5-1. 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. 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.
b. 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.
c. Sequence Valves. Sequence valves control the operating sequence between two branches of a circuit. The valves are commonly used to regulate an operating sequence of two separate work cylinders so that one cylinder begins stroking when the other completes stroking. Sequence valves used in this manner ensure that there is minimum pressure equal to its setting on the first cylinder during the subsequent operations at a lower pressure.
Figure 5-7, diagram A, shows how to obtain the operation of a sequencing pressure by adjusting a spring's compression, which holds piston 1 in the closed position. Liquid enters the valve at inlet port C, flows freely past piston 1, and enters the primary circuit through port D. When pressure of the liquid flowing through the valve is below the valve's setting, the force acting upward on piston 1 is less than the downward force of the spring 2. The piston is held down and the valve is in the closed position.
When resistance in the primary circuit causes the pressure to rise so it overcomes the force of spring 2, piston 1 rises. The valve is then open (Figure 5-7, diagram B). Liquid entering the valve can now flow through port E to the secondary circuit.
Figure 5-8 shows an application of a sequence valve. The valve is set at a pressure in excess of that required to start cylinder 1 (primary cylinder). In its first operating sequence, pump flow goes through ports A and C (primary ports) to force cylinder 1 to stroke. The valve stays closed because the pressure of cylinder 1 is below the valve's setting. When cylinder 1 finishes stroking, flow is blocked, and the system pressure instantly increases to the valve setting to open the valve. Pump flow then starts cylinder 2 (secondary cylinder).
During this phase, the flow of pilot oil through the balance orifice governs the position of the main piston. This piston throttles flow to port B (secondary port) so that pressure equal to the valve setting is maintained on the primary circuit during movement of cylinder 2 at a lower pressure. Back pressure created by the resistance of cylinder 2 cannot interfere with the throttling action because the secondary pressure below the stem of the main piston also is applied through the drain hole to the top of the stem and thereby canceled out. When cylinder 2 is retracted, the return flow from it bypasses the sequence valve through the check valve.
d. Counterbalance Valves. A counterbalance valve allows free flow of fluid in one direction and maintains a resistance to flow in another direction until a certain pressure is reached. A valve is normally located in a line between a directional-control valve and an outlet of a vertically mounted actuating cylinder, which supports weight or must be held in position for a period of time. A counterbalance valve serves as a hydraulic resistance to an actuating cylinder. For example, a counterbalance valve is used in some hydraulically operated fork lifts. It offers a resistance to the flow from an actuating cylinder when a fork is lowered. It also helps support a fork in the up position.
Figure 5-9 shows a counterbalance valve. The valve element is balance-spool valve 4 that consists of two pistons which are permanently fixed on either end of the shaft. The inner piston areas are equal; therefore, pressure acts equally on both areas regardless of the position of the valve, and has no effect on the movement of the valve, hence, the term balanced. A small pilot piston is attached to the bottom of the spool valve.
When the valve is in the closed position, the top piston of the spool valve blocks discharge port 8. If fluid from the actuating unit enters inlet port 5, it cannot flow through the valve because discharge port 8 is blocked. However, fluid will flow through the pilot passage 6 to the small pilot piston. As the pressure increases, it acts on the pilot piston until it overcomes the preset pressure of spring 3. This forces the spool up and allows the fluid to flow around the shaft of the spool valve and out the discharge port 8.
During reverse flow, the fluid enters port 8. Spring 3 forces spool valve 4 to the closed position. The fluid pressure overcomes the spring tension of check valve 7. It opens and allows free flow around the shaft of the spool valve and out port 5. The operating pressure of the valve can be adjusted by turning adjustment screw 1, which increases or decreases the tension of the spring. This adjustment depends on the weight that the valve must support.
Small amounts of fluid will leak around the top piston of the spool valve and into the area around spring 3. An accumulation would cause a hydraulic lock on the top of the spool valve (since a liquid cannot be compressed). Drain 2 provides a passage for this fluid to flow to port 8.
e. Pressure Switches. Pressure switches are used in various applications that require an adjus-table, pressure-actuated electrical switch to make or break an electrical circuit at a predetermined pressure. An electrical circuit may be used to actuate an electrically controlled valve or control an electric-motor starter or a signal light. Figure 5-10 shows a pressure switch. Liquid, under pressure, enters chamber A. If the pressure exceeds the adjusted pressure setting of the spring behind ball 1, the ball is unseated. The liquid flows into chamber B and moves piston 2 to the right, actuating the limit to make or break an electrical circuit.
When pressure in chamber A falls below the setting of the spring behind ball 1, the spring reseats ball 1. The liquid in chamber B is throttled past valve 3 and ball 4 because of the action of the spring behind piston 2. The time required for the limit switch to return to its normal position is determined by valve 3's setting.