The compressed air generated by the compressor is subject to fluctuations. Pressure fluctuations in the piping system can negatively influence the switching characteristics of valves, cylinder operating times and time regulation of flow control and double pilot valves.
Smooth operation of a pneumatic system calls for a constant working pressure. To guarantee a constant pressure level, pressure regulators are connected centrally to the compressed air network and, independent of the pressure fluctuations in the main control circuit (primary pressure), ensure a constant pressure supply in the system (secondary pressure). The pressure reducer or pressure regulator is connected downstream of the compressed air filter and keeps the working pressure constant. The pressure level should always be adapted to the requirements of the respective system section.
A working pressure of
• 600 kPa (6 bar) in the power section and
• 300 to 400 kPa (3 to 4 bar) in the control section
has proven the best economical and technical compromise between compressed air generation and performance of the components in practice.
A higher operating pressure would result in poor energy utilisation and greater wear while a lower pressure would reduce efficiency, particular in the power section.
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3 3
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Figure 3.15: Pressure regulator with relief port – sectional views and symbol
Pressure regulator with relief port – how it works
The supply pressure (primary pressure) at the pressure regulator must always be higher than the output pressure (secondary pressure). Pressure regulation itself takes place via a diaphragm. The output pressure acts on one side of the diaphragm, the force of a spring on the other side. The spring force can be set via an adjusting screw.
If the pressure on the secondary side increases, for example during a load change at the cylinder, the diaphragm is pressed against the spring and the outlet cross-sectional area at the valve seat is reduced or closed. The valve seat of the diaphragm opens and the compressed air can escape into the atmosphere through the relief ports in the housing.
If the pressure on the secondary side decreases, the spring force opens the valve. Regulating the air pressure to the preset operating pressure therefore means constant opening and closing of the valve seat, triggered by the air volume flowing through. The operating pressure is displayed on a measuring device.
Pressure regulator without relief port – how it works
If the operating pressure (secondary pressure) is too high, the pressure in the valve seat increases and presses the diaphragm against the force of the spring. At the same time, the outlet cross-sectional area at the seal seat is reduced or closed. The air flow is reduced or cut off. The compressed air can only start flowing again when the operating pressure is less than on the primary side.
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Figure 3.16: Pressure regulator without relief port – sectional view and symbol
3.6.3 Lubricator
The generated compressed air should generally not be lubricated. If moving parts in valves and cylinders need external lubrication, the compressed air must be sufficiently and continuously enriched with oil.
Lubrication of the compressed air should always be restricted to the sections of a system where lubricated air is needed. The oil added to the compressed air by the compressor is not suitable for lubricating pneumatic components.
Cylinders with heat-resistant seals should not be operated with lubricated compressed air as the oil can flush out the special grease.
If systems that were operated with lubrication are switched over to unlubricated compressed air, the original lubrication of the valves and cylinders must be renewed as it may have been flushed out.
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1: Riser line; 2: Valve throttle point; 3: Ball seat; 4: Riser pipe; 5: Oil; 6: Non-return valve; 7: Duct; 8: Drip chamber
The compressed air should only be lubricated if:
• Extremely fast motion sequences are required
• Cylinders with large diameters are used
(the lubricator should be directly upstream of the cylinder in this case) Excessive lubrication can result in the following problems:
• Malfunctioning components
• Increased environmental pollution
• Components seizing up after extended idle periods
Operational principle
The compressed air flows through the lubricator and generates a vacuum when it passes through a restriction/restricted passage. This vacuum sucks oil from the reservoir via a riser pipe. The oil reaches a drip chamber, is atomised by the oil flow and then transported onwards.
Setting the lubricator
The correct metering can be checked as follows: hold a piece of white card approx. 10 cm away from the discharge port of the control element on the cylinder furthest away from the lubricator. Allow the system to work for some time; the card should show a pale yellow colouration. Dripping oil is a sign of overlubrication.
Lubricator maintenance
The oil deposited by the compressor cannot be used as lubricant for the drive components. The heat generated in the compressor burns up and cokes the oil. It would have an abrasive effect on cylinders and valves and significantly reduce their performance.
A further problem when maintaining systems operated with lubricated compressed air is the oil deposits on the inner walls of the supply line pipes. These oil deposits can be absorbed uncontrolled into the air flow and increase the amount of dirt in the compressed air lines. Maintaining dirty systems of this type is extremely time-consuming as a pipe soiled by oil deposits can only be cleaned by dismantling it.
Oil deposits can also result in components seizing up, particularly after long idle times. After a weekend or holiday, lubricated components may no longer work properly.
Lubrication of the compressed air should be restricted to those system parts that really do need to be lubricated. Lubricators are best installed directly upstream of the consuming components. Components with self-lubrication should be chosen for the control section of a pneumatic system.
The basic rule should therefore be: compressed air should be prepared unlubricated.
In summary, the following points should be noted:
• Compressor oils should not get into the compressed air network (install oil separators).
• Only components that can also be operated with unlubricated air should be installed.
• Once a system has been operated with oil, it must continue to be operated with oil as the original lubrication of the components is flushed out over time.