Basic Hydraulics - Pressure Control

CHAPTER 4 - Pressure Control

Figure 4.1 - schematic illustrations of various pressure relief valves
The primary concern in fluid power circuits is to control either the rate of flow or the pressure level. One misconception has been that pressure may be controlled with an orifice or flow control device. This is never accomplished with any degree of accuracy. For accurate control of force, six types of pressure controls have been developed. They are: relief valve, unloading valve, sequence valve, reducing valve, counterbalance valve, and brake valve. By symbol, these valves closely resemble one another. Often only their location in the hydraulic circuit will designate what type of pressure valve they are.

Maximum system pressure can be controlled with the use of a normally closed pressure valve. With the primary port of the valve connected to the system pressure and the secondary port connected to the tank, the poppet is actuated by a predetermined pressure level, at which point primary and secondary passages are connected, and flow is diverted to the tank. This type of pressure control is known as a relief valve. A direct acting relief valve is one in which the poppet is held closed by the direct force of a mechanical spring which is usually adjustable. Spring tension is set on the knob to keep the poppet closed until system pressure working against the poppet reaches the desired cracking pressure. When the system pressure reaches full relief value, all fluid is passed across the poppet to the tank passage. It should be noted that direct acting relief valves are usually available in only relatively small sizes, because it is difficult to design a strong enough spring to keep the poppet closed at high pressure and high flow.
Figure 4.2 - direct acting pressure relief valve

Pilot operated relief valves are designed to accommodate higher pressures with higher flows being
confined to smaller frame size than a direct acting relief valve with the same rate of flow capacity. The valve is built in two stages. The first stage includes the main spool held in a normally closed position by a light non-adjustable spring. The stage is large enough to handle the maximum flow rating of the valve. The second stage is a small direct acting relief valve usually mounted as a cross head on the main valve body, and includes a poppet, spring, and adjustable knob. The first stage handles full rate of flow to the tank. The second stage controls and limits pilot pressure level in the main spring chamber.
Figure 4.3 - a simplified cutaway view of a pilot operated relief valve
This restricted flow caused by the orifice creates a pressure difference between the pump line and the area across the pilot orifice. This pressure imbalance causes the main poppet to move off its seat. This will discharge enough of the pump flow to prevent any further rise in the pump line pressure. When pump line pressure drops below the control knob setting, the pilot relief closes, flow through the orifice ceases, and the main spring can re-seat the main poppet.

The pilot operated pressure relief valve comprises a valve body, a main spool cartridge, and a pilot valve with a pressure-setting adjustment.

The pressure present in the primary port acts on the bottom of the main spool, and at the same time the pressure is fed to the spring-loaded side of the main spool via the control lines and containing orifices. The pressure is also present at the ball of the pilot valve. If the pressure increases to a level above the spring setting of the pilot valve, the ball opens against the spring.

The pilot fluid on the spring side of the main spool cartridge now flows into the spring chamber of the pilot valve and is directed internally to the secondary port and back to the tank.
Figure 4.4 - a cutaway view of a pilot operated pressure relief valve
Due to the orifices in the control line between the primary port and the pilot valve, a pressure drop, or pressure differential, exists between the bottom of the main spool and the spring side of the main spool. This pressure differential lifts the main spool off its seat and connects the primary pressure port of the valve to the secondary, or tank port. Fluid now flows to the tank, maintaining the set operating pressure of the valve.

A sequence valve is a normally closed pressure control valve that ensures that one operation will occur before another, based on pressure. In the clamp and drill system illustrated here, the clamp cylinder extends completely before the drill cylinder. To accomplish this a sequence valve is placed just before the drill cylinder. The sequence valve is set to 500 psi. This will ensure that the drill will not extend until the clamp cylinder reaches 500 psi.
Figure 4.5 - an example of a sequence valve in a clamp and drill circuit

A pressure reducing valve is a normally open pressure control valve used to limit pressure in one
or more legs of a hydraulic circuit. Reduced pressure results in a reduced force being generated. A
pressure reducing valve is the only pressure control valve that is normally open. A normally open
pressure control valve has primary and secondary passages connected. Pressure at the bottom of the spool is sensed from the pilot line, which is connected to the secondary port. Remember, a pressure reducing valve is normally open.

