Unit: 5 MAINTENANCE OF HYDRAULIC SYSTEMS

Transcription

1 Unit: 5 MAINTENANCE OF HYDRAULIC SYSTEMS 5.1 INTRODUCTION The following is a list of the most common causes of hydraulic system breakdown: 1) Clogged or dirty oil filters 2) Inadequate supply of oil in the reservoir 3) Leaking seals 4) Loose inlet lines that cause the pump to take in air 5) Incorrect type of oil 6) Excessive oil temperature 7) Excessive oil pressure Most of these kinds of problems can be eliminated if a planned preventive maintenance program is undertaken. This starts with the fluid power designer in the selection of high quality, properly sized components. The next step is the proper assembly of the various components. This includes applying the correct amount of torque to the various tube fittings to prevent leaks 5.2 GENERAL TYPE OF FLUIDS The broad tasks of hydraulic oil can be classified broadly as follows: 1) to transfer hydraulic energy 2) to lubricate all parts 3) to avoid corrosion 4) to remove impurities and abrasion 5) to dissipate heat There are innumerable types of materials in use as hydraulic fluids. These range from water to inorganic salt solutions to water oil emulsions, synthetic and naturally occurring organic materials. Though water was the first hydraulic fluid and was used during the early stages of Industrial Revolution, petroleum based hydrocarbon type fluids are widely used today. There are basically four different types of fire-resistant hydraulic fluids in common use: 1) Water-glycol solutions: This type consists of an actual solution of about 40% water and 60% glycol. These solutions have high viscosity index values, but the viscosity rises as the water evaporates. However, metals such as zinc, cadmium, and magnesium react with water-glycol solutions and therefore should not be used. In addition, special paints must be used. 2) Water- in-oil emulsions: This type consists of about 40% water completely dispersed in a special oil base. It is characterized by the small droplets of water completely surrounded by oil. The water provides a good coolant property but tends to make the fluid more corrosive. Thus, greater amounts of corrosion inhibitor additives are necessary. Water-in-oil emulsions are compatible with most rubber seal materials found in petroleum-based hydraulic systems. 3) Straight synthetics: This type is chemically formulated to inhibit combustion and in general has the highest fire-resistant temperature. Typical fluids in this category are the phosphate esters or chlorinated hydrocarbons. Disadvantages of straight synthetics include low viscosity index, incompatibility with most natural or synthetic rubber seals, and high costs. 4) High-water-content fluids: This type consists of about 90% water and 10% concentrate. The concentrate consists of fluid additives that improve viscosity, lubricity, rust protection, and protection against bacteria growth. Advantages of high-water-content fluids include high fire resistance, outstanding cooling characteristics, and low cost, which is about 20% of the cost of petroleum-based hydraulic fluids. 5.3 DESIRABLE PROPERTIES 1) Good Lubricity The components in a hydraulic system contain many surfaces which are in close contact and which move in relation to each other. The hydraulic fluid must separate and lubricate such surfaces. Hareesha N G, Lecturer, Don Bosco Institute of Technology 1

2 Protection against wear is a principal reason for selecting a fluid having good lubricating characteristics as a hydraulic medium. 2) Stable Viscosity Characteristics Viscosity is a very important fluid property from the point of view of actual use. Viscosity may be considered as the resistance of the fluid to flow or as a measure internal friction. Viscosity varies with temperature and pressure. Fluids having large changes of viscosity with temperature are commonly referred as low viscosity index fluids and those having small changes of viscosity with temperature are known as high viscosity index fluids. Viscosity is also important with regard to the ability of fluid to lubricate. 3) Stable Chemically and Physically Fluid characteristics should remain unchanged during an extended useful life and during storage. 4) System Compatibility From the design point of view, it is expected that the hydraulic fluid should be inert to those materials used in or near the hydraulic equipment. 5) Good Heat Dissipation An important requirement of the fluid is to carry heat away from the working parts. Pressure drops, mechanical friction, fluid friction, leakages, all generate heat. The fluid must carry the generated heat away and readily dissipate it to the atmosphere or coolers. Therefore high thermal conductivity and high specific heat values are desirable in the fluid chosen. 