Tetrix axle set collar set screws

[FTC0839]

I don't know if we just got a bad batch or what, but is anyone else having problems with the collar set screw socket stripping? We used to round the allen wrenches, now it seems like the set screw is losing the battle. It's easy enough to refresh an allen wrench on the belt sander, but set screws ... not so much.

[http://www.tetrixrobotics.com/TETRIX_Axle_Set_Collars]

[EV713]

We haven't had trouble with the set screws stripping, but we did have trouble with them working loose. We sometimes replace the set screws with short bolts (used to hold TETRIX parts together). The bolts don't work loose as easily and allow us to use the same allen wrench for both the collars and the bolts holding TETRIX parts together.

[Cheer4FTC]

We often have problems with the Tetrix shaft collar set screws. I believe that the hex hole in the set screws is not the correct size for the tetrix wrenches.

We've had better luck going to McMaster and purchasing either replacement set screws for the tetrix shaft collars or even new shaft collars from McMaster that come with better set screws in them.

[FTC0839]

Dear Cheer4FTC, thanks for the response!
We've NEVER had problems before this season with the set screws - we usually twist the allen wrench and mush the soft stainless steel axles.
I went to HomeDepot to see if I could buy higher-quality replacement set screws retail. It appears that the set screws are M4-0.7 x 4mm (4mm dia x 0.7 threads/mm x 4mm long), so it probably would require a metric allen wrench - that's one thing I'll try to address with the team. Being so small, the near-equivalent US Customary (inch) size may not be the correct size, and may be what's rounding off the hex socket.
I was also disturbed by the material on the Everbilt bags on the set screws at HD - it said "steel" and/or "Plain Steel" [I forget]; however the tag on the bin said "Alloy Steel". As a metallurgist, this is disturbing on several accounts. First, there are many classes of "steel" that it could be: mild (low carbon) steel, medium-carbon steel or alloy steel - generally in increasing order of strength. [They're not stainless because they're black in color - from heat treat or a coating, I'm not sure.] If these are mild steel, they're doomed to failure - the allen wrench will always win. If these are heat-treated (medium-carbon steel or alloy steel), those would more likely be the proper materials. Per the description on the website [http://www.homedepot.com/p/Everbilt-...778/204275887], these are supposedly heat-treated alloy steel, so maybe the reference to "Plain" means non-stainless?
Since I also work in my professional life with codes and standards, I looked for the ASTM specification(s) on [steel] set screws. I could only find specifications for alloy and stainless steel set screws, none for "plain" steel. ASTM F912M is for metric alloy steel set screws, and requires that they be medium-carbon (0.30-0.48%) plus Cr, Ni, Mo and/or V to alloy for strength, and that they be heat treated for strength by quenching and tempering. It also has a minimum specification torque of 2 N-m (17.7 in-lbs) - I doubt that we torque them nearly that hard (~8 lbs force [~weight of a gallon of milk] on a ~2" long allen wrench). So, there appears to be a possible quality control issue (along with the wrong allen wrench size?).
Thanks for the McMaster suggestion. I'll try MSC and Grainger too.

[FTC0839]

Dear EV713, that may not be a bad suggestion either. Shouldn't be too much of a safety issue, and commonality of tooling is a good idea.

[busyb97]

We've had problems with the screws coming loose on the All-Terrain Wheel shafts as well. We replaced the set screws with regular small flat-tip screws, and it HELPS, but hasn't eliminated the problem completely. We're just having to remember to retighten them every so often. (And we've got at least one set screw that was stripped and we had to just let it stay where it was and rearrange the config to make it work. We'll deal with it later. I'm glad to know it at least wasn't just us doing some stupid rookie-mistake!

[energyelites]

We've had the same problem with the All-Terrain Wheel shafts as well. Ours aren't stripping, but they are working themselves loose. I'll let the team know that others are having this issue and have them look for additional screws.

[RRLedford]

We rarely need to use collars on axles because we have mostly eliminated the use of Tetrix axles in favor of long (up to 5") partially threaded 10-32 socket head cap screws which have the head and nylon insert nut at either end.
We attach sprockets or gears directly to the driven items and have the SHCS axles squeezing the driven item enough that the screw free spins within the sleeve bearing module.
So, no collars are needed to retain the stack if items in the axle. The unthreaded portion of SHCS (after just a bit of polishing in the drill press) rides nicely fitted into the sleeve bearings, and the nylon insert lock nut outside the wheel adjusts for optimum axial free play. The doubled up channel pieces, with one leg edge of each shortened enough to have hole pattern remain aligned when they nest together, also aligns and supports the two sleeve bearings better. Avoiding the 6-32 socket heads, and instead using button head or flat head 6-32 screws with countersinking hub/gear/sprocket faces as needed, helps keep the the component stack tightly packed together along the axle without spacers.



If you must use collars (or set screw hubs), one easy fix is to replace the set screws with matching thread socket head cap screws to allow use of a bigger size wrench that is less likely to strip its bigger socket. Of course you must have extra swing clearance for the now protruding head.
When collars or hubs that use set screws are simply your only option to hold location against high axial forces and/or transfer high levels of torque on flatted axles/shafts , and you want to minimize the chances of screw loosening, if space allows, replace set screws with long enough socket head caps screws so that when screw is tight will still be a gap in which to locate a jam nut between the head and the collar/hub. Then after tightening the SHCS to the max, you use a wrench to tighten the jam nut against the collar/hub OD to further lock it in place. This will dramatically reduce the frequency of screw loosening.

In the case of hubs, when axial space allows, you can attach double hubs to gears or sprockets (with one on each side). Having two hubs & two set screws significantly lowers the wiggling amplitude of gears and sprockets on the axles/shafts, and it is this wiggling that triggers the set screw loosening process.

For just locking a 16 tooth sprocket on a 6mm shaft motor, this split clamping 2-piece 16-tooth sprocket design with integrated sleeve-hub from
AndyMark → S25-16T 6mm Bore Aluminum Sprocket (am-3185) ← is near bullet proof, especially if you use a touch of grease the threads of the clamping screw.
LINK→http://www.andymark.com/product-p/am-3185.htm


-Dick Ledford

[FTC0839]

I just received an email from Pitsco that [one of] their vendor(s) mistakenly shipped collars with metric set screws that would require a 2 mm Allen wrench / "hex key", which is 5.04/64th's of an inch.
Per http://www.tetrixrobotics.com/gettin...ools_guide.pdf, the correctly-manufactured collars require the 1/16" (or 4/64") hex key.
Since the hex socket is 0.016" bigger than the 1/16" hex key, this explains why we are rounding the sockets.

It had also confused me why I had found that they were 4 mm set screws, but another Mentor found that a #6 screw fit perfectly - just luck of the draw.

We will segregate (discard) the metric collars. Pitsco has said they would replace the ones with the wrong set screws.

[FTC0839]

And busyb97, you should look into a thread-locker like Loctite 222 (e.g., http://www.mscdirect.com/product/details/88545157). Or we have a Mentor who works in industry who uses as a shortcut nail polish on the nut-bolt junction after the nut is torqued.