Wheel Axle Material

[JamesCH95]

Originally Posted by craigboez

Has anyone ever seen a snap ring/e-clip give way?

I haven't seen a snap ring give out directly, but there was a case last year where a team had a snap ring groove in a torque-loaded section of the drive shaft. That is a serious stress-riser and it caused the shaft to fail.

A few thoughts: While 7075 Al has a higher yield strength than 4140 steel, 66ksi vs 60ksi, 7075 has a lower ultimate strength than 4140, 78ksi vs 95ksi. That means that the steel shaft will start to permanently bend at a lower load, but the aluminum will actually break in half first in a bending-load situation.

Aluminum and steel do not have the same tensile-strength to shear-strength relation. A rule of thumb is that for steel, shear strength = 0.75*tensile strength, but for aluminum shear strength = 0.65*tensile strength. This means that 4140 has a slight strength advantage in shear strength, the strength required to resist torsion. 4140 has the option to be heat-treated by most small machine shops to be considerably stronger, easily twice the strength of 7075 T6 aluminum.

I would use 4140 steel for the reasons I discussed above, and because I'd have the option to weld features onto the shaft if I wanted to.

[Chris is me]

Originally Posted by JamesCH95

A few thoughts: While 7075 Al has a higher yield strength than 4140 steel, 66ksi vs 60ksi, 7075 has a lower ultimate strength than 4140, 78ksi vs 95ksi. That means that the steel shaft will start to permanently bend at a lower load, but the aluminum will actually break in half first in a bending-load situation.

A shaft that yielded is just as useless as a shaft that broke.

[JamesCH95]

Originally Posted by Chris is me

A shaft that yielded is just as useless as a shaft that broke.

Not necessarily true. A bent dead-axle could still function and drive. A bent live cantilevered axle might throw a chain, but will still rotate. One might also get penalized if a part of the robot broke off and was left on the field.

Some yielding, especially localized yielding, does not render a part as failed. It was a hard mindset to get into after I got out of college. I now always think through "what if this part yields a little bit right here?" and evaluate if it will still function afterwards.

On the subject of failure, a steel shaft will deflect much less at yield than a similarly sized aluminum shaft despite having a lower yield strength. And either material will recover its shape after bending. In my experience forming both materials, I found that alloy steel will recover it's shape (spring back) more than aluminum will.

Sure you can argue it either way, I'm just offering my point of view and my experience.