-- Last Updated: Jun-23-13 9:40 AM EST --
... but not a factor that's properly applicable to this situation either. If the hub creates a lot of friction (like with a poor-quality pulley), a larger wheel will make that friction less apparent to the operator when using the pulley to do work. That's for the same reason you can open a door by twisting a doorknob but you can't if the knob is removed and you try to twist the bare shaft with your fingers (bigger wheel creates greater torque, and torsional resistance is less apparent). Also, if a large-diameter rope is needed to accomplish the job, forcing it to kink tightly around a too-small pulley wheel adds a bit of resistance that you probably will feel when pushing your own limits.
Former p-netter DuluthMoose built a fancy z-drag system using very tiny pulleys, but they were mucho-expensive pulleys made for the tree-trimming business. Each one had a tiny, high-quality ball bearing inside the hub, and they were exceptionally smooth running in spite of being very small relative to the work load they were designed for. I'd put one of them up against a hardware-store pulley ten-times its size as far as ease of action goes (just a guess of course, but trust me, it was pretty dramatic how smoothly and effortlessly they operated under heavy load (we had the chance to try the system out on a badly pinned boat!)).
In the case of this hoist as originally built, one COULD attempt to balance the friction among the pulleys so that the boat would stay level while being lifted, but you'd never get it exactly right, as it would be like trying to balance a lever on a pin point, and even changing to a different rope would alter things enough to ruin your efforts (back to that situation of different ropes bending around the wheel with differing amounts of resistance). Further, even if by some miracle you got it right, the friction that would "balance out" while lifting would be totally wrong while lowering. You can see this on any basic pulley hoist where there's friction, especially with cheap pulleys and there's a lot of friction, in that the rope on the side of the pulley closer to the operator has much more tension on it while raising the load than while lowering the load, but the tension on the rope that carries the load is the same in both situations. This indicates greater friction while lifting than when lowering. The less friction in your pulley wheels and due to "flexing" of the rope, the less you'd see this effect, but you could never eliminate it, and thus you could never make all friction forces balance out for both lifting and lowering.
Bottom line, I have no doubt you noticed an advantage to using a bigger pulley when lifting bundles of bricks, but for fixing this boat hoist it just makes more sense to put the ropes in control of keeping the boat level rather than hope to control the situation via internal resistance to operation. That's what Carlelo was saying when he said relying on friction as part of the design means it's a bad design.
Kindle / iPad Cases
|Table of Contents|