# Torque and Force

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1. Jan 25, 2015

### Cyril Marty

Hi guys. I am doing a project involving a pedal powered rotary vane pump. I would like to ask how do I measure the Torque. I used a hollow steel shaft with a diameter of 0.75 inches. I was trying out the formula T=Fr but I have no idea how to measure the perpendicular force.
In the picture you can see the bicycle chain attached to the shaft then the pump (orange)
Any insights would be helpful. Thanks

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2. Jan 25, 2015

### sophiecentaur

Hi and welcome to PF.
From the picture, the shaft seems to be a reasonable diameter and I think you could probably manage to hang a webbing belt over it with a weight on one side and a spring balance on the other. This is a poor man's version of the type of dynamometer that's used to measure car engine brake horse power and torque. As you turn the shaft, the reading on the spring balance will increase to show you the friction force on the belt*. You can multiply this by the radius of the shaft to get the torque that's being delivered. The reading on the dial may vary a bit, due to uneven surface on the shaft but you could policy it up and reduce this variation. It would be a matter of doing some mental 'averaging' of the reading and a steady peddler. Of course, this is not the whole story as you need to find the Power as well. This needs you to know the angular velocity of the shaft and that can be found in various ways. Bicycles and exercise machines have meters to do this.

*which is the "normal" force you wanted.

3. Jan 25, 2015

### Cyril Marty

thank you very much for your response. I will get right on it.

4. Jan 25, 2015

### sophiecentaur

Good luck with it - if you are using pedals then the torque could vary a lot over the cycle and I wonder whether a flywheel (which would not waste any energy) could help - both in the measurement and in delivering a steady power to the pump.

OMG - I just saw the words "policy it up" in the previous post. Damned automatic spelling correction! POLISH!!!

5. Jan 25, 2015

### CWatters

Errr I don't think that works. The spring scale would display the torque delivered to the webbing/brake not to the pump. eg If the shaft isn't accelerating the relevant equation would be...

Tcyclist + Tbrake + Tpump = 0

I think you would need a different approach. Perhaps mount the pump casing on some sort of bracket and measure the strain in the bracket?

6. Jan 25, 2015

### sophiecentaur

I'm assuming that the pump would not be under load - of course. To measure the on-load torque would require some strain gauge on the shaft or measuring the tension in the chain. Much harder and less convenient. (I must say, I sort of took that for granted - as you do when measuring bhp for any engine.

7. Jan 26, 2015

### Cyril Marty

guys im having a hard time securing a dynamometer. is there another way for me to get the Power?

8. Jan 27, 2015

### billy_joule

How accurate does it have to be?
Being a positive displacement pump flow rate is linear with rpm.
You can calculate pump power vs rpm based on the inlet and outlet conditions & the swept volume. Find the volume/rpm from the spec sheet or a 5 minute experiment. A piece of clear tube can measure the pressure difference across the pump (U-tube manometer). get pump efficiency from the manufacturer or estimate (rotary vanes are >90% IIRC)

All you need is a cheap tacho to calc. instantaneous power (bicycle speedometers can be had for <\$10). Divide by theta to get instantaneous torque.

There are various ways to measure the losses from pedal to pipe and get more accurate data.