The hydrostatic transmissions that power construction and maintenance equipment from ride-on lawn mowers and walk-behind loaders to skid-steer loaders and dozers are marvels of engineering design and operating efficiency.
Using pressurized hydraulic fluid rather than gears, clutches, disks and drive shafts to harness power from the engine for propulsion, they pack big performance in a small package. “No other type of transmission, mechanical or electrical, offers more power per pound,” says Steve Zumbusch, manager of mobile application and commercial engineering for Eaton Hydraulics.
Closed systems
Most hydrostatic transmissions are closed systems in which the hydraulic fluid circulates between a variable displacement pump that supplies the oil, and a motor, which uses the oil to drive the machine’s wheels or tracks. A supplemental or charge pump, powered by the engine, maintains fluid supply to make up for leakage of fluid used to cool the system.
Both the right and left side of a skid-steer loader, for example, have a pump motor, typically connected by chains to drive the front and rear wheels. In other machines, the motor may be coupled directly to a wheel or track sprocket.
Normally, a piston in the pump moves a swash plate, which controls the flow of hydraulic fluid. The greater the flow, the faster the motor turns. Depending on how far you push or pull the control lever, which operates this piston, you can control travel speed infinitely throughout the speed range and go from full forward to full reverse without shifting. It’s simple, fast and efficient.
“A hydrostatic transmission provides a perfect match between travel speed and engine power, automatically providing the optimum power for a given speed,” explains Diego Navarro, service marketing manager with John Deere Construction and Forestry.
Another big advantage is dynamic braking, which eliminates the need to apply brakes to slow down or stop, even on an incline. That’s done by pulling back on the control lever.
A hydrostatic transmission is designed for long, dependable life. With proper care, it can last as long as the engine. “If a hydrostatic transmission fails, it’s usually because of improper maintenance,” says Zumbusch. Failure to maintain a hydrostatic transmission properly can cause a variety of problems.
Contamination
The main culprits behind the failure of a hydrostatic transmission are dirt, water and other contaminants that can foul components and the hydraulic fluid. “The silica and alumina in dirt particles are harder than any metal used in a hydrostatic transmission,” says Navarro.
These particles can score and erode surfaces of motors, pistons and sleeves and the swash plates of pumps. And, they can enlarge finely machined clearances, some no wider than a just a few microns, leading to internal leakage and loss of transmission performance. Meanwhile, water in hydraulic fluid can impair its lubricating abilities and corrode system components.
“Dirt and contamination in a closed hydraulic system can create a tremendous amount of wear,” says Bob Beesley, product manager for skid-steer and compact track loaders with Komatsu. “The wear feeds on itself, creating more particles, which create more wear.”
Zumbusch adds, “When wear creates internal leakage of hydraulic fluid, efficiency of the transmission drops. That increases the load on the system, which leads to higher fluid temperatures. That, in turn, causes the fluid to break down, reducing its ability to lubricate and remove particles, leaving the system even more vulnerable to contamination.”
The source of dirt and water in the hydrostatic system is the machine’s hydraulic reservoir, which supplies oil not only for the transmission but for hydraulic operations. Most dirt enters the machine through the hydraulic cylinders that operate lift arms, buckets, and the like.
“There’s a film of oil on the cylinder, which attracts dust and dirt,” Zumbusch says. “If the seals are worn or damaged, these particles can be drawn inside the hydraulic system when the cylinder retracts.”
Dirt and moisture in the air can also enter through the reservoir breather. When you operate the various systems on the machine, the hydraulic fluid level in the reservoir falls and rises as the oil circulates and draws in air.
Failure to clean the hydraulic reservoir cap before removing it, using dirty containers to top off the hydraulic reservoir, installing hoses that contain dirt and other debris on the inside, and even changing attachments, all can introduce contaminants. So can make-shift hydraulic tank caps. “I’ve seen machines brought into the shop for hydrostatic problems with the hydrostatic tank cap missing and rags stuffed into the opening,” says Beesley.
Cavitation
Although cavitation is much more common with hydraulic systems, it can also seriously damage hydrostatic transmissions. Caused when gases in the fluid vaporize, it’s characterized by noisy, erratic transmission operation.
“If your machine runs low on hydraulic fluid, the hydrostatic pump can draw in air,” says Beesley. “As the pump compresses these pockets of entrapped vapor, they collapse, releasing extreme pressure that can gouge metal and cause machined surfaces to implode. This can damage the moving parts of the pump, such as the cylinder bore, the swash plate, the piston and the pivot connection between the two. This damage creates contamination, and the failure cycle begins.”
Cavitation can also be caused by a restriction in the pump’s inlet line, like a kink, a worn or failed charge pump, a broken or loose hose, or hydraulic fluid that is too thick, either because it is heavier than recommended or has thickened due to cold weather.
Overheating
When hydraulic fluid becomes too hot, it oxidizes. This reduces the oil’s ability to lubricate, increasing wear. It also damages seals and gaskets, allowing contaminants to enter the system. Excessive heat can be caused by a dirty oil cooler or by worn pump components. Fluid leaking through passages widened by wear can build up heat quickly.
These potential problems highlight the value of following the manufacturer’s maintenance recommendations. They include:
Use the correct oil. Use only hydraulic fluid that meets the manufacturer’s specifications. This ensures that it will have the proper viscosity and will contain any additives needed to provide the necessary lubrication and heat and corrosion resistance.
Keep the fluid clean. In addition to changing filters as scheduled, replace old ones with those that meet manufacturer specifications — otherwise the filter may not capture the smaller particles. Repair leaking seals and gaskets and replace broken lines or hoses. Keep parts clean when repairing. Use clean storage containers and clean the filler cap before removing it to check levels or add fluid. Zumbusch advises using a water gate filter on the hydraulic tank breather to remove moisture in the air.
Keep the cooler clean. Clean the oil cooler regularly with an air hose or pressure washer to clear dirt and debris, which reduce cooling capacity. Beesley also recommends keeping fan belts tight and cleaning the air intakes on the hood or tailgate to keep air flowing through the engine compartment. Use a thermometer to check for overheating of the hydraulic fluid in the reservoir. Although hydrostatic systems may run as hot as 100 degrees F above ambient temperature, cooler is almost always better.
Follow proper warm-up procedures. To prevent cavitation in cold weather, Navarro says, let the hydrostatics warm up sufficiently before operating at full power.
Greg Northcutt is a freelance writer based in Port Orchard, Wash. He can be reached by e-mailing this publication at editor@onsiteinstaller.com.
