Hydraulic fluids perform four basic functions. Their primary function is to create force and motion as flow is converted to pressure near the point of use. Second, by occupying the space between metal surfaces, the fluid forms a seal, which provides a pressure barrier and helps exclude contaminants. A third function - often misunderstood - is lubrication of metal surfaces. The fourth and final function provided by hydraulic fluid is cooling of system components.
If any one of these functions is impaired, the hydraulic system will not perform as designed. Worse yet, sudden and catastrophic failure is possible. The resulting downtime can easily cost a large manufacturing plant thousands of dollars an hour. Hydraulic-fluid maintenance helps prevent or reduce unplanned downtime. It is accomplished through a continuous program to minimize and remove contaminants.
Aside from human interference, the most common source of system impairment is fluid contamination. Contamination can exist as solid particles, water, air, or reactive chemicals. All impair fluid functions in one way or another.
Sources of contaminants
Contaminants enter a hydraulic system in a variety of ways. They may be:
- built-in during manufacturing and assembly processes
- internally generated during normal operation, and
- ingested from outside the system during normal operation.
If not properly flushed out, contaminants from manufacturing and assembly will be left in the system. These contaminants include dust, welding slag, rubber particles from hoses and seals, sand from castings, and metal debris from machined components. Also, when fluid is initially added to the system, a certain amount of contamination probably comes with it. Typically, this contamination includes various kinds of dust particles and water.
During system operation, dust also enters through breather caps, imperfect seals, and any other openings. System operation also generates internal contamination. This occurs as component wear debris and chemical byproducts from fluid and additive breakdown due to heat or chemical reactions. Such materials then react with component surfaces to create even more contaminants.
In broad terms, contaminant interference manifests itself as either mechanical or chemical interaction with components, fluid, or fluid additives.
Mechanical interactions, Figure 1, include blockage of passages by hard or soft solid particles, and wear between particles and component surfaces.
Chemical reactions include: formation of rust or other oxidation, conversion of the fluid into unwanted compounds, depletion of additives - sometimes involving harmful byproducts - and production of biochemicals by microbes in the fluid.
Any of these interactions will be harmful. Without preventive measures and fluid conditioning, their negative effects can escalate to the point of component failure. One of the most common failure modes is excessive wear due to loss of lubrication.
Lubrication and wear
The pressures required in modern hydraulic systems demand sturdy, precisely matched components. And precision machining leaves very small clearances between moving parts. For example, it is not uncommon for control valves to have pistons and bores matched and fitted within a mechanical tolerance of ±0.0002 in. (two ten-thousandths of an inch). In metric units, this is about 5 µm (five millionths of a meter). In modern electrohydraulic devices, tolerances may be even tighter and clearances can be less than 1 mm. 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, Figure 2. This condition is known as hydrodynamic or full-film lubrication and 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 elastohydrodynamic (EHD) or thin-film lubrication. If loads become high enough, the film will be punctured by the asperities of the two moving parts. The result is boundary lubrication.
Component and system designers try to avoid boundary lubrication by making sure that fluid has the proper viscosity. However, viscosity changes 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.
Fluid chemists continually try to minimize potential lubrication problems by improving fluids with additives. Viscosity-index (VI) improvers are added to help keep viscosity stable as temperature changes. Antiwear additives increase film strength. If very heavy loads will be applied, the fluid should contain an extreme pressure (EP) additive that reacts with metal surfaces to form a hard protective film. For fluids in circulating systems, defoamant, demulsifier, detergent, or dispersant may be added. Rust and oxidation (RO) inhibitors are used in most hydraulic fluids because air and water are always present to some extent.
The symptoms of wear are diminished system performance and shorter component life. In pumps, wear may first 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 start-up 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 valves used to control flow, 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, Figure 3, 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.