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Tài liệu đào tạo Fluid Power Ebook, Edition 1: Fluid Power Basics

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  1. hochoi
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    Fluid Power Basics starts with background information about simple air and hydraulic circuits, principles of fluid power operation and physical laws governing fluid power. Subsequent chapters cover different types of hydraulic fluids, fluid rating, operating parameters, and how to apply them. Next, a discussion on plumbing of fluid power systems covers tubing, pipe, and hose installations. A short section on vacuum and its applications is followed by basic circuit information. Coverage then shifts to discussing different components that make up a complete hydraulic or pneumatic system: reservoirs, filters, pumps, flow meters, gauges, and valves.
    Subsequent chapters cover flow and pressure controls, special-purpose valves, and accumulators. The book also covers all types of actuators, including cylinders, rams, motors, and rotary actuators. Application of these components in different circuits gives a general overall view of how they are used.
    Circuit diagrams are intended to show the function of the components and do not necessarily show all the components to make a safe and reliable system. Drawing practices and symbols according to ISO standards have been used when possible.
     
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    Chapter 1: Fluid Power Basics

    Any media (liquid or gas) that flows naturally or can be forced to flow could be used to transmit energy in a fluid power system. The earliest fluid used was water hence the name hydraulics was applied to systems using liquids. In modern terminology, hydraulics implies a circuit using mineral oil. Figure 1-1 shows a basic power unit for a hydraulic system. (Note that water is making something of a comeback in the late '90s; and some fluid power systems today even operate on seawater.) The other common fluid in fluid power circuits is compressed air. As indicated in Figure 1-2, atmospheric air -- compressed 7 to 10 times -- is readily available and flows easily through pipes, tubes, or hoses to transmit energy to do work. Other gasses, such as nitrogen or argon, could be used but they are expensive to produce and process.
    [​IMG]
    Fig. 1-1: Basic hydraulic power unit.
    Of the three main methods of transmitting energy mechanical, electrical, and fluid fluid power is least understood by industry in general. In most plants there are few persons with direct responsibility for fluid power circuit design or maintenance. Often, general mechanics maintain fluid power circuits that originally were designed by a fluid-power-distributor salesperson. In most facilities, the responsibility for fluid power systems is part of the mechanical engineers' job description. The problem is that mechanical engineers normally receive little if any fluid power training at college, so they are ill equipped to carry out this duty. With a modest amount of fluid power training and more than enough work to handle, the engineer often depends on a fluid power distributor's expertise. To get an order, the distributor salesperson is happy to design the circuit and often assists in installation and startup. This arrangement works reasonably well, but as other technologies advance, fluid power is being turned down on many machine functions. There is always a tendency to use the equipment most understood by those involved.
    [​IMG]
    Fig. 1-2: Basic pneumatic power arrangement.
    Fluid power cylinders and motors are compact and have high energy potential. They fit in small spaces and do not clutter the machine. These devices can be stalled for extended time periods, are instantly reversible, have infinitely variable speed, and often replace mechanical linkages at a much lower cost. With good circuit design, the power source, valves, and actuators will run with little maintenance for extended times. The main disadvantages are lack of understanding of the equipment and poor circuit design, which can result in overheating and leaks. Overheating occurs when the machine uses less energy than the power unit provides. (Overheating usually is easy to design out of a circuit.) Controlling leaks is a matter of using straight-thread O-ring fittings to make tubing connections or hose and SAE flange fittings with larger pipe sizes. Designing the circuit for minimal shock and cool operation also reduces leaks.
    A general rule to use in choosing between hydraulics or pneumatics for cylinders is: if the specified force requires an air cylinder bore of 4 or 5 in. or larger, choose hydraulics. Most pneumatic circuits are under 3 hp because the efficiency of air compression is low. A system that requires 10 hp for hydraulics would use approximately 30 to 50 air-compressor horsepower. Air circuits are less expensive to build because a separate prime mover is not required, but operating costs are much higher and can quickly offset low component expenses. Situations where a 20-in. bore air cylinder could be economical would be if it cycled only a few times a day or was used to hold tension and never cycled. Both air and hydraulic circuits are capable of operating in hazardous areas when used with air logic controls or explosion-proof electric controls. With certain precautions, cylinders and motors of both types can operate in high-humidity atmospheres . . . or even under water.
    When using fluid power around food or medical supplies, it is best to pipe the air exhausts outside the clean area and to use a vegetable-based fluid for hydraulic circuits.
    Some applications need the rigidity of liquids so it might seem necessary to use hydraulics in these cases even with low power needs. For these systems, use a combination of air for the power source and oil as the working fluid to cut cost and still have lunge-free control with options for accurate stopping and holding as well. Air-oil tank systems, tandem cylinder systems, cylinders with integral controls, and intensifiers are a few of the available components.
    The reason fluids can transmit energy when contained is best stated by a man from the 17th century named Blaise Pascal. Pascal's Law is one of the basic laws of fluid power. This law says: Pressure in a confined body of fluid acts equally in all directions and at right angles to the containing surfaces. Another way of saying this is: If I poke a hole in a pressurized container or line, I will get PSO. PSO stands for pressure squirting out and puncturing a pressurized liquid line will get you wet. Figure 1-3 shows how this law works in a cylinder application. Oil from a pump flows into a cylinder that is lifting a load. The resistance of the load causes pressure to build inside the cylinder until the load starts moving. While the load is in motion, pressure in the entire circuit stays nearly constant. The pressurized oil is trying to get out of the pump, pipe, and cylinder, but these mechanisms are strong enough to contain the fluid. When pressure against the piston area becomes high enough to overcome the load resistance, the oil forces the load to move upward. Understanding Pascal's Law makes it easy to see how all hydraulic and pneumatic circuits function.
    [​IMG]
    Fig. 1-3: How Pascals Law affects a cylinder Notice two important things in this example. First, the pump did not make pressure; it only produced flow. Pumps never make pressure. They only give flow. Resistance to pump flow causes pressure. This is one of the basic principles of fluid power that is of prime importance to troubleshooting hydraulic circuits. Suppose a machine with the pump running shows almost 0 psi on its pressure gauge. Does this mean the pump is bad? Without a flow meter at the pump outlet, mechanics might change the pump, because many of them think pumps make pressure. The problem with this circuit could simply be an open valve that allows all pump flow to go directly to tank. Because the pump outlet flow sees no resistance, a pressure gauge shows little or no pressure. With a flow meter installed, it would be obvious that the pump was all right and other causes such as an open path to tank must be found and corrected.


