Lecture 32 MAINTENANCE OF FLUID POWER SYSTEMS

Transcription

1 Lecture 32 MAINTENANCE OF FLUID POWER SYSTEMS Learning Objectives Upon completion of this chapter, the student should be able to: List the most common causes of hydraulic system breakdown. Explain the importance of cleanliness of hydraulic systems. Explain the problems caused by air in hydraulic systems. List the causes and remedy for excessive noise, incorrect flow, pressure and faulty operations. Describe the various general safety rules for electricity and electronics. List possible faults in solenoid valves. Explain the method for maintaining and disposing of fluids. 1.1 Introduction The working medium in hydraulic systems is a fluid. Till the early 20th century, water was used as a fluid. Water as a working fluid had many drawbacks, such as low freezing point, corrosive (rust formation) nature and poor lubrication characteristics. Gradually, various oil-based fluids that had the desirable properties were developed for use in hydraulic systems. In a hydraulic system, a hydraulic fluid has to perform various functions, such as follows: To transmit power, which is the primary function? To lubricate various moving parts, so as to avoid metal-to-metal contact, and reduce wear and noise. To carry the heat generated in the system due to friction between moving parts and moving fluid, and to dissipate to the environment either through a suitable heat exchanger or through the reservoir. To perform these functions and make the system work efficiently, a hydraulic fluid must be clean and should possess certain properties.a hydraulic system is fairly easy to maintain: the fluid provides a lubricant and protects against overload. But like any other mechanism, it must be operated properly. You can damage a hydraulic system by too much speed, too much heat, too much pressure or too much contamination. The following is a list of most common causes of hydraulic system breakdown: Clogged or dirty oil filters. Inadequate supply of oil in the reservoir. Leaking seals. Loose inlet lines that cause the pump to take in air. Incorrect type of oil. Excessive oil temperature. Excessive oil pressure. Most of these and similar kinds of problems can be eliminated if a plant-preventive maintenance program is undertaken. This starts with the fluid power designer in the selection of high-quality, properly sized components. It is important for the total system to provide easy access to components requiring periodic inspection such as filters, strainers, sight gauges, drain and fill plugs, flow meters, and pressure and temperature gauges. 1

2 Over half of all hydraulic system problems have been traced directly to the oil. The test kit may be used on the spot to determine whether fluid quality permits continued use. Test that can be performed include the determination of viscosity, water content and particulate contamination level. Viscosity is measured using a visage viscosity comparator. Water content is determined by the hot-plate method. Contamination is eliminated by filtering a measured amount of hydraulic fluid, examining the particles caught on the filters under microscope and comparing what is seen with the series of photos indicating contamination levels. For preventive maintenance techniques to be truly effective, it is necessary to have a good report and record system. This report should include the following: The types of symptoms encountered, how they were detected and the date. The description of the maintenance performed. This should include the replacement of parts, the amount of downtime and the date. Records of dates when oil was tested added or changed. Dates of filter changes should also be recorded. Proper maintenance reduces hydraulic troubles. By caring for the system using a regular maintenance program, we can eliminate common problems and anticipate special ones. 1.2 The Importance of Cleanliness Cleanliness is the first requirement when it comes to servicing hydraulic systems. Keep dirt and other contaminants out of the system. Small particles can score valves, seize pumps, clog orifices and cause expensive repair jobs. How to keep the hydraulic system clean? Let us put it this way: Keep the oil clean. Keep the system clean. Keep your work area clean. Be careful when you change or add oil. 1.3 Importance of Oil and Filter Changes We cannot get peak performance out of a hydraulic system that is not clean. Despite all the precautions taken when working with the hydraulic system, some contaminants get into the system anyway. Good hydraulic oils hold these contaminants in suspension and filters collect them as the oil passes through. Good hydraulic oil contains many additives that work to keep contaminants from damaging or plugging the system. However, these additives lose their effectiveness after a period of time. Therefore, oil should be changed at the recommended intervals to make sure that the additives do their job. The system filters can absorb only a limited amount of dirt particles and other contaminants from the oil. After that the filters stop working. At this point, the filters should be cleaned or replaced with new ones so that the cleaning process can be maintained. 2

