I have heard people say not to use shaft collars like this
in their designs because they fall off. What do you use instead? One team told me that they thread the shaft and use nuts. How does this method work? What other methods are there? My team only has a bandsaw and a drill press so our manufacturing is quite limited but we may be able to go ask other teams to use their machines.
I have heard people say not to use shaft collars like this
In certain cases you can use ThunderHex and tap the ends. You can use spacers in conjunction with this.
If you can find the cash for a lathe, you could use retaining rings as well. This would make spacers easier as well as many other things.
So basically with the tapped axle technique, you’re relying on the compression from the screw PLUS a washer to make the end of the shaft larger, in a sense, preventing it from falling out of a bearing. Unfortunately, the sort-of downside with this technique is that if you cut your shafts too short, you’re going to end up clamping whatever’s spinning to the bearing, which reduces efficiency. Most teams that utilize this technique end up cutting shafts a little large, even if that means stuff can move axially. This can be mitigated by using spacers on the shaft. Furthermore, you can’t really retain things in the middle of a shaft - with bolt & washer, you’re primarily concerned with the shaft as a whole, opposed to stuff that needs to be positioned precisely in the middle of the shaft.
As for other techniques, there’s a part called “external snap rings”, which rely on you having access to a lathe to machine a groove in the axle. You expand these snap rings with snap ring pliers, and they expand over the shaft and sit in your machined groove. A benefit of snap rings is that they’re pretty light when compared to bolt and washer, but they involve a bit more machining to get right. Also, it might just be my team, but we’ve always found snap rings a bit tricky to get set on the shaft when using the pliers – there’s been more than one occasion where a snap ring flew into the dirt! (If anybody has any tips, please do share)
There are also “e-clips” which also require you to machine a groove, but do not require tools to install.
To be honest, shaft collars are fine, as long as you take the time to check them before matches! From what it seems like from your machining resources, it might be a good idea to stick with shaft collars, at least for applications where screw and washer doesn’t work. You didn’t mention having a tap set in your post, so you should consider getting a nice tap wrench and some 1/4-20 & 10-32 taps. It’s also worthwhile to consider getting a lathe for the upcoming season!
Good explanation. Thank you!
As mentioned for retaining at the ends tapping Thunderhex for 1/4-20 bolts and a flanged bolt or bolt and washer are the way to go.
For actual shaft collars we always put them on our FIRST Choice list and usually get a few through that. However you can buy them at AndyMark too, though they don’t offer the color selection available in FIRST Choice. They are larger and heavier than the Vex ones but they are more secure.
If you use shaft collars, use the head-duty ones with 2 screws. Single-screw shaft collars fail easily and move around.
We did Thunderhex + tapped ends this year, and it worked ok. But, it makes interfacing with encoders pretty hard, and it can fail if you don’t loctite your screws or use screws with a nylon locking patch. Next year we’ll have a better lathe and will try using circlips for most shafts.
The snap ring pliers I’ve used usually have a hardstop you can set to your snap ring, so you can hold the pliers really firmly while keeping the snap ring stretched to the right size. With the hardstop set, I’ve never had trouble with snap rings.
That said, thunderhex + tapped ends is the way to go.
We have used the lighter duty Vex shaft collars with no failures - by only using them in light duty situations. That is, in situations where minimal forces along the shaft length are expected, like an otherwise constrained lift. If we expect longitudinal forces, we go to the heavy duty collars, and if we expect heavy longitudinal forces, we go to a shoulder or threaded solution. This year we pivoted our intake arms (originally driven by oversize servos) on Toughbox Mini shafts. The bottom, which supported the weight of the arms, was the 3/8" end of the shaft supported in a radial bearing from the hardware store. On top, we used heavy duty shaft collars because we lifted the cubes out of the intake with a separate arm, and wanted to be sure we didn’t pull the arms off the shafts.
Not sure if “external snap rings” includes “E” rings, but that’s normally my preference. You can cut the groove with a hacksaw: lathe or drill-press optional.
An e-ring weighs less than a shaft collar and doesn’t cost three bucks. Only takes up a tenth of an inch.
You don’t need a special tool to remove an e-ring, you can just pull it with a pair of pliers or a screwdriver.
The person making the groove will screw-up at least a couple times before they learn how deep to make the grooves, so give them some practice stock.
Also, if you have a shaft that is removable, try smashing one end to form a ‘head’ like a nail head. only one end needs to be removable, right?
Shaft collars work great with shafts that only have rotational/tangential or perpendicular/radial loading, it creates a simple assembly and provides flexibility. If there is side/axial loading on a shaft like on a wheel that scrubs when turning, you’re only relying on friction to retain the shaft. A better option would be the retaining clips or bolt + washer mentioned by others, with spacers to locate the shaft as needed. Bolt + washer would be best with your team’s capabilities, they don’t have to be on center to be effective and could be done on a drill press and and then tapped.
Another option with a lathe available is to turn down the end of the shaft to a smaller diameter on both ends and use the new edges to capture the shaft in the assembly. This option is commonly used for gearboxes; it eliminates the retaining clips from the assembly.
