In the majority of pneumatic applications, more than one cylinder is used in a circuit. Depending upon the application, the cylinders may be required to operate at different time intervals and be expected to exert different forces. There are a wide variety of pneumatic circuit designs that enable this type of operation to meet the requirements of an application.

1 typical application using more than one cylinder is a pneumatic-powered robotic arm. 1 robotic arm uses two or more cylinders, which must be synchronized with each other to perform precise movements for exact positioning (Figure 20-5).


The purpose of estimating the air consumption of a cylinder is usually to find the horsepower capacity that must be available from the air compressor to operate the cyl­inder on a continuous cycling application. 1ir consumption can be estimated from Table 20-1. The consumption can then be converted into compressor horsepower.

Figures in the body of the table are air consumption for cylinders with standard diam­eter piston rods. The saving of air for cylinders with larger diameter rods is negligible


for most calculations. Air consumption was calculated assuming the cylinder piston will be allowed to stall, at least momentarily, at each end of its stroke, giving it time to fill up with air to the pressure regulator setting. If reversed at either end of its stroke before full stall occurs, air consumption will be less than that shown in the table.

The first step in the calculation is to be sure that the bore size of the selected cylinder will just balance the load at a pressure of 75 percent or less of the maximum pressure available to the system. This leaves about 25 percent of available pressure, which can be used to overcome flow losses through piping and valving. This surplus pressure must be available or the cylinder cannot travel at normal speed.

Determine the exact air pressure needed to just balance the load resistance. Add about 25 percent for flow losses and set the system regulator to this pressure. This is also the pressure figure, which should be used when going into the table. After determining the regulator pressure, go into the proper column of the table. The figures shown in the table are the air consumption for a l-inch stroke, forward and return. Take this fig­ ure and multiply by the number of inches of actual stroke and by the number of com­ plete cycles forward and back that the cylinder is expected to make in 1 minute. This gives the standard cubic feet per minute (scfm) for the application.

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