Flash back about 40 years. Vane pumps in hydraulic systems were operating at or below 1,000 pounds per square inch (psi). Do you know what the preferred hydraulic fluid was during that time? It was monograde engine oil, containing the usual number of additives and chemistries. Why engine oil? One of the chemistries in that oil, zinc dithiophosphate (ZnDTP), provided the antiwear protection required by those vane pumps.

Now, fast-forward a decade or so. The industry discovered that some of the other chemistries in that same engine oil were, in fact, detrimental to pump performance. The oil contained a lot of dispersants, detergents and other additives that prevented the oil from separating water readily. (Hydraulic fluids, particularly those used for in-plant industrial applications, need to be able to get rid of a lot of water quickly.) The water caused the formation of sediment, deposits and other undesirables. Also, vane pump outputs began to climb, with pressures reaching 3,000 to 4,000 psi. With those higher outputs came higher operating temperatures. As these pumps ran harder and hotter, parts began to wear and corrode. Some pumps failed.

Meanwhile, in the world of higher-pressure piston pumps, the story was different, but the ending was often the same. Unlike vane pumps, piston pumps did not need the engine oil’s ZnDTP antiwear protection; in fact, in some cases, the antiwear chemistry was harmful. Piston pumps using engine oil showed high rates of wear on the copper used in the pumps’ piston shoes and cylinder liners. More pumps failed.

There had to be a better way. In 1980, Cleveland, Ohio-based Lubrizol Corporation pioneered the development of stabilized ZnDTP chemistry. Stabilized simply means that the ZnDTP is chemically modified to improve both its thermal and hydrolytic stability. Working with the major pump and equipment manufacturers, Lubrizol developed these stabilized ZnDTP chemistries to meet Denison HF-0 Hydraulic specifications, which are still current (Figure 1). Close to 90 percent of the hydraulic fluids in use today are still ZnDTP-based, and most are based on stabilized ZnDTP.

Figure 1. Market Overview and Trends Timeline

Go Additive-Free?
This question surfaces every so often. However, very little hydraulic fluid is sold without additives, and for good reasons.

First, it has been proven over many decades that hydraulic fluid must have both an antiwear agent and an antioxidant to extend pump and equipment life. That’s where additives earn their keep.

Also, additives protect more than the pump; they protect the base oil itself. An additive is a sacrificial material. It is put into a system to be used up, so that the base oil will not be used up. Though additives can be, and sometimes are replenished in a system, it is often not a good practice.

Temperature, oxidation (potential corrosion from water and acids), wear and other factors limit the useful life of hydraulic fluids. Additives clearly improve their performance characteristics and extend their useful lives. Today, almost all commercial lubricants contain additives to enhance their performance, with additive amounts ranging from 0.5 percent to more than 15 percent.

Zinc Dithiophosphates
Today, ZnDTP chemistry is somewhat of an art, in addition to being a science. It’s produced through a reaction between zinc oxide and an organic thiophosphoric acid. ZnDTP introduces special properties into many types of lubricants, including antiwear lubricants like hydraulic fluids. When added to the base oil, ZnDTP delivers multifunctional protection in both vane and piston pumps. The primary function of the additive is as an antiwear agent; its secondary purpose is as an antioxidant.

Today, there are three basic ZnDTP derivatives used in hydraulic fluids and other lubricants: aryl, primary alkyl and secondary alkyl. Each has its own set of strengths (Table 1).

Both aryl and alkyl derivatives are employed commercially. Aryl derivatives have a higher degree of thermal stability, while alkyl derivatives are generally more effective as antiwear additives. Both the antiwear and thermal stability characteristics of the alkyl compounds can be varied by using different alcohols.

Zinc-based hydraulic fluids continue to perform well. That’s why they’re widely used in so many demanding applications:

  • Off-road bulldozers, graders, excavators, shovels, walking cams
  • Forestry and agriculture - cherry pickers, tree harvesters, tractors
  • Transportation - ships, aircraft, haul trucks, dumptrucks, towmotors
  • In-plant machinery - any hydraulic system requiring precise control such as steel manufacturing, paper manufacturing, and auto and tire manufacturing

Benefits Outweigh the Risks
Today’s hydraulic systems are efficient, but not perfect. Equipment operators and maintenance personnel aren’t perfect, either. As such, hydraulic fluids occasionally find their way into our fields and waterways, mostly by accident, but occasionally through negligence.

There is a move within the industry to go “nonzinc,” and it’s based on the fact that zinc is a heavy metal. Water treatment facilities consider zinc an issue. Right now, however, there’s nothing that mandates against the use of ZnDTP. It simply has too many advantages. By following common-sense maintenance, use and disposal practices, the environment issues should be no greater than with any other industrial liquid.

First, zinc-based additives have a long history of successful use. In addition, ZnDTP is a multifunctional additive. With ZnDTP, only one additive is needed to add antioxidant and antiwear properties. Without ZnDTP, achieving those same needed properties requires two separate additives: one antioxidant and a separate antiwear additive. Furthermore, these two additives must be added at levels higher than the zinc-based additive. All of this adds significant cost.

Lubricant makers continue to seek high-performance, cost-effective, environmentally safe alternatives to zinc-based hydraulic fluids. While the results are certainly promising, zinc-based fluids continue to be the preferred choice. Actually, today’s state-of-the-art non-zinc fluids work quite well, but they’re expensive and not fully tested. Until costs come into line, zinc-based hydraulic fluids will continue to provide an attractive performance/value combination.

Proactive Maintenance
Are you one of those people who tries to make your hydraulic fluid last as long as possible? If so, it’s time to start viewing hydraulic fluid maintenance as an investment, not an expense.

As mentioned earlier, oil additives are meant to be consumed as they perform their functions. But, how do you know if the hydraulic fluid - the additives, specifically - is still up to the task?

Most people, if they’re really serious about it, will subscribe to a maintenance program or work with an oil analysis company. They’ll check the fluid perhaps every thousand operating hours or so. They’ll look at the zinc analysis, wear metals, acid numbers and many other factors. Some people never really change the fluid at all - they just top it off and replenish the oil, adding a small percentage of additives back in during the process. It’s not uncommon to see some fluids that have been in use for four, five, even six years and have no additives left in them at all.

It’s a totally different game in the construction and off-highway equipment industries. These people do change their oils regularly. Every construction company has its own drain intervals - typically every 2,000, 4,000 or 6,000 operating hours. Every equipment builder has a specification for when fluids should be changed. Most of the equipment builders will also insist that you comply with their own analysis program.

At the low end, a piece of highway construction equipment costs about $20,000 to $25,000. Conversely, a large hydraulic excavator can cost up to a half million dollars. The construction industry has a lot of money invested in these pieces of equipment and has become knowledgeable and diligent about maintenance. Here and in every other industry, the cost of maintenance is clearly a drop in the oil bucket when compared with the cost of machine replacement. Unless you have an unlimited capital equipment budget, it’s smart to be proactive about maintenance.

Serious About Hydraulic Fluid
In the past, hydraulic fluid was considered an afterthought by equipment designers and builders - something required to help make the piece of equipment move.

Today, hydraulic fluid is often viewed as a key component of a machine’s design - as much as the pump, the motor or the hoses - or it certainly should be. Without essential additives such as ZnDTP, the machines and equipment you see running today wouldn’t be operating as efficiently and effectively as they are. Nor would they be as compact. Today’s better hydraulic fluids allow us to get more work out of smaller equipment running at higher temperatures. One of those fluids is zinc-based hydraulic fluid, with its combination of high performance, good value and proven track record.