Contamination and cavitation lead to premature hydrostatic transmission failure. Addressing these problems can slash repair costs and downtime.
"When properly maintained, hydrostatic transmissions often outlive the other components in the system, or the entire machine itself, and may never require replacement," says Steve Zumbusch, manager, application and commercial engineering, Eaton Hydraulics Operation.
But you can't ignore proper service. "Like any transmission parts, hydrostatic transmission parts will eventually show signs of wear. Preventive maintenance can help make sure that catastrophic system failures do not occur," says Michael Gidaspow, product marketing manager, wheel loaders, Komatsu America Corp.
Oil analysis is an important part of this program. "When failures occur, it's typically an issue of contamination in the fluid or temperature affecting the fluid," says Zumbusch. "Studies have shown that up to 80% of pump failures can be attributed to contamination."
Cleanliness is to . . .
When servicing a hydrostatic system observe cleanliness at all times. "Any extra time cleaning will pay dividends by adding life to the components, as well as the overall life of the machine," says Dan Cahalan, technical support group, Volvo Construction Equipment.
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.
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."