Very small torsion axles

[Forhire]

Our design this season called for some small torsion axles. It turned out to be a fairly involved project to manufacture them. I figured I'd share the build method in the event anyone else needs something similar. One of the challenges we encountered was the figuring out how to put the rubber rods (quarter inch o-ring stock) into the corner of the square tubes. While we were brain storming how to stuff the rubber into the housing I flippantly suggested freezing the rubber rods with liquid nitrogen. After some further research that is exactly how they make trailer torsion axles. Crazy.

It took some preparation and a lot of trial and error. When we started the whole team turned up for the liquid nitrogen portion. After a few failed attempts we got side tracked freezing apples and carrots along with a few other tricks. Pretty soon it was just a couple students and after a couple more hours we finally figured it out.

Step one: Freeze all the components including the dies. This give you a lot more working time.


Step two: Stretch each o-ring section and place it in the V die and quickly press it. It will then freeze into the deformed triangle shape. Work fast otherwise the o-ring may freeze round preventing it from being deformed properly without cracking. This was a three person job, one to stretch, one to close the die, and one to operate the press. Wood was used to help insulate the aluminum dies.


Step three:
Trim one end off the frozen rods with dikes. Then insert the 4 rods into the housing with the axle. Use the thawed end to hold onto the frozen rod. Without the extra end the heat from your fingers will thaw the rod.


Step four:
Thaw the assembly in warm water. The rubber rods will contract back into their original size and shape.


Step five:
Trim the excess rubber with a razor knife.


The rubber holds the axle in place. The axle has about 30 degrees of travel in each direction. The stiffness is controlled by the durometer of the rubber o-ring material used.

[Forhire]

Axles were made from cold rolled 4130 and tapped on the ends. Corners were knocked off to clear the 3/4" square tube.


Connecting arms:



Idler assembly:



Here's some idler machining at the school:
https://www.youtube.com/watch?v=EDhaW5d9uDk

In the end we didn't use the assemblies in competition but we certainly learned a lot. Maybe in the fall we'll get them sorted... even though the student that designed them has graduated.

[sanddrag]

Sounds cool, but a picture would really help me here. Also, does that mill not have ballscrews, or is there some other reason you're conventional cutting?

[Forhire]

Originally Posted by sanddrag

Sounds cool, but a picture would really help me here. Also, does that mill not have ballscrews, or is there some other reason you're conventional cutting?

My web host is offline this morning due to a fiber outage... the pics will re-appear once the issue is sorted.

The machine has ball screws. The pocketing was done conventional because the CAM software generated a cleaner path convention rather than climb. It is something odd with the CAM software. Being that time was critical we just ran with it. This was one of the first times messing with hsm paths on this mill. For being a convention mill with limited rpms it did ok.

[Richard Wallace]

I got a look a the pictures early this morning before the server issue. Looks very cool. I was inspecting in Hopper and recall getting a brief look at your drive train, but not at this. Do you have a picture showing the complete track drive?

[Forhire]

Originally Posted by Richard Wallace

I got a look a the pictures early this morning before the server issue. Looks very cool. I was inspecting in Hopper and recall getting a brief look at your drive train, but not at this. Do you have a picture showing the complete track drive?

This design was to replace the limited suspension used in completion but the upper arm design wasn't quite right. They took them to St. Louis but didn't end up installing them. They made for a great conversation piece. My understanding is that the upper arms have been modified and they will be installed in the fall for Girl's Generation. We are off for the summer. I don't think I have a picture from testing... they don't let me out of the shop much.

I'll see if I can dig up a photo.

[sanddrag]

Photos are showing now. Very cool. (literally!) Thanks for sharing!

[s_forbes]

This is a really neat process, thanks for posting it!

Are you using Buna-N type of o-ring material, or does it have to be some other exotic material for this process to work?

As for the die, how did you decide on the dimensions for the triangular shape and o-ring diameter to use?


Edit: apparently I can't hand out more green dots for your post, but have to say that I love these threads that document building processes!

[Lil' Lavery]

What made you opt for torsion axles over other suspension systems?

[cbale2000]

Would anyone care to explain what building this type of axle accomplishes? I think I'm missing something here.

[asid61]

Originally Posted by cbale2000

Would anyone care to explain what building this type of axle accomplishes? I think I'm missing something here.

I googled it, and it said that these can be used as suspensions for trucks and the like as opposed to the other method of using leaf springs. That being said I have no idea how these work, although they look really cool.

[RoboChair]

Originally Posted by asid61

I googled it, and it said that these can be used as suspensions for trucks and the like as opposed to the other method of using leaf springs. That being said I have no idea how these work, although they look really cool.

It's a small non-rotating square axle around which there is flexible rubber in a tube. The axle cannot rotate in relation to it's mounting hardware. The tube WOULD be able to move freely around the axle except that there is rubber preventing continuous rotation and providing a spring effect relative to the axle.

[sanddrag]

I wonder if it's possible to shove one of these together while soaked in alcohol or something, without needing to freeze anything.

[Tom Line]

I bet a pour-able molding urethane might accomplish this.

[Forhire]

Originally Posted by s_forbes

This is a really neat process, thanks for posting it!

Are you using Buna-N type of o-ring material, or does it have to be some other exotic material for this process to work?

As for the die, how did you decide on the dimensions for the triangular shape and o-ring diameter to use?

Thanks. Standard buna-n. Stiffness is dependent on the shore hardness of the
rubber. We modeled the parts in solidworks and then estimated space required. We ended up facing the die about 20 thou to get the compression just right. The V was a standard 90 degree cutter.

Originally Posted by Lil' Lavery

What made you opt for torsion axles over other suspension systems?

Lightweight, compact, no stops required.

Originally Posted by cbale2000

Would anyone care to explain what building this type of axle accomplishes? I think I'm missing something here.

This page may help explain everything:
http://www.lovejoy-inc.com/products/...uspension.aspx

Originally Posted by asid61

I googled it, and it said that these can be used as suspensions for trucks and the like as opposed to the other method of using leaf springs. That being said I have no idea how these work, although they look really cool.

Common on trailers. They are also used in track systems like on skid steers. I believe this video was part of the inspiration.
https://www.youtube.com/watch?v=dIImhJMbc88

Originally Posted by sanddrag

I wonder if it's possible to shove one of these together while soaked in alcohol or something, without needing to freeze anything.

No. We tried a bunch of methods... pulling, pushing, stretching, and etc. Rubber acts like a liquid under compression. Freezing worked. Liquid nitrogen came from the local welding supply.

Originally Posted by Tom Line

I bet a pour-able molding urethane might accomplish this.

After reading a few patents it was clear that potting the axle in rubber or urethane wouldn't accomplish the correct motion and tension. We made a very crude prototype from wood and learned exactly how they work. As the square rotates the rubber rods roll tighter and tighter into the corner. The rolling action was key.