Fluid Maintenance. The hydraulic oil performs four functions in the hydraulic system:

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Jodie Bell
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1 The hydraulic oil performs four functions in the hydraulic system: Transmits Energy Coolant Lubricant Sealant Viscosity Viscosity is the rating of the oil thickness or resistance to flow. Viscosity is rated in SUS (Saybolt Universal Seconds). To measure viscosity, the oil is heated to a specific temperature (usually 100 For38 C ) and then poured through an orifice into a container of known size. The time required to fill the container is the SUS rating or number. Most hydraulic oils have a viscosity of approximately 150 SUS at 100 F (38 C). The important fact for you to know about viscosity is that it changes with temperature. Hot Oil - Low Viscosity Cold Oil - High Viscosity Basic Hydraulic Troubleshooting Page 7-1

2 Oil that is too hot will more readily bypass in pumps, valves and cylinders. The bypassing causes additional heat and a slow down of the system. The major problem with cold oil is pump cavitation. Cavitation is discussed earlier in the pump section. Essentially, air is pulled from the oil when a pump cavitates. The air is compressed inside the pump causing eventual pump failure. Cold oil will also cause the system to operate sluggish or erratic. Page 7-2 Basic Hydraulic Troubleshooting

3 1. Reservoir The reservoir has four primary functions in the hydraulic system: Allows the contaminants to settle. Oil Storage It requires more oil to extend a cylinder than to retract it. Presses that use hydraulic rams require a large reservoir to hold the fluid when the press is fully open. Air Purging Any air in the hydraulic oil will be dissipated to the atmosphere at the reservoir through the breather cap. Basic Hydraulic Troubleshooting Page 7-3

4 Oil Cooling The reservoir acts as a heat sink by dissipating the heat in the oil to the surrounding air. The reservoir should be cleaned on a regularly scheduled basis. A thick layer of sludge can form on the bottom of the tank and on the reservoir walls. The sludge will prevent the heat from dissipating to the atmosphere. 2. Heater In many plants, particularly in the South, the heaters are not wired up or defective. The purpose of the heater is to keep the oil warm when the system is shut down. Heaters normally have a thermostat for setting the temperature range. The heater should normally come on at 70 F (21 C) and turn off at 105 F (41 C). The heater is a high current device that is rated in kilowatts. Because of the high current flow, the heater contacts can fail closed and continuously heat the oil. The heater should be located relatively near the pump suction line or strainer. 3. Clean Out Plates There are normally two plates, one on each end of the reservoir. These doors permit access for cleaning out the reservoir. The plate gasket is normally reusable. 4. Sight Glass Oil should always be visible in the sight glass between the high and low marks. The glass should be mounted so that when the oil is at the low mark, the level is 3 above the suction strainer. Whenever the reservoir is drained for cleaning, the sight glass should also be cleaned. It can become dark and discolored over a long period of time. 5. Suction Access Plate The suction line and access plate can be removed to easily pull the suction strainer out of the reservoir. Although this plate is not a standard component on all reservoirs, it allows easy inspection of the suction strainer. If this plate is not included, then the oil has to be drained to check and clean the strainer. 6. Suction Strainer The purpose of the suction strainer is to keep the large particles of contamination out of the pump. Large in this case are particles above 74 microns in size. Some strainers may have a 149 micron rating. The strainer should be removed and cleaned a minimum of once a year. It can be cleaned Page 7-4 Basic Hydraulic Troubleshooting

