View Full Version : OT (or maybe not) - any members play with hydraulic motors?

10-20-2008, 08:58 PM
has anyone here used one? do they slip, or if you're going up a steep hill will the tires break loose before the motor starts turning in reverse or stalls (assuming you are sending fluid to it in the forward direction)? what if you had a four-wheeled vehicle with the two right tires on one circuit and the two left on a second circuit, and all four tires had an individual hydraulic motor. if say the left front tire got jammed, would all of the fluid attempt to flow to the left rear motor? what if the left front tire popped up into the air, would all fluid attempt to take the path of least resistance and flow to the left front motor? i'm assuming that if you had four individual control valves that fluid would always flow to the tire you directed it to, correct?

how are these motors rated? if you use the (RPMxTorque)/5252=HP formula do you use the highest rated RPM and torque to calculate it, or does the conversion not work for hydraulic motors?

is a calculated HP number realistic for comparisons? how does a 7HP hydro motor compare to a 7HP gasoline engine or a 7HP electric motor? if you have four 7HP hydro motors, do you need a 28HP gasoline engine to drive the hydro pump to power them? what if you only have a 18HP motor, will the RPM drop or will the torque drop? i'm guessing if it's a 10 GPM motor and you have four of them, that you need at least a 40 GPM pump to run them. how does power transmission loss compare to a mechanical transmission or a hydrostatic drive?

anyone have any links to sites that deal with hydraulic motors and applications for the home shop tinkerer? :)

andy b.

10-20-2008, 09:51 PM
Hi Andy:

A Mother earth news article in the 80s, it had a vw chassis, 8hp sideshaft engine, no mods.. a hydraulic accumulator and sun-motor-pump driving through the transaxle.

WITH all the shortcomings left on the chassis, it still got 80mpg.. During braking, when you laid your foot on the brake it went into regen in hydraulics.. when you depressed further it operated the brakes.
Imagine a gps hooked to this rig with a laptop.. the motor runs building power ahead of time before you get to a hill or accellerate onto the interstate. Planned trips=more mileage. Drive by wire is coming.

Yes I have experience with hydraulics, servos mostly. I can't think of a place to point you thou.

HYD Motors from memory are 1.5x electric, near 2x gasoline in torque.

I had a rock crawler laid out using four hyd motors and a jap bike engine.. Never got finished. The guy with the money got distracted. My plan was to use fwd spindles and drive through center.

10-20-2008, 11:53 PM
Get yourself one of these first,worth the money,covers all the common pump and motor,mounts,fittings and shaft configurations.Also contains useful formulas and rules of thumbs,plus a lot of other good info on other topics such as piping and compressed air.


Next would be keep your eyes open for hydraulic componets laying around.I found a beat up Toro commercial front deck mower for $75.The deck was rotten and it was missing the engine,but it had two 15hp wheel motors and three 5hp deck motors plus valving and a six section pump all of which has worked fine.

10-20-2008, 11:56 PM
It would divert the power to the free wheel just like an open differential unless you installed a flow divider/ priority valve which would make the system divert a given amount of fluid (depending on the valve) regardless to one of the wheels. Kind of a limited slip hydraulic system. You can get these flow dividers in pretty much any gpm size you want. I know on the heavy equipment we run, one machine in particular (a chipspreader/ stonespreader) has independent hydraulic motors for each wheel with a flow divider between the front and rear so that there is guaranteed 2 wheel drive. The process could be done again side to side but it could cause some scrubbing problems and make for un-even flow from side to side when the gpm of the pump changes... if that makes sense. Couldn't tell you the equation off the top of my head for your other problem.

10-21-2008, 03:23 AM
In the early 80's I built a hydraulic hybrid car; the pump/motor was variable displacement and used a hydraulic accumulator to store the energy... this was my masters of engineering project at UC Davis.

I placed the hydraulic pump/motor in parallel w/ the engine (which I replaced w/ a small industrial engine).. worked well but desperately needed better computer control.

