Many variables impact the life of hydrostatic transmissions. At the top of the list is oil cleanliness. “The most common failures are due to fluid cleanliness and improper maintenance,” says Michael Kanitz, marketing product manager for heavy--duty hydrostatics, Eaton Hydraulics.

“Life expectancy is also heavily dependent on the duty cycle of the machine,” says Brad Nicol, Applications Engineering, Parker Denison. “High operating speeds, high pressures, high cam angles and high shock levels will all shorten the life of a hydrostatic transmission.”

Keep It Clean

Ensuring the proper level of oil cleanliness goes a long way toward achieving maximum life. “If viscosities and cleanliness levels are maintained in the proper range, you could expect more than 5,000 hours out of a hydrostatic transmission,” says Nicol. “Even operating at the max conditions, our transmissions should last longer than 5,000 hours.”

But tight tolerances demand clean fluids. “One of the main advantages of piston pumps and motors is their high volumetric efficiency,” says Nicol. “In order to obtain these efficiencies, clearances must be very tight, so very small particles can do large amounts of damage. Remember that most damage in high--pressure hydraulic systems is done by particles too small to see, so visually inspecting the oil is not good enough.”

It doesn’t take a lot of contamination to wreak havoc. “A small number of particles can do a lot of damage in a transmission,” notes Nicol. “For example, if the flow rate through a transmission is 100 gpm, and there is 5 gal. of trapped volume in the loop, the contaminant goes through the pump and motor 20 times a minute.”

Oil sampling provides a method to ensure the system conforms to manufacturer cleanliness specs.

The cleanliness standard varies by manufacturer. “For hydrostatic transmissions used in mobile applications, our typical recommendation is an ISO 20/18/13,” says Eaton’s Kanitz.

“We recommend an ISO cleanliness of 17/15/13 or better for our piston products,” says Nicol at Denison.

But everyone agrees the cleaner you can keep your oil, the better. “Cleaner oil typically leads to longer service life for a component,” explains Nicol.

Rick Sporrer, manager for technical services, Sauer--Danfoss, reports that, according to a recent study, 19% of all incident reports received by a particular large original equipment manufacturer were caused by degradation of the oil being used in the hydraulic system. “It had to do with cleanliness of the oil,” he states.

Fluid quality also plays an important role in transmission life. “There are certain fluids that Eaton does not recommend for hydrostatic transmissions,” says Kanitz. “With some universal tractor fluids, without proper additive packages and under the wrong set of operating conditions, we have found instances where the fluid can attack the yellow metals in the pumps and motors — specifically the bronze, which is contained in the shoes, cylinder bores and bearing faces of the cylinder block.”

A good quality hydraulic fluid is a better choice. “Petroleum--based hydraulic fluids with rust and oxidation inhibitors typically work the best,” says Nicol.

You also need to be aware of the risk of cross contamination. Sporrer reports one company was experiencing failures due to contamination from attachments sent to a repair shop. “The repair shop used a low--quality grade of oil,” he explains. “This low--quality oil would then go throughout the hydraulic system and ultimately it would react with some of the yellow metals in the hydrostatic system.”

Owners Play Important Role

Sporrer cites three factors that every fleet owner can control to maximize hydrostatic transmission life: fluid maintenance, hydraulic circuit preventive maintenance and service procedures.

Fluid maintenance includes making sure the hydraulic reservoir is filled using quality refill fluids that meet cleanliness standards. Prior to adding fluid to the reservoir, always clean the cap with a lint--free rag to minimize contamination entering the system.