CHAPTER 5

Valves

Valves are used in hydraulic systems to control the operation of the actuators. Valves regulate pressure by creating special pressure conditions and by controlling how much oil will flow in portions of a circuit and where it will go. The three categories of hydraulic valves are pressure-control, flow- (volume-) control, and directional-control (see Figure 5-1). Some valves have multiple functions, placing them into more than one category. Valves are rated by their size, pressure capabilities, and pressure drop/flow.

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.

5-2. Directional-Control Valves. Directional-control valves also control flow direction. However, they vary considerably in physical characteristics and operation. The valves may be a-

• • Poppet type, in which a piston or ball moves on and off a seat.
  • Rotary-spool type, in which a spool rotates about its axis.
  • Sliding-spool type, in which a spool slides axially in a bore. In this type, a spool is often classified according to the flow conditions created when it is in the normal or neutral position. A closed-center spool blocks all valve ports from each other when in the normal position. In an open-center spool, all valve ports are open to each other when the spool is in the normal position.

  Directional-control valves may also be classified according to the method used to actuate the valve element. A poppet-type valve is usually hydraulically operated. A rotary-spool type may be manually (lever or plunger action), mechanically (cam or trip action), or electrically (solenoid action) operated. A sliding-spool type may be manually, mechanically, electrically, or hydraulically operated, or it may be operated in combination.

  Directional-control valves may also be classified according to the number of positions of the valve elements or the total number of flow paths provided in the extreme position. For example, a three-position, four-way valve has two extreme positions and a center or neutral position. In each of the two extreme positions, there are two flow paths, making a total of four flow paths.

  

  Spool valves (see Figure 5-11) are popular on modern hydraulic systems because they-

  • Can be precision-ground for fine-oil metering.
  • Can be made to handle flows in many directions by adding extra lands and oil ports.
  • Stack easily into one compact control package, which is important on mobile systems.

  Spool valves, however, require good maintenance. Dirty oil will damage the mating surfaces of the valve lands, causing them to lose their accuracy. Dirt will cause these valves to stick or work erratically. Also, spool valves must be accurately machined and fitted to their bores.

  a. Poppet Valve. Figure 5-12 shows a simple poppet valve. It consists primarily of a movable poppet that closes against a valve seat. Pressure from the inlet tends to hold the valve tightly closed. A slight force applied to the poppet stem opens the poppet. The action is similar to the valves of an automobile engine. The poppet stem usually has an O-ring seal to prevent leakage. In some valves, the poppets are held in the seated position by springs. The number of poppets in a valve depends on the purpose of the valve.

  

  b. Sliding-Spool Valve. Figure 5-13 shows a sliding-spool valve. The valve element slides back and forth to block and uncover ports in the housing. Sometimes called a piston type, the sliding-spool valve has a piston of which the inner areas are equal. Pressure from the inlet ports acts equally on both inner piston areas regardless of the position of the spool. Sealing is done by a machine fit between the spool and valve body or sleeve.

  

  c. Check Valves. Check valves are the most commonly used in fluid-powered systems. They allow flow in one direction and prevent flow in the other direction. They may be installed independently in a line, or they may be incorporated as an integral part of a sequence, counterbalance, or pressure-reducing valve. The valve element may be a sleeve, cone, ball, poppet, piston, spool, or disc. Force of the moving fluid opens a check valve; backflow, a spring, or gravity closes the valve. Figures 5-14, 5-15 and 5-16 show various types of check valves.

  

  

  (1) Standard Type (Figure 5-17). This valve may be a right-angle or an in-line type, depending on the relative location of the ports. Both types operate on the same principle. The valve consists essentially of a poppet or ball 1 held on a seat by the force of spring 2. Flow directed to the inlet port acts against spring 2 to unseat poppet 1 and open the valve for through flow (see Figure 5-17, diagram B, for both valve types). Flow entering the valve through the outlet port combines with spring action to hold poppet 1 on its seat to check reverse flow.

  

  These valves are available with various cracking pressures. Conventional applications usually use the light spring because it ensures reseating the poppet regardless of the valve's mounting position. Heavy spring units are generally used to ensure the availability of at least the minimum pressure required for pilot operations.

