We are looking at using an elevator/telescoping arm for climbing the pyramid this year. Right now the design requires a 2 stage elevator which makes me lean towards using cable and pulleys to drive the arm since we could use a single motor/gearbox to drive both stages. I found some good information in old threads on cable types to use and how it needs to be routed but I didn’t find a whole lot on what teams actually used for pulleys and anchors.
So I’ll pose the question to teams who’ve done elevators before, where do you get pulleys from and what do you actually tie your cable to as an anchor? I found some pulleys on McMaster but I’d like to use small diameters (~1") to make it fit better in the mechanism. The few pulleys with bearings around that size were only rated for 175 pounds which doesn’t give much of a safety factor if our robot is near full weight. The pulleys without the low friction bearings could support much higher weights but I’m not sure if the bearings would be needed.
For our FTC robot this year, and with the expanded range of allowed materials, we decided to use the 80/20 liner motion slide bearing options.
Pictured below is our two stage lift using 600 lb test kevlar core, braided polyester covered cord. It is thin yet strong and can handle sharp radius pulleys.
We used shoulder screws with 8mm OD shoulder and 6mm thread to mount our pulleys both in the frame slots and on the slide plates.
We made our aluminum pulleys on the lathe and pressed them onto cheap roller skate bearings of the “mini” size having 8mm IDs by 19mm ODs (Skate bearing types: micro-16mm OD, mini-19mm OD, std. 22mm OD). You can also Locktite the pulleys onto the bearing ODs.
For an even smaller pulley, you can also forgo bearings and just use the shoulder screw as a precision ground axle and have the aluminum pulley spin right on it, with just a little lubrication, but the pulley bore must still be a nice slip fit to work well.
We really liked the infinitely variable positioning that the shoulder screw on slotted framing gave us. You have to use a washer against the slot and the end of the shoulder to have proper grip on the frame, and you can use a larger ID washer fitting on the shoulder OD to handle the spacing & location of the pulley along the shoulder length.
The cord end anchor point was also a shoulder screw without a pulley, and just a small bowline knotted loop captured by the head of the shoulder screw.
1802 used pulleys with bearings for our cable powered lift that we re-purposed from replacement sliding patio door rollers (they were cheap and worked very well). Bought them at lowes for something like 3$ a pair.
Pulleys - McMaster has some real cheap solid plastic pulleys we used when we were still doing lifts.
Cable - 3/32 steel stranded cable with a nylon jacket. Use crimps and thimbles on the ends you want to hook to. You need to strip the jacket before using crimps. This cable seems to bend very well when needed yet resisit kinking.
Grounding - or making a permanent end. Drill 3 1/8" holes in a row. Thread through the holes and pull tight. We never had one of these fail
