I recently conducted failure analysis and a reliability audit on a 300-kilowatt (400-hp) hydrostatic transmission. This hydraulic system was running a synthetic-ester, biodegradable hydraulic fluid, and the original set of pumps failed inside a year.
The system was built and installed by a reputable firm. From a hydraulic-engineering perspective, the circuit was adequately designed and the system well built. But from a maintenance and reliability perspective, there was plenty of room for improvement.
When I arrived on site, one of the first things I noticed was the hydraulic oil appeared dark in the sight glass, whereas the unused oil was the color of light honey. A subsequent check of the oil-analysis reports indicated viscosity was increasing. Suspecting oxidative failure of the oil, I requested acid number and water content by Karl Fischer. These weren’t included in the original test slate.
In the meantime, I turned my attention to analysis of the failed pumps. It was obvious the oil had been polymerizing for some time before the pumps failed. Internal components were heavily coated with a gum-like sludge (Fig. 1). These deposits can block lubrication passages, reduce heat transfer, and cause valve stiction.
The valve plate and cylinder barrel of both pumps exhibited damage from cavitation erosion—another possible side effect of oxidative failure. The oxidation process typically diminishes foaming resistance and air-release properties of the oil, which in turn causes damage through increased air entrainment and gaseous cavitation.
Acid number came back at 9.5 mg KOH/g and the Karl Fischer at 2,200 ppm. For this particular oil, an acid number of between 4 and 5 mg KOH/g is the trigger for an oil change. And although water contamination was much higher than desirable for this application, hydrolysis was ruled out as a significant factor in the formation of the sludge deposits based on the oil manufacturer’s experience.
All the evidence pointed to the fact that this $12/liter hydraulic fluid had suffered oxidative failure well inside 12 months. So with $20,000 worth of hydraulic fluid and $50,000 worth of pumps ruined in short order, my client was understandably wondering what had gone wrong.
Suspecting high-temperature operation as the cause of the accelerated oxidation, I turned my attention to the historical operating parameters of the hydraulic system. As built, the system had a temperature sender installed in the hydraulic reservoir with alarm levels and shutdowns programmed into the system’s PLC. Tests showed this instrumentation to be functioning correctly. But the operators advised the system never had a temperature alarm or shutdown on over temperature.
This didn’t come as a total surprise to me, because the temperature sender was in the wrong location. Let me explain. As many readers would be aware, a hydrostatic transmission consists of a variable-displacement pump and a fixed-displacement or variable-displacement motor operating together in a closed circuit. In a closed circuit, fluid from the motor outlet flows directly to the pump inlet without returning to the reservoir.
Because the pump and motor leak internally, which allows fluid to escape from the loop and drain back to the reservoir, a fixed-displacement pump called a “charge pump” is used to ensure that the loop remains full of fluid during normal operation.
In practice, the charge pump not only keeps the loop full of fluid; it pressurizes the loop to between 110 psi and 360 psi, depending on the transmission manufacturer. A simple charge pressure circuit comprises the charge pump, a relief valve, and two check valves through which the charge pump can replenish the transmission loop. Once the loop is charged to the pressure setting of the relief valve, the flow from the charge pump passes over the relief valve and back to the reservoir.
Apart from losses through internal leakage, which are made up by the charge pump, the same fluid circulates continuously between transmission pump and motor. This means if the transmission is heavily loaded, the fluid circulating in this loop can overheat. To ensure the fluid in the transmission loop is positively exchanged with that in the reservoir and subsequently cooled, a flushing or hot-oil purge valve is installed in the circuit.
When the hydrostatic transmission is in neutral, the flushing valve has no function and the charge relief valve, which is usually located in the transmission pump, maintains charge pressure. When the transmission is operated in either forward or reverse, the flushing valve operates so that the “purge” relief valve incorporated in the flushing valve maintains charge pressure in the low-pressure side of the loop. This purge relief valve is set around 30 psi lower than the charge pump relief valve (Fig. 2).
The effect of this is that cool, conditioned fluid drawn from the reservoir by the charge pump charges the low-pressure side of the loop through a check valve located close to the transmission pump inlet. The volume of hot fluid leaving the motor outlet that is not required to maintain charge pressure in the low-pressure side of the loop vents across the flushing valve purge relief and back to the reservoir.
The important point here and what’s relevant to this case is if the flushing valve malfunctions or is not configured correctly, there is no positive exchange of the fluid in the loop with that in the reservoir. This means the transmission loop can be operating in over-temperature conditions, while the fluid in the reservoir remains relatively cool.
As you can see, in a hydrostatic transmission, the correct location for the temperature sender on which over-temperature alarms and/or shutdowns are based, is in the transmission loop—not the reservoir.
So was this case a failure of maintenance or a failure of design? It could be argued that it’s both. A failure of design in that had the temperature sender been correctly located in the transmission loop, the failure wouldn’t have occurred. A failure in maintenance in that had the early warning signs of oxidative failure of the oil been picked up through observation and better oil analysis test slate selection, the failure could still have been prevented.
ABOUT THE AUTHOR
Brendan Casey is the founder of HydraulicSupermarket.com and the author of Insider Secrets to Hydraulics, Preventing Hydraulic Failures, Hydraulics Made Easy, Advanced Hydraulic Control and The Definitive Guide to Hydraulic Troubleshooting. A fluid power specialist with an MBA, he has more than 20 years experience in the design, maintenance and repair of mobile and industrial hydraulic equipment. Visit his website: www.HydraulicSupermarket.com