Liz, welcome everybody. This is our second series in our webinars for all our instructors. My name is Jason Blake, the Executive Vice President for the Foundation. I'm here today to thank Tim for taking time out of his day to talk about the hydrostatic drive fundamentals. For those of you who are new to these webinars, this is something that the Foundation has been putting on. We felt a need for the instructors to collaborate and to learn with each other. As everyone, we've talked about the themes that we're really seeing from our instructors is that there's some changes going on, not only with technical changes, but in addition to that, how we teach our students going forward. AAD has really taken the lead for the construction technicians. We will be continuing to develop standards and give guidance based on what we hear from our instructors on best practices. This is the second series in this. We're going to dive a little deeper into the technical aspect of that. We also are offering a CEA credit for those who are on the call. Again, I'd like to just thank Tim for taking time out of his day to present in front of us. We look forward to some great information from Tim. I will pass it on to Lindley at this point. She will go through a couple highlights of the rules of this call. Lindley, it's all yours. Thank you, Jason. First of all, I'd like to introduce you all to our guest today, Dr. Tim Dell. He is an automotive technology professor at Pittsburgh State University for the past 21 years. He's also the department's diesel and heavy equipment coordinator. Dr. Dell received his doctoral degree in curriculum and instruction from Kansas State University. He got his master's in science and technology education from Pittsburgh State and a bachelor of science in automotive technology with an emphasis in diesel and heavy equipment. Today's webinar, Hydrostatic Drive Fundamentals, will be based off of two of his Goodheart Wilcox textbooks, Chapter 23 of Hydraulic Systems for Mobile Equipment and Chapter 13 in Heavy Equipment, Power Trains, and Systems. Before I turn it over to Tim, I'd like to let those of you who are live with us today that you should submit your questions during the webinar via the Q&A tab at the bottom of your screen. This webinar is also going to be recorded, so you can re-watch or watch at your demand. With that, I will turn it over to Dr. Tim Dell. Welcome, everybody. Thanks for joining us today. As everybody said, we're going to focus on hydrostatic drive fundamentals. As Linley mentioned, if you have that textbook there handy, you can follow along with Chapter 23 of the Blue Hydraulics Book or Chapter 13 of the Power Trains Book. Tomorrow's webinar basically is going to go a little bit deeper into the service and diagnostics of hydrostatic drive. Let's just go ahead and jump into today's topic. First of all, let's start with the fluid drive. You can actually have two different types. You can have a hydrodynamic drive. You can also have a hydrostatic drive. Now, both require fluid. So without the right quality, quantity, the fluid itself, you don't have anything, right? So we can lose the drive itself, whether it's hydrodynamic or hydrostatic. So you could first say, okay, well, let's qualify what is hydrodynamic. In our world, we know that those are torque converters, and so within the torque converter, we'll have an impeller being driven at engine RPM. Its purpose is to impart flow and energy to the turbine, so hopefully our turbine can then provide an input to our transmission input shaft. That's not the focus of today's webinar, but if we want to try to make some comparisons, we know that there is not tight sealing surfaces within a hydrodynamic drive, so we can have slippage between the impeller and the turbine. We know that the pressures are relatively low, let's say 150 PSI or less, approximately, and that we use energy that is kinetic energy, high velocity, and the weight of the fluid that's doing the drive, whereas hydrostatic drive, they do have tight sealing surfaces. We can have pressures all the way up to 7,000 PSI and potentially higher, and so there are significant differences between the two. So let's then say, okay, well, let's define what's a hydrostatic drive. Within a classroom, I want the students to be able to just basically say, you know, it's a hydraulic pump that is going to drive a hydraulic motor, and so, you know, we can have lots of different types, and I'll provide some examples here in a second. So here shows an Eaton hydraulic pump, it's reversible variable displacement, and it's going to send fluid to this fixed displacement motor, and in this case, it's an example of a transmission. Now, technically, when you think of hydrostatic drive, it could be used in place of a power shift transmission, an automatic transmission, a synchro shift transmission. It's actually going to be providing the power transmission for the machine itself in some applications. So you could say, well, what are those applications? We think a lot of times the most common type, which is used for propulsion, right? So you take the average student or instructor and say, well, what's a hydrostatic transmission? The thing that comes to mind is transmission for propulsion. So we can get a change of direction from forward to reverse, or we can get a change of speed or change of torque, but we have other applications of hydrostatic drive. So in the concrete world, we need to spin that drum drive, and we'll use a closed-loop hydrostatic drive. We can use it on cranes for moving the wire and on the winch drive, and we can also use it to cause the excavator or the crane to swing to the right or to the left. If you have a twin-track tractor that uses differential steer, then it is possible you could have a closed-loop hydrostatic drive for your steering inputs, your hydraulic steering input. And then, of course, you can also use it for your variator within a continuously variable transmission as well. So where might you find them in the construction equipment world? You can find them, as we said, on concrete trucks. You can find them in dozers, excavators, pavers, trenchers, skid steers, rubber track loaders, track loaders, and sales wheel loaders. They're used in lots and lots of different locations. That's not an exact comprehensive list. That's just an example of some applications. In the agricultural world, you'll find them in harvesters and compact utility tractors, lawn mowers, sprayers, and swathers. So when I'm in a classroom, I'll typically like to ask the students, say, hey, who here has operated a hydrostatic drive? And most of the time, students have a decent idea of what a hydrostatic drive is. Not always. For those who might not quite