High strength shaft installation

VEX V5 General Discussion Technical Discussion
#1

Now that the new high strength shaft and bearing flats are available I have a question for those thinking of using them. The shaft is larger than the 0.182 hole in the structure pieces, will you drill out the structure so the shaft can pass through in the same way as the small shafts or will you cut the shaft to be the exact length and just support it in the bearing flat?

#2

Another option:

Or will you turn down (with a lathe) the end of the axle to make it fit in the normal-sized bearing flats?

That’s what I would do.

#3

That would just defeat the whole purpose of the HS axle. If you make the axle smaller at any point it will weaken the axle.

I would drill out the structure that way it will support the axle better and if you need to take it out for any reason at all you would just need to pull the axle out.

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#4

If you don’t install the axle into the metal, I think that it would just tear the bearing apart. I would just drill out a larger hole that would fit the full axle.

#5

This is why I thought they should have also released high strength pillow blocks so that the shafts could run beside the metal instead of going through a custom cut hole.

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New Products - August 2022
#6

The 2" 3" and 4" shafts are designed to be about 1mm shorter than the 2" 3" and 4" Standoffs. This means if you use a standoff to hold together the two pieces of metal your bearing blocks are attached to no cutting is required.

Good Luck!
, Bryan

#7

I like the idea of metal locking in the shafts. No need for shaft collars.

#8

This was a smart move on Vex’s part, I was wondering if we were going to have to do a bunch of drilling to work with the new parts!:stuck_out_tongue:

Might I ask that this bit of info be mentioned on the high strength shaft’s product page? A lot of people (myself included) might not catch this on their own…

#9

Thanks, I’ll make sure this information gets added to the product page.

, Bryan

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#10

:eek: It all makes sense now

Can I just say that the vex engineers are amazing in all forms. I just love how the engineers place as much, or even more thought into a single part than a competitor places into an entire design. It never ceases to amaze me the profusion cool things you can do without modifying vex pieces.

#11

Nope! Axles rarely (if ever) suffer from shear failure. Shear force is constant along the length of a beam, and shear strength depends on the smallest cross-section, so my method would in fact increase shear stress where the axle is narrower. But if your axle ever fails in shear, you are doing something very wrong.

Bending failure is much more likely. Bending stress is highest in the center of a beam, and reduces to zero at free ends (ends in bearing blocks), so a lower cross-sectional area at the ends will have little to no effect on bending failure.

You would of course never reduce the diameter of a shaft segment in significant torsion (e.g. between a motor and a loaded HS gear). But VEX motors cannot produce enough torque to cause twisting failure in shafts small enough to pass through holes in VEX plates, so this really isn’t a concern.

The best use of the new axles is in cases where there is very high transverse force applied to an axle, such as in compound gearing assemblies.

I’ve had many more headaches caused by VEX shaft collars (the old set screw ones) failing and shafts sliding axially than caused by needing to remove axles, so having axles that cannot fit through holes in plates is actually not a bad thing.

I applaud VEX for their ongoing consideration in making VEX easy and fun to use for everyone! This really is the best low-cost robotics design system out there.

#12

I’m sure this is what you meant, but just to avoid any confusion:

In the typical case with a Vex axle, where both ends of the beam are able to flex slightly and a point load is applied in the middle, the stress is highest wherever the load is applied.

Here is a picture which shows this for both the case where the load is in the centre of the beam and the case where the load is a quarter of the way along the beam:

The peak on the second graph for each case shows the location of greatest bending stress.

Image source: http://www.roymech.co.uk/Useful_Tables/Beams/Shear_Bending.html

#13

Yes! Yeah by “center” I meant “where the load is applied” (words are hard…).
And thanks for the pretty pictures :slight_smile:

I suppose now it’s time for some numbers…

Using the bending moment from the above graphs (labeled at “BM”) as M:

VEX 1/8" axle: 3.2mm square
Second moment of area I = (3.2mm)^4 / 12 = 8.74mm^4
Distance to edge of beam y = 1.6mm
Normal stress at edge of beam σ = M * y / I = M * 0.183mm^-3

VEX 1/4" axle: 6.4mm square
Second moment of area I = (6.4mm)^4 / 12 = 140.mm^4
Distance to edge of beam y = 3.2mm
Normal stress at edge of beam σ = M * y / I = M * 0.0229mm^-3

So the old axles experience a normal stress 8x higher (cubic relationship) than the new axles under the same load.

And if you’re worried about shearing…

Using the shear force from the above graphs (labeled as “Shear Force”) as V:
VEX 1/8" axle:
Cross-sectional area A = 10.1mm^2 area
Shear stress τ = V / A = V * 0.1mm^-2

VEX 1/4" axle:
Cross-sectional area A = 40.3mm^2 area
Shear stress τ = V / A = V * 0.025mm^-2

…and if you’re still worried about shearing…

Assume 3" (76.2mm) long beam, with load w applied at center:
VEX 1/8" axle:
Maximum normal stress σ = w * 3.5mm^-2
Shear stress τ = w * 0.05mm^-2 (70x lower)

VEX 1/4" axle:
Maximum normal stress σ = w * 0.44mm^-2
Shear stress τ = w * 0.012mm^-2 (35x lower)

…don’t be. You’d need to decrease the area of the ends of your 1/4" axle 35 times (to about 1/3 the size of a 1/8" axle) for it to be the same effective “strength” as the center of the 1/4" axle (bending failure).

#14

This may be slightly off-topic, but does anyone know if there are plans to release pillow-block bearings for the new high strength shafts?

#18

Does anyone know if pillow-block bearings are needed for high strength shafts? My team is using high strength shafts for all moving and load bearing places but they have so far opted to not use pillow blocks. Will friction be more without pillow blocks?

#19

You might end up cutting through the metal with your HS shafts. Overall, use bearings whenever you need to, dont cut corners.
Although I guess it’s ok if you dont

#20

from just trying to hand spin the shaft, it feels that it is less friction without the bearing, which is one of the reasons I posted the question here.

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