    [​IMG]Fig. 1-4: Comparison of mechanical and hydraulic leverage

    Another area that shows the effect of Pascal's law is a comparison of hydraulic and mechanical leverage. Figure 1-4 shows how both of these systems work. In either case, a large force is offset by a much smaller force due to the difference in lever-arm length or piston area.
    Notice that hydraulic leverage is not restricted to a certain distance, height, or physical location like mechanical leverage is. This is a decided advantage for many mechanisms because most designs using fluid power take less space and are not restricted by position considerations. A cylinder, rotary actuator, or fluid motor with almost limitless force or torque can directly push or rotate the machine member. These actions only require flow lines to and from the actuator and feedback devices to indicate position. The main advantage of linkage actuation is precision positioning and the ability to control without feedback.
    At first look, it may appear that mechanical or hydraulic leverage is capable of saving energy. For example: 40,000 lb is held in place by 10,000 lb in Figure 1-4. However, notice that the ratio of the lever arms and the piston areas is 4:1. This means by adding extra force say to the 10,000-lb side, it lowers and the 40,000-lb side rises. When the 10,000-lb weight moves down a distance of 10 in., the 40,000-lb weight only moves up 2.5 in.
    Work is the measure of a force traversing through a distance. (Work = Force X Distance.). Work usually is expressed in foot-pounds and, as the formula states, it is the product of force in pounds times distance in feet. When a cylinder lifts a 20,000-lb load a distance of 10 ft, the cylinder performs 200,000 ft-lb of work. This action could happen in three seconds, three minutes, or three hours without changing the amount of work.
    When work is done in a certain time, it is called power. {Power = (Force X Distance) / Time.} A common measure of power is horsepower - a term taken from early days when most persons could relate to a horse's strength. This allowed the average person to evaluate to new means of power, such as the steam engine. Power is the rate of doing work. One horsepower is defined as the weight in pounds (force) a horse could lift one foot (distance) in one second (time). For the average horse this turned out to be 550 lbs. one foot in one second. Changing the time to 60 seconds (one minute), it is normally stated as 33,000 ft-lb per minute.
    No consideration for compressibility is necessary in most hydraulic circuits because oil can only be compressed a very small amount. Normally, liquids are considered to be incompressible, but almost all hydraulic systems have some air trapped in them. The air bubbles are so small even persons with good eyesight cannot see them, but these bubbles allow for compressibility of approximately 0.5% per 1000 psi. Applications where this small amount of compressibility does have an adverse effect include: single-stroke air-oil intensifiers; systems that operate at very high cycle rates; servo systems that maintain close-tolerance positioning or pressures; and circuits that contain large volumes of fluid. In this book, when presenting circuits where compressibility is a factor, it will be pointed out along with ways to reduce or allow for it.
    Another situation that makes it appear there is more compressibility than stated previously is if pipes, hoses, and cylinder tubes expand when pressurized. This requires more fluid volume to build pressure and perform the desired work. In addition, when cylinders push against a load, the machine members resisting this force may stretch, again making it necessary for more fluid to enter the cylinder before the cycle can finish.
    As anyone knows, gasses are very compressible. Some applications use this feature. In most fluid power circuits, compressibility is not advantageous; in many, it is a disadvantage. This means it is best to eliminate any trapped air in a hydraulic circuit to allow faster cycle times and to make the system more rigid.
    Boyle's Law