3 1.3.1 Draining the System Periodic draining of the entire hydraulic system is very important. This is the only positive way to completely remove contaminants, oxidized fluid and other substances from the system. The machine operator s manual tells the method to be used, and the frequency, depending on conditions Cleaning and Flushing the System In some hydraulic systems, there might be deposits left in the system. It is advisable to clean and flush the system after draining the oil out. After draining the system, clean any sediment from the reservoir and clean or replace the filter elements. Flush out the old oil remaining in the system after draining, particularly if the oil is badly contaminated. Drain out the flushing oil and refill the system with clean hydraulic oil of the recommended type. Be sure to clean or replace the system filters before refilling the system Filling the System Before filling the system, be sure the area around the filler cap is clean. Fill the reservoir to the specified level with the recommended hydraulic oil. Use only clean oil and funnels or containers, and then be sure to replace the filler cap before operating the equipment Preventing Leaks There are internal and external leakages. Internal leakage does not result in actual loss of oil but it does reduce the efficiency of the system. External leakage does result in direct loss of oil and can have other undesirable effects as well. A hydraulic system should always be monitored for leaks and remedial actions should be taken immediately Preventing Overheating Heat causes hydraulic oil to breakdown faster and lose its effectiveness. This is why cooling of the oil is needed. In many systems, enough heat is dissipated through the lines, the components and the reservoir to keep the oil fairly cool. But on high-pressure, high-speed circuits, oil coolers are needed to dissipate the extra heat. To help prevent overheating, keep the oil at the proper level; clean dirt and mud from lines, reservoirs and coolers; check for dented and kinks lines; and keep relief valves adjusted properly. Also be careful to not over speed or overload the system and never hold the control valve in power position longer than necessary. 1.4 Problems Caused By Gases in Hydraulic Fluids Gases can be present in a hydraulic fluid (or any other fluid) 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 a spongy and unstable operation of hydraulic actuators. 3

4 1.4.2 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 be created when the pressure drops below the vapor pressure of the hydraulic fluid. When this happens, the bubbles of hydraulic fluid are created within the fluid stream. Entrained gas can cause cavitation problems in pumps and valves. These gases can greatly reduce the effective bulk modulus of hydraulic fluids, resulting in spongy and unstable operation of hydraulic actuators. Vapor pressure is defined as the pressure at which a liquid starts to boil and thus begins changing into a vapor. The vapor pressure of a hydraulic fluid (or any other liquid) increases with an increase in temperature. Petroleum-based and fire-resistant phosphate ester fluids have very low vapor pressures. Cavitation occurs because the vapor bubbles collapse as they are exposed to the high pressure at the outlet port of the pump, creating extremely high local fluid velocities. This high -velocity fluid impacts internal metal surfaces of the pump. The resulting high impact forces cause flaking or pit ting the surfaces of internal components such as gears, vanes, etc. Cavitation also interferes with the lubrication of mating moving surfaces and thus produces increased wear. One indication of cavitation is a loud noise emanating from the pump. The rapid collapsing of gas bubbles produces vibrations of pump components, which are transmitted into pump noise. Cavitation also causes a decrease in the pump flow rate because the pumping chambers do not completely fill with the hydraulic fluid. As a result, the system pressure becomes erratic Dissolved Air Dissolved air is in the solution and thus cannot be seen and does not add to the volume of the hydraulic fluid. Hydraulic fluids can hold an amazingly large amount of air in the solution. A hydraulic fluid, as received at atmospheric pressure, typically contains about 6% of dissolved air by volume that increases to 10% when pumped. Dissolved air creates no problem in hydraulic systems as long as the air remains dissolved. However, if the dissolved air comes out of the solution, it forms bubbles in the hydraulic fluid and thus becomes entrained air. The following will help control or eliminate pump cavitation by keeping the suction pressure above the vapor pressure of the fluid: Keep suction velocities below 1.5 m/s. Keep pump inlet lines as short as possible. Minimize the number of fittings in the pump inlet line. Mount the pump as close as possible to the reservoir. Use low-pressure drop-pump inlet filters or strainers. Use a properly designed reservoir that can remove entrained air from the fluid before it enters the pump inlet line. Use proper oil as recommended by the manufacturer. Keep the oil from exceeding the recommended maximum temperature level. 4

