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 de energized, a spring returns a spool to its original position.
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.
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.
Figure 5-23, 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 combination 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 over travel 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.
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.
|Path-of-flow type||Two way||Allows a total of two possible flow paths in two
extreme spool positions
|Four way||Allows a total of four possible flow paths in two
extreme spool positions
|Control type||Manual operated||Hand lever is used to shift the spool.|
|Pilot operated||Hydraulic pressure is used to shift the spool.|
|Solenoid operated||Solenoid action is used to shift the spool.|
|Solenoid controlled, pilot operated||Solenoid action is used to shift the integral pilot
spool, which directs the pilot flow to shift the main spool.
|Position type||Two position||Spool has two extreme positions of dwell.|
|Three position||Spool has two extreme positions plus one intermediate or center position.|
|Spring type||Spring offset||Spring action automatically returns the spool to the normal offset position as soon as shifter force is released. (Spring offset is always a two-way valve.)|
|No spring||Spool is not spring-loaded; it is moved only by shifter force, and it remains where it is shifted (may be two- or three-position type, but three-position type uses detent).|
|Spring centered||Spring action automatically returns the spool to the center position as soon as the shifter force is released. (Spring-centered is always a three position valve.)|
|Spool type||Open center||These are five of the more common spool types.|
|Closed center||They refer to the flow pattern allowed when the spool is in the center position (three-position valves) or in the cross-over position (two-position valves).|
|Partially closed center|
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.
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.
Pressure switches are used in various applications that require an adjustable, 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.
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).
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.
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.