6) High Bulk Modulus In general, oil is taken as incompressible. However, in practice, all materials are compressible and so is oil. The bulk modulus is a measure of the degree of compressibility of the fluid and is the reciprocal of compressibility. The higher the bulk modulus, the lesser the material will be compressed with increasing pressure. Bulk modulus is an important characteristic of a hydraulic fluid because of control problems, especially in servo hydraulics. 7) Adequate Low-temperature Properties This is an important consideration for hydraulic systems which must operate in outdoors, in low temperature environments or at high altitudes. Low-temperature properties may be described by the pour point or viscosity-temperature characteristics of the fluid. 8) Flash Point The flash point of hydraulic oil is defined as the temperature at which flashes will be generated when the oil is brought into contact with any heated matter, e.g., a heated stick. The fire point is actually the ignition point of the oil. 9) Low Foaming Tendency A liquid has a property to absorb a portion of gas or air with which it comes in contact. Though the accumulation of air is not detrimental when it is within a certain limit, it may create acute problems in proper functioning of the system if the limit is crossed. The ability of a fluid to release air or other gases without the formation of foam is an important characteristic of a hydraulic fluid. Excessive foaming results in loss of fluid if the volume of the hydraulic system is exceeded. 10) Fire Resistant Fire resistance is one of the properties that is optional in a good usable hydraulic fluid. The commonly used hydraulic liquids are petroleum derivatives, and consequently they burn vigorously once they pass the fire point. For critical applications, artificial or synthetic hydraulic fluids are used which have high fire resistance. 11) Prevent Rust Formation Moisture may be present to some extent in hydraulic systems. Moisture and oxygen cause rusting of iron parts in the system. Rust particles can cause abrasive wear of system components and also act as catalyst to increase the rate of oxidation of the fluid. Fluids with rust inhibitors help to minimise rust formation in the system. 12) Low in Volatility The fluid should have a low volatility, i.e. low vapour pressure or high boiling point characteristic. Hareesha N G, Lecturer, Don Bosco Institute of Technology 2

3 13) Good Demulsibility Moisture or water may enter a hydraulic system through contamination or condensation. This water may either dissolve in the fluid or form two layers. Dissolved water may produce corrosion, rusting or sludge in the fluid. Fluids with emulsifiers easily separate the water from its main body. Generally used or contaminated fluids are more likely to emulsify with water than new fluids. 14) Low Coefficient of Expansion A low coefficient of expansion is usually desirable in a hydraulic fluid to minimise the total volume of the system required at the operating temperature. 15) Low Specific Gravity Specific gravity of fluid is of importance only in those cases where the overall system weight must be kept to a minimum. High specific weight means more weight for a given volume of fluid. Heavy fluids can also cause pump cavitation and malfunction. This aspect is important especially in the aircraft industry. 16) Non-toxic, Easy to Handle and Available These characteristics refer to the interaction of the fluid with people who repair, handle, use or pay for the hydraulic system or hydraulic fluid. Obviously, it is desirable that the fluid be as simple to handle and as available and cheap as possible. 5.4 SEALING DEVICES Introduction Oil leakage, located anywhere in a hydraulic system, reduces efficiency and increases power losses. Internal leakage does not result in loss of fluid from the system because the fluid returns to the reservoir. Most hydraulic components possess clearances that permit a small amount of internal leakage. This leakage increases as component clearances between mating parts increase due to wear. If the entire system leakage becomes large enough, most of the pump's output is bypassed, and the actuators will not operate properly. External leakage represents a loss of fluid from the system. In addition, it is unsightly and represents a safety hazard. Improper assembly of pipe fittings is the most common cause of external leakage. Over tightened fittings may become damaged, or vibration can cause properly tightened fittings to become loose. Shaft seals on pumps and cylinders may become damaged due to misalignment