The plant gauge shows 6 bar. Will four pneumatic vises still clamp in 0.3 seconds? The gauge alone cannot answer that question. It shows pressure at one place and one moment. It does not show how much air each motion needs or how far pressure falls while other loads are active.
Use three separate values. Air per cycle, multiplied by cycle rate, sets average branch and compressor demand. Flow during the shortest motion sizes the filter, regulator, valve, tube, fittings, and exhaust. Minimum pressure at the fixture while it moves is the final acceptance result.
The release rule:
Pfixture,min ≥ Pdevice,min + approved project margin, while the clamp or release motion also finishes within its time limit. If either condition fails, the air path is not accepted.
Three air values answer three different questions
| Value | How to get it | What it decides |
|---|---|---|
| Reference air per cycle | Add the air that fills and vents for every clamp and release, plus line volume that is filled and vented each cycle. | Input to average branch demand and the energy estimate. |
| Stroke-average required flow | Divide the reference air for one motion by the allowed motion time. Add loads that overlap. | First-pass flow target for the local air path. |
| Minimum dynamic fixture pressure | Log pressure at the fixture inlet during the worst planned overlap. | Pass or fail against the exact workholding device requirement. |
Do not call the second value an exact instantaneous peak. Real flow changes through the stroke as pressure builds. Use the selected component's flow curve or ISO 6358 data for the final choice. The SMC engineering calculator also separates cycle consumption from required flow.
FRL means filter, regulator, and lubricator. The letters name three functions, not a rule that every circuit needs all three. The device manuals decide the required air class and whether oil is required, allowed, or forbidden.
The sizing sheet needs eight exact inputs
Complete this sheet for the real cell. A blank field is a stop sign for sizing, not an invitation to copy a value from another machine.
| Input | Record this | Evidence |
|---|---|---|
| Workholding device | Exact vise, plate, cylinder, or booster model and circuit type. | Current drawing and manual. |
| Actuator data | Supplier air per operation, or bore, rod, stroke, and working pressure for each powered chamber. | Model data sheet. |
| Motion target | Allowed time for clamp and release, measured from command to the required confirmation. | Cell sequence and acceptance plan. |
| Demand timing | Cycles per minute, quantity moving together, and the start and stop time of every air load. | Programmable logic controller (PLC) timing chart or dry-cycle log. |
| Supply | Lowest header pressure while the plant and this cell are running. | Logged pressure, not compressor nameplate pressure. |
| Air path | Exact filter, regulator, valve, silencer, quick coupling, fitting, tube inside diameter, and tube length. | Part numbers and layout. |
| Other loads | Robot gripper, seat check, air purge, chip blow, leak, and any booster demand on the same branch. | Supplier curves and measured duty time. |
| Acceptance limits | Device minimum pressure, lowest applicable maximum pressure, allowed motion time, approved margin, and air purity class. | Design review and exact device manual. |
Why the two cylinder chambers need separate calculations
Use the supplier's published air-per-operation value when it matches the exact model and pressure. If the device uses a known single-rod, double-acting cylinder, calculate the cap end and rod end separately. The rod takes up space, so multiplying one chamber by two is wrong.
With bore D, rod diameter d, and stroke L in millimetres:
Vextend = π × D² × L ÷ (4 × 1,000,000)
Vretract = π × (D² − d²) × L ÷ (4 × 1,000,000)
Both results are the geometric chamber volume in litres. Convert a chamber to reference litres with Vref = Vchamber × (Pabs ÷ Pref) × (Tref ÷ Tabs). Use absolute pressure and absolute temperature.
The worked example below defines one reference litre as the amount of air at 1.00 bar absolute and 20°C. Record the humidity rule used by the component supplier. If another data sheet uses standard cubic feet per minute (SCFM), normal litres per minute (Nl/min), or L/min at its ANR reference state, copy its stated reference pressure, temperature, and humidity before converting the number. ISO 8778 defines a standard reference atmosphere for pneumatic data.
Worked example: one small double-acting actuator
This is a planning example, not a NEXTAS product rating. The cylinder has a 50 mm bore, a 20 mm rod, and a 25 mm stroke. Supply pressure is 6 bar gauge. Reference pressure and local atmospheric pressure are both taken as 1 bar absolute. Supply and reference temperature are equal.