know what it is, then you might ask, say, well, perhaps you already have operated one. Maybe you have a hydrostatic propelled lawn mower at the house. And then, of course, you can start talking about the noise that you can hear with a hydrostatic drive and kind of relate to that. So let's get into a few classifications here. I'm going to jump into open loop and closed loop. I'm trying to cover quite a bit of content today as well as tomorrow. Those two hours of content, I expand on that in a normal academic year. I typically take five hours to basically discuss all of this. And so I will quickly go through this, and then we'll have time, of course, for questions and answers at the end. So what's an open loop hydrostatic drive? So first of all, let's say this is our pump, and this is our motor, and if we're trying to draw a loop, you'll notice that it is open. It is not a closed loop. So in that context, we're going to draw oil from the reservoir into the pump. The pump's going to deliver oil, drive oil to the motor, cause the motor to spin, and then the motor's going to exhaust that oil back to the reservoir. And so that means the pump each time has to start over from scratch and draw that oil again from the reservoir. It is an open loop design. We find these in excavator applications. So in this case, you can see this excavator pump is drawing oil from the reservoir, makes itself available to the directional control valve, and then the directional control valves will send oil to the left and right track propulsion drive. Now, there are some other differences between an open loop excavator, hydrostatically driven machine than a closed loop, but for the sake of time, we'll just kind of keep it there and we can elaborate more if somebody wants to elaborate more here in a minute. Whereas on a closed loop, a couple of things. You have two legs, we could call this leg A, we could call this leg B, and you'll notice that it's actually a closed loop. So the pump's going to draw oil into its inlet, and it's going to send drive oil to the motor. The motor is then going to spin. It's going to exhaust its oil and send it back to the inlet of the pump. So now the pump has the assistance of the oil as it has left the motor to help supercharge the pump's inlet, and that is a closed loop design. Now, many of you have used U2 and probably have probably the same videos that you probably use in the classroom. I like to use them. I can elaborate extensively on them in the classroom. I'm probably not going to go into a lot of detail today, but basically, I like to share with students that, okay, first of all, this is our swashplate. We've actuated the swashplate where we have a positive angle, and we're looking at the end views of the pump, and we're looking at the end views of the motor. So this is the valve plate of the pump. These little circles are the pistons reciprocating in and out of the actual barrel. So now you can see the swashplate's been brought back to neutral, and then you can see that the motor is now being rotated in the opposite direction. You can also see that the closed loop leg went from positive or dry pressure here to low, and then now you can see high. So you could literally spend probably 20 minutes just on this discussion in a classroom looking at this particular video. And so students can see that the further that you basically move this lever, the farther you move it, then basically the more speed you get out of the motor because you're going to send more oil to the motor, and the direction of the actual swashplate will determine which way the motor is going. So in closed loop hydrostatic dries, we know that the motor's direction is determined by the swashplate angle of the pump. So when it's in a neutral position, the motor stops, and then when the swashplate goes to either a forward or reverse, then the motor will spin either in a forward or reverse direction. Now this is another slide that we can look at, or excuse me, another video. It is produced by the same person, and this time it shows the side view. So you actually get to see the swashplate angle, and you can see basically the electric motor spinning, the rotating group, and then you can see the output of the motor also rotating and also the valve plate from the end view as well. And so basically as the swashplate is moved, you can see oil coming out of the pump that's now basically going to the motor, causing the pistons to move, which then slide against the swashplate of the motor causing it to rotate. So for the sake of time, I'm not going to elaborate too much more on that just because there's a lot of information I'd like to get through today. So let's talk about closed loop drives and potential failures. So this is a tough subject for our young people that are working in a dealership that has a customer that walks in and says, hey, I have a closed loop piston pump, which they probably don't know it's closed loop, but this piston pump has had a catastrophic failure. It's worn, it needs replaced, and so I'd like to order this $4,000 or $5,000 pump, and let's get it on the way where I want you to rebuild it. That's when the technician in the service department needs to have that difficult conversation with that customer saying, you have a closed loop drive, and because of that, you're going to need to also have the motor serviced or rebuilt or replaced as well. It's difficult for a customer saying, wait a second, I'm spending $4,000 or $5,000 for the pump. Now you want me to spend $4,000 or $5,000 for the motor, and you're hoping that our technicians got the confidence to be able to say, well, you can spend $4,000 or $5,000 today, or you can also then come back later and spend another $8,000 to $10,000 buying another pump and another motor, and here's the explanation why. In a closed loop, we know that the pump is going to feed the motor, and the motor is going to feed the pump. So whatever trash we have in that system is going to feed the other components. Now you can see in this illustration on page 556 of the Blue Hydraulic book that it is possible to put a filter in the legs, the closed loop legs, drive A and drive B of a closed loop circuit. It is rare though. Why is it rare? Because you're asking two challenging feats of that filter. First of all, that filter has to be able to withstand tremendous amounts of drive pressure, potentially up to 7,000 PSI. That's a very expensive filter housing. Second thing that you're going to ask of that filter is it also has to be able to filter in both directions. So in a forward direction, it's filtering oil in this direction, and then in a reverse direction, it's filtering in this direction, and because of that, it's quite rare to have filtration in