5 1.5 Troubleshooting Guides The following troubleshooting guides are arranged in five main categories. The heading of each is a symptom that indicates some malfunction in the system. The causes and remedies are given in Tables Fluid Maintenance Fluid maintenance can be done in the following ways: Before opening a drum, clean the top and the bung Use only clean containers and hoses to transfer the hydraulic fluid. Provide a 200-mesh screen in the reservoir filter pipe. Store drums indoor or under a roof In-Operation Care of Hydraulic Fluid This can be done in the following ways: Keep the system tight and repair all leaks immediately. Use proper air and fluid filtration. Establish fluid change intervals. Keep the reservoir filled properly to take the advantage of its heat-dissipating characteristics and prevent moisture from condensing on inside walls. Table 1.1 Excessive noise Symptom Cause Remedy Pump noisy Cavitation Any or all of the following: Replace dirty filters. Wash strainers. Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Change to proper pump drive motor speed. Overhaul or replace the pump. Check fluid temperature. Air in fluid Coupling misaligned Pump worn or damaged Motor noisy Coupling misaligned Any or all of the following: Tighten leaky inlet connections. Fill the reservoir to proper level. Bleed air from the system. Replace the pump shaft seal. All of the following: Align unit. Check the condition of seals, bearings and couplings. Overhaul or replace defective parts All of the following: Align unit. Check the condition of seals, bearings and couplings. 5

6 Motor or Overhaul or replace defective parts Relief valve noisy coupling worn or Setting too low or too Install and adjust pressure gauge Table 1.2 Excessive close to heat Symptom Cause Remedy Pump heated Fluid heated See symptom fluid heated Cavitation Any or all of the following: Replace dirty filters. Wash strainers. Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Change to proper pump drive motor speed. Overhaul or replace the pump. Check fluid temperature. Air in fluid Excessive load Any or all of the following: Tighten leaky inlet connections. Fill the reservoir to proper level. Bleed air from the system. Replace the pump shaft seal. All of the following: Align unit. Check the condition of seals, bearings and couplings. Locate and correct mechanical binding. Check for workload in excess of circuit design. Pump worn or Overhaul or replace defective parts damaged Relief or unloading Install and adjust pressure gauge Motor heated valve set too Fluid high heated See symptom fluid heated Relief or unloading Install and adjust pressure gauge valve set too Excessive high loading All of the following: Align unit. Relief valve heated Motor or coupling worn or damaged Fluid heated Valve setting incorrect 6 Locate and correct mechanical binding. seals, bearings and couplings. Check for workload in excess of circuit design. Overhaul or replace defective parts See symptom fluid heated Install and adjust pressure gauge

7 Fluid heated Worn or damaged valve System pressure too high Unloading valve set too Fluid high dirty or low supply Incorrect fluid viscosity Faulty fluid cooling system Overhaul or replace defective parts Install and adjust pressure gauge Install and adjust pressure gauge Change filters. Check system fluid viscosity, change if necessary. Fill the reservoir to proper level. Change filters. Check system fluid viscosity, change if necessary. Fill the reservoir to proper level. Clean the cooler and/or strainer. Replace the cooler control valve. Repair or replace the cooler. Worn pump, valve, motor, cylinder or other Overhaul or replace defective parts Table 1.3 Incorrect flow Symptom Cause Remedy No flow Pump not receiving fluid Any or all of the following: Replace dirty filters. Clean the clogged inlet line. Clean the reservoir breather vent. Change the system fluid. Overhaul or replace the pump. Pump drive motor not operating Overhaul 7

