Here’s the final part of our 3 part series on hydraulic trouble shooting 101.
STEP 5 – Relief Valve…
If the test in STEP 3 has indicated the trouble to be in the relief valve, point D, the quickest remedy is to replace the valve with one known to be good. The faulty valve may later be disassembled for inspection and cleaning.
Pilot-operated relief valves have small orifices which may be blocked with accumulations of dirt. Blow out all passages with an air hose and run a small wire through orifices.
Check also for free movement of the spool. In a relief valve with pipe thread connections in the body, the spool may bind if pipe fittings are over-tightened. If possible, test the spool for bind before unscrewing threaded connections from the body, or screw in fittings tightly during inspection of the valve.
STEP 6 – Cylinder…
If the pump will deliver full pressure when operating across the relief valve in STEP 2, both pump and relief valve can be considered good, and the trouble is further downstream. The cylinder should be tested first for worn-out or defective packing by the method described in our guide “Cylinder and Valve Testing”. Other Components…
Check other components such as bypass flow controls, hydraulic motors, etc. Solenoid 4-way valves of the pilot-operated type with tandem or open center spools may not have sufficient pilot pressure to shift the spool.
If you still have problems…
If you still have questions or problems after trying to troubleshoot your hydraulic system, feel free to give us a call and have one of our hydraulic specialists come and give you a hand.
Here’s part 2 of our 3 part series on hydraulic trouble shooting 101.
STEP 3 – Pump or Relief Valve…
If high pressure cannot be obtained in STEP 2 by running the pump against the relief valve, further testing must be conducted to see whether the fault lies in the pump or in the relief valve. Proceed as follows: If possible, disconnect the reservoir return line from the relief valve at point H. Attach a short length of hose to the relief valve outlet. Hold the open end of this hose over the reservoir filler opening so the rate of oil flow can be observed. Start the pump and run the relief valve adjustment up and down while observing the flow through the hose.
If the pump is bad, there will probably be a full stream of oil when the relief adjustment is backed off, but this flow will diminish or stop as the adjustment is increased. If a flowmeter is available, the flow can be measured and compared with the pump catalog rating. If a flowmeter is not available, the rate of flow on small pumps can be measured by discharging the hose into a bucket while timing with a watch.
For example if a volume of 10 gallons is collected in 15 seconds, the pumping rate is 40 GPM, etc. If the gauge pressure does not rise above a low value, say 100 PSI, and if the volume of flow does not substantially decrease as teh relief valve adjustment is tightened, the relief valve is probably at fault and should be cleaned or replaced as instructed in STEP 5.
If the oil substantially decreases as the relief valve adjustment is tightened, and if only a low or moderate pressure can be developed, this indicates trouble in the pump. Proceed to STEP 4.
STEP 4 – Pump…
If a full stream of oil is not obtained in STEP 3, or if the stream diminishes as the relief valve adjustment is tightened, the pump is probably at fault. Assuming that the suction strainer has already been cleaned and the inlet plumbing has been examined for air leaks, as in STEP 1, the oil is slipping across the pumping elements inside the pump. This can mean a worn-out pump, or too high an oil temperature.
High slippage in the pump will cause the pump to run considerably hotter than the oil reservoir temperature. In normal operation, with a good pump, the pump case will probably run about 20F above the reservoir temperature.
If greater than this, excess slippage, caused by wear, may be the cause. check also for slipping belts, sheared shaft pin or key, broken shaft, broken coupling, or loosened set screw.
Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.
Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system, other components could also be at fault, but this would require the services of an experienced technician.
By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, and then if necessary, each component in that area can be tested or replaced.
STEP 1 – Pump Suction Strainer
Probably the trouble encountered most often is cavitation of the hydraulic pump inlet caused by restriction due to a dirt build-up on the suction strainer. This can happen on a new as well as an older system. It produces the symptoms described above: increased pump noise, loss of high pressure and/or speed. If the strainer is not located in the pump suction line it will be found immersed below the oil level in the reservoir (point A).
Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump. The suction strainer should be removed for inspection and should be cleaned before re-installation. Wire mesh strainers can best be cleaned with an air hose, blowing from inside out. They can slso be washed in a solvent which is compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum base hydraulic oil. Do not use gasoline or other explosive or flammable solvents.
