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 Relief Valves

Relief valves are the most common type of pressure-control valves. The relief valves’ function may vary, depending on a system’s needs. They can provide overload protection for circuit components or limit the force or torque exerted by a linear actuator or rotary motor.

The internal design of all relief valves is basically similar. The valves consist of two sections: a body section containing a piston that is retained on its seat by a spring(s), depending on the model, and a cover or pilot-valve section that hydraulically controls a body piston’s movement. The adjusting screw adjusts this control within the range of the valves.

Valves that provide emergency overload protection do not operate as often since other valve types are used to load and unload a pump. However, relief valves should be cleaned regularly by reducing their pressure adjustments to flush out any possible sludge deposits that may accumulate. Operating under reduced pressure will clean out sludge deposits and ensure that the valves operate properly after the pressure is adjusted to its prescribed setting.

(1) Simple Type. Figure 5-2 shows a simple-type relief valve. This valve is installed so that one port is connected to the pressure line or the inlet and the other port to the reservoir. The ball is held on its seat by thrust of the spring, which can be changed by turning the adjusting screw. When pressure at the valve’s inlet is insufficient to overcome spring force, the ball remains on its seat and the valve is closed, preventing flow through it. When pressure at the valve’s inlet exceeds the adjusted spring force, the ball is forced off its seat and the valve is opened. Liquid flows from the pressure line through the valve to the reservoir. This diversion of flow prevents further pressure increase in the pressure line. When pressure decreases below the valve’s setting, the spring reseats the ball and the valve is again closed.

The pressure at which a valve first begins to pass flow is the cracking pressure of a valve. The pressure at which a valve passes its full-rated capacity is the full-flow pressure of a valve. Because of spring rate, a full-flow pressure is higher than a cracking pressure. This condition is referred to as pressure override. A disadvantage of a simple-type relief valve is its relatively high-pressure override at its rated capacity.

(2) Compound Type. Figure 5-3 shows a compound type relief valve. Passage C is used to keep the piston in hydraulic balance when the valve’s inlet pressure is less than its setting (diagram A). The valve setting is determined by an adjusted thrust of spring 3 against poppet 4. When pressure at the valve’s inlet reaches the valve’s setting, pressure in passage D also rises to overcome the thrust of spring 3. When flow through passage C creates a sufficient pressure drop to overcome the thrust of spring 2, the piston is raised off its seat (diagram B). This allows flow to pass through the discharge port to the reservoir and prevents further rise in pressure.

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.