The competitive nature of the hydraulics market has created a trend of increasing power outputs along with smaller fluid reservoirs. This produces consequences for manufacturing plants. The operating temperatures of hydraulic fluids in these systems are significantly higher, leading to greater risk of oxidation and thermal degradation of the additives in the fluid. The result of that degradation is varnish, and it can cause unplanned downtime, higher maintenance costs and reduced profitability.
Over time, varnish buildup increases the friction inside the valves, especially those with fine tolerances such as servo and proportional types, where this phenomenon can be especially troublesome. The effect of hydraulic-system valves sticking and operating erratically leads to impaired responsiveness and reduced oil flow, which reduces efficiency and increases maintenance costs.
The lowdown on varnish
Oxidation, thermal decomposition and the natural process of additive consumption is what causes fluid to degrade as it ages. Additives — the performance-enhancing chemicals — are consumed during the fluid lifespan. The concentration of degradation byproducts increases as oil ages, eventually forming varnish.
Because varnish is a polar material, it’s attracted to metal surfaces such as servo valves. Varnish starts as a sticky, soft residue. The sticky nature of this material allows it to attract wear debris, resulting in formation of a sandpaper-like surface. Eventually, the soft, sticky material changes to a hard lacquer that can be difficult to remove (Figure 1).
Be assured that varnish harms equipment. Oxidized oil generally doesn’t lubricate very well. The result can be reduced oil flow, plugged filters, plugged valves, greater friction, poor heat transfer and elevated operating temperature. Because varnish acts as an insulator, it compromises the system’s cooling capacity.
For example, in high-performance vane pumps, varnish adhering to the vanes can cause a vane to jam in the rotor slot. The consequences are increased noise, decreased volumetric and mechanical efficiency, increased energy consumption, side plate scuffing, rotary seal damage and possible bearing damage.
If varnish is present in the sump, you can count on it being present in servo valves and other places in the system.
Electrostatic filtration systems can remove contaminants, but they can’t address the causes of varnish formation. They also tend to be expensive, and can be susceptible to water contamination.
So, some end users routinely change or clean servo valves in hydraulic equipment to keep their systems running. One new valve can cost $3,000, and the cost to clean and refurbish a valve can be about $2,000. Don’t forget to add the labor and lost production costs.
Neither approach is particularly economical. The ideal solution is to use hydraulic fluids that don’t deposit varnish on metallic surfaces. The additive packages in such fluids react with varnish precursors, thus minimizing the formation of tenacious, hard films on system hardware (Figure 2).
Laboratory testing confirms the keep-clean feature of fluids using this technology. In industry-accepted pump tests, most additive technologies exhibit varnish formation within 500 hours of operation. In the same pump tests, fluids containing varnish-reducing additive technology showed no evidence of varnish formation after 1,000 hours of operation.
Using fluid that contains varnish-reducing additives results in a clean sump.
Hydraulic fluids are being subjected to increasingly tough operating conditions. Demands that raise production while reducing oil volume emphasize the need for high-quality hydraulic fluids. A varnish-reducing additive chemistry is the perfect partner for hydraulic fluids used in those harsher operating conditions.
Rob Profilet is Commerical Manager for Industrial Hydraulic and Gear Oil Additives for The Lubrizol Corp., Wickliffe, Ohio. Contact him at [email protected] and (440) 943-4200.