Hydraulic Pump Troubleshooting

a. Overloading. One risk of overloading is the danger of excess torque on a drive shaft. Torque is circular force on an object. An increase in pressure/pump displacement will increase the torque on a shaft if pump displacement/pressure remains constant. Often in a given package size, a higher GPM pump will have a lower pressure rating than a lower GPM pump. Sometimes a field conversion to get more speed out of an actuator will cause a pump to be overloaded. You may need a larger pump.

b. Excess Speed. Running a pump at too high a speed causes loss of lubrication, which can cause early failure. If a needed delivery requires a higher drive speed than a pump’s rating, use a higher displacement pump. Excess speed also runs a risk of damage from cavitation.

c. Cavitation. Cavitation occurs where available fluid does not fill an existing space. It often occurs in a pump’s inlet when conditions are not right to supply enough oil to keep an inlet flooded. Cavitation causes the metal in an inlet to erode and the hydraulic oil to deteriorate quicker. Cavitation can occur if there is too much resistance in an inlet’s line, if a reservoir’s oil level is too far below the inlet, or if an oil’s viscosity is too high. It can also occur if there is a vacuum or even a slight positive pressure at the inlet. A badly cavitating pump has oil bubbles exploding in the void. The only way to be sure a pump is not cavitating is to check the inlet with a vacuum gauge.

To prevent cavitation, keep the inlet clean and free of obstructions by using the correct length of an inlet’s line with minimum bends. Another method is to charge an inlet. The easiest way to do this is to flood it by locating the reservoir above the pump’s inlet. If this is not possible and you cannot create good inlet conditions, use a pressurized reservoir. You can also use an auxiliary pump to maintain a supply of oil to an inlet at low pressure. You could use a centrifugal pump, but it is more common to use a positive-displacement gear pump with a pressure-relief valve that is set to maintain the desired charging pressure.

d. Operating Problems. Pressure loss, slow operation, no delivery, and noise are common operating problems in a pump.

(1) Pressure Loss. Pressure loss means that there is a high leakage path in a system. A badly worn pump could cause pressure loss. A pump will lose its efficiency gradually. The actuator speed slows down as a pump wears. However, pressure loss is more often caused by leaks somewhere else in a system (relief valve, cylinders, motors).

(2) Slow Operation. This can be caused by a worn pump or by a partial oil leak in a system. Pressure will not drop, however, if a load moves at all. Therefore, hp is still being used and is being converted into heat at a leakage point. To find this point, feel the components for unusual heat.

(3) No Delivery. If oil is not being pumped, a pump—
• Could be assembled incorrectly.
• Could be driven in the wrong direction.
• Has not been primed. The reasons for no prime are usually improper start-up, inlet restrictions, or low oil level in a reservoir.
• Has a broken drive shaft.

(4) Noise. If you hear any unusual noise, shut down a pump immediately. Cavitation noise is caused by a restriction in an inlet line, a dirty inlet filter, or too high a drive speed. Air in a system also causes noise. Air will severely damage a pump because it will not have enough lubrication. This can occur from low oil in a reservoir, a loose connection in an inlet, a leaking shaft seal, or no oil in a pump before starting. Also, noise can be caused by worn or damaged parts, which will spread harmful particles through a system, causing more damage if an operation continues.

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