Larger OD Washers for High Strength shafts

I already agreed with this, but I’m going to agree with this again because I hope the VEX folks are reading. I really hate those square holed spacers. (Also, wouldn’t it be cheaper for vex to just sell a plain circular type?)

Speaking of high strength shafts, I’d kind of like to see a version of the EDR motors that accomodates high strength shafts.

Me, too. And I think a lot of people would love to see that. Can you start a new thread for that request?

This has been discussed at length before. It shouldn’t be necessary. In any system in which you require high strength shafts you will be compound gearing therefore the first set of gears (off the motor) will be just fine using the 1/8 shafts (if built properly).It is the slower moving shafts (where the high torque is) that requires the 1/4 shafts.

I’ll admit, we have twisted/bent axles and perhaps a 1/4 motor adapter could have fixed that. But my guess is that if we did have that part available, all that would happen is the motor gears would be the next week point and strip out anyways. Then people will be complaining about the motor gears being “weak”.

I don’t think that I’m the right person for that job, to be quite honest. I don’t think I’d be able to make a very good case for why it would be worth it. :L

I would agree with you (and I did before last year) but after running torsional stress analysis on the primary drive shaft on our flywheel, 5 motors on a regular shaft operating at stall torque would shatter an 1/8" shaft nearly immediately. It places nearly 3 times the yield stress of the metal. Of course motors can’t operate at stall torque, but it’s still enough to twist 1/8" shafts with ease. When the motors operate at 75% output the shafts are fine, but during spin-up the motors get close to stall torque and that’s when the twisting/shearing occurs. I think that makes a pretty decent bid.

I thought I had to use an 11/32 drill bit for free spinning. (0.34375")

This doesn’t make sense to me? How can you possible hook 5 motors to the same 1/8 shaft? At most you can have two (one on each end).

So, you use 1/8" shafts directly off the motor, then on the driven side of the system you can use 1/4" shafts. No bending.

Gears? We had a gearbox with 5 motors on the side all using 1/8" shafts with one of them going to a 35:1 gear ratio to the flywheel. The shaft that connected the motors to the flywheel twisted.

We’ve turned many 1/8" shafts into drill bits over the years.

Have not done that to 1/4" shafts yet. (I say yet… but let’s do some math…)

http://www.engineeringtoolbox.com/torsion-shafts-d_947.html

This is the max torsion before bad things happen and you don’t go back to your original shape. We look at the material properties of the metal and the shape of the object in this equation. Here we have a square shaft and we will assume they are the same material. Can’t say for sure now but I think they are. That would effect σmax in the equation below…

Max_torsion_moment_square_shaft = (2 / 9) *σmax *b^3

0.125" cubed = 0.001953

0.25" cubed = 0.015625

So the high strength shafts are over a factor of 10 better in regards to twist.

And maybe 11/32" is the best actual common drill size. I can’t say for sure. I was just going by a quick caliper measurement I had made last night. But if that’s true, then I think that further supports my argument that there is very little margin for error between pass-through holes and the ODs of the square-holed spacers.

(0.390 - 0.34375)/2 = 0.023 inch, which is about half the thickness of my thumbnail.

Since you need to use HS axle bearing flats anyway, can’t you put one between the spacers and the metal instead of on the other side of the metal? That would probably solve the issue of the spacers getting jammed into the drilled-out hole.

Most times the bearing flat is in place as you drive the shaft through the components of the lift. Start on one side that has a bigger hole drilled in it, push the HS shaft through the bearing block, other spacers, the gear, then more spacers.

Before you get to the final bearing block you typically have a hard time to insert that last spacer. So you want a nice easy washer to slide on as you hold it with tweezers or your fingers. Aligning the square hole to the square shaft can be a bear as you lose sight of the square hole. Drop it about 4-5 times and frustration ensues.

So if you have the last spacer be a round washer, you can slip it on easier and get on with things. You many times do this job in replacing a gear whose teeth broke, or replacing a motor whose screws are covered up by the gear. You don’t want to take apart the whole tower, just that section.

Yes, it’s possible to do that, but if they are making gear boxes, it can add thickness to their design. Also, if they place bearing flats on both sides of one metal hole, it can sometimes greatly increase the friction on the shaft. Generally speaking, you want them to have no more than two bearings supporting a shaft, otherwise they increase the risk of the shaft jamming simply because it’s a lot harder to adequately line up 3 holes instead of 2 holes.

But this is exactly my point. You could have simply used a 1/4 shaft on that FINAL shaft that goes to the flywheel. No twisting would have occurred. You can’t have a 35:1 ratio when directly driving a flywheel with a motor. (unless I am misunderstanding what you are trying to say)

As have we, but I would argue that this typically happens only when compound gearing. Which means you can use 1/4" shafts anyways (on the driven side) and motor adapters for them wouldn’t really help. If teams are twisting shafts that are directly driven by motors, then there is most likely a design problem. Also, like I said before, if we do get the ability to direct drive 1/4" shafts then all that will happen is the gears in the motor will be the next weak point. So instead of turning a 1/8" shaft into a drill bit you will shred the motor gears. Not sure if that is any better.

This is exactly what we were forced to do. It was a real challenge because it wasn’t a roomy area. But if I quote

It was definitely built properly but still twisted the shafts that “will be just fine”. My point is that 4 motors on a 1:1 output enough torque to twist an 1/8" shaft with relative ease.

Not sure if I understand. In this case, you actually have 4 motors with 4 separate 1/8" shafts driving a 1/4" shaft with a flywheel at the end? We have had far more extreme use cases that have held up without twisting?

Honestly I think I am completely misunderstanding what you are saying…sorry.

Yes, 1/8" shafts will hold up if the motors are running at around 60% speed or more, but lower than that you begin to twist (i.e. starting up a flywheel). Speed and torque have a near linear relationship on electric motors (including VEX 393s) so shafts are fine at higher speeds but when more load is applied and the closer the motors get closer to stall torque the shafts simply cannot hold up.

Backing up to “drilling the nylon spacers,” when drilling plastics, it really helps to modify the drill before drilling. Here is a graphic of how to do it. The 90° point is not as important as the zero-rake angle. I just take the drill to a belt grinder and touch the two cutting edges to straighten them out. The goal is to make a scraper out of the drill, so it doesn’t try to “screw” itself into the plastic. (this is great for brass too, by the way). Once the drill is modified, keep it in a safe place, because it won’t work for steel any more.

Holding the part (with safety in mind): If you have one available, a collet in a lathe is the best way to drill out bushings and spacers. The collet retains the shape of the bushing, and lathes drill on-axis. If you don’t have a lathe, you can use use a collet with a drill press by getting a “5-C Collet Fixture” (about $40 from Grizzly or similar tool importers, 5-C Collets run $8).