Hydraulic equipment users routinely change or clean servo valves. But is that practice really necessary? What are the cost implications? Are there alternatives?
Often, valves are changed because they are stuck in one position or do not respond properly to commands. When the system no longer operates effectively, the only choice is to stop the machine and replace the valve. Unfortunately, changing valves is the norm in some operations, and this expense is rolled into the overall cost.
An electrostatic filtration system is one way to remove contaminants that affect hydraulic system efficiency. Electrostatic filters require physical space in your plant and may not be the optimum solution, depending on the condition of your fluid.
There is another solution: New additive technology for hydraulic oil can reduce the need to frequently clean or change the servo valves. Such technology works to prevent varnish from depositing on critical work surfaces. The result is longer component life and a hydraulic system that stays clean. That translates into improved equipment efficiency and reduced cost.
A major use for industrial lubricants is in hydraulic equipment, and varnish formation is a major issue for many of these applications.
This shows a varnish-laden sump. If varnish is present in the sump, it is also present in other places in the system.
Varnish Defined
As oil ages, it degrades through oxidation and thermal decomposition. The additives, which are performance-enhancing chemicals, are consumed during the life of the fluid. The decomposition by-products of aging increase over the life of the oil, ultimately forming varnish.
Because varnish is polar, it is attracted to metal surfaces, including servo valves. It starts as a sticky, soft residue and attracts wear debris, forming a sandpaper-like surface. In time, it ends up as a tenacious, hard lacquer.
Why It's Bad
Oil that has oxidized generally does not lubricate well. It can reduce oil flow, plug filters, cause valves (especially proportional and servo types) to stick, increase friction, inhibit heat transfer and elevate operating temperature. Because varnish acts as an insulator, cooling capacity can be diminished. In addition, oxidation shortens component life, affecting valves, filters, pumps, bearings and seals. The result is diminished hydraulic system performance.
For example, when varnish adheres to vanes in high-performance vane pumps, the vanes can stick in the rotor slot. This can result in increased noise, decreased volumetric and mechanical efficiency with an equivalent increase in energy consumption, side plate scuffing, rotary seal damage and possibly bearing damage. The phenomenon is most obvious at low pressures when there is little centrifugal force and low fluid pressure.
In the case of piston pumps, varnish can increase piston land friction against the wear plate, leading to leakage and possible seizure. It also is well known that sticking valves, such as the regulation valves on piston pumps, can cause unscheduled stoppages during equipment use. Finally, the varnish that forms can block filters, leading to high use of filter cartridges and increased maintenance costs.
Having to change or clean hydraulic system servo valves and risk system failure are some of the results of varnish. A new servo valve can cost $3,000, and approximately $2,000 to clean and refurbish. But the expense doesn't end there. Don't forget the associated labor and shutdown costs, both of which impact your bottom line. And if the system fails, your income declines as well.
Let's put that into real-world terms in the following example: A large plastic injection molding company produces between 20 million and 30 million parts per month with more than 200 machines that range from 33 to 770 tons. The hydraulic fluid reservoirs in these machines range from 80 to 250 gallons. The equipment operates 24 hours a day, five days a week. Excluding lost production, the estimate of the yearly cost due to varnish is approximately $135,000.
In addition to potentially triggering premature replacement of control valves, if left uncorrected, varnish can reduce filter load-carrying capacity and plug supplemental cooling system orifices.
Solution to Varnish
In the past, equipment users replaced servo valves or cleaned them as needed to keep their systems operating. Electrostatic filters and precipitators have been used successfully but have some shortfalls, including their cost and the loss of productive floor space in the plant.
The ideal solution is to use hydraulic fluid that does not allow varnish to deposit on metal surfaces while it provides important wear and corrosion prevention and water separation capabilities. Because hydraulic formulations are carefully balanced to meet OEM requirements, adding a new varnish-mitigating feature to the fluid's performance profile requires a unique solution.
Fluids are now available that incorporate additive chemistry that reacts with the precursors to varnish, minimizing the formation of resinous films on system hardware. This technology recently achieved Denison HF-0-approval.
Laboratory testing demonstrates the clean feature offered by these new fluids. In industry-accepted pump tests, many widely used fluids show varnish formation within 500 hours of beginning operation. Compare that to the results found in the new additive technology solution to the age-old varnish problem: Even after 1,000 hours of use, there is no evidence of varnish formation.
Industrial hydraulic systems typically operate at approximately 140°F, although temperature spikes up to 180°F are common. High-temperature applications that place thermal stresses on the oil - such as plastic injection molding machines, glass transfer systems, heavy presses and mobile equipment - are ideal candidates for this new technology. It also is a good choice if you want to improve the productivity of your equipment and extend the life of oil, equipment and components such as valves, filters and pumps.
Today's hydraulic fluids are subjected to increasingly tough operating conditions. Demands to increase production at the same time that oil volume is decreasing emphasize the importance of using high-quality hydraulic fluids. Increased operating temperatures have resulted in today's hydraulic systems developing varnish deposits over time that can lead to problems. The availability of new additive chemistries goes hand-in-hand with today's harsher operating conditions.
About the Author
Rob Profilet is the commercial manager for industrial hydraulic and gear oils with the The Lubrizol Corporation. For more information, visit www.lubrizol.com.
Challenges for Hydraulic Oils
Original equipment manufacturers report that the most frequent problems with higher-performance hydraulic systems are:
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Systems are getting smaller while the flow rates in the reservoir have decreased.
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The reservoir size and shape are not optimum for fluid life.
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Oil flow rates are high compared to oil volumes.
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Hydraulic systems are designed to have higher power densities.
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Oil temperatures are higher.
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Oil pressures have increased in general.
