Monday, March 14, 2011

Horizontal Hip Joint Prototype

I've been (slowly) working on the prototype design for the horizontal hip joint. This is by far the trickiest joint of each leg, as the ~500 pounds it needs to support will be torquing the bearing instead of being a clean radial load. It's also the limiting factor in the overall speed of the hexapod: in a normal walking gait, the other two joints will only move very slightly, while the horizontal hip joint will swing through 90 degrees of travel. The smoother the action is here, the faster the top speed is likely to be.

I decided to use car wheel hub bearing assemblies, as they are cheap and robust. After playing with one under very moderate load, I decided that I just didn't like the torque it was being put under. So I went with two, stacked one top of the other. Now all that torque is converted into a much nicer radial load within each bearing. You can get an idea of the arrangement in this rough sketch. (Except I grew the bearing support frames upside down, oh well.)

This is the completed prototype. (The large shelf is only there for testing purposes.) The action is very smooth, and while a bit tedious, making 6 of these would not be a problem. I'm quite happy with it. Unless some major problems are discovered in later testing, I think this design will work nicely. Making just one of them is giving me a greater appreciation of just how big this thing is going to be.

Here's a closeup of one of the bearing assemblies. They're simply bolted to plates which are welded to the support frames. Everything is under compression.

And here it is with about 175 pounds of static load on it, which it handled quite nicely. Once I get a better test stand for it, I'm hoping to do some tests with it loaded closer to the design goal of 500 pounds. I'm running out of large chunks of steel to hang off of it, though.