Hydraulic motors 101

[zmoz]

Lately I've been really thinking hard about building a small hydrostatic driven vehicle, similar to a "Power Trac" or something, maybe smaller like the mini skid steers. I've never done any projects with hydraulics before, so this is new territory for me. Basically I want a really small "hybrid" loader/forklift for my shop.

I've been doing a ton of reading and searching, but still having a little trouble figuring out how the hydrostatic drive works. Basically, I want to make a small vehicle with 2 or 4 hydraulic wheel motors. It would be driven with, lets say, a 20hp engine, coupled to a hydraulic pump that takes the full output.

Here's my confusion - how do I control the speed of the wheel motors? Where is the "throttle"? I've been looking all over surplus center's site for parts, not sure what I'll need to control the motors. If I'm supplying full hydraulic power to the wheel motors, do I just throttle the engine to vary the motor speed? I know a normal setup is to have a forward pedal and a reverse pedal for the hydraulic control - but do these actually control the hydraulic flow rate and speed, or are they just on/off valves?

 

[dcyrilc]

I've had little experiance with hydraulic drive equipment, but the one I used years ago had a proportional valve for forward/reverse and you ran the engine at operating RPM. It used a single control similar to a FEL joystick which controled left/right and forward/reverse.

 

[brokenot]

Hydrostatic drive systems vary the wheel motor rpm by changing the pump's displacement. The speed control linkage moves a swashplate in the piston pump. Moving the swashplate controls the stroke of the pistons. Varying the piston stroke varies the pump output to control the speed of the motors.

You *can* build a simple system that uses a proportional valve to control wheel motor speed, but it's an inefficient way to do it. Fixed displacement pumps pump oil at a given rate. Any of that flow you're not "using" for propulsion is essentially wasted. A proportional valve basically closes down a "leak" back to tank in the drive circuit. By opening/closing that leak back to tank, you can control the motor speed. But the fixed displacement pump still has to be pumping a given volume of oil all the time. Much of the flow is wasted....along with engine hp/fuel/etc.

Here's a short clip of a piston pump showing how moving the speed control varies the output of the pump by controlling the pistons' stroke and displacement:

YouTube - Hydraulics_How a hydraulic transmission piston pump works

 

[dcyrilc]

Thanks brokenot. I probably used the wrong terminology as I'm sure the equipment I used years back was a closed center system with a variable displacement pump.

I was trying to convey to the OP that engine throttle isn't a good way to control the speed, but I think you presented better info.:thumbsup:

 

[kennyd]

What is the budget? Hydraulic drive systems are VERY expensive to build from scratch!

 

[J_J]

20 HP will not give you much, as you will have to have several pumps for things like Variable Speed Pump for the drive, Gear pump for steering and loader pump, and maybe a PTO pump for hyd implements.

My PT has 3 pumps, and a 45 HP engine to power a 3900 lbs machine, with the ability to steer and lift 1200 lbs, and power a hyd implement, with the PTO pump. With everything working, it takes about all the HP available. You might do just as well to use a hyd garden tractor, lock the steering and put dozer treads on the front and rear tires.

 

[zmoz]

Hmmm...thanks for clearing some of that up. I was thinking one single large pump for everything. Basically apply all power to the wheels, then stop, do my other hydraulic work, and drive again. But maybe not so much...

So far my inspiration has been the Toro Dingo and the Powertrac 422, or something in between. I may end up just buying one...the Power Tracs look much more affordable than some of the other models. I want something that is less than 48" wide tire to tire.

What kind of pump would a machine like this have with only 22hp and 4 wheel motors?
H Class
Does it use the same pump for the wheels and the attachments - or multiple pumps?

 

[J_J]

22 HP will power a pump with a displacement of .7 cu in, and will pump 10.5 GPM at 3000 psi. I believe that all the PT's have 2 or 3 pumps. You might find some used PT's around, like Ebay or Craigs list. On the PT web site, there are used machines for sale.

Used Power Tracs

 

[zmoz]

Alright, so this would be the kind of pump I would use for driving (2) wheels?
https://www.surpluscenter.com/item.asp?item=9-7686&catname=hydraulic
A little on the small side, but really on the cheap side.

How long exactly is "intermittent" when they're talking about the maximum pressure for the pump and motors?

 

[brokenot]

So far my inspiration has been the Toro Dingo...

We have four Dingos in the rental fleet, two are gas units and the others are diesel.

Here's a Mediafire link to the parts manual I just uploaded for one of our gas units....it will give you some idea of what's involved. Click the link, and then click the download icon in the yellow box once the page loads:

Toro Dingo Parts Manual 55 15 4.pdf

The Dingo uses 3 pumps, two individual hydros for propulsion that are belt-driven off of the engine's pto end, and a tandem fixed-displacement gear pump that's directly coupled to the flywheel end of the crank. The gear pump is tandem because one section of it provides power for the onboard hydraulics for the loader arms and such, and the other section provides power for the machine's auxiliary hydraulics for attachments.

Toro really has these machines well-sorted. They're very compact, powerful, and user-friendly.