Gears Archives - Page 3 of 3 - Mentored Engineer

In this video, Corey explains the basics of gear materials, the types of runout, how to set backlash and why gears wear. https://youtu.be/oCXJVmi0TdY Video Transcript In this one, we're going to talk about a few subjects. The first one being fatigue and wear. Now fatigue is more of a problem that usually evidence itself by a tooth breaking off. And a lot of times that's just due to high load. So, the first thing you can do is get your diameters up.And what that really does is it sometimes eliminates the amount of torque that needs to be produced and lowers your tangential force. The other thing it does is if you go to a bigger number of gears, it actually raises your Lewis form factor. We're still working under this equation. The other things we can do are we can raise our allowable stress. That could be done just by using a stronger material or possibly hardening that material.There is a cutoff as to how much that will help you in fatigue. Obviously, if you're getting up to 180, 200 KSI on your gear teeth, you may want to change and try a different design. We can increase our rack width or just our gear width sorry or we could decrease our pitch diameter and with all these things there are trade-offs with power transmission and just the size to put all these things in maybe you find that gears just aren't the right method of transmitting that power. Maybe something, a V-belt or a chain and sprocket system would be better.Who knows? So, one thing you can do with gears to make sure they don't have premature wear is to make sure that they don't mess with the same teeth every time they go through. Alright, so if we look at this, this gear has 24 teeth, and this one has 40 teeth. Alright, I could divide both by 8, so this would be 3 and 5. So for every 5 revolutions, it's going to line up perfectly again.With the teeth on this one which will have completed three revolutions and that's not terrible but if I had let's just say this was 24 and 48 that means every two revolutions I’m hitting on the same teeth and that's you know that's getting down there but if I had 23 and 48 wow that's quite a few revolutions away that they're that they'll mesh again and that just helps us to keep the pinion teeth wearing because generally speaking the smaller gear is going to wear faster than the bigger gear will all right so we want to make sure that we look into that when we're designing our system and just see you know can I just make this of a weird number of teethThe second thing I want to talk about is material. So, your materials are an important thing. You basically have three categories. You've got plastics, you've got brass and its subsidiaries, and then you've got steel. And you'll see gears made out of different things, but we'll just kind of go through them and highlight the good and bad things of them.We won't get very deep into this. There's a lot of research that can be done here. So plastics are more for precision, low torque things. Lego gears are made of plastic because they can withstand High speeds, fairly well. They don't shave on each other much.Basically, they're going to last as long as you're not transmitting a lot of power. As you get down to the harder materials, brass, probably the first chance that you're going to actually be able to transmit some sort of power. The wear is pretty good on that, but as with metal on metal, you're going to have more heat building up, so you're going to need something to remove that heat. Lubrication is a great way to, one, prevent the heat from starting, and two, just remove the heat immediately. It'll flow right into the fluid and be okay.And then steel, steel you can get that really hard. You can transfer a lot of force through gears and that's probably the number one choice of bigger gears is steel. Obviously, they cost more than plastic gears but they'll last a lot longer under the loads consuming. One of the things that I haven't touched on is with these is speed. You know, plastic gear, plastic is much, much lighter than steel, probably about fourth to an eighth of the weight.So, you're not going to have a bunch of mass trying to fling itself. You don't have to worry about out of balance as much. So, if you're using it at high speed, plastic is going to be a much better material than steel will. Alright, well that's all I wanted to focus on materials. So, the last thing I want to talk about in this video is run out.Run out is the eccentricity of the gear to the shaft. So, there's three different kinds. There's axial, angular, and radial. Alright, so radial is very simply put, I have a shaft and then I have a clear you know distance that it is out of round all right and that's you know shown right here as it goes around it's going to flop around like that that's radial run out axial run out is if I have a gear on a shaft and I put a an indicator right on the surfaceand I watch the dials I go around you know what is it what is it moving and you know probably have a tolerance of what is acceptable if you're doing it on a large crane rotation bearing quarter inch may not hurt you but if you're doing some something precise like a clock or a watch or something like that ten thousands maybe way too much And then angularity this would be you know how the how the gear is pressed on the shaft You know you want to find out essentially What is this angle right here?And that's going to tell you something that kind of tied closely with the angular or the axial road run out so Just so you're aware of those things and you can keep an eye on them. Now, there's two types of gears. There are normal gears that you'll buy and then you can buy precision gears.And these things, somebody spent the time, and you will spend the money if you use them to get these where... All these runouts are very small. The teeth are almost exactly identical. They've probably even been put on a CMM machine, a coordinate measuring machine. Well, they will go through and trace the profiles at various depths through the gear tooth.it's there you can buy it and it's expensive but these things will be minimized however for a lot of cases probably just don't need it the last thing we want to talk about here is backlash is something that all gears have and we want to try to minimize it in certain conditions it's basically a sloppiness between the two gears so we run a balance between The two gears meshing on this side and having a gap over here, you can see this gap right here. If we have a crane pivot where it's rotating side to side, we probably want to keep that fairly tight.because we don't want the crane going jerk, jerk, jerk, jerk, and then it's got some momentum every time it hits each side of the teeth just it's going to wear it might even fatigue a tooth off if we were to do that you know several hundred times so you want to get that fairly tight just so it feels snug and as I mentioned earlier you basically want to go in and be able to adjust that dimension. My favorite way to adjust these is to take one of them, usually the pinion, and make an eccentric hole for it.So, I'd have my hole directly on top of that and then I'd have Another hole. No, pretend those are round. I'm sorry. My freehand drawing is not that good. But you can clearly see that, let's just say, the center of this hole, the inner hole, is there, and the center of the outer hole is there.And maybe, I don't know, depending on your application, that's 1 16th to 1 8th of an inch off-center. Whoops. 1 8th, not 18th. And you can... Adjust that as needed by rotating this around and they go closer together or further apart. Now what you'll need is some way to lock them in place.There's some way to do it. You can kind of put almost teeth on this and then have some sort of plate, lock that in, or something like that. You'll have to get a little creative there. I've got some ideas, but they're proprietary, so I'm not going to share those. So that's just one way to do it.You can just clamp them together. clamp this to something else and just allow friction to account for that. That might loosen up over time. You probably want more of a positive engagement. So, the way you would measure that is you would take whichever gear remains steady.Let's just say it's this one. This is the pinion gear that's driving this gear. What we want to do is get in as close as we can and we're going to mount a dial gauge here so that's not a good so what we want to do is we want to come up on this tooth and get as perpendicular as we can and put a dial gauge here and have it mounted back to this gear whatever holds this gear that whole mechanismand then we want to just rotate it back and forth until we get whatever amount of backlash we're okay with but it basically just measures how much change is going to happen when we when we rock this thing back and forth generally you want to see you know five to ten thousandths or something of that nature if you get tighter than that you're going to start to buying this up right here also a lot of gear manufacturers know that their gears aren't perfect and they will actually indicate to you usually by painting on there which teeth are the highest or the most oversized the target so that way you can mesh those ones and then put your dial indicator on that and do that.So, if you don't know which ones are the high ones, you may have to do this several times. Okay, well thank you for watching this episode. Be sure to join us on the next one.

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