Go BottomHow does a Hydrostatic work
 | Duane Pearl
Join Date: Posts: 1 
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2000-10-03 20282
I was just wondering how a Hydrostatic transmission works. Is it basically just hydraulic fluid pressed through a hydraulic motor? Are there any clutches inside the transmission (other than the one on the back of the motor) that are prone to wear? Are there both clutches and hydraulic motors? Just wondering. My machine is a Kubota B7500.
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-10-04 20295
I'm not really a hydrostatic type of guy, but I'll keep the ball rolling here. There might be different types, and I've only looked at a few. The JD's of X55 vintage use just what you describe: A hydraulic pump driving a hydraulic motor. What's more, the two are near identical and in the same housing. Wobble plate type. No clutches, just flow control. Output goes to 2 speed transaxle and PTO.
Some larger tractors use a series of nested rotor vanes where one set of rotors drive another through forces on the fluid. Think of a pelton wheel with a multiplicity of nozzles. Fluid flows from one set of vanes to the next and also radially outwards to return to the sump as work is done. Again no clutches.
Both of the systems above turn an output shaft which goes into a transaxle. The transaxle is a typical rear end but with a set of sliding gears for low/high range. A clutch is optional there, and also there may be one to operate the PTO.
I don't have any idea of how the Kubota's hydrostat works, but would be interested to find out.
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York 
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2000-10-04 20305
Roger hit it right with one of the descriptions although we have worked on a lot of the hydro's I can not say I've seen the vane style. Normally the disconect on a hydro is in front of the pump system not on the rear on those I'm used to working with. ....
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| Duane Pearl
Join Date: Posts: 1
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2000-10-04 20315
So then is it safe to say that a hydrostatic transmission is a fairly rugged type of drivetrain? I was worried that the hydrostatic would fail sooner than a gear transmission, but couldn's resist the lure of the super smooth operation, so I bought one. Since then I have had this gnawing sensation that someday down the road I might regret the decision. If there are no clutch packs inside like you would find in the Kubota L35's shuttle shift tranny, I would feel pretty safe. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2000-10-05 20329
I could use a little concept help as well. I think I've got about a 1/2 or less an understanding of HST. What I can't get past is the idea of gear ratio. I wonder what the equivalent is in a hydraulic motor, or if the ratio just doesn't change in an HST? I guess the equivalent of gear ratio in a hydraulic cylinder is it's diameter in relation the pump volume. I don't know if there is a similar concept for hydraulic motors--variable displacement pump maybe. I guess the angle of nozzles (if they have nozzles) could change in relation to the vanes in a vane type. I really just don't know, but I think there's got to be some way to change engine rpm in relation to transmission output shaft speed. ....
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| Steve in Buffalo NY
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2000-10-05 20334
As I understand HST, it is somewhat similar to a Train Locomotive. We all know that trains are diesel / electric. A Big Diesel turns a generator, which turns the big electric motor which moves the train. Why? Because it starts from a big fat heavy stop and has to lug 100+ cars. Try that with a clutch. So to the HST. Your engine turns the hydraulic pump. The pump turns the hydraulic motor. It will start from a stop easier than with a clutch slipping. Any down side? Yes, some. There is some loss of power thru the system and any farmer running heavy tillage will tell you that a HST won't cut it because of fuel consumption. ....
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| Brent B
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2000-10-05 20340
The typical hydrostatic transmission consists of a variable displacement
piston pump that drives a fixed displacement motor. By varying the angle of
the swash plate of the pump (with your foot pedal) you vary the stroke of the pistons in the pump, and therefore the flow of oil out of the pump per revolution.
The motor is basically a piston pump operated in reverse. The flow and pressure
from the pump pushes on the pistons in the motor and is converted to rotary motion. The motor shaft then drives a conventional 2 or 3 speed transmission
to give you your speed ranges.
Hyd pumps and motors operate at 90-95% efficiency each, so together only put out
80 to 90% of the input power. Gear meshes operate at 99% efficiency, and so
have much less system loss.
Brent
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| Duane Pearl
Join Date: Posts: 1
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2000-10-05 20344
You guys have been very helpful at explaining how a hydrostatic works, and given me some neat numbers as far as effeciency, but I still would like to hear from someone about durability and reliability of these transmissions. They sound like they would be very durable, as the flow of a lubricating fluid is what ultimately powers the wheels. It seems like all those parts, being submersed in hydraulic fluid, would be hard pressed to wear out. Does that make sense? ....
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-10-05 20346
Duane.... I think it took me a long time before it finally struck me just how reliable tractors are compared with just about anything else mechanical. We tend to think of reliability in terms of automobiles, power tools, and other high-revving machines. But tractors are lots more durable. Fifty-year-old tractors are for sale everywhere in the country every day. A surprising amount of these have the original engine and transmission - all without any major rebuilds having been done. The compact tractors seem to be just as reliable. I've worked my little compact hard for 20 years. It never has needed anything and still doesn't. This isn't unusual. Just look back through the archives of this tractor board. Threads discussing major repairs are almost nonexistant.
Back to hydros....Yes, they are full of fluid, but then so are gear transmissions. Some gear transmissions even have internal oil pumps with pressurized and filtered oil fed to the proper parts. Hard to beat that kind of lubrication! And having a hydro doesn't exempt you from having a gear transmission. Your high/low range is a very standard type of gear transmission which has only two speeds. The hydro is acting as a clutch when you shift the ranges.
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| Peter Accorti
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2000-10-05 20348
Another great discussion! I have a question though. If the hydro motor is a fixed displacement pump (I assume a piston and connecting rod), then how does a hydro avoid having a "dead spot" in power when the hydro motor piston (not the hydro pump) is fully extended or retracted? Are there perhaps two hydro motor pistons at 90 degrees of each other? ....
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| Brent B
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2000-10-05 20350
The motors I'm familiar with are called bent axis motors. They contain 5 to 9 pistons push against a rotating plate at somewhere around 25 deg from perpendicular. Similar to a gas engine with a crank, they are timed so that only some of them are pressurized at one time, causing rotation in one direction. To reverse the motor, the pistons on the other side are energized.
So at any point of time, some pistons are pushing against the plate (which is attached to the output shaft)some are open to tank and are being pushed back to be ready accept oil for the next revolution.
Its hard to describe in words, isn't it?
Brent ....
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| Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York
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2000-10-06 20371
Brent you did a great job explaining the workings of a hydro. The hydro trans is a very reliable transmission for all but tillage work and at that it is not that it is unreliable it just burns more fuel than a gear drive. We have found that in most cases the hydro in general work has better service history than the gear drive tractors. The biggest thing with hydros is that the oil needs to be kept clean and the filters changed when needed. ....
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| Duane Pearl
Join Date: Posts: 1
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2000-10-06 20382
You guys are great. Thank you so much for all of the info. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2000-10-07 20390
I recall somebody saying that the only place gears work better is for a calibrated speed operation. Some planters I think. Maybe that comes under Art's heading of tillage. I've got gears myself--it was the right tractor at the right time for me and still is for the most part. Still, after a day of loader work, HST might be nice, even though the tractor has something like a shuttle-shift. ....
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