your closed loop leg. Now you say, how rare? Well, it's possible to find it. This is one of the only references that I've stumbled across in terms of closed loop filtration that I found in a Caterpillar piece of literature, but for the most part, our closed loop hydrostatic drives do not use filtration in the closed loop legs. You might find filtration in the charge circuit, you might find filtration in the case drain circuit. You might even have an offline kidney loop filtration circuit, but you're not going to typically find it commonly found in the closed loop legs. So now let's talk about single path versus dual path hydrostatic drives. The single path is just basically a single transmission where you just have one pump that's going to drive one motor. So you might find this in a wheel loader application, you might find this in a compact utility tractor, might find it in a harvester application. But if it's closed loop, then we have basically the reversible variable displacement pump that's driving a motor, and the motor could be fixed displacement or it could be variable in that case, whereas on a dual path, it's different. Now what do I want my students to learn on dual path? There are about three things. First of all, I want them to see that you have two separate hydrostatic drives for each side of the machine. So you have a dedicated pump and motor for the left side of the machine. Let's say this is a track type tractor, whether it's a dozer or a track loader, and then you also have a pump that's dedicated to drive a motor for the right side. We can also find this in other applications like skid steers, we'll look at some of those here in a second. But what are the other things that I want the students to realize on this? Yes, these are independent. Yes, this is dedicated just to the left, and this is dedicated just to the right. But this gives us propulsion, and it gives us steering. So whether we're going to drive this side a little bit faster than this side, that will then cause us to have a heading a little bit to the right, or if we spin this one forward and this one backwards, we're now counter rotating. So dual path is unique in that it has two separate transmissions. So not only provides propulsion, but it also gives you steering. Now you could say, well, how is that different than a traditional excavator? As an excavator, you're going to have a motor that drives this track, you're going to have a motor that drives this track. Well, one of the differences on an excavator is, yeah, you'll have a couple of pumps, but they're not dedicated exclusively just to the motor itself. And so the pump could be used for one or the other motor, and it could also be used for the boom, the stick, the bucket, the swing, and the other hydraulic functions on the actual excavator itself. So let's look at a skid steer, it is also a dual path. So this would be your tandem pump. So basically a pump for the left, a pump for the right. And then this would be your motor, basically to drive your hydrostatic propelled skid steer. So when a person says, my skid steer is hydrostatically propelled, they're probably not thinking that you have two transmissions inside of there. They may not be thinking that. And so you have a pump that's dedicated to driving the motor for the left side, which then drives your chain drives for your wheels on the left. And then you have a pump that's driving your motor for the right side, which is driving the chain drive to drive the wheels on the right. So that would be an example of a skid steer. You can also find it in the ag world or self-propelled wind rower or a swather, where you have a pump that's going to drive the motor on the left, the pump is going to drive the motor on the right. And then you'd have these rear caster wheels. If you happen to have agricultural students in your program, you probably are aware that the old school swather, self-propelled swathers, had a very tremendously complex hydrostatic control system. Fortunately today, we do not have to rely on the mechanical side of this control system. In case your students are going to stumble across this in the real world, then it's a good idea to address that. And so as a way of understanding a non-electronically controlled system, you have basically a pump for let's say the left and the pump for the right or vice versa. And then this would be the swashplate control arm. This would be the swashplate control arm. And so this control arm and this control arm have to be controlled for steering as well as propulsion. So if you have steering, then you have your steering wheel, it's going to spin the steering shaft. And when it spins the steering shaft, it's going to turn basically, let's say a left-hand nut here in one direction, let's say a right-hand nut in the opposite direction. And those control arms are going to move independently. I got a video on the next slide, I'll show you that video, how they work in opposite directions. But then you also have to have propulsion. So then you would have basically a control cable coming from the operator's cab. That's going to move your directional control valve actuation cylinder, which then when it's actuated, it's going to move a pivot plate, pivot plate's going to then move your actual control strap, control strap is then attached to the control arm, which will cause this control arm to move forward or reverse. And then through the connection of your steering shaft will then move the other control arm. So what are the points to emphasize here? If you're propelling, then basically both arms are moving in the same direction, either forward or reverse, depending on the machine, right? But then if you're steering, then these arms are moving in the opposite direction. And then if you're steering and propelling, then you're getting basically independent movement of both. So this is a video clip that actually some students recorded for me, if I can go back to the right screen here, apologize on that. Where some students actually recorded this for me when we were out in Heston, Kansas, looking at one of their swathers. So as the steering shaft is spinning, you'll see that these control arms are moving in the opposite direction. So steering to the right, steering to the left. And then if you were not steering, that control strap would then basically move both of those in the same direction. And then fortunately today, we do not have to worry about the complexities of setting that system to neutral and having it not drift to the right or drift to the left. We can rely exclusively on electronic software and soil noise to make it happen. So we've talked about single path, talked about dual path. And then you can also have other applications