How to make this pulley system more efficient?

Question

I am trying to make a portable pulley system to lift heavy stuff. I made the following prototype using 8 individual pulleys:

When I pull the rope, the load should seem approximately 8 times lighter since there are 8 shortening per one lengthening rope segment.

However, the load doesn't seem dramatically lighter. If the concrete block weighs 30 kg, then I should be able to lift it with around 5 kg of force. I haven't measured it, but it seems that I need to pull much more strongly.

At first, I tried an 8mm diameter polypropylene rope. This didn't work at all. When I pulled the rope, it was as if all but the first two pulleys were actually moving. Oiling all of them didn't help, so I tried a thinner and probably less elastic rope. The results are not ideal, as you can see in this video.

I have no idea where the most force is lost. There is the stretching and bending of the rope, the pulley wheels rotating, the wobbling of the load, and that's pretty much it. What is the most likely problem? Does it make sense to get pulleys with ball bearings? Or a different rope?

Answer 1

It looks you're using the standard very cheap tackles with plain bearings. Plain bearings are cheap, but unless they're well made and maintained they offer significant friction while under load.

You can improve this by oiling it, or even better - replace it with a sheave with a roller or ball bearing. Roller/ball bearings have magnitudes lower friction than plain bearings.

Answer 2

One problem I see with multiple discrete pulleys, particularly if the line is routed linearly as suggested in another answer, is that the load is likely to tilt as it is moved. Suppose the pulleys are numbered 1-8 in the order that the line passes through them.

I admit I'm not inclined to write out the force equations to entirely convince myself, but intuitively one would think that it's much easier to get line pulled out from between pulleys 1 and 2 than from between pulley 8 and the fixed end. The former requires overcoming only the friction of pulley 1, plus some force to rotate the load (but not necessarily to raise its center of gravity a significant amount), while the latter requires overcoming the friction of 8 pulleys and force to raise the entire load.

Perhaps the staggered routing of the line is intended to compensate for this tendency to rotate?

I think you'll do better with a block, which is multiple pulleys assembled into a single frame, rather than an array of discrete pulleys.

Friction of each individual pulley, and their cumulative friction, is ignored in the Mechanical Advantage section of the Wikipedia article on Block and Tackle. At the end of the section on Friction, though, it is noted that:

Frictional losses also mean there is a practical point at which the benefit of adding a further sheave is offset by the incremental increase in friction which would require additional force to be applied in order to lift the load.

This is likely a factor in your system.

Answer 3

vidarlo is correct--friction is the culprit. I recently attempted something similar to allow my elderly father to lift a maple sap boiling pan off the wood stove. I saw a similarly disappointing reduction in effort, even after lubricating the bushings.

Rather than spending even more for high-grade hardware I installed a manual boat winch in conjunction. This not only provides another compounding of leverage, but gives a substantial level of safety with the ratchet system. I also eliminated one degree of the system since it was no longer necessary.

Note that your rope must be up to the task presented by this compounding. I used 1/4" Dyneema winch rope, which has load limits in the thousands of pounds. I see about a 200 lb. load limit for 3/8" polypropylene, so consider upgrading that if you go this route.

Answer 4

The rope placement in your pulley system is incorrect.

The rope should be continually progressive from one pulley to the next such as this.

The rope should terminate at the location of the last pulley ( to the far left.) OR continue through the last pulley and terminate at an anchor in the beam.

You current rope placement is skipping pulleys and will limit the effectiveness of having multiple pulleys.