  (2) Restriction Type (Figure 5-18). This valve has orifice plug 1 in the nose of poppet 2, which makes it different from a conventional, right-angle check valve. Flow directed to the inlet port opens the valve, allowing free flow through the valve. Reverse flow directed in through the outlet port seats poppet 2. Flow is restricted to the amount of oil, which can be altered, to allow a suitable bleed when the poppet is closed. Uses of a restriction check valve can be to control the lowering speed of a down-moving piston and the rate of decompression in large presses.

  

  (3) Pilot-Operated Type (Figure 5-19). In diagram A, the valve has poppet 1 seated on stationary sleeve 2 by spring 3. This valve opens the same as a conventional check valve. Pressure at the inlet ports must be sufficient to overcome the combined forces of any pressure at the outlet port and the light thrust of spring 3 so that poppet 1 raises and allows flow from the inlet ports through the outlet port. In this situation, there is no pressure required at the pilot port.

  

  In diagram B, the valve is closed to prevent reverse flow. Pressure at the outlet port and the thrust of spring 3 hold poppet 1 on its seat to block the flow. In this case, the pilot port has no pressure.

  In diagram C, pressure applied at the pilot port is sufficient to overcome the thrust of spring 3. The net forces exerted by pressures at the other ports raise piston 4 to unseat poppet 1 and allow controlled flow from the outlet to the inlet ports. With no pressure at the inlet ports, pilot pressure must exceed 40 percent of that imposed at outlet to open the poppet.

  Figure 5-20 shows another pilot-operated check valve. This valve consists of poppet 1 secured to piston 3. Poppet 1 is held against seat 4 by the action of spring 2 on piston 3. In diagram A, the valve is in the free-flow position. Pressure at the inlet port, acting downward against poppet 1, is sufficient to overcome the combined forces of spring 2 against piston 3 and the pressure, if any, at the outlet port. (The pressure at the outlet port is exerted over a greater effective area than that at the inlet because of the poppet stem.) The drain post is open to the tank, and there is no pressure at the pilot port. Diagram B shows the valve in a position to prevent reverse flow, with no pressure at the pilot port and the drain opening to the tank.

  

  Diagram C shows the pilot operation of the valve. When sufficient pressure is applied at the pilot port to overcome the thrust of spring 2 plus the net effect of pressure at the other ports, poppet 1 is unseated to allow reverse flow. Pilot pressure must be equal to about 80 percent of that imposed at the outlet port to open the valve and allow reverse flow.

  d. Two-Way Valve. A two-way valve is generally used to control the direction of fluid flow in a hydraulic circuit and is a sliding-spool type. Figure 5-21 shows a two-way, sliding-spool, directional-control valve. As the spool moves back and forth, it either allows or prevents fluid flow through the valve. In either shifted position in a two-way valve, a pressure port is open to one cylinder port, but the opposite cylinder port is not open to a tank. A tank port on this valve is used primarily for draining.

  

  e. Four-Way Valves. Four-way, directional-control valves are used to control the direction of fluid flow in a hydraulic circuit, which controls the direction of movement of a work cylinder or the rotation of a fluid motor. These valves are usually the sliding-spool type. A typical four-way, directional-control valve has four ports:

  • One pressure port is connected to a pressure line.
  • One return or exhaust port is connected to a reservoir.
  • Two working ports are connected, by lines, to an actuating unit.

  Four-way valves consist of a rectangular cast body, a sliding spool, and a way to position a spool. A spool is precision-fitted to a bore through the longitudinal axis of a valve's body. The lands of a spool divide this bore into a series of separate chambers. Ports in a valve's body lead into a chamber so that a spool's position determines which ports are open to each other and which ones are sealed off from each other. Ports that are sealed off from each other in one position may be interconnected in another position. Spool positioning is accomplished manually, mechanically, electrically, or hydraulically or by combing any of the four.

  Figure 5-22 shows how the spool position determines the possible flow conditions in the circuit. The four ports are marked P, T, A, and B: P is connected to the flow source; T to the tank; and A and B to the respective ports of the work cylinder, hydraulic motor, or some other valve in the circuit. In diagram A, the spool is in such a position that port P is open to port A, and port B is open to port T. Ports A and B are connected to the ports of the cylinder, flow through port P, and cause the piston of the cylinder to move to the right. Return flow from the cylinder passes through ports B and T. In diagram B, port P is open to port B, and the piston moves to the left. Return flow from the cylinder passes through ports A and T.

