# Thread: Nitrile O-ring belt drive - suggested stretch percentage..?

1. 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

2. ----------------------------------------------------------------------------------------------------------

- Nick
Last edited by magicniner; 12-05-2014 at 08:38 PM.

3. 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%?
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.
Last edited by Jonathan; 09-05-2014 at 10:55 PM.

4. 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.
Foooking hell all that just to say get a fatter O-ring.!!! . .

5. 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.
Last edited by EddyCurrent; 10-05-2014 at 05:40 PM.

6. Originally Posted by JAZZCNC
Foooking hell all that just to say get a fatter O-ring.!!! . .
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.

7. 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.
Last edited by longy; 12-05-2014 at 10:58 PM. Reason: eddy's post

8. 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.
The reason has been stated before:
Wal: I didn't want to get into bigger motor territory - 12v small and quiet(ish) was the aim!
Possibly not to get into 230V territory ..Clive

9. Originally Posted by Clive S
The reason has been stated before: Possibly not to get into 230V territory ..Clive
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

10. 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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