Calculating mechanical advantage: the T-system
The subtle but significant difference to this system… a pulley on the load and the main line terminated at head of the tripod. We have a 6:1 mechanical advantage in the pulley system.
1. Assume continuous strands of rope have the same amount of tension.
If we start at the point where effort is applied and assume that we apply one unit of tension (1T) then we can assume that the tension along this single strand of rope will be the same throughout. Any time this strand passes 180 degrees around a pulley there are effectively two strands of rope applying tension to the pulley attachment. This means that the pulley must oppose these strands with a force of 2T,
2. Rope grabs add tension.
Previously we saw that pulleys apply two units of tension (2T) to their attachment carabiner in order to oppose the tension of the two input strands of rope. Whenever a rope grab is placed on a rope under tension, the rope grab effectively adds its tension to that of the host rope. So the pulley in this example has the 2T on the carabiner added to the 1T on the host rope to produce a new strand tension of 3T and an overall Ideal Mechanical Advantage of 3:1.
3. Redirection pulleys.
Some pulleys add mechanical advantage while others perform the task of ‘folding’ systems to fit within operational spaces. Here we have a high redirection pulley in the apex of a tripod which allows a horizontal effort to raise a load vertically.
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