- Cap-end geometric volume: 0.0491 L.
- Rod-end geometric volume: 0.0412 L.
- Extend air at the reference state: 0.3436 L.
- Retract air at the reference state: 0.2886 L.
- One full extend-and-retract cycle: 0.6322 reference L, before any line volume or leakage is added.
At six full cycles per minute, the actuator averages about 3.8 reference L/min. If extension must finish in 0.25 seconds, the stroke-average required flow is about 82.5 reference L/min. Retraction in 0.25 seconds needs about 69.3 reference L/min. The 82.5 L/min event, not the 3.8 L/min cycle average, is the first-pass sizing case.
Add only tube or dead volume that fills and vents during that motion. Add continuous purge or leakage as flow, multiplied by its real duty time. Do not add a volume in litres directly to a flow in litres per minute.
The overlap window sets the highest demand
A cell can have a low average demand and a severe quarter-second demand. Put every load on one timeline. Then add flow only inside the time windows that overlap.
| Time window | Active load | Flow used in the worksheet |
|---|---|---|
| 0.00–0.25 s | Two example cylinders extend together. | 2 × 82.5 = 165 reference L/min. |
| 0.10–0.60 s | A chip-blow nozzle runs. | Add the exact nozzle flow from its pressure/flow curve. Call it Qblow. |
| 0.10–0.25 s | Both actions overlap. | 165 + Qblow. This is the largest worksheet event. |
| 0.25–0.60 s | Only chip blow remains. | Qblow. |
Moving the blow command to start at 0.25 seconds would change the combined case from 165 + Qblow to the larger of 165 or Qblow. That timing change may cut the required peak delivery without changing the vise. Check that it does not harm chip clearing, seating, or cycle logic.
A normal pressure switch consumes almost no process air. An air-sensing circuit or open nozzle can consume air continuously. Use the exact sensing or nozzle curve. A booster also has its own air use and refill time. Do not treat either as a fixed allowance.
Port size is not a flow rating
A 1/4-inch port tells you the thread size. It does not tell you how much outlet pressure remains at 165 L/min. Two regulators with the same port can have different flow curves and different pressure loss.
- Filter: take the required air-purity class from the most sensitive downstream device. On the exact filter curve, find the loss at the combined event flow. A finer filter adds resistance, so do not specify one without a device or process need.
- Regulator: use the lowest measured inlet pressure, the chosen set pressure, and the event flow. The curve must keep its outlet above the fixture minimum plus the approved margin.
- Valve: check both clamp and release at their real inlet and outlet pressures. Use the manufacturer's flow curve or stated flow-capacity data, including ISO 6358 data when supplied. Include the effect of the exhaust port and silencer.
- Tube and fittings: use actual inside diameter, length, bends, banjo fittings, quick couplings, and manifolds. Check supplier pressure-drop data or test the assembled path.
- Lubricator: install it only when every affected downstream component requires or permits that oil. If one component forbids oil, a common lubricator is not acceptable.
The U.S. Department of Energy compressed-air sourcebook identifies undersized point-of-use parts as a cause of pressure drop and advises using manufacturer pressure-drop data. Festo's operating guide also notes that each filter adds flow resistance and that service-unit size depends on total air use.
Record the exact air-purity class required by the device manual using ISO 8573-1. Set the inspection and replacement rule from the selected filter manual or its condition indicator. Do not substitute a universal micron grade or calendar interval.
A receiver covers only a calculated shortfall
A local receiver can bridge a short flow gap. It works only if it has enough usable pressure range and enough time to refill before the next demand. A first-pass, near-isothermal estimate is:
Vreceiver ≈ Qnet × t × Pref ÷ (Phigh,abs − Plow,abs), where Qnet is the demand still missing after upstream flow is counted, in reference L/s; t is seconds; Pref is the absolute reference pressure; and Phigh,abs − Plow,abs is the allowed receiver pressure fall from event start to event end. Use the same pressure unit for all three pressures.
Recheck the result for temperature, valve response, refill time, and the real event trace. The receiver also needs a valid pressure rating, relief protection, drainage, isolation, and compliance with local pressure-vessel rules. It is not a repair for a blocked fitting or restricted exhaust.
