Hydraulic Pump Trouble Shooting


Your machine's hydraulic pump probably takes the most punishment. As a result, it's the component most likely to wear out causing gradual or sudden failure.

Signs your pump is starting to go will usually be noise, excessive heat, cylinders that don't move smoothly, weak output, decreased speed, or when the functions aren't working at all.

One of the following problems will most likely be the cause if any of the above symptoms appear, if they are indeed caused by the pump.

A major problem is called cavitation. That is when the pump system has the inability to draw in a full charge of oil. When this happens, the pump will get noisy and probably overheat around the shaft and front bearing. Hydraulic cylinders may start acting up or hydraulic oil could become milky. If you suspect cavitation check the following:

  1. Inspect the pump suction strainer. Clean it even if it's not dirty. Use a solvent then blow dry with an air hose. Tiny deposits in the wire mesh may be restricting the oil flow. If you find varnish deposits on internal surfaces of pumps or valves, the system is operating too hot. A heat exchanger should be installed.
  2. Check for a clogged pump inlet. Be sure hoses are not collapsed. Only use hoses designed for the pump inlet. These hoses have an internal wire spiral to prevent collapse.
  3. Be sure the air breather on top of the reservoir isn't clogged. On systems where the air volume above the oil is relatively small, the pump could experience cavitation during its extension stroke.
  4. Oil viscosity could be too high for the particular pump. Some pumps cannot pick up the prime on heavy oil.

Cold weather start-up is particularly damaging to a pump. Running a pump for several hours until the oil warms up when it is experiencing cavitation will cause excessive wear. On equipment operating outdoors, use an oil that is as high as possible on the viscosity index.

5. Check the suction strainer size to make sure it is correct. Increasing its size if possible may help.

6. The use of high quality oil may reduce formation of varnish and sludge.

7. Follow the recommended speed of the pump. Check the pulley and gear ratios. Be sure the original electric motor has not been replaced with one which runs at a higher speed.

8. Be sure the pump has not been replaced with one that delivers a higher flow, if so, that could overload the suction strainer. Increase suction strainer size if necessary.

Air in a new system purges after about 30 minutes. For this reason, you should let a machine idle for that amount of time before operating.

Trapped air will slowly dissolve in oil and transfer into the reservoir and escape. This process can be accelerated by bleeding air from high points in the plumbing and from cylinder ports.

Air that comes into the system from leaks will cause the oil to look milky a short time after the machine starts running. The oil will usually clear up about an hour after shutdown. Here is how to find where air is entering the system:

  1. Be sure the oil reserve is filled to the proper level, and that the pump intake is well below the minimum oil level. The NFPA reservoir specifications call for the highest point on the suction strainer to be at least three inches below minimum oil level.
  2. Check the oil level when all cylinders are extended to be sure it is not below the low mark on the gauge. Do not overfill the reservoir when cylinders are extended. You can overflow the oil when the cylinders are retracted.
  3. Air may be entering around the pump shaft seal. Gear and vane pumps that suction oil from a reservoir located below them will have a slight vacuum behind the shaft seal. When this seal wears out, air will get in. Piston pumps usually have up to 15 PSI behind the shaft seal. Air is unlikely to enter these pumps through the seal.
  4. Check all plumbing and joints in the pump inlet line, especially unions. Check for leaks in hoses used in the inlet line. One way to check for plumbing leaks is to pour oil over a suspected leak. If the pump noise stops this is likely where the leak is located.
  5. Check around the inlet port. Screwing a tapered pipe fitting into a straight thread port will damage the thread which will cause air to leak.
  6. Air may be entering through the rod seal of a cylinder. This can happen on cylinders mounted with the rod up that aren't properly counterbalanced. On the downstroke the load may cause a partial vacuum to appear in the rod end of the cylinder. Cylinder seals are not usually designed to prevent air from entering.
  7. Check the main tank return line to see that it discharges well below the minimum oil level and not on top of the oil. On new designs it is helpful to increase the diameter of the tank return line before it discharges. This causes oil velocity to decrease, reducing turbulence inside the reservoir.

