Hydraulics- pumps motors hydrostatic

Maybe there should be a discussion section just for Hydraulics on this site.
Have a misfit dozer with a 32 hp diesel engine and a hydraulic pump that puts out 3gpm
and a hydraulic motor that maxes out at 240 rpm. However it is doing less than 100 rpm at 3 gpm.
I need a motor that has 106 ft lbs at 900 RPM and 32 ft lbs at 3000 rpm and then a pump to provide
the flow and pressure to meet those needs.
Where to start looking.
Short story is this used to have 16 hp gas and 3 speed stick shift that got replaced with the above problem.
So I am trying to find a solution that will work.
Currently the dozer would lose a race to a slug at the speed it can go.
Thoughts. Who to call. Nice to find a used hydrostatic that would work or something to keep cost down.
Marvin

Here is one pump. Motors here as well. Jim
Surplus Center

if there were a hydraulic forum on this site ,,. it would have to be manned by one hek of a guru.. not a bad idea,but I think getting correct answers every time will be a challenge .. the case board has limited knowledge when it comes to hydraulic problems ..

How are you intending to vary ground speed? With a hydraulic flow control valve?

If a unit capable of 18 to 20 HP would work, and you are mechanically-inclined, you could take the hydrostatic unit from an older GARDEN tractor and modify it to connect to the drivetrain you have.

Example would be ebay auction #262369909629.

That is a self-contained hydro unit, charge pump, hydro pump and motor are all in one unit.

Has input shaft with pass through PTO output shaft, and bevel gear speed-controlled output. Even has a spin-on filter mounting provision.

Input is 3600 RPM, output is 0 - 3600 RPM, and they are capable of 20+ HP.

You would need to remove the output bevel gear and have a custom hub made up for whatever exactly you will be driving with it, and provide some type of oil seal and retainer at that area.

If you can figure out an output hub and seal, it's pretty reasonable at $175.00.

<img src = "http://i.imgur.com/9iOQYdQ.jpg">

<img src = "http://i.imgur.com/Saw2usu.jpg">

If you feel that isn't large enough after your repower, you could try to find a similar system from a wrecked skid-steer, or, if in ag country, a windrower.

When designing hydraulically-driven machines you have to match the power able to be transmitted through the oil to the power at the engine and the power you want available at the motor shaft. The power in the oil can be calculated with the equation GPM * PSI / 1714 assuming perfect efficiency. (Hydraulic drives are anything but efficient but this makes the math easier and it will be in the ballpark.) Using the 3 gpm you said your pump puts out gives only about 5 horsepower at 3000 psi that can be transmitted through the hydraulics. It makes no difference what kind of motor you have on the other end, you simply won't get more than 5 hp out of it at this pressure. To transmit the full engine power at 3000 psi will require a pump that can put out more like 18 gpm. Next, use the equation DISPLACEMENT (in^3) = GPM ? 231 / RPM to size the motor to run at the speed you want. You can also use the equation TORQUE (in-lb) = DISPLACEMENT (in^3) * PSI / 6.28 to determine the motor's torque output. Assuming you aren't hydraulic power limited the torque output from a motor is dependent entirely on displacement and pressure - the rotational speed doesn't factor in.

As Bob noted, you'll need some way to vary the flow rate to the motor. A flow control valve can be used but this would be an extremely inefficient way of doing it as the entire pump flow rate will be raised to whatever pressure the motor requires to turn, even if the valve is sending only a small percentage of the pump's flow to the motor. The rest of the engine's power will be converted into heat through the valve. A better way would be to use a hydrostatic pump and motor (also like Bob mentioned) where the pump's flow rate can be varied from zero to the maximum. However, when you get into hydrostatic drives you also need to provide filtered charge oil and a variety of other considerations. This would be a difficult project to get working right the first time for a person without a background in such things and the trial-and-error method could be prohibitively expensive given the cost of hydraulic components.

I agree with Brendon, he did all the math.