A pneumatic-to-hydraulic booster is also model-specific. Record its ratio, air use, oil displacement, maximum air inlet, maximum hydraulic outlet, refill time, and safe loss-of-air state. Never infer those values from a generic ratio. If the selected workholding device already gives consumption per actuation, use that model data.
Three pressure traces locate the restriction
Use a pressure logger at three points: P0 before the local air-preparation unit, P1 after its final filter, regulator, or lubricator, and P2 at the fixture inlet. Record the sample interval. Repeat the worst event at a faster interval until the measured minimum pressure is stable within the approved test tolerance. Use the longest real tube, installed valve and silencer, lowest planned header pressure, and worst allowed overlap.
| What the trace shows | First place to investigate |
|---|---|
| P0 falls during the event. | Plant header, branch capacity, upstream storage, or another shared load. |
| P0 is stable but P1 falls too far. | Local filter, regulator, lubricator if fitted, pressure setting, contamination, or drain condition. |
| P1 is stable but P2 falls too far. | Valve, manifold, coupling, fitting, tube, or fixture inlet restriction. |
| P2 passes, but motion is late. | Actuator load, mechanical drag, valve command, exhaust restriction, or confirmation sensor. |
For every test run, record P0 minimum, P1 minimum, P2 minimum, command time, clamp-confirm time, release-confirm time, and pressure-recovery time. The pass line is explicit:
P2 minimum ≥ device minimum + approved margin;actual motion time ≤ allowed motion time;highest observed pressure ≤ lowest component maximum rating;- pressure recovers before the next planned demand; and
- no clamp, seat, low-pressure, or sequence fault is hidden by an automatic retry.
If the project has not approved the pressure margin or motion limit, the test has no pass criterion. Do not raise regulator pressure to hide a loss. First locate the loss with P0, P1, and P2. Then change the restricted component or sequence and repeat the same test.
Safety boundary: a normal pressure switch is a process signal. It does not prove part seating, mechanical locking, or a safety-rated function. Define the cell response to low pressure, lost air, emergency stop, and restart. Before service, isolate and lock the supply, vent stored pressure, and verify zero pressure.
ISO 4414 covers general pneumatic-system safety requirements. The machine builder must still apply the cell risk assessment and validate its safety functions.
What a reviewable air-sizing package contains
Send the completed sizing sheet, demand timeline, supplier flow curves, and worst-event P0/P1/P2 log. Mark every file with its part number, revision, and test date. NEXTAS can review the named workholding interface and supplied air-path data. Final cell sizing and safety acceptance remain tied to the selected components and measured test.
Calculation and selection sources
- SMC air consumption and required-flow calculator — separate cycle consumption and motion flow.
- Festo general operating conditions — cylinder chamber calculation, air preparation, and lubrication limits.
- U.S. Department of Energy compressed-air sourcebook — point-of-use pressure drop and component selection.
- ISO 8778, ISO 8573-1, and ISO 4414 — reference atmosphere, air-purity classes, and pneumatic safety.
Common questions about pneumatic air sizing
Why is port size not enough to size an FRL?
Port size tells you what thread connects to the component. It does not show pressure loss at your inlet pressure, outlet pressure, and flow. Select the exact filter and regulator from their manufacturer flow curves, then test the pressure at the fixture.
What is the difference between air per cycle and flow during a stroke?
Air per cycle is the total reference volume filled and vented in one cycle. It helps estimate average capacity and energy use. Stroke flow divides the air for one motion by its allowed time. That shorter event often sets the filter, regulator, valve, and tube size.
Can a local receiver fix every pressure-drop problem?
No. A receiver can cover a calculated short demand when it has enough usable pressure range and can refill before the next event. It cannot repair a restricted valve, fitting, exhaust path, or a supply that stays short during a long demand.
Why must SCFM and Nl/min include their reference conditions?
Both express air as a reference volume, but suppliers may use different reference pressure, temperature, and humidity. Record the definition from each data sheet before comparing or converting values.
How do I prove that a pneumatic fixture has enough air?
Log pressure before the local air-preparation unit, after its final component, and at the fixture during the worst planned overlap. Pass only when fixture pressure stays above the selected device minimum plus the approved margin, motion finishes within its time limit, and pressure recovers before the next demand.