By: Nathan Irvine – AVENTICS Corp
Rightfully, pneumatics is considered extremely simple, flexible and economical. Add to that, air-powered systems are quite durable—they routinely run for years trouble-free with little maintenance. That’s why they’re used in countless applications ranging from food, beverage and packaging operations to metalworking and automotive applications.
To get the most out of pneumatic systems, considering a few basic tips can help engineers save time and optimize system performance. Obviously, the components in a circuit need to be sized to provide the necessary force, speed and precision to do a job. But right-sized components are a prerequisite for low energy consumption, too. Energy costs make up more than half of total cost of ownership of pneumatics today, so not wasting compressed air pays off.
While every part of a pneumatic system plays an important role in proper engineering, arguably a good place to begin a design is where the application demands force and motion—at the cylinder. Here are some basic tips on specifying cylinders for load, speed and energy efficiency and, ultimately, lower purchase and operating costs.
Cylinders come in countless standard and special versions, including compact, mini and short-stroke designs; with round or square profiles; in light-duty “throwaway” units and rugged tie-rod constructions; and in rod-type as well as rodless designs.
Regardless of the specific type, however, sizing an air cylinder for an application is essential. Engineers can rely on “hand” calculations to come up with suitable options, or turn to the latest software tools to generate results. Let’s look at each alternative.\
For manual calculations, start with the load. Knowing the force and stroke requirements, and the available air pressure, engineers can readily determine the minimum piston diameter to get the job done. Consider the math for the venerable double-acting cylinder. Calculate required cylinder force from:
F = PA
Where P = pressure, psi; and A = cylinder piston area, in.2, noting that the area of the piston rod reduces the working area on the retract stroke.
So calculate extend force:
Fe = Pπdc2/4
Where dc = piston diameter, in. And retract force is:
Fr = Pπ(dc2 – dr2)/4
Where dr = rod diameter, in.
But never design a cylinder to just barely move the load, always allow a bit of margin. Select cylinder bore sizes to handle the expected load plus a reasonable safety factor. Depending on the application, experts generally recommend that the cylinder provide an extra 33 to 100% of calculated force to overcome internal friction from seals, bearings, guides and other external forces; pressure losses from clogged filters or restrictions from other components in the circuit; and pressure losses due to leaks that can develop throughout a system over time.
However, the specifics of the application determine the amount of force margin necessary to ensure proper operation. Be aware that cylinders with larger-than-necessary diameters increase air consumption and cycle time, and larger systems cost more to purchase and operate than properly sized ones. They are also heavier and take up more space, which can be critical when weight or mounting space is at a premium.
It also goes without saying that cylinder stroke should be no more than required. Longer-than-needed cylinders cost more, waste energy and, again, add to cycle time.
Another factor in sizing a cylinder is air consumption, and how it relates to other parts of the system. This takes into consideration the forces required to move a load at the specified pressure, extend and retract volumes and cycle times, and relating all that to air flow through the cylinder. Many U.S. manufacturers use standard cubic feet per minute (scfm) to size components, and also correlate that to flow coefficient Cv for other components. Calculating cylinder air flow lets engineers determine the correct size tubing, fittings, valves, filters and other components in the system.
First, size the cylinder for motion requirements. A good rule of thumb for attaining the necessary speed is to size the cylinder to handle double the load. (However, if speed is not important, using a force multiplier between 1.25 and 2.0 times the load may result in smaller cylinders and lower air consumption.)
From that, calculate the total volume per cycle. Recognize that in double-acting cylinders, extend and retract volumes differ due to the volume the rod displaces and must be calculated separately. For a basic cylinder:
Extend volume Ve = (πdc2/4)l, where l = stroke length, in.
Retract volume Vr = (π(dc2 – dr2)/4)l
Total volume per cycle V = Ve + Vr
Multiply total volume per cycle by cycles/min to calculate total volume/min, in.3/min. Multiply in.3/min by conversion factor 1728 in.3/ft3 to determine cubic feet per minute (cfm) flow. Finally, convert cfm to scfm. This conversion requires the compression ratio of compressed air. This converts compressed air to standard conditions (14.7 psia, 36% relative humidity, and 68°F) and gives the working pressure in absolute terms. In most industrial applications, ambient temperature and humidity can be ignored because they have little impact on the calculations. Compression ratio for air at 80 psi, for example, is (80 + 14.7)/14.7 = 6.44. Multiply compression ratio by cfm to get scfm.
Knowing the cylinder’s air requirements, use this information and manufacturers’ catalog data for scfm and Cv to properly size valves, fittings, flow controls, FRLs and other system components. Note that all components that conduct air resist flow to some degree, and pressure drop across each device will increase with flow. Cv specifications help evaluate typical circuits for potential bottlenecks, as the total system Cv is less than that of the component with the smallest Cv.
Such tedious and, sometimes, error-prone calculations for forces, flows and pressure drops may be necessary when an OEM or designer needs to fine-tune a design for exacting performance or minimal costs. But often, engineers are better off turning to online calculators for fast, efficient and accurate component selection and overall design.
That’s why many pneumatics manufacturers have developed software tools that encompass theoretical operating parameters, well-established sizing equations, predetermined safety factors and real world experience to arrive at conservative, but not overly designed, product selections.