    Boyle's Law for gasses states: It is the principle that, for relatively low pressures, the absolute pressure of an ideal gas kept at constant temperature varies inversely with the volume of the gas. In down-home language this means if a ten cubic foot volume of atmospheric air is squeezed into a one cubic foot container, pressure increases ten times. (10 X 14.7 psia = 147 psia.) Notice that pressure is stated as psia.
    [​IMG]
    Fig. 1-5: Measurement of gauge and absolute pressure Normally, pressure gauges read in psi (with no additional letter). Commonly called gauge pressure, psi disregards the earth's atmospheric pressure of 14.7 psia, because it has no effect either negative or positive on a fluid power circuit. The a on the end of psia stands for absolute, and would be shown on a gauge with a pointer that never goes to zero unless it is measuring vacuum. Another type of gauge that shows both negative and positive pressures would have a pointer with an inches-of-mercury (in. Hg) scale below zero and a psig scale above zero. Both of these gauges could read pressure or vacuum. (They are always found in a refrigeration repairperson's tool kit. Refrigeration units have both vacuum and pressure in different sections of the system at the same time.) Figure 1-5 pictures a typical psig gauge and one type of psia gauge.
    In the example above, when ten cubic feet of air was squeezed into a one cubic-foot space, both pressures were given in psia. To see what gauge pressure (psig) would be, subtract one atmosphere from the 147-psia reading. (147 psia 14.7 psia = 132.3 psig.) To calculate the amount of compression of air in a system, always use absolute pressure, or psia, not psig. For example: the cylinder in Figure 1-6 contains eight cubic feet of air at 70 psig. To what will pressure increase when an external force pushes the piston back until the space behind the piston is two cubic foot? It is obvious the pressure will rise four times. At first it might look easy to take 70 psig X 4 = 280 psig, but this answer is wrong. For the correct answer, gauge pressure must be changed to absolute pressure. In this case by adding one atmosphere to the 70-psig reading. (70 psig + 14.7 psia = 84.7 psia.) Now multiply the 84.7-psia pressure by 4 to see what the absolute pressure is when the cylinder stops at one cubic foot volume. (84.7 X 4 = 338.8 psia.) Finally, to return to gauge pressure, subtract one atmosphere from the absolute pressure. (338.8 psia 14.7 psia = 324.1 psig.) Notice that the correct pressure is 44.1 psig higher than when gauge pressure is the multiplier.
    [​IMG]
    Fig. 1-6: Pressure change as air is compressed Temperature was not considered in both preceding cases, but notice that the law says kept at constant temperature. Compressing a gas always increases its temperature because the heat in the larger volume is now packed into a smaller space. The next law says that increasing temperature increases pressure if the gas cannot expand. This means the pressures given are measured after the gas temperature returns to what it was originally.
    Gauges today read in psi and bar. Bar is a metric or SI unit for pressure and is equal to approximately the barometer reading or one atmosphere. One atmosphere is actually 14.696 psi but the SI unit for bar is 14.5 psi.