out there. So this would be an ag application where you have a sprayer. Sprayer potentially could be a hydrostatically propelled machine, not all sprayers are hydrostatically propelled. For instance, you can find a sprayer out there that uses an Allison 1000 series transmission. But if this is a hydrostatically propelled machine, then they typically use one pump to drive two motors. And then they also have another pump to drive the other two motors. And it's interesting that at least on a deer application, that the pump that drives the motors, the motors are located diagonally, not on the same axle, they're diagonal. All right, let's move on to some other concepts in hydrostatic transmissions. Let's just look at some motors. This shows a simple fixed displacement motor. So when you see or hear fixed displacement, this is the fixed displacement swashplate. So this swashplate inside of this housing is stationary. So at that angle, let's say 15 degrees, right? It's always going to be that particular degree, it's not going to vary. We can sometimes call this a single speed motor, we can sometimes call it a fixed displacement motor. Then we can also have variable displacement motors and those variable displacement motors can be infinitely variable within its limits and I'll explain that in a second or it can be discrete in the sense of two speeds. So you guys probably familiar that two speeds are found on let's say skid steers and dozers, maybe track loaders and probably other applications as well as harvesters. But then you can also have infinitely variable motors such as let's say, on the front of a motor grader or on wheel loaders. So this is a Bosch Rexroth dense axis motor. Now technically speaking, even though this one is infinitely variable, it also could be designed to operate in discrete positions such as high and low, right? So in this example, when I say it's infinitely variable within its limits, let's say that in a neutral angle, we're at zero, right? And then if we have a swivel angle of 40 degrees, if that's the maximum swivel angle that we have, well, it's going to be infinitely variable within that range of zero to 40 degrees, whatever those maximum angles are at, right? This is an example of an Eaton two speed motor, you might say it looks a lot like an Eaton pump, the housing itself could be used for a pump. But if it is a two speed motor, then you would say one of the servo pistons is used for high, one of the servo pistons is used for low. In this example, it was a harvester. And honestly, the difference between high and low on this application was a swashplate angle between 15 degrees and 18 degrees, only a three degree difference, which seems kind of surprising, right? So we can elaborate a lot on that. But for the sake of time, I'm going to keep moving on. I would not want to spend a lot of time talking about the advantages. I think this is pretty self explanatory that you can visit with students about on page 565 of the blue book, you know, students can see our advantages and disadvantages of pumps and I should say hydrostatic drives are, I probably spend quite a bit of time talking about how hydrostatic drives are incredibly productive, they make the machines very, very productive. So the name of the game is getting that material in the back of that truck or getting it smoothed out or getting the crop harvested, whatever. And it makes the machine productive, which at the end of the day, is profitable, right? But it comes at a cost. So what are those disadvantages? Well, they're not extremely mechanically efficient, right? They're going to be less efficient than a mechanical drive. But it means more fuel, they're noisy, they're susceptible to contamination and heat. So there are some costs that occur with hydrostatic drives. Let's move on to the next concept, and that would be split versus interval drive. A split hydrostatic drive is by far the most common type that we find in our world. This shows the closed loop split drive, where the pump supplies oil to the motor, the motor sends it directly right back into the pump and in between the two, we have our hoses or our drive lines. And so it allows the manufacturer designer to basically place the pump right up at the engine at the best location, and then strategically place the motor wherever it needs to be right down by a final drive or a mechanical gearbox or something of that nature, and then just simply route hoses or tubing to the actual motor as needed. So it provides tremendous flexibility in designing the machine. So that's great. Whereas you can also have hydrostatic drive, where basically you couple the pump directly to the motor. So this shows an interval type system. And let me just jump ahead a second. So this would be the interval design, where basically this is your input shaft pulley that's driven by a belt from the engine that provides input into the pump, and this is your control valve. And then inside of the pump, we're going to send oil directly to the motor. So the motor is directly coupled to the pump, and then we don't have any external drive line hoses between the pump and the motor. Another thing that you could point out with the student is that this is your input shaft to the pump, and it is not related to the output shaft of the motor. So we could be spinning this input shaft proportional to the engine RPM, and this output shaft of the motor could be stationary until we actually move the control valve on the actual pump itself. And so then we could cause this output shaft to rotate forward or reverse at different speeds, et cetera. We can also have interval shapes that are S-shaped. So basically the input comes in from the engine, drives the pump, and then basically the pump's going to send oil to the motor, which then sends output through this particular output shaft of the motor. You can also have a U-shape. These are not near as popular as split design. This is an example of a harvester application, an older harvester application that we typically do not see on late model machines. Now let's move on to the next topic, which is charge circuits, charge flow. Charge is so, so important with hydrostatic drives today. So we typically use a charge pump, but a charge pump is going to do four things for us. It's going to supply oil to the piston pump. So here's our charge pump. In this schematic, you can see oil's going to go through the makeup valve and make its way available to supercharge the piston pump inlet. That's number one. Number two, it's going to provide oil to our closed loop. So this is our closed loop. So there's leg A, and then it's going to make oil available to leg B. As our machine operates, due to internal clearances of the motor and the pump, we're