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Hydrostatic Drive Design

  1. Jan 11, 2010 #1
    Hi all. I don't know if I have posted this in the right section, but I am hoping someone might be able to lend me a hand with a project.

    A friend on mine and I are always looking to build something challenging and unique. We recently finished building a truck completely from the ground up, designing all our own parts (besides engine, tranny, etc..) Anyway, inspired by our commercial lawn mower and our bobcat, we would like to build a hydraulic driven dune buggy-ish looking vehicle. Many have asked why, and the answer is simple, its something different and we would like to learn more about hydraulics in the process. I have researched power requirements, done many of the formulas, but I am having a tough time translating data into the real world. In other words, will it work)

    My DATA:

    Vehicle Mass - 2000lbs
    Maximum Speed - 70mph
    Maximum Incline - 20degrees
    Wheel Radius - 13"
    Axle Ratio - 2.5:1
    Rolling Resistance - 0.037

    With these values, I have concluded this:

    Drawbar Pull - 758
    Wheel Torque - 9854.524 in/lb
    Motor Torque - 3941 in/lb
    RPM - 2261rpm
    Power Required - 141hp

    If I calculate based on 3000psi system and 4 drive motors I get:

    Flow Rate - 80gpm
    Torque on each motor - 985 in/lb

    Concluding a minimum motor size of 2 cu in/rev motor

    Now for my questions. 1st, is my math correct? I did this all on paper, then created a spreadsheet that solves all the equations so I can change the variables to meet the end goal.

    2nd question, am I going about this the right way. Where my confusion lies mostly is in the motors and pumps. The math does not seem to match manufacturers specs for there motors. So is a formula not accurate enough given the different types of hydraulic motors?

    I am wondering if there is anyone out there with any hydraulic experience that can point us in the right direction. Maybe with component selection, and even possibly helping us coming up with the schematic.

    We are just starting the research, so any suggestions would be helpful. Do don't want a traditional steering wheel. We want it to operate with two handles just like the commercial mower works. Both handles forward it goes straight. Left handle up, steers right, handles back, reverse. A gas pedal to control engine speed. Currently the thought is one motor for each wheel. Front left and rear left on the same circuit. Same with the right side. Front wheels would be on steering spindles, tied to each other, but left to free steer. Possibly use stabilizers to help with hitting bumps and ruts and returning to straight. This would just be a concept to test, don't know if it would steer well that way or not.

    I am still looking for the best method for braking. On the mower, you just easy back on the handles and it stops. If you let go of the handles, it comes to a abrupt stop. Which I don't think would be very safe at higher speeds. Maybe at valve center, a bypass which allows the wheels to free spin, and then use traditional braking. I have read about using accumulators for hydraulic braking. I don't care about fuel efficiency, this isn't a car, which it what this is used for such as the UPS test truck.

    Anyway, this post has gotten long enough. If anyone wants to give me a hand I would be very grateful. And just so everyone knows, this is not a design for a commercial vehicle, just boys and there toys, so lets not think in terms of production.

    Thanks again,

  2. jcsd
  3. Jan 11, 2010 #2


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    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.
  4. Jan 11, 2010 #3
    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
  5. Jan 11, 2010 #4
    Also, one additional note. This is in no way going to be driven on any road. This is strictly an off-road vehicle.
  6. Jan 11, 2010 #5


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    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.
  7. Jan 13, 2010 #6
    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
  8. Jan 14, 2010 #7
    Sounds like a fun project. But extrememly challenging. Fluid power has been my occupation for the last ten years. There are many reasons you do not see hydrostatic buggies running around.

    Your math is correct on the flow and pressure required to get 141HP. But, being a mobile application using an internal combustion engine you are going to need 200+ HP motor. The reason for this that with an electric motor varying loads are instantly met by an increase in current. A combustion engine cannot react to loads by increasing fuel consumption quick enough so the engine needs to be oversized to accomodate load changes. Rule of thumb is 2x theoretical HP required.

    Basically, take that engine out of the truck you built and put it in the buggy. That will be the easiest part of the project.

    You won't want to regulate the speed of the engine. You want this to maintain an optimum RPM to accomodate the loads. You simply want to control flow to your motors to control your speed. There are a couple of ways to do this but let's say we do it at the pump swash plate.

