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Homework Help: Barbell spring collar

  1. Nov 27, 2008 #1
    1. The problem statement, all variables and given/known data

    The project i am working on is determining the force of a helical spring on a barbell. Like this (http://bodybuilderfitness.com/library/2York_Barbell_Spring_Collar_Side_View.JPG" [Broken].

    Once i find the amount of force the on the barbell via the spring collar, i need to find how much weight (free weights in 25 kg increments) & and what angle is needed overcome the friction and spring collar force.

    I am not sure what equations i need to use exactly. To solve this lets make a few assumptions.

    I think the angular spring rate tells me how much force is on the bar, maybe?

    50 kgs of Iron weights on each side of the barbell
    A steel spring, E=28Mpsi, G=10Mpsi
    Angle of Barbell = 30 deg
    mu of iron on steel = 0.4 static, 0.23 sliding
    spring handles are 2" long
    Diameter outer of spring, Do = 1.5"
    diameter of the wire, d = 0.2"
    # body coils, Nb (before handles are squeezed which increases the number of coils) = 4.375

    2. Relevant equations

    **i think the angular spring rate is what tells me how much force is on the bar

    force on barbell via spring:
    mean diameter, D = Do-d
    angular spring rate, ko = M/deg,rev = (E*d^4)/(10.186*D*Na)
    # of active coils, Na = Nb + Ne
    # of end coils, Ne = (l1 +l2)/(3*pi*D)

    sliding weights:

    I am sure i am missing a lot and maybe chose wrong eq's.

    3. The attempt at a solution

    Ne = 0.33
    Na = 4.7
    ko = 719.61 psi

    F@30deg = 547.352 (units? ft/s^2?)

    my assumption on angular spring rate is assuming the force it takes to open the spring wider (squeeze handles) is then applied onto the bar because it cannot return to its original size.
    Last edited by a moderator: May 3, 2017
  2. jcsd
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