A reduction in machine performance is often the first indication of a problem with a hydraulic system. This usually manifests itself in longer cycle times or slow operation. Because flow determines hydraulic actuator speed, a loss of speed indicates a loss of flow.
Flow can escape from a hydraulic circuit through external or internal leakage. External leakage such as a burst hose is usually obvious and therefore easy to find. Internal leakage can occur in the pump, valves or actuators and is more difficult to isolate. A hydraulic flow-tester (also known as a flow meter) is commonly used for this purpose.
A flow-tester is a portable instrument that comprises a turbine for measuring flow rate, an adjustable orifice that is used to increase the resistance to flow (load valve) and a pressure gauge, which measures pressure upstream of the load valve. When fitted into the circuit, the flow-tester allows the flow rate to be monitored while the resistance to flow (and therefore pressure) is increased using the load valve.
Using a flow-tester in a troubleshooting situation involves a process of elimination. The location where the flow-tester is fitted into the circuit determines the conclusions. To illustrate this point, consider a simple hydraulic circuit comprising only four components of interest: a fixed-displacement pump, a relief valve, a directional control valve and a double-acting cylinder (Figure 1). The pump has a rated flow of 10 gpm and the pressure setting of the relief valve is 3,000 psi.
Figure 1. Simple Open Center Circuit Showing
Four Components of Interest
It is noticed that cylinder speed slows under load. Flow determines speed, and fluid under pressure takes the path of least resistance. Therefore, we can assert that when the load comes on the cylinder, some of the available flow is taking an easier path back to the reservoir. The question is, which of the four components in the system is allowing this to happen?
Consider that fluid is leaking past the piston seal of the cylinder. In this case, the flow-tester would be connected into the circuit after the pump, relief valve and directional control valve but before the cylinder. To do this, we remove the two hoses from the service ports of the cylinder and connect these hoses directly to the flow-tester. This takes the cylinder out of the circuit for the purpose of this test.
Because the cylinder was isolated, there are only two possible conclusions for this particular test: either the cylinder is leaking internally or the cylinder is not leaking internally. This is an exercise in logic.
With the prime mover running and the directional control valve activated, pump flow circulates through the flow-tester and back to the reservoir. Because the load valve on the flow-tester is fully open, there is little resistance to flow and therefore minimal pressure develops.
At this point, note the flow and pressure readings on the flow-tester, which for the purposes of this example are 9.8 gpm and 50 psi. To proceed with the test, increase the resistance to flow (and therefore pressure) using the load valve on the flow-tester, while taking note of the flow rate.
The assumed results of this test are shown in Table 1.
The flow available to the cylinder is 9.5 gpm at 500 psi, decreasing to 5.9 gpm at 2,500 psi. This represents a 38 percent reduction in flow between 500 psi and 2,500 psi (9.5 - 5.9 = 3.6 and 3.6 ÷ 9.5 x 100 = 38). This means that a 38 percent reduction in cylinder speed between 500 and 2,500 psi should be expected! This indicates that a major leak is occurring in components upstream of the cylinder.
While we have not eliminated the cylinder completely from suspicion, we know that we must identify the leading contributor and repair that problem at a minimum. We may later find that the cylinder is contributing something to the problem, but for now, we will proceed with the check on the balance of the system components. What these results do not reflect, unfortunately, is which of the other three components is causing the problem.
If the cylinder is not leaking internally, then it must be the pump, right? To prove this assumption, relocate the flow-tester so that it is positioned after the pump, but before the relief valve, directional control valve and cylinder.
The results of the pump test look like those shown in Table 2.
Pump flow is 9.8 gpm at 50 psi and 9.0 gpm at 3,000 psi. This represents a volumetric efficiency of 90 percent at 3,000 psi (9.0 ÷ 10 x 100 = 90), indicating that the pump is not the cause of the problem.
Note that in the first test, flow decreased significantly at 3,000 psi (Table 1) because the relief valve opened, allowing the remaining flow to bypass the flow-tester. In the second test, the flow-tester was connected into the circuit upstream of the relief valve; therefore, the relief valve had no influence on the flow readings.
It has now been established that neither the cylinder nor the pump is causing the problem. It must be the relief valve that is leaking internally, right? To prove this assumption, relocate the flow-tester so that it is positioned after the pump and relief valve but before the directional control valve and cylinder.
The results of this test look like those shown in Table 3.
The test results are identical to the previous test - apart from the influence of the relief valve at 3,000 psi, when the flow decreases significantly due to the relief valve opening and allowing the remaining flow to bypass the flow-tester. This indicates that the relief valve is not leaking internally.
By process of elimination, the problem has been isolated to the directional control valve. This is confirmed when disassembly of the directional control valve reveals a crack in the casting, which is allowing fluid to pass from the pressure gallery to the tank gallery.
This hypothetical example illustrates how a flow-tester is used to locate internal leakage in a hydraulic system. But more importantly, it demonstrates how easy it can be to jump to the wrong conclusions in a troubleshooting situation.
This leads to incorrect diagnosis of the problem, which usually results in the unnecessary repair or replacement of serviceable components. To avoid such costly mistakes, the correct equipment and a logical approach are required.