Hydraulic Valve Troubleshooting

Listed below are areas that you can diagnose in hydraulic valves. When working on a specific machine, refer to a machine’s technical manual for more information.

a. Pressure-Control Valves. The following lists information when troubleshooting relief, pressure-reducing, pressure sequence, and unloading valves:

(1) Relief Valves. Consider the following when troubleshooting relief valves because they have low or erratic pressure:

• Adjustment is incorrect.
• Dirt, chip, or burrs are holding the valve partially open.
• Poppets or seats are worn or damaged.
• Valve piston in the main body is sticking.
• Spring is weak.
• Spring ends are damaged.
• Valve in the body or on the seat is cocking.
• Orifice or balance hold is blocked.

Consider the following when troubleshooting relief valves because they have no pressure:

• Orifice or balance hole is plugged.
• Poppet does not seat.
• Valve has a loose fit.
• Valve in the body or the cover binds.
• Spring is broken.
• Dirt, chip, or burrs are holding the valve partially open.
• Poppet or seat is worn or damaged.
• Valve in the body or on the seat is cocking.

Consider the following when troubleshooting relief valves because they have excessive noise or chatter:

• Oil viscosity is too high.
• Poppet or seat is faulty or worn.
• Line pressure has excessive return.
• Pressure setting is too close to that of another valve in the circuit.
• An improper spring is used behind the valve.

Consider the following when troubleshooting relief valves because you cannot adjust them properly without getting excessive system pressure:

• Spring is broken.
• Spring is fatigued.
• Valve has an improper spring.
• Drain line is restricted.

Consider the following when troubleshooting relief valves because they might be overheating the system:

• Operation is continuous at the relief setting.
• Oil viscosity is too high.
• Valve seat is leaking.

(2) Pressure-Reducing Valves. Consider the following when troubleshooting pressure reducing valves because they have erratic pressure:

• Dirt is in the oil.
• Poppet or seat is worn.
• Orifice or balance hole is restricted.
• Valve spool binds in the body.
• Drain line is not open freely to a reservoir.
• Spring ends are not square.
• Valve has an improper spring.
• Spring is fatigued.
• Valve needs an adjustment.
• Spool bore is worn.

(3) Pressure-Sequence Valves. Consider the following when troubleshooting pressure sequence valves because the valve is not functioning properly:

• Installation was improper.
• Adjustment was improper.
• Spring is broken.
• Foreign matter is on a plunger seat or in the orifices.
• Gasket is leaky or blown.
• Drain line is plugged.
• Valve covers are not tightened properly or are installed wrong.
• Valve plunger is worn or scored.
• Valve-stem seat is worn or scored.
• Orifices are too large, which causes a jerky operation.
• Binding occurs because moving parts are coated with oil impurities (due to overheating or using improper oil).

Consider the following when troubleshooting pressure-sequence valves because there is a premature movement to the secondary operation:

• Valve setting is too low.
• An excessive load is on a primary cylinder.
• A high inertia load is on a primary cylinder.

Consider the following when troubleshooting pressure-sequence valves because there is no movement or the secondary operation is slow:

• Valve setting is too high.
• Relief-valve setting is too close to that of a sequence valve.
• Valve spool binds in the body.

(4) Unloading Valves. Consider the following when troubleshooting these valves because a valve fails to completely unload a pump:

• Valve setting is too high.
• Pump does not build up to the unloading valve pressure.
• Valve spool binds in the body.

b. Directional-Control Valves. Directional-control valves include spool, rotary, and check valves. Consider the following when troubleshooting these valves because there is faulty or incomplete shifting:

• Control linkage is worn or is binding.
• Pilot pressure is insufficient.
• Solenoid is burned out or faulty.
• Centering spring is defective.
• Spool adjustment is improper.

Consider the following when troubleshooting directional-control valves because the actuating cylinder creeps or drifts:

• Valve spool is not centering properly.
• Valve spool is not shifted completely.
• Valve-spool body is worn.
• Leakage occurs past the piston in a cylinder.
• Valve seats are leaking.

Consider the following when troubleshooting directional-control valves because a cylinder load drops with the spool in the centered position:

• Lines from the valve housing are loose.
• O-rings on lockout springs or plugs are leaking.
• Lockout spring is broken.
• Relief valves are leaking.

Consider the following when troubleshooting directional-control valves because a cylinder load drops slightly when it is raised:

• Check-valve spring or seat is defective.
• Spool valve’s position is adjusted improperly.

Consider the following when troubleshooting directional-control valves because the oil heats (closed-center systems):

• Valve seat leaks (pressure or return circuit).
• Valves are not adjusted properly.

c. Volume-Control Valves. Volume-control valves include flow-control and flow-divider valves. Consider the following when troubleshooting these valves because there are variations in flow:

• Valve spool binds in the body.
• Cylinder or motor leaks.
• Oil viscosity is too high.
• Pressure drop is insufficient across a valve.
• Oil is dirty.

Consider the following when troubleshooting volume-control valves because of erratic pressure:

• Valve’s poppet or seat is worn.
• Oil is dirty.

Consider the following when troubleshooting volume-control valves because of improper flow:

• Valve was not adjusted properly.
• Valve-piston travel is restricted.
• Passages or orifice is restricted.
• Valve piston is cocked.
• Relief valves leak.
• Oil is too hot.

