# Hydrostatic Drive Design

by larkinja
Tags: design, drive, hydrostatic
 P: 2,497 You mention 4 drive motors which implies wheel hub motors. 70 MPH is not in your budget. Have you researched the cost of wheel motors and the planetary systems that go along with them? - Oh I just reread your post. Front wheels are simply on casters at 70 MPH? Steering a vehicle with a zero turn radius at 70 MPH can't be anything but, well, I can't think of words other than crazy, impossible, etc. BTW, what you describe IS a zero turn radius vehicle even if you hadn't mentioned it specifically.
 P: 63 Actually, I wasn't thinking wheel hub motors. I was thinking stationary motors, with a gear reduction box, and a cv shaft to the wheel hubs. The question more is do you use a high rpm motor with a gear box, or a motor that already has gear reduction that already has the appropriate rpm out put to be couple directly. Either way, I was thinking of mounting a motor to the frame and using a cv shaft. This eliminated side load, etc... As far as the steering, no I am not after zero turn. Zero turn would allow a direct pivot of the vehicle. I don't need or even want that. As for 70mph, I realize that may not be practical, just a design consideration. The ideal speed would be more in the 40-45 range. But just because the wife's minivan can go 130mph, doesn't mean you should. Also, please I am looking for some help with the design, not just a "you're crazy" response. I can get that from the wife anytime. If anyone has an application suggestion, or any specific components in mind please let me know. Or even an engineering firm that won't charge a fortune to draw something up. Thanks again
 P: 63 Hydrostatic Drive Design Also, one additional note. This is in no way going to be driven on any road. This is strictly an off-road vehicle.
 P: 2,497 I was in a hurry when I posted earlier today. You mentioned 4 drive motors. You mentioned that the front wheels will caster. So it's obvious to me that the 2 back wheels are driven off of one hydraulic motor each(that's what you said), but then where will the other two motors be used? You mention that you intend to steer the vehicle by allowing the front to caster freely (no problem with that, at low speeds anyway) and varying the speed to the back wheels to pull one side around more than the other. No problem there either. If you intend to drive the vehicle with CV joints and a drive shaft, I would then assume you don't intend to drive the front wheels? This is the reason I asked what you intend to do with the other two hydraulic motors. The reason I assume you don't intend to drive the front wheels is because if you allow them to caster freely how will you attach a driveline to them? Think about this. There are several reasons why vehicles use hydrostatic drive. 1) Infinitely variable speed. 2) Difficulty getting power transmitted to the wheels with conventional drive shafts. 3) Zero turn radius. By using two hydraulic motors to drive the back wheels along with CV joint drive shafts you are proving that you DON'T need hydrostatic drive because of my reason #2. You stated that you didn't want zero turn radius so you have eliminated reason #3. Is reason #1 that important to you? - Have you priced hydraulic motors with a built in gear reduction? I have, but it was a wheel motor. Expect to pay between $5000 and$10,000 for just one. - I see this project as a worst case. You are accomplishing things by using the least cost effective design. - So, to wrap my post up, please elaborate on what you intend to do EXACTLY. I want to know what you intend to do with the front wheels castering and if you intend to drive them. I personally can't imagine freely castering front wheels even at 30 MPH.
 P: 4,663 Torque is measured in Newton-meters, or Lb-force feet, not in in/lb. Power (watts) = HP/746 = torque (N-m) x 2 pi RPM/60 Bob S
 P: 5 [QUOTE=larkinja;2526484]Actually, I wasn't thinking wheel hub motors. I was thinking stationary motors, with a gear reduction box, and a cv shaft to the wheel hubs. The question more is do you use a high rpm motor with a gear box, or a motor that already has gear reduction that already has the appropriate rpm out put to be couple directly. Either way, I was thinking of mounting a motor to the frame and using a cv shaft. This eliminated side load, etc... As far as the steering, no I am not after zero turn. Zero turn would allow a direct pivot of the vehicle. I don't need or even want that. As for 70mph, I realize that may not be practical, just a design consideration. The ideal speed would be more in the 40-45 range. But just because the wife's minivan can go 130mph, doesn't mean you should. Also, please I am looking for some help with the design, not just a "you're crazy" response. I can get that from the wife anytime. If anyone has an application suggestion, or any specific components in mind please let me know. Or even an engineering firm that won't charge a fortune to draw something up. Thanks again[shaft connection and gears]
 P: 63 Thank you so much for the reply. Yes, I do full understand it is a challenge. From a financial standpoint I can definitely see why you wouldn't see this as a norm, but I have found a few people that have done it, so I see encouragement in that. The horsepower requirements shouldn't be a problem. We have several engines we can use for this project. I have a basically stock 350ci engine that should do 240hp easily. The challenge I see is that most pumps seem to have a limit of 2-3k rpm and a gas engine makes its peak around 4000, so I will have to find the hp curve and see if it will still make enough power at lower rpm. As for the pump. A single displacement motor seems to be the most cost effective, but I see this as being a problem at slower speeds not having enough torque. It seems that the pump would have to be extremely oversized to make up for the torque at slower speeds. A variable displacement pump seems to be a better fit, although more expensive. However I have seen used ones that are reasonable. What are your thoughts on this. Am I thinking right on this? I think you might be right on the braking. I'm now thinking that maybe a relief valve or someway to free spin the motors when pressure is removed and just using traditional disk brakes. If I stick with the auto spindle and hub, that wouldn't be a problem at all. As for the traction issue. I am definately going to need some help with that. I have heard of a couple ways of doing this. I have heard of using a manifold or valve of some sort that would allow free flowing fluid as you described. Would that be parallel? Then you could flip a switch or push a valve and it would redirect to a series flow "locking" all motors together for full traction. I may be mistaken, but doesn't some heavy equipment do something like this. We rented a case backhoe last spring at it had a function like this? As for the motors, the math seems to say that the motors don't have to be all that big. Based on my math in the first post, these can be had for a few hundred buck each, maybe less used or from a surplus industrial store. Don't know if you have any experience in these types of parts. I do realize this may seem like something not worth the effort, but myself it's more about the challenge. I like learning about new things and trying new things. To many people spend their life in a "comfort zone". But think about the advantages this affords if it does work. At least from a custom vehicle build standpoint. No longer does the engine have to be in one exact spot. Rear transaxle transmissions that can take 240hp are very expensive. Less expensive transaxles have a short and long side for axles which limits the travel on the suspension. CG can be fine tuned since components can be placed anywhere. Not to mention making the buggy 4 wheel drive is near impossible without custom tranfer cases. This buggy could easily cost $15000 to build with 240hp, rear engine, and 4 wheel drive, maybe more. Heck high performance rear tranaxles can cost$10,000 alone. So maybe now what I want to do doesn't sound so bad? I guess the real question comes down to, can a hydraulic system provide both power and speed realistically. I know I stated 70 mph, but more important to me would be acceleration. Most of the driving would be in the 0-40 mph range, high speed I guess would be a luxury, since most of my builds do both it is hard to think on not having it. Hopefully this gives a clearer picture. And again, I want to thank you for any help you can provide. Let me know if there is anything I can do in return. Jason
 P: 175 Get back to you tonight Jason. We can start building a conceptual schematic.
 P: 171 To start with hydrostatic drives are not nearly as efficient as mechanical drives. As for brakes you could look into recovering some of it. Research hydraulic hybrids. Chapter 6 in this book has information on hydrostatic drives and he works through an example: http://www.scribd.com/doc/10512227/F...d-Applications This web site has members that build hydraulic drives: http://www.hydraulicinnovations.com/
 P: 63 Thanks drankin, I look forward to your ideas. I like the flow divider idea. If we can make it so it doesn't get the open differential effect on loose ground, maybe I will forget the steering concept. Maybe instead use the hydraulics for hydraulic steering. Maybe using a joystick method. Forward on the stick makes is go forward, the side to side movement could control the hydraulic steering, the way a skidsteer works, although I'm thinking of it more like a fighterjet!! :) Anyway, I can't wait to hear your thoughts.
P: 175
 Quote by larkinja Thanks drankin, I look forward to your ideas. I like the flow divider idea. If we can make it so it doesn't get the open differential effect on loose ground, maybe I will forget the steering concept. Maybe instead use the hydraulics for hydraulic steering. Maybe using a joystick method. Forward on the stick makes is go forward, the side to side movement could control the hydraulic steering, the way a skidsteer works, although I'm thinking of it more like a fighterjet!! :) Anyway, I can't wait to hear your thoughts.
How big are we talking?

