Broke a swingaxle axle

[dustymojave]

Back about 40 years ago, in my Hi Jumper off road race buggy, which was then state-of-the-art swingaxle, I broke some expensive aftermarket axle shafts.. What wound up being the most durable was a pair of stock axle shafts, that were chucked in a lathe and polished from the base of the ear out to the thrust flange at the outer end. Nothing any more fancy. No special material. Not shot peened or heat treated or anything else.

[Leatherneck]

What did the polishing do that shot peen wouldn’t?

[Ol'fogasaurus]

What did the polishing do that shot peen wouldn’t?

As I remember shot-peening compacts the metal while polishing takes away any flaws in the shafts surface that could cause problems.

Lee

[dustymojave]

Correct Lee. A machined metal part has a surface made up of scratches from the cutting tool that shaped it. A part that is cast has a rough surface. A forged part also has a rough surface. By removing all the high spots and blending them down to the lowest common denominator, ie the smooth polished surface, there are no grooves or scratches initiate cracks in.

When I was a young guy working in a plastics research laboratory, I did a lot of material testing using a device called an "Izod-Charpy Tensile Impact Tester". It's a pendulum with a vise part on the end to clamp a test piece in. There is another such clamp in a loose piece, so that clamping the test piece into both clamps attaches the loose piece to the pendulum. The test piece is flat and wider at the ends where it clamps than it is in the center. So it looks sort of like a squared dog bone. The parts are generally punched out of a sheet of the material with a punch press with a specific die set, or molded into the shape. Before inserting the piece into the clamps, you measure the width and thickness of the center of the dog bone to determine the cross-sectional area of the test material. The pendulum frame has a latch that you swing the pendulum up into. There is an anvil at the bottom for the loose piece to strike when released. Then there is a dial gauge and a needle pushed by the pendulum that indicates how far the pendulum swings beyond the stop point where the loose clamp strikes the anvil. Once you have tested a certain number of samples of the material in the basic dog bone shape. You are measuring the force to break the sample in force per area. Like pounds per square inch.Then you take another set of samples of the same material and cut a notch in the side of the dog bone. The size of the notch is specified in the standard for the test. The force to break of course is reduced by the reduction in area of material. But some materials, like glass or Plexiglas will react dramatically to a small notch. Other materials like Kevlar, the notch will hardly make any difference in force/area. I've done test where there was NO difference in notched or un-notched in force/area. But some materials...WOW!.

Note that you "cut" plexiglas by scratching one side, then bending at the scratch. If you try the same technique with Kevlar, it won't give a damn about the scratch and you would have to cut essentially all the way through to separate the pieces.

All of this applies to steel as well as plastics and there are similar tensile impact tests for steel. Polishing the axle shaft removes all the micro scratches and grooves that are start points for notch effect and cracks and thus breaks. I was amazed at how much more durable the stock axle was after polishing, especially compared to expensive aftermarket fancy alloy axles.

[Ol'fogasaurus]

I got thinking about just what all is shot peened and even I was surprised. Torsion bars, spring and many other things are shot peened. One of the reasons that there is a coating on the torsion bars to protect them from scratches. Also the marking of R and L are there as torsion bars can take a directional set to them which can lead to failure if not set in proper side of the torsion housing.

https://www.amtechinternational.com/sho ... its-gears/

For what it is worth.

Lee