Pneumatics and hydraulics
When something has to be pushed, clamped or moved with force, the energy almost never travels in electrical form all the way to the end: it is converted into compressed air or pressurized oil. The electrical automation system merely opens and closes the valves.
01Compressed air
| Stage | What it does |
|---|---|
| Compressor | Takes air from the atmosphere and compresses it, typically to 8 bar for distribution and use at 6 bar. |
| Aftercooler and receiver | Lowers the temperature, condenses a good part of the moisture and smooths out demand peaks. |
| Dryer | Removes the remaining moisture. Without it, water reaches the cylinders and ruins seals and valves. |
| Distribution network | Piping with a slope and drains at the low points. Branch lines leave from the top of the pipe, so that condensate does not travel down with the air. |
| FRL unit (filter-regulator-lubricator) | Filter, regulator and, where applicable, lubricator. It is what sits immediately before each machine. |
Producing it is extremely inefficient: most of the compressor’s electrical energy ends up as heat. That is why a leak is not a detail. A 3 mm hole at 6 bar lets roughly 500 liters per minute escape, and that flow is paid for in electricity twenty-four hours a day, whether the machine is working or not. Finding and repairing leaks is one of the maintenance tasks that pay for themselves fastest.
02Cylinders and force
Double-acting cylinder, 50 mm piston, 20 mm rod, working pressure 6 bar (6 × 10⁵ Pa).
- Extension area: π × 0.050²/4 = 1963 mm² = 1.963 × 10⁻³ m².
- Extension force: 6 × 10⁵ × 1.963 × 10⁻³ = 1178 N, about 120 kgf.
- Retraction area: π × (0.050² − 0.020²)/4 = 1649 mm².
- Retraction force: 6 × 10⁵ × 1.649 × 10⁻³ = 989 N, 16 % less.
Friction, on the order of 10 %, must still be subtracted from that. That is why cylinders are sized with a margin of at least 1.5 over the theoretical force required, and why a machine that pushes well in one direction can fall short in the other.
| Cylinder type | Characteristics |
|---|---|
| Single-acting | Air pushes in one direction and a spring returns it. Uses less air and needs only a 3/2 valve. Short stroke and small return force. |
| Double-acting | Air acts in both directions. The most widely used: controllable force in both directions and long strokes. |
| With cushioning | Slows the piston at the ends with an adjustable restriction. Without it, the impact destroys the cylinder and makes a characteristic noise. |
| Rodless | Very long strokes in little space, with the load coupled magnetically or mechanically. |
| Rotary | Converts pressure into limited rotation, typically 90 or 180°. |
03Valves and control
- The first number is the ways—connections—, the second the positions.
- 3/2: three ways and two positions. For single-acting cylinders and for piloting.
- 5/2: five ways and two positions. The typical valve for a double-acting cylinder.
- 5/3: adds a center position, which can leave the cylinder locked or free.
- Monostable: returns by itself by spring when the signal is released. Bistable: remembers the last position even if the signal is cut.
- Direct actuation: the solenoid moves the valve. Simple, for small flows.
- Indirect (pilot-operated) actuation: the solenoid moves a small valve that in turn moves the large one using the line pressure itself. This is the usual approach for higher power.
- Speed control: with flow control valves on the exhaust, not on the inlet. Throttling the inlet gives a jerky movement; throttling the outlet gives a smooth advance, because the cylinder always works against an air cushion.
This is the question to ask before choosing the valve. With a monostable valve, the cylinder returns by itself to its rest position: that may be the safest outcome, or exactly the opposite if it releases a clamped part. With a bistable valve, it stays where it was.
And there is a third situation that gets forgotten: when air or power is restored, no actuator must move on its own. That is why machines carry a soft-start valve and the program requires a deliberate reset.
04Hydraulics
| Aspect | Pneumatics | Hydraulics |
|---|---|---|
| Typical pressure | 6 to 8 bar | 100 to 350 bar |
| Force | Up to a few thousand newtons | Tens or hundreds of tonnes |
| Fluid | Air: compressible, free, released to the atmosphere | Oil: practically incompressible, returns to the tank |
| Stiffness | Low: air compresses, an intermediate position is not firm | High: allows precise positioning and holding |
| Speed | High, fast movements | Lower, controlled movements |
| Hazards | Hose whip, flying debris, noise | High-pressure jet that pierces the skin, fire, temperature |
- Tank with a return filter: it is also where the oil settles and cools.
