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What Is a Hydrostatic Transmission?

Maintenance and service of a hydrostatic drive transmission is more costly than a clutch unit in a traditional transmission.
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  • Originally Written By: Lori Kilchermann
  • Revised By: C. Mitchell
  • Edited By: Jacob Harkins
  • Last Modified Date: 24 October 2014
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Hydrostatic transmission is a category of engine mechanics, and basically describes a system in which power is generated and transmitted by pressurizing and releasing fluid through specialized pumps. It’s also sometimes called “continuously variable transmission,” and it works by converting energy harnessed from the movement fluid through hydraulic pumps to the drive train of a vehicle. This sort of transmission can technically be used to power almost any type of machine, though it’s most commonly seen in heavy-duty machinery like tractors and backhoes. The transmission has the ability to provide a lot of power very quickly, but it isn’t usually very efficient when it comes to acceleration and maintaining high speeds. As such, it isn’t usually a good choice for passenger cars or trucks that spend most of their time on the highways.

How it Works

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Motor vehicle engines usually come in several different types, and in most cases they’re named based at least in part of how they work. Passenger cars, for instance, are typically marketed with either “automatic” or “standard” transmissions, and both are terms that describe how the technology works — which is to say, whether the engine gears will shift automatically in response to driver speed, or whether they require driver engagement. Describing an engine as having a hydrostatic transmission is usually a statement about how the engine actually operates internally. In these cases, fluid is transferred from a piston pump to the motor through a series of tubes, and the energy generated in the process is what actually powers the vehicle.

There are no provisions for a clutch or a need for changing gears with this kind of transmission. To change speeds the operator simply moves the speed selector in the direction that represents the desired reaction. Typically, the farther forward the speed selector is pushed, the faster forward the tractor or vehicle will go, while pulling the lever to the rear while in motion will allow the vehicle to slow down. From a stopped position, pulling the lever backward places the vehicle in reverse.

Most Common Applications

This sort of transmission works really well for machinery that needs to have a lot of power without a lot of movement. Most machines that use it also have big enough engine boxes to hold its typically large size; it’s common in land-moving equipment and industrial machinery, particularly tractors, excavators, and forklifts. Anything that needs a lot of power and energy in short bursts can usually benefit from this sort of energy arrangement, though it’s not usually a good choice for vehicles that need speed. This includes cars, trucks, and equipment that needs to move in traffic.

Why People Want It

Ease of operation is one of the biggest advantages to hydrostatic transmission. By eliminating the clutch pedal, the machine becomes much less complicated to operate. The vehicle's speed and power can be easily controlled with levers instead of a throttle, too, and the engine can also be used to control the descent down a hill or grade by using speed and direction rather than brakes.

This sort of transmission also makes certain jobs more efficient, and may allow the operator to get work done more comfortably than with the tiresome pushing of the clutch and the changing of gears. Once acclimated to the hydrostatic drive, most operators are able to manipulate the machinery with one hand without ever touching the throttle or the braking system until finished with the task.

Drawbacks and Disadvantages

The hydrostatic transmission model isn’t without its faults, though. The mechanics involved in transferring power from the hydraulic shafts to the to gearbox increases the odds of something becoming stuck. The power application often directs far too much torque to the wheels in a soft terrain situation, making them prone to spinning; also, in most cases, the tire speed is not as effectively managed as by a throttle. This can create a control issue that typically results in the operator losing the fight between gravity and soft surfaces.

Expense might also be a factor. Hydraulically powered drive systems tend to be more costly than more typical clutch and gear models. The fluid alone is often pricey to buy, and maintenance and service tends to be a lot more expensive, too. For these reasons, some people choose traditional clutch transmission systems for their machinery, particularly if it’s something that’s only used sometimes or for limited projects.

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Discuss this Article

anon967801
Post 5

Even oil has small amounts of water in it.

anon946797
Post 4

Hydro is Latin/Greek for water. I don't believe there is water in a hydro transmission.

anon936812
Post 3

I would like to know if a "Hydrostatic transmission" would work for a golf cart application?

Wisedly33
Post 2

@Lostnfound: Actually, most of the vehicles that have hydrostatic transmissions are heavy duty vehicles like tractors, rather than passenger vehicles. The thinking is that this kind of transmission is not very efficient on the street at high speeds. Not sure why, but I'm no engineer!

Lostnfound
Post 1

I know I've heard the term "hydrostatic" when referring to a transmission, but I didn't really know what it meant. I figured with the "hydro," it had something to do with hydraulics, but beyond that, I didn't know what it was. I sure didn’t know the difference between a regular transmission and this kind.

I would be interested in knowing what kinds of cars have these transmissions. I'm sure they're way out of my price range, but I'd still be interested in seeing which cars carried them. I suspect they all have European labels on them and are probably manufactured by hand in a small, rural factory.

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