Zinc-free hydraulic oils – the highperformance, environmentally friendly option
Keywords: Hydraulics, Oils, Environment
Steve Travell, Technical Services Adviser for Castrol Industrial, examinesthe compelling reasons why hydraulic engineers should specify zinc-freehydraulic oils.
Despite zinc-free hydraulic oils being generally available for a number ofyears and despite the fact that their environmental and performance advantagesshould command serious attention – acceptance levels are still low.
Can it be that for many engineers the extra cost of zinc-free oils stilloutweighs the benefits, especially when price is often the only considerationwhen specifying conventional hydraulic oils?
But times are changing. Even if the environmental lobby is not exactlywinning the day, "added-value" benefits such as performance and reliability areincreasingly being acknowledged. So what are these benefits and why shouldengineers change?
Hydraulic power is used in practically every industry from machine tools toautomotive, from aerospace to baking machinery. The reason for this widespreaduse is that a fluid is one of the most versatile means of transmitting power andmodifying motion. A fluid is infinitely flexible, yet is as unyielding as steelwhen under pressure. It can readily change its shape; it can be divided intoparts to do work in different locations; it can move rapidly in one applicationand slowly in another; and it can transmit a force in any or all directions. Noother medium combines the same degree of positiveness, accuracy and flexibilityof control with the ability to transmit maximum power in a minimum of bulk andweight.
However, when a fluid flows in a hydraulic circuit the resulting frictionproduces heat, which means that some of the energy being transferred is lost inthe form of heat energy. Although friction can never be eliminated entirely, itcan be controlled to some extent using fluid additives.
Additives are also needed to provide an efficient hydraulic function whenpumping fluids. Since the different types of hydraulic pumps (e.g. gear driven,centrifugal/radial vane and piston) require different anti-wear and extremepressure agents to make them work efficiently and reduce wear, oils need to beformulated to operate in all pump types. For example, in geared pumps, extremepressure is important whilst in radial pump cleanliness and anti-wear propertiesare essential. Hydraulic fluids must also lubricate pumps and motors withoutimpairing the function of other components in the hydraulic system.
So the ideal hydraulic fluid must offer a comprehensive list of performancebenefits including rust inhibition; demulsibility; filterability; compatibility;hydrolytic stability; corrosion control; oxidation stability; thermal stabilityand anti-wear.
A comprehensive additive system is required to achieve these properties,starting with a good quality solvent refined base stock, which will itself havea partial resistance to oxidation and sludging. However, it will need to befortified with other anti-oxidants and additives. Typical hydraulic fluidadditives include: EP agents; anti-wear; corrosion inhibitors/metaldeactivators; rust inhibitors; anti-oxidants; anti-foam; demulsifiers; VIimprovers; pour point depressants; biocides; and seal degrading retardants.
EP agents are used to prevent metal-to- metal contact between the matingparts under boundary lubrication conditions. Anti-wear agents are generallydefined as additives that prevent metal-to-metal contact under lower operatingpressures and temperatures. Corrosion inhibitors react chemically to form aprotective film. Many of these inhibitors are also metal deactivators that workby rendering the metal surface passive to additional chemical attack. Rustinhibitors protect iron and steel from attack by acidic contaminants and water.
Anti-oxidants reduce oil oxidation by interfering with the complex chainreaction that causes oil to break down, with oxidation promoted by free radicalmolecules, being dealt with by special filtration machines that are being testedto remove these molecules in order to extend the life of zinc-free hydraulicoils further.
Anti-foam agents reduce the level of foaming by altering the surface tensioncharacteristics of oil. Demulsifiers also alter the surface tensioncharacteristics so that water will separate from the fluid.
VI Improvers reduce the rate at which the viscosity of a fluid varies withchanges in temperature. Pour Point Depressants reduce the size of wax crystalsformed in the oil at low temperatures and thus increases the fluidity of oil.
Dieseling is a phenomenon caused by air ingress and cavitation, specificallyin injection moulding where the extremes of pressure cause variations thatcontract gases trapped within the oil during the pumping process, these thenexpand rapidly when these pressure obstructions or cavitations are clearedcausing mini explosions that locally burn the oil. Modern non-zinc additivesdramatically aid the reduction in dieseling reactions with consequentcleanliness and oil life enhancement.
The three basic types of hydraulic fluid additive systems are high zinc (0.07or greater wt per cent zinc in the fluid); low zinc (less than 0.07 per cent wtzinc in the fluid); and "Ashless" (no zinc or other metals present in thefluid), which commonly uses sulphur-phosphorus as an additive system.
Most conventional hydraulic oils use a high-zinc additive package, which hasprovided a satisfactory performance for many years in anti-wear applications.However, they are not always acceptable in some of the current higherperformance pump applications and users should check with the manufacturer.
Low-zinc additives do provide a compromise between the anti-wear requirementsof vane pumps and the anti- corrosion requirements of piston pumps.
Ashless technology is used in hydraulic oils designed to meet anti-wear,anti-rust and anti-oxidation applications and generally provide very goodperformance. The additives used in ashless fluids are usually tricresolphosphates (TCPs) or other sulphur phosphorus amine combinations. Whilst thepresent ashless technology does not meet all high-pressure vane pump testrequirements, it is expected that the use of ashless formulations will increaseas the technology improves.
Whilst there are specific bio-degradable and vegetable hydraulic oilsavailable that will reduce the impact on the environment, they are currentlymore expensive; so industry still widely uses the conventional mineral- basedproducts. This despite the fact that the elimination of heavy metals in oilssuch as Castrol's Hyspin zero zinc (ZZ) range does greatly reduce environmentalimpact through accidental spillage, misting from "air lubricators", disposaltreatments and burn off from swarf smelting/furnaces.
Another important advantage of zinc-free hydraulic oils is their ability tooperate in fine clearance spool valves where cleanliness is critical.
Hydrolytic stability is a problem faced by all systems made from metal asthey will inevitably suffer corrosion if exposed to water, even if this waterexists as only droplets within oil. Whilst all hydraulic oils may suffer wateringress through contact with wet air, coolant ingress or process watercontamination, conventional zinc additive oils do tend to suffer more, due tothe reaction of the zinc additive and water forming deposits and gels. This is amajor concern for filtration, where filter blockages can either cause oilstarvation leading to failure of the system or, if oil vents through a reliefvalve, can deliver unfiltered oil containing contaminants to critical areas.Radial pumps can also be affected as the vanes of the pump are reliant onunimpeded movement and gelling can jam them – reducing pump efficiency andincreasing wear.
In summary, currently zinc-free mineral hydraulic oil is a finely tunedpackage that can be used to significantly reduce the actual costs. It employsthe latest technology to make it safer and more economical and is extensivelytested to ensure high performance in areas such as oxidation stability,demulsibility, wear, corrosion inhibition and hydrolytic stability, with theadvantages greatly outweighing cost differentials of older technology fluids.
For further information, contact: Castrol Customer Service, Castrol (UK) Ltd,Wakefield House, Pipers Way, Swindon, SN3 IRE, UK. E-mail: thrussc@castrol.com; Web site: www.castroladvantage.com
