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Choosing an air cylinder without a full understanding of how it will perform in a pneumatic circuit can have long lasting, costly consequences.

There may be no stronger ally of electrical power utilities than industrial size air compressors, as they drone on every day, taking atmospheric air and transforming it into useful energy. Countless kilowatt hours are gobbled up in a mechanical conversion of electrical power into pneumatic power, a process that is wickedly inefficient, with one horsepower of pneumatic energy costing six times as much to generate when compared to one horsepower of electrical energy. Nevertheless, with its tremendous versatility, efficiency, and widespread use throughout many sectors, compressed air provides a clean, reliable source of pneumatic power that has a value outweighing its cost to produce.

The cost of having access to that source of pneumatic power is often accepted as the price of doing business. But, for those unwilling to accept the high costs of producing and using compressed air, there are cost reduction options. This article will look at one type of pneumatic component, air cylinders, and investigate how taking the time to correctly size a cylinder and its related components can dramatically reduce compressed air costs.

Pneumatic Energy

Figure 1. A round-body style pneumatic cylinder and a square-body NFPA style cylinder from Parker Hannifin. The round-body style reduces cost in light-duty applications.

Air cylinders, or linear actuators as they are commonly called, are used to convert pneumatic energy into a usable motion (Figure 1). Compressed air enters one end of the cylinder through a port, moving a piston rod at a desired speed and force. This force can be used to move an object or hold that object in place. The theoretical force that a cylinder is capable of generating is calculated by multiplying the system air pressure by the area of the piston face; pressure losses in the system will decrease the actual force generated. In the simplest of terms, when sizing an air cylinder for an application, the force generated by the cylinder must be greater than the load that is to be moved. For instance, a 100-pound steel plate that has to be moved into place on a machining center would require a linear actuator capable of generating at least 100 pounds of force. Given the equation of Force = Pressure × Area, greater forces are achieved by either increasing air pressure or increasing the bore and therefore the piston size of the cylinder.

With simple force equations and catalogs full of numerous configurations of cylinders of varying bores, the selection of a suitable cylinder for an application can be fairly straightforward. But, what if the same machining center from the previous example required that the actuator move the 100-pound steel plate into position every 2 seconds, with a dwell time of 2 seconds, at 15 cycles per minute? What about variables such as friction, pressure drop, and the environment where the cylinder will be used? The choice now becomes a little more complex, and making that choice without a full understanding of how that cylinder will perform in a pneumatic circuit can have long lasting, costly consequences resulting from wasting of precious compressed air.

An experienced engineer who can design a new pneumatic circuit on a clean piece of paper is typically able to enlist the components needed to construct a functional system that will provide the requisite air flow and pressure to generate the force needed to move a load with a linear actuator. Conversely, an engineer or maintenance person charged with the duty of sizing components to fit into an existing system or having to design a circuit based on system pressures, flows, or line sizes that are incompatible with the forces required will have a much tougher time putting an efficient system in place. Unfortunately, the second scenario is more the norm, with mismatched components being used for the sake of convenience or getting a line that is down back in business in short order.

Oversized Actuator

Figure 2. The cost differences between a range of cylinders. NFPA types are heavy-duty, long-life units. Disposable types are lighter duty and non-repairable.

When pneumatic component sizing decisions are made in these types of situations, it almost always ends up with an oversized actuator being selected, with the engineer wanting to ensure that the cylinder will be able to do its job. Those short-sighted decisions come with costs. Larger cylinders are more expensive than their smaller counterparts, which is a relatively obvious cost. But, what about the larger volume of compressed air that will be required to enable the larger cylinder to perform as needed when compared to a smaller cylinder that would perform adequately?

Let’s take a closer look at these potential cost differences with the following example. A pneumatic actuator is needed to move a 250-pound load a distance of 28 inches in one second, and then retract in one second with a dwell time of one second. Thankfully, a doctorate in fluid mechanics is not needed to take the requirements of the application and figure out what actuator would be suitable for the task. Air cylinder manufacturers typically will have software that they use to do the heavy lifting of calculating force, flow, and pressure figures. (Parker Hannifin, for instance, has made their suite of sizing calculators and selection tools available both as an app for mobile phones, and on their website.) With our given set of parameters, we can take a look at some cylinders and their ability to meet those parameters, and their associated costs (Figure 2).

Each of these cylinders in Figure 2 would meet the requirements of the task at hand, providing the necessary force within the desired cycle time. Based solely on the requirements of the cylinder, it does not make much sense to pay 55% more for a cylinder that will not move that 250-pound load any better than the least expensive unit.

The initial cost of the cylinder only tells part of the story, though. Let’s take a closer look at the costs to power the cylinders from the above example to reveal the costly nature of compressed air, and the importance of using air cylinders that are correctly sized to fit their purpose (Figure 3).

Figure 3 shows that for this particular application, using an oversized cylinder would result in an increased energy cost of approximately $500 to $800 annually. It should be noted that these are annual operating costs for one cylinder, operating for one eight-hour shift, five days a week, for 50 weeks a year, which would be considered very light usage. When these dollar figures are multiplied by the dozens or hundreds of cylinders that are typically used in a manufacturing or industrial facility that operates around the clock, seven days a week, it is easy to see how compressed air that is used to power an oversized cylinder can waste a lot of money.

Ancillary Components

Figure 3. Operating costs for air cylinders. These calculations are based on 2,000 operating hours per year.

Equal consideration must also be given to properly sizing the ancillary components required to operate an air cylinder. It does little good to go through the work of diligently selecting a cylinder and then placing it into a circuit with a mismatched valve, regulator, and filter. The old adage of “the line size is the right size” makes life easier for maintenance personnel who might be more concerned with getting a machine operational than they are with maintaining a proper coefficient of flow for a pneumatic system. But, this common practice can lead to unacceptable pressure drops or system pressures that will adversely affect the operation of the entire pneumatic circuit.

With the cost of electricity continuing to rise, companies that are not already employing strategies to reduce their power consumption will find themselves at a distinct competitive disadvantage. Compressed air, with its high cost of generation, should be an early target of any company looking to reduce electricity usage. Proper sizing of air cylinders is a tool that can be used to reduce compressed air consumption, reducing costs while improving overall system efficiency.

This article was written by Rick Hand, product manager for Parker Hannifin Pneumatic Division North America, Richland, MI. For more information, Click Here.

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