Why requirements come before components
A list of a cylinder, a directional valve, a regulator and two sensors is not a specification. It does not say whether the cylinder may move when air is first applied, what proves that a clamp has gripped, what happens if supply pressure drops mid-cycle or how an operator recovers a stopped sequence. When these questions are left open, they surface during commissioning as rework, delays and safety arguments. Good requirements describe behaviour; the drawing, valve selection and control program then implement it.
The guiding question is not “which valve operates this cylinder?” but “which arrangement delivers the required behaviour across all agreed conditions, and how will we prove it?”
What a complete specification covers
| Requirement area | Questions to answer |
|---|---|
| Function | What each actuator does, required stroke, force or torque and speed |
| Sequence | Order of motions, interlocks, what must be confirmed before each step |
| Feedback | Which sensors prove position, pressure or presence, and what happens if they disagree |
| Supply | Minimum and maximum pressure at the machine, flow and air quality |
| Abnormal conditions | Behaviour on loss of air, loss of electrical power, emergency stop, sensor failure, jam |
| Energy isolation | How stored air is dumped, which parts stay pressurised, how gravity loads are secured |
| Start-up and recovery | Soft start, homing, restart after stop |
| Acceptance | Tests, measurements and records that prove each requirement |
Force, speed and air consumption
Size each actuator from the required force at the minimum pressure that will reliably be available at the machine, not the compressor set-point. Theoretical cylinder force is pressure × piston area, but friction, back-pressure and dynamic loads reduce the useful figure, so designers commonly apply a load ratio so that the cylinder is not working near its theoretical limit. Speed is governed by valve flow capacity, tube size and length, and flow controls; it should be specified as a cycle time or velocity with tolerance.
- Use meter-out flow control for most double-acting cylinders to give stable speed under load.
- Check valve flow ratings (for example Cv, Kv or sonic conductance) against required speed.
- Keep tube runs between valve and cylinder short; long lines add dead volume, delay and air use.
- Calculate air consumption per cycle from swept volume and dead volume at working pressure, then convert to free air for compressor sizing.
Average consumption can look small while transient demand during simultaneous motions is large. Local receivers or larger supply lines may be needed to stop pressure dips that slow other motions or trip pressure switches.
Defining safe states and valve behaviour
Each actuator needs a defined state for loss of electrical signal, loss of air and emergency stop. The valve choice follows from this, not the other way round.
| Valve or function | Behaviour on loss of signal | Typical use |
|---|---|---|
| 5/2 single solenoid, spring return | Returns to a defined position | Actuators that must go to a known safe position |
| 5/2 double solenoid (memory) | Stays in last position | Where holding the last position is safer than moving |
| 5/3 closed centre | Traps air in both chambers | Stopping mid-stroke, though air compressibility and leaks allow drift |
| 5/3 exhaust centre | Vents both chambers | Allowing manual movement after stop |
| 5/3 pressure centre | Pressurises both sides | Balancing, with differential-area effects to check |
| Soft-start dump valve | Builds pressure slowly, dumps on demand | Preventing violent movement on start-up and isolating energy |
| Pilot-operated check or rod lock | Holds load mechanically or pneumatically | Vertical loads that must not fall |
Vertical axes and gravity loads need particular attention: venting a cylinder lets the load fall. Specify a holding method and test it.
Sequences, interlocks and feedback
- Write the normal sequence as numbered steps, each with its start condition and confirmation.
- Specify how each confirmation is proven: end-of-stroke sensor, pressure switch, vacuum switch or part-present sensor.
- Define timeouts: what happens if confirmation does not arrive within the expected time.
- List interlocks: motions that must never occur together, such as a press stroke while a guard is open.
- Describe the stop and recovery behaviour for each step, including manual operation in setup mode.
Where a pneumatic function has a safety role, such as preventing unexpected motion, the risk assessment and the relevant machinery safety standards (for example ISO 4414 for pneumatic systems and ISO 13849 for safety-related parts of control systems) decide the architecture and performance level. A general requirement list is not a substitute for that assessment.
