Hi, we are a small second year team quickly approaching the end of build season. I was wondering if there was anyone out there who could help us with some unforeseen difficulties. We are doing a basic elevator design posted by cables and pulleys. However when we tried running the lift yesterday the churro(where the line spooled) instantly began to bend with the pressure. Any advice for how to solve this issue and or to get the cables to pull more evenly is greatly appreciated. Thanks:)
Churro is a wonderful thing…but not for a load this high. I would switch it out for a more proper hex shaft, which does not have the weight relief features Churro does. You can get that from AndyMark, Vex, McMaster, The Robot Space, or whatever vendor you usually like.
Yep, swap that churro out. Churros were designed for light power transmission (conveyors of light game pieces), structure, and spacers.
You will want to use 1/2" steel hex shaft or maybe 1/2" aluminum 7075 hex shaft. I would not recommend a weaker aluminum hex shaft than 7075 for this application. Whatever you get, make sure that your flat to flat width is under 0.500" If it’s wider than this, then your bearings and hubs will have difficulty fitting. Most steel hex shaft is suitably undersized. Usually. aluminum shaft, for some reasons runs 3-5 thousands over 0.500".
Something else that helps - support the shaft closer to the pulley. Right now it looks like your supports are fairly wide apart (20" or so?). When there’s a high load situation like this, I prefer to get good supports right on either side of the pulley. I would run a 2x1 horizontally above or below the VersaPlanetaries, then a short horizontal section vertically with a bearing to help support the shaft. Use some strong gusset plates on both front and back to hold the vertical and horizontal pieces in place.
The hard part is getting everything lined up. Cut the horizontal to the proper length, cut the vertical as close as you can and put in a bearing hole. Mount the vertical piece to the horizontal, and then run the shaft through. This will let you bolt the entire support structure on while knowing the shaft is properly aligned.
Unrelated, but it also looks like you have some room to shorten your battery cables. Tat should help lower your overall system resistance, increasing the voltage each motor gets and lengthening your battery life.
An alternative would be to attach a plate spanning the two vertical 2 x 1’s below the two drums. Attach one short piece of 2 x 1 oriented the same way as the existing ones to this plate. This will transfer the vertical forces from the space between the two drums into the two 2 x 1’s you already have.
To get the holes for the bearings for the churro (or hex shaft) to line up, you will have to manufacture the parts in a particular order. Disassemble what you have now. Clamp one of the short 2 x 1’s onto one of the existing 2 x 1’s and line up the edges. Then cut the bearing hole in the new piece using whatever tool was originally used. This match-drilling technique should get it really close. It would be best to do this outside of your robot so you do not get bits of metal falling into your electronics and shorting it out. Reassemble everything with the new tube in between the two drums. Then drill the holes necessary to attach the new piece of 2 x 1 to the plate.
We’ve used this steel hex stock whenever we have a long hex shaft supporting significant cross forces, such as our climber last year and our arm pivot this year. The dimensions have been good for us three orders out of three, except that we sometimes have to take off a fraction of an inch at the ends due to mushrooming during shipment. While cold rolled steel isn’t much stronger than 7075, it’s significantly stiffer.