Let us consider a hydraulic circuit in which two cylinders are used to execute two separate operations as shown in Figure 6.24.
Now, let us assume that cylinder A is required to extend completely before cylinder B extends. This can be accomplished by placing a sequencing valve just before cylinder B as shown. The pressure value of the valve is set to a predetermined value say 28 kg/cm2 (400 psi). This ensures that the operation involving cylinder B will occur after the operation involving cylinder A or in other words, cylinder B will not extend before a pressure of 28 kg/cm2 (400 psi) is reached on cylinder A.
Let us consider a hydraulic circuit where one cylinder is required to apply a lesser force than the other as shown in Figure 6.20. Here cylinder B is required to apply a lesser force than cylinder A. This is accomplished as follows.
A pressure-reducing valve is placed just before cylinder B in the hydraulic circuit as shown. This arrangement allows flow to the cylinder, until the set pressure value on the valve is reached. At this point where the set pressure is reached, the valve shuts off, thereby preventing any further buildup of pressure. The fluid is bled to the tank through the drain valve passage resulting in the easing-off of the pressure, as a result of which the valve opens again. Finally a reduced modulated pressure equal to the valve results.
A pressure switch is an instrument that automatically senses a change in pressure and opens or closes an electrical switching element, when a predetermined pressure point is reached. A pressure-sensing element is that part of a pressure switch that moves due to the change in pressure. There are oasically three types of sensing elements commonly used in pressure switches:
1. Diaphragm: This model (Figure 6.44) can operate from vacuum pressure up to a pressure of 10.5 kg/cm2 (150 psi). It consists of a weld-sealed metal diaphragm acting directly on a snap action switch.
2. Bourdon tube-type sensing element: This model (Figure 6.45) can operate with pressures ranging from 3.5 kg/cm2 (50psi) to 1265 kg/cm2 (18 000 psi). It has a weldsealed bourdon tube acting on a snap action switch.
3. Sealed piston-type sensing element: This type of sensing element can operate with pressures ranging from 1 kg/cm^ (15 psi) to 844kg/cm^ (12 000 psi). It consists of an O-ring-type-sealed piston direct acting on a snap action switch (Figure 6.46).
The electrical switching element in a pressure switch, opens and closes an electrical circuit in response to the actuating force received from the pressure-sensing element.
There are two types of switching elements:
1. Normally open
2. Normally closed.
A normally open switching element is one in which the current can flow through the switching element only when it is actuated. The plunger pin is held down by a snap action leaf spring and force must be applied to the plunger pin to close the circuit. This is done by an electrical coil which generates an electromagnetic field, when current flows through it. In a normally closed switch, current flows through the switching element until the element is actuated, at which point it opens and breaks the current flow.
A hydraulic fuse is analogous to an electric fuse and its application in a hydraulic system is much the same as that of an electric fuse in an electrical circuit. A simple illustration of a hydraulic fuse is shown in Figure 6.43.
A hydraulic fuse when incorporated in a hydraulic system, prevents the hydraulic pressure from exceeding the allowable value in order to protect the circuit components from damage. When the hydraulic pressure exceeds the design value, the thin metal disk ruptures, to relieve the pressure and the fluid is drained back to the tank. After rupture, a new metal disk needs to be inserted before the start of the operation.
Hydraulic fuses are used mainly in pressure-compensated pumps with fail-safe overload protection, in case the compensator control on the pump fails to operate. A hydraulic fuse is analogous to an electric fuse because they both are ‘one-shot’ devices. On the other hand, the pressure relief valve is analogous to an electrical circuit breaker because they both are resetable devices.
A sequencing valve again is a normally closed pressure control valve used for ensuring a sequential operation in a hydraulic circuit, based on pressure. In other words, sequencing valves ensure the occurrence of one operation before the other. A sectional view of a sequencing valve is shown in Figure 6.23.
When the components connected to port A of the valve reach the pressure set on the valve, the fluid is passed by the valve through port B to do additional work in a different portion of the system. The high-flow poppet of the sequence valve is controlled by the spring-loaded cone. At low pressures, the poppet blocks the flow of fluid from entering port A. The pressure signal at port A passes through the orifices to the top side of the poppet and to the cone. There is no flow through the valve unless the pressure at port A exceeds the maximum set pressure on the spring-loaded cone. When the pressure reaches the set valve, the main poppet Hfts, allowing the flow to pass through port B. It maintains the adjusted pressure at port A until the pressure at port B rises to the same value. A small pilot flow (about VA gpm) goes through the control piston and past the pilot cone to the external drain. When there is subsequent pressure increase in port B, the control piston acts to prevent further pilot flow loss. The main poppet opens fully and allows the pressures at port A and B to rise together. Flow may go either way during this condition.
A compound pressure relief valve is one which operates in two stages. They are designed to accommodate higher pressures than direct acting relief valves at the same flow rate capacity. To have a broad understanding of how a compound pressure relief is internally designed, a cutaway view of an actual valve manufactured by Vickers INC., Detroit is shown in Figure 6.18.
The first stage of the pilot relief valve includes the main spool which is normally closed and kept in position by a non-adjustable spring. The pilot stage is located in the upper valve body and contains a pressure-limiting poppet, which is held against a seat by an adjustable spring. The lower body contains the port connections. The balanced piston in the lower part of the body accomplishes diversion of the full pump flow.
In normal operation, the balanced piston is in a condition of hydraulic balance. Pressure at the inlet port acts on both sides of the piston, through an orifice, that is drilled through the large land. For pressures less than the valve setting, the piston is held on its seat by a light spring. As soon as the pressure reaches the setting of the adjustable spring, the
poppet is forced off its seat. This limits the pressure in the upper chamber. The restricted flow through the orifice into the upper chamber results in an increase in pressure in the lower chamber. This causes an imbalance in the hydraulic forces, which tends to raise the piston off its seat. When the pressure difference between the upper and the lower chamber reaches approximately 1.5kg/cm3 (approx. 21 psi) the large piston lifts off its seat to permit flow directly to the tank.
If there is a flow increase through the valve, the piston lifts further off its seat. However, this compresses only the light spring and hence very little override occurs. Compound relief valves can also be operated remotely by using the outlet port from the chamber above the piston. This chamber in turn can be vented to the tank through a solenoid-operated direction control valve.