8 Pump to drive coupling sheared Pump drive motor turning in the wrong direction Directional control set in the wrong direction Check for the damaged pump Replace and align coupling. Reverse rotation Any or all of the following: Check the position of manually operated controls. Check the electrical circuit on solenoid- operated controls. Repair or replace pilot pressure pump. Entire flow passing over the relief valve Damaged pump Adjust part Check for the damaged pump Replace and align coupling. Incorrectly assembled pump Excessive flow Flow control set too high Adjust part Yoke actuating device inoperative (variable displacement Rotation per minute pumps) (RPM) of pump Replace with correct unit drive incorrect Improper size pump used for Replace with correct unit Low flow replacement Flow control set too low Adjust part Relief or unloading valve set too low Adjust part Flow bypassing through the partially or any open valve or all of the following: Check the position of manually operated controls Check the electrical circuit on solenoid- operated controls. Repair or replace the pilot pressure pump. External leak in the system Yoke actuating device inoperative (variable displacement pump) RPM of pump drive motor incorrect Worn pump, valve motor, cylinder or other Bleed air from the system. Replace with correct unit 8

9 Table 1.4 Incorrect pressure Symptom Cause Remedy No pressure No flop See incorrect flow, symptom no flow Low pressure Pressure relief path exists See incorrect flow, symptom no flow and low flow Pressure-reducing valve set too low Adjust part Pressure-reducing valve damaged Damaged pump, motor or cylinder Erratic pressure Air in fluid Tighten leaky connections. Fill the reservoir to proper level. Bleed air from the system. Excessive pressure Worn relief valve Contamination in fluid Accumulator defective or had lost charge Worn pump, motor or cylinder Pressure-reducing, relief or unloading valve misadjusted Yoke actuating device inoperative (variable displacement pumps) Pressure-reducing, relief or unloading valve worn or damaged Replace dirty filters and system fluid Check the gas valve for leakage. Change to correct pressure. Overhaul if defective. Adjust part 9

10 Table 1.5 Faulty operation Symptom Cause Remedy No movement No flow or pressure See incorrect flow Limit or sequence device inoperative or misadjusted Mechanical bind Locate bind and repair No command signal to the servo amplifier Repair command console or interconnecting wires Inoperative or misadjusted Adjust, repair or replace part servo amplifier Inoperative servo valve Worn or damaged cylinder or motor Slow movement Low flow See incorrect flow Fluid viscosity to high Insufficient control pressure for valves No lubrication of machine ways or linkage Misadjusted or malfunctioning servo amplifier Sticking servo valve Lubricate Check fluid temperature. Check system fluid viscosity, change if necessary. See incorrect pressure Adjust, repair or replace part Clean and adjust or replace part. Check the condition of system fluid and filters. Worn or damaged cylinder or motor Erratic movement Erratic pressure See incorrect pressure Air in fluid See excessive noise No lubrication of machine ways Lubricate or linkage Erratic command signal Repair command console or interconnecting wires Misadjustment of Adjust, repair or replace part malfunctioning servo amplifier Malfunctioning feedback transducer Sticking servo valve Clean and adjust or replace part. Check the condition of system fluid and filters. Worn or damaged cylinder or motor Excessive speed or movement Excessive flow See incorrect flow Feedback transducer 10

