A Simple Planetary Gear Set... Let's make an OER!

In this episode Harnek and Ian discuss a simple planetary gear set and review some basic gear theory. This episode of the Talking with SOT podcast will be licensed in a special way! Using a Creative Commons license this podcast is open source to be used in any way for students to help understand a simple planetary gear set across the globe! For more information on Open Educational Resources contact icampbell@centennialcollege.ca. 

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Transcript

0:08

Welcome to the Talking with the School of Transportation podcast, presented by Centennial College. With your hosts, Ian Campbell and Harna Gill. Let's get into it now. Welcome back to the Talking with the SOT podcast. We have a special episode based on something we're working with with Centennial College. That's our OERs, Open Educational Resources. So these are resources that are available to anyone, uh free of charge. And the idea is that we want to promote learning and reduce barriers for those who aren't traditionally able to access uh some of the information. So this OER will be available uh for anyone who's interested in learning about uh what are we covering today, you know? Planetary gears, one of my favorite subjects. So a planetary gear is something that's utilized in an automatic transmission. Yeah, a bunch of different uh applications. We can see them in starter motors and in the car where we can want to change the gear ratio for maybe more torque. We can see them in uh electric power steering sometimes. You see them in there. And uh mainly, I guess one of the most predominant places would be the transmission, specifically an automatic transmission. Uh because we can create a whole bunch of different gear ratios from three different gears. Now, in a transmission, we have to add a couple uh two of these together to make it work because of how many uh components can be grabbed or held and applied. Um, but just the simple planetary gear set is uh pretty impressive overall for just having three gears. Uh and those three gears, do you know what they might be? I believe I have an idea. So we have our ring gear, okay, also known as our annulus, and then we have a uh planetary gear. What's the planetary gear look like? So it's generally composed of uh multiple gears. Is that fair to say? On a housing on a housing that the housing together called the carrier, essentially. And those little gears, do you know what they might be called? The planetary pinions. Okay. So we have like little planet planetary pinions that kind of rotate around another gear that you haven't said yet. And do you know what that is? Which is the sun. Yeah. So we have planets that rotate around the sun, and then a ring gear that kind of encapsulates encapsulates it all. Holds it all together. Yeah. When we talk about planetary gears, I think we need to start with some basic gear theory. Before we know what the planetaries can do, we kind of have to know what gears do themselves. Sure. So gears themselves are basically like little levers pushing on another lever that's going to rotate or transmit torque or power or movement from one to the next. Does that kind of make sense? Absolutely. And those levers or or gears that we are rotating now, we can change the size or the ratio of them to give us a different output. So what we could have is a either a larger gear and a smaller gear, either a large gear driving a small gear or a small gear driving a larger gear, but then I guess that how does that affect our output? Well, that's that's a huge factor. You think about a bicycle, right? And when you're riding a bicycle, you could have a small gear at the front and a big gear at the back, and that's gonna change the amount of effort needed for different applications, right? Now a bicycle has a chain on that, but that's still essentially sprockets and gears. Uh so if we have a real small gear, that's gonna be the input. And I like to use the words input and output instead of driving or driven. I feel it just kind of sends it home a little easier. When we say driving and driven, it can get really muddled sometimes when you're thinking about how many components are doing what. So if we have the input as a small gear, right? Let's say it has 10 teeth. Okay. Okay, and the output is a bigger gear, let's say it has 30 teeth. What do you think that output might be? Like a ratio? Yeah. So like a three to one ratio. How'd you come up with three to one? That is a great question. So what I did was I took my driven or my output gear divided by my drive or input gear. Exactly. So, and I'm glad that you said that with the input beside drive and the output beside driven, because that can really kind of remember what it is. It's kind of hard when it's like drive over driven and all this kind of stuff. So if you can remember the input, right? And we have the output over the input will give us that. So driven over