5 by taking an air hose and blowing from the inside out. Some strainers have internal check valves, which open and allow contaminated oil to the pump when the strainer wire mesh is plugged. Those strainers without check valves will cause the hydraulic pump to cavitate (pull the air out of the oil) when contaminated. The problem with the strainer is that it is out of sight and out of mind. Although more expensive, an external suction filter mounted outside the reservoir is a better method of filtering. These external filters usually have visual or electrical indicators to tell when the element is becoming contaminated. 7. Level Switches On many reservoirs two level switches are used, a low level warning and an electrical motor cut off. The low level warning usually illuminates a light on the operator s panel. 8. Air Breather The reservoir fluid level constantly changes as cylinders extend and retract. The air breather is the first line of defense against contamination. A dessicant type breather is recommended which will remove moisture from the air. Many dessicant filters also have an internal filter to remove solid particles. The breather element should have a rating of 40 micron or smaller. Breathers are often ignored and not changed or cleaned. A breather should be used that has a visual indicator. If the breather element is contaminated then the air will come in the reservoir through a bad gasket or rusty mounting bolt. The breather cap should be replaced at least once a year. 9. Temperature Gauge The oil temperature of a well designed system should be between F (38-49 C). This will vary of course, with the season and reservoir location. The operating temperature of a system should be recorded as a reference for future troubleshooting purposes. 10. High Temperature Switch This switch may be preset from the manufacturer anywhere from F (60-71 C). Hydraulic oil starts breaking down at 140 F (60 C). For every 15 above C) the life of the oil is cut in half. Usually when the high temperature switch actuates, the unit is shut down. Sometimes a decision is then made to jumper the switch. The switch should never be jumpered! Maximum setting of the switch should be 160 F (71 C). Basic Hydraulic Troubleshooting Page 7-5

6 11. Drain Valve All reservoirs have a drain plug located near the bottom of the tank. On systems where water enters by rain or washdown the reservoir can be slightly tilted and drained by this valve. 12. Coolers Two types of coolers are normally used in the industry, water and air. Water Coolers The heat in the oil is transferred to the water flowing in the tubes. Water flow is usually 1/4-1/3 of the oil flow through the cooler. A modulating valve should be connected in the inlet water line to control the water flow relative to the oil temperature. The time of year to check a water cooler is in the winter. A significant temperature difference should be felt between the inlet and outlet water lines. Water coolers are normally more efficient than air cooled units. Air Coolers The oil flow is directed through tubes that are bonded to fins. An electric motor driven fan blows air over the tubes and fins, transferring the heat to the air. The cooler fins should be checked and cleaned regularly. The cooler should be located where cool inlet air is available, if possible. Bypass Check Valve A65PSIcheck valve should be provided to protect the cooler from becoming over pressurized. If the oil viscosity is too high (cold oil) or the cooler stops up, then the oil will bypass through the check valve. The check valve will also prevent damage to the cooler when high flow surges exist in the system. Page 7-6 Basic Hydraulic Troubleshooting

7 Cooler Locations When installing a cooler in the line, the question that should be answered is Where is the heat generated?. Since coolers are low pressure devices ( PSI) they must be installed in the low pressure lines in the system. They may be in the following locations: In the case drain line of a variable displacement pump. There is always a small amount of flow in this line. A low pressure check valve (5 PSI) should be installed as a bypass to prevent the pump shaft seal from blowing out. Basic Hydraulic Troubleshooting Page 7-7

8 In the tank line of a relief valve. This is only an effective location if there is flow through the relief valve while the system is operating. On pressure compensating pump systems oil only flows through the relief when the compensator fails closed. Page 7-8 Basic Hydraulic Troubleshooting

9 In the tank line of a directional valve. When the directional valve is de-energized, the pump volume will recirculate through the cooler. Energizing either of the the valve solenoids will direct the pump flow to the system. In a dedicated filtering and cooling circuit supplied by a separate pump. Basic Hydraulic Troubleshooting Page 7-9

10 Contamination Particles of contamination in a hydraulic system are measured in microns. 1 micron equals 1 millionth of a meter. The table below compares microns to inches. The tolerances inside valves and pumps are very close requiring good system filtration. Hydraulic Pumps and Valves 5 microns (.0002 ) 8 micron (.0003 ). Servo Valves 3 micron (.0001 ) Sources of Contamination New Oil New oil leaving the refinery is relatively clean. It is transported through a variety of pipes and hoses when transported to storage drums or the plant bulk storage tank. During this process it picks up metal particles, rubber and other particles of contamination. Condensate inside drums and tanks can cause Page 7-10 Basic Hydraulic Troubleshooting