  

  Table 5-1 lists some of the classifications of directional-control valves. These valves could be identified according to the-

  • Number of spool positions.
  • Number of flow paths in the extreme positions.
  • Flow pattern in the center or crossover position.
  • Method of shifting a spool.
  • Method of providing spool return.

(1) Poppet-Type Valve. Figure 5-23, page 5-16, shows a typical four-way, poppet-type, directional-control valve. It is a manually operated valve and consists of a group of conventional spring-loaded poppets. The poppets are enclosed in a common housing and are interconnected by ducts so as to direct the fluid flow in the desired direction.

The poppets are actuated by cams on the camshaft. They are arranged so that the shaft, which is rotated by its controlling lever, will open the correct poppet combinations to direct the fluid flow through the desired line to the actuating unit. At the same time, fluid will be directed from the opposite line of the actuating unit through the valve and back to the reservoir or exhausted to the atmosphere.

Springs hold the poppets to their seats. A camshaft unseats them to allow fluid flow through the valve. The camshaft is controlled by moving the handle. The valve is operated by moving the handle manually or by connecting the handle, by mechanical linkage, to a control handle. On the camshaft are three O-ring packings to prevent internal and external leakage. The camshaft has two lobes (raised portions). The contour (shape) of these lobes is such that when the shaft is placed in the neutral position, the lobes will not touch any of the poppets.

One cam lobe operates the two pressure poppets; the other lobe operates the two return/exhaust poppets. To stop the rotating camshaft at the exact position, a stop pin is secured to the body and extended through a cutout section of the camshaft flange. This stop pin prevents overtravel by ensuring that the cam lobes stop rotating when the poppets have unseated as high as they can go.

Figure 5-23 shows a working view of a poppet-type, four-way valve. The camshaft rotates by moving the control handle in either direction from neutral. The lobes rotate, unseating one pressure poppet and one return/exhaust poppet. The valve is now in a working position. Pressure fluid, entering the pressure port, travels through the vertical fluid passages in both pressure poppet seats. Since only one pressure poppet is unseated by the cam lobe, the fluid flows past the open poppet to the inside of the poppet seat. It then flows out one working port and to the actuating unit. Return fluid from the actuating unit enters the other working port. It then flows through the diagonal fluid passages, past the unseated return poppet, through the vertical fluid passages, and out the return/exhaust port. By rotating the camshaft in the opposite direction until the stop pin hits, the opposite pressure and return poppets are unseated, and the fluid flow is reversed. This causes the actuating unit to move in the opposite direction.

(2) Sliding-Spool Valve. The four-way, sliding-spool, directional-control valve is simple in operation principle and is probably the most durable and trouble free of all four-way, directional-control valves in current use. Figure 5-24 shows a typical four-way, sliding-spool, directional-control valve. The valve body contains four fluid ports: pressure, return/exhaust, and two working ports (referred to as cylinder ports). A hollow steel sleeve fits into the main bore of the body. Around the outside diameter of the sleeve are O-ring gaskets. These O-rings form a seal between the sleeve and the body.

In Figure 5-24, diagram A, the valve is at the far right in its cylinder. Liquid from the pump flows to the right end of the cylinder port, while liquid from the left end returns to the reservoir. In diagram C, the situation is reverse. The piston is to the far left in its cylinder. Liquid from the pump flows to the left end of the cylinder port, while liquid from the right end returns to the reservoir. In diagram B, the piston is in an intermediate position. Flow through the valve from the pump is shut off, and both ends of the cylinder can drain to the reservoir unless other valves are set to control the flow.

In a closed-center spool valve, a piston is solid, and all passages through a valve are blocked when a piston is centered in its cylinder (see Figure 5-24, diagram B). A closed-center valve is used when a single pump or an accumulator performs more than one operation and where there must be no pressure loss in shifting a stroke direction at a work point.

In an open-center spool valve, the spools on a piston are slotted or channeled so that all passages are open to each other when a piston is centered (see Figure 5-25). In some open-center valves, passages to a cylinder port are blocked when a valve is centered and liquid from a pump is carried through a piston and out the other side of a valve to a reservoir (see Figure 5-26). Liquid must be carried to both ends of a piston of a directional valve to keep it balanced. Instead of discharging into a reservoir when a valve is centered, liquid may be directed to other valves so that a set of operations is performed in sequence.