    Charles' Law

    Heating a gas or liquid causes it to expand. Continuing to heat a liquid will result in it changing to the gaseous state and perhaps spontaneous combustion. If the gas or liquid cannot expand because it is confined, pressure in the contained area increases. This is stated in Charles' Law as: The volume of a fixed mass of gas varies directly with absolute temperature, provided the pressure remains constant. Because fluid power systems have some areas in which fluid is trapped, it is possible that heating this confined fluid could result in part damage or an explosion. If a circuit must operate in a hot atmosphere, provide over pressure protection such as a relief valve or a heat- or pressure-sensitive rupture device. Never heat or weld on any fluid power components without proper preparation of the unit.
    Static head pressure

    The weight of a fluid in a container exerts pressure on the containing vessel's sides and bottom. This is called static head pressure. It is caused by earth's gravitational pull. A good example of head pressure is a community water system. Figure 1-7 shows a water tower with a topmost water level of 80 feet. A cubic inch of water weighs 0.0361 pounds. Therefore a one square-inch column of water will exert a force of 0.0361 psi for every inch of elevation. This works out to .433 psi per foot of elevation. For the water tower in Figure 1-7, the pressure at the base would be: 80 ft X 0.433 psi/ft = 34.6 psi. This pressure is always available, even when no pumps are running. Of course, if the water level drops, static head pressure also will drop.
    [​IMG]
    Fig. 1-7: Pressure measurement for water tower The specific gravity of hydraulic oil is approximately 0.9, so multiplying water's 0.433 psi per foot by 0.9 shows oil exerts 0.39 psi per foot of elevation. Usually this fraction is rounded to 0.4 for simplicity. If the water tower were filled to 80 ft with oil, it would exert a pressure of 32 psi at ground level. Other fluids would develop a higher or lower static pressure according to their specific gravities.
    This pressure is only realized at ground level at the tower. Outlets at other levels would be higher or lower according to their distance below the fluid surface.
    Tanks seen on most water towers simply store volume. Pressure does not drop rapidly or require frequent pump starts to maintain the fluid level. The size or shape of the tank does not affect pressure at the base. Pressure at the base of a straight 80-ft pipe would be the same, but useful volume before pressure drop would change drastically. Always remember: it is not the physical size of a body of fluid that determines pressure but how deep it is.
    Head pressure can have an adverse effect on a hydraulic system because many pumps are installed above the fluid level. This means the pump must first create enough vacuum to raise the fluid and then create even higher vacuum to accelerate and move it. Therefore there is a limit to how far a pump can be located above the oil level. Most pumps specify a maximum suction pressure of 3 psi. At 4- to 5-psi suction pressure, pumps start to cavitate . . . causing internal damage. At 6- to 7-psi vacuum, cavitation damage is severe and noise levels increase noticeably. (The effects of cavitation are covered fully in Chapter 8, Fluid power pumps and accessory items.) Axial- or in-line-piston pumps are especially vulnerable to high inlet vacuum damage and should be set up below the fluid level to produce a positive head pressure.
    Many modern hydraulic systems place the pump next to the reservoir so the fluid level is always above the pump inlet. With this type of installation the pump always has oil at startup and has a positive head pressure at its inlet. A better arrangement puts the tank above the pump to take advantage of even greater head pressure. Everything possible should be done to keep pressure drop low in the pump inlet line because the highest possible pressure drop allowable is one atmosphere (14.7 psi at sea level).
    The earth's atmosphere the air we breathe exerts a force of 14.7 psi at sea level on an average day. This pressure covers the whole earth's surface, but at elevations higher than sea level, it is reduced by approximately 0.5 psi per 1000 feet. This pressure of earth's atmosphere is the source of the power of vacuum. The highest possible vacuum reading at any location is the weight of the air above it at that time. A reading of maximum vacuum available is given during the local weather forecast as the barometer reading. Divide the barometer reading by two to get the approximate atmospheric pressure in psi. This force could be directly measured if it were possible to isolate a one square-inch column of air one atmosphere tall at a sea level location. Because this is not possible, the method used to measure vacuum is demonstrated in Figure 1-8.
