Plastic shaft strength

What is the tensile strength and the twist stength of the Vex IQ plastic shafts?
I don’t have the tools (or enough parts) to test this myself.

I don’t know the exact number, but I would say 10 pounds. I don’t use them a lot because I prefer metal axles.

I would not recommend plastic axes, as they cannot be cut, and all of the sizes are now available as metal. Plastic axes easily bend or twist. I personally am not too concerned about how much strength an the plastic axle has, but rather that it twists over time, so you would have to switch axles constantly.

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i recommend to use regular metal shafts because my team used metal motor mounting shafts for our flywheel last year and it snaped clean in have during practice

I would like to know how much stress the plastic shafts can take so I can use (and buy) less metal shafts

Hey welcome to the VEX forum, land of helpful opinions and sometimes not so helpful answers! This one is a combination of both.

As I recall, VEXIQ parts are made of ABS (Acrylonitrile Butadiene Styrene) plastic. Not sure where you live, so not sure what units (Mpa, Newtons, etc. ) you want.

In discussions of plastics materials there are details you may or may not care about. For instance, you asked about tensile strength, There are three types of tensile strength:

a)Yield strength - The stress a material can withstand without permanent deformation.
b) Ultimate strength - The maximum stress a material can withstand
c) Breaking strength - The stress coordinate on the stress-strain curve at the point of rupture

If you look here, you can find all the gory details with the math behind it.

(site also has info about ABS plastics to give you the other numbers you want)

You ask about twist strength, and while you can find the numbers on that (see pages above), there is some additional engineering you need to do. The key factor is how long is the axle. If you were to build a setup where the fixed points are next to each other, the breaking point is going to be close to the shear value. If you are using the 4" long plastic shafts, then the elasticity of the ABS plays a part in the “twist” if the points are at the far end of the shafts. Your best bet would be to use the low end of yield strength for your calculations.

A selection of plastic shafts is $8.99, a similar set of metal shafts will set you back $24 ($11.99 (axle) + $11.99) capped) or a difference of $15. Assuming that you value your time at $5 per hour, it only takes 3 hours of rebuilding to replace twisted plastic shafts to break even.

Buying the cheaper part, because it is cheaper is an aspect of “Foster’s rule on buying Harbor Freight Tools”. :tm: While the cheaper part is serviceable, in the long run it may turn out to bite you.

This is why people are saying “Use plastic axles for pure shear loads on short distances and metal for anything that has a rotational load

For example, upper support of a 4 bar linkage on Linq, it’s supported on both sides and the stress points are 1/4" apart, there is no twist. Or in use as an idler gear on a drive train, again no twist load. Metal axles on the ends of a drive train and the motive ends of a 4 bar linkage. Remember the quote from C. Checker " twist all night, Twist Round and 'round and 'round"

I applaud your efforts to apply the science. Keep it up!

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