going to lose oil. And as we lose oil, you can't just rely on the oil coming from the pump, going right back from the motor to the pump, as being the same quantity. It's going to be less. And the reason it is less is due to the losses that occur due to your internal clearances. And because of that, we need to have make up oil, which comes from a charge pump. It's basically made available to the closed loop legs. See, charge pump is also going to supply oil to your control valve or your servo valve, whichever you'd like to call it. This is what's going to control the actual transmission itself. Where does it get its oil? It gets its oil from the charge circuit, from the charge itself, charge pump. And then the last thing that our charge pumps do for us, they help cool the transmission. And the reason they help cool the transmission is that remaining balance of oil that is not needed in our closed loop legs, it's going to dump across either our charge valve in our pump while we're in neutral, or across our flushing valve in our motor when we're moving forward or reverse. And when it dumps across that valve, where is it sent? It's sent into the case of the motor, which flushes out the hot oil from the motor. And then we typically route that to the pump to flush out the hot oil within the pump. And then we'll send it to basically a cooler and then back to the reservoir. So those are four important traits of the charge circuit. And so when we elaborate more on this on tomorrow's webinar, we may have to come back and rediscuss just how important these charge circuits are. So here's an example of a charge pump from an Eaton hydrostatic pump. So you'll see that this is the charge pump shaft. This is the notch that basically aligns with the rotating group inside the piston pump. And so they're directly coupled together in that the charge pump will rotate proportionally to the same speed as the piston pump rotating group, which is located down here. So it's typically a gear pump, typically coupled to the actual piston pump itself. But let's please make a note that that's not 100% always the case. So again, in a harvester example, the Keysight flagship machine, you should know that their charge pumps is located on the backside of that Rexroth piston pump. Technically, even though it's coupled together and it's designed to run with that piston pump is actually used for a separate purpose. And the actual charge pump flow comes from a separate pump. So keep that in mind when you're learning about hydrostatic transmissions that is just it might not always appear as you think it appears. One other thing to consider is that on a Rexroth system, they will necessarily call it a charge pump, they will call it a boost pump in that particular case. So how large should your charge pump be? A rule of thumb is 19% or if you want to round it to 20% is approximately 20% of the piston pumps displacement. So excuse me, to make this a simple thing to consider, let's say we have a very large hydrostatic drive. Let's say that the piston pump displacement is 10 cubic inches. That's an incredibly large hydrostatic drive. Well, in that case, you would want the charge pump displacement to be at least two cubic inches because the piston pump was 10 cubic inches. So you could say, well, why is that? Well, when you start working with designers and when we're building transmissions, they might say originally that the piston pump might have a brand new volumetric efficiency of 96%. They might say that the motor has a volumetric efficiency of 97%. Well, you multiply those two together and you might say, okay, we got an overall efficiency of, I don't know, 93%. You got to calculate, you can do the percentages there. But what happens over a long period of time as this system wears? Well, the pump and the motor are going to wear and the volumetric efficiency is going to drop. Well, you need that charge pump to be able to overcome those losses within the pump and the motor. And so if that pump and that motor fall to, let's say, 90% volumetric efficiency, then if you take a 90% volumetric efficient pump and a 90% volumetric efficient motor and multiply that, well, what's 90% times 90%? You know that that's 81%. Well, all of a sudden that 81%, you can start to see where 19 comes from. Well, if you have a pump at 90% and a motor at 90%, then that means the total loss of 19% has to be the actual charge pump. And so in that case, if you have a charge pump and it's only capable of delivering 19% of the piston pump and basically your overall volumetric efficiency drops below 81%, as you suspect, you have now, again, start to cause cavitation. And we basically are going to destroy this hydrostatic transmission and you're going to have noise and customers are going to be upset that their machine lacks the torque and the power to do its work. So what are the different types of piston pump frame designs? We know that the two common types are the old school trunnion bearing, where basically you have two servo pistons that are going to actuate a swashplate. And so you can find these in lots of different brands. This would be an example of an Eaton. You can find other brands that are out there as well. So one servo would be dedicated to, let's say, forward. One servo would be dedicated to reverse. I want to cautiously say though, you would never say on this pump, this is always forward and this is always reverse. That is more of a function of your motor. Is your motor positioned, let's say, one direction or is it turned upside down in the other direction or are your hydrostatic drive lines flip-flop, et cetera. So I don't want to elaborate on that too much right now, but that would be the old school two servo trunnion bearing design. This would be an example of the more modern type that we see today, where you have a single servo piston that is going to be either moved, let's say, up or down. And as it's moved up or down, it's tied to, let's say, a cradle bearing shape swashplate through a rectangular key here, so that as the piston moves up or down, it's going to move the swashplate up or down from a neutral angle to a positive or a reverse angle in that particular case. So let's talk about some of the control. This shows an example of a control valve from, let's say, an Eaton manually controlled hydrostatic pump. So you have a lever in the cab, the cab moves a cable, the lever in the cab moves a cable, which is then attached to a lever on the actual linkage. The linkage is then going through a set of linkages here to move the actual spool valve. This would be the centering spring that's holding the spool valve in a neutral position. And then you'd have a little bit of orifice located under here sometimes, depends on