11 1.6 General Safety Rules for Electricity and Electronics Following are the general safety rules for electricity and electronics: Use approved tools, equipment and protective devices. Avoid wearing rings, bracelets and similar meal items when working around exposed electric circuits. Never assume that a circuit is OFF. Double check it with an instrument that is supposed to be surely operational. Some situations require a buddy system to guarantee that power would not be turned ON Never tamper with or try to override safety devices such as interlock (a type of switch that automatically removes power when a door is opened or a panel removes). Keep tools and test equipment clean and in good working condition. Replace insulated probes and leads at the first sign of deterioration. Some devices, such as capacitors, can store a lethal charge. They may store this charge for long periods of time. It must be certain that these devices are discharged before working around them. Do not remove grounds and do not use adapters that defeat the equipment ground. Use protective clothing and safety glasses when handling high vacuum devices such as picture tubes and cathode ray tubes. Do not work on equipment before knowing proper procedures and having awareness of any potential safety hazards Solenoid Valves Pneumatics will continue to dominate the power section and retain its positions in the control section. On the other hand, it is impossible to deny the advances made by electronic controls, systems and components due to the following advantages: Low power consumption. Short switching times. Higher contact ratings. Long service life. Maintenance-free operation Possible Faults in Solenoid Valves Following are the possible faults: Directed short (at power supply, electrical bus and load): A direct short is when too much current is sent back to the power supply overloading it, generally blowing a fuse. Cross short: A cross short is created by one or more wires (cables) bypassing the load causing a direct short to occur. High resistance connections (too many connections at the terminal eye). Low voltage or over voltage at the solenoid. Corrosion. Partially or fully blocked hoses. 15

12 Wire connections are open internally Lack of source pressure (at the compressor or on the service unit). Sticking spool. Diaphragm not working. Exhaust ports blocked. Gaskets mounted incorrectly. Faults caused by wear or external influences. Caution: Short circuiting of the power supply is not recommended without the installation of a circuit breaker to protect the equipment and the user. Causes and remedy for troubleshooting for direct shorts and faults in relay coil is given in Tables 1.6 and 1.7. Table 1.6 Troubleshooting for direct shorts Fault Cause Remedy When the push button is pressed, solenoid Y 1 is not activated and the circuit breaker Relay coil K 1 has been shorted directly to the ground bypassing the load (relay coil K 1 ) must be reset. When proximity limit switch a 1 and pressure switch a 2 are activated, solenoid Y 3 is not activated and the circuit breaker must be reset. Solenoid Y 1 has been shorted directly to ground bypassing the load (solenoid coil). This causes the relay coil to buzz (noise like a bee) and the circuit breaker must be reset. Relay coil K 2 has been shorted directly to ground, bypassing the load (relay load). Solenoid Y 3 has been shorted directly to ground, bypassing the load (solenoid). The circuit breaker must be reset. Pull the ground cable connected to solenoid Y 1, retry the circuit. If the power supply continues to short, the location of the direct short is at relay coil K 1. If not, the direct short is at solenoid Y 1. Pull the ground cable connected to solenoid Y 3. Retry the circuit. If power supply continues to short, the location of the direct short is at relay coil K 2. If not, the direct short is at solenoid Y 3. 16

13 Table 1.7 Faults in relay coils Fault Cause Remedy When powered, the relay coil does not function. One or more of the wires (leads) have an infinite resistance. Open (infinite resistance) in the coil. Low voltage: Below specification. Use the voltmeter to check the potential difference across each cable and the push button when switch is open, then closed. If the problem is with any of these components Relay coil is not energized by electrical limit switch (or proximity limit switch). The electrical limit switch is not being activated. One of the cables (wires) is either not connected or there is infinite resistance in the cable (wire). The relay coil itself is malfunctioning, that is, low voltage, an open (infinite resistance) circuit, loose connection, high-resistance connection. Visually examine the electrical limit switch to make sure that the roller is fully activated. If not, reposition the sensor so that physical contact is achieved (or see LED). Use the voltmeter to test the difference in potential across the limit switch (24 V when open, 0 V when closed); replace if not functioning or remove the limit switch from the circuit and use the ohmmeter to measure the resistance of the limit switch (infinite pressure when open, approximately 0 when closed). Use the multimeter to check the difference in potential across each cable (wire) in the corresponding ring. Then remove the suspect cable(s) and use the ohmmeter to confirm your findings, replace the cables if required. If the voltmeter identifies low voltage, then check the power supply and original source. Replace or modify the source as required. If an open infinite resistance, loose or high connection is suspected, use the ohmmeter to determine the exact location of the problem and repair/replace as required. 15