If you put all motors in series, the motors will all run at the same speed w/ different torque - turning will be hard. If you put two motors in parallel, it will be like a differential - both motors will have the same torque, but different speeds. If you put the front two motors in parallel, and the back two as well, and then place both sets in series, you'll have the equiv. of a 4wd truck.

Power transmission losses are about 15-20 % overall for good quality components - axial piston, etc. Losses are higher for georoter or gear pumps/motors.

Flow divider valves are very lossy; they're like a limited slip rear end.

> If you have four 7HP hydro motors, do you need a 28HP gasoline
> engine to drive the hydro pump to power them? what if you only
> have a 18HP motor, will the RPM drop or will the torque drop?

A hydraulic motor & pump is like a gear train. If you don't have enough
power to drive the vehicle, it will slow down. If the prime mover develops more torque at lower speed or the load decreases (because it's wind resistance), the vehicle will stop slowing down... otherwise it will continue to slow down until the engine stalls. If you have a variable displacement pump, you can reduce the displacement - this is like downshifting.

Idea: If the control computer had a gps input, it could learn your daily commute and adjust when to charge up the accumulator, discharge etc. This would be particularly important w/ a hydraulic hybrid, since while the total energy storage is low, the power levels are not - you can get 40 HP from a hydraulic accumulator w/ o problems, which was difficult w/ small batteries in the 80's.

- Bart

10-21-2008, 07:03 PM
If you want full torque out of the motors you need to be able to supply the rated flow and pressure of oil to the motors, Any less and you will get less than the rated output of the motor.

This is a little trick used on man lifts. High speed plumbs the motors in series and delivers lower torque (and behaves sort of like posi-traction), low speed plumbs the motors in parallel and delivers higher torque (and behaves like a differential).

Hydrostatic drives have a variable output pump connected to a motor with a small make-up pump (ideally running totally closed loop, but in reality there is also a small make-up pump keeping the hydrostatic system charged).

10-21-2008, 09:33 PM
I have a Tenant rider scrubber and a sweeper, both with hydrostatic drives. The sweeper is lighter duty, with a single motor driving a swivelling rear wheel, and a 2-cylinder Kohler engine. The scrubber has 2-wheel drive, with direct-coupled motors, driven from the variable-volume pump. It has a 4-cylinder Continental engine. I've dreamed for a long time about putting some of this stuff to use, maybe some sort of off-road car, an all-hydraulic lawn mower or snowblower, or...??? How's about a small paddle-wheel boat, with 2 stern wheels, each with its own motor/pump combination?

Hydraulics are really fun!

Oh! You can get a lot of information for free by studying the Northern Tool catalog, they sell all sorts of components.

W.S. has the right idea also, keep your eyes open for components like I've described above. Like him, I also have a Toro lawn mower, it's a 3-reel golf course "greens" mower. Hydraulic wheel motors, plus small motors on the reels. Of course, it came with valves, a double or triple pump, etc. I attend a large consignment auction every Labor Day that invaribly has some of this sort of stuff.

10-22-2008, 08:11 PM
thanks for the replies!!! a lot of good info. i plan on picking up a copy of the mini reference book, and i'll keep my eyes open at the local spring consighment sales for a Toro golf greens mower. i think that would be a good source for parts to experiment with.

andy b.

10-22-2008, 10:04 PM
You fellers speaking hydraulics: When your equipment utilizes a smallish (10 HP, or less) hydrostatic pump, does it creep when it is put in neutral? Other words, when it is supposed to be in neutral, with the prime mover driving it, does the pump tend to slowly move fluid one way or the other?

We've utilized the old, 700 dollar Sundstrand HS for a throughput of 4 HP max, some can be put in neutral and will stay there. Others, for the luv of mike, will creep ever so slowly and can/will cause a heck of a mess if not caught. Any suggestions or solutions, all appreciated.