11 rust. When storing drums lay them on the side to prevent water from seeping through and causing rust. Oil should always be filtered prior to putting it into the reservoir. New oil meets a micron standard. A portable filter cart should be used when transferring oil from the drum to the reservoir. The filter cart element should have a 10 micron rating when filling non servo valve systems. Systems with servo valves should be filled through a 3 micron element. Built In Contamination When a system is first built and installed contamination can be in the form of: Metal Chips Dirt Sand Pipe Sealant Burrs Dust Weld Splatter Paint The way to remove these contaminants is through system flushing. Prior to starting a new system, it should be flushed for hours. The filters should be monitored during this time period and changed as necessary. Ingressed Contamination There are four ways that contamination can enter the system from the outside: Breather Access Plate Hose and Component Replacement Cylinder Seals Basic Hydraulic Troubleshooting Page 7-11

12 The number one way that dirt gets in the system is in hoses. When hoses are made, pieces of metal, rubber, dust and dirt can remain in the hose. The hose should be flushed or, at a minimum, blown out with clean, dry, air. Generated Contamination As the system operates, contamination can lodge between the tight tolerances of pumps and valves. System filters must be checked and changed on a regularly scheduled basis to control the contamination. Fluid Sampling The biggest problem in hydraulic systems is CONTAMINATION! The key to controlling contamination is through an effective fluid sampling and filter maintenance program. Fluid contamination, viscosity, and water content can be determined by fluid analysis. One place to sample the fluid is immediately downstream of the pump. Sampling from this location on a regular schedule will indicate pump wear caused by contamination. In some cases the sample may be taken upstream of the return filter. A tee with a quick disconnect can be permanently installed in the outlet line of the pump to sample the fluid. The sample should be taken while the system is running or immediately after shutdown so contaminants do not have time to settle. The quick coupler on the sampler hose can then be attached to the disconnect to run the fluid into the sample bottle. After the sample has been taken the hose should be removed. The disconnect can also be used to attach a gauge when necessary to check the system pressure. Page 7-12 Basic Hydraulic Troubleshooting

13 Once the sample is taken, usually 1 ml from the sample is tested. The size and number of particles are then determined by an automatic particle counter. An ISO code number is then assigned to the number of particles for a given range. For example, if the sample contains 50 particles that are between 4 and 6 micron then the ISO code would be 13. Figure 1 Basic Hydraulic Troubleshooting Page 7-13

14 To determine the target cleanliness level, the most sensitive component used in the system must be identified. The target ISO 4406 cleanliness code can then be determined using a chart similar to the one below. The ISO Target Levels in the table below represent the particles that are 4 micron and larger, 6 micron and larger and 14 micron and larger. For example, a system that uses a proportional valve operating at 2500 PSI would require a 3 micron filter to achieve the 17/15/12 target level. Figure 2 Page 7-14 Basic Hydraulic Troubleshooting

15 Let s take a look at the contamination level of two systems where a proportional valve is the most contaminant sensitive component. The maximum pressure in both systems is 2500 PSI. These two systems can be charted on the Cleanliness Code graph to determine the ISO cleanliness level. In figure 3, you can see that in Figure 3 System No.1 the number of 4 micron particles (44) falls in the code 13 range. The number of 6 micron particles (26) falls within the code 12 range, and the number of 14 micron particles (13) falls within the 11 range. Therefore, the ISO cleanliness level for this system is 13/22/11. The recommended ISO code for a system with proportional valves is normally 17/15/12. In figure 2, the table indicates that a 3 micron absolute filter rating will be required to meet this code. The term absolute refers to the diameter of the largest spherical particle that can pass through the element. Since all numbers (13/12/11) are lower than the standard of 17/15/12, the system exceeds the cleanliness standard. System No.2, however, does not meet the ISO 16/14/11 standard. The number of 4 micron particles (1340) falls in the code 18 range. The number of 6 micron particles (173) falls in the code 15 range, and the number 14 micron particles (94) falls in the code 14 range. The ISO cleanliness code for this system is therefore 18/15/14. Basic Hydraulic Troubleshooting Page 7-15