Full size buggy or skid steer size? This will determine how we steer. Turning the front wheels or opposing?

There are a few ways to design this circuit but I'll need a little more of an idea of what it's supposed to look like. The trick is how to drive the wheels in a way that's efficient and doesn't let the motors self destruct by way of intensification or cavitation.

I suggest going with something on a smaller scale. It could be a 4x4 ATV with crazy torque and climbing performance. :) Then graduate to something larger.
P: 175
200lb frame seems light but the size will work fine. I like the rear wheel drive mode idea. That's easy enough to include.

Need you to find a rotating joint setup that allows the wheel to "free-wheel" so that the motor can't be driven by the wheel. Like a ten-speed bicycle, you propel pedaling forward but free-wheel pedaling backwards. That will simplify the circuit and protect the motors. This will also allow us to go faster. If the free wheel mode can be engaged/disengaged then that solves our reverse function.

If we make the circuit closed-loop we won't need a large reservoir, just a decent size cooler. Open-loop would require a larger reservoir which we don't want sloshing around while climbing/descending/turning at speed.
 P: 63 This free-spiining joint, is this something you have heard of, or an idea? I don't even know what you would call that. Is there an application that uses something like is? Otherwise, I do like that idea. Let me know, and I will start looking for something like that. Jason
P: 175
 Quote by larkinja This free-spiining joint, is this something you have heard of, or an idea? I don't even know what you would call that. Is there an application that uses something like is? Otherwise, I do like that idea. Let me know, and I will start looking for something like that. Jason
Start looking. I'm not as familiar with automotive type parts as you might be. Don't know if this would even be an automotive type part but you might recognize something that would work in this fashion.
P: 63
 Quote by drankin Start looking. I'm not as familiar with automotive type parts as you might be. Don't know if this would even be an automotive type part but you might recognize something that would work in this fashion.
Ok, I'll see what I can find. Thanks!

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