My team continuously struggles getting certain drive shafts to pass through the metal square inserts for the high strength gears. Some go through just fine and dandy, others are impossibly tight (most are like this in fact). This makes a simple job of sliding a shaft through an arm, for instance, incredibly difficult. We have often times gotten these darn inserts so stuck on shafts while trying to assemble a robot that we have to put them in a vise and hammer them out. I have thrown countless shafts/inserts away due to this.
This makes us sound like we don’t know what we are doing, but I assure you it is the tolerances to which the shafts/inserts are produced are not very good. I am wondering if anyone else has this problem.
Also, one time we received an order in which the drive shafts were literally rectangular, not square, and could not be used. I complained to VEX in that case and they did replace them for free. I feel their quality control/tolerance allowances are not acceptable for these parts. It wouldn’t be such a big deal, but every moving part of a robot uses drive shafts…kind of an important part.
I feel like we are alone on this because it seems like a big problem, but yet I hear nothing on the forms about it…makes us feel like we are doing something really stupid?
I generally don’t have severe issues with this, but I agree they do fit quite tightly and sometimes get stuck. Generally, once there are scuffs on the shaft from the shaft locks, or oxidation from age, they get stuck more often. A recommendation is to sand the shafts in high-grit sand paper and reduce their size this way before using them. I do realize the importance of having a tight fit, though.
My team always sanded the axles before using them, as well. The set screws in the old steel shaft collars really did a number on the axles’ nice finish.
We bought some 1/8" steel bar stock from another source once to save a few bucks, but they didn’t have the same rounded corners that the VEX ones do, and as a result didn’t fit without significant sanding.
The tolerances are made to be tight to ensure as little backlash as possible, and I think the trade-off is well worth it.
You are not alone! My team has problems like this all the time. I think the reason you don’t hear about is just the fact that some people find it an issue and some don’t and it really comes down to the set of shafts in the order. You are absolutely right, pretty much every moving part of a robot needs a drive shaft.
I agree with you in every way.
You are not alone. As has been mentioned already, the set screws of the metal shaft collars can put nearly-microscopic scuffs on the shafts that the inserts seem to locate, magnify, then turn into friction welds.
You need to make sure the shafts are clean and that their ends are burr-free before pushing the shafts into the inserts. Feelings count - use your fingers to feel around the shafts for ANY rough spots, then smooth them out with super-fine sand paper. Don’t over sand the shafts, however, else their cross sections become nearly circular and your inserts might lose their grip. Also, look at the inserts with a magnifier to ascertain that the metal has not become dinged on their edges, which can lead to friction welding, too.
A tiny amount of grease might help, too.
I’ve been encouraging my kids to use the new, larger shafts this year when and where they can just to avoid the irritation of un-sticking stuck shafts. The kids almost always come crying to me to fix these friction welds and it’s a depressing thing to have to deal with.
In my past, I’ve also had some troubles with fitting the drive shafts on some VEX parts (particularly metal gears, not so much inserts). Usually it’s because wear and tear on the drive shaft makes it kind of uneven, rough, or bent in some places. I just check for those things. Usually it’s rough because so many things have been sliding on the shafts and it’s just wear and tear. I just use a file to sand down any irregularities on the shafts and they work just fine.
Just knowing we are not alone makes me feel better! haha. Still frustrating though. I understand the tolerances should be tight, but when they are so tight they are not even usable…that isn’t any good!
I totally hear you regarding shafts that have nicks on them from set screws, and end burrs. That is understandable though. A little sanding does the trick. However, I am talking about brand new shafts and inserts that still don’t even fit well, if at all! I think I’m going to bring my caliper to my classroom tomorrow if I remember and see how “out of square” some of the shafts are.
Just to clarify. I am talking about brand new shafts and inserts not fitting properly. It is completely understandable that even after one use, a shaft will not slide as well through inserts and gears. They get so nicked up from set screws and the like. That is normal wear and tear and I am not upset by that.
In terms of performance, a shaft that has to be hammered into the gear is preferable to one that moves around inside the gear. Of course there is a tradeoff to be made between performance and ease of use, but I think you’ll find most of the people who spend time on the forums prioritise performance.
This implies that the shafts that slide nicely into the inserts and gears are sloppy and fail often. Of the shafts that I have had that do slide nicely…not loose…just actually slide, I have NEVER had one fail. They are simply “snug” and still slide, not Godzilla tight. I do not believe you are making a valid arguement. My arguement is that the tolerances to which these parts are built is crappy. One shaft will fit through one insert, and another shaft will not fit through the exact same insert. Or visa versa. Again, I am talking about BRAND NEW parts.
My original point was that often times the shafts are quite literally impossible to push through several gears/inserts at a time. I’ll try to give an example. Lets say you have a compound gear system with just two gears on a single shaft. Each gear has inserts. Meaning each gear has two metal inserts. What happens to us is IF we can even get the shaft to go through the 1st of four inserts, as you continue to push towards the insert towards the “backside” of the gear, because the fit is so unbelievably tight it simply pushes the insert out of the gear instead of passing through.
Also, we often have the problem that an insert will go onto a shaft from one side, but not the other. Meaning the shaft is not even parallel down its length. On these shafts if you push the insert down the “good” side of the shaft, it will because seized somewhere along the shafts length. I’m not talking about just a little tight. I’m talking about impossible to move any further no matter how much pounding. This infuriates me because a pack of 4 shafts costs something like $9, and we are lucky if 2 of the 4 shafts are any good.
I have had this problem from several orders of shafts, so it’s not like we just got one bad batch, which is why I have to believe many other teams have this problem (as a few have already replied to this thread that they do indeed)
I’d grab a micrometer and measure the dimensions of the axles. Record the apparent size tolerance and parallelism and send those numbers to VEX. The tolerance should be ~ +0.0000" -0.0020" for this kind of part (assuming ⌀0.0010" oversized hole).
I’m surprised you’re having this problem, because I’ve never seen a defective VEX product; they’re generally very good about quality control.
I have this problem occasionally, there are a few issues, including the potential for tolerance errors but the biggest problem I see is that the gear inserts are not installed correctly.
This is the ideal condition, everything is square and fits properly.
If one or both of the gear inserts are installed at an angle, then the shaft will bind.
I measured a few shafts and they are all slightly undersized, 0.123". I checked a few gear inserts and they all work with these shafts and slide smoothly, my best efforts to measure the hole puts it at exactly 0.125".
When press fitting the gear inserts use something that pushes them in perpendicular to the gear, I use a small vice with plastic jaws and press fit both sides together. Then check that the shaft passes through both inserts easily, if needed use a piece of scrap shaft to realign one or both inserts.