16 Filter Selection Hydraulic filters today are selected by what is known as the Beta rating or Beta ratio. The Beta rating of a filter is the ratio of the number of particles above a specific size upstream of the filter compared with the downstream particles. For example, a Beta Rating of 3 = 2 means that for every 2 particles above 3 micron and above that enters the filter, 1 particle will pass through to the system. This filter is said to be 50% efficient. Filters in hydraulic systems should have a Beta rating of 75 to 100. The 75 rated filter is 98.7% efficient. For every 75 particles at and above the rated size that enter the filter, only 1 will pass through. The 100 rated filter is 99% efficient, which means that for every 100 particles at and above the rated size, only one will pass through. Filter Placement There are primarily three locations for filters in the system (other than the suction filter, which was previously discussed earlier in this section). Pressure Line Separate Recirculating System Return Line Pressure Line A pressure line filter should be installed if any one of the following three conditions exist: 1. A servo or proportional valve is used in the system. The filter should normally be installed as close as possible to the servo or proportional valve. The filter should be a non bypassing type. Page 7-16 Basic Hydraulic Troubleshooting

17 2. A fixed displacement pump is used and the pressure exceeds 2250 PSI. At higher pressures the pump may have more wear and generate more contamination than at low pressures. 3. A variable displacement pump that operates in a system where pressure exceeds 1500 PSI. As with the fixed volume pump operating above 2250 PSI, the higher pressure can increase the wear inside the pump. Basic Hydraulic Troubleshooting Page 7-17

18 Return Line The return filter is a low pressure filter. It filters the return oil from the system just prior to entering the reservoir. The return line filter can be used as the main contamination removal device as long as 20% of the system flow is ported through it each minute. In some pressure compensating pump systems the pump may compensate more than 80% of the time, making the return filter ineffective. Separate Re-circulating System One of the most desirable methods of filtering is through the use of a dedicated pump. The pump volume is usually GPM. This method is essential if a pressure filter is not used or the system volume does not pass through the return filter at least 20% of the time. In many cases a cooler is also installed in the recirculating loop. Page 7-18 Basic Hydraulic Troubleshooting

19 Maintaining Fluids and Filters Once the ISO cleanliness level has been established and the filters are in the proper location in the system, the oil and filters must be maintained. The system filters should have mechanical or electrical dirt alarms to indicate when the filters are contaminated. The mechanical indicator will usually indicate when the filter is clean, near bypassing, and totally bypassing. These filters should be checked on a regular basis and changed normally three or four times a year. The ideal method of monitoring the filter condition is through the use of a differential pressure switch. In the example, the pressure switch will actuate when the difference in pressure between the inlet and outlet of the filter reaches 20 PSI. An electrical signal is then sent to activate a light, alarm or other type of warning device. If the element is not changed and the pressure drop across the filter reaches 25 PSI, the oil will flow through the bypass check valve. Basic Hydraulic Troubleshooting Page 7-19

20 Oil sampling also should be done at regular intervals. This will indicate that the filters are doing their job. If the cleanliness level is not being met, then a filter may need to be added to the system. It may also mean that the elements may need to be changed at shorter intervals. Hydraulic Leakage Control Leakage is one of the biggest problems in a hydraulic system. Many times leaks are accepted and lived with instead of finding the cause of the leak. Several problems exist when leaks occur: Expense Hydraulic oil costs $9.00 per gallon. A leak that drips once per second will total 1 1/8 gallons per day ($10.13/day). In a month s time the loss will be 33 3/4 gallons ($305/month). In one year s time the loss will be 405 gallons ($3660/year). Keep in mind the above figures are for a single leak. Safety Dangerous conditions exist to mill personnel due to leaked fluid. Environmentally Hazardous The EPA is setting stricter standards and penalties for oil that is lost through leakage. Causes of Leaks The main reason that a hydraulic system leaks is because of a bad installation. We will examine several causes and cures for leaks. System Piping Use Proper Schedule schedule 80 or 160 should be used when piping pressure lines. Schedule 80 has nearly twice the thread thickness of schedule 40 pipe. Schedule 40 can be used in the low pressure lines in the system (suction and return lines). Apply Sealant Properly pipe dope or Teflon tape is not recommended. A good hydraulic sealant should be used and applied to the male end only. Start two threads from the end to prevent contaminating the system. Page 7-20 Basic Hydraulic Troubleshooting

Proper Clamping A beam or conduit clamp is not acceptable in the pressure line of a hydraulic system. They are not built to hold the pipe when pressure spikes occur in the system.

