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The transmission service life depends on these cleanliness levels. "Typical minimum cleanliness level is ISO 4406 18/16/13 down to 16/14/11," Casey states.

Today, the tolerances between parts are very close. "Many of the sliding surfaces may have as little as 5-micron gaps during operation," Wickenhauser points out. "Larger gaps cause more leakage, heat and sluggish operation."

Contaminant particles that get in between the gaps can cause significant wear. "Particles the same size as the internal clearance cause heavy friction and wear through a process known as three-body abrasion," says Casey.

As tolerances decrease, the size of the particles causing the abrasion also decrease. This requires finer filtration to control particle distributions in the lower micron range. "The most dangerous particles to a hydrostatic transmission, in the long term, are smaller than the components' internal clearances," says Casey.

This makes filter selection and maintenance the most important steps to maximize life. "Filters with the wrong micron rating allow excess contamination into the system," says Gidaspow. "Always install the filters recommended by the manufacturer."