The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced. When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints (points B, E, G, H, J, and K). There must be no air leaks in the suction line.
Check the reservoir oil level to be sure it covers the top of the strainer by at least 3″ at minimum oil level, with all cylinders extended. If it does not cover to this depth there is danger of a vortex forming which may allow air to enter the system when the pump is running.
STEP 2 – Pump and Relief Valve
If cleaning the pump suction strainer does not correct the trouble, isolate the pump and relief valve from the rest of the circuit by disconnecting at point E so that only the pump, relief valve, and pressure gauge remain in the pump circuit. Cap or plug both ends of the plumbing which was disconnected. The pump is now deadheaded into the relief valve.
Check out this recent troubleshooting situation by one of our guys:
The machine in question had a complex hydraulic system, the heart of which comprised two engines driving ten hydraulic pumps. Six of the pumps were variable displacement and four of these had electronic horsepower control.
The symptoms of the problem were slow cycle times in combination with lug-down of the engines (loss of engine rpm). The machine had just been fitted with a new set of pumps.
The diagnosis of the mechanic in charge was that the hydraulic system was tuned above the power curve of the engines, that is the hydraulics were demanding more power than the engines could produce, resulting in lug-down and therefore, slow cycle times.
The other possible explanation of course, was that the engines were not producing their rated horsepower.
Due to the complexity of the hydraulic system, he knew that it would take around four hours to run a complete system check and tune-up. So in order to eliminate the easy things first, when he arrived on site he inquired about the condition of the engines and their service history.
The mechanic in charge not only assured him that the engines were in top shape, he was adamant that this was a “hydraulic” problem.
Four hours later, after running a complete check of the hydraulic system without finding anything significant, he was not totally surprised that the problem remained unchanged.
After a lengthy discussion, he managed to convince the mechanic to change the fuel filters and air cleaner elements on both engines.
This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which were preventing the engines from developing their rated horsepower.
Had the relatively simple task of changing the engine fuel filters been carried out when the problem was first noticed, an expensive service call and four hours of downtime could have been avoided.
ALWAYS check and eliminate the easy things FIRST.
Here is a hypothetical hydraulic problem and how to trouble shoot it.
“We have a hydraulic system that operates two cylinders. The pump (piston-type) has failed – for reasons unknown at this time. The tank, valves and cylinders were cleaned and a replacement pump installed. The new pump is delivering a maximum pressure of 1,000 PSI and appears to be creating heat.”
In any troubleshooting situation, no matter how simple or complex the hydraulic system, always start with the basics. This ensures that the obvious is never overlooked. In order for the ‘obvious’ to be obvious, the fundamental laws of hydraulics must be kept in mind:
Theory is great, but it always makes more sense when put into practice. So let’s apply these fundamentals to the above situation in a way that ensures the obvious things are not overlooked.
“The new pump is delivering a maximum pressure of 1,000 PSI…”
We know that a hydraulic pump can only produce flow (pressure is created by resistance to flow). It follows that if the pump can’t get oil it can’t produce flow. So, check that the reservoir is filled to the correct level, the breather is not clogged, the suction strainer or filter (if fitted) is not clogged, the pump intake isolation valve is fully open, and the pump intake line is otherwise unrestricted.
If the pump is producing flow, then an absence of pressure indicates an absence of resistance to flow. Knowing this, and that fluid under pressure always takes the path of least resistance, the task now is to find the point at which pump flow is escaping from the circuit. If you’re skilled in reading and interpreting hydraulic symbols, the system’s schematic diagram (if available) can be useful in identifying possible locations.
“The new pump… appears to be creating heat.”
Because heat is generated when there is a pressure drop, using an infrared thermometer to check the temperature of individual components will quickly lead us to the hottest part of the system – and the probable location of the internal leakage. Note that in a properly functioning system fitted with a piston pump, it is not unusual for the pump case to be the hottest part of the circuit.
The above checks should have taken less than 10 minutes. If nothing conclusive was revealed, I would continue the process of elimination using a flow-tester to conduct a direct pump test.