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)
Click edit button to change this text.
|Pressure Drop Factor PD for Various Pressure Drops|
|Supply Pressure||Compression Factor||2||5||10||15||20|
Compression Factor= Taken from Table 20 (based on supply pressure rating).
- 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).
Connect with Us