Machine Design

A few guidelines for figuring pneumatic-actuator load requirements

Cylinders are rather simple devices, and calculating the theoretical force output is fairly straightforward. But sizing a cylinder for a specific application can be a more challenging task.

Undersizing, for instance, is a common mistake. However, by following a few simple guidelines, engineers and technicians can quickly determine the right cylinder to fit a specific need.

Determine the force. The theoretical force output of a cylinder is the usable piston area multiplied by the applied air pressure (F = PA). For example, a cylinder with a 112-in. bore and an extend force of 80 psi generates 141 lb of force (1.767 × 80 = 141). This is the theoretical force output, but there are several factors that lower the actual output force. Keep the following issues in mind when sizing a cylinder.

Subtract the rod area, if applicable. The theoretical force output is the usable piston area multiplied by air pressure. If using the return stroke, or a double-rod cylinder, then subtract the rod area to calculate the usable piston area.

Know the actual operating pressure. Although a compressor is rated for a certain pressure, the pressure that the cylinder actually sees in operation can be much lower due to factors such as other equipment's air consumption or other air-supply restrictions. An air system that runs at 100 psi may drop to 80 psi or lower during peak air usage times.

Allow for internal friction. Because of internal friction, cylinders never reach their theoretical force output. Seals, bushings, wear bands and other load-support items all produce internal friction. The force loss is typically 1 to 10 psi of theoretical force output of the cylinder, depending on the type of seal. Cylinders with side loads, misalignment, or specialty features have even higher internal friction. Remember that cylinders convert pressure to linear force. Considerable side loads and bending moments should be handled separately.

Know the true load. Unless lifting the load vertically, it can be difficult to determine the true load. Determining force loss due to sliding friction is difficult because friction force = coefficient of friction × normal force, but the coefficient of friction is often difficult to determine. Tables list coefficients of friction for various materials, but small variances in this number produce large differences in required force.

If sizing a cylinder for an existing application, try to physically measure the required force. If it is a new application, conduct as many physical experiments as possible to prove the initial calculations. Even if lifting a load vertically, there will be friction if the load is guided.

Add speed requirements into the equation. After summing all forces, it is excess force that causes movement. Acceleration equals excess force divided by the total mass being moved (A = F/m). Total mass includes the mass of the cylinder piston-rod assembly. Keep in mind that air must get to the piston quickly enough to build adequate pressure to produce the required acceleration force. If this is a concern, contact your distributor to analyze your system and determine actual performance characteristics.

Consider the angles. If you are dealing with linkages or force-transfer angles, be sure to allow for force losses in angles. For example, the forces transferred to applications diminish when they work against a pin or other pivoting member instead of directly on the load.

The force transferred to the application equals force × sin(transfer angle). Force absorbed by the pivot equals force × cos(transfer angle). When the transfer angle exceeds 135° or is less than 45°, more cylinder force acts against the pivot than transfers to the application. Transfer angles above 150° and below 30° transfer less than half the cylinder force to the application and should be avoided.

Allow for future changes. Designing cylinders "to the edge" does not allow leeway for the future. It is a good idea to allow for slight increases in production requirements or unforeseen force losses. Adding other new equipment may also affect the available air pressure. A regulator can always reduce output force, but it's not so easy to add force.

Consider kinetic energy. Don't overlook the kinetic energy associated with the moving load. Cylinders have some ability to absorb kinetic energy, but their primary purpose is to convert pressure to linear force. Effective use of a shock absorber can transform a potentially destructive moving mass into a good application.

Test the results. Sizing a cylinder exactly to an application, especially without any testing, can be a mistake. Once an application has been built it can be very costly to make changes to allow a larger bore. Tandem cylinders can be used to increase force and extend the cylinder length, but they are less efficient than using a larger-bore cylinder. Unless there are strict air consumption or time requirements, it is a good idea to oversize the cylinder.

Information for this article was provided by Joe Malloy, Chief Engineer, Numatics Actuator, Franklin, Tenn.

TAGS: Hydraulics
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