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Certain applications require specialized fluids, such as fire resistant or environmentally-friendly fluids. "In these cases, fluid selection guides are available to help make the proper choice," says Zumbusch. "The Eaton-Vickers 35Q 25-vane pump test (ASTM D6973) is one of the most widely accepted methods to evaluate the performance of petroleum-based AW hydraulic fluids."

Many OEMs recommend specially formulated hydraulic oils. "Using oils that do not conform to OEM specifications can shorten the life of HST components," says Gidaspow.

Don't be tempted to use a single type of oil to service all of the hydrostatic transmissions in your fleet without carefully checking the

compatibility.

For example, Volvo uses hydrostatic transmissions in many of its products. "Even though they all use hydrostatic transmissions, the fluid recommendations as to the type, change interval and filter requirements are made with regards to the complete hydrostatic system and the overall life and performance of the machine," says Cahalan. "One machine's requirements may not be good enough for another's."

Watch for out of spec components
"When an operator begins to feel a decline in output drive power, it is a good indicator of an excessively worn pump or motor," says Wickenhauser. "At that point, I recommend to remove and inspect the filtration system on the machine. Look for iron and brass material in the filter media. The sooner a worn pump or motor is found, the better chance there is of not damaging other components."