In industrial production processes, it often happens that a pneumatic cylinder has to make a compound movement: first approach the workpiece calmly and with limited force, and only deliver the maximum pressing force in the event of physical contact. A recognizable practical example is flow drilling (Flowdrilling), where a rotating mandrel must first be positioned gently against a metal pipe or plate before the full pressing pressure is applied to deform the material.

Without the correct pneumatic switching, this often leads to faltering movements, unsafe situations or valves that refuse to switch. Below we explain step by step how to build a reliable two-step pressure system, avoid common pitfalls and fully automate the changeover point with a digital pressure switch.

1. The pneumatic scheme: Low force versus full working pressure

If you want to work with two different pressure levels (e.g. 0.5 to 1.5 bar during the start-up and 6 bar during machining), you will soon run into the mechanical limitations of standard valves.

Why the pre-pressure before squeezing the valve does not work

A logical first thought is to place a pressure regulator directly in front of the main valve. In practice, however, this causes malfunctions:

• Most indirectly (pneumatically steered) 5/2 valves require a minimum control pressure of approximately 2 bar for reliable switching.

• If you lower the supply pressure in front of the valve to, for example, 0.5 bar, the internal sliding mechanism cannot switch and the valve blocks.

The bypass circuit around the pressure regulator The correct set-up controls the 5/2 valve with the full network pressure (approx. 6 bar). The switching between the two pressure levels takes place between the main valve and the cylinder port: 5/2-Main valve: Determines the direction of movement of the cylinder (retracting or extending). Pressure reducer with non-return valve: Adjusts the air supply to the cylinder to a safe low supply pressure. The integrated non-return valve prevents the regulator's vent from continuously blowing out air as soon as high pressure is applied to the pipe later. 2/2 bypass valve: This valve is mounted parallel to the pressure regulator. At rest, the compressed air flows through the regulator (low pressure). As soon as the 2/2 valve is energized, the air bypasses the pressure regulator and the cylinder immediately receives the full 6 bar network pressure. Important control tip for the return stroke: When the cylinder retracts after machining, it must be possible to vent the air from the working chamber via the outlet of the 5/2 valve. Because the check valve blocks this return flow through the pressure regulator, the 2/2 bypass valve must remain energized until the cylinder is fully back in its starting position.

 

2. Pressure versus flow: Preventing the stick-slip effect

A common mistake when balancing compressed air cylinders is using pressure to determine the speed. However, these are two fundamentally different physical quantities:

• Pressure (bar): Determines only the applied force of the piston rod.

• Flow / Flow rate (l/min): Determines the volume of air that flows into or out of the room per minute, and thus dictates the linear speed of the cylinder.

Why a cylinder will 'stutter' at 0.5 bar

When a cylinder is set to an extremely low pressure of approximately 0.5 bar, the so-called stick-slip effect often occurs. The piston then moves forward jerking or stuttering.

This is due to the fact that the static friction (breakaway force) of the internal cylinder seals is higher than the dynamic sliding friction. The cylinder builds up just enough pressure to break loose, shoots a bit, loses its pressure, stops and has to build up pressure again.

The solution: Throttle flow, increase pressure

To ensure smooth, slow movement without jerks, split your power and speed:

• Use a speed control valve (flow regulator) at the cylinder port to throttle the airflow (liters per minute).

• Increase the starting pressure via the secondary regulator from, for example, 0.5 bar to approximately 1.5 bar.

This gives the cylinder more than enough power reserve to effortlessly overcome the friction of the seals, while the adjusted flow ensures that the piston rod runs smoothly and constantly towards the workpiece.

 

3. Automatic switching with a digital pressure switch

In simple setups, an operator first operates a button for the run-up, and then a second switch to open the 2/2 bypass for the working stroke. However, for a reliable and fast production process, you want to automate this changeover as soon as the tool makes contact with the product.

The principle of operation

As long as the piston rod moves forward unhindered, a constant dynamic pressure is created in the cylinder chamber that is equal to the frictional resistance of the cylinder and the mechanism.

As soon as the tip touches the material, the linear movement stops abruptly. Because compressed air is still flowing into the chamber via the regulator, the pressure rises to the maximum set value of the regulator within fractions of a second. This sudden increase in pressure is the ideal signal to activate the bypass.

Integration of a digital pressure sensor

With a compact digital pressure switch (such as the DPS43P30), this transition can be detected directly:

• Easy installation: The sensor is placed directly in the work line between the bypass section and the cylinder port via a push-in T-piece.

• Signal processing: The sensor requires a 24V power supply and features adjustable PNP switching outputs and/or a 4-20 mA analog output for communication with a PLC.

• Setting the switching point: The desired threshold value can be programmed directly on the digital display, whereby the PNP output switches and controls the coil of the 2/2 valve.

Reliability and bandwidth

For trouble-free operation, it is essential that the sensor does not trigger too early during the filling of the pipe and the start-up movement.

Therefore, use sufficient bandwidth between the two pressure levels:

• Make sure that the dynamic pressure required to move the cylinder remains well below the switching point.

• Set the sensor's switching point just below the maximum setting of the secondary pressure regulator.

• The greater the pressure difference between the free stroke and the final resistance to the material, the more accurate and insensitive the system is to pressure peaks in the compressed air network.

Component overview for this system

The following core components are required to build this circuit:

Component 5/2-Solenoid valve Pneumatic pressure regulator Plug-in check valve 2/2-Solenoid valve Speed Control Valve Digital pressure switch Push-in T-coupling & PU hose

Function in the system Directional control of the cylinder (retracting and extending) at mains pressure (approx. 6 bar) Reduces the compressed air for the start-up phase (e.g. 1.5 bar). Prevents the pressure regulator from blowing off when switched on at 6 bar. Bypass function to release the full 6 bar at the right time. Precise limitation of the flow (l/min) for smooth movement. Detects the pressure jump on contact with the workpiece and controls the bypass. For quick and leak-free integration of the sensor into the work pipe.

By mechanically separating force and speed, you prevent stuttering cylinders and damage to tools or workpieces. The combination of bypass control and a digital pressure sensor offers a compact, reliable automation step without the need for heavy external limit switches or complex mechanical sensors.

 

Frequently Asked Questions FAQ 1. Why does an indirectly actuated 5/2 valve lock at low inlet pressure? Indirectly actuated 5/2 valves require at least approximately 2 bar of internal control pressure. At a lower supply pressure (such as 0.5 bar), the internal sliding mechanism cannot move and the valve blocks. 2. How do you build up a two-pressure control for a safe start-up? Control the 5/2 valve with full mains pressure (approx. 6 bar). Place a pressure regulator with a non-return valve for the low start pressure between this valve and the cylinder port, with a 2/2 bypass valve in parallel for switching on the full working pressure. 3. Why does a pneumatic cylinder stutter at low pressure (stick-slip effect)? If the pressure is too low (e.g. 0.5 bar), the static breakaway force of the seals is greater than the sliding friction. The cylinder shoots forward briefly each time, stops as soon as the pressure drops and repeats this shockingly. 4. How do you solve the stick-slip effect with a slow cylinder movement? Separate force and speed: increase the starting pressure to about 1.5 bar to overcome the friction of the seals, and throttle the speed with a speed control valve (flow controller) at the cylinder port. 5. How does a digital pressure switch detect contact with the workpiece? As soon as the cylinder touches the workpiece, the movement stops and the pressure in the chamber immediately rises to the maximum start-up value. The pressure switch measures this sudden jump and controls the 2/2 bypass for the full 6 bar.

 

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