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Finding torque on a rotating disk.
2

Finding torque on a rotating disk.

Finding torque on a rotating disk.

(OP)
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?  

 

RE: Finding torque on a rotating disk.

(OP)
Uh, yeah.  So I understand how to calculate torque.  My question was how to measure it, and how to turn those measurements into a torque value.  

Maybe I should word it like this.  I have axial and radial strain at some point on the face of a rotating disk.  How do I go from the strain to the torque being applied to the disk?

RE: Finding torque on a rotating disk.

CStoos,

   Is your torque due to acceleration, or steady state friction?

               JHG

RE: Finding torque on a rotating disk.

Why use axial and radial strain of the disc? Is there not an axial connection rod (or component) of the sort you can use to directly measure the 1D strain and correlate this into a force...then torque?
Or, why not measure the 2D acceleration of the disc....this can then translate into a torque (using the inertia of all the rotating mass). (Of course this is not valid if the system is connected to a brake or is static) (also assumes negligible friction)

peace
Fe

RE: Finding torque on a rotating disk.

Why not verify the torque at the end of the shaft that is coneected to the disk?

radial torque sensor?

Mfgenggear

RE: Finding torque on a rotating disk.

(OP)
Well what I am hoping to do is to be able to calculate generator efficiency.  I need to know the exact power coming from an engine to the generator.  The issue is size, space, and repeatability.

The only instrumentation option is the flex plate, or rotating disk.  Any other location is either too involved (meaning days of labor) or will result in false results by not accouning for engine accessory power loss and bearing friction.

Oh, and the measurement will be taken at steady state.  Well, steady RPM.  Because there are two forces involved it will never reach a true steady state as one force will be lagging or advanced at any point.

RE: Finding torque on a rotating disk.

You can either do the job properly, in which case you need a strain gage guy, or you can do it my way.

Stick an accelerometer onto the disk. zero the gage. Applya  known torque to to the disk. Observe the new reading on the gage.

Draw a straight line.

 

Cheers

Greg Locock


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RE: Finding torque on a rotating disk.

I'm not sure if this is what you have in mind but we used to put bellows (from which you can measure the pressure from compressing the bellows) between the engine and the frame to measure the rotation of the engine against the "flexible" engine mounts to determine engine output torque.  You need to calibrate the "flexibility" of the engine mounts vs. torque but this can be done with a torque wrench.

RE: Finding torque on a rotating disk.

(OP)
Bobber, that is an interesting concept and seems like it would work well on small engines.  The issue here is the engines I am working on weigh around 40,000 lbs and are enclosed, making any movement incredibly time consuming.  The goal is to cut cost while achieving accurate results.

This is what has put me on the strain gauge path.  I am thinking of a four gauge rosette placed on 45s with the radius.  My initial thought is this will eliminate any strain due to flexure of the plate.  

I made a quick drawing of the system and will upload it, along with a picture to see if it helps clarify the issues invloved.

RE: Finding torque on a rotating disk.

You could also put compression load cells under the engine and/or generator mounts to measure the torque transmitted between the two, since the reaction torque is absorbed by the frame or base.  If your shims or chocks are thick enough you may be able to use the engine/gen jacking bolts to get enough clearance to slide a load cell in.  

As mentioned above, your best option, if you've got a few of these White-Superior units to test, is to buy or rent a coupler that will measure running torque.

There have been some discussions on here regarding measuring flywheel torque in real time, you may want to try searching these forums as well.

Brian Bobyk - Hoerbiger Canada

RE: Finding torque on a rotating disk.

CStoos,

   Can you disconnect your shaft and generator, and do a dynamometer on the engine?

   It is easy to rig up a brake.  You need to monitor speed, fuel usage and throttle settings for a batch of torque values.  

   Connect the shaft and generator and monitor the engine settings again.

               JHG

RE: Finding torque on a rotating disk.

(OP)
drawoh:  The only way to disconnect the generator is to move the engine forward 6-12 inches.  If that has to be done it will make it less cost effective than our current method.  

Bribyk:  The problem with the load cells is two fold.  First, it won't account for other accessories and since the test will be on different engines with different accessories we cannot accurately estimate their power draw.  The second is that the shaft is dialed in at 0.002in and any movement up could very well get it out of balance.  I like the thought though.

 

RE: Finding torque on a rotating disk.

You can certainly gage it to pick up torque.  Have you though about how you would get the power to the rotating gages and the signal out from them?

RE: Finding torque on a rotating disk.

If you're trying to measure the overall efficiency of the generator train, including the engine's driven accessories, my advice would be to measure the energy input to the engine, as with a flow meter.  Such that you can measure the fuel rate of the engine in Btu/hr or whatever units you like.  You should already be able to measure the Voltage and current output of the generator.  The generator itself you should know is going to be efficient by itself (around 95%)--the biggest loss in efficiency is going to be the engine itself--it could be anywhere between 30 and 40% efficient.

RE: Finding torque on a rotating disk.

Go to www.binsfeld.com for strain gage telemetry system and equations to relate strain to torque. I have their system that measures both static and dynamic torque.

Walt

RE: Finding torque on a rotating disk.

CStoos,

   Can you measure the reaction forces at the motor mounts?  You should fairly easily be able to calculate motor torque from this.

               JHG

RE: Finding torque on a rotating disk.

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.

RE: Finding torque on a rotating disk.

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.   

RE: Finding torque on a rotating disk.

(OP)
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.

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