Shop Talk: Hydrostatic drivetrains

Hydrostatic drives are perfect for most of the equipment used by landscapers. They offer unprecedented machine control coupled with efficient power transmission and low-maintenance demands. They also offer higher ground speeds, says Mark Naperala, director of marketing for the Ariens Company, which also owns Gravely Commercial Equipment. “Hydrostatics provide our mowers with travel speeds of 11 to 13 mph, which is higher than most geared systems. Mowing speeds are generally faster with hydrostatics, too.”

But for most landscapers, the initial attraction to hydrostatic drive systems is the efficiency boost it will give their operations, notes Eric Mangum, marketing project engineer for skid steer and multi-terrain loaders at Caterpillar. “Skid steer and multi-terrain loaders, commercial mowing equipment, track-type tractors – any machine that has quick movements and needs infinitely variable speed capability is a good match for hydrostatic drive,” he says. “These can include machines that make repetitive movements as they work, or machines that require quick, precise control inputs while moving across a lawn or open ground.”

Hydrostatic drives have two key components, a pump and a drive motor. Final drive can be via a drive shaft or chains. Direction is controlled by a three-position (F-N-R) toggle switch, by joystick controls, or by levers with linkages. There are no clutches or shift linkages. Most machines have two modes, such as travel/work or hi/lo, and switching between modes can be done on the fly.

“Hydrostatic drives are easier to learn to operate than mechanical drive units,” notes Doug Laufenberg, product marketing manager of attachments and compact wheel loaders in John Deere’s Construction and Forestry Division. “Equipment typically defaults to the low range or work mode on startup, so an inexperienced operator is less likely to overstress the machine. When traveling, the farther you push the gas down or the drive sticks forward, the faster you’re going to go. There’s no shifting. The fluid in a hydrostatic system acts as a shock absorber during abrupt changes in direction. As long as you have oil in the system, it’s pretty abuse-resistant.”

Perfectly suited for small machines
Given these advantages, it’s not surprising that hydrostat drive machines are common in landscaping fleets. But hydrostatic drive systems have some quirks you should be aware of on the job, and there are limitations on which machine types are suitable for hydrostatics. High-horsepower equipment, for example, can overwhelm a hydrostatic drive, causing excessive heat and slippage. Even in smaller equipment, drivetrain slippage can be apparent. Travel speed will vary for a given engine speed depending on the load being carried. And because there is no mechanical connection between the final drive and the engine, there is limited engine braking on descents.

Laufenberg says hydrostatics are not inherently more or less efficient than mechanical drives. “The question of efficiency really depends on how well the nature of the work matches the characteristics of hydrostatic drive systems.”

Mangum agrees and offered an example. “In applications requiring frequent changes in direction – as with a skid-steer loader or zero-turn mower – a hydrostatic system will be much more efficient than other types of equipment, thus cutting down on the time it takes to complete a job.”

Because of this, hydrostatic drives are often perfectly suited for smaller machines – particularly excavators and zero-turn mowers. Since they’re so common, other factors enter into the equation when selecting the right hydrostatic system on a given piece of power equipment.

Mangum recommends hydrostatics with sophisticated control systems such as pilot- or electronically controlled hydraulics. These systems are generally easier to manipulate and are often fully-integrated with the hydraulic system for optimum control. Other machines use mechanical linkages to initiate hydrostat control. “Consider the efficiency of the pumps and motors,” Mangum says. “Pilot hydraulics and electronically-operated hydrostatic systems last longer than mechanically actuated systems and need far less maintenance. A mechanically actuated system’s parts will wear, diminishing the controllability of the machine. Pilot hydraulics and electronic operated systems will also maintain a higher residual value than mechanical systems.”

Perhaps the most serious challenger to hydrostatic drive systems on the market today are units powered by continuously-variable transmissions (CVTs). Like a hydrostatic system, a true CVT offers infinitely variable travel speeds, although it changes ground speed by effectively changing transmission ratios, whereas a hydrostatic transmission changes ground speed in response to changes in engine speed.

A CVT allows OEMs to maximize engine performance in a narrow RPM band and operate the equipment at that optimal engine speed all the time. This provides maximum engine efficiency and helps reduce engine emissions. On the other hand a CVT is still a mechanical system and is not as forgiving of shock loading as hydrostatic drives are. Another limitation is that CVTs have tighter design requirements than hydrostat systems; they won’t wrap around frame rails or turn corners the way hydraulic lines do.

And, again, the simplicity of a hydrostatic system is a bonus in compact equipment and other
landscaping-specific machines. In machine types where drive speed differential is used for steering, such as skid steer and track loaders, hydrostatics simply vary the flow to the right and left side drive motors. Flow can even be in opposite directions from one side to the other. A CVT requires some fairly tricky additional transmission components to accomplish the same thing.

Hydraulic oil is the key
When buying a used hydrostat machine, Laufenberg recommends oil analysis be performed to evaluate the condition of the hydrostatic system. “There’s a time factor here, however,” he cautions. “Dealers don’t do oil analysis at their facilities. They send the samples out to labs. You’re looking at a day or two turn-around time, at least. This isn’t a big deal if you’re buying in your local market, but it can be a problem if you’re looking at equipment in another state or buying off the Internet. If you’re the seller, you may want to provide the results of a recent oil sampling from the equipment you’re listing.”

Oil may be the Achilles’ heel of hydrostatics. Keeping sufficient oil in the system is just part of the challenge. The other part is keeping the oil clean. “We recommend a 25 micron filter on our systems,” said Brian Gloudemans, engineering leader for Ariens.”

“It’s important to follow the proper maintenance intervals,” says Mangum. “Keep the fluid filled. Change the filters regularly. In severe cold-weather applications, synthetic oils may be required.” In fact, Ariens uses synthetic oil in all its hydrostatic systems.

Despite their unique servicing requirements, hydrostatic drives are easier to maintain than traditional mechanical drives, said Laufenberg. “There are no adjustments to make, no gaps to maintain, and no clutch plates to replace. Service is as simple as periodic oil and filter changes. If there’s an oil cooler, check to make sure its fins are clean and that air flow over them is unimpeded.”

Ariens had an oil cooler on a few mower models for a short time, but the additional cooling effect was minimal. The company now uses a fan mounted on the pump drive shaft to cool the pump and the oil. “The systems itself provides ample cooling,” says Gloudemans. “We like to keep the oil at or below 210 degrees [Fahrenheit] and not exceed 230 degrees. Non-synthetic oils start to break down soon after that, and synthetics start to break down around 300 degrees.”

Preventive maintenance tasks are easily accomplished by a fleet’s service personnel, but Laufenberg recommends rebuilds be left to a dealer’s service department, especially if the rebuild is brought about due to catastrophic failure. “Dealers’ service departments have evacuation systems that will thoroughly clean out the tank and lines to ensure there’s no debris or foreign material left in there.”

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