Lubrication and wear
The pressures required in modern hydraulic systems demand tough, precisely machined components. Precision machining leaves very small clearances between moving parts. For example, it is not uncommon for control valves to have spools and bores matched and fitted within a mechanical tolerance of ±0.0002 in. (5 µm). With some of today’s electrohydraulic valves, tolerances may be even tighter, with clearances of 1 µm or less. The surface finishes on high-pressure bearings and gears can result in rolling clearances as small as 0.1 µin.
The hydraulic fluid is expected to create a lubricating film to keep these precision parts separated. Ideally, the film is thick enough to completely fill the clearance between moving parts. This condition is known as hydrodynamic or full-film lubrication, and it results in low wear rates. When the wear rate is kept low enough, a component is likely to reach its intended service life expectancy, which may be millions of pressurization cycles.
The actual thickness of a lubricating film depends on fluid viscosity, applied load, and the relative speed of the two dynamic surfaces. In many applications, mechanical loads are so high that they squeeze the lubricant into a very thin film, less than 1 µin. thick. This is known as thin-film lubrication. If loads become high enough, the asperities of the two moving parts will puncture the film. The result is boundary lubrication, which allows metal-to-metal contact and the surface wear resulting from it.
Component and system designers try to avoid boundary lubrication by making sure that fluid is of the proper viscosity. However, viscosity can change as the fluid temperature changes. Also, loads and speed may vary widely during normal operating cycles. Therefore, most hydraulic components operate at least part of the time with only boundary lubrication. When that happens, parts of moving surfaces contact each other and are torn away from the parent material. The resulting particles then enter the fluid stream and travel throughout the system. If not removed by filtration, they react with other metal parts to create even more wear.
Lubricant manufacturers continually strive to reduce potential lubrication issues by improving fluids with additives. Viscosity-index (VI) improvers are added to help keep viscosity stable as temperature changes. Antiwear additives increase film strength. For hydraulic fluids, defoamers, demulsifiers, detergents, or dispersants may be added. Rust and oxidation (R&O) inhibitors are used in most hydraulic fluids because air and water are always present to some extent.
Symptoms of component wear are sluggish or erratic system operation, poor efficiency, and short component life. In pumps, wear first may be detected as reduced flow rate. This is because abrasive wear has increased internal clearance dimensions. Sometimes called increased slippage, this condition means that the pump is less efficient than it was when new.
When pump flow rate decreases, the fluid system may become sluggish, as evidenced by hydraulic actuators moving slower. Pressure at some locations in the system also may decrease. Eventually, there can be a sudden catastrophic failure of the pump. In extreme cases, this can occur within a few minutes after initial startup of the system.
In valves, wear increases internal leakage. The effect this leakage has on the system depends on the type of valve. For example, in flow-control valves, increased leakage usually means increased flow. In valves designed to control pressure, increased leakage may reduce the circuit pressure set by the valve. Silting interference causes valves and variable-flow pump parts to become sticky and operate erratically. Erratic operation shows up as flow and pressure surges, causing jerky motion in actuators.
Contamination control involves preventing contaminants from entering a hydraulic system. For example, filters may be placed in strategic locations throughout the system to trap any contaminants that do find their way into the fluid.
But for critical equipment, a successful contamination control program also must include regular assessment of the hydraulic fluid’s cleanliness. Often, fluid must be checked every two to six months or after every 500 or 1,000 hours of operation, depending on the equipment’s duty cycle, operating environment, and how critical it is to overall operation.
Experts also recommend that fluid be tested immediately after any maintenance event that exposes the hydraulic system to the external environment, such as when a hose or other component is replaced or fluid is added to the reservoir. Fluid replenishment can be particularly troublesome because new fluid is notorious for being dirty, often from improper storage and handling practices.
Test labs or do it yourself?
Before the advent of portable contamination detection instruments, fluid testing was conducted for only the most critical equipment and sent to a laboratory for analysis. This is still the most practical route for companies that do not require fluid testing often enough to justify purchasing their own diagnostic equipment and training their personnel. Even if a company has equipment, test labs still prove valuable for running multiple tests, interpreting results, troubleshooting, and recommending appropriate action.
Test labs also should be consulted for periodic chemical analysis of hydraulic fluid. Even if contamination has been brought to within acceptable limits, certain contaminants can alter a fluid’s chemical composition (primarily the additives) and render it ineffective. For example, additive depletion, over time, can reduce a fluid’s lubricity, oxidation resistance, or anti-foaming characteristics.
Excessive water and overheating can upset a fluid’s chemical balance relatively quickly. If excessive water is found in a fluid or a system overheats, experts recommend not only finding and correcting the source of the problem, but conducting chemical analysis of the fluid as well. Even if these problems do not occur, hydraulic fluid suppliers generally recommend having fluid analyzed chemically at regular intervals, such as annually, to identify potential problems and prevent them from occurring.
Portable particle counters and other diagnostic equipment have made it easy and convenient to monitor the fluid cleanliness of even non-critical equipment. In fact, many companies that have invested in their own particle counters and other instruments monitor the cleanliness of more equipment more often.
The higher reliability that results from this more intense preventive maintenance adds to the return on their investment. Furthermore, advanced techniques are being developed to make fluid-monitoring instruments even more sophisticated. Equipment currently is under development to continuously monitor the condition of hydraulic fluid while equipment is running.