Ways of retaining hex shafts?

For the 2022 season, my team used hex shafts as “rollers” for our intake and collection system. We decided that we did not want to use the shaft retainers as they were tedious and the hex-heads kept stripping due to their size and the amount of clamping force we needed from them. We ended up cutting shafts so that they were flush with the beings they went through, drilled and tapped them, and used a 1/4-20 and washer to secure them. The problem was that the bolts kept backing out, causing our shafts to come out of the bearings, even after using blue Loctite.

Any ideas for potentially fixing this issue or a new form of retainment?

I’d use these washers to secure them. They are used in industrial robotics and on Navy ships to eliminate bolts loosening due to vibration. We have used them successfully on many applications like this.

https://www.mcmaster.com/91074A129/

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  • cut a groove into the outside of the hex shaft, put a retaining ring into the groove.
  • Use a longer 1/4x20 screw – more threads, the harder it is to back-out.
  • Use reverse-threaded screws

I’m more curious what you’re doing that a normal shaft-collar just doesn’t provide enough clamping force. In a previous year, we just wrapped rubber bands around a shaft and found that was frequently sufficient to hold it in place.

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Maybe this isn’t your problem, but we found that students were putting those against the bearing and it scrape and work itself loose as it turns against the fixed part of the bearing.

We fixed this by putting a small washer that was a smaller diameter than the outside fixed part, but bigger than the moving portion.

It didn’t take much force on the collars to keep everything in place this way.

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This really surprises me. We’ve done this exact same procedure for most of our shafts for many years and never had this issue (pre-hex shaft, but our 2010 robot lived as a demo bot for 7 years before we finally took it apart, and the bolts holding the drive train shafts on didn’t need tightening that whole time). How deep are you taping, and how long are your bolts? We typically use 3/4" long bolts for our shafts, and make sure to tap far enough that they fully engage. Tighten them well, and they can be hard to take out, even with blue loctite.

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It seems that something is wrong if you can’t get clamping shaft collars to hold and you are backing out 1/4-20 screws with loctite. There must be some substantial axial force being applied to your hex shafts. Finding and removing the source of that axial force seems like the best bet.

Short of removing the root cause of the problem…

I’ll give an additional vote to external retaining rings if you have access to a lathe.

An option not yet mentioned is a clamping collar with a more substantial tightening screw and also a boss to run against the rotating part of your bearing and not rub against the stationary part.

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A quick fix that we have used put the shaft through the bearing, let it extend about an inch and put a few wraps of electrical tape on it. With that added thickness, the hex shaft will not be able to slide through the bearing.

Duct tape is a surprisingly good way to keep things stuck together.

Other than that, +1 to the suggestion for reverse threaded bolts, but I’d like to know what forces you’re putting on that since I don’t think I’ve seen a 1/4-20 bolt back out of a 1/2" hex shaft on a drivetrain in 7 years.
E-clips/retaining rings, longer screws, shaft collars (might be the retainers you mentioned as stripping) are all things to try but if those aren’t working for you, it may be an idea to look at what the shaft is doing and what the heck those forces are.
You could also try welding but that’s likely out of reach for most teams, and is probably more effort than it’s worth.

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I would suggest 1/4-28 bolts with loctite, but the thought of what happens when they inevitably get mixed up with the 1/4-20s prevents that suggestion.

I have the same question as most with “what kind of load ARE you putting on this shaft?” We’ve rarely had a problem with shaft collars (as long as they’re actually HEX shaft collars–if you’re using round I’ve got some words for you along the lines of “there’s your problem!”).

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You might be doing something wrong there, possibly using an oversized tap or not cleaning the surfaces enough. Or my favorite to explain to people: Loctite threadlocker needs like 24 hours to cure: it’s right there on the bottle. Or on their website: image

There are accelerants, such as Loctite 7649, but that’s getting a bit too far into the weeds for FRC. Easier to just use super glue at that point.

As Eric mentioned, finer threads are a nice option to have and make sense for this sort of application.

Of course, you can always reach faaaaar back in the “securing thing to shaft” playbook and get some left hand threaded 1/4-20s.

We use Vex Pro shaft collars, and replace the hex 4-40 screw with a Torx, then crank on them so they’re really tight

Haven’t had one of those slip yet

Ideally we would use an E clip but we don’t have a working lathe yet

We have had big success with almost all loctite brand accelerants when it comes to blue on high vibration applications like an intake. We struggled with this early on in the season, and after getting disciplined about using accelerator had 0 failures from that point on.

I’m a fan of the aerosol spray for ease of use.

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Have y’all noticed any weird evaporation on those? I have some 7649 in various forms and it seems to just sort of disappear out of its container after a few months.

We’ve had good luck with: Loctite® Primer 7649, 0.88 oz. Aerosol Can

https://www.mcmaster.com/7614A27

Nowhere near as bad as the loctite sticks drying out :upside_down_face:

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If you are using stainless steel screws in your Aluminum hex shaft the loctite will never set, or will take days… copper sulfate loctite primer will fix this issue. It’s a classic failure for a First-timer doing vibration tests… “But I loctited all the screws! Why did it fall apart???”

Wedge lock washers are the only lockwasher type object thst actually works… all the rest make your bolts loosen faster :wink:

Buying some fender washers at 0.6" OD or a little less helps this fastening work better.

I’ve had very poor results with snap rings in FRC. They work out of the grooves and/or get over stretched on installation. A shaft collar seated on the snap ring isolates it from the jiggling gears and makes it a reliable locating point in OUR use. YMMV.

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Let me guess were they the Vex collars? Never had any luck with those myself and I don’t allow them to be used on the robot anymore.

The solution is nylock bolts which you can buy at McMaster in regular and button head versions.

Nord Lockwashers are, hands down, the best anti-loosening solution I’ve ever used.

However… and it’s a big however… @Jcichop we have used nothing, or maybe a splash of medium loctite, for tons of similar hex shaft retention cases with no significant failure rate (or any memorable failures). This includes intake, drivetrain, and even shooters.

To guess, it seems like you may have something spinning or dragging on whatever flat washer(s) you use.

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I second this! They strip out way too easily and do not hold in place.

In terms of the screw in the ends I recommend these screws, just know that you will need a little bit of a countersink in your threads. https://www.mcmaster.com/#90835A626

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Wow! Those are super cool! I was looking for one that had a head the right size, but sidewalk bolts are too big!