drive, and if you've taken any classes with me, that's hammered home, uh, gives you that ratio of three to one. So, what does that mean, three to one, though? Okay, that's a great question. Well, uh, for me to turn that big gear at the back, how many rotations do you think I'd have to go on the on the smaller gear? So I'd have to go three rotations to get one to get one full rotation on that other one. Now that creates a ton of torque because you're moving that gear so much. Uh you know, you could wheelie on the bicycle pretty easily or whatever that might be. Or if we're talking about a car, which we're uh eventually going to be talking about, that can get the vehicle up to speed quick. Now, the difference between torque and speed, though, is something else we need to talk about. So there's a direct uh relationship. Yeah. And they're like inversely proportional. Is that fair to say? Yeah, the uh you can only have one without the other, essentially. You can have a balance between them at times, and that would probably be a one-to-one ratio, which would be, you know, straight through the same speed, maybe a BMX bike or something like that, depending on on who you are. Uh, or you could have a high torque, which we just talked about, that three to one or higher, depending on what the application is, or then you could have a speed. And what do you think a speed ratio might look like? So would this be a value that is maybe less less than one? Exactly. Like a 0.8 to one or uh 0.25 to 1 would be an extreme overdrive condition. So in this overdrive condition, what's changed? What's how did I achieve this overdrive? Well, instead of having a small gear driving a big gear giving us that four to one or three to one ratio, we're gonna have a big gear driving a smaller gear. So if I have uh a big gear that spins once, let's let's use the same teeth tooth count, right? I have a big gear that's that has 30 teeth, and I'm spinning a small gear with 10 teeth. For one rotation of that big gear, that other gear is gonna move a third three times. Three times. Yeah, three times faster. So it's moving faster with that. We have a large gear that's gonna fly that small gear around and create this overdrive condition. And in a vehicle, we want that overdrive condition for the highway speeds because we don't need torque, right? So at low speed, we need torque. We need to, especially when we're at a stop. Exactly. We need that breakaway torque to get the vehicle moving. And once we're at highway speeds and we're already moving, we don't require as much torque, but we do want more speed. Exactly. So a gear ratio in the car and engineers building vehicles or transmissions will take that all into account. How heavy is the vehicle? Uh, you think of a uh 18-wheeler truck, the the ratio of those gears are going to be so high because they need to get all that weight and payload to start moving at the stoplight versus you know uh a Chevy Sprint or a Firefly back then. Firefly, yeah. You know, I would have gone with Honda Civic or something. Okay, I'm old. I'm old, I understand. So now if we kind of relate that gear theory to a planetary gear set, oh, there's one thing I forgot actually. When you have an external gear rotating with an external gear, what happens to the rotation of those? So just so we can visualize that, when we say external gear, we mean that if you look at a gear almost in the traditional sense that the teeth aren't on the outside, and there's two sets of gears that have teeth on the outside, mesh external gear. Yeah, meshing to one another. So when they rotate together directly, they'll rotate in opposite directions. Exactly. So it's gonna change the direction of rotation. So if one's rotating clockwise, that other one's gonna rotate counterclockwise. Now, another thing before we get into the planetaries and how they're built, is if you have an internal gear, which means the tooth, the the teeth are on the inside of the gear meshing with another one. If that's so there's a there's a gear that uh has the teeth on the inside, and then on the inside of that, there's another gear that has external and they are uh in mesh with each other. So they would move in the same direction. Exactly. Instead of going opposite at this point, that internal gear is going to drive it the same direction. And we need to know that for the planetary gear set. So now moving into the planetary gear set, we have those three components we talked about. The sun would be the smallest of the gears. Okay, and the sun has uh external teeth. External teeth. Then we have the ring. Now, this is kind of a tricky subject because the ring goes over top of everything, right? And has the internal teeth. So physically it's the biggest gear. Okay, but when we do the math, it can't be the biggest gear because the carrier or the planetary carrier has more has if you do the proper equation, comes out with the most tooth count based on that ring gear and the sun gear. So the