Load-bearing Knee Joint

I'm trying to work on a larger project, am on a budget, and as such, I do not have a welder.

I'm looking for a way to make a leg frame with threadall and/or PVC, but am stuck on knee construction.
Every search comes up either for human knee robotics, or weaker leg joints, such as spiders under a food in size.

My issue I'm running into, is while I'm using a linear actuator to contract/expand the leg, I can't find a way to fabricate a joint that can sustain, for safety's sake, 200-300 lbs. In reality, it will be up to this weight at most distributed across 4 legs, but I prefer if possible to err on the sake of caution. I'm open to other ways of going this than initially intended, but feel a bit limited due to not having money for a machinist. 

Any ideas or suggestions would be appreciated, as I've spend a few hours at Home Depot and Lowe's trying to find a way. Thanks!

A couple of ideas come to

A couple of ideas come to mind:

1.  How about using several ball bearing turntables on their sides?  If you screwed three or four of those together, that might give you enough strength to meet your specifications.  


2.  Home Depot approach:  buy 1/2" pipe with a 1/2" rod, grease well.  The end of the thigh and top of tibia/fibia would have the pipe go through it.  Depending on what you are using for the thigh would really define what the actual joint would look like.

With both of these ideas, you could then install pulleys on the joint and use either a motor with rope or an elastic to relieve weight or control the leg ala how tendons or muscles on a biological leg.

Good luck!  Let us know how you make out.



I’m not following the second

I’m not following the second one, as I’m finding myself unable to visualize what you’re taking about where the “pipe would go through it”. Also, I’m lost on how to do the pulleys: I’m planning to use a linear actuator for this, and not sure how to implement the pulleys.

For the first, I looked and it seems it could work; but how would I mount them to make the joint is where I’m lost. Thanks!

knee joint

For option 1 I think 2 parallel square rods for the upper section (thigh) and 2 turntables for the knee joint (one to each rod). Then a single square rod for the lower section (tibia) would connect to the 2 inner turntable plates. This is something that would be easy to model with a 3d cad tool such as OpenScad.


Thing is, I’m not sure what square rods I can get that would be durable enough without fabrication: If I buy them from a hardware store (can I?), is it within the same price boundries as threadall seems to be?


Square Rod

I specified square rod because:

  1. Readily available in steel and alumimum
  2. Easy to drill and attach to flat bearing plates
  3. Do not expect a machine that supports 200kg to be cheap.

You might want to give serious thought to modeling first. At a minimum you would see what the final product would look like and have some details to cut and dill to. Then there are the issues of torque requirements at each joint to be calculated. 

Sorry, not very good

Sorry, not very good description.  I was in a hurry.

1.  I think ggallant did a better job of explaining what I had in mind.  Thank you!

2.  I was thinking what you would end up with is that the thigh would be attached to the 1/2" pipe.  Then the tibia/fibia would be attached to the 1/2" bolt.  You could weld or attach the pipe to the thigh or possibly use a heavy duty door hinge as the actual joint.  Or basically create your own heavy duty door hinge out of pipe and a bolt.  

If you ran pulleys off of each end of that 1/2" bolt, you could then tie strong elastics to the mid points of the fibia/tibia and the thigh bones and across those pulleys to transfer some of the weight on the joint to those attachments.  Kind of like the tendons that run across your knee and attach to the thigh and fibia/tibia. 

Since the ball bearing turn tables are so cheap, I might try a prototype of that first.  Since you are using them in a way they aren’t designed to be used, you really might want to do some destructive testing to see how they hold up with worst case scenarios for weight and abuse. 

I hope my clarification helps.  Good luck and let us know howyou make out.






Knee joint with openscad

I played with openscad and did a rough  femur, knee joint and tibia based on 1 inch square rod and 3 inch lazy susan ball bearing plates. You can find (and use) the scad files at