    [​IMG]
    Fig. 1-8: Vacuum measurement with mercury Submerge a clear tube with one closed end in a container of mercury and allow it to fill completely. (The tube must be more than 30-in. long for this example to work when mercury is the liquid.) After the mercury displaces all the air in the tube, carefully raise the tube's closed end, keeping the open end submerged so the mercury can't run out and be replaced by air. When the tube is positioned vertically, the liquid mercury level will lower to give the atmospheric pressure reading in inches of mercury (29.92-in. Hg at sea level). The mercury level will fluctuate from this point as high and low-pressure weather systems move past. If the tube had been 100-in. tall, the mercury level would still have dropped to whatever the atmospheric pressure was at its location. The reason the mercury does not all flow out is that atmospheric pressure holds it in.
    This barometer could have been built using another liquid but the tube would have to be longer because most other liquids have a much lower specific gravity than mercury's 13.546. Water, with a specific gravity of 1.0, would require a closed-end tube at least 33.8 ft long, while oil, with a specific gravity of approximately 0.9, would have to be even longer.
    Vacuum pumps can be similar in design to air compressors. There are reciprocating-piston, diaphragm, rotary-screw, and lobed-rotor designs. (See air compressor types in Chapter 8, Fluid power pumps and accessory items.) Imagine hooking the inlet of an air compressor to a receiver tank and leaving the outlet open to atmosphere. As the pump runs, it evacuates air from the receiver and causes a negative pressure in it.
    [​IMG]
    Fig. 1-9: Cross-sectional view of venturi vacuum generator Vacuum pumps are an added expense and normally are only found in facilities that use a constant supply of negative pressure to operate machines or make products.
    Vacuum generators that use plant compressed air as a power source are also available. These components have no moving parts but use plant air flowing through a venturi to produce a small supply of negative pressure. Figure 1-9 shows a simplified cutaway view of a venturi-type vacuum generator. The device consists of body A with compressed-air inlet B that passes air flow through venturi nozzle C. The air exhausts at a higher velocity to atmosphere through orifice D. As air at increasing velocity flows past opening E near the venturi nozzle, it creates a negative pressure and draws in atmospheric air through port F. Port F can connect to any external device that needs a vacuum source. A vacuum gauge at port F shows negative pressure when compressed air is supplied to port B.
    Vacuum generators are inexpensive, but can be costly to operate. For every 4 cfm of air supply required to power them, they use approximately one compressor horsepower. For this reason, venturi-type vacuum generators usually are installed with a control valve to turn them on only when needed.
    Vacuum is limited to one atmosphere maximum at any location, and standard vacuum pumps only reach about 85% (approximately 12 psi) of this on average. As a result, vacuum is not powerful enough to do much work unless it acts on a large area.
    [​IMG]Fig. 1-10: Simplified representation of lifting with vacuum Many industrial vacuum applications have to do with handling parts. Large-area suction cups can lift a large heavy part with ease, as illustrated in Figure 1-10. When the lift rises, negative pressure (vacuum) inside the suction cups causes atmospheric pressure on the opposite side of the part to push it up.


    [​IMG]
    Fig. 1-11: Simplified representation of work holding with vacuum Industries such as glass and wood manufacturing use vacuum to hold work pieces during machining or other operations, as shown in Figure 1-11. The pieces are held firmly in place as the negative pressure under them causes atmospheric pressure to push against them. A resilient seal laid in a groove in the fixture keeps atmospheric air from entering the cavity beneath the part. This groove can be cut to match the contour of the part. In machining operations, the seals can isolate interior cutouts, allowing them to be removed while firmly holding the rest of the piece.


