are quite but the drum might need lining, or use the gear you have and put the belt around the drum and the motor pulleys the same as a tumble dryer.

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Originally Posted by JAZZCNC

Foooking hell all that just to say get a fatter O-ring.!!! . .:hysterical:

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Hehe, I see your point, Dean, but thanks and maximum respect to Jonathan for a) knowing this stuff and b) taking the time and effort to explain the technicalities - even if they are somewhat wasted on a pea-brain such as mine..!

After all that, it turns out (no pun intended) that the motor does have sufficient torque to spin the tub - just. I need to leave a part in there for a day and see what happens. I have a feeling that 15rpm on a smaller drum such as mine isn't going to be enough...

Wal.

Find an old tumble dryer that has a plastic drum complete with motor. Or just use one motor and a long belt around your tub, like the TD does. Cut a couple of rings out of some ply to fit round the tub glued and screwed to stop the belt from coming off. Simple and cheap just like me LOL
Just read eddy's and he's thinking along the same lines as me.

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Originally Posted by longy

Find an old tumble dryer that has a plastic drum complete with motor. Or just use one motor and a long belt around your tub, like the TD does. Cut a couple of rings out of some ply to fit round the tub glued and screwed to stop the belt from coming off. Simple and cheap just like me LOL
Just read eddy's and he's thinking along the same lines as me.

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The reason has been stated before:

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Wal: I didn't want to get into bigger motor territory - 12v small and quiet(ish) was the aim!

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Possibly not to get into 230V territory ..Clive

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Originally Posted by Clive S

The reason has been stated before: Possibly not to get into 230V territory ..Clive

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Well that's true but it may be that Wal does not feel confident working with 230v or he may not want to be hassled with the associated control gear for a larger 230v motor ?
That's why I suggested the tumble dryer, it's just a plug and play device, the motor is quite small, it's quiet

Some where i've got a 12/24Vdc motor with geabox that would do this but not sure how fast it spins. I'll check it out and if it's fast enough your welcome to it just pay carriage.

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Originally Posted by EddyCurrent

Well that's true but it may be that Wal does not feel confident working with 230v or he may not want to be hassled with the associated control gear for a larger 230v motor ?
That's why I suggested the tumble dryer, it's just a plug and play device, the motor is quite small, it's quiet

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Hi Eddy,

Yeah, the main reason I'd like to stick to 12VDC is that it's compact and straightforward. Out of interest I had a look at some dryer motors on ebay, this seems like a decent, cheap example:

Hotpoibt Vtd Indesit Creda Series Tumble Dryer Motor | eBay

...and as luck would have it, here's a vid of what appears to be a very similar motor in action:

https://www.youtube.com/watch?v=WGJtecRxzHo

You can see that it would require a fair bit of gearing to get it down to 30RPM - there goes the compact and straightforward..!

With my approach, I'm a gnat's chuff away from something that I'd really be quite pleased with - if I could just find a 12VDC 30RPM motor with a torque rating of 300Ncm... Hey, I'd even try a compact 240AC... Something along these lines would be great:

High Torque AC 220V 30RPM Gear-Box Electric Synchronous Gear Motor Replacement | eBay

if only it had 4 times the torque...

As ever, thanks to all for your input!

Wal.

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Originally Posted by JAZZCNC

Some where i've got a 12/24Vdc motor with geabox that would do this but not sure how fast it spins. I'll check it out and if it's fast enough your welcome to it just pay carriage.

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Nice one Dean. No mad rush, drop us a line when you find it..!
:beer:

Wal.

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With my approach, I'm a gnat's chuff away from something that I'd really be quite pleased with - if I could just find a 12VDC 30RPM motor with a torque rating of 300Ncm... Hey, I'd even try a compact 240AC... Something along these lines would be great:

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Wal is that bigger or smaller than a bees dick or a knats cock? :: We will have to catch up some time. ..Clve

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Originally Posted by Clive S

Wal is that bigger or smaller than a bees dick or a knats cock? :: We will have to catch up some time. ..Clve

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Hehe... Hard to call that one, Clive - you're comparing gasses with solids...

EDIT - Actually you're not - never mind arse/elbow, I think I've mixed my chuffs up...

Deffo catch up one of these weekends - hope all's well your way.

Wal.