Air-to-hydraulic pressure booster

Air-to-hydraulic pressure booster is a device used to convert workshop air into a higher hydraulic pressure needed for operating cylinders requiring small to medium volumes of high-pressure oil (Figure 2.7(a)).

air-hydraulic

It consists of an air cylinder with a large diameter driving a small diameter hydraulic cylinder. Any workshop equipped with an airline can easily obtain hydraulic power from an air-to-hydraulic booster hooked into the airline. Figure 2.7(b) shows an application of the air-to-hydraulic booster. Here the booster is seen supplying high-pressure oil to a hydraulic cylinder used to clamp a work piece to a machine tool table.

air-hydraulic-booster

Since the workshop air pressure normally operates at around 100 psi, a pneumatically operated clamp would require a relatively larger cylinder to hold the work piece while it is being machined.

Let us assume that the air piston has a 10 sq. in. area and subjected to a pressure of 100 psi. This produces a 1000 lb force on the hydraulic cylinder piston. Thus if the area of the hydraulic piston is 1 sq. in., the hydraulic discharge oil pressure will be 1000 psi. As per Pascal’s law this produces a 1000 psi oil pressure at the small hydraulic clamping cylinder mounted on the machine tool table.

The pressure ratio of the pressure booster can be determined as follows:

equ-1

Servo Valve Port Shape

Servo valve bodies are machined with three common port shapes as shown in Fig. 11.13. The effect of port shape on flow is shown in Fig. 11.14. A full annulus port gives the highest flow per unit of spool displacement or, correspondingly, per unit of current input to the torque motor. The valve flow characteristics will be important when the valve transfer function is defined in a later section.

servo-valve-body

servo-valve-body-flow

Hydrostatics

The term hydro-static pressure is common in Physics. It is the pressure which acts on the base of an open container filled with fluid, and which is dependent on the height of the head of liquid inside the container. A hydraulic paradox occurs here, which is that the shape of the container is irrelevant, and only the height of the head of liquid determines the pressure. Hence, this also means that the pressure at the bottom of the container is higher than at the top of the container. This fact is well-known, if you consider the pressure of water deep down in the open sea. The behaviour is the same in a “sea of air”.

In statics, care must be taken that the forces are balanced. This is also true for analogue forces in hydrostatics. At the base of a container, at the bottom of the sea, or at a particular height in the place to be measured, the pressure present does not create any changes in the existing relationships.

If the fluid is enclosed in a closed container, as for example, in a hydraulic cylinder in fluid power, and if much higher pressures are needed than exist due to gravity at a certain height in a fluid, then these pressures are created via appropriate technical measures, e.g. by a hydraulic pump. Fluid is pumped into the closed container at a pressure produced by the hydraulic pump, and this pressure exerts itself equally on all sides of the container. This fact may be made use of, by making the base of the container movable. The base then moves,when pressure is applied, and providing that the hydraulic pump continues to supply fluid under pressure, a head of liquid is moved.

If the hydraulic cylinder (also under pressure) is at rest, e.g. in clamping hydraulics the forces are in equilibrium. This effect may be described as hydro-static. However, if the piston in the cylinder is moved by a supply of flow under pressure, then not only is the pressure produced from potential energy effective, but a boost pressure is also effective which is created by the kinetic energy. This pressure must be and is taken into account in fluid power systems. The relationships in this process or system may not really be described wholly as hydro-static, but the hydro-static relationships predominate.

Systems of this type, where hydro-static relationships are predominant and the transfer of pressure is most important, operate at relatively high pressures and low flow velocities in order to keep the influence of hydro-kinetics1′ as low as possible.

Meter-out operation

In the meter-out operation shown in Figure 6.39, the direction of the flow through the circuit is simply changed as can be made out from the diagram. It is the opposite of a meter-in operation as this change in direction will cause the fluid leaving the actuator to be metered. The advantage with the meter-out operation is that unlike in the case of meter-in operation, the cylinder here is prevented from overrunning and consequent cavitating.

One major problem confronting the meter-out operation is the intensification of pressure in the circuit which can in turn occur on account of a substantial differential area ratio between the piston and the rods. Pressure intensification occurs on the rod side when the meter-out operation is carried out without a load on the rod side of the cylinder and can result in failure of the rod seals. It is therefore seen that both the meter-in and meterout operations have their relative advantages and disadvantages and only the application determines the type and nature of flow valve placement.

