Specifying Pneumatic Cylinders
Air cylinders are offered in a variety of industry standards. Within these standards, air cylinders come in an assortment of shapes, sizes, and types, as well as with numerous optional features. At first glance, the number of permutations can be a bit overwhelming. The good news is that each pneumatic actuator type and configuration has a place in today’s motion-centric automation environment.
Even though the air cylinder market includes a multitude of standard options, pneumatic actuators are still selected by their ability to perform a specific function. These functions are literally endless. Here are a few examples of some common air cylinder applications.
• Opening and closing the gate on a knife gate valve.
• Allowing motion and movement in the animatronics industry.
• Diverting goods on a conveyer system.
• Raising and lowering rides at an amusement park.
• Operating gates to rapidly unload a railcar commodity.
• Press operation in the dry cleaning industry.
• Brush movement in the auto wash industry
Sometimes the job at hand simply falls outside the standard product offering and only a tailored or custom air cylinder will suffice. The development of custom air cylinders can often be both expensive and time consuming. We have outlined a step-by-step process to ensure that your time and investment is well spent.
(1) Two Questions
In order to properly specify an air cylinder for any application, it requires that two main questions be answered before moving into the heart of the design. The first question is: what do I need the cylinder to do (what type of work will it be performing)? The second is: what types of cylinders do I have to select from? As previously mentioned, there are several standard cylinders types available to fit most applications, but often times there are 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.
(2) Push and pull forces
First you’ll need to determine the force required in order to size the cylinder properly. When the force is known, you can determine the bore size or the power factor (effective piston area) of the cylinder you need by using the equation:
Force = (Pressure Available) x (Power Factor)
Power Factor = Force ÷ (Pressure Available)
In this calculation we have not considered any safety factors. So, as a starting point, let’s use a 50% factor of safety. Therefore, multiply our Cylinder Power Factor above by 1.5 and use the result to calculate the required cylinder bore from the equation:
(Cylinder Power Factor) x 1.5 = π (Bore)2 ÷ 4.
Or we can find the appropriate bore using force factor tables in the manufacturers’ catalogs as shown below in a typical example. Here, Sizing Guide “A” shows the actual