14 1.7 Maintaining and Disposing of Fluids Controlling pollution and conserving natural resources are important goals to achieve for the benefit of society. Thus, it is important to minimize the generation of waste hydraulic fluids and to dispose them in an environmentally friendly manner. The following are some recommendations that should be adhered to strictly for properly maintaining and disposing hydraulic fluids: Select the optimum fluid for the application involved. This includes the consideration of the system operating pressures and temperatures as well as the desired fluid properties. Utilize a well-designed filtration system to reduce contamination and increase the useful life of the fluid. Filtration should be continued and filters should be changed at regular intervals. 3. Follow a proper storage procedure for the unused fluid supply. For example, outdoor storage is not recommended, especially if the fluid is stored in drums as it is affected by increment weather and resulting weakening of drum seams may occur and cause leakage and contamination. Fluids from the storage containers to the hydraulic systems should be transported carefully as the chances of contamination depend to a large extent on handling. The transfer containers should be covered when not in use. Operating fluids should be checked regularly for viscosity, acidity, bulk modulus, specific gravity, water content, additive levels and particle contamination. The entire hydraulic system, including pumps, piping, filters, actuators and reservoir, should be maintained according to the manufacturer s specifications. Corrective action should be taken to reduce or eliminate leakage from operating hydraulic systems. Typically leakage occurs due to worn seals or loose fittings. A preventive maintenance program should be implemented to ensure ideal operating conditions and reduce contamination, leakage, etc. Fluids must be disposed properly because a hydraulic fluid is considered to be a waste material when it has deteriorated to the point where it is no longer suitable for use in hydraulic systems. The various environmental government agencies also suggest against mixing hazardous wastes with waste hydraulic fluids being disposed. It is also not allowed to burn these waste fluids in non-industrial boilers. Pollution control and conservation of natural resources are critical environmental issues for society. Properly maintaining and disposing of fluids not only protects the environment but also conserves our natural resources. 16

15 Objective-Type Questions Fill in the Blanks 1. The primary function of a hydraulic fluid is to transmit. 2. Over half of all hydraulic system problems have been traced directly to the. 3. Entrained gas can also occur when the pressure drops below the of the hydraulic fluid. 4. Cavitation occurs because the vapor bubbles collapse as they are exposed to the pressure at the outlet port of the pump, creating extremely high local fluid velocities. 5. Oxidation is caused by the chemical reaction of oxygen from the air with particles of. State True or False 1. Dissolved air creates no problem in hydraulic systems as long as the air remains dissolved. 2. Most of fire-resistant fluids are compatible with most natural or synthetic rubber seals. 3. The neutralization number is a measure of the relative acidity. 4. A 10 µm filter is one capable of removing contaminants as small as 10 µm in size. 5. Free air can cause the hydraulic fluid to possess a much lower stiffness. Review Questions 1. Name five reasons for the overheating of the fluid in a hydraulic system. 2. Name four causes of low or erratic pressure. 3. What three devices are commonly used in the troubleshooting of hydraulic circuits? 4. Name five of the most common causes of hydraulic system breakdown. 5.List eight recommendations that should be followed for properly maintaining and disposing hydraulic fluid. 6.Name two items that should be included in reports dealing with a maintenance procedure. 7. Name the three ways in which a hydraulic fluid becomes contaminated. 8. Name five things that can cause a noisy pump. 9.Name four causes of low or erratic pressure. 10.Name four causes of no pressure. 11.If an actuator fails to move, name five possible causes. 12.If an actuator has slow or erratic motion, name five possible causes. 13. Why is loss of pressure in a hydraulic system not a symptom of pump malfunction? 15

16 Answers Fill in the Blanks 1. Power 2. Oil 3. Vapor pressure 4. High 5. Oil State True or False 1. True 2. False 3. True 4. True 5. True 16