How can I rig a cabled pulley system for a homemade weight machine?

That gives both variable resistance through range of motion (lighter at start, full weight at end) AND force multiplying where e.g. 20 lbs becomes 80 lbs (added weight effect with less plates).

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Re-design5 years ago
Try something like this - The excentric pulley coud be made out of 3 or 5 layers of 3/4" plywood and cut on a band saw. The cable should be attached to the moving bar. Most of these types of machines use twice the needed weight and a pulley system that divides by two to keep the speed of the weight down. If it's too fast it won't stop at the top when you do unless you're going slowly. and it'll bang at the bottom. Your framework is going to have to be pretty strong to keep it from falling apart on you in use.
vwchest (author)  Re-design5 years ago
Ok, it looks as though I need to tack this design onto the 4:1 inverse pulley design and then add my redirecting pulleys for the other functions. So the excentric pulley changes the 'point of attachment' on the lever arm according to the curve design (more leverage less weight at start and then ramping up with less leverage more weight at end of range of motion). By the way this compliments the mechanical (dis)advantage of muscle/tendon attachments to bones. So much science in picking something up huh? Thanks
LIke Ork says, you need cams. I'd be tempted to use eccentric MTD pulleys, they use ribbed belts, which will stay locked to the right position on the cam all the time.
I agree with steveastrouk if you can use MTD pulleys do so for you own safety because they are much safer and the only downside is cost. Also in your design a small tip make the weight move far and whatever is pulling move less. here is a pic of a simple design sorry for the bad drawing but just putting point to picture please criticize and offer a better design
vwchest (author)  jj375 years ago
Thanks to all. I want to add some redirectors for more functions, look feasible? Could you explain the cam components (how it works and how the resistance curve would behave for different fulcrums).
orksecurity5 years ago
Force multiplying is easy -- that's what compound pulley systems are for. Websearch on that should find what you need. (Basically, you want to set up the opposite of the usual where force is divided but the rope must be pulled a greater distance.) Variable resistance requires eccentric or odd-shaped pulleys. Your best bet here might be to go to a gym and take lots of photos and measurements of their system. The real problem is going to be keeping those eccentrics in phase with the intended range of motion.