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I have to study several industrial pulleys made for lifting heavy weights.

It is important to select one which has a low coefficient of friction because the system is based in a hoist and several corner angle transmissions under heavy load. So that there is lots of strength lost by friction. I also have many other constraints of course. I'm not searching for pulley brand but for a test mechanism I could use to check the already selected candidate pulleys' coefficient of friction.

I could test the static coefficient of friction quite easily using a weight at one side and a dynamometer at the other side.

However my application wile involve several lifts a day with up to 20 meters of rope running under heavy load (1~2 metric tons per pulley). The speed is relatively low. 1m/s in the rope would be the absolute maximum.

I don't know if the static coefficient of friction will be enough to make a relevant selection. What do you think about it ?

I had several ideas to build a testing machine. All seems to be quite complicated :

  1. I firstly imagined 2 big hydraulic jacks with a dynamometer at each side. I'm not sure of the minimum length of the jacks but I had in the idea that 2m would be a minimum to get something representative.

  2. I then imagined a relatively compact system with an endless rope running under load between a motor and pulley. I would measure the torque between the motor axis and the wheel actuator. I'm not sure of what I would exactly be measuring here but I could make some comparisons.

  3. I also imagined a system with 2 opposite pulleys under tension. I would measure the strength needed to move the rope with a dynamometer. I'm quite sure I should not do it manually but with a constant speed or constant strength system. I'm afraid to build a labyrinthine system.

Do you have better ideas or insights on how to do it ?

Regards

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    $\begingroup$ You don't need anything complicated. Just hang two heavy weights either side of the pulley. Then measure how much extra weight you have to add to one side, to make the rope move. (Repeat adding weight to the other side, since the "equal" weights won't be exactly equal) $\endgroup$ – alephzero Oct 2 '18 at 22:26
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    $\begingroup$ This really shouldn't be necessary. You haven't provided us enough to go on, but several pulls of 20 meters at 2 tons isn't much for a fit person. Pulley efficiency tends to be shockingly bad when you start looking into it. The simple and normal solution is to buy big, cheap pulleys and live with the inefficiency. If you start with 3/8 inch Dynema, you have about 10 to 1 safety factor at the lift point. If your system of sheaves is 33% efficient, you still have 3 to 1 at the winch. This would require 3" pulleys or a little bigger. Upping diameter will often gain a little in efficiency. $\endgroup$ – Phil Sweet Oct 2 '18 at 22:26
  • $\begingroup$ You could even spin unloaded pulleys and measure the decay time, but if the bearing friction is nonlinear with load, that won't work any better than the schemes you listed. $\endgroup$ – Carl Witthoft Oct 3 '18 at 19:23
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The parameters are what you would experience on a sail boat.

If you want to measure your dynamic friction run your line through the a block mounted on the ceiling (at your desired speed and load) and measure the force between the block and the ceiling.

(on a boat: block = pulley)

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  • $\begingroup$ This will not do it. You seem to have force vectors going everywhere except where's wanted. $\endgroup$ – Carl Witthoft Oct 3 '18 at 19:20
  • $\begingroup$ Do you think so ? I would put the dynamometer at the point of traction (not between the pulley and the roof) and compare it with the load weight. All the force lost would be in the pulleys. The only problem is that I would have to lift a very heavy load. That is not very safe... $\endgroup$ – user6131 Oct 5 '18 at 19:06

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