Why correctly sizing pneumatic components is important

The most important function in the design of a pneumatic system is to correctly size the pneumatic components. Too often, components are chosen primarily because of what is readily available at the time or because the component is already currently being used throughout a facility. Improperly sourced components can result in an inefficient system leading to decreased productivity and increased cycle times, along with increased component and energy costs associated with compressed air. Following is an example of the necessary steps to a properly designed system.

Actuator Selection

Actuator selection is the most critical aspect in designing a system. If the actuator is too large, all other components will need to be oversized to meet required cycled times and energy cost will increase. If the actuator is too small, the load will either move erratically or not at all. Sizing an actuator includes these key factors:

  1. Determine the force in pounds required to move the load. Add 25% to provide enough power to overcome friction and move the load at less than 4 inches/second, add 50% to move the load at 4 to 16 inches/second, and/or add 100% to move the load at speeds above 16 inches/second.
  2. Find out the minimum air pressure in pounds/square inch (PSI) that will be present to operate the actuator.
  3. Divide the force by the pressure to determine the power factor required.
  4. Choose the next largest power factor in the chart below to determine the bore size.

actuator-selection

Example: Estimated force needed is 220 pounds and air pressure available is 80 PSI. Therefore, 220 ÷ 80 = 2.75 power factor. For this application and using the chart above, it’s then necessary to round up the power factor to 3.1 and have the bore size be 2”.

Valve Selection

Calculating the Cv required is the best method for selecting a valve. Cv, or flow coefficient, is a way to measure a components ability to flow air. The higher the Cv, the more the valve will flow. Most valve manufacturers will list their Cv ratings, making it relatively easy to compare the many different valves available. Valves today are reduced in size while flow ratings are increased, so you can no longer judge the valve by its port size.

The following equation can be used to determine the Cv required for your application.

Cv = (Area x Stroke x C) ÷ (Time x 29).

Area = π x radius squared, also the same as power factor in the cylinder sizing chart.

Stroke = Cylinder travel in inches

C = Constant (see table)

Time = in secondsvalve-selection

* “C” constant based in a 5 PSI pressure drop for most normal applications

 

Example:  2” bore actuator with a 6” stroke cycling 60 times a minute at 80 psi. Cv = (3.1 x 12 x .3072) ÷ ( 1 x 29) = 11.4278 ÷ 29 = .394 Cv. Therefore, our valve selection will need a minimum Cv of .394.

For more information on general guidelines for selecting the proper equipment, please contact one of our Fluid Power specialists today at fluidpower@jhfoster.com.

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