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a hydraulic cylinder. the cartridge valves are operated by a 4-way directional control pilot valve. pilot pressure to operate the cartridge valves is applied or relieved at the f ports. When the directional control pilot valve is in the center position, pilot pressure is applied to the f port of all four cartridge valves, locking the cylinder rod in position.

the cylinder rod is extended by shifting the directional control pilot valve to the right (b) envelope. this relieves pilot pressure from valve 1, allowing flow from the pressure line to flow through the cartridge valve from the a port to B port, and then to the cap end of the cylinder. pilot pressure also is relieved from cartridge valve 3, allowing return oil from the rod end of the cylinder to flow through the valve from the a port to B port, and then to the reservoir. pilot pressure is applied to the f ports of cartridge valves 2 and 4, keeping them closed.

the cylinder rod is retracted in a similar way by shifting the directional control valve to the left (a) envelope. this relieves pilot pressure from cartridge valves 2 and 4, providing a path through valve 2 for pressurized oil to the rod end of the cylinder, and through valve 4 for return oil flowing to the reservoir.

fig. 3-14: differential area directional control din Valve cylinder circuit.

Review 3.4.2.3: What would occur if an over-

running load were applied to extend and retract the cylinder rod when the directional control pilot valve in figure 3-14 is in the center position? assume the induced pressure of the over-running load exceeds the pressure at the p port.

a. the cylinder rod will drift in when the pilot valve is in the center position.

b. the cylinder rod will drift out when the pilot valve is in the center position.

c. the cylinder rod is free to float because the pilot valve has a float center.

d. pilot pressure to all spring chambers locks the cylinder in position.

e. Valve springs in the cartridge valves lock the cylinder rod in place.

3 - 4 • Hydraulic Specialist • Study Guide HS Manual # 40 - 07/0/06

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Outcome 3.4.3: Recognize the characteristics of proportional control

valves.

proportional valves are used to control the output through the full range of operation of the valve. they are applied as pressure control valves, flow control valves, and directional control valves for cylinders and motors. the two most common types of actuators used to power the valves are solenoids and torque motors. Solenoid controlled proportional valves use either force or position solenoids.

proportional valves are distinguished from servo valves by the use of proportional force solenoids instead of torque motors or recently, by high force solenoids. mechanical feedback (mfB) servo valves utilize the traditional torque motor and internal feedback. electronic feedback servo valves (efB) use a linear force motor and electronic feedback. proportional valves utilize proportional solenoids that have lower force than the linear force motors used in efB servo valves. in addition, proportional valves may or may not have electronic feedback. the performance differences (frequency response capability) between proportional valves have narrowed greatly in recent years. in the case of single stage valves, proportional solenoids and linear force motors act directly on the valve spool, while torque motors control a hydraulic amplifier which in turn controls the position of the valve spool.

While an in-depth analysis of static and dynamic design calculations are beyond the scope of the following discussion, the hydraulic specialist should be familiar with operating curves used to select spools, cycle times, and components on the amplifier card that can be adjusted to tailor operation of the valve to the specific application.

proportional force solenoids are modified versions of the dc powered solenoids used on directional control valves. Where a standard dc solenoid travels full stroke when it is energized, a proportional force solenoid generates a fairly linear force in proportion to the current input over a stroke of about 1.5 mm (0.060 in.). theoretically, a current setting of 800 milliamps would generate twice the force of a 400 milliamp setting through full travel of the solenoid. the input/output relationship is not linear at the ends of the range of the coil, though the spring against which the coil acts is very linear. Higher quality solenoids are more linear than are lesser quality solenoids, and are linear over a greater range of stroke. proportional solenoids produce forces on the order of 14 lbs whereas linear force motors produce forces as great as 45 lbs. pressure relief and pressure reducing valves use force solenoids to actuate single stage valves or the pilot stage of two stage relief valves. the use of a proportional valve allows the operator to electrically change relief pressures at will from the operator’s station, both between and within operating cycles. the same is true for pressure reducing valves that use proportional force solenoids to actuate the pilot stage.

When used to pilot the second stage of a directional control valve, spool movement in the second stage is proportional to the pilot pressure signal, which ranges from about 20 psi to 365 psi.

a simplified illustration of a solenoid operated sliding spool four-way, three position proportional directional control valve is shown in figure 3-15. What must be understood at the outset is that for a proportional

Review 3.4.3.1: if a force solenoid that actuates

a proportional valve exerts a maximum force of 14 lbs at 800 milliamps, approximately how much force will the solenoid exert at 300 milliamps?

a. 3.25 lbs b. 4.50 lbs c. 5.25 lbs d. 6.00 lbs e. 7.50 lbs

Hydraulic Specialist • Study Guide • 3 -  HS Manual # 40 - 07/0/06

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