3/2 Way Pressure Control Valves
Installation, Function, Usage Example
The valve is connected in series in the pressure line (between the supply pump and the fluid outlet).
Physical procedure during pressure control
The inline valve (pressure build-up) and the bypass valve (pressure reduction) have, with the valve seals, a pressure-active area AM. In combination with the dynamic pressure of the medium pM a medium force FM develops. This medium force effects a closing of the inline valve (pressure build-up is decreased) and an opening of the bypass valve (pressure reduction is increased):
FMedium = AMedium · pMedium
This medium force FM acts against the driving force FL. FL is determined by the area of the drive piston AL and the variable control-air pressure pL. This driving force effects an opening of the inline valve (pressure build-up is increased) and a closing of the bypass valve (pressure reduction is decreased). With the inline valve, the spring resistance FF must be taken into account for the equilibrium of forces:
FAir = AAir · pAir
In the emptied work line (connection A), an increase in the control-air pressure effects an opening of the Inline valve and a closing of the Bypass valve. The system pressure available at connection A can therefore generate the working pressure to be regulated in the work line P:
pactual < ptarget
Cycle: Maintaining Pressure
The more the working pressure reaches its target value, the greater the medium force that effects a closing of the inline valve (pressure build-up is interrupted). Equilibrium of forces (pressure balance) at the inline valve is then prevalent if:
FL = FM + FF and therefore pactual = ptarget
Cycle: Reducing Pressure
A reduction of the control-air pressure pL effects a decrease in the driving force on the control piston. Consequently, the greater medium pressure pM effects a closing of the inline valve (pressure build-up is interrupted completely). Simultaneously, the medium pressure effects an opening of the bypass valve, which leads to a continued reduction in pressure at connection R until an equilibrium of forces prevails. An equilibrium of forces (pressure balance) at the bypass valve is then always prevalent if the following is true:
FL = FM and therefore pactual = ptarget
A complete reduction of the control-air pressure pL effects a complete reduction of the driving force on the control piston FL = 0. Despite this, the strong closing springs effect a secure closing of the inline valve (pressure build-up completely interrupted). Simultaneously, the still available medium pressure pM effects a complete opening of the bypass valve (pressure is reduced completely). Now, e.g. a tool mould can be opened and the work-piece removed:
pactual > ptarget
In this example, a build-up in pressure in the working line is carried out in two phases. For this, the full system pressure must already be available at the valve. By increasing the control-air pressure twice, the return flow at the bypass valve is closed and the inline valve opened to build up the pressure. When the desired working pressure has been reached, the intake of the inline valve is closed by the working pressure. The working pressure is now confined in the system. The pressure build-up often takes place at very high flow speeds. Due to this it may happen, that a higher working pressure than desired is generated before the inline valve has interrupted the intake. This is then reduced by the bypass valve until the desired working pressure is achieved.
In this example, a reduction in pressure in the working line is carried out in two phases. For this, the full working pressure has already been generated. By reducing the control air, the intake at the inline valve is completely interrupted. The bypass valve now opens in two stages as long as the return flow, until the desired working pressure has been achieved.
In this example, the basic manufacture of plastic components by the internal gas-pressure process is illustrated. For this, first the full system pressure is generated. Subsequently, the hot plastic blank is injected into the tool mould. Now, by increasing the control air, a preliminary pressure is generated in the work line that preforms the plastic bottle. After that the full working pressure is generated, the bottle formed complete and the pressure maintained for a while. Subsequently the working pressure is reduced completely in two stages, the mould opened and the work-piece removed.