We’ve been using exterior retaining rings for a couple years now and never had problems to make the grove we use a thin parting tool thats what i would recommend if you have a lathe but you could do it in the same way hrench described
Shaft collars were one of our major fail points that we addressed in our FMEA process this season. However, because of the extensive training we went through, we did not have time to redesign our drive train. So we had to find a fail-proof method. One of our mentors suggested that we use a Rocklinizer to help the shaft collar ‘grip’ the shaft. Holy crap it worked - so we rocklinized ALL of our collars - to a tune of 70 of them (maybe more?). It took some time - but it was worth it in the end. Almost all of the rocklinized collars are still in use.
We also found that the students have to be trained to not over-tighten the set screw as they were stripping them.
Are you planning to replace all your shaft collars each year as they’re no longer COTS?
We’ve had this issue with the Vex shaft collars as well. It turns out that the socket is not as deep as the similar screw available from McMaster-Carr. We keep a few boxes of those around and any shaft collars that come in the shop get the screw swap before they get put in the storage bins. Still possible to over-tighten and strip them out but much harder. It’s cheap insurance.
This thing looks cool. Which model are you using? Any opinion on how much it tears up the shaft?
We have used shaft collars as part of a primary torsion member in our chassis (2016), and many other less-exciting applications, and did not have issues with them loosening.
A properly sized, assembled, and torqued one or two piece shaft collar should not come loose in nearly any FRC application. The CoF of dry aluminum to aluminum is somewhere between 1.0 and 1.4, so it would take 1.0 to 1.4x the fastener clamping force to start to move the collar (which is how loosening occurs). To put some numbers to this:
VP shaft collars appear to use #4-40 screws](https://www.vexrobotics.com/shaft-collars.html#Drawing). Assuming that the screw is torqued to a conservative 1/2 of proof load, it should be generating around 386/2=193lbf of clamping force. Combined with the CoF this equates to 193lbf to 270lbf of lateral load support capability. This should be more than enough to hold even a drivetrain wheel in place.
Note that the Al-Al CoF goes straight in the toilet (0.3) if grease, oil, or other lubricants are present. **It is important to have a clean, dry, assembly when putting these together! Otherwise your holding capacity is somewhere between ~20 and ~30% of what it should be! ** We love to clean parts with acetone or 90%+IPA (iso-propyl alcohol) before assembly.
If set-screws cannot be avoided, I use cone-point screws on a D-shaft or other flattened shaft.
The Vex collars are not dry aluminum though they have been anodized. The other problem is that with a single clamping bolt, combine that with the fact that they are sized so they will slip easily over the shaft (in other words not properly sized) and the clamping area is very small. The ones from AndyMark on the other hand are sized such that you have to open them up for them to slide easily onto the shaft.
The Vex ones failed our team and I’ve seen it cause failures on other teams and those were cases where the load was quite small. So for me the ones we have in stock are only to be used as spacers going forward.
I’ve had a lot of issues with the standard VexPro shaft collars coming loose, however their “High Strength Clamping Shaft Collars” have generally held up fairly well (we used one to attach our climbing winch cord this year and never had any issues with it coming loose).
Back in the day we used to machine our own shaft collars and used a set screw to press into the shaft itself rather than using a clamping hold. This method was rock solid (assuming you used locktite on the set screw), but damaged the shaft over time making it harder to slide things on and off. These are certainly easy enough to make yourself with a drill press and a tap (you can probably even modify the Vex or AM collars to attach this way).
Almost every VEX shaft collar that has ever come off on either of my past teams have been due to this. Set screw gets stripped, student tries to make it work and doesn’t get it tight enough. Other than that we love using the low profile single screw ones and only opt out for the high strength ones in critical or high stress areas.
We make custom drivetrain shafts every year. We simply buy eight foot lengths of 0.5" 7075 Hex Shaft from a local warehouse (Online Metals)
We use external E Clips (aka Side Mounted External Rings) on the interior of the robot. This allows for a shorter interior shaft that can be easily removed with needle nose pliers in a sometimes crowed robot interior. On the outside end we tap a ¾+ inch 10-24 thread. We than install (with Loctite) a 10-24 fastener and oversize washer for wheel retention.
For cutting E-clip groves we use a Nickole tool holder with a 1.5 mm (0.059”) grooving insert. A 1/2 “ E-Clip is specified for a 0.046” wide grove with a 0.396 inner diameter. You can either use a blade micrometer (or simply trail fits with an Eclip) to obtain the right depth of cut on the first shaft. Subsequent shafts are simply cut by adjusting the coss-slide depth to the same value as the first.
There are several cheap (<$20) 2.0 mm grooving tools out there using MGMN200 inserts on amazon that would work fine; and are “way better than a hacksaw”.
For tapping holes on the lathe, I use a Brown & Sharpe spring tensioned tap guide in the drill chuck.
With decent attention to detail, there is no issue machining all the axles to the correct CAD length +/- 0.001 inch.
The use of standard c-clips isn’t really worth the effort on hex shaft. C-clips (circlips) can only be fitted over a round shaft into their grove with snap ring pliers. I would only use them on an already round shaft. Making replacement hex gearbox output shafts is the one area where we have used them.
Shaft collars might be ok for some game specific mechanism, particularly for a shaft with no real side loading. I simply would never use one for any wheel retention on a drivetrain.