When most people think of preventive maintenance and reliability procedures for a hydraulic system, regularly changing filters and checking the oil level are the only things considered. When the machine fails, there is often very little information about the system to refer to when troubleshooting. However, proper reliability checks should be done with the system running under normal operating conditions. These checks are vital for preventing equipment failures and downtime.
Most hydraulic filter assemblies have a bypass check valve to prevent damage in the event the element becomes plugged with contamination. This valve will open whenever the differential pressure across the filter reaches the spring rating of the valve (usually 25 to 90 pounds per square inch, depending on the filter design). When these valves fail, they normally fail in the open condition due to contamination or mechanical malfunction. When this occurs, the oil will bypass the element without being filtered. This will lead to premature failure of the downstream components.
In many cases, this valve can be removed from the housing and inspected for wear and contamination. Consult the filter manufacturer's documentation for the specific location of this valve and the proper removal and inspection procedures. This valve should be inspected periodically when the filter assembly is being serviced.
Leakage is one of the biggest problems in a hydraulic system. Proper hose assembly and replacement of defective hoses is one of the best ways to reduce leakage and prevent unnecessary downtime. Hoses should be inspected regularly for leakage and deterioration. Hoses that show wear on the outer jackets or leakage at the hose ends should be replaced as soon as possible. A hose that has "blisters" indicates a failure of the inner lining of the hose, allowing the oil to leak through the metal braid and collect under the outer jacket.
Whenever possible, hose length should not exceed 4-6 feet. Excessive hose length will increase the possibility of the hose rubbing against other hoses, a catwalk or a beam. This will lead to premature hose failure. In addition, when a pressure spike occurs in the system, the hose can absorb some of the shock. When this occurs, the hose length can change slightly. The hose should be made long enough so that it has a slight bend to absorb these shock spikes.
Where possible, hoses should be routed in a way that prevents them from rubbing against each other. This will prevent premature failure of the outer jacket of the hose. In the event it is not possible to route hoses to prevent rubbing, protective sleeves should be used. Several types of sleeves are commercially available for this purpose. Sleeves can also be made by cutting an old hose to length and slitting it lengthwise. The sleeve can be placed over the hose at the rub point. Plastic cable ties should also be used to fasten the hoses together. This prevents relative movement of the hoses at the rub points.
Proper hydraulic pipe clamps should be used. Conduit clamps are generally not acceptable for hydraulic lines due to the vibration and pressure spikes inherent in a hydraulic system. Clamps should be inspected regularly for loose mounting bolts. Broken clamps should be replaced. In addition, clamps should be properly spaced. A good rule of thumb is to space clamps approximately 5-8 feet apart and within 6 inches from where the pipe terminates.
Breather caps are some of the most neglected items in a hydraulic system, but keep in mind that a breather cap is a filter. The reservoir oil level constantly changes as cylinders extend and retract, and the breather cap (filter) is the first line of defense against contamination. To prevent contamination from entering the reservoir from the outside, a proper breather filter of the appropriate micron rating should be used. Several manufacturers offer breather filters in the 3-micron range that also remove water from the air by using desiccant materials. The desiccant will change color when saturated with moisture. These filter assemblies will pay for themselves many times over when changed on a regular basis.
The amount of power required to drive a hydraulic pump is dependent on the system pressure and flow. As the pump wears, the internal bypassing increases due to increased internal clearances. This results in decreased pump output. With the pump delivering less flow to the system, the power required to drive the pump will decrease proportionally. Therefore, the drive motor current draw will decrease. A record should be made of the current draw when the system is relatively new to establish a baseline reference.
As system components wear, the internal clearances increase. This leads to increased bypassing. Whenever this bypassing occurs, heat is generated. This heat does no useful work in the system; therefore, energy is wasted. By using an infrared camera or some other type of heat-detecting device, this bypassing can be found. Bear in mind that heat is generated whenever a pressure drop occurs, so localized heat will always occur in any device that meters flow, such as a flow control or proportional valve. Making regular inlet and outlet oil temperature checks on heat exchangers will give an indication of the overall heat exchanger's efficiency.
Regular sound checks should be made, especially with hydraulic pumps. Cavitation occurs whenever the pump cannot get the total amount of oil it is calling for at the suction port. This will produce a steady, high-pitched whining sound. If not corrected, the pump will deliver a reduced output until it destroys itself. The most common cause of cavitation is a plugged suction strainer. It may also be caused by excessive oil viscosity (low temperature) or excessive drive motor revolutions per minute (RPM). Aeration occurs whenever outside air enters the suction port of the pump. It will produce a more erratic sound. Causes of aeration include an air leak in the suction line, low fluid level or a bad shaft seal on a fixed displacement pump.
Pressure checks should be made regularly. This will give an indication of the condition of several components in the system, such as accumulators and various pressure control valves. If the pressure drops more than 200 pounds per square inch (PSI) while the actuators are moving, this may indicate a problem. A record should be made of these pressures when the system is operating normally to establish a baseline reference.