Open-center valves are used when a work cylinder does not have to be held in position by pressure and where power is used to perform a single operation. These valves also avoid shock to a system when a valve spool is moved from one position to another, since in the intermediate position, pressure is temporarily relieved by liquid passing from a pump directly to the reservoir.

(3). Manually Operated Four-Way Valve. This valve is used to control the flow direction manually. A spool is shifted by operating a hand lever (Figure 5-27). In a spring-offset model, a spool is normally in an extreme out position and is shifted to an extreme in position by moving a lever toward a valve. Spring action automatically returns both spool and lever to the normal out position when a lever is released. In a two-position, no-spring model, a spool is shifted back to its original position. (Figure 5-27 does not show this valve.) In a three-position no-spring model, a detent (a devise which locks the movement) retains a spool in any one of the three selected positions after lever force is released. In a three-position, spring-centered model, a lever is used to shift a spool to either extreme position away from the center. Spring action automatically returns a spool to the center position when a lever is released.

(4) Pilot-Operated, Four-Way Valve. This type of valve is used to control the flow direction by using a pilot pressure. Figure 5-28 shows two units in which the spool is shifted by applying the pilot pressure at either end of the spool. In the spring-offset model, the spool is held in its normal offset position by spring thrust and shifted to its other position by applying pilot pressure to the free end of the spool. Removing pilot pressure shifts the spool back to its normal offset position. A detent does not hold this valve, so pilot pressure should be maintained as long as the valve is in the shifted position.

(5) Solenoid-Operated, Two- and Four-Way Valves. These valves are used to control the direction of hydraulic flow by electrical means. A spool is shifted by energizing a solenoid that is located at one or both ends of the spool. When a solenoid is energized, it forces a push rod against the end of a spool. A spool shifts away from the solenoid and toward the opposite end of the valve body (see Figure 5-29). In a spring-offset model, a single solenoid shifts a spring-loaded spool. When a solenoid is deenergized, a spring returns a spool to its original position.

5-3. Flow-Control Valves. Flow-control valves are used to control an actuator's speed by metering flow. Metering is measuring or regulating the flow rate to or from an actuator. A water faucet is an example of a flow-control valve. Flow rate varies as a faucet handle is turned clockwise or counterclockwise. In a closed position, flow stops. Many flow-control valves used in fluid-powered systems are similar in design and operation to a water faucet's.

In hydraulic circuits, flow-control valves are generally used to control the speed of hydraulic motors and work spindles and the travel rates of tool heads or slides. Flow-control valves incorporate an integral pressure compensator, which causes the flow rate to remain substantially uniform regardless of changes in workload. A nonpressure, compensated flow control, such as a needle valve or fixed restriction, allows changes in the flow rate when pressure drop through it changes.

Variations of the basic flow-control valves are the flow-control-and-check valves and the flow-control-and-overload relief valves. Models in the flow-control-and-check-valve series incorporate an integral check valve to allow reverse free flow. Models in the flow-control-and-overload-relief-valve series incorporate an integral relief valve to limit system pressure. Some of these valves are gasket-mounted, and some are panel-mounted.

a. Gate Valve. In this type of valve, a wedge or gate controls the flow. To open and close a passage, a handwheel moves a wedge or gate up and down across a flow line. Figure 5-30 shows the principal elements of a gate valve. Area A shows the line connection and the outside structure of the valve; area B shows the wedge or gate inside the valve and the stem to which the gate and the handwheel are attached. When the valve is opened, the gate stands up inside the bonnet with its bottom flush with the wall of the line. When the valve is closed, the gate blocks the flow by standing straight across the line where it rests firmly against the two seats that extend completely around the line.

