What is in this article?:
- Does your fluid provide enough protection?
- Air as a contaminant
Contamination doesnt just reduce system performance and reliability it can also degrade the hydraulic fluid itself.
You’ve spent a lot of time and effort designing and building your hydraulic system; now you’re ready for production. After the equipment with its brand new hydraulic system has been shipped, your colleagues in accounting can calculate your profit.
But what will happen when (not if) contaminants enter the hydraulic system? Will contamination degrade fluid performance? Will it cause equipment malfunctions and downtime? If so, you can bet it will eventually jeopardize future orders from customers. Of course, adequate filtration will eliminate or reduce many contamination related failures, but your choice of hydraulic fluid help can help mitigate the problems associated with contamination.
Hydraulic systems have changed dramatically in the last 20 years. Generally, they are smaller, contain less hydraulic oil, and require the same or more output as the systems of yesteryear. The smaller size places a greater burden on the hydraulic oil to help protect the equipment and components.
One thing that hasn’t changed over time, however, is the fact that hydraulic oil can become contaminated in a variety of ways and from a variety of sources. Contaminants in a hydraulic system can be:
- present when the system is new,
- generated by the system during operation, or
- be introduced to the system during operation or even during idle periods.
Examples of contaminants that could be present in a new system include metal chips or dust left from manufacturing processes. Contaminants generated by the system include wear debris and thermal degradation by-products from heat, such as sludge and varnish. Contaminants, including dirt, water, and even other fluids, can enter the system through reservoir breathers, rod and shaft seals, and whenever a hydraulic component is replaced.
You can also classify contaminants by type, such as dirt and wear debris, water, air, by-products of heat, and other fluids such as gear oils, cleaners, and motor oils. Systems are designed to have filters in place to remove many of these contaminants. But what role does the hydraulic fluid itself play with respect to these contaminants?
Dirt and wear debris
Eaton Vickers has stated that 90% of failures due to contamination come from abrasive wear, which is caused by metal-to-metal contact between components. The result is that small particles of the components break off and infiltrate the hydraulic fluid. In turn, they can cause wear of other components, clog filters, and chemically react with the fluid. Vane and gear pumps especially need protection against abrasive wear. Hydraulic fluids help prevent abrasive wear by using an effective anti-wear additive. Zinc dialkyl dithiophosphate (ZDDP) is typically the anti-wear additive used. ZDDP is both relatively inexpensive and very effective at reducing mild wear by preventing metal surfaces from contacting each other. The result is reduced abrasive wear. Over time, anti-wear additives can become less effective, therefore it is important to use a fluid that exhibits wear protection over an extended time period. These fluids are said to have good durability, meaning that their performance does not diminish over extended periods of time. The timing depends on system operating conditions such as temperatures, pressures, and the operating environment.
Water as a contaminant
It is not uncommon for water to enter a hydraulic system through access plates in reservoirs or simply condensed from ambient air drawn into the reservoir. Because water can cause poor lubrication, hinder filtration, and potentially form rust, water content should be monitored closely by analyzing the oil. Depending on the application, water content of more than 1000 ppm (0.1%) generally is considered excessive.
Hydraulic fluids are designed to separate from water (demulsify) quickly so it can be drained from the bottom of a reservoir. Some hydraulic fluid additives containing some of the less stable ZDDPs also can react with water to form acids that can corrode yellow metals, such as copper and brass.
When a fluid does not react with water, it is hydrolytically stable. It is important to use a fluid with high demulsibility and hydrolytic stability, referred to on fluid suppliers’ technical data sheets as hydrolytic stability — ASTM D 2619 and water separation (or demulsibility) ASTM D 1401.
Rust in a hydraulic system often can be tracked back to water ingression. The interaction of iron, water, and oxygen forms rust. Rust can be prevented by using a fluid containing rust inhibitors built into the additive system.
Water also can cause problems with filtration. Today, many systems are designed to use very fine filters, 3 μm or less, to help remove wear debris and other contaminants. In the development of hydraulic fluids, filtration is measured to make sure that the fluid can be filtered, both in its original condition and as contaminated with water. Generally, fluids will filter much more slowly when contaminated with water. Some OEMs have set specifications to ensure that fluids filter well both in their original condition and when contaminated with water.