or excessive pressure. Seals are used in hydraulic systems to prevent excessive internal and external leakage and to keep out contamination. Seals can be of the positive or non-positive type and can be designed for static or dynamic applications. Positive seals do not allow any leakage whatsoever (external or internal). Non-positive seals (such as the clearance used to provide a lubricating film between a valve spool and its housing bore) permit a small amount of internal leakage. Static seals are used between mating parts that do not move relative to each other. Dynamic seals are assembled between mating parts that move relative to each other. Hence, dynamic seals are subject to wear because one of the mating parts rubs against the seal. The following represent the most widely used types of seal configurations: 1) O-rings 2) Compression packings (V- and U-shapes) 3) Piston cup packings 4) Piston rings 5) Wiper rings Hareesha N G, Lecturer, Don Bosco Institute of Technology 3

4 5.4.2 O-Rings The O-ring is one of the most widely used seals for hydraulic systems. It is a molded, synthetic rubber seal that has a round cross section in its free state. Fig.5.1: O- Ring operation Fig.5.2: Backup ring prevents the extrusion Of O-ring As illustrated in Figure 5.1, an O-ring is installed in an annular groove machined into one of the mating parts. When it is initially installed, it is compressed at both its inside and outside diameters. When pressure is applied, the O-ring is forced against a third surface to create a positive seal. The applied pressure also forces the O-ring to push even harder against the surfaces in contact with its inside and outside diameters. As a result, the O-ring is capable of sealing against high pressures. However, O-rings are not generally suited for sealing rotating shafts or where vibration is a problem. At very high pressures, the O-ring may extrude into the clearance space between mating parts, as illustrated in Figure 5.2. This is unacceptable in a dynamic application because of the rapid resulting seal wear. This extrusion is prevented by installing a backup ring, as shown in Figure 5.2. If the pressure is applied in both directions, a backup ring must be installed on both sides of the O-ring Compression packings: V-ring packings are compression-type seals that are used in virtually all types of reciprocating motion applications. These include rod and piston seals in hydraulic and pneumatic cylinders, press rams, jacks, and seals on plungers and pistons in reciprocating pumps. They are also readily suited to certain slow rotary applications such as valve stems. These packings (which can be molded into U-shapes as well as V-shapes) are frequently installed in multiple quantities for more effective sealing. These packings are compressed by tightening a flanged follower ring against them. Proper adjustment is essential since excessive tightening will hasten wear: Piston Cup Packings Piston cup packings are designed specifically for pistons in reciprocating pumps and pneumatic and hydraulic cylinders. They offer the best service life for this type of application, require a minimum recess space and minimum recess machining, and are simply and quickly installed. Sealing is accomplished when pressure pushes the cup lip outward against the cylinder barrel. The backing plate and retainers clamp the cup packing tightly in place, allowing it to handle very high pressures. Hareesha N G, Lecturer, Don Bosco Institute of Technology 4

5 5.4.5 Piston Rings Piston rings are seals that are universally used for cylinder pistons, as shown in Figure 5.3. Metallic piston rings are made of cast iron or steel and are usually plated or given an outer coating of materials such as zinc phosphate or manganese phosphate to prevent rusting and corrosion. Piston rings offer substantially less opposition to motion than do synthetic rubber (elastomer) seals. Sealing against high pressures is readily handled if several rings are used, as illustrated in Figure 5.3. Fig.5.3: Use of piston rings for cylinder pistons Wiper Rings Wiper rings are seals designed to prevent foreign abrasive or corrosive materials from entering a cylinder. They are not designed to seal against pressure. Figure 5.4 shows a typical installation arrangement. The wiper ring is molded from a synthetic rubber, which is stiff enough to wipe all dust or dirt from the rod. Fig.5.4: Installation arrangement of wiper rings 5.5 RESERVOIR SYSTEM Design and Construction Features The proper design of a suitable reservoir for a hydraulic system is essential to the overall performance and life of the individual components. The reservoir serves the following functions: 