the machine, depends on if it's IPOR, internal pressure override, kind of depends on, we can elaborate maybe later on that. But basically, charge pump sends oil to the orifice, orifice supplies oil to the spool valve, and if you move the control valve, then the spool valve will direct oil to either the forward or the reverse servos in that particular case. So this is what it looks like schematically. Oils come from the charge pumps made available to the control valve via the orifice. You move the control valve or the servo valve, it's going to then send oil, which we call control oil, metered oil, servo oil, to the actual servo piston. We could call this the forward piston if you like, or the reverse, it doesn't matter. It's going to move the swashplate. That piston moves swashplate, causes the piston pump to generate flow because we have a positive angle on the piston pump, so it sends oil to the motor, causes the motor to spin. So a couple things to think about. In today's discussion, we're mainly thinking about large equipment, right? So because it's large equipment, we need the effectiveness of hydraulic oil pressure acting on servo pistons to have enough force to move that swashplate to move. So oil pressure, use Pascal's law, times the area of the piston, it's going to cause our swashplates to move. Why is that? Well, if you get in a real small application, you don't need servo pistons. The operator can use a long enough lever just to mechanically move the actual swashplate itself. And you don't have to get into very large applications. It doesn't take too much horsepower before all of a sudden we're physically using servo pistons with the effectiveness of Pascal's law to physically move the swashplates themselves. So this shows an example of the actual mechanical lever that's basically moving a manually controlled spool valve. This one shows where you can actually have a neutral safety switch on the control valve. In this particular fashion, when we're in a neutral position, the switch is closed so that we can have an electrical circuit from our key switch that'll send current through the closed neutral safety switch. When this is in a neutral position that allows us to start the engine, I'll cautiously say I personally have experienced basically starting the machine that was not in a neutral position, that was not correct. This was specifically at a training center in Racine, Wisconsin. It's firing up this harvester and the person that was in there last had the propulsion lever in the fourth position and I had about 10 feet to bring the machine to a stop before I was going to hit a wall because the neutral safety switch was not doing what it should be doing. So just because you have a switch doesn't mean it's doing its job, so be cautious about that. Another application that you have is an electronically controlled pump. This is a Bosch Rexroth pump. You can see this is basically one solenoid, let's say, for forward, one solenoid for reverse. We'll have two wires that are basically used for the current or the positive and the negative for controlling, let's say, the forward and the same two wires over here for the reverse. This would be your single servo piston that's going to operate that cradle bearing design. So this shows basically in place of having a manual lever, you can have a solenoid to push the solenoid to the right, you can have a solenoid that pushes it to the left. I'll cautiously share with you guys that I have stumbled across a hydrostatic drive used as a variator in an actual transmission, a CBT transmission, that only used a single solenoid to give you forward, neutral, and reverse. But normally we're going to have a dedicated solenoid for forward and a solenoid for reverse. Now if you look at this illustration, you might say, hey wait a second, you're showing twin solenoids and you're showing two servo pistons. Yeah, you are correct. It is possible to find those designs where you have basically electronically controlled and still use two servo pistons. But if you say, I want to find a schematic example of where you have the cradle bearing design and your solenoids, then go to page 606 of the Blue Hydraulic Book and you'll see that's what that single servo piston looks like, where basically you actuate the controls from the solenoids and then it's physically moving a single servo piston either to the right or to the left, which then actuates your swashplate, cradle bearing swashplate to then drive your hydrostatic drive. Within some hydrostatic drives, we have feedback links. And that feedback link is going to basically act as a communication device. Technically you can have mechanical communication feedback, hydraulics such as mode sensing, and you can also use electric as well. So mechanical, hydraulic, or electrical feedback. If you have mechanical feedback, it's basically tying the swashplate to the control valve so that when this control valve tells one of these servo pistons to move, that you can communicate back to the actual control valve that the swashplate has achieved the commanded position that the operator requested, and we can then therefore hold that spool valve in the last commanded position so that our swashplate just doesn't completely run away and have an exponential increase of machine speed. Now what happens if you don't have a mechanical feedback link? In that particular case, you most likely have inputs that are electrical, such as a swashplate angle position sensor, as well as pressure sensors, and another host of other electrical inputs to your computer. So you're not always dedicated to have a mechanical feedback link. So let's get into what I'd say the remaining meat and potatoes of hydrostatic drives. We have basically the oil flow and operation. As I said a second ago, your charge pump's going to deliver oil to your piston pump inlet. It's going to make oil available to your control valve, and it's going to send oil to your closed loop. In terms of your servo oil, it's going to do a few things for you. One, it's going to, well, it's going to do one thing. It's going to basically cause the piston to move, and when the piston moves, it's directly attached to the swashplate, and it's going to cause the swashplate to move, which causes your transmission to move. So when you're in a neutral angle, you have, on this example, zero servo oil to your servo piston. And so basically the control valve would have this basically zero psi and have this piston at zero psi. You get into other applications, that's different. It just kind of depends. We'll elaborate more on that tomorrow. But for now, when you move the control valve, it takes oil from your charge pump. Your charge pump sends oil to the control valve. The