What is a dead axle in FRC

Technical
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So I know that when talking about a dead axle it’s mainly talked about in a car where the back axle is not driven but can move cause the front one is driven. In FRC I hear teams calling their arms/wrist dead axles although they are driven directly which is very confusing so, what is a dead axle, how would you determine something is a dead axle, why would you use one, and how would you make one?

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Live axle: The shaft rotates, driving whatever it’s attached to.
Dead axle: The shaft doesn’t intentionally rotate, but whatever is attached to it is driven by whichever method the team uses.

Using a wheel as an example: If a sprocket is attached to the wheel, and driven by chain, the axle the wheel rides on is a dead axle. If the same sprocket and chain are attached to the axle, and the wheel is attached to the axle but not directly to the sprocket, it’s a live axle.

Dead axles can be as simple as a sufficiently-large bolt, or as complex as a custom hex shaft in hex bearings.

Why would you use one? That’s going to depend on applications. If you need a high-torque setup (e.g. a long arm) it may be a lot easier to use a large sprocket and dead-axle it than to use a live axle that needs to be super-strong. Drives and intakes tend to use live axle more than arms and wrists, in my experience–though many drives do use dead axles.

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Thanks a lot! The wheel example helps explain it, along with the long arm cause that’s what I usually hear people talk about.

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Typically to avoid stressing the shaft. Live axle setups load the shaft in torsion which can cause unwanted deflection or in extreme cases, complete failure, which is particularity detrimental on arms. Whatever interfaces between the arm and a live axle is particularly prone to failure and worsening backlash over time. Dead-axle gets rid of the extra set of adapters meaning the entire assembly is stronger and stiffer.

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There have been several great threads on this!

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