Assembly Presses / Motion Control

How to Specify Pneumatic Cylinders

Stroke length, force requirements, and operating environment are among the many considerations when specifying pneumatic cylinders.

September 3, 2014
Trans

Air cylinders are offered in a variety of industry standards. Within these standards, air cylinders come in an assortment of shapes, sizes and types. Numerous optional features are also available.

At first glance, the permutations can be a bit overwhelming. The good news is that each pneumatic actuator type and configuration has a place in today’s factory automation.

Despite the multitude of standard options, pneumatic actuators are still selected for their ability to perform a specific function. The applications for air cylinders are endless. Here are a few examples:

  • Opening and closing a knife gate valve.
  • Raising and lowering an automatic screwdriving spindle.
  • Diverting goods on a conveyer.
  • Raising and lowering a 15-foot oven door.
  • Operating the gates of a railcar to rapidly unload grain or other bulk commodity.
  • Shuttling parts into and out of a semiautomatic assembly station.
  • Clamping parts to be welded or bonded.

Sometimes the application falls outside standard product offerings and only a custom cylinder will suffice. However, developing custom air cylinders can often be expensive and time-consuming.

Whether you need a standard or custom cylinder, this step-by-step process will ensure that your time and investment are well spent.

Two Questions

To properly specify an air cylinder for any application, two questions must be answered before moving into the heart of the design. What type of work will it be performing? What types of cylinders are available to choose from? Several standard cylinders are usually available to fit any one application, but there are often design issues that keep a standard unit from meeting your specific requirements. Before charging into the specifics of your application, you should get a handle on the basics of what you need.

To size a cylinder properly, you’ll need to determine how much force it needs to produce. When the force is known, you can determine the bore size or the power factor (effective piston area) of the cylinder with the equation: force = air pressure x power factor. Or, put another way: power factor = force ÷ air pressure.

In this calculation, we have not considered any safety factors. So, as a starting point, let’s use a 50 percent factor of safety. Therefore, multiply the power factor by 1.5 and use the result to calculate the cylinder bore from the equation: Power Factor x 1.5 = p (Bore)2 ÷ 4.

Don’t want to do math? No worries. Cylinder catalogs typically have force factor tables that list the force produced by different piston sizes at various air pressures.

Pulling force, produced when the cylinder retracts, brings the diameter of the piston rod into account. In retract mode, air pressure can act on only part of the piston, because the rod blocks the center portion of the piston. Thus, the power factor for retraction force is calculated as the piston area minus the rod area. Again, cylinder catalogs typically list rod areas to speed your calculations of pulling power factors.

If you do not have enough pressure to produce the desired force using the preferred bore size cylinder, then you must go to a larger unit. This will affect the package size and may create some space requirement issues. As a result, engineers may need to make some trade-offs.

Cylinder Stroke

Stroke is usually a given in most applications. Decide on it early so you can determine how large a package size you’ll have when it comes time to address the mounting style.

How much stroke do you need? It depends on the application. Closing the door to a large oven might require a 15-foot stroke. Lifting a stop gate on a conveyor could require only a 2-inch motion, whereas pushing a load off a conveyor might require 30 inches or more. Staking a rivet wouldn’t require much stroke at all—most likely only a fraction of an inch.

Whatever the task, knowing the stroke starts to define the type of cylinder you’ll need and the envelope size required for mounting it. For the purposes of discussion, cylinders can be classified into four categories according to their strokes: short stroke, intermediate stroke, long stroke and specialty stroke. Note that some cylinders may overlap all four categories.

Compact, short-stroke cylinders come in a variety of body styles and have strokes as short as 0.0625 inch and bores as small as 0.5 inch.

Intermediate-stroke cylinders have strokes up to 36 inches. These tie-rod cylinders are popular for fast-acting, light-duty automation applications. Aluminum construction reduces weight and cuts manufacturing costs.

NFPA-interchangeable, long-stroke cylinders can have strokes up to 99 inches, but they are well-adapted to applications calling for strokes as small as 4 inches. (NFPA stands for National Fluid Power Association. This organization set up standards for all manufacturers of tie-rod cylinders. Mounting codes were established to ensure that all such cylinders, regardless of brand, would be dimensionally interchangeable.)