control valve, when you move it, causes the control valve to send servo oil to actuate the piston to cause the swashplate to move. It's on page 575 of your textbook. I'm also going to share with you a little bit of information about the flushing valve. So when we are in a neutral position, you have two different charge valves. The charge valve located inside your piston pump is in command, and it's typically set 20 to 30 psi higher than the flushing charge valve in the motor when we're in a neutral position. And so when we leave neutral, meaning when we go to forward or reverse, we have a shuttle valve in the motor. That shuttle valve senses your drive loop leg. So drive loop leg A, drive loop leg B. And anytime we're moving, we have drive pressure, and that drive pressure will be higher than the low-pressure leg. That low-pressure leg could be called charge or it could be the low-pressure leg, whichever you'd like to say. But when we're in a neutral position, both of these legs are low pressure. Let's say 270 or 300 psi, whatever, and they're both charged. Well, that means you have 300 psi here, 300 psi here, and that this shuttle valve is basically neutralized, and therefore it's now blocking off the passage to this flushing valve. But when you exit neutral, then let me show you a better illustration here. When you exit neutral, now you have a higher pressure that's going to shift the shuttle valve in one direction that opens a passageway from the low-pressure leg, which can now have that oil go and be exposed to the low-pressure flushing valve, which is located here. And that flushing valve, you can call it the flushing valve, the replenishing valve, the hot oil purge valve. Most generally, it's located in the motor. It is set 20 to 30 psi lower than the pump. So why in the world is it that way? For one reason, if you have a bent-axis motor, you have an additional bearing inside the motor. The more bearings you have, the more heat you're going to have, and so we want to get rid of that heat. And so the excess balance oil that you have from your charge pump is now going to flow across your flushing valve into your motor case drain, flush the hot oil out of your motor case drain, and then preferably, we're going to have our designers send that oil to go flush out the case drain of your pump and then flush it out of the pump back to the actual reservoir. So if you're going to ask a student, hey, I'd like you to just take an essay question and just tell me what to take in order to see that drop in charge pressure. When you leave neutral, that charge pressure should drop 20 to 30 psi. So get out a piece of paper and a pencil and ask that student, please tell me what are the actions it's going to take to make that happen? Well, you got to operate the control valve, right? When you operate the control valve, it's going to have to actuate a servo piston. That servo piston is going to have to move the swash plate. That swash plate is going to have to cause the pump to send flow to the motor. That flushing valve inside the motor is going to have to sense that higher drive pressure and shift down. And then when it shifts or up or down, then it's going to allow that low pressure circuit to basically be exposed to the lower pressure setting valve inside the motor, the flushing valve, and cause it to drop to 30 psi. So if you have students that have that inquisitive mind, you can share with them, hey, if we're on decel, if we're allowing this machine to decelerate, then actually the pump kind of becomes a motor and the motor kind of becomes a pump. And actually that shuttle valve will actually shift in the opposite direction. So for the sake of time, I don't want to spend lots of time on that right now. So this is that flushing valve that we're looking at. It senses both legs, leg A, leg B. And when we have high pressure or drive oil on one side, it causes it to shift. Then that flushing valve will take the low pressure leg oil and drop it across the flushing valve and then send the balance of that charge oil into the case drain of the motor to flush that hot oil out of the motor. This illustration, you'll also see we have crossover high pressure relief valves. And you'll notice that they sent one drive leg and dumped the oil to the opposite drive leg. So high pressure crossover relief valve you'll see there. So when are those used? Excessive high pressure stall type conditions is where you'll find those. And so if you look at it, this is an example of an E where they are located in the motor. Here's the flushing shuttle valve spool cap on one side, the flushing charge valve. And then this would be one high crossover relief valve. And on the opposite side, you'd have the other. So you look at the reverse propulsion, basically the actual squash plate moves in the opposite direction. Earlier, you heard me say, if you have a pump that is operating the opposite direction, I said, I don't like to say this is forward or this reverse. It really truly is a function of this line coming in here, this line coming in here, and this motor squash plate being in this position. If we flip this motor squash plate upside down, then we're moving in the opposite direction. What do I say is because when this pump leaves, let's say the Eaton plant, the Eaton manufacturing plant, we're not sure if this is forward, we're not sure if this is reverse. We only know that once this hose is connected to this motor, this hose is connected to this motor, and when this squash plate is located in this position, that is when we know which is forward and which is reverse. So I'm getting to the end here, folks. Let me just share a couple things. This is an Eaton cross-sectional drawing. I typically like to put some PowerPoint animation to it to help my students understand what's going on. I said a second ago, we have a neutral relief located in the pump. It's set 20, 30 psi higher than the charge relief located in the motor. So let's start this oil flow. Let's say we're spinning our engine, which is spinning our pump. We're going to draw oil from the reservoir into our charge pump. Charge pump is going to deliver oil to the neutral relief charge. It's going to send oil into the piston pump to supercharge the piston pump. It's going to send oil into the inlet of the control valve. If the control valve is actuated, then we're going to send servo or control oil to a servo piston. It's going to actuate the swashplate. When the swashplate actuates, we now develop oil. That oil is going to basically send a flow to the motor and it's going to act on the piston. When it has enough energy to push the piston out of its bore, it's going to ride down the ramp of the motor and cause the motor to spin. The motor is then