hydraulic system failure

Part 1 Of Hydraulic Troubleshooting Guide 101

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.

Start the pump and watch for pressure build-up on the gauge while tightening the adjustment on the relief valve. If full pressure can be developed, obviously the pump and relief valve are operating correctly, and the trouble is to be found further down the line. If full pressure cannot be developed in this test, continue with step 3.

Step 3 and 4 will be included in next week’s post. Stay tuned.

Craig Cook

Hydraulic Troubleshooting Basics

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:

Hydraulic pumps create flow – not pressure.
Resistance to flow creates pressure.
Flow determines actuator speed.
Pressure determines actuator force.
Fluid under pressure takes the path of least resistance.
When fluid moves from an area of high pressure to an area of low pressure (pressure drop) without performing useful work, heat is generated.

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.

Craig Cook