Cylinder Bore Selection Guide: Sizing Principles, Formulas, and Airtac Specifications

July 3, 2026

In pneumatic automation design, selecting the correct cylinder bore size is a foundational engineering decision that directly impacts system force output, operational velocity, kinetic energy management, and air consumption efficiency. The "bore" refers to the internal diameter of the pneumatic cylinder barrel.

Sizing a cylinder bore incorrectly introduces systemic vulnerabilities: an undersized bore fails to overcome load inertia, causing system stalls, while an oversized bore increases procurement costs, expands the physical installation footprint, and wastes significant volumes of compressed air. This technical guide outlines the mathematical principles of bore selection, details the application of load factors, and references standard bore specifications within the Airtac product architecture.

1. The Physics of Bore Sizing: Core Theoretical Formulas

The primary function of a pneumatic cylinder is to convert compressed air pressure into linear force. The theoretical force output is governed directly by the surface area of the internal bore and the operating pressure of the system.

A. Theoretical Force Equations

The relationship between force ( $F$ ), pressure ( $P$ ), and cross-sectional area ( $A$ ) is expressed by the standard physical formula:

F = P \times A

Because a standard single-rod cylinder features different effective surface areas depending on the direction of movement, calculations must be divided into extension and retraction strokes.

Extension Stroke (Push Force): The compressed air acts upon the entire circular surface area of the cylinder bore.

$F_{\text{ext}} = P \times \left( \frac{\pi \times D^2}{4} \right)$

(where $D$ represents the cylinder bore diameter).
Retraction Stroke (Pull Force): The effective surface area is reduced because the piston rod cross-section occupies a portion of the cylinder bore space.

$F_{\text{ret}} = P \times \left( \frac{\pi \times (D^2 - d^2)}{4} \right)$

(where $D$ represents the cylinder bore diameter, and $d$ represents the piston rod diameter).

B. Standard Metric Units for Engineering Calculation

To ensure computational accuracy within Airtac metric specification frameworks, engineers apply the following unit standards:

Force ( $F$ ): Measured in Newtons ( $\text{N}$ ).
Pressure ( $P$ ): Measured in MegaPascals ( $\text{MPa}$ ). (Note: $1\text{ MPa} = 10\text{ bar} \approx 145\text{ psi}$ ).
Bore Diameter ( $D$ ) and Rod Diameter ( $d$ ): Measured in millimeters ( $\text{mm}$ ).

When units are configured this way, the formula evaluates as:

F = P \times \frac{\pi \times D^2}{4}

2. The Critical Factor: Load Factor ( $\eta$ ) Sizing Metric

In real-world factory automation, a cylinder cannot be sized purely on theoretical maximum force output. Static friction, dynamic friction of internal elastomeric seals, air flow resistance through upstream solenoid valves, and load inertia require the application of a Load Factor (η).

The load factor is defined as the ratio of the actual external load force ( $F_{\text{load}}$ ) to the theoretical cylinder force output ( $F_{\text{theoretical}}$ ):

\eta = \frac{F_{\text{load}}}{F_{\text{theoretical}}} \times 100\%

To guarantee stable velocity profiles and ensure the cylinder overcomes dynamic transitions, engineering standards dictate specific load factor limits based on the operational velocity and state of the application:

Application State / Motion Profile	Recommended Load Factor (η)	Sizing Context
Static / Clamping Tasks	$\eta \le 70\%$	Low velocity, primarily holding a fixed force.
Low Speed / Standard Motion	$\eta \le 50\%$	Operating velocities between $50\text{ mm/s}$ and $500\text{ mm/s}$ .
High Speed / High Inertia	$\eta \le 30\% \text{ to } 40\%$	Operating velocities exceeding $500\text{ mm/s}$ or heavy mass movement.

Engineering Sizing Criterion

The final bore selection must satisfy the following condition to ensure operational stability:

F_{\text{load}} \le P \times \frac{\pi \times D^2}{4} \times \eta

3. Standard Cylinder Bore Matrices Across Airtac Series

Airtac manufactures pneumatic cylinders categorized into distinct standardized series, each featuring a fixed array of bore dimensions aligned with international standards (such as ISO 15552).

A. Standard Standard-Bore Series (e.g., SE, SI Series)

Designed for heavy-duty industrial movement, these series utilize traditional tie-rod or profile barrel layouts.

Standard Available Bores ( $D$ ): Ø $\phi 32, \phi 40, \phi 50, \phi 63, \phi 80, \phi 100, \phi 125, \phi 160, \phi 200\text{ mm}$ .
Structural Note: As the bore diameter increases, the structural cross-section of the corresponding piston rod ( $d$ ) increases proportionally to prevent structural buckling under high compression forces. For example, a standard Ø $\phi 32\text{ mm}$ bore cylinder typically incorporates a Ø $\phi 12\text{ mm}$ piston rod, whereas a Ø $\phi 100\text{ mm}$ bore cylinder incorporates a Ø $\phi 25\text{ mm}$ piston rod.

B. Compact Series (e.g., SDA Series)

Optimized for tight installation footprints where space constraints are severe and stroke lengths are short.

