I hope everyone is having a great season so far. We had our first competition last weekend, and did OK, but not as well as we hoped. We tipped over like a turtle on its back twice (3 times?) early on in each match and were down for the count. It was ugly. Our first match of the day was awesome, and that’s the video we’ll burn to disc. 😉
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We have already had to replace and repair several motors on our robot this year, so if you haven’t yet, your team will probably have to soon. It may seem a bit daunting at first—like this is something you’re not really supposed to be doing because it doesn’t open with a standard allen wrench, but really it’s OK. And if you’re doing this on a motor that you think is broken anyway, there’s probably even less to worry about.
First, you do need to find yourself a small Phillips-head screwdriver that fits the motor screws, either via a VEX seller or just at the hardware store.
Second, get yourself 1 or 2 tissues or paper towels. The gears inside the motor are greasy, so you’ll need one place to put them down, and another thing to wipe your hands on, because they will get greasy too. When you take the motor screws out, be sure to put all 4 of them in one, deliberate location; you’ll thank me later.
Where Are the Gears?
Both sides of the motor can be unscrewed, and there are gears to replace or inspect on both sides! This is something that was rather News To Me when I was starting out. I just thought that there were the gears on the shaft side of the motor and that was it, but there are other gears that can be replaced if you open up the green cap on the other side of the motor. This is the best diagram I could find. It’s a little overly technical, but it just shows the idea:
The Gears that Control Speed
If you unscrew the side of the motor that the shaft goes through (the black side of the motor), that’s where you’ll find the gears that control how fast the motor can go (or conversely, how much torque is has/how strong your motor is). When you buy a VEX motor, it comes with “normal speed” gears installed, and high-speed gears in a little plastic packet. If you’re like our team, at some point you are going to end up with a big bag of mixed normal-speed and high-speed gears and you’re going to have to figure out what’s what. So here’s a little primer:
The motor uses a pair of gears for each speed level and they mesh together, one nested on top of the other as in the somewhat-hokey picture at right. So you can see that if the stem of the gear on the left gets fatter, then the base of the gear on the right must get smaller to fit into the motor casing. Returning to the image above, normal speed gears have the thin-stemmed grooved gear, and high-speed motors have the fat stem. If you have a big bag of these things all mixed together, someone will just have to go through them one at a time and figure out what’s big & what’s small and make piles (set out one paper towel ahead of time for each type). This requires paying a bit of attention for the gear with the barrel-shaped stem (right-hand gear in the image at right), as the difference between the 2 types is very small. You need to hold them up to each other to figure out which is which.
Checking the Speed Gears
Now that you’ve got the motor opened up, take out the 2 gears and take a very close look at them, especially the gear with the grooved stem, as this is the one that usually has the problem (in our experience, anyway). Look at it under good lighting, compare it against a new gear if you have one, and make sure you rotate it all around. Often-times you will see a groove running around the stem like a ring that has been cut there by its partner gear. Next, point the end of the stem toward yourself and look at the teeth on the stem from this angle. Sometimes you can see from this viewpoint if they look junky or worn down. Last, look at all the teeth on the big round part of this gear and the barrel-gear.
If *any* of these things look bad, and you have some spare gears hanging around, replace the damaged gear and THROW THE BAD GEAR IN THE TRASH NOW to make sure it never goes back into a motor by accident. If you do not have any spare gears, you’ll probably need to order the VEX motor refurb kit ($5).
If everything looks fine on this end, put everything back in, and screw the black plastic cap back on this end of the motor. If any of those little tiny motor screws seem like they’re starting to get stripped, TRHOW THEM IN THE TRASH NOW. Every VEX motor comes with a bunch of extra screws, so there’s no need to penny-pinch, and believe me, you do not want to have to get a stripped screw out of a critical motor in an emergency—or, really, ever. Really, you do not.
The Green-Cap End
So, there is another side to this whole thing, and this is where we have had a number of our motor gear problems, especially last year with the high-speed flywheel on our Nothing But Net robot. So do the same here: get your workspace set up with your screwdriver and some tissues or paper towels, and take the green cap off of the back of the motor.
This end also has 2 gears, shown in the image at right. If you are in need of replacement versions of these, they only come in the motor refurb kit, unfortunately, and there’s only 1 of each gear in the kit. We’ve had to buy the whole kit because we needed just one of these 2 gears.
As I mentioned, we have had problems with these gears on a number of occasions, including once a gear that looked like this photo at left (that I found from another team), with a serious groove cut into the stem by one of the other gears inside the motor.
The other thing we have experienced more than once on this back side is a broken or missing tooth from the big round part of the gear! That is Not Good. Again here: inspect each gear in good light from every angle — stem-on, from the side, the teeth, and rotate it all the way around.
Sometimes you’ll see a ring of gunk, but nothing that seems actually broken or damaged, and in those cases, I don’t know what to tell you, because I don’t know what to tell my own team. We usually just end up putting the gears back in the motor, or comparing it against the extra gears we have sitting around to see if we have a better one, in which case we will swap it out, since we have the thing all opened up anyway.
Again, if you have a damaged or broken gear that you are replacing, take the damaged gear and THROW IT IN THE TRASH RIGHT NOW. This is the sure-fire way of not getting it mixed up with a functional part that goes back into someone’s robot someday. (Do you detect a pattern here?)
The Gears Look OK, But It Still Makes a Weird Noise
This happened to us recently.
On one occasion, one of our chassis wheels made a clicking noise, even just when you rotated it with your hand. So it was time to check it out. We investigated both ends of the motor and all the gears looked OK. While we had the green cap off the back, and the back-side gears out, we stuck a shaft & wheel on the motor and turned it by hand, and it was still making the clicking noise, and we also noticed that when you shook it, there seemed to be things rattle-ey inside. So we concluded that (a) there was indeed a problem, (b) we still didn’t know where it was, and (c) it didn’t matter because the motor was a goner.
On another occasion, the back-side gears looked OK on visual inspection, but as an experiment, with the green cap still off, we put a shaft & wheel in the bottom side (lots of hands holding things here) and turned it. We could see that the top-most gear was meshing with the fixed-in-place, tiny gear in the motor almost all the way around, but then it would get to a point where it would lose connection and skip. Again, we concluded that there was a problem with the alignment of one of the gear shafts, and that there was nothing we could do about it.
Junked Motor? Save the Gears!
If you do have a motor that’s headed for the graveyard, take it apart and salvage whatever gears inside still look good. Especially those gears on the back side; they’re hard to come by, and we have been in a pinch with them in the past. This may save you from having to buy the motor refurb kit some day in the future, or even more important, this may save you a week of inaction on your robot while you’re waiting for stuff to arrive in the mail.
On the flip side, we have a bag of motors that are missing the gear shown in the photo above on the left—the one with the long skinny stem. We had to put new motors on the NBN robot in their place, but instead of throwing away the motor just because it was missing one gear, we carefully labeled it as missing the gear in the back and put it in a bag with others of its kind (we broke a lot of these last year), and we will make it whole again at some point in the future—a Frankenstein motor with some parts from here, some from there… (And you can save yourself a little time down the road if you just attach the green cap with blue tape instead of screwing it on, since you know the very next thing you will need to do is take off the green cap.)
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I hope this motor-gear primer has been helpful, and taken away some of the scary-unknown-I-don’t-want-to-break-anything feeling you may have about diving in with motor maintenance. Sorry for the long gap since my last post; I’ll try to be more regular with my musings in the future!