caz_in_a_teacup (caz_in_a_teacup) wrote in little_details,

Lifting weights with pulleys

Setting: fantasy world with a rather mediaeval tech-level

Searches: Google: "manual handling" + "maximum weights" + pulling + female + women + pulleys in various combinations. Wikipedia pages on pulleys, mechanical advantage and manual handling. (And weightlifting sites and sailing sites, none of which were specific enough.)

The Question: I have a female character who has rigged up a pulley system to move a load. What type of pulley system depends on the answers, as I can't work out how much force she could reasonably be expected to apply. She's about 5' 6", but sturdily built (weighs about 70kg, 155lbs) and used to manual labour. I want her to be able to lift this load, and do it every few days, but with some difficulty. I know the manual handling guideline weights for lifting objects (16kgs in ideal conditions for a woman), but I can't work out how they would apply to this kind of work.

In purely mechanical terms it would seem she could lift slightly less than her body weight (with a fixed pulley) just by hanging off the rope as a counterweight. But I don't want this, I want her hauling on the rope. I'm assuming she could apply less force by hauling on the rope than by using herself as a counterweight, but how much less?

She's lifting the load about 3 metres, from one floor to the next of a building, so it doesn't need to be sustained for too long. The load is about 72kg, although this is already subdivided, so I can mess around with this as appropriate. Will she be able to lift the equivalent of 36kg if I use a pulley with a mechanical advantage of 2? Or will this be incredibly easy? Do I need a mechanical advantage of 3 to reduce the weight to 24kgs, or even further? Ideally I can start with a simple pulley and then she can modify it to increase the mechanical advantage.

I'm trying to get my head around this from personal experience, but I've never used a pulley system and the most I can lift from the floor is about 25kg (and that only a few centimetres from the floor).

Thank you!
Tags: ~science (misc), ~science: physics

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