Hydraulic oils are the lifeblood of much of today’s industrial machinery. The oils must be designed to satisfy the requirements of today and the future, however sophisticated hydraulic systems may become.
Originally, all hydraulic systems used water. And even today, water-based hydraulic fluids are still extensively used, particularly where there is a fire risk. But water has serious drawbacks as hydraulic fluid. It is corrosive and can freeze. Under these water filled hydraulic buffers, a flame must be kept burning in winter. The boiling point of water limits its operating range whereas mineral oil can be used at higher operating temperatures. Water is always at one viscosity but mineral oils vary from thick to thin. They have a range of viscosities.
Temperature changes affect the viscosity of some oils more than others. At room temperature, both these oils have about the same viscosity. But in boiling water, the left hand sample becomes very thin. The ball drops faster. Packed in ice, the left hand sample thickens. Its viscosity has been affected by temperature change. The viscosity of the oil on the right has changed less because it has a higher viscosity index.
Base oils of different viscosities are blended to produce hydraulic oils for a wide range of uses. A thin oil is needed for operating in the Arctic. A thick one in the tropics. Automated machine tools which demand the sensitive response of hydraulic systems need an oil with precise viscosity characteristics. Equipment that is working hard will raise the temperature of the hydraulic oil even on a cold day. But if the oil has the correct viscosity index, it will not become too thin. By selecting base oils which have a natural high viscosity index, flow resistance at startup will be reduced as will viscous drag and internal pump leakage at high operating temperatures.
In conditions where the oil is operating at high temperatures and pressures, oxidation is likely to occur. With the growing demand for small units designed to work at high operating pressures, oxidation stability is even more essential. Compare fresh oil on the right with the same oil after excessive use. Oxidation has thickened it until it is almost useless. Filtering the heavily oxidized oil reveals a sludge which would block ports and filters. Oil oxidation coats pistons and other parts with gums and lacquers and causes acidic attack on metal surfaces. Not all hydraulic oils have the same useful lifespan. These samples were run in identical hydraulic systems for a fixed time. The darker oil is badly oxidized. The others show better oxidation stability and will last much longer. Good refining is the most important step in producing high-quality hydraulic oils.
After distillation, the oil is treated in three plants to improve its quality. Here is the oil after distillation but before it is treated in the first plant the solvent extractor. In the solvent extractor, the aromatics are removed to improve the oxidation stability and the viscosity index of the oil. This is the output from the solvent extractor. Compare it with the input. The second plant removes wax from the oil; it is wax which limits the use of oil at very low temperatures. This is the oil before it goes into the third plant the finishing unit. The finishing unit removes trace materials which affect the water separation properties of the oil and which may also affect oxidation stability. The oil looks bright and polished after the finishing unit, the last of the refining processes. A variety of crude oils may be used and good refining is essential to ensure that base oils are of a consistently high-quality. The use of high-quality base stocks enables oxidation to be further reduced and the life of the oil increased. Also, new additives have been devised which will inhibit oxidation in hydraulic oils.
The working life of a hydraulic oil depends on its oxidation stability. In a clean well-maintained system, an oil will last several years.
Oxidation stability can be assessed in this apparatus which speeds up the oxidation process. The oil is agitated with pure oxygen while it is kept at high temperature in the bath. The equipment works continuously. The amount of oxygen taken up by the oil is recorded. For a long period, oxidation is steady, but then it increases rapidly and the useful life of the oil is over. Another test shows the difference in oxidation between a general hydraulic oil and a high-grade hydraulic oil designed to meet current developments. But poor system design or bad management can cause rapid oxidation even in high-grade hydraulic oil. A supply tank too small or half-empty means that the hot oil is reused. And overheating increases the rate of oxidation. For every 10 degrees centigrade rise in temperature, the oxidation rate doubles so cooling may be necessary.
Modern industrial equipment is designed to work at higher ratings. Pumps are getting smaller and more heavily loaded which can cause increased wear. The effective wear can be seen on the edges of these pump veins. The hydraulic systems of container handling equipment are very highly stressed. Reliability and safety are vital. And excessive wear of components could lead to costly system failures.
To protect metal surfaces, modern hydraulic fluids contain load-carrying and anti-wear compounds. These are tested under load during an FZG Test. Gears are run in the oil under test at a constant speed for a fixed period. The test assesses the load carrying capacity of the oil. The gears were loaded against each other in stages and the maximum load transmitted by the oil recorded. Samples from the gears are examined in an electron microscope. Chemicals present are detected with pinpoint accuracy and it can be determined how well an anti-wear additive has formed a protective film. Wear is also evaluated in sliding vane pumps. The pump is run under standard conditions for 250 hours and then stripped and examined. Wear is evaluated by measuring the loss in weight of the pump ring and veins. Soils are further evaluated in hydraulic test rigs.
A variety of pumps are run for extended periods to check likely oil performance in service. Current developments in pump design emphasized the need for low wear figures. Good water separation properties are essential in hydraulic oils. Water in the atmosphere condenses on the walls of supply tanks and gets into hydraulic systems where it can cause damage. Here, oil and water are stirred into an emulsion. Well refined clean oils demulsify rapidly and the oil floats to the top. Once separated from the oil, water can easily be drained from the hydraulic system reservoir. Good quality oils contain an anti-rust additive. The steel rod on the left shows severe rusting. The oil on the right contains an anti-rust additive which protects the metal surface.
In metalworking machinery, water-based cutting fluids can get into the hydraulic system. Their synthetic components could settle on valves and clutches, preventing accurate machining. The hydraulic oil in these units requires special additives to prevent contaminants settling out. These are detergent hydraulic oils, with controlled demulsibility characteristics. Excessive water will settle out in the sump where it can be drained off. Water in oil can also cause blocked filters. The oil sample on the left is contaminated with water, the one on the right is not. The sample on the right flows faster because it passes freely through the filter. The flow on the left is slower the filter has become blocked due to the interaction of the water with some of the oils components. When the blocked filter is examined in an electron microscope, it can be seen that some of the components have separated from the oil as a result of water contamination. Compare the blocked filter with the clean one, this analysis is an essential step in the selection of hydraulic oil additives.
Air must also be kept out of hydraulic systems. It is compressible and makes controls spongy. It has made the feed on this grinder unsteady, giving a poor surface finish. Air can be drawn into hydraulic systems through leaks in the suction side and get compressed in the pump. The bubbles of compressed air heat up and produce conditions suitable for rapid oxidation of the oil. The ability of an oil to release entrained air is measured during an air release test. Compressed air is blown through preheated oil. After seven minutes, the air flow is stopped. The time taken for the volume of entrained air tube reduced is recorded. When pressure in hydraulic fluid drops, any air dissolved in it will be released and cause foaming. Foaming is also caused by poor system design such as return pipes located above or close to the fluid surface. To minimize foaming, the return pipe must be below the fluid level and the tank should contain baffles or screens. An anti-foam additive is used in most hydraulic oils as on the right. This test measures the difference in foaming, but additive treatment is very critical. The oil should not be overdosed or air release will be retarded. In a properly designed and operated system, there would be no entrained air to caused foam. System design must also take into account the effect of pressure on certain properties of hydraulic oils. This pressure viscometer measures change in viscosity with change in pressure. At the top of the cylinder is a ball. When the ball is released it starts a timer which stops when the ball reaches the bottom. At a pressure of 350 atm, the ball falls in eight seconds. Double the pressure to 700 atm and the oil becomes much thicker. In this instance, the increase in pressure has nearly doubled the viscosity of the oil.