|Bore Dia. & Cylinder Area|
The equations below & chart at right show the relationship between the force required to move a load, the pressure available and the required cylinder bore size. Always factor in 25% more force than actually required to overcome friction, pressure drop, and other factors.
Given an estimated force required of 900lbs (including the 25% overage for friction, pressure drop, etc.) and that the system will operate at 80PSI:
The force a cylinder can apply during extension is a simple calculation (the inverse of the above formula): the effective surface area of the cylinder's piston × the differential pressure. For example: a 4" cylinder has a surface area of 12.57 square in. (πr²). If that cylinder is extended with 100 PSI of air pressure, it can supply a force of 1257 pounds.
When retracting a double acting cylinder, the cylinder rod blocks a portion of the effective surface area. If that same 4" cylinder has a 1" rod, the effective surface area is reduced to 11.78 sq. in. and it can only supply 1178 pounds of force while retracting (given the same 100 PSI differential pressure).The following chart shows the available force (in pounds) for both extension and retraction. White rows show extension force, which takes advantage of the full piston area. Grey rows show retraction forces with the rod diameter taken into account.
If you choose a cylinder with spring return, be sure to factor in the additional force needed to overcome the spring during extension.
Pressures shown across the top of the chart are differential pressures across the two cylinder ports. In practice, the air supply line must supply another 5% of pressure to make up for cylinder loss, and must supply 25-50% additional pressure to make up for flow losses in lines and valving so the cylinder will have sufficient travel speed.
Systems should always be designed with calculated forces at least 25% above the actual requirements.
|Cylinder Force (Pounds)|
|Differential Pressure (PSI)|