ring gear would be considered the middle gear, even though it's the biggest, and that really kind of messes people up, uh, especially when you're learning in the classroom. And then the carrier we're going to consider the biggest gear. Okay, so let's let's just sum that up. That's the sun gear, which is the smallest gear. It has external teeth. Yeah. The um the planetary carrier um has external. External teeth. External teeth as well. And then finally we get um the the annulus or the ring gear, which has the internal teeth. Internal. Okay, so depending on the component, we have either internal or external teeth. And we said the sun is the smallest, the ring is the second largest, yeah, or middle, I guess you would say, and then the largest overall is the planetary carrier. Yes. And now that we've kind of identified what the largest gear is, we can kind of manipulate that overdrive and underdrive condition based on that. Because if the carrier or the planetary carrier is considered the largest gear, if we are then making that the input or drive gear, everything below that will be in an overdrive condition. Now, if we flip that and make that the output or the driven gear, everything's gonna be underdrive and underdrive condition. So we can manipulate what we want to do based on uh what the carrier is doing, right? So do you want to try and like find the slowest gear we could possibly make based on that? Sure. Now to get our slowest gear where we'd have the most torque, we want a small gear driving the largest possible gear. Exactly. So our smallest gear is the sun. Yeah. Okay, and our largest gear is the planetary carrier. Perfect. So we want the sun to drive the planetary carrier. Now there's one thing we haven't talked about yet. We can have an input and an output, but what about that last thing? We have to do something with that. We gotta hold it, we gotta hold it in place. We want it not to spin so that therefore that it can transmit the torque through that planetary set. How do we do that in a transmission? In a transmission, we use bands or clutches that can hold that gear to the case. Okay, so they apply pressure to stop it from rotating and make it a held gear. A held component, yes. Okay. Okay. So if we were to use a band or clutch to hold our uh our ring gear, yeah, okay, and we had our sun gear drive the planetary, yeah, we'd be in an underdrive. The the most underdrive. The most underdrive. Yeah, the most extreme underdrive that a that that planetary can make. And I guess that's a good point, too. We'll have multiple uh potential underdrive gears, yeah, right, because we have a different combination. So when you say most underdrive, it is the the greatest ratio of underdrive. Yeah, well, and we teach this in the lab, but you can get up to eight different functions from one simple planetary gear set. From those three components, we can make up to eight. So we could get a slow underdrive, which I call super slow, which is what we just discussed. Which is what we just discussed. We can get a fast underdrive. So let's pause here for a second. How would I achieve a fast underdrive? Well, when we talk about fast underdrive, the carrier still needs to be the output because it's in underdrive. But then we want the medium gear, which is the ring, to be driving that carrier. So we're gonna hold the sun, the ring will drive the carrier, and we're in a higher underdrive. It's not super slow, it's just slow. Now, if we were to look at a ratio for that. Sure. And we said the three to one ratio was our super underdrive, I think you called it. Yeah. What would the ratio look like for our next underdrive? Uh it could be anything, depending on how they engineer it. But probably, you know, if that first gear was three to one, maybe we're at one and a half to one or something like that, right? So that that second gear essentially, if we're thinking about a car, that's kind of that second gear stage. So we're already rolling, so we don't need as much torque, but we want to start getting some speed. Yeah, exactly that. And then you third gear in the in the grand scheme of things would be kind of like direct drive. A one to one. And direct drive would be a one-to-one ratio. Now, in order to achieve this, it's kind of simple. We don't need to hold anything now. We just want two inputs, any two inputs. If we have one input on the sun and one put input on the ring, that carrier is going to follow in the same speed as the other. It's going to kind of lock them all together and rotate at the same speed. We could have the carrier in the sun or the carrier in the ring as well. Just hold any two inputs. A lot of students are like, we have to hold two. And I'm like, no, you would go nowhere because it's not driving. We need to have two inputs in that direction. So direct drive, two inputs, any two inputs will create a one-to-one ratio. Okay. So far, we have our super slow underdrive, we have our next uh underdrive, and then we have