Meter-in Operation

Meter-in is a method by which a flow control valve is placed in a hydraulic circuit in such a manner that there is a restriction in the amount of fluid flowing to the actuator. Figure 6.38(a) shows a meter-in operation in a hydraulic system.

If the flow control valve were not to be located, the extension and retraction of the actuator which in this case is a cylinder, would have proceeded at an unrestricted rate. The presence of the flow control valve enables restriction in the fluid flow to the cylinder and thereby slowing down its extension. In the event of the flow direction being reversed, the check valve ensures that the return flow bypasses the flow control valve.

For the same meter-in operation, Figure 6.38(b) shows shifting of the flow control to the other line. This enables the actuator to extend at an unrestricted rate but conversely the flow to the actuator during the retracting operation can be restricted so that the operation takes place at a reduced rate. The meter-in operation is quite accurate with a  positive load. But with an overrunning load over which the actuator has no control, the cylinder begins to cavitate.

Pressure-compensated flow control valves

Figure 6.35 illustrates the operation of a pressure-compensated valve. The design incorporates a hydrostat which maintains a constant 1.4kg/cm2 (20 psi) pressure differential across the throttle which is an orifice, whose area can be adjusted by an external knob setting. The orifice area setting determines the flow rate to be controlled. The hydrostat is normally held open by a light spring. However, it starts to close as inlet pressure increases and overcomes the spring tension. This closes the opening through the hydrostat, thereby blocking all the flow in excess of the throttle setting. As a result, the only amount of fluid that can flow through the valve is that amount which a 1.4 kg/cm2 (20 psi) pressure can force through the throttle.

To understand better the concept of pressure compensation in flow control valves, let us try and distinguish between flow control in a fixed displacement pump and that in a pressure-compensated pump. Figure 6.36 is an example of flow control in a hydraulic circuit with fixed volume pumps.

In this system, a portion of the fluid is bypassed over the relief valve in order to reduce flow to the actuator. Pressure increases upstream as the flow control valve, which in this case is a needle valve, is closed. As the relief pressure is approached, the relief valve begins to open, bypassing a portion of the fluid to the tank.

Flow control in a pressure-compensated pump as illustrated in Figure 6.37, is different in that the fluid is not passed over the reUef valve. As the compensator setting pressure is approached, the pump begins the de-stroking operation, thereby reducing the outward flow.

The design of a pressure-compensated flow control valve is such that it makes allowances for variations in pressure, before or after the orifice. In a pressure-compensated flow control valve, the actuator speed does not vary with variation in load.

Throttling only or non-pressure-compensated valve

This type of valve is used where the system pressures are relatively constant and the motoring speeds are not too critical. They work on the principle that the flow through an orifice will be constant if the pressure drop remains constant.

The figure of a non-pressure-compensated valve shown in Figure 6.34 also includes a check valve which permits free flow in the direction opposite to the flow control direction. When the load on the actuator changes significantly, the system pressure changes. Thus, the flow rate through the non-pressure-compensated valve will change for the same flow rate setting.

Ball Valves

This is another type of flow control valve shown in Figure 6.31.

It is made up of a ball with a through hole which is rotated inside a machined seat. The manner in which flow control is exercised can be understood better with the help of Figures 6.32(a) and (b).

From Figure 6.32(a), it can be seen how flow assists opening and opposes closing of the valve. Conversely, from Figure 6.32(b), the flow is seen to assist closing and oppose opening of the valve.

Figure 6.33 shows the balanced version of a ball valve. This valve uses two plugs and two seats with opposite flows resulting in very little dynamic reaction onto the actuator shaft, although at the expense of higher leakage.

Butterfly Valve

This is another type of flow control valve. It consists of a large disk which is rotated inside a pipe, the restriction in flow being determined by the angle. Figure 6.30 shows a simple design of a butterfly valve.

The advantage with this valve is that it can be constructed to almost any size. These valves are widely used for controlling gas flow. But a major problem associated with these valves is the high amount of leakage in the shut-off position.

Globe Valve

This is the simplest form of a flow control valve. The globe valve gets its name from the disk element ‘globe’ that presses against the valve seat to close the valve.

A simplified view of a globe valve has been illustrated in Figure 6.29.

The fluid flow through the valve is at right angles to the direction of flow in pipes. When this valve is opened, the entire surface of the globe moves away from the valve seat at once. Due to this action, a globe valve provides an excellent means of throttling the flow. In a hydraulic system, the globe valve can be operated either manually by means of a hand wheel or mechanically by means of an actuator.