1) Used as a storage space for the hydraulic fluid used by the system 2) Used as the principal location where the fluid is conditioned 3) The reservoir is where sludge, water, and metal chips settle 4) Entrained air picked up by the oil is allowed to escape in the reservoir 5) The dissipation of heat is also accomplished by reservoir The reservoir is constructed of welded steel plates. The inside surfaces are painted with a sealer to prevent rust, which can occur due to condensed moisture. The bottom plate is dished and contains a drain plug at its lowest point to allow complete drainage of the tank when required. Removable covers are included to provide easy access for cleaning. A sight glass is also included to permit a visual check of the fluid level. A vented breather cap is also included and contains an air filtering screen. This allows the tank to breathe as the oil level changes due to system demand requirements. In this way, the tank is always vented to the atmosphere. Hareesha N G, Lecturer, Don Bosco Institute of Technology 5

6 As shown in Figure 5.5, a baffle plate extends lengthwise across the center of the tank. Its height is about 70% of the height of the oil level. The purpose of the baffle plate is to separate the pump inlet line from the return line to prevent the same fluid from re-circulating continuously within the tank. In this way all the fluid is uniformly used by the system. Fig.5.5: Baffle plate controls the direction of flow in reservoir Essentially the baffle serves the following functions: 1) Permits foreign substances to settle to the bottom 2) Allows entrained air to escape from oil 3) Prevents localized turbulence in reservoir 4) Promotes heat dissipation through reservoir walls Sizing of Reservoirs The sizing of a reservoir is based on the following criteria: 1) It must make allowance for dirt and chips to settle and for air to escape. 2) It must be able to hold all the oil that might drain into the reservoir from the system. 3) It must maintain the oil level high enough to prevent a whirlpool effect at the pump inlet line opening. Otherwise, air will be drawn into the pump. 4) It should have a surface area large enough to dissipate most of the heat generated by the system. 5) It should have adequate air space to allow for thermal expansion of the oil A reservoir having a capacity of three times the volume flow-rate of the pump has been found to be adequate for most hydraulic systems where average demands are expected. This relationship is given by Reservoir size (m 3 ) = 3 x pump flow-rate (m 3 /min) 5.6 FILTERS AND STRAINERS Introduction Modern hydraulic systems must be dependable and provide high accuracy. This requires highly precision-machined components. The worst enemy of a precision-made hydraulic component is contamination of the fluid. Essentially, contamination is any foreign material in the fluid that results in detrimental operation of any component of the system. Contamination may be in the form of a liquid, gas, or solid and can be caused by any of the following: 1) Built into system during component maintenance and assembly Contaminants here include metal chips, bits of pipe threads, tubing burrs, pipe dope, shreds of plastic tape, bits of seal material, welding beads, bits of hose, and dirt. 2) Generated within system during operation During the operation of a hydraulic system, many sources of contamination exist. They include moisture due to water condensation inside the reservoir, entrained gases, scale caused by rust, bits of worn seal materials, particles of metal due to wear, and sludges and varnishes due to oxidation of the oil. 3) Introduced into system from external environment The main source of contamination here is due to the use of dirty maintenance equipment such as funnels, rags, and tools. Hareesha N G, Lecturer, Don Bosco Institute of Technology 6

7 5.6.2 Strainers The reservoirs help to keep the hydraulic fluid clean. In fact, some reservoirs contain magnetic plugs at their bottom to trap iron and steel particles carried by the fluid. However, this is not adequate, and in reality the main job of keeping the fluid clean is performed by filters and strainers. Filters and strainers are devices for trapping contaminants. Specifically, a filter is a device whose primary function is to retain, by some porous medium, insoluble contaminants from a fluid. Basically, a strainer is a coarse filter. Strainers are constructed of a wire screen that rarely contains openings less than 100 mesh. The screen is wrapped around a metal frame. A strainer removes only the larger particles. Observe that the lower the mesh number, the coarser the screen. Because strainers have