Air supply and preparation at the machine
- State the minimum working pressure at the machine inlet and design for it; typical plant air is about 6–7 bar (87–101.5 psig), but pressure at a machine is often lower under load.
- Specify the air quality to ISO 8573-1 classes needed by valves and any process contact.
- Include an inlet isolation and dump valve, filter regulator and pressure switch to detect low supply.
- Do not raise the plant header pressure to cover a poor machine circuit: each extra bar costs about 7% more compressor power.
- Give the machine’s peak and average air demand to the plant engineer so the compressor and piping can be checked.
Documentation and acceptance
A circuit specification is complete when it can be tested. Agree the evidence at the start:
- Circuit diagram to ISO 1219 symbols, with component list and settings.
- Sequence description and interlock list matching the control program.
- Recorded cycle times, forces or pressures at the actuators.
- Tests of loss of air, loss of power, emergency stop and sensor failure, with observed behaviour recorded.
- Measured air consumption at the machine inlet during production.
Interfaces between disciplines
Pneumatic machines fail at boundaries more often than in components. A circuit specification should name who owns each interface and what passes across it.
- Plant air to machine: the plant owner guarantees minimum pressure, flow and air quality at the machine isolation valve; the machine builder designs for that minimum.
- Pneumatics to controls: valve coil voltages, sensor types, signal meanings, timeouts and fault codes are agreed in a shared input and output list.
- Pneumatics to mechanics: mounting, side loads, cushioning, end stops and guarding of moving parts are agreed with the mechanical designer.
- Machine to safety: the risk assessment defines which pneumatic functions are safety-related and the validation evidence needed.
- Machine to maintenance: access to filters, drains, valves and manual overrides, plus spare-part lists and isolation procedures for the plant team.
Hold a review of the specification with all parties before detailed design begins, and again before commissioning against the agreed tests. Changes after that should be recorded with their effect on air demand and safety, so that the compressed-air supply and the risk assessment stay current with the machine as built.
A short revision log attached to the specification is usually enough: what changed, why, who approved it and which tests were repeated.
Common design faults we see
- Cylinders sized at compressor pressure, then stalling when the network sags.
- Machines that jump violently on start-up because no soft-start valve was fitted.
- Vertical loads drifting down after stop because valves and cylinders leak.
- Undersized FRLs and quick couplings limiting speed.
- Leaking fittings and continuous blow-offs inflating air demand.
How Reddy Compressors helps
We look after the air supply that every pneumatic circuit depends on. From our office at Tarnaka, Secunderabad, our team of more than 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply equipment and spares across India. As an independent multi-brand specialist we can:
- Measure pressure and flow at machine inlets to show what the circuit really receives.
- Size and supply new or used/refurbished compressors, dryers, filters and receivers from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes for new machines and line expansions.
- Carry out leak surveys and pressure-drop audits.
- Provide service, AMC and rental compressors so machine air stays available.
Circuit design, safety validation and control programming remain with the machine builder and your engineering team; we make sure the supply meets what they specify.
What to send us
- Machine description and its air requirement, pressure and quality.
- Circuit diagram or component list if available.
- Number of machines and running pattern.
- Existing compressor and dryer nameplate photos.
- Any pressure readings at the machine and problems observed.
Frequently asked questions
What should a pneumatic circuit specification include?
It should state each actuator’s function, force and speed, the sequence of motions with confirmations, sensors and timeouts, minimum supply pressure and air quality, behaviour on loss of air, power or emergency stop, how stored energy is isolated, start-up and recovery, and the tests that prove each point. Components are selected afterwards to meet these requirements.
How do I size a pneumatic cylinder?
Calculate the force needed, including friction and dynamic loads, then divide by the minimum reliable pressure at the machine to find piston area, adding a margin so the cylinder is not working at its theoretical limit. Check stroke, cushioning, speed and side loads. Use the pressure under load at the machine, not the compressor set-point.
Why does my pneumatic cylinder move slowly?
Common causes are low pressure at the machine, undersized valves, tubes or quick couplings, flow controls set too tight, long tube runs and restrictive silencers. Worn seals and high friction also slow motion. Measure pressure at the valve inlet while the cylinder moves; if it sags, the supply or preparation unit is the restriction.