The illustrated clamp circuit in Figure 4.5 requires that clamp cylinder B apply a lesser force than clamp cylinder A. A pressure reducing valve placed just before the clamp cylinder B will allow flow to go to the cylinder until pressure reaches the setting of the valve.

At this point, the valve begins to close off, limiting any further buildup of pressure. As fluid bleeds to
the tank through the valve drain passage, pressure will begin to decay off and the valve will again open. The result is a reduced modulated pressure equal to the setting of the valve.

An unloading valve is a remotely piloted, normally closed pressure control valve that directs flow to the tank when pressure at that location reaches a predetermined level. A good example of an unloading valve application would be a high-low system. A high-low system may consist of two pumps; one high volume pump, the other a low volume pump. The system is designed to give a rapid approach or return on the work cylinder. The total volume of both pumps is delivered to the work cylinder until the load is contacted.
Figure 4.6 - an example of a high-low system with an unloading valve
At this point the system pressure increases, causing the unloading valve to open. The flow from the high volume pump is directed back to the tank at a minimal pressure. The low volume pump continues to deliver flow for the higher-pressure requirement of the work cycle. Both pumps join again for rapid return of the cylinder. This application allows less input horsepower for speed and force requirements.

  • Example 4.1 - Consider a high-low pump circuit that incorporates an 18 gpm pump which unloads at 1000 psi and a 10 gpm pump which relieves at 3000 psi. What is the maximum theoretical input fluid hp required?
  • Solution: Just prior to unloading, the system will supply 28 gpm (18 gpm + 10 gpm) at 1000 psi. Based on the theoretical input horsepower formula, the required hp equals: Hydraulic HP = 28 gpm x 1000 psi/ 1714 = 16.3 HP
  • With the 18 gpm pump unloading, only 10 gpm is supplied at 3000 psi. Again, using the formula, the theoretical input horsepower equals: Hydraulic HP = 10 gpm x 3000 psi/ 1714 = 17.5 HP
  • Answer: 17.5 hp (theoretical)

A counterbalance valve is a normally closed pressure valve used with cylinders to counter a weight or potentially overrunning load. In this circuit, without a counterbalance valve the load would fall
uncontrolled, or overrun, and pump flow would not be able to keep up. To avoid the uncontrolled
operation, a counterbalance valve is placed just after the cylinder. The pressure setting of the
counterbalance valve is set slightly above the load-induced pressure of 1100 psi. This counters the
load. As the cylinder extends, pressure must slightly rise to drive the load down.
Figure 4.7 - a counterbalance valve is placed in a hydraulic circuit to prevent the load from overrunning

A brake valve is a normally closed pressure control valve with both direct and remote pilot connected simultaneously for its operation. This valve is frequently used with hydraulic motors for dynamic braking.
Figure 4.8 - a brake valve used with a hydraulic motor
Because any downstream resistance will add to the load on the hydraulic motor, a remote pilot is used along with working pressure to keep the valve open during running. This eliminates backpressure on the motor. When the directional valve is de-energized, remote pilot pressure is lost, allowing the valve to close. The inertia of the load will now drive the valve open via the internal pilot, giving the system dynamic braking.

  • Pressure relief valve: This valve limits the maximum system pressure.
  • Sequence valve: If properly adjusted, the sequence valve ensures that the cylinder will fully extend before the motor starts.
  • Pressure reducing valve: The reducing valve will limit the pressure to the motor, thus limiting the output torque of the motor.
  • Unloading valve: When the system pressure reaches the unloading valve setting, the valve opens, thus diverting flow from the larger pump back to the tank at minimum pressure.
  • Counterbalance valve: Counterbalance valves are used to aid a cylinder in lowering a load at a controlled rate.
  • Brake valve: The brake valve serves two purposes. It prevents a load from over speeding the motor, and when the directional control valve is centered, it brings the motor to a stop at a controlled rate of speed.

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