Stain gauge on a crank arm

Discussion in 'The Projects Forum' started by dempsey, Apr 4, 2014.

  1. dempsey

    Thread Starter New Member

    Apr 4, 2014
    3
    0
    Hi Chaps

    This is my first post on here so please do tell me if I have the topic in the wrong section etc?

    I have a problem whereby I have an axle on a crank arm which has a load imposed in a vertical direction. I'm looking to strain gauge the crank axle which is having the vertical load imposed on it as can be seen by the picture. The problem arrises where by you go out of plane of the strain gauge that is bonded to the axle, as you end up with the stress looking like it is reducing uptill it comes 90 DEG from vertical.

    I've had thoughts about setting up three lots of strain gauges at 0 DEG, 45 DEG and 60 DEG and then looking at the equations but not sure where to start there.
    Another idea is to attached two rosettes at 90DEG to each other as I'm not worried about the principle strain, just the maximum strain.

    Any ideas of how to solve this problem?
     
  2. Johann

    Senior Member

    Nov 27, 2006
    190
    30
    Am I correct in assuming your stain gauge wire is running in line with the rotating axle?

    Not exactly sure what you want to do, but I would rather put the strain gauge wire on the supporting bearing/bearing mounts of the axle in such a way that the vertical force on the axle will be measured by stretching the strain gauge wire on the mounts carrying the bearings. In this way, it does not matter in which position of rotation the axle finds itself as the force is downwards on the mounts (with the strain gauge on it instead of on the axle). You just need to calibrate your output in such a way as to offset the static vertical force due to the weight of the axle etc. without the vertical force.
     
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  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    A lot has been done to measure and study the forces applied by a bike rider to the crank. You might learn relevant techniques from those studies. The book "Bicycle Physics" covers the topic and would offer references.
     
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  4. dempsey

    Thread Starter New Member

    Apr 4, 2014
    3
    0
    Hi Johann

    Are you meaning that the strain gauges are going on the axle that has the force applied to or indeed the stantionary axle which the crank is rotating around?
    I'd like to measure nearer the force so hence having the strain guages on the axle that is having the force exerted onto.

    One idea is to use a bearing encoder, although the bearings being used at present are specially made for this system so having them made with an encoder will prove too expensive.

    My other idea is to use two rosettes at 45DEG to each other as I'm not bothered about the principle strain but actually the today strain.

    What do you think?
     
  5. dempsey

    Thread Starter New Member

    Apr 4, 2014
    3
    0
    I've had a quick look and it doesn't really have anymore advice that I don't already know, but it has been good to reafirm some principles that i've already gone through. I think this little task is going to be a tough one!:)
     
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