low-pressure drops, they are usually installed in the pump suction line to remove contaminants large enough to damage the pump. A pressure gage is normally installed in the suction line between the pump and strainer to indicate the condition of the strainer. A drop in pressure indicates that the strainer is becoming clogged Filters A filter can consist of materials in addition to a screen. Particle sizes removed by filters are measured in micrometers (or microns). The smallest-sized particle that can normally be removed by a strainer is in or approximately 150 microns. On the other hand, filters can remove particles as small as 1 micron. There are three basic types of filtering methods used in hydraulic systems: mechanical, absorbent, and adsorbent. 1) Mechanical. This type normally contains a metal or cloth screen or a series of metal disks separated by thin spacers. Mechanical-type filters are capable of removing only relatively coarse particles from the fluid. 2) Absorbent. These filters are porous and permeable materials such as paper, wood soft tissue, cloth, cellulose, and asbestos. Paper filters are normally impregnated with a resin to provide added strength. In this type of filter, the particles are actually absorbed as the fluid permeates the material. As a result, these filters are used for extremely small particle filtration. 3) Adsorbent. Adsorption is a surface phenomenon and refers to the tendency of particles to adhere to the surface of the filter. Thus, the capacity of such a filter depends on the amount of surface area available. Adsorbent materials used include activated clay and chemically treated paper Location of a filter in hydraulic circuit Figure 5.6 shows the four typical locations where filters are installed in the hydraulic circuit. Figure 5.6: Four common circuit locations for filters Hareesha N G, Lecturer, Don Bosco Institute of Technology 7

8 5.6.5 Beta Ratio of Filters Filters are rated according to the smallest size of particles they can trap. Filter ratings used to be identified by nominal and absolute values in micrometers. A filter with a nominal rating of 10 µm is supposed to trap 95% of the entering particles greater than 10 µm in size. The absolute rating represents the size of the largest opening or pore in the filter and thus indicates the largest-size particle that could pass through the filter. Hence, the absolute rating of a 10 µm nominal size filter would be larger than 10 µm. A better parameter for establishing how well a filter traps particles is called the Beta ratio, or Beta rating. The Beta ratio is determined during laboratory testing of a filter receiving a specified steadystate flow containing a fine dust of selected particle size. The test begins with a clean filter and ends when the pressure drop across the filter reaches a specified value indicating that the filter has reached the saturation point. This is when the contaminant capacity has been reached, which is a measure of the service life or acceptable time interval between filter element changes in an actual operating system. By mathematical definition, the Beta ratio equals the number of upstream particles of greater size than N µm divided by the number of downstream particles greater in size than N µm (as counted during the test), where N is the selected particle size for the given filter. This ratio is represented by the following equation No. upstream particles of size > N µm Beta ratio = No. upstream particles of size > N µm A Beta ratio of 1 would mean that no particles above the specified size N are trapped by the filter. A Beta ratio of 50 means that 50 particles are trapped for every one that gets through the filter. Most filters have Beta ratings greater than 75 when N equals the absolute rating. A filter efficiency value can be calculated using the following equation: No. upstream particles - no. downstream particles Beta efficiency = No. upstream particles Where the particle size is greater than a specified value of N µm Thus, we have the following relationship between Beta efficiency and Beta ratio: 1 Beta efficiency = 1 (Eq.5.1) Beta ratio Hence, a filter with a Beta ratio of 50 would have an efficiency of 1-1/50 = 98%. Note from Equation (5.1), that the higher the Beta ratio the higher the Beta efficiency. The designation B 20 = 50 identifies a particle size of 20 µm and a Beta ratio of 50 for a particular filter. Thus, a designation of B 20 = 50 means that 98% of the particles larger than 20 µm would be trapped by the filter during the time a clean filter becomes saturated. 