A gate valve allows a straight flow and offers little or no resistance to the fluid flow when the valve is completely open. Sometimes a gate valve is in the partially open position to restrict the flow rate. However, its main use is in the fully open or fully closed positions. If the valve is left partly open, the valve's face stands in the fluid flow, which will act on the face and cause it to erode.

b. Globe Valve. A disc, which is screwed directly on the end of the stem, is the controlling member of a globe valve. A valve is closed by lowering a disc into a valve seat. Since fluid flows equally on all sides of the center of support when a valve is open, there is no unbalanced pressure on a disc to cause uneven wear. Figure 5-31 shows a globe valve.

c. Needle Valve. A needle valve is similar in design and operation to a globe valve. Instead of a disc, a needle valve has a long, tapered point at the end of a valve stem. Figure 5-32 shows a sectional view of a needle valve. A long taper allows a needle valve to open or close gradually. A needle valve is used to control flow-

• Into delicate gauges, which could be damaged if high-pressure fluid was suddenly delivered.
• At the end of an operation when work motion should halt slowly.
• At other points where precise flow adjustments are necessary.
• At points where a small flow rate is desired.

d. Restrictor. A restrictor is used in liquid-powered systems to limit the movement speed of certain actuating devices by limiting flow rate in a line. Figure 5-33 shows a fixed restrictor. Figure 5-34 shows a variable restrictor, which varies the restriction amount and is a modified needle valve. This valve can be preadjusted to alter the operating time of a particular subsystem. Also, it can be adjusted to meet the requirements of a particular system.

e. Orifice Check Valve. This valve is used in liquid-powered systems to allow normal speed of operation in one direction and limited speed in another. Figure 5-35 shows two orifice check valves.

f. Flow Equalizer. A flow equalizer (flow divider) is used in some hydraulic systems to synchronize the operation of two actuating units. An equalizer divides a single stream of fluid from a directional-control valve into two equal streams. Each actuating unit receives the same flow rate; both move in unison. When the two streams of return fluid operate in opposite directions, a flow equalizer combines them at an equal rate. Thus, a flow equalizer synchronizes the actuating units' movements during both operational directions.

Figure 5-36 shows one type of flow equalizer; the valve is in the splitting (divided-flow) position. Fluid, under pressure from the directional-control valve, enters port 3. This pressure overcomes spring tension and forces plug 4 down and uncovers the two orifices in sleeve 2. The fluid then splits and should flow equally through side passages 1 and 5. The fluid flows through-

• Splitting check valves 7 and 15.
• Metering grooves 10 and 14.
• Ports 9 and 13.
• The connecting lines to the actuating cylinders.

Any difference in the flow rate between the two passages results in a pressure differential between these two passages. Free-floating metering piston 11 shifts to equalize the internal pressure, equalizing the flow.

5-4. Valve Installation. Since a flow-control valve meters flow in one direction only, the inlet and outlet ports must be correctly connected in a circuit in relation to the flow direction to be metered. A valve's drain connection must be piped to a tank so that a connection will not be subjected to possible pressure surges. The location of a flow-control valve with respect to workload has an affect on a circuit's operating characteristics. The three basic types of flow-control-valve installations are the meter-in, meter-out, and bleed-off circuits.

a. Meter-In Circuit (Figure 5-37). With this circuit, a flow-control valve is installed in a pressure line that leads to a work cylinder. All flow entering a work cylinder is first metered through a flow-control valve. Since this metering action involves reducing flow from a pump to a work cylinder, a pump must deliver more fluid than is required to actuate a cylinder at the desired speed. Excess fluid returns to a tank through a relief valve. To conserve power and avoid undue stress on a pump, a relief valve's setting should be only slightly higher than a working pressure's, which a cylinder requires.

A meter-in circuit is ideal in applications where a load always offers a positive resistance to flow during a controlled stroke. Examples would be feeding grinder tables, welding machines, milling machines, and rotary hydraulic motor drives. A flow-control-and-check valve used in this type of circuit would allow reverse free flow for the return stroke of a cylinder, but it would not provide control of return stroke speed.

b. Meter-Out Circuit (Figure 5-38). With a meter-out circuit, a flow-control valve is installed on the return side of a cylinder so that it controls a cylinder's actuation by metering its discharge flow. A relief valve is set slightly above the operating pressure that is required by the type of work.