    [​IMG]
    Fig. 1-12: Simplified representation of plastic-sheet forming with vacuum
     
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    Chapter 2: Hydraulic Fluids

    For long service life, safety reasons, and reliable operation of hydraulic circuits, it is very important to use the correct fluid for the application. The most common fluid is based on mineral oil, but some systems require fire resistance because of their proximity to a heat source or other fire hazard. (Water is also making its return to some hydraulic systems because it is inexpensive, fireproof, and does not harm the environment.
    Transmit energy.
    The main purpose of the fluid in any system is to transmit energy. Electric, internal combustion, steam powered, or other prime movers drive a pump that sends oil through lines to valves that control actuators. The fluid in these lines must transmit the prime movers energy to the actuator so it can perform work. The fluid must flow easily to reduce power losses and make the circuit respond quickly.

    Lubricate.
    In most hydraulic systems, the fluid must have good lubrication
    qualities. Pumps, motors, and cylinders need ample lubrication to make them efficient and extend their service life. Mineral oils with anti-wear additives work well and are available from most suppliers. Some fluids may need special considerations in component design to overcome their lack of lubricity. Seal.
    Fluid thickness can be important also because one of its requirements is for sealing. Almost all pumps and many valves have metal to metal sealing fits that have minimal clearance but can leak at elevated pressures. Thin watery fluid can flow through these clearances, reducing efficiency and eroding the mating surfaces. Thicker fluids keep leakage to a minimum and efficiency high.
    There are several areas that apply to specifying fluids for a hydraulic circuit. Viscosity is the measure of the fluids thickness. Hydraulic oils thickness is specified by a SUS or SSU designation, similar to the SAE designation used for automotive fluids. SUS stands for Saybolt Universal Seconds (or as some put it, Saybolt Seconds Universal). It is a measuring system set up by a man named Saybolt. Simply stated, the system takes a sample of fluid, heats it to 100° F, and them measures how much fluid passes through a specific orifice in a certain number of seconds.
    Viscosity is most important as it applies to pumps. Most manufacturers specify viscosity limits for their pumps and it is best to stay within the limits they suggest. The prime reason for specifying a maximum viscosity is that pressure drop in the pump suction line typically is low and if the oil is too thick, the pump will be damaged due to cavitation. A pump can move fluid of any viscosity if the inlet is amply supplied. On the other end, if fluids are too thin, pump bypass wastes energy and generates extra heat. All other components in the circuit could operate on any viscosity fluid because they only use what is fed to them. However, thicker fluids waste energy because they are hard to move. Thin fluids waste energy because they allow too much bypass.
    Viscosity index (or VI) is a measure of viscosity change from one temperature to another. It is common knowledge that heating any oil makes it thinner. A normal industrial hydraulic circuit runs at temperatures between 100° and 130° F. Cold starts could be as low as 40° to 50° F. Using an oil with a low VI number might start well but wind up with excessive leakage and wear or cause cavitation damage at startup and run well at temperature. Most industrial hydraulic oils run in the 90- to 105-VI range and are satisfactory for most applications.
    Pour point is the lowest temperature at which a fluid still flows. It should be at least lower than the lowest temperature to which the system will be exposed so the pump can always have some lubrication. Consider installing a reservoir heater and a circulation loop on circuits that start or operate below 60° F.
    Refined mineral oil does not have enough lubricating qualities to meet the needs of modern day hydraulic systems. Several lubricity additives to enhance that property are added to mineral oil as a specific manufacturers package. These additives are formulated to work together and should not be mixed with others additives because some components may be incompatible.
    Refined mineral oil also is very much affected by temperature change. In its raw state it not only has low lubricity but also would thin out noticeably with only a small increase in temperature. Viscosity modifiers enhance the oils ability to remain at a workable viscosity through a broad temperature range.
    There are several causes of hydraulic oil oxidation. These include contamination, air, and heat. The interaction of these outside influences cause sludge and acids to form. Oxidation inhibitors slow or stop the fluids degradation and allow it to perform as intended.
    Wear inhibitors are additives that bond with metal parts inside a hydraulic system and leave a thin film that reduces metal-to-metal contact. When these additives are working, they extend part life by reducing wear.
    In most hydraulic systems, fast and turbulent fluid flow can lead to foaming. Anti-foaming agents make the fluid less likely to form bubbles and allow those that do form to dissipate more rapidly.
