Hydrostatic ATV

[kdlklm]

I've been doing a little research on building my own hydrostatic 4x4 ATV with high and low range. I would like to be able to get between 20-25 mph out of it. It will have articulating steering. I'm looking at an engine between 30-40 hp. I'm looking for advice on what size of pump and motors to run. The nature of the design will require it to have four hydraulic motors, plus the steering. It will be for a wheelchair user, and this is a similar design attached.

Thanks
Keith

 

[BruceWard]

Very cool idea. I think the biggest challenge will be finding suitable tranmissions for each of the motors.

 

[Code54]

Very cool idea

 

[workinprogress]

Sounds like a great project. Below is a good web site with some calculating tools. I would suggest starting with the tire size and going from there. So something like this:

Garden Tractor rear tires or ATV around 23" diameter
So 300 -400 rpms would get you 20-27 mphs. (Using Vehicle speed calculator)

A simple design would be a hydraulic wheel motor. This would result in a 4 wheel drive system (one motor for each wheel). A quick check found a few that max out at 300 rpms but most were more like 150 rpms. So for a rough calculation one of these motor give 150 rpm at 8 gpm.

With 4 motors you will need 32 gpm to get your 150 rpm but this only gives you 10 mphs wiht those 23" diameter wheels. The combinations are endless so I suggest a few minutes with the below link might get you some focus.



Baum Hydraulics Corp :: Spec Calculator

 

[brokenot]

A simple design would be a hydraulic wheel motor. This would result in a 4 wheel drive system (one motor for each wheel).

There's really nothing simple about having four individual wheel motors for a drive system, especially since the OP is looking for some speed out of the vehicle. For example during turns, the outside wheel motors are going to want to push the inside motors. Designing a drive system that isn't going to skid the tires in turns will get pretty complex.

Additionally, hydrostatic drive isn't really the way to go for the kinds of speeds mentioned. You're going to need some serious hp to move the kind of oil it takes to go 20-25 mph....especially with four wheel drive. If you have the horsepower/pumps/motors to obtain those kinds of speeds, then you're also going to need a pretty good sized reservoir or a big cooler to dissipate the heat generated.

While I admire the ambition to tackle this project, I'd hold off on purchasing any bits to get started without having a solid plan in place. First on the agenda should be (trying to) find an existing design to get a better idea of what's going to be required. It will probably be easier said than done though. Hydrostatic drive systems offer several advantages, and are the power transmission method "go-to" choice in many applications.

High ground speed applications are definitely not their forte though. Too much input hp required, too much heat generated, too many losses due to the various component inefficiencies, etc.

Another thing to consider is the control setup. We have some Kubota RTVs at work that are hydrostatic drive. Operators often complain about the on/off nature of the drive system. When you step on the pedal to go....you go. But if you're cruising along at 10 mph or so and let off on the "go" pedal, the braking effect will toss you through the windshield if you're not used to it. You don't coast at all like you would with a regular transmission. And these are factory-built machines that are pretty well sorted....

 

[Iplayfarmer]

Take a look at golf course reel mowers. They have the pumps, the hydraulic motors, the variable flow valves, and everything you'd need. I've picked up a few here and there because of the hydraulic parts they can donate. I have one sitting in the pasture right now with a 35 hp diesel engine. It sounds like the perfect start for whatever project you'd be tackling.

 

[Renze]

make sure you dont overspeed them wheel motors. Lots of hydrostatic wheel loaders die that way, by traveling from jobsite to jobsite. The oil turns hot at high speed, low torque.

 

[kdlklm]

I might have to sacrifice some speed because of the obvious heating issues, and a massive amount of horsepower that it takes. As for steering, the unit above in the photo doesn't use any kind of sophisticated steering system to slow the motor down on the one side, most steering will be done at low speeds anyway. I've still been hunting for something to compare the horsepower to speed and torque ratio, like the mowers suggested.

Braking is a big consideration as well, the comparisons I have are my zero turn lawnmower and my hydrostatic tractor. They both coast when the engines are running and reversing the unit will slow it quickly.

There is the option of going to the electric hybrid type of system, but they tend to lack the low-end torque that I would like to have for pulling stuff. And electric motors can heat up pretty good to.

Keith

 

[J_J]

The Power-Tracs all use VSP , variable speed pumps, and some large hyd wheel motors. The ground speed on our units are about 8 to 12 MPH. A foot control turns the lever on the the VSP swash plate, and controls the speed of the fluid through the wheel motors. Power-Trac also produces a hydro ATV, 6 wheel drive using wheel motors.

You could use use one hyd motor each side and drive the wheels by chain if it is not articulated.

Some of the wheel motors are in a series/parallel configuration.

So in essence, you need to compute the size of the wheel motors in CC, and the torque developed to push your machine at a certain speed, including the weight of all the components, and an avg person of about 200 lbs. . That will dictate the size of the pump, and the size of the pump will dictate the size of the engine HP necessary to power your machine. Keep in mind the 85% efficiency of the pump and 85% on the motors.

Might be best to use a small engine and transmission out of a small car to power the unit. Radiator and fan, etc.


ATV Hunt Vee

http://www.power-trac.com/VideoClips/WindowsMedia/UVclass.wmv

http://www.power-trac.com/uvclass.htm#atv

 

[kdlklm]

Does that ATV use six separate motors for each wheel, and one pump. Or does it have more than one pump. Because the drive system is virtually exactly what I'm looking for. I do like the idea of having articulating steering, because I do a lot of driving in my yard and on my grass, and I think the skid steer would chew up my grass too much. I also have to have a kneeling system for the front, so a wheelchair can be back in, so that kind of eliminates having a front axle.

Keith