Torsion bar spinning live-load meter?

Discussion in 'Prony Brake' started by DMahalko, Sep 5, 2008.

  1. DMahalko

    DMahalko Intermediate Poster

    While the Prony is useful for measuring engine power as the load for the engine all by itself, I am wondering about the possible ways to measure the operating load of an engine while it is supplying power to an implement other than the Prony.


    One of the simplest methods I can think of is to use a calibrated spinning torsion bar. This would basically just be two pulleys on a long steel shaft, with angle markers on each pulley. Attach an engine to the input pulley and a load to the output pulley.

    The greater the torque of the engine and the higher the implement load, the more this torsion bar will twist under tension and the greater the angle of separation will be between the two pointers.


    Direct observation of the spinning pointers would be possible with a strobe light, though it could also be handled with rotating digital encoders on each pulley.

    Gearing the two pulleys together with an axle differential would also show the angle difference with a steady pointer. As long as the two pulleys are geared to spin the differential axles at exact equal speed in opposite directions, the differential box will not move. If one pulley leads the other then the differential box will turn slightly:


    A torque measurement device like this probably already exists, but I do not know what it would be formally called.

    - Dale
  2. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member

    You're always thinking Dale... thats a good thing.

    How would you factor in the amount of HP lost by turning the extra load of your "torque measurement device"?
  3. DMahalko

    DMahalko Intermediate Poster

    All measurement devices affect the subject being tested to some degree. The scientists dealing with quantum physics know all about this little problem. :)

    Generally I would expect very low losses from a simple two-pulley axle since it floats on bearings and draws no load other than from bearing losses.

    There would be a measurement skew due to the inertia of the pulleys. The inertia causes a lagging of measurements behind real-time values. A direct PTO drive without pulleys would keep inertia to a minimum. None of this is a problem if you aren't looking for instant measurements recorded every hundreth of a second.

    The big problem to me would be gear backlash/slop in the differential measurement system. Using an actual vehicle differential could work but I would expect a large amount of slop in the gearing, probably more than the total measurement range of the torsion bar.

    It would need a way to tighten up the backlash, and this gearing tightness would be the main power loss since it adds more resistance against rotation.


    Heh, I have had an opportunity for several months now to buy some old WWII precision analog measurement devices known as Selsyns or Sychro motors.

    All About Circuits: Selsyn (synchro) motors

    These were used in early analog (non-digital) computers and were components used in torpedo guidance, radar systems, and anything requiring precision positional data to be transmitted over wires.

    But what the heck am I going to do with a pile of sychro-motors? Hmm, maybe I've found a use for them. ;)

    Selsyns could replace all the messy gearing and differential system, and they're very small. They look like motors and are about 3 inches in diameter and 5 inches long.

    Alas they require 60Hz AC to drive the field coils so this would need a small half-amp 110v generator to power the Selsyns. But I suppose they could run off a battery and a small AC inverter.

    - Dale
  4. M Kirschenman

    M Kirschenman Intermediate Poster WMSTR Lifetime Member

    Inline hp measurement

    If you really want to do this simply, forget the pointer buisiness and attach strain gages to the shaft and commutate the bridge signal out. This would give you torque and a standard tachometer would give you the RPM and hence hp.

    There is also a non-contact technology out there that use twisting of the mgnetic field to measure torque. I could dig info on that out if you're interested.

    It wouldn't be to tough to make a HP measurement device that you could put between any belted engine and a load; It could be kinda fun to see the power transfered when Jim B rams a 4' dia log through his sawmill of 4 guys try to slug a dual feed separater.

    Hey Jerry C -I think you need to build another piece of equipment (and tell Jane you need a longer trailer):rof:

  5. Spencer M

    Spencer M Intermediate Poster WMSTR Lifetime Member

    Build it! I would love to hear more about this planned device, it appears to me at least that it measures real time power output while a machine is being used.
  6. DMahalko

    DMahalko Intermediate Poster

    I am mainly just thinking out loud here. With three of these Pronies already and one person to run them, I don't mean to be piling more work on one person.

    Also I understand the purpose is to demonstrate antique technologies, and I don't think it would be very "historical" to use a load meter with digital strain gauges.

    - Dale
  7. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member

    Hi Dale,

    We had a small misscommunication. I am NOT the only person that runs the three Prony Brakes at Rollag. We have a very good group of people that spend a lot of time running the Red Brake. My problem is that I want to be involved with all of them!

    The live-load meter sounds like a good idea. Like all good ideas at Rollag it needs a champion. It needs someone to take the idea from conception to completion and then operate it during the show and/or assemble a crew to operate it. I am not that person for this project at this time. Maybe someone else will get fired-up about taking this on.

    Jerry Christiansen
  8. M Kirschenman

    M Kirschenman Intermediate Poster WMSTR Lifetime Member

    Antique vs modern for demos

    I wouldn't get too concerned about using some modern tech to help demonstrate an antique device - keep in mind that the red prony uses a digital tach, Jerry C's blue proney is all electronic (strain and tach) and we use amplifying and recording equipment all over the grounds to explain and demonstrate equipment. - just thought, not to be taken too seriously.
  9. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member

    And you should see our electric bill for the months of August / September! :sad:
  10. M Kirschenman

    M Kirschenman Intermediate Poster WMSTR Lifetime Member


    Ok, a solar powered brake!!!

    OK, I'll put my crack pipe down now......
  11. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member


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