our one-to-one ratio. Yeah. And then where do we go from here? Well, we need to go to overdrive. Overdrive. Yeah. So now instead of having a higher ratio, like four to one, three to one, we're gonna have a less than one ratio as we talked about earlier. So if I wanted to get an overdrive condition, I then need to drive or have an input on which gear. So we need the carrier to be driving something else. So we could have the carrier driving the ring. Yep. And that would give us an overdrive. It would. Now is that fast overdrive or slow overdrive? I'm gonna call that a slow overdrive. You nailed it. That's the slow overdrive because we have the biggest gear driving the second biggest gear. So that's gonna give us an overdrive condition, which would be less than one to one, but it's not the fastest or that fastest overdrive we can make. And ratio-wise, that would probably look like something like a 0.9 or 0.8 to one. Yeah, around that. And then if we want to get our fastest overdrive, we would have our uh carrier driving the sun. We have the largest gear, which is the carrier, considered largest gear driving that sun gear. So if our carrier is 40 teeth and our sun is 10 teeth, that's that's a crazy ratio, right? What's that gonna be? 0.25 to 1. 0.25 to 1. So that's gonna be moving super fast. And you if you have a planetary gear set or uh, you know, if you look one up, you'll see that move really quickly based on the rotation of the carrier. Uh, and that works great in a highway application or a highway speeds. So we've done uh a few now. I think we have three spots left. Yeah, we did five. Yeah. So the next one we can talk about is neutral. Sure. Pretty easy. You can have one input, and as long as you don't hold anything, it's just spinning freely. Okay. So it's freewheeling. So you can have one input moving, and in a transmission, we usually have that input shaft that's rotating, right? But nothing else is being held. So it's just spinning inside the transmission, doing nothing. Okay. So neutral is pretty straightforward. Very straightforward. And then lastly, we need to get it in reverse, right? We need that car to back up. So with this planetary set, we can get two reverse speeds, but in a car, we can't really do that because when we hold it in our hands or on a planetary gear trainer, you can move those things pretty freely, hold anything, move anything. In a car, we can only hold certain components. And usually we have to double up this planetary gear set to have two of them to make it really work for us. So um, when we talk about reverse, is there a way to get that? So, well, we sort of talked about this with our uh gears. If we have two external gears driving each other, yeah, then we're gonna move in reverse. Okay. So our external gears were our um sun and our planetary carrier. Yeah. So if we hold the carrier or allow it not to spin, those planetary opinions still can rotate in the carrier itself. They're on bearings, they can move nice and freely. So if we drive the sun then and hold the carrier, the sun will be spinning with an external tooth meshing with those planetary opinions. That means the planetary opinions rotate the opposite way. Okay, and then our ring gear is going to be driven which way? Uh the same direction. As the carrier, as the carrier, which is in reverse. Right. Which is pretty cool, actually. That's nifty. Yeah. So we can create a fairly easy reverse just by holding that carrier housing and allowing the gear three to take its, take us through that. And we said we drove the sun gear or had the sun gear as the input and had the ring gear as the output. Was that overdrive or underdrive? So we have our uh sun gear driving our ring. So we are in a underdrive. Exactly. We have the smallest gear driving the mid-gear and underdrive condition. Now, something you probably will never see on an automotive vehicle. We can flip that around and have the ring gear, which is middle, drive the sun gear, which is smallest, and have an overdrive in reverse. Why do you think we wouldn't see that in a car? Well, in reverse, we don't really want high speed. We we want some torque. Need that torque, right? That 3,000 pound vehicle needs torque to start moving. Yeah. So we would use that underdrive reverse in a vehicle. And again, that can be held by clutches and bands and whatnot. Uh, and uh, the apply pistons or whatnot that are we're gonna grab the different components and start driving them will be all applied. And that's another podcast, I think. Yeah, I think where we can go in a little bit more detail with some of the more advanced components. Yeah, absolutely. Okay, so that about sums up our uh planetary gear set and our gear theory. Um, hopefully, instructors, students, anyone who's interested can use this information to help them understand how a simple planetary gear set works. And that's awesome. This has been the Talking with SOT podcast with Harnett Gill and Ian Campbell. Thanks for listening.