Consider the following when troubleshooting volume-control valves because the oil heats:

• Pump speed is improper.
• Hydraulic functions are holding in relief.
• Connections are incorrect.

Hydraulic Pressure Reducing Valves

These valves limit pressure on a branch circuit to a lesser amount than required in a main circuit. For example, in a system, a branch-circuit pressure is limited to 300 psi, but a main circuit must operate at 800 psi. A relief valve in a main circuit is adjusted to a setting above 800 psi to meet a main circuit’s requirements. However, it would surpass a branch- circuit pressure of 300 psi. Therefore, besides a relief valve in a main circuit, a pressure-reducing valve must be installed in a branch circuit and set at 300 psi. Figure 5-4 shows a pressure reducing valve.

In a pressure reducing valve (diagram A), adjusting the spring’s compression obtains the maximum branch circuit pressure. The spring also holds spool 1 in the open position. Liquid from the main circuit enters the valve at the inlet port C, flows past the valve spool, and enters the branch circuit through the outlet port D. Pressure at the outlet port acts through the passage E to the bottom of spool. If the pressure is insufficient to overcome the thrust of the spring, the valve remains open.

The pressure at the outlet port (diagram B) and under the spool exceeds the equivalent thrust of the spring. The spool rises and the valve is partially closed. This increases the valve’s resistance to flow, creates a greater pressure drop through the valve, and reduces the pressure at the outlet port. The spool will position itself to limit maximum pressure at the outlet port regardless of pressure fluctuations at the inlet port, as long as workload does not cause back flow at the outlet port. Back flow would close the valve and pressure would increase.

(1) X-Series Type. Figure 5-5 shows the internal construction of an X-series pressure reducing valve. The two major assemblies are an adjustable pilot-valve assembly in the cover, which determines the operating pressure of the valve, and a spool assembly in the body, which responds to the action of the pilot valve to limit maximum pressure at the outlet port.

The pilot-valve assembly consists of a poppet 1, spring 2, and adjusting screw 3. The position of the adjusting screw sets the spring load on the poppet, which determines the setting of the valve. The spool assembly consists of spool 4 and spring 5. The spring is a low rate spring, which tends to force the spool downward and hold the valve open. The position of the spool determines the size of passage C.

When pressure at the valve inlet (diagram A) does not exceed the pressure setting, the valve is completely open. Fluid passes from the inlet to the outlet with minimal resistance in the rated capacity of the valve. Passage D connects the outlet port to the bottom of the spool. Passage E connects the chambers at each end of the spool. Fluid pressure at the outlet port is present on both ends of the spool. When these pressures are equal, the spool is hydraulically balanced. The only effective force on the spool is the downward thrust of the spring, which positions the spool and tends to maintain passage C at its maximum size.

When the pressure at the valve’s outlet (diagram B) approaches the pressure setting of the valve, the liquid’s pressure in chamber H is sufficient to overcome the thrust of the spring and force the poppet off its seat. The pilot valve limits the pressure in chamber F. More pressure rises as the outlet pushes the spool upward against the combined force of the spring and the pressure in chamber F.

As the spool moves upward, it restricts the opening to create a pressure drop between the inlet and outlet ports. Pressure at the outlet is limited to the sum of the equivalent forces of springs 2 and 5. In normal operation, passage C never closes completely. Flow must pass through to meet any work requirements on the low-pressure side of the valve plus the flow required through passage E to maintain the pressure drop needed to hold the spool at the control position. Flow through restricted passage E is continual when the valve is controlling the reduced pressure. This flow is out the drain port and should be returned directly to the tank.

(2) XC-Series Type. An XC-series pressure-reducing valve limits pressure at the outlet in the same way the X-series does when flow is from its inlet port to its outlet port. An integral check valve allows reverse free flow from outlet to inlet port even at pressures above the valve setting. However, the same pressure-reducing action is not provided for this direction of flow. Figure 5-6 shows the internal construction of an XC series valve.

Hydraulic Pressure Control Valves

A pressure-control valve may limit or regulate pressure, create a particular pressure condition required for control, or cause actuators to operate in a specific order. All pure pressure-control valves operate in a condition approaching hydraulic balance. Usually the balance is very simple: pressure is effective on one side or end of a ball, poppet, or spool and is opposed by a spring. In operation, a valve takes a position where hydraulic pressure balances a spring force. Since spring force varies with compression, distance and pressure also can vary. Pressure-control valves are said to be infinite positioning. This means that they can take a position anywhere between two finite flow conditions, which changes a large volume of flow to a small volume, or pass no flow.

Most pressure-control valves are classified as normally closed. This means that flow to a valve’s inlet port is blocked from an outlet port until there is enough pressure to cause an unbalanced operation. In normally open valves, free flow occurs through the valves until they begin to operate in balance. Flow is partially restricted or cut off. Pressure override is a characteristic of normally closed-pressure controls when they are operating in balance. Because the force of a compression spring increases as it lowers, pressure when the valves first crack is less than when they are passing a large volume or full flow. The difference between a full flow and cracking pressure is called override.

a. Relief Valves.
b. Pressure-Reducing Valves.
c. Sequence Valves.
d. Counterbalance Valves.
e. Pressure Switches.