Finding torque on a rotating disk. 2

[CStoos]

I need to find a way to measure torque on a rotating disk.

The input force is applied on the outer radius (R) of the disk. The disk then outputs a force to a shaft on the inner radius (r).

I cannot instrument the shaft because space is very limited and a dyno can in no way be used for this measurement.

I have some instrumentation ideas as far as using strain gauges goes but I don't know how to get from the strain measurements to torque accurately.

Any ideas?

 

[TheTick]

Since you already know the math...

If there is a way to accurately monitor and record the speed in fine enough increments, you can get acceleration, thus torque.

 

[BrianE22]

You can get the relationship between strain and torque via two methods:

1) You can calculate the shear strain at your point of interest for a given torque and then use the gage factor to get the change in resistance of the gages. Go through the bridge balance equations and you will have volts/torque.

2) Apply a static torque and measure the output from the bridge. Can you lock up the output and use a crow bar of some sort to apply a known torque?

You've probably already figured this out, put the gages in a location that has the best combination of high torsional strain and low disk bending strains.

 

[CStoos]

Brian, that is pretty much what I am hoping to do. I just haven't been able to find a sufficient equation that can relate shear strain to torque in a rotating disk with a hole in the center.

Marks' Standard Handbook for Mechanical Engineers comes close, and Shigley's book throws in a couple of pointers, but nothing exactly correct. So I need to either find a solution to this problem or go in a different direction.

I don't have the stuff in front of me or I would throw some equations down. It seems as though I may have to measure deflection angle too according to one of my other books.