Steve Travell, a technical services adviser for Castrol Industrial, examines the compelling reasons why hydraulic engineers should specify zinc-free hydraulic oils. Despite zinc-free hydraulic oils being generally available for a number of years and despite the fact that their environmental and performance advantages should command serious attention - acceptance levels are still low. Can it be that for many engineers the extra cost of zinc-free oils still outweighs the benefits, especially when price is often the only consideration when specifying conventional hydraulic oils?
But times are changing.
And even if the environmental lobby isn't exactly winning the day, 'added-value' benefits such as performance and reliability are increasingly being acknowledged.
So what are these benefits and why should engineers change?
Hydraulic power is used in practically every industry from machine tools to automotive, from aerospace to baking machinery.
The reason for this widespread use is that a fluid is one of the most versatile means of transmitting power and modifying motion.
A fluid is infinitely flexible, yet is as unyielding as steel when under pressure.
It can readily change its shape; it can be divided into parts to do work in different locations; it can move rapidly in one application and slowly in another; and it can transmit a force in any or all directions.
No other medium combines the same degree of positiveness, accuracy and flexibility of control with the ability to transmit maximum power in a minimum of bulk and weight.
However, when a fluid flows in a hydraulic circuit the resulting friction produces heat, which means that some of the energy being transferred is lost in the form of heat energy.
Although friction can never be eliminated entirely, it can be controlled to some extent using fluid additives.
Additives are also needed to provide an efficient hydraulic function when pumping fluids.
And because the different types of hydraulic pumps (e.g gear driven, centrifugal/radial vane and piston) require different anti-wear and extreme pressure agents to make them work efficiently and reduce wear, oils need to be formulated to operate in all pump types.
For example, in geared pumps extreme pressure is important whilst in radial pump cleanliness and anti-wear properties are essential.
Hydraulic fluids must also lubricate pumps and motors without impairing the function of other components in the hydraulic system.
So the ideal hydraulic fluid must offer a comprehensive list of performance benefits including rust inhibition; demulsibility; filterability; compatibility; hydrolytic stability; corrosion control; oxidation stability; thermal stability and anti-wear.
A comprehensive additive system is required to achieve these properties, starting with a good quality solvent refined base stock, which will itself have a partial resistance to oxidation and sludging.
However, it will need to be fortified with other anti-oxidants and additives.
Typical hydraulic fluid additives include: EP agents; anti-wear; corrosion inhibitors/metal deactivators; rust inhibitors; anti-oxidants; anti-foam; demulsifiers; VI improvers; pour point depressants; biocides; and seal degrading retardants.
EP agents are used to prevent metal-to-metal contact between mating parts under boundary lubrication conditions.
Anti-wear agents are generally defined as additives that prevent metal-to-metal contact under lower operating pressures and temperatures.
Corrosion inhibitors react chemically to form a protective film.
Many of these inhibitors are also metal deactivators that work by rendering the metal surface passive to additional chemical attack.
Rust inhibitors protect iron and steel from attack by acidic contaminants and water.
Anti-oxidants reduce oil oxidation by interfering with the complex chain reaction that causes oil to break down, with oxidation promoted by free radical molecules, being dealt with by special filtration machines that are being tested to remove these molecules in order to extend the life of zinc free hydraulic oils further.
Anti-foam agents reduce the level of foaming by altering the surface tension characteristics of oil.
Demulsifiers also alter the surface tension characteristics so that water will separate from the fluid.
VI Improvers reduce the rate at which the viscosity of a fluid varies with changes in temperature.
Pour Point Depressants reduce the size of wax crystals formed in the oil at low temperatures and thus increases the fluidity of oil.
Anti-Dieseling is a phenomenon caused by air ingress and cavitation, specifically in injection moulding where the extremes of pressure cause variations that contract gasses trapped within the oil during the pumping process, these then expand rapidly when these pressure obstructions/cavitation is cleared causing mini explosions that locally burn the oil.
Modern non-zinc additives dramatically aid the reduction in Dieseling reactions with consequent cleanliness and oil life enhancement.
The three basic types of hydraulic fluid additive systems are High Zinc (0.07 or greater wt % zinc in the fluid); Low Zinc (less than 0.07% wt zinc in the fluid); and 'Ashless' (no zinc or other metals present in the fluid), which commonly uses sulphur-phosphorus as an additive system.
Most conventional hydraulic oils use a high-zinc additive package, which has provided a satisfactory performance for many years in anti-wear applications.
However, they are not always acceptable in some of today's higher performance pump applications and users should check with the manufacturer.
Low-zinc additives do provide a compromise between the anti-wear requirements of 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 good performance.
The additives used in ashless fluids are usually tricresol phosphates (TCPs) or other sulphur phosphorus amine combinations.
Whilst today's ashless technology does not meet all high-pressure vane pump test requirements, it is expected that the use of ashless formulations will grow as the technology improves.
Whilst there are specific bio-degradable and vegetable hydraulic oils available that will reduce the impact on the environment, they are currently more expensive; so industry still widely uses the conventional mineral based products.
This despite the fact that the elimination of heavy metals in oils such as Castrol's Hyspin ZZ (Zero Zinc) range does greatly reduce environmental impact through accidental spillage, misting from 'air lubricators', disposal treatments and burn off from swarf smelting/furnaces.
Another important advantage of zinc-free hydraulic oils is their ability to operate in fine clearance spool valves where cleanliness is critical.
Hydrolytic stability is a problem faced by all systems made from metal as they will inevitably suffer corrosion if exposed to water, even if this water exists as only droplets within oil.
Whilst all hydraulic oils may suffer water ingress through contact with wet air, coolant ingress or process water contamination, conventional zinc additive oils do tend to suffer more, due to the reaction of the zinc additive and water forming deposits and gels.
This is a major concern for filtration, where filter blockages can either cause oil starvation leading to failure of the system or, if oil vents through a relief valve, can deliver unfiltered oil containing contaminants to critical areas.
Radial pumps can also be affected as the vanes of the pump are reliant on unimpeded movement and gelling can jam them - reducing pump efficiency and increasing wear.
In summary, today's zinc-free mineral hydraulic oil is a finely tuned package that can be used to significantly reduce actual costs.
It employs the latest technology to make it safer and more economical and is extensively tested to ensure high performance in areas such as oxidation stability, demulsibility, wear, corrosion inhibition and hydrolytic stability, with the advantages greatly outweighing cost differentials of older technology fluids.