The pressure’s on: Hydrostatic transmissions

There’s nothing new about the principles of hydraulics – using a fluid under pressure to accomplish some type of work. That much even the ancient Romans knew. But in the past few years, advances in manufacturing techniques and materials have allowed hydraulic component makers to lower the weight, reduce the size and increase the performance of hydrostatic transmissions.

These changes have enabled equipment manufacturers to build machines that can move quicker and more nimbly and dig, load and carry more material faster and more profitably than ever before. It’s been a quiet revolution, but there is no question today’s equipment can dance circles around the machines of just a decade ago – and hydraulics get the lion’s share of the credit for this.

From excavators to dozers
Hydrostatic transmissions have long been the preferred method for moving most excavators around. Excavators don’t need high travel speeds or torque, and hydraulic drive motors were sufficient to propel all but the biggest excavators. But as the reliability and quality of hydraulic componentry improved, manufacturers were quick to take advantage. “The components evolved into the kind we needed,” says Bob Post, Komatsu’s product manager for wheel loaders and motor graders.

Today, roughly half the heavy equipment produced by many OEMs is hydrostatically driven. And these hydrostatics don’t just crawl excavators along at 4 or 5 mph. They can propel huge wheel loaders around the jobsite at speeds up to 25 mph and put down the kind of torque needed for 90,000-pound dozers.

Thanks to the versatility of hydraulic systems, high speeds and high torque are no longer mutually exclusive properties. And freed from the linkages and metal-to-metal contact mechanical and traditional transmissions require, hydrostatic transmissions can be guided by sophisticated electronics that in many cases eliminate shifting and give the operator superb control of his machine.

“Control systems became more advanced to enable the fine control of a transmission without the high lever effort that would otherwise exist with larger components,” says Brett Errthum, John Deere’s business analysis manager for crawler dozers.

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Komatsu in the past year and half has put hydrostatic transmissions into increasingly larger equipment such as this 30,000-pound WA 320.

Hydrostatic advantages
Brendan Casey, founder of, cites six advantages a hydrostatic transmission offers over a traditional mechanical transmission:
· High power density (high power output per unit mass)
· Infinitely variable and step-less speed (and torque) control – both forward and reverse
· Up to 90 percent of maximum torque available at start-up or break-out
· Low inertia of rotating parts permits rapid starting, stopping and reversing
· Simple overload protection
· In the case of traction drives, the engine can be located anywhere in the machine without the need to consider complicated driveshaft and drive axle arrangements.

The wheel loader is perhaps the one machine that takes the most advantage of these properties. “The operator is going to think he has a powershift transmission that’s automatic because the shifting is seamless; it’s all done hydrostatically,” Post says. “You get good power going into the pile for digging. And then you’ve got immediate response to get rolling if you’re going to load a truck or do a load-and-carry operation.”

The variable flow characteristics of hydrostatics also “gear down” sufficiently to meet low speed/high torque needs such as dozing, says Errthum, but without gears. Hydrostatic dozer transmissions enable infinitely variable and independent power to each track. This permits full power turns and the ability to make a dozer counter-rotate in its tracks by reversing one track while powering the other forward.

Another big advantage of hydrostatics, says Errthum, is the power is transferred through flexible hoses. This reduces the number of components in a machine, resulting in fewer service complications. It also allows manufacturers to deliver power to places where it would be impossible to run a drive shaft or chain drive. Freed from having to arrange a linear sequence of mechanical drivelines, equipment designers are creating more compact and better balanced machines with better service access points and larger operator stations.

Piston vs. gear pumps
The revolution in hydrostatics has been driven by the ability of component manufacturers to build better piston-style pumps, as compared to the older gear pump technology. Gear pumps are dependent on the rpms of the engine – higher revolutions give you higher pressure – whereas piston pumps use a swash plate to control flow (and hence pressure). The variable control means you can exert a great deal of force without having to rev the engine to full throttle.

Piston pumps are smaller and more efficient than gear-type pumps, Casey says. They are also more expensive and sensitive to damage and contamination, but their benefits clearly outweigh their limits.

