Keep Hydrostatic Equipment Productive

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 (, 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."

Your choice of filter is determined by several criterion. "When selecting filters, it's important to consider how clean the fluid needs to be (ISO cleanliness requirements), how well the filter cleans (Beta rating) and the capacity of the filter," says Zumbusch. "Vehicle manufacturers work closely with their hydraulic component manufacturers to determine the proper filter selection for a particular application."

The goal is to capture particle sizes that could damage the system. "The particle-blocking size and efficiency of installed filters must be matched to the target cleanliness level," says Casey. "Generally speaking, the finer the filtration, the better the achieved cleanliness and the longer the transmission service life - all other things being equal. Ten micron, Beta 200 would be the minimum level of filtration for most hydrostatic applications, down to 6 micron, Beta 200."

Wickenhauser adds, "A charge filter can provide higher performance than a suction filter. Charge filter ratings of 4 micron or better are best to maintain ISO levels of 18-15."

In addition to fluid cleanliness, radiators also need attention. "The radiator and cooling packages need to be cleaned on a periodic basis, and more frequently when operating in very dusty conditions," says Wickenhauser. Filters also need to be replaced immediately if the bypass warning system is on, and the machine should be shut down if the temperature warning comes on.

Cavitation is the enemy of pumps
"Cavitation happens when a pump or a motor tries to compress air in the hydraulic system fluid," says Cahalan. "When that air is compressed rapidly, it actually explodes and removes little bits of metal from the pump or motor internal parts (creating contamination)."

System leaks are one source of cavitation. "Leaks letting oil out of the system also have the possibility of allowing air into the system," says Cahalan. Plus, as the oil level decreases, the oil can foam (air in the oil).

Charge pump failure is another source. "The charge pump makes up internal leakage, and therefore keeps the transmission loop full of fluid. If the charge pump is damaged and is unable to keep the loop charged with fluid, the transmission pump will cavitate," says Casey. This can rapidly result in catastrophic failure.

"Prevention of charge pump and fluid failure comes back to effective contamination control, and maintaining operating temperature - and therefore fluid viscosity within the limits permissible by the weight of the fluid used in the system," adds Casey.

Using an oil with too high a viscosity can result in cavitation. "In cold climates where the machine experiences frequent cold starts, replace the hydraulic oil with a lower viscosity oil that is within the temperature range recommended in the service manual," Wickenhauser advises.

Given there are no leaks in the system, eliminating cavitation comes down a few simple checks. "Cavitation can be prevented by making sure that the suction line allows oil to move freely, ensuring the return oil is cooled, using the proper oil for the hydrostatic transmission and increasing the charge pump output, to name just a few," says Gidaspow.

The best way to prevent cavitation damage is to ensure that the transmission has a charge pressure gauge or transducer installed so the operator can monitor this aspect of operation. "The operator must be trained to stop operating the transmission if charge pressure falls below a set value," says Casey. "And in some applications, a low charge pressure alarm and/or shutdown is advisable."

Inspect hoses and pipes
"A weak link in hydrostatic transmissions is the connecting hoses or pipes between the transmission pump and motor," says Casey. "A burst hose or pipe in service can result in the destruction of the pump and/or motor through cavitation."

Regularly inspect the high-pressure hoses or pipes between the pump and motor, and replace suspect lines before they fail.

Even small leaks can be catastrophic. "Cavitation can result from air entering the system (aeration) through loose suction fittings or damaged hoses," says Zumbusch.

But take care when replacing these components. The hoses and connectors are part of an engineered system. "Vehicle manufacturers usually work with the component suppliers to properly size not only the pumps and motors, but also the hoses and connectors to ensure proper, free-flowing connections," Zumbusch notes.

"Replacing hoses with the same lengths and diameters can help make sure the system works properly," he continues. "Avoiding pinched lines or other damage to connectors and hoses will also make sure proper flow and pressure drops are maintained."

Fluid selection tips
Don't overlook the fluid itself as a common cause of failure. "If the fluid is compromised through contamination (hard or soft particles, water or air), oxidation (thermal damage) or low viscosity (high-temperature operation), then the fluid can't carry out one of its critical functions - keeping the transmission adequately lubricated," says Casey.

Also make sure you use the right fluid. "The choice of fluid has a large impact on hydrostatic transmission life," says Wickenhauser. "I recommend the highest viscosity level fluid in the service manual for the ambient temperature range the machine is operating in."

Zumbusch agrees, noting, "While a hydrostatic transmission can run with a number of different fluid types, proper fluid selection that takes into account viscosity and additive packages will deliver longer life, cooler temperatures and better system efficiency. For many systems, we recommend a premium grade anti-wear (AW) hydraulic fluid."

"It is essential that the fluid used in any high-performance hydrostatic transmission has a robust anti-wear additive package," Casey states. "This usually means (but not always) a high zinc content (900 ppm or higher)."

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


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."

As components wear, higher levels of debris will build up in the hydraulic oil. "This debris can cause secondary damage to the HST system," says Gidaspow. "So it is very possible to get further damage when operating a hydrostatic transmission that is worn excessively or damaged."

This circulating debris intensifies the wear process. "These particles can be more dangerous to a system since they are 'work hardened' to a greater hardness than the surfaces from which they came, and are very aggressive in causing further wear," says Zumbusch.

Any hydraulic component performing out of specification also causes heat, which in turn causes exponential decay of all hydraulic system components. "Heat breaks down the oil quicker and prevents it from doing all of its jobs in the hydraulic system," says Cahalan. "Heat damages seals and hoses, causing leaks which can allow contamination to enter the system. Operating a vehicle in this manner is just begging for a major hydraulic system failure and downtime."

A few easy checks can determine if your hydrostatic system is in good working order. "One quick way to check a hydrostatic system (both motor and pump) is to time the wheel or track revolutions," says Cahalan. "Most manufacturers provide revolutions per minute specifications. However, the only way to determine which component - the pump or the motor - is not performing up to specification is to check each with pressure gauges and flow meters."