**Common Pneumatic Formulas**

Air Valves help to regulate pressure in the cylinder and are sized for flow capacity **(Cv)**. **Cv** is based on the specific cylinder stroke & travel time requirements, as well as piston size. **Cv** is a coefficient that measures the quantity of air a device can pass.

**Cv**= __Area (in²) x Length (ins.) x Compression Factor__

Pressure Drop Factor x Time (secs) x 29

**Area**= Effective cylinder piston area* (square inches)*

**A**= **πr² (3.14 x radius²)**

**PLEASE NOTE**: The same formulas apply for the rod end of the cylinder. However, to make precise calculations, one must take the cylinder area (in²) minus the rod end area (in²) when utilizing this valve sizing formula to determine return stroke Cv rating.

**Length**= The total cylinder stroke length in inches (in.)

**Compression Factor**= Taken from the table (based on supply pressure rating).

**Pressure drop factor**= 10 or 15 psi drop is a good guideline for using this formula (refer to Table 20 for further details)

**Time**= Required cylinder stroke time (in seconds)

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Pressure Drop Factor PD for Various Pressure Drops |
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Supply Pressure |
Compression Factor |
2 |
5 |
10 |
15 |
20 |

40 | 3.7 | 9.9 | 15.3 | 20.5 | 23.6 | N/A |

50 | 4.4 | 10.8 | 16.7 | 22.6 | 26.4 | 29 |

60 | 5.1 | 11.7 | 18.1 | 24.6 | 29 | 32 |

70 | 5.8 | 12.5 | 19.3 | 26.5 | 31.3 | 34.8 |

80 | 6.4 | 13.2 | 20.5 | 28.2 | 33.5 | 37.4 |

90 | 7.1 | 13.9 | 21.6 | 29.8 | 35.5 | 39.9 |

100 | 7.8 | 14.5 | 22.7 | 31.3 | 37.4 | 42.1 |

110 | 8.5 | 15.2 | 23.7 | 32.8 | 39.3 | 44.3 |

120 | 9.2 | 15.8 | 24.7 | 34.2 | 41 | 46.4 |

130 | 9.8 | 16.4 | 25.6 | 35.5 | 42.7 | 48.4 |

140 | 10.5 | 16.9 | 26.5 | 36.8 | 44.3 | 50.3 |

150 | 11.2 | 17.5 | 27.4 | 38.1 | 45.9 | 52.1 |

**Table 20 **

**Compression Factor**= Taken from Table 20 (based on supply pressure rating).

**Sizing Example**

- 6 inches bore cylinder with 2-inch rod thickness and 15-inch total stroke
- Travel time=2 seconds
- 100 psi supply pressure
- 15 psi pressure drop factor to be used

**Calculate the 6” diameter piston bore area (in square inches) for extend calculations**

**A**= 6 ins. x 6 ins. x .7854 = 28.27 (in²)

**PLEASE NOTE**: This is for the cylinder extend area. To calculate the cylinder return area, the rod area must be calculated (in²) and then this value must be subtracted from the piston bore area (in²).

**Calculate the 2” diameter rod end area in square inches**

**A**= 2 ins. x 2 ins. x .7854 = 3.1416 (in²)

Thus, Cylinder Return Area is 28.27 in² – 3.1416 in² = 25.12 in²

**Apply these application variables to the Cv sizing formula**:

Cv= |
28.27 in² x 15 ins. X 7.8 |
= | 3,307 |
1.52 Cv (to extend) |

37.4 x 2 secs. X 29 | 2,169 |

Cv= |
25.12 in² x 15 in x 7.8 |
= | 2,939 |
1.52 Cv (to extend) |

37.4 x 2 secs x 29 | 2,169 |

**Select a valve that meets this 1.52 Cv rating**

**Air Flow Rates**

**SCFM – Standard Cubic Feet per Minute** – One cubic foot of gas (air) per minute at conditions of:

14.69 Pounds per Square Inch (psi)

68 degrees F

Relative humidity of 36%

**Cubic Feet Per Minute (CFM): **A particular unit of measurement for airflow volume. It’s determined by how many cubic feet of air passes by a stationary point per minute.

**Free Air Flow**: How much flow is actually generated (in standard condition).

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