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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.”