Basically the system consists of one stage and a carriage. There are a couple of pulleys and a rope. The pulleys marked with blue are attached to the stage and the ones marked with black are attached to the carriage. Lastly, the part of the rope is fixed on the aluminum bar marked green as in the figure below:
My question is about the lower pullies attached to the stage. Since the rope above them is fixed on the aluminum bar, I couldn’t understand their role in the system. I might even misunderstood the mechanism as a whole. If the stage goes up x meters the carriage needs to go 2x because of the upper pulleys. However, don’t the rope wrapped in lower pullies get loose when when it goes up and then down. How do you maintain the tension on that side?
Maybe If you can give a brief explanation to system, that can be useful as well.
You’re making a basic mistake that’s throwing you off. The ratio over the top (and bottom) pulleys is 1:1, not 2:1. So as the stage rises, it will pull up the carriage by the same distance within its frame. The bottom rope will always stay tight and is there to provide pull back down when the stage lowers. The pulleys on the carriage are just there as part of the tensioning system (along with the cams next to them) and are essentially static in relation to the lifting of the carriage (i.e., the rope doesn’t move through them when the carriage goes up and down.) The carriage only seems to be traveling at 2:1 because it’s also moving with the stage as it goes up and down. So it is effectively moving twice the distance, but not within the stage, only within the whole elevator system.
Thanks for your reply, but the point I didn’t understand is that since the lower pullies of the stage are attached to lower tensioning pieces of the carriage with rope, if the carriage goes up it seems to me like it will pull the rope while going up. Can you help me with that point? thanks in advance
The way to think about this is as a closed loop. The carriage is just one section of the loop formed by the rope running through the upper and lower pulleys. That loop is attached to the cross-bar that is fixed to the base (not the stage), so that as the stage rises in the base, the loop is pulled along its entire length equally. It’s essentially rotating with one fixed point along the length, where it’s attached to the base. To visualize this in a tangible manner, tie a loop in a rope around two pegs (nails or screws will work fine, but you can use pulleys if you have them), then grab opposite sides of the loop with your hands. If you keep your hands in the same place on the rope but move one hand up and the other down at the same rate (as is happening with the elevator) the ends of the oval will slide over the pegs even as the whole loop stays in the same place. One hand is the fixed point in the rope on the bar attached to the base, the other is the carriage.
