Pulley Mechanical Advantage Question

[jd93]

I can't figure out this seemingly simple problem. I've sketched a very rough idea of what I'm trying to figure out.

The sketch is basically an enclosed lift with a pulley and motor system hanging from a ceiling. The cable runs down from the ceiling, around the floating pulley, and is wrapped around a shaft by a motor. For simplicity ignore potential leaning from the center of gravity being off, moments caused by spooling on the motor shaft etc. My basic question is that when lifting, would the motor still experience a mechanical advantage from the pulley (i.e. M/2) even though it is only fixed at one end?

 

[handleman]

The pulley isn't really "floating", is it? Its position relative to the motor is fixed? Then no. No benefit.

 

[rb1957]

my 2c ...

if you let out 1" of cable the mass will drop 1", no? therefore no mechanical advantage.

the tension in the cable is equal to the weight, (make a FBD, cutting the cable above the "box") no? therefore no mechanical advantage.

Like above, I don't really get "floating" pulley. Is it attached to the "box" then it's doing nothing ? If it's attached outside the "box", like to the rest of the world, then it's fixed in place as the box moves.

another day in paradise, or is paradise one day closer ?

 

[JGard1985]

Agree with posts above. Draw a free body diagram of the box with the winch in it. The only external forces are the weight to be lifted and the tension in a single cable...therefore tension in cable equqls weight to be lifted (....plus self weight of winch)...There is no mechanical advantage.


If you're thinking about the motor size and required torque...then consider as drum size and number of wraps on the drum increase, there is a greater moment arm and it will require more torque to lift the load

Jeff
Pipe Stress Analysis
Finite Element Analysis

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