The tools are very well-accepted. Users trust they will receive an answer that technically works well, with the understanding that there may be some room for minor adjustments away from the conservative side. However, different manufacturers’ tools may apply different safety factors in their calculation software, or adjust them based on specific cases like cylinder orientation, the effects of gravity, or even the type of application. But results from one tool to another are usually similar.
Interactive tools such as the AVENTICS CylinderFinder let the user describe the intended application in terms of loads, cycle times, system pressure, mounting orientation and the like, and in a few seconds the tool replies with the recommended cylinder dimensions and applicable products.
Results also include operating margins in terms of maximum load handling and speed, plus recommended valve sizes and hose dimensions. Other calculation tools are also available online, including air consumption calculators as a special feature.
With online tools such as product configurators, engineers can conveniently choose functions, weigh alternatives, and select styles, mountings, and other accessories to create custom packages. Product data, including CAD models and pricing are also immediately available.
Speed and cushioning
Another benefit of online calculators is that they recognize the limits of a cylinder. For example, thanks to ongoing seal and bearing improvements, cylinder speeds of 6 to 15 ft/sec and higher are certainly attainable. But not all cylinders are suited for such quick movements, and the software tools instantly recognize restrictions concerning a particular product.
It’s one thing to get a cylinder moving fast. It’s equally important to bring it to a stop without excessive impact loads that generate noise and vibration and can damage the cylinder and the machine itself. Software tools not only know maximum cylinder speeds, they take deceleration requirements into consideration as well.
Cylinder cushioning may not be necessary in low-energy applications that involve low speeds and light loads. In fact, probably the majority of “throw-away” type cylinders are made without cushioning. But any cylinder larger than about 1-in. bore with a stroke exceeding a few inches would expect to warrant some kind of cushioning—whether it’s an elastomeric bumper, pneumatic cushioning or even an external shock absorber. Today, more and more manufacturers are gravitating toward to building cushions into the cylinder.
AVENTICS, for example, offers what we call “Ideal Cushioning,” and it is a critical part of the cylinder selection process. Ideal Cushioning is a proprietary, adjustable method to optimize cushioning and reduce shock/vibration, noise and cycle times. It includes both adjustable, precision pneumatic cushions and elastic elements for impact cushioning. It’s designed such that the direction of travel of the piston is the same throughout the entire cushioning sequence (no piston bounce), and such that the velocity can be exactly zero when the piston reaches the end of its travel, so impact and noise generation on contact are minimized. A shorter total cycle time is an additional valuable benefit; many OEMs and end users have used this method in rod-type and rodless cylinders to increase machinery productivity.
While this discussion focuses on the cylinder, getting air efficiently in and out of the actuator is important, too. Another recommendation is to make air-line lengths as short as possible. Reducing the tube volume between the valve and cylinder saves energy and shortens cylinder response times, because that volume pressurizes and empties every cycle. In fact, from an efficiency standpoint, the ideal place to mount a valve is directly on the cylinder, almost completely eliminating the tubing.
Also, each application has an optimum air-line ID. Choking flow to the cylinder from undersized tubing, fitting and valves will definitely limit cylinder power, stroke time and maximum acceleration. On the other hand, a larger line diameter increases Cv, but it also increases the volume that must be filled and emptied each cycle. And in short stroke, high-frequency applications, larger tubing can actually increase the cycle time and decrease throughput.
Fortunately, online configuration tools and air consumption calculators also recommend the valve flow rate and tubing ID based on the application and the tubing length between valve and cylinder, to produce
One additional note regarding cylinders: The vast majority of pneumatic controls apply the same pressure for both cylinder extend and retract strokes, and that frequently wastes energy and money by supplying higher pressure than an actuator actually needs. For instance, in many applications cylinders either pull or push the load, but not both.
Fortunately, pressure regulators with a reverse flow/backflow mechanism can independently control a cylinder’s load and non-load pressure. Studies have shown that supplying the right pressure for each operation by using pressure regulators on a machine can lower air consumption and produce energy savings on the order of 25%. The key to non-load pressure regulation is to look for cylinders with significant differences in the forces required for each stroke action. And, in general, the larger the cylinder, the greater the efficiency gain.
Properly adjusted manual regulators are one option. And electro-pneumatic pressure regulators are useful in cases where loads can vary, say in flexible automation systems. For example, E/P regulators can be programmed to supply exactly the pressure needed to perform a specific operation on one type of parts, and then reset the pressure for a different product or operation. Savings increase even more because the user does not need to specify the maximum pressure needed for a range of operations—the level can be tailored to each task.
Another benefit of pneumatic pressure regulation is higher cylinder speed. In addition to wasting energy, charging a cylinder to a higher-than-necessary pressure also reduces cylinder speed because the actuator wastes time charging beyond the level required, and it takes longer to empty the chamber. And overpressurizing can magnify internal and external leaks.
And one final note. Machine operators commonly increase supply pressure on regulators in hopes of improving performance. But this wastes air and increases costs for no actual benefit—if components are sized correctly. It’s important to monitor and ensure machine pressure remains within designated limits to avoid wasting energy.