This type of circuit is ideal for overhauling load applications in which a workload tends to pull an operating piston faster than a pump's delivery would warrant. Examples would be for drilling, reaming, boring, turning, threading, tapping, cutting off, and cold sawing machines. A flow-control-and-check valve used in this circuit would allow reverse free flow, but it would not provide a control of return stroke speed.

c. Bleed-Off Circuit. A typical bleed-off circuit is not installed directly in a feed line. It is Td into this line with its outlet connected to a return line. A valve regulates flow to a cylinder by diverting an adjustable portion of a pump's flow to a tank. Since fluid delivered to a work cylinder does not have to pass through a flow-control valve, excess fluid does not have to be dumped through a relief valve. This type of circuit usually involves less heat generation because pressure on a pump equals the work resistance during a feed operation.

d. Compensated Flow. The flow-control valves previously discussed do not compensate for changes in fluid temperature or pressure and are considered noncompensating valves. Flow rate through these valves can vary at a fixed setting if either the pressure or the fluid's temperature changes. Viscosity is the internal resistance of a fluid that can stop it from flowing. A liquid that flows easily has a high viscosity. Viscosity changes, which can result from temperature changes, can cause low variations through a valve. Such a valve can be used in liquid-powered systems where slight flow variations are not critical consideration factors.

However, some systems require extremely accurate control of an actuating device. In such a system, a compensated flow-control valve is used. This valve automatically changes the adjustment or pressure drop across a restriction to provide a constant flow at a given setting. A valve meters a constant flow regardless of variation in system pressure. A compensated flow-control valve is used mainly to meter fluid flowing into a circuit; however, it can be used to meter fluid as it leaves a circuit. For clarity, this manual will refer to this valve as a flow regulator.

5-5. Valve Failures and Remedies. Hydraulic valves are precision-made and must be very accurate in controlling a fluid's pressure, direction, and volume within a system. Generally, no packings are used on valves since leakage is slight, as long as the valves are carefully fitted and kept in good condition.

Contaminants, such as dirt in the oil, are the major problems in valve failures. Small amounts of dirt, lint, rust, or sludge can cause annoying malfunctions and extensively damage valve parts. Such material will cause a valve to stick, plug small openings, or abrade the mating surfaces until a valve leaks. Any of these conditions will result in poor machine operation, or even complete stoppage. This damage may be eliminated if operators use care in keeping out dirt.

Use only the specified oils in a hydraulic system. Follow the recommendations in a machine's operator's manual. Because oxidation produces rust particles, use an oil that will not oxidize. Change the oil and service the filters regularly.

a. Servicing Valves. Do the following before servicing a valve:

• Disconnect the electrical power source before removing a hydraulic valve's components. Doing so eliminates starting the equipment accidentally or shorting out the tools.
• Move a valve's control lever in all directions to release the system's hydraulic pressure before disconnecting any hydraulic valve components.
• Block up or lower all hydraulic working units to the ground before disconnecting any parts.
• Clean a valve and its surrounding area before removing any part for service. Use steam-cleaning equipment if available; however, do not allow water to enter a system. Be certain that all hose and line connections are tight.
• Use fuel oil or other suitable solvents to clean with if steam cleaning is not possible. However, never use paint thinner or acetone. Plug the port holes immediately after disconnecting the lines.

b. Disassembling Valves. Do the following when disassembling a valve:

• Do not perform service work on a hydraulic valve's interior on the shop floor, on the ground, or where there is danger of dust or dirt being blown into the parts. Use only a clean bench area. Be certain that all tools are clean and free of grease and dirt.
• Be careful to identify the parts when disassembling for later reassembly. Spools are selectively fitted to valve bodies and must be returned to those same bodies. You must reassemble the valve sections in the same order.
• Be very careful when you have to clamp a valve housing in a vise. Do not damage the component. If possible, use a vise equipped with lead or brass jaws, or protect the component by wrapping it in a protective covering.
• Make sure that you seal all the valve's housing openings when you remove the components during service work. Doing so will prevent foreign material from entering the housing.
• Use a press to remove springs that are under high pressure.
• Wash all valve components in a clean mineral-oil solvent (or other noncorrosive cleaner). Dry the parts with compressed air, and place them on a clean surface for inspection. Do not wipe a valve with waste paper or rags. Lint deposits on any parts may enter the hydraulic system and cause trouble.

• • • Do not use carbon tetrachloride as a cleaning solvent; it can deteriorate the rubber seals.
    • Coat the parts with a rust-inhibiting hydraulic oil immediately after cleaning and drying them. Make sure to keep the parts clean and free from moisture until you reinstall them.
    • Inspect the valve springs carefully when disassembling them. Replace all the springs that show signs of being cocked or crooked or ones that contain broken, fractured, or rusty coils.
    • Use a spring tester to check the strength of the springs, in pounds, compressed to a specified length (see Figure 5-39).

