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Accountant needs help with spring

  1. Mar 9, 2010 #1
    Greetings all.

    I'm working on a 3dof platform project as the basis for a flight simulator and am pretty much out of my element.

    Here's is a picture of the typical platform: http://www.flickr.com/photos/46403450@N02/4420935812/ [Broken]

    I have constructed the following lightweight prototype using 16ga 1” square steel tubing. Here is a quick vid:

    There are two center column tubes in my prototype: The tube attached to the weld yoke from the underside of the upper platform fits inside the slightly larger tube attached to the lower platform, this allows for up and down movement.

    I have three small 12vdc 50rpm gear motors that that are powerful enough to induce pitch and roll movements in the upper platform (which sits on a u-joint) but they are not strong enough to lift the upper platform; therefore I need to introduce a spring to the system.

    I have initially specified the following:

    Compression spring:
    Inside dia: 3.00” min
    Free length: 24”
    Load 70 lbs +/- 10% at 16” (8” deflection)
    Solid height max: 7”
    Closed ground flat end’s
    Material: music wire.

    I want the upper platform to be able to heave 5" above and 5" below the 16" deflection point of the spring at 70lbs (10 in of travel). I’ve constructed mounts for the gear motors, reduced the rpm to 25 using pulleys and a 2L belt, and attached a parallel lever to the shaft of the large pulley with holes for all thread at 1”, 2”, 3”, 4”, and 5”. When a perpendicular shaft of all thread is attached to the 5” hole then the up and down heave will be 10” per complete rotation.

    My question is how much force, based on the above sprint specification, is required to lift the 70lb platform up 5” from the 8” deflection point and how much to lower the 70lb platform down 5” from the 8” deflection point? My very limited understanding of springs is that they act linear on the middle 60% of travel (?) so is it fair to say that the force required per inch of travel will be the same?

    I would appreciate any feedback and please remember the simpler the better.


    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Mar 10, 2010 #2


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    binto100: The spring can be assumed approximately linear, having spring constant k = 1.532 N/mm. Therefore, the additional force required to lift your platform upward 127 mm will be 194.6 N. The additional force required to lower your platform downward 127 mm will be 194.6 N.
  4. Mar 10, 2010 #3
    Thank you very much for your reply.

    Using convert.exe can I say that 194.6n = 43.6 lbf and if I have three motors all working equally then they must be able to supply 14.54 lbf ea to lift the platform? In other words there must be 14.54 lbs of force produced at some point on the perpendicular arm that is attached to the motor shaft?

  5. Mar 10, 2010 #4


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    That is correct (or actually, 43.75 and 14.58).
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