5.7 PROBLEM CAUSED BY GASES IN HYDRAULIC FLUIDS Gases can be present in a hydraulic fluid (or any other liquid) in three ways: free air, entrained gas, and dissolved air. Free Air Air can exist in a free pocket located at some high point of a hydraulic system (such as the highest elevation of a given pipeline). This free air either existed in the system when it was initially filled or was formed due to air bubbles in the hydraulic fluid rising into the free pocket. Free air can cause the hydraulic fluid to possess a much lower stiffness (bulk modulus), resulting in spongy and unstable operation of hydraulic actuators. Entrained Gas Entrained gas (gas bubbles within the hydraulic fluid) is created in two ways. Air bubbles can be created when the flowing hydraulic fluid sweeps air out of a free pocket and carries it along the fluid stream. Entrained gas can also occur when the pressure drops below the vapor pressure of the hydraulic fluid. When this happens, bubbles of hydraulic fluid vapor are created within the fluid stream. Entrained gases (either in the form of air bubbles or fluid vapor bubbles) can cause cavitation Hareesha N G, Lecturer, Don Bosco Institute of Technology 8

9 problems in pumps and valves. Entrained gases can also greatly reduce the hydraulic fluid's effective bulk modulus, resulting in spongy and unstable operation of hydraulic actuators. Dissolved air Dissolved air creates no problem in hydraulic systems, as long as the air remains dissolved. However, if the dissolved air comes out of solution, it forms bubbles in the hydraulic fluid and thus becomes entrained air. The amount of air that can be dissolved in the hydraulic fluid increases with pressure and decreases with temperature. Thus, dissolved air will come out of solution as the pressure decreases or the temperature increases. To avoid pump cavitation, pump manufacturers specify a minimum allowable vacuum pressure at the pump inlet port based on the type of fluid being pumped, the maximum operating temperature, and the rated pump speed. The following rules will control or eliminate pump cavitation by keeping the suction pressure above the vapor pressure of the fluid: 1) Keep suction line velocities below 4 ft/s (1.2 m/s). 2) Keep pump inlet lines as short as possible. 3) Minimize the number of fittings in the pump inlet line. 4) Mount the pump as close as possible to the reservoir. 5) Use low-pressure drop-pump inlet filters or strainers. 6) Use a properly designed reservoir that will remove the entrained air from the fluid before it enters the pump inlet line. 7) Use proper oil, as recommended by the pump manufacturer. 8) Keep the oil temperature from exceeding the recommended maximum temperature level. 5.8 WEAR OF MOVING PARTS DUE TO SOLID-PARTICLE CONTAMINATION All hydraulic fluids contain solid contaminants (dirt) to one degree or another. Excessive solid contaminants in the hydraulic fluid will cause premature failure of even excellently designed hydraulic systems. One of the major problems caused by solid contaminants is that they prevent the hydraulic fluid from providing proper lubrication of moving internal parts of hydraulic components such as pumps, hydraulic motors, valves, and actuators. As an example, Figure 5.7 shows a hydraulic cylinder having a radial clearance between the bore of the cylinder and the piston's outer cylindrical surface. This figure shows the cylinder bore surface to be worn over a given axial length due to excessive solid particle contamination of the fluid. Such a wear problem often includes a scored piston seal and cylinder bore. This problem typically means that channels are cut through the outer surface of the seal and tiny grooves are cut into the cylinder bore surface. This wear causes excessive internal leakage, prevents the cylinder from positioning accurately, and results in premature cylinder failure. Fig.5.7: Hydraulic cylinder with a bore that is worn due to solid-particle contamination of the fluid. Solid contaminants can be classified by their size relative to the clearance between the moving parts of a hydraulic component, such as the radial clearance between the piston and bore of the hydraulic cylinder of Figure 5.7. There are three relative sizes: smaller than, equal to, and larger than the clearance. All three contaminant sizes can contribute to wear problems. Contaminants that are smaller than the clearance can collect inside the clearance when the hydraulic cylinder is not operating. These contaminants block lubricant flow through the clearance when cylinder actuation is initiated. Contaminants of the same size as the clearance rub against the mating surfaces, causing a Hareesha N G, Lecturer, Don Bosco Institute of Technology 9