going to exhaust the oil back through the closed loop to help supercharge the piston pump inlet in that particular case. And then because our shuttle valve has shifted, because it has sensed this high pressure oil on one side of the shuttle valve, because it shifts down, now this low pressure leg can now act on the lower value flushing valve and basically dump across it into the case of the motor, flush out the oil from the motor, send that flushing oil to the pump, flush out the hot oil through the pump, back through the cooler, back to the reservoir. The last couple slides I typically share with students are, if you just want to know the oil flow through the low pressure circuit, then again, this is on page 580 of the blue book. Basically, we start with the charge pump since it draws oil from the reservoir. We're going to send it to the piston pump inlet, direction control valve in to your closed loop. We're going to actuate our directional control valve. It's going to send servo oil to the piston. Servo piston is going to actuate the pump. The pump's going to then deliver drive oil to the motor. That motor is going to have its shuttle valve shift. Then it's going to basically expose that low pressure oil to the flushing valve. Flushing valve is going to dump oil into the motor case drain. And then hopefully our motor case drain will send oil back to cool the pump and then back to the reservoir. But what happens if we want to look at the high pressure circuit? High pressure circuit, we start off the same, but then if we stall, let's say we reach high, high pressure, where does that oil go? It goes to the charge circuit. And then of course, charge circuit dumps into the case drain. Case drain then goes, flushes the oil out of the motor to the pump and then back to the reservoir. So before we get into questions here, I wanted to share with everybody that I'm going to be conducting some hands-on hydraulic workshops here in Southeast Kansas. This is a function of a grant that I received from the USDA NIFA federal grant that they allowed us to purchase a Bosch Rexroth hydraulic training board. They are very, very good boards. This is a photo I took, taken of one. They have some very unique things to them, such as some overrunning loads. It has some visible cavitation that we'll be able to look at. It also has a graduated glass cylinder. It allows you to look inside a hydraulic system at some oil that you normally can't see. And then we also have three hydrostatic trainers, specifically a skid steer, infinitely variable, electronically controlled, and manually controlled. We'll talk about open center pressure pumps and preschool load sensing, flow sharing, and spend a good four days hands-on instruction. It's actually at a reduced cost of $900 per class. Because of the grant, once I get through the three-year grant, I'm sure that those dollars are going to go up. So with that in mind, I've tried to do my very level best to keep us under an hour. And so, Lindley, if you would like to share with me some questions. If you guys want to know more information about this hydraulic systems hands-on training, you can go to kccte.pittstate.edu slash industry dash training to learn more information about it. So, Lindley, I'll let you field some questions for us here. Great, thanks, Tim. I just wanted to, again, thank you for your time again today. And that was an amazing presentation. While we're waiting for additional questions, and I'll start with one. Is there any way you could share your email address, Tim, with everyone, just in case people have questions again later? Yeah, so it's t-bell, like computer. So t-b-e-l-l at Pitt State, p-i-t-t-s-t-a-t-e dot e-d-u. Great, thank you, Tim. Well, our question today comes down from Charles Siegel at San Jacinto College. He wants to know, why is a systems approach so very critical in the repair of hydraulic equipment? Say the question again, systems approach? Mm-hmm. He told me that he really loves the systems approach to avoid the root causes that are critical for proper repair, especially in hydraulic equipment. And his question was, why is a systems approach so very critical in repair of hydraulic equipment? Okay, well, I'm going to take that question as implement hydraulic systems, such as open center, pressure compensating, preschool load sensing, flow sharing. And if that's the case, why understanding a systems approach? I say it's absolutely critical, because they all behave so differently. So, you know, if a person goes out to a machine, and it has a pressure compensated system, and they go out there, and they see that it's behaving as an open center system, if they don't correctly understand how the system is supposed to behave, such as open center, pressure compensated, load sensing, etc., then they could easily be troubleshooting a system as if it is a different type of system. And so, I like to personally say, let's start with those fundamentals on how does an open center system, how is it supposed to behave versus pressure compensating versus load sensing. So, for instance, one of my case studies, and I think it's chapter 16 on open center, I went to go help a veterinary that had a machine system inside their actual clinic. It was behaving as a pressure compensated system. They said, hey, it sounds like it's on high pressure, sounds like it's going to blow up, and we're going to continue to use it. So, basically, what's wrong? Well, if I was going to troubleshoot that as a pressure compensated system, I'd say, well, hey, it's behaving as it should. But because it was an open center system, it allows us to then diagnose and find out why is it on high pressure demand, and let's back up and find the actual root cause. Because in that sense, knowing how each system is separate allows you to then, I guess, troubleshoot how they're normally supposed to behave. I hope that answers that question. Great. Thank you very much, Tim. I'm going to leave it up to the rest of the group to send Tim emails upon their questions that they have. I wanted to thank you all for participating in the instructor series today, and we're looking forward to seeing you all again tomorrow at 11 a.m. for Dr. Tim Dell's second installment of the hydrostatic drive and service diagnostics, which will be a more advanced class upon today that will build upon what you learned this afternoon or this morning in some cases. So, I want to put a hats off to all of you. Thank Tim again very much for participating, and we'll see you all tomorrow. Have a great rest of your day. Thanks, guys. All right. Take care, all.

webinar

hydrostatic drive fundamentals

instructors

Jason Blake

Foundation

collaboration

American Association of Diesel

standards

guidance