Roller Chain Fulcrum

Sometime ago I saw a product called a Zizag made by Pretzel. You can see it here Mechanical Prusik. I had some roller chain and cable, so I experimented with different size chain and cable combinations and made a speed reducer to use on one of my Lifter models Planetary Gear. By adjusting the expansion of the roller chain around a cable I was able to slow or nearly stop the downward motion of the carriage. The behavior of the roller chain and cable closely resembles that of other Lifters using gripper style fulcrums.

The video shows a Lifter using roller chains and cables to control the positioning and placement of the carriage on the vertical support. At the top and bottom of the vertical support are projections. The bottom projection has a roller chain and pulley wheel assembly. A cable attached to the bottom of the carriage fulcrum position runs down through the roller chain, around the pulley wheel and up to the top projection. The top projection has a pulley wheel and the cable coming up from the bottom projection goes over the this pulley wheel and down to a fulcrum position on the carriage. The top of the roller chain is held stationary on the bottom projection and the bottom of the roller chain is allowed to collapse up or expand down and grip the cable going through it. Attached to the bottom of the chain is a weight which assists pulling the chain to an extended position. When the chain is extended the cable is held tightly and supports the carriage at a fulcrum position. When the carriage is tilted the cable is pulled up through the roller chain. This action causes it to collapse and it releases its grip on the cable allowing the carriage to tilt. When the force to tilt is removed the carriage attempts to pull the cable back through the roller chain.

Roller Chain and Cable Fulcrum Supported Lifter

With the help of the weight on the bottom end of the roller chain, the roller chain extends and grips the cable. To allow the carriage to come down the vertical support the weight on the bottom of the roller chain is removed releasing the gripping action on the cable. In this example I placed two roller chain and cable assemblies on each side of the vertical support. I used a #40 roller chain and one-eighth inch cable. The cable size needs to be able to slide freely through the collapsed roller chain and the number of links of roller chain needs to be sufficient to grip the cable and support the weight in the carriage. The roller chain has a response distance when gripping the cable and by varying the number of roller chain links along with the roller chain and wire size one can reduce this response distance. Each roller in the chain supports a portion of the total weight in the carriage. By using multiple roller chain and wire rope assemblies one can design a roller chain fulcrum supported Lifter to handle small or large carriage loads.