21 Proper Clamping A beam or conduit clamp is not acceptable in the pressure line of a hydraulic system. They are not built to hold the pipe when pressure spikes occur in the system. A good hydraulic clamp should be used with a polypropylene or UHMW type insert. The clamp can be bolted down, however a better mount is usually achieved when it is welded down. A good rule of thumb when installing clamps is to space them approximately 5 feet apart. A clamp should also be placed 6 inches from where the pipe terminates. The pipe should rarely terminate rigidly into a valve manifold or cylinder. A hose should always be installed to absorb hydraulic shock. The only exception is a cylinder or motor that must maintain a load in the raised position. Basic Hydraulic Troubleshooting Page 7-21

22 Use Pickled Pipe pickled pipe is simply pipe that has been chemically treated internally to flush out pipe scale and other contaminants. Pipe scale, burrs, and thread flakes can cause pump, cylinder and valve seals to leak. Although pickled pipe is not always readily available it should be used whenever possible. Use Socket Weld Flanges socket weld flanges are recommended over threaded flanges. By eliminating the pipe threads a leak path is prevented. Also, 4-bolt socket weld connections are recommended for connecting pipe instead of pipe unions. Hose Installation Proper Crimping Usually nearly everyone in the maintenance department makes hoses. Hoses are many times made hurriedly because the machine is down. A crimper should be used that everyone understands and is easy to use. One of the main reasons hoses fail is because of a bad crimp. The crimp ferrule diameter should be measured and checked to ensure a good crimp. Page 7-22 Basic Hydraulic Troubleshooting

23 Proper Length Hose length should normally not exceed 3-4 feet. Hoses that are too long end up rubbing on another hose, a catwalk, or a beam. When a system pressure spike occurs, the hose will absorb some of the shock. The length of the hose can change by +2% or 6%. The hose should be made long enough so that the hose has a slight bend to absorb these shock spikes. The hose should not be twisted as this will tend to loosen the fittings Protective Sleeves When there are several hoses in close proximity to one another, a protective sleeve should be installed to prevent rubbing. Sleeves may also be required where the hose may come in contact with a hot machine surface. Basic Hydraulic Troubleshooting Page 7-23

24 Drain Lines Hydraulic Pump and Motor Drains Variable displacement pumps usually have an external case drain line that should be piped directly back to tank. Maximum pressure in the drain line is normally 5-10 PSI. If this pressure is exceeded the pump shaft seal will blow out causing leakage. On most variable displacement pumps and hydraulic motors the case drain line should terminate below the oil level. This prevents siphoning of the fluid out of the case when the component is turned off and it coasts to a stop. Pressure Settings and Shock Pressure settings in a hydraulic system that are set too high result in excess force. This excessive force is taken up within the system itself. It shows up as leaky O-Rings, cylinder seals, piping, and valve bodies. Properly setting the system pressure is discussed in detail in the pump section. Contamination Cylinder rod seals are not 100% efficient. A cylinder rod that is exposed to chips, bark, metal or other contaminants will eventually have rod seal leakage. A protective cover or rod boot should be installed to keep the contamination off the rod. Page 7-24 Basic Hydraulic Troubleshooting

Hydrostatic Drive. 1. Main Pump. Hydrostatic Drive

Hydrostatic Drive. 1. Main Pump. Hydrostatic Drive Hydrostatic Drive The Hydrostatic drive is used to drive a hydraulic motor at variable speed. A bi-directional, variable displacement pump controls the direction and speed of the hydraulic motor. This