The swash plates in a piston pump also mate well with the latest technology in electro-hydraulic controls, says Rudy Urbano, senior training consultant for Caterpillar. So instead of mechanical linkages or small hydraulic circuits trying to control large pressures and forces, a simple solenoid moves the valves that move the swash plate, what many call “fly by wire.” The electro-hydraulic controls are linked with a machine controller (essentially a small computer hooked up to sensors on the engine and transmission) that get feedback on everything from torque needs to wheel slippage and can respond instantaneously with no operator input.

Lifecycle and maintenance considerations
According to Post, the lifecycle on hydrostatic transmissions should closely match that of a mechanical transmission. The key – as with hydraulic systems for implement control – is maintaining the cleanliness of the system. The close tolerances and high pressures experienced by today’s more advanced piston pumps are more easily damaged and worn by contamination in the hydraulic fluid, including dirt, tiny pieces of hose, water and wear metals.

Hydrostatic transmissions can also be damaged by faulty or incorrectly adjusted circuit protection devices or a loss of charge pressure as a result of a burst hose, wear or damage to its charge pump, Casey says. Nonetheless, in a correctly designed, well-maintained hydraulic system the service life of piston pumps should exceed that of their gear-type counterparts, he says.

Normal life on a piston pump or motor should range from 8,000 hours to 10,000 hours or even 12,000 hours under ideal conditions, Urbano says. As the equipment designs evolve, manufacturers are working to make the hydraulic systems easier to repair. Disassembly and assembly is being taken into consideration, driven by the parts and service side of the business. “We now have designs of components able to be taken apart and put back together again without any special tools,” Urbano says. “If you can take a pump or motor down from 180 parts to 90 parts it makes a big difference on serviceability.”

Rebuilding a hydrostatic transmission is usually less expensive, Post says, because you are dealing with fewer components than a mechanical transmission.

Where’s the limit?
The trend over the past few years has been to put hydrostatics on increasingly larger pieces of equipment. Deere, in conjunction with Liebherr, fields a 324-horsepower, 90,000-pound hydrostatically driven dozer, model 1050C. And Komatsu’s hydrostatic WA 320 wheel loader pushes the scales to 30,000 pounds. Liebherr also makes a pair of big hydrostatic wheel loaders – the 54,600-pound L580 and the 33,740-pound L544.

But beyond those chart toppers, the efficiency and practicality of hydraulics begins to diminish. A hydraulic piston pump pushing fluid though a piston motor gets an overall efficiency of 85 percent, not counting flow losses, Casey says. A mechanical gearbox transfers energy at efficiencies of 95 to 99 percent and is coupled with a torque converter with an efficiency of 80 to 95 percent. Once a machine reaches a certain weight or torque requirement, getting the same efficiency would require hydraulic pumps that are too large and expensive to be practical, Post says.

Sidebar: Torque and speed: As much or as little as you need
In conventional transmissions speed and torque quickly get to the point where they are mutually exclusive. A race car goes fast, but can’t carry much weight. A loader can carry tons of earth, just not very fast.

But thanks to the development of hydraulically driven, dual-motor transmissions, as shown in the diagram above, you can somewhat expand these limitations. Komatsu’s Bob Post explains how it works:

The engine flywheel drives the piston pump. The speed of the flywheel and the angle of the pump swash plate determine the amount of flow the pump will provide to the motors. When torque demand is high, such as when starting up or digging, both the low- and high-speed motors receive flow. This provides low-speed, high-torque output from the motors. When torque demand drops and speed demand increases, the low-speed motor is virtually removed from the system by a clutch, sending all flow to the high-speed motor. The high-speed motor rotates faster and provides higher output speed with lower torque.

“All this is done seamlessly for the operator,” Post says. “All he does is set a maximum speed with a dial and then control the ground speed with his throttle. Since it’s hydrostatic, there isn’t any shifting.”

The dual-motor configuration shown here is considered state of the art for wheel loaders. Some brands and some smaller sizes of loaders will use a single pump with a two-speed transmission.