Standard Available Bores ( $D$ ): Ø $\phi 12, \phi 16, \phi 20, \phi 25, \phi 32, \phi 40, \phi 50, \phi 63, \phi 80, \phi 100\text{ mm}$ .
Application Context: Smaller bore compact cylinders (Ø $\phi 12$ to Ø $\phi 25$ ) are frequently utilized in electronics assembly, pick-and-place modules, and small fixture clamping.

C. Miniature Round-Body Series (e.g., MAL Series)

Standard Available Bores ( $D$ ): Ø $\phi 16, \phi 20, \phi 25, \phi 32, \phi 40\text{ mm}$ .

4. Step-by-Step Technical Bore Selection Framework

When designing a pneumatic circuit or processing a parts list for Airtac actuators, engineers follow this structured verification layout to determine the exact bore code:

Step 1: Quantify the Actual External Load

Determine the physical mass to be moved and calculate the required force in Newtons. For vertical lifting tasks, force is calculated as mass ( $m$ ) multiplied by gravitational acceleration ( $g \approx 9.8\ \text{m/s}^2$ ). Consider any mechanical resistance or guide friction present in the external track system.

Step 2: Establish Minimum Operating Pressure

Identify the lowest reliable system air pressure at the machine interface after accounts for pressure drops across upstream FRL filters and directional control valves. Do not size based on maximum compressor plant pressure; size based on local minimum operating limits (standard industrial assumptions use $0.5\text{ MPa}$ or $0.6\text{ MPa}$ ).

Step 3: Compute Minimum Required Cylinder Area

Apply the required load factor (η) corresponding to the target velocity profile. Calculate the target cross-sectional area:

A_{\text{required}} = \frac{F_{\text{load}}}{P_{\text{minimum}} \times \eta}

Step 4: Derive and Round Up the Bore Diameter

Calculate the absolute minimum bore diameter:

D = \sqrt{\frac{4 \times A_{\text{required}}}{\pi}}

Cross-reference the calculated value of $D$ against standard Airtac catalog bore size increments (Ø $\phi 32, \phi 40, \phi 50$ , etc.). Always round up to the nearest larger standard manufactured bore size to ensure safety margins are maintained.

5. Field Procurement Verification Checklist

Prior to final processing of an Airtac cylinder part number sequence, verify the following configuration interactions:

Verify Valve Flow Capacity Balance: Ensure the upstream solenoid valve (e.g., Airtac 4V Series) has a flow coefficient ( $Cv$ ) or nominal flow rate sufficient to fill the selected cylinder bore volume at the target velocity. Large bore cylinders (≥ Ø $\ge \phi 80\text{ mm}$ ) combined with restrictive small-ported valves will stall or drop in speed due to restricted flow volume.
Confirm Pressure Envelope Compliance: Ensure the working pressure of the system falls within the standard Airtac cylinder operational envelope of $0.15\text{ to } 1.0\text{ MPa}$ ( $21\text{ to } 145\text{ psi}$ ).
Check Cushioning Requirements: For large bores (≥Ø $\ge \phi 50\text{ mm}$ ) executing high-speed movements, ensure the selected series features internal adjustable pneumatic cushions (standard on SE/SI series) to safely dissipate kinetic energy and protect the cylinder end caps.
Verify Mounting Bracket Mechanics: Confirm that all selected structural mounting accessories (such as LB foot mounts, FA flange plates, or CB clevis joints) match the exact bore dimension code specified in the cylinder part number schedule. Mounting hardware dimensions change for every distinct bore size.

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Cylinder Bore Selection Guide: Sizing Principles, Formulas, and Airtac Specifications

1. The Physics of Bore Sizing: Core Theoretical Formulas

A. Theoretical Force Equations

F = P x A

B. Standard Metric Units for Engineering Calculation

2. The Critical Factor: Load Factor ( $\eta$ ) Sizing Metric

Engineering Sizing Criterion

3. Standard Cylinder Bore Matrices Across Airtac Series

A. Standard Standard-Bore Series (e.g., SE, SI Series)

B. Compact Series (e.g., SDA Series)

C. Miniature Round-Body Series (e.g., MAL Series)

4. Step-by-Step Technical Bore Selection Framework

Step 1: Quantify the Actual External Load

Step 2: Establish Minimum Operating Pressure

Step 3: Compute Minimum Required Cylinder Area

Step 4: Derive and Round Up the Bore Diameter

5. Field Procurement Verification Checklist

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1. The Physics of Bore Sizing: Core Theoretical Formulas

A. Theoretical Force Equations

F = P x A

B. Standard Metric Units for Engineering Calculation

2. The Critical Factor: Load Factor (η) Sizing Metric

Engineering Sizing Criterion

3. Standard Cylinder Bore Matrices Across Airtac Series

A. Standard Standard-Bore Series (e.g., SE, SI Series)

B. Compact Series (e.g., SDA Series)

C. Miniature Round-Body Series (e.g., MAL Series)

4. Step-by-Step Technical Bore Selection Framework

Step 1: Quantify the Actual External Load

Step 2: Establish Minimum Operating Pressure

Step 3: Compute Minimum Required Cylinder Area

Step 4: Derive and Round Up the Bore Diameter

5. Field Procurement Verification Checklist

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2. The Critical Factor: Load Factor ( $\eta$ ) Sizing Metric