What is a soft-start valve and why is it needed?
A soft-start valve raises machine pressure gradually when air is switched on, so cylinders that are out of position move slowly into place instead of jumping. Combined with a dump function, it also vents the machine’s air on stop or isolation. It is a standard part of safe start-up and energy isolation on many pneumatic machines.
Which valve centre position should I use?
It depends on the required behaviour when the valve is de-energised. Closed centre traps air to stop mid-stroke, though drift is possible. Exhaust centre vents both sides so the actuator can be moved by hand. Pressure centre pressurises both sides. Choose from the safe-state requirement in your risk assessment, and add mechanical holding for vertical loads.
How much compressed air will my machine use?
Add each actuator’s swept and dead volume per cycle at working pressure, convert to free air, multiply by cycles per minute and add continuous users such as blow-offs and leakage. Peak demand during simultaneous motions can be much higher than the average. We can measure actual consumption at the machine inlet to confirm compressor and pipe sizing.
Related pages
Field guide
The pneumatic component chain, from compressor to actuator
A pneumatic system turns stored air energy into controlled motion. Its performance depends on every part between the compressor and the actuator, not on the compressor alone. The aim is clean, dry, regulated air at the right pressure and flow where the work happens.
| Part | What it does |
|---|---|
| Filter-regulator (FRL) unit | Removes dust, rust, oil aerosol and condensate, and sets the working pressure. Add lubrication only where the component maker requires it; many modern parts are pre-lubricated and oil mist can harm them |
| Air receiver | Buffers pressure, covers peaks, reduces compressor cycling and drops out moisture |
| Air dryer | Removes water vapour to prevent corrosion, sticking valves, frozen lines and instrument faults |
| Ball valve | Fast, low-loss manual isolation for maintenance and emergencies |
| Directional control valve (3/2, 5/2, 5/3) | Sets airflow direction and so the cylinder cycle |
| Flow control valve | Sets actuator speed, positioning and shock |
| Quick exhaust valve | Vents the cylinder directly for faster movement |
| Silencer | Cuts exhaust noise and keeps dirt out of exhaust ports |
| Check valve | Stops reverse flow and isolates circuits |
| Pressure relief valve | Protects compressors, receivers, pipework and people from over-pressure |
| Pressure gauge | Shows pressure loss and regulator or compressor trouble |
| Shuttle valve | An OR element that passes the higher of two supplies |
| Cylinder | Linear motion: push, pull, clamp, lift, position |
| Rotary actuator | Quarter-turn motion for ball, butterfly and plug valves |
| Tubing and push-in fittings | Distribution with low loss and fast, leak-resistant connections |
Related guidance
See end-use air preparation and FRL selection, cylinder sizing, valve flow and response and machine air consumption acceptance.
Where the money goes
Producing the air costs far more over the years than the hardware. Plants that run well find and fix leaks, hold air quality, watch pressure and flow, load compressors sensibly and cut pressure drop. See leak detection and loss management.
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Pneumatic cylinder air consumption
Work out the free air a cylinder uses per cycle and per minute.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Bore (mm): 63
- Stroke (mm): 200
- Working pressure (bar g): 6
- Cycles per minute: 20
- Number of cylinders: 8
- Double acting? (2 = yes, 1 = single): 2
- Rod diameter (mm; 0 to ignore): 20
Result
- Free air per cycle (each): 8.2 litres
- Total consumption: 21.9 L/s
- 1.311 m³/min = 46.3 cfm
- Theoretical force, outstroke: 1,870 N (before friction; allow 10–20% less)
- Theoretical force, return stroke: 1,682 N
Method. Air per stroke (free air) = swept area × stroke × (P + 1.013) / 1.013. The outstroke uses the full bore area π/4 × D²; the return stroke of a double-acting cylinder uses the bore area less the rod area π/4 × d². Enter rod 0 to ignore it (a small over-estimate).
What to take from it. Many small cylinders add up. Reducing stroke, pressure or cycle rate lowers consumption in direct proportion.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.