    Moisture in the air can condense in a hydraulic reservoir and mix with the fluid. Rust inhibitors negate the effect of this unwanted water and protect the surfaces of the systems metal components. All of these additives are necessary to extend system life and improve reliability.
    Overheating the fluid can counteract the additives and decrease system efficiency. Overheating also thins the oil and reduces efficiency because of internal bypassing. Clearances in pump and valve spools let fluid pass as pressure increases, causing more heating until the fluid breaks down. External leaks through fittings and seals also increase as fluid temperatures rise. Another problem caused by overheating is a breakdown of some seal materials. Most rubber compounds are cured by controlled heat over a specific period of time. Continued heating inside the hydraulic system over long periods keeps the curing process going until the seals lose their resiliency and their ability to seal. It is best if hydraulic oil never exceeds 130° F for any extended period. Installing heat exchangers is the most common cure for overheating but designing heat out of a circuit is the better way.
    Cold oil is not a problem as far as the oil is concerned but cooling does increase viscosity. When viscosity gets too high, it can cause a pump to cavitate and damage itself internally. Thermostatically controlled reservoir heaters easily eliminate this problem in most cases.
    Fire-resistant fluids
    Certain applications must operate near a heat source with elevated temperatures or even open flames or electrical heating units. Mineral oil is very flammable. It not only catches fire easily but will continue to burn even after removing the heat source. This fire hazard situation can be eliminated by several different choices of fluids. These fluids are not fireproof, only fire-resistant, which means they will burn if heated past a certain temperature but they will not continue to burn after removing them from the heat source.
    Generally, the fire-resistant fluids do not have the same specifications as mineral oil-based fluids. Pumps often must be down rated because the fluids lubricity or specific gravity is different and would shorten the pumps service life drastically at elevated pressures or high rotary speeds. Some fire-resistant fluids are not compatible with standard seal materials so seals must be changed. Always check with the pump manufacturer and fluid supplier before using or changing to a fire-resistant fluid.
    Water
    Originally, hydraulic circuits used water to transmit energy (hence the word hydraulics). The main problem with water-filled circuits was either low-pressure operation or very expensive pumps and valves to operate with this low viscosity fluid above 500 to 600 psi. When huge oil deposits were discovered, mineral oil replaced water because of its additional benefits. Water made a brief comeback during an oil shortage crisis but quickly succumbed when oil flowed freely again.
    In the late 90s, water again made inroads into oil-hydraulic systems. Several companies have developed reliable pumps and valves for water that operate at 1500 to 2000 psi. There are still limitations (such as freezing) to using water, but in certain applications it has many benefits. One big advantage is that there are fewer environmental problems during operation or in disposing of the fluid. Price also is a factor because water costs so little and is readily available almost anywhere.
    Some suppliers are making equipment that operates on seawater to eliminate possible contamination of the earths potable water sources. These systems operate at elevated pressures without performance loss.
    High water-content fluids
    Some types of manufacturing still use water as a base and add some soluble oil for lubrication. This type of fluid is known as high water-content fluid (or HWCF). The common mixture is 95% water and 5% soluble oil. This mixture takes care of most of the lubricity problems but does not address low viscosity concerns. Therefore, systems using HWCF still need expensive pumps and valves to make them efficient and extend their life.
    Rolling mills and other applications with molten metals are one area where HWCF is prevalent. Often the soluble oil is the same compound used for coolant in the metal-rolling process. This eliminates concerns about cross-contamination of fluids and the problems it can cause.
    Water-in-oil emulsions
    Some systems use around 40% water for fire resistance and 60% oil for lubrication and viscosity considerations. Again, these are not common fluids because they require special oil and continuous maintenance to keep them mixed well and their ratio within limits. Most manufacturers do not want the problems associated with water-in-oil emulsions so their use is very limited.
    Water glycol
    A very common fire-resistant fluid is water glycol. This fluid uses water for fire resistance and a product like ethylene glycol (permanent anti-freeze) for lubricity, along with thickeners to enhance viscosity. Ethylene glycol will burn, but the energy it takes to vaporize the water present quickly quells the fire once it leaves the heat source. This means a fire would not spread to other parts of the plant. Always remember fire-resistant not fireproof.
    Water glycol fluids are heavier than mineral oil and do not have its lubricating qualities, so most pump manufacturers specify reduced rpm and lower operating pressures for water glycol. In addition, the water in this fluid can evaporate, especially at elevated temperatures, so it must be tested regularly for the correct mixture.
