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First, you must understand what fluid cleanliness means. "By cleanliness, manufacturers are referring to solid particles (dirt), as well as other forms of contamination such as water or chemical contamination," says Zumbusch. "Most manufacturers have guidelines for fluid particle cleanliness. Eaton recommends systems run at a cleanliness level (ISO 4406) of 20/18/13 or better."

Cleanliness becomes increasingly important as tolerances continue to shrink. "Over the years, the power density and pressures in the transmissions have been increasing," says Tom Wickenhauser, OEM Solutions Group, Caterpillar.

This is a direct response to demands from contractors and improved manufacturing processes. "Contractors want faster machines and more power at the wheels or track," says Cahalan. "To get more speed you need more flow, and to get more power you need higher pressures. For a hydrostatic transmission to produce higher flows and pressures, you need tighter tolerances."

Brendan Casey, author of Insider Secrets to Hydraulics (www.InsiderSecretsToHydraulics.com), adds, "These days, when you talk about a hydrostatic transmission, this means a high-performance piston pump and motor. [These components] have close internal tolerances, operating together in a closed circuit at pressures as high as 6,500 psi."

High-pressure systems are more sensitive to contamination and require stringent cleanliness guidelines. "We recommend an ISO level of 18-15 or better in most Caterpillar hydrostatic transmissions," says Wickenhauser.