Water leaking into the system will cause the oil to have a milky appearance while the system is running. The oil will usually clear up a short time after the system is shut down as water settles to the bottom of the reservoir. Water could get into the system the following ways:

  1. A leak in a shell and tube heat exchanger may allow water to mix with the oil.
  2. There could be condensation on the inside walls of the reservoir. This is virtually unavoidable on systems operating in areas where temperatures significantly rise and fall. Condensation can be prevented by tapping off a small amount of fluid from the bottom of the reservoir through the drain valve.
  3. Be sure any tubing or piping which carries cool water inside the air space of the reservoir enters and leaves below the oil level.
  4. There could be leakage around the shaft. On pumps with overhead reserviors, there may be pressure behind the shaft seal. As the seal becomes worn external leakage may appear. This will usually be more pronounced while the pump is running and may disappear while the pump idle.
  5. Other pumps, like gear and vane pumps, usually run with a slight vacuum behind the seal. If leakage happens, it will occur after the pump has been stopped.

Excessive oil temperatures reaching 200°F and higher will shorten the life of shaft seals.

Abrasives in the oil also will wear seals out and will also cause scoring of the shaft. To see if there are abrasives, take an oil sample and let the oil settle for about an hour. Check all points where abrasives can enter. The most common entry point is through the air breather on the reservoir. To solve this problem, seal the reservoir air tight and maintain a maximum of 2 PSI on top of the oil.

Leakage around a pump port sometimes is caused from screwing a taper pipe thread fitting into a straight thread port. Once the threads have been damaged there is no easy way to repair the pump.

  1. Check the tightness of fittings. If dry-seal pipe threads are used, there should be no need to use a pipe thread sealant. Beware of screwing taper pipe threads too tightly into a pump body. This may cause the casting to crack.
  2. If leakage is from a small crack in the pump body, this most likely was caused either by screwing a pipe fitting too tightly or from operating the pump in a system where either the relief valve is set too high, or where high transient pressure spikes are caused by shocks.
  3. If a shaft was turned in the wrong direction, shut the pump down immediately. Reversed leads on a 3-phase motor are the most common cause for wrong rotation. Pumps must be run in the direction marked on their nameplate or case.
  4. To see if an intake is clogged, check the suction strainer for dirt, and check for collapsed intake hoses.
  5. Check to see if the oil level in the reservoir is low.
  6. Check for stuck vanes, valves, or pistons caused by either varnish in the oil or from rust or corrosion. Varnish indicates the system is running too hot. Rust or corrosion may mean water is getting into the oil.
  7. The oil may be too thin. One obvious reason is the wrong oil is being used. Another reason for this could be operating temperature is too high. If that's the case, problems will start cropping up a few hours into operation.
  8. You could have mechanical problems like a broken shaft or coupling, sheared key or pin.
  9. The pump could also be running too slow. Most pumps deliver a flow at all speeds, proportional to RPM. But some vane pumps, which depend on centrifugal force to extend the vanes, could deliver fail to produce enough flow at low speeds.
  10. If the driving motor has been replaced, make sure it is the correct speed for the pump.

If the pump has become noisy, you're probably dealing with one of the following problems:

  1. Cavitation of pump inlet.
  2. Air leaking into the system from low oil or other cause.
  3. A loose or worn coupling, set screws, or badly worn internal parts.
  4. The system may be running too hot.
  5. The pump may be running too fast.

If the pump flat out fails, especially after a short time into its life, the cause could be one of the following reasons:

  1. The pump was run above the pressure rating for an extended period of time.
  2. The wrong oil or viscosity was used.
  3. Operating the pump above the recommended temperature.
  4. Poor filters.
  5. Failure to keep suction strainer clean.
  6. A pump shaft that wasn't balanced with the power source.
  7. Air or water leaked into the system.
  8. The pump was ran too fast or too slow.
  9. There was inlet cavitation from other causes.