    Cost is also a consideration. Water glycol is more expensive than oil and requires most of the same considerations when disposing of it.
    Always check with the pump manufacturer before specifying water glycol fluid to see what changes are necessary to run the pump with this fluid. Seal compatibility is usually not a problem, but always check each manufacturers specifications before implementing this fluid. In addition, it is imperative to completely flush a system of any other fluids before refilling with water glycol.
    Synthetics
    The other main fire-resistant fluids are synthetic types. They are made from mineral oil, but have been processed and contain additives to obtain a much higher flash point. It takes more heat to start them burning but there is not enough volatile materials in them to sustain burning. These fluids may catch fire from a pot of hot metal but quickly self-extinguish after leaving the heat source.
    Synthetic fluids retain most of the qualities of the mineral oil from which they are derived, so most hydraulic components specify no operating restrictions. However, most of these fluids are not compatible with common seal materials so seal specification changes are usually necessary. Special consideration must be given to handling of synthetics because they can cause skin irritation and other health hazards. Also most synthetic fluids require protective epoxy paint for all components in contact with them.
    Of all the fluids discussed, synthetics are the most expensive. They can cost up to five times more than mineral oil.
    No matter which fluid is chosen, design the circuit to work in a reasonable temperature range; install good filters and maintain them; and check the fluids regularly to see if they are within specification limits.
    A good operating temperature range is between 70° and 130° F with the optimum being around 110° F. A rule of thumb would be: warm enough to feel hot to the touch but cool enough to hold tightly for an extended period. Overheating hydraulic fluids is second only to contamination when it comes to reasons for fluid failure.
    Continuous filtration of any hydraulic system is necessary for long component life. Fluids seldom wear out but they can become so contaminated that the parts they drive can fail. (The filter section of this book offers some good recommendations on keeping fluid clean.)
    Even with the best of care, any hydraulic fluid should be checked at least twice a year. Systems located in dirty atmospheres may need to be checked more often to see if a pattern exists that requires special consideration. Pay close attention to the sampling process and packaging procedures recommended by the test facility that will process the sample. Expect a report on the level of contamination plus an analysis of the additive contents, water content, ferrous and non-ferrous material amounts, and any other problem areas the test facility finds. Use this information to know when to change fluids and to check for abnormal part wear problems.
    [​IMG]
    Fig. 2-1. Filter cart (used to transfer hydraulic fluids) and its circuit schematic diagram New oil or other fluids from the supplier are not necessarily clean. The fluids are shipped in drums or by bulk, and there is no way of knowing how clean these containers are. Some suppliers offer filtered oil with a guaranteed contamination level at added cost. Otherwise, about the lowest level of contamination from most manufacturers is 25 microns.
    Anytime a system needs new fluid, it is best to use a transfer unit, Figure 2-1, with a 10-micron or finer filter in the loop. Another way of filtering new or refill fluid is with a filter permanently attached to the reservoir, Figure 2-2. In this arrangement, the breather or other possible fill points should be made inaccessible.


    [​IMG]
    Fig. 2-2 Hydraulic power unit and circuit diagram of its filter arrangement The filter cart shown in Figure 2-1 can also be used to filter any hydraulic unit in the plant. Instead of this filter unit sitting idle except when filling systems, set it up at a machines power unit for a timed run. Place the suction hose in one end of the reservoir and the return hose in the opposite end. This adds a continual filtration loop to any machine even when the machines main pump is shut off. Run the cart until the fluid is clean and then move is to another power unit. Repeating this process on a regular schedule can assist the hydraulic units filters and add extra life to the fluid and the hydraulic components. This process may also show a pattern on machines that have a contamination problem.

    Hydraulic fluids should be stored in a clean dry atmosphere. Keep all containers closed tightly and reinstall covers on any partially used drums.
    Never mix fluids in any hydraulic system. Make sure all containers are clearly marked and segregated so fluids will not be mixed with one another. Mixing fluids can result in damage to components and some combinations are very difficult to clean up. Be especially careful when mineral oils and synthetic or water-glycol fluids are used in different parts of the same plant.
    Fluids are the lifeblood of any hydraulic system and should be given the utmost care.
     

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