    Now we have to determine the pump configuration. Are we utilizing a single variable displacement pressure compensated pump (with overcenter swash plate capability for a reverse function) or one for each motor? If we utilize one for each motor we have a few options for pump mounting. We may be able to stack them inline or mount them on pump drives, something like this: http://www.twindisc.com/IndustrialProducts/IndustrialPumpDrives.html" [Broken].

    If we use a single pump, which would be more efficient from the pump side of things, it's pretty easy. The problem is prioritizing flow to your motors. Oil will go the path of least resistence. If a wheel somehow sees no load, let's say it caught some air going over a bump, all your oil is going to want to go towards that wheel causing an immediate drop in power to the rest of the wheels. To counter this we need to introduce a flow divider. This will require some research. You will want a configuration that is accurate but has enough play that when you turn your inside wheels won't be fighting with your outside wheels. Kind of like an old school 4wd dif.

    Braking, this will be touchy. Feathering the brake won't be so easy. But I'll continue after I hear what you think so far...
    Last edited by a moderator: May 4, 2017
  9. Jan 14, 2010 #8
  10. Jan 14, 2010 #9
    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.

  11. Jan 14, 2010 #10
    Get back to you tonight Jason. We can start building a conceptual schematic.
  12. Jan 14, 2010 #11
    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/Fluid-Power-Circuits-and-Controls-Fundamentals-and-Applications [Broken]
    This web site has members that build hydraulic drives:
    Last edited by a moderator: May 4, 2017
  13. Jan 14, 2010 #12
    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.
  14. Jan 15, 2010 #13
    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.
  15. Jan 15, 2010 #14
    The current frame we have now is a little smaller than a full size buggy. The original concept was to make it as small as possible, but the width of the drivetrain we were going to use dictated it's width. The drivetrain came from a Pontiac Fiero, so if you've seen one of those recently, its about that size. The frame itself doesn't weigh that much, currently less than 200lbs. We can definately make weight a top priority. I used 2000 lb for my numbers. Somewhat of a guess. It is designed for two seats, so figure 400 lb for people, another 200lb for seats and misc.. The variables are engine/pump/motor weight since we dont know what those are yet. The 350 weighs 520lbs, plus whatever pump we choose, 4 motors, and the oil tank and of course gas tank, brakes, wheels, and tires.

    Can some sort of large oil cooler be used to cut down on the size of the oil tank? This could help save weight, but I don't know anything about that end of it yet.

    Definately bigger than an atv, more like a offroad go-kart on steroids I guess.

    The ideal operating range would be in the 0-40ish mph range. But if possible I would like the ability to go faster. I read once about someone who had some sort of valve that would allow in one position fluid to go to all for wheels with higher torque and a top speed of 45, then could move a valve and divert all the flow to the rear wheels giving him more top end. I don't know if that makes sense or is even really possible, but sounded like a good idea.

    I guess to sum up, I would like to use the frame I already built. If it is really THAT bad of and idea, I can shelf that and start over with the frame, although I would rather not.

    Are you recommending a smaller size to start with based on component cost, or complexity, or something else? If it's just purely cost, then lets design it so I can start looking for parts. It it just becomes to expensive, then that will tell me we have to go smaller. Honestly most of the size is based on being a 2 seater. I don't want a single seater. To be honest, if this one worked, I was going to contemplate a 4 seater next.

    Hope this helps, let me know what else your thinking.
  16. Jan 15, 2010 #15
    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.
  17. Jan 15, 2010 #16
    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.

  18. Jan 15, 2010 #17
    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.
  19. Jan 15, 2010 #18
    Ok, I'll see what I can find. Thanks!
  20. Jan 17, 2010 #19
    Is there a way do do this with fluid? Maybe a a valve that allows the fluid to free flow through the motor? I haven't had to much luck finding any application that would use something like a free sprinning joint. At least something that is strong enough to handle the torque and abuse. I'll still keep looking though.

    Let me know if it is possible.

  21. Jan 17, 2010 #20


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    You will need to make one or two changes maybe, but look at the inside of an automatic transmission torque converter, the stator clutch is a slip design that freewheels in one direction and locks up in the other direction, they transfer a lot of torque.

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