- Pump driven by an electric motor, which delivers flow, not pressure.
- Pressure relief valve: sets the maximum system pressure by returning the excess to the tank. Without it, the pressure would rise until something breaks.
- Directional valves, the same in concept as the pneumatic ones.
- Actuators: hydraulic cylinders or motors.
- Accumulator: stores energy for demand peaks and damps pulsations.
The idea that organizes everything: the pump provides flow and the load determines the pressure. It is the opposite of what is usually assumed, and it explains why the pressure rises on its own when the cylinder reaches the end of its stroke.
A jet of oil at 200 bar from a pinhole in a hose penetrates the skin and causes a serious injury that at first is barely visible. Leaks are never searched for by passing a hand over them: a piece of cardboard is used. Before disconnecting any line, the system is depressurized and this is verified with the pressure gauge, bearing in mind that an accumulator can remain charged with the pump stopped.
05In the lab
With a double-acting cylinder and a force gauge, measure the extension and retraction force at different pressures. Compare with the calculation and estimate the efficiency. The difference due to friction is what later justifies the design margin.
Fit flow control valves first on the inlet and then on the exhaust of the cylinder, and compare the movement under load. With inlet throttling the jerky advance appears; with exhaust throttling, the movement is smooth.
Implement the sequence with two cylinders, 5/2 valves and end-of-stroke sensors, controlled by the PLC with the Grafcet from the previous topic. Then add the condition that the sequence does not start if either cylinder is not in its initial position.
With the installation pressurized and no consumption, measure how long it takes for the pressure to drop and how often the compressor starts per hour. Locate the leaks with soapy water, repair them and repeat the measurement. Estimate the annual energy savings.
06Common mistakes
| Mistake | Consequence |
|---|---|
| Sizing the cylinder by the extension force | On the return stroke there is up to 20 % less and the machine falls short. |
| Controlling speed by throttling the inlet | Jerky, irregular movement under variable load. |
| No dryer or drains | Water reaches the valves and cylinders, and ruins the seals. |
| Branch lines taken from below the main pipe | Condensate runs straight down to the machine. |
| Choosing monostable or bistable without thinking | On a power cut, the actuator does exactly what it should not. |
| No soft start | When the air is restored, the actuators move suddenly and on their own. |
| Ignoring leaks | You pay for electricity twenty-four hours a day for air that is not used. |
| Searching for a hydraulic leak with the hand | The high-pressure jet penetrates the skin and causes a serious injury. |
07Self-assessment
Calculate the extension force of a 63 mm cylinder at 6 bar.
A = π × 0.063²/4 = 3.117 × 10⁻³ m². F = 6 × 10⁵ × 3.117 × 10⁻³ = 1870 N, about 190 kgf, before subtracting friction.
Why is the retraction force smaller?
Because the piston rod takes up part of the piston area, and the pressure acts on a smaller surface.
What does a 5/2 valve mean?
Five ways—connections—and two positions. It is the typical valve for a double-acting cylinder.
Difference between a monostable and a bistable valve.
The monostable returns by itself to its rest position by spring when the signal is released; the bistable remembers the last position even if the signal is cut.
Where is the flow control valve placed, and why?
On the exhaust. That way the cylinder works against an air cushion and the advance is smooth. Throttling the inlet, the movement comes out jerky.
Why do air branch lines leave from the top of the pipe?
So that the condensate, which runs along the bottom of the pipe, does not travel down to the machine.
What function does the pressure relief valve serve in hydraulics?
It sets the maximum system pressure by returning the excess flow to the tank. Without it the pressure would rise until a component breaks.
Does the hydraulic pump generate pressure?
No: it generates flow. The pressure is determined by the resistance the fluid meets, that is, the load.
Why does hydraulics allow precise positioning and pneumatics does not?
Because oil is practically incompressible and air is not: in pneumatics, an intermediate position is “soft” and yields under load.
How do you look for a leak in a hydraulic circuit?
With a piece of cardboard, never with the hand: the high-pressure jet penetrates the skin. And before disconnecting any line it is depressurized and checked with a pressure gauge, remembering that the accumulator may still be charged.