    

  c. Repairing Valves. The following paragraphs address repair of directional-control, volume-control, and pressure-control valves:

  (1) Directional-Control Valves. Directional-control-valve spools are installed in the valve housing by a select hone fit. This is done to provide the closest possible fit between a housing and a spool for minimum internal leakage and maximum holding qualities. To make this close fit, you would need special factory techniques and equipment. Therefore, most valve spools and bodies are furnished for service only in matched sets and are not available individually for replacement.

  When repairing these valves, inspect the valve spools and bores for burrs and scoring as shown in Figure 5-40. The spools may become coated with impurities from the hydraulic oil. When scoring or coating is not deep enough to cause a leakage problem, polish the surfaces with crocus cloth. Do not remove any of the valve material. Replace a valve's body and spool if scoring or coating is excessive. If a valve's action was erratic or sticky before you removed it, it may be unbalanced because of wear on the spools or body; replace the valve.

  

  (2) Volume-Control Valve. On valve spools with orifices, inspect for clogging from dirt or other foreign matter (see Figure 5-41). Clean a valve with compressed air or a small wire. Rewash all the parts thoroughly to remove all emery or metal particles. Any such abrasives could quickly damage an entire hydraulic system. Check a valve spool for freedom of movement in a bore. When lightly oiled, a valve should slide into a bore from its own weight.

  

  (3) Pressure-Control Valve (Figure 5-42). Check for a weak relief-valve spring with a spring tester if system checks have indicated low pressure. You can remedy this by replacing a spring or by adding shims to increase the compression of a spring, in some cases. Never add so many shims that a spring is compressed solid.

  

  (4) Valve Seats and Poppets. Check the valve seats for possible leakage by scoring. Replace a valve if flat spots appear on a seat or on the poppets. You can surface polish the metal valve seats and poppets if the scoring is not deep. Do not remove any valve material.

  Some seats and valve poppets are made of nylon, which is long wearing and elastic enough to conform perfectly to mating surfaces, giving a tight seal. The nylon seats on the poppet valves will take wear, with no damage to the mating metal point. When repairing these valves, always replace the nylon parts with new nylon service parts.

  (5) Nonadjustable, Cartridge-Type Relief Valves. If a relief valve's screen or orifice becomes plugged, oil cannot enter its body to equalize the pressure in an area between an orifice plate and a pilot assembly (see Figure 5-43). This plugging causes a valve to open at lower pressures than it should. The result is sluggish operating hydraulic units. Keep a relief valve's screen and orifice clean at all times. Also check the O-rings for damage, which might cause leakage.

  

  Each relief valve's cartridge is stamped with a part number, a pressure limit, and the date of manufacture (see Figure 5-44,). Use this code when testing the cartridges. Test a valve's cartridges for pressure setting by installing them in a system and operating it until you reach the valve's opening pressure. Read the pressure on a gauge that is installed in a valve's circuit.

  

5-6. Valve Assembly. Do the following when assembling valves:

• • Ensure that the valves are clean. Wash their parts in kerosene, blow dry them with air, and then dip them in hydraulic oil with rust inhibitor to prevent rusting. Doing so will aid in assembly and provide initial lubrication. You can use petroleum jelly to hold the sealing rings in place during assembly.
  • Double check to make sure that a valve's mating surfaces are free of burrs and paint.
  • Replace all the seals and gaskets when repairing a valve assembly. Soak the new seals and gaskets in clean hydraulic oil before assembling. Doing so will prevent damage and help seal a valve's parts.
  • Make sure that you insert a valve's spools in their matched bores. You must assemble a valve's sections in their correct order.
  • Make sure that there is no distortion when mounting valves. Distortion can be caused by uneven tension on the mounting bolts and oil-line flanges, uneven mounting surfaces, improper valve location, or insufficient allowance for line expansion when the oil temperature rises. Any of these could result in valve-spool binding.
  • Check the action of a valve's spools after you tighten the bolts. If there is any sticking or binding, adjust the tension of the mounting bolts.

5-7. Troubleshooting Valves. 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.

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