10 breakdown in the fluid lubricating film. Large contaminants interfere with lubrication by collecting at the entrance to the clearance and blocking fluid flow between the mating surfaces. In addition to the internal leakage between the piston and cylinder bore, a similar wear and leakage problem can occur around the rod seal of a hydraulic cylinder. This wear produces an external leakage that can create a messy leak as well as become a safety hazard to personnel in the area. The majority of hydraulic system breakdowns are due to excessive contamination of the hydraulic fluid. Wear of moving parts due to this contamination is one of the major reasons for these failures. 5.9 TROUBLESHOOTING HYDRAULIC SYSTEMS Introduction Hydraulic systems depend on proper flow and pressure from the pump to provide the necessary actuator motion for producing useful work. Therefore, flow and pressure measurements are two important means of troubleshooting faulty operating hydraulic circuits. Temperature is a third parameter, which is frequently monitored when troubleshooting hydraulic systems because it affects the viscosity of the oil Viscosity, in turn, affects leakage, pressure drops, and lubrication. The use of flow meters can tell whether or not the pump is producing proper flow. Flow meters can also indicate whether or not a particular actuator is receiving the expected flow-rate Pressure measurements can provide a good indication of leakage problems and faulty components such as pumps, flow control valves, pressure relief valves, strainers, and actuators. Excessive pressure drops in pipelines can also be detected by the use of pressure measurements. A portable hydraulic circuit measures pressure and flow-rate and temperature. By connecting this tester to the hydraulic circuit, a visual means is provided to determine the efficiency of the system and to determine which component in the system, if any, is not working properly. Testing a hydraulic system with this tester consists of the following: 1) Measure pump flow at no-load conditions 2) Apply desired pressure with the tester load valve on each component to find out how much of the fluid is not available for power because it may be a) Flowing at a lower rate because of slippage inside the pump due to worn parts. b) Flowing over pressure relief valves due to worn seats or weak or improperly set springs. c) Leaking past valve spools and seats back into the fluid supply reservoir without having reached the working cylinder or motor. d) Leaking past the cylinder packing or motor parts directly into the return line without having produced any useful work Probable Causes of Hydraulic System Problems When troubleshooting hydraulic circuits, it should be kept in mind that a pump produces the flow of a fluid. However, there must be resistance to flow in order to have pressure. The following is a list of hydraulic system operating problems and the corresponding probable causes that should be investigated during troubleshooting: S. N. Hydraulic System Operating Problems 1 Noisy pump Corresponding Probable Causes a) Air entering pump inlet b) Misalignment of pump and drive unit c) Excessive oil viscosity d) Dirty inlet strainer e) Chattering relief valve f) Damaged pump g) Excessive pump speed h) Loose or damaged inlet line Hareesha N G, Lecturer, Don Bosco Institute of Technology 10

11 2 Low or erratic pressure a) Air in the fluid b) Pressure relief valve set too low c) Pressure relief valve not properly seated d) Leak in hydraulic line e) Defective or worn pump f) Defective or worn actuator 3 No pressure a) Pump turning in wrong direction b) Ruptured hydraulic line c) Low oil level in reservoir d) Pressure relief valve stuck open e) Broken pump shaft f) Full pump flow bypassed to tank due to faulty valve or actuator 4 Actuator fails to move a) Faulty pump b) Directional control valve fails to shift c) System pressure too low d) Defective actuator e) Pressure relief valve stuck open f) Actuator load is excessive g) Check valve in backwards 5 Slow or erratic motion of actuator 6 Overheating of hydraulic fluid a) Air in system b) Viscosity of fluid too high c) Worn or damaged pump d) Pump speed too low e) Excessive leakage through actuators or valves f) Faulty or dirty flow control valves g) Blocked air breather in reservoir h) Low fluid level in reservoir i) Faulty check valve j) Defective pressure relief valve a) Heat exchanger turned off or faulty b) Undersized components or piping c) Incorrect fluid d) Continuous operation of pressure relief valve e) Overloaded system f) Dirty fluid g) Reservoir too small h) Inadequate supply of oil in reservoir i) Excessive pump speed j) Clogged or inadequate-sized air breather Hareesha N G, Lecturer, Don Bosco Institute of Technology 11