Specialty-stroke cylinders can have strokes of more than 99 inches. Cable cylinders, made by several manufactures, are one example. With this type of cylinder, a clamp can be pulled left or right by a cable attached to the cylinder’s piston. A cable cylinder with a 15-foot stroke could be used to control that 15-foot oven door I mentioned earlier. Because the cable can be any length, the cylinder can be mounted anywhere that is convenient for your design—directly on the oven or across the room, if need be.

Mounting Considerations

Mounting can be a challenging issue. At this point, it’s time to determine how you want to mount the unit and decide if you have the proper structure and space to do so. This seems like a basic concept, but failure to plan here can demolish your plans quickly. For example, if you want to flange-mount a unit, but a fitting is in the way of the mounting surface, you may be forced to redesign the system for a bottom mount. In that case, an additional plate may need to be added.

Here’s a question to help you determine how to mount a cylinder: Will it be pushing a load linearly, or turning a crank arm?

For pushing, pulling or lifting along a straight line, you want the cylinder to be rigidly mounted. You could bolt the cylinder to your equipment either by tapped holes in the bottom, or stand it on end and run bolts into sleeve nut mounts in the end cap. Or, you could choose any of 16 standard, interchangeable, rigid mounts established by the NFPA.

Rigid through-hole mounting is available on many short-stroke cylinder models. It provides counterbored holes drilled through the cylinder body for easy mounting with socket-head cap screws.

If the cylinder will be turning a crank arm, it would need to pivot. A rear clevis mount attached to the cylinder will allow it to pivot, but restrain lateral motion. By attaching the crank to the piston rod with a flexible coupling, we can secure the desired motion.

Short stroke cylinders have many of the same mounts as the big boys. For example, eye mounts can be attached to a round-body, short-stroke unit. These mounts also provide a pivot point attachment to allow pivotal motion of the cylinder. To further assist with these types of flexible mounts, rod clevises, rod eyes and mounting brackets are widely available.

Most cylinder styles have mounts similar to the NFPA mounts, even if they aren’t the standard tie-rod design. As example, trunnion mounts are available on stainless steel body cylinders. Dimensions differ from the NFPA mounts, but they function the same.

Whatever cylinder style you choose, be sure to dig deep in the product catalogs to find the mount best-suited to your application.

Specialty Cylinder Applications

If you have addressed these preliminary questions and determined that a standard cylinder will not meet your specific requirements, it’s time to consider a custom cylinder.

When designing in a custom cylinder, it’s easier if you start with a particular family of cylinders—such as NFPA-interchangeable, compact, or non-repairable—that will be conducive to your application.

Is the application heavy duty? Is there plenty of room available? If yes, then you may wish to start with an NFPA-style cylinder.

Is real estate at a premium? If yes, then you may be forced into starting with a compact style.

Is price the main concern? If so then a non-repairable unit may become your first choice.

Remember that you do have several choices. Once you have determined the style of cylinder, you can get to the details of your design.

You probably know how you plan to mount the unit. Engineers often find that mounting a cylinder will require a special bolt pattern or mounting style that is nonstandard. In that case, you’ll have to ask how the cylinder can be modified to fit the pattern for a minimum cost. Will it require special parts to be manufactured? Will it need unique mounting hardware, such as plates, flanges or brackets?

Beware: In cases where the mount is built into or uniquely attached to your cylinder, costs will increase along with manufacturing lead times. For example, nose mounting might require that unique end caps be produced. Integral lug mounts could require special extrusions, welding or other creative attachment concept to be employed. In lieu of lugs, is there room to drill and tap your cylinder’s end caps (or body) with special bottom- or side-mounting holes? Would you have enough depth of thread?

Motion Elements

Now, you’ll want to address the motion elements of your cylinder. Are there special movements, sensing, or side loads being applied that will require special modifications to the cylinder? If so, you’ll need to accommodate them.

If the load must stop at an intermediate position, you’re in luck. You can get three or more rod positions from a single cylinder! Many cylinder styles are offered with three-position options. Short-stroke, tie-rod cylinders are available that are essentially two cylinder bodies combined in a single package. You can specify the same or different stroke lengths to set your work positions as required.

What’s more, numerous cylinder styles are available in in back-to-back configurations that enable positioning at up to four end points. As the name implies, two single-rod cylinders are assembled with their back end caps attached. By anchoring one rod end and allowing the cylinder body to “float,” four distinct end points can be obtained.

For applications in which anti-rotation and registration are critical, there are solutions. Maintaining the load’s fixed orientation can be accomplished in several ways. In one method used on tie rod cylinders, two guide pins incorporated inside the cylinder pass through the piston head. These guide pins prevent rotation of the rod with a tolerance of ±1 degree. A rubber disk is included at the end of each guide pin to take up end play and firmly seat the pins in the precision guide-pin holes.

Because the guide pins are inside the cylinder, they are protected from the environment and physical damage, and they are lubricated by the system lubrication. They require no additional space, leaving the rod end area free for attachments and tooling as required by your application.

Internal guide pins are also available on a number of short stroke cylinder models.

Another solution uses an external guide block securely attached to the piston rod. A steel guide shaft, attached to the guide block, assures anti-rotation of less than 0.8 degree.

Twin piston rods can also be incorporated into the cylinder head to provide anti-rotation. The rods are securely fastened to the piston and tied together externally by a rod end tool bar. The tool bar ensures that the rods move in tandem and provides an ideal mounting surface for attachments required by your application. The tool bar is furnished with threaded mounting holes or optional counterbored mounting holes.

Stroke-adjustment styles may be needed when the stroke can change either on the extension or the retraction of the unit.

An adjusting screw with a thread-sealing locknut mounted in the rear end cap provides a simple, yet rugged adjustment of the cylinder stroke in the retract direction. A fine thread on the adjusting screw will provide precision adjustment. Adjustable retract strokes are offered as optional features for many cylinder styles.

It is also possible to use the back end of a double-rod cylinder to adjust the extend stroke. A stop collar, bumper and some kind of impact plate could do the trick. However, if taking this approach, use caution and consider a safety cover to avoid leaving the pinch point exposed!

The need for sensing can often change the cylinder design, depending on what type of sensor is needed. Standard electronic switching will require magnets to be added to the piston. Proximity switches may require internal or external changes to the cylinder, so that the sensing probes will have targets that they can read. Transducers may also require a variety of internal or external changes to a unit.

Side loads often suggest a need for items such as stop tubes or heavier bushings because of the wear produced when the cylinder is in motion.

Cylinder piston rods are supported by a bearing in the front head of the cylinder and the piston itself running inside the cylinder walls. As the rod nears full extension, the distance between support surfaces becomes shorter. The piston rod assembly tends to cock causing uneven wear on the bearing surfaces and shortening seal life.

One solution to the problem is to install an internal stop tube. The stop tube blocks the piston from reaching the front head, thereby increasing the minimum distance between support points. Component wear is reduced and cylinder operating life is extended.

However, to maintain the same work stroke, the length of the cylinder body must be increased by the length of the stop tube. Dealing with the increased package size may present issues.

If you have room available, a double-rod cylinder gives you the best piston rod assembly support. Rod bearings in both end caps reduce the load on the piston. This design also ensures maximum distance between support points.

Environmental Issues

Lastly, we come to the environment. External issues should be addressed first before considering internal issues.

External issues are items that will cause harm to the outside of the cylinder. Certain “wash down” or wet environments often require that material changes be made to the basic cylinder components. End caps, tubing, tie rods and other parts might have to be made of stainless steel or a unique type of plastic. In lieu of material changes, a chemical coating process may be required to use the cylinder in a harsh environment.

Heat may be another external issue that demands material changes.

Internal issues include the fluids being used to operate the cylinder and contaminants that could enter into the cylinder.

If fluids other than air are required to run the cylinder, then seal changes may be needed.

If abrasive materials could enter into the unit, rod wipers or scrapers may be needed. External chemicals may affect the internal components as well. Seal materials may also need to change.

Special cylinder applications can get quite complicated. But, by providing cylinder manufacturers with answers to these questions, you will greatly help them in their efforts to provide exactly what you need.

 Keep in mind that each deviation from the standard product may cause special parts to be manufactured, purchased or designed. When you are looking at your next design project, I would suggest that you try to fit it into a standard product if at all possible. The fewer parts that have to change from a standard product, the less likely the cost will have to increase. However, when a total custom cylinder is required, you will need to plan on a longer lead-time because these items will be designed to your specific needs. 

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