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Calculations: cam strap tensioning

cam-strap-fg9.pngWhen you pull on the free end of a cam strap (Figure 9) in a direction that is approximately parallel to the tensioned strap (Figure 10a) you are in effect putting a purchase on the fixed end webbing segment that is attached to the VBL. If there was no friction between the webbing and the radius on the cam buckle and you pulled on the free end parallel to the tensioned strap (Figure 10b), the tension applied to the free end c (Tc) would equal the tension in the fixed end b (Tb). The tension of fixed end a equals the sum of the tension applied to the free end c and the tension of fixed end b (Ta = Tb +Tc) or (Ta = 2 x Tc).

cam-strap-fg10.pngIn the real world we, of course, have friction and we can rarely pull parallel to the tensioned strap. The coefficient of dynamic friction between polyester and steel is approximately 0.18. This means that Tb = Tc – 0.18 x Tc. If we assume that we lose an additional 0.10 (10%) due to departure from pulling on the free end parallel to the tensioned strap (Figure 10b), then Tb = Tc - .28 x Tc or Tb = .72 Tc. This means that Ta = Tb + Tc or Ta = .72Tc + Tc or Ta = 1.72Tc. So if we wanted 150 lbs. tension in the tie-down strap we would need to pull with 87 lbs. force on the free end (87 lbs. x 1.72 = 150 lbs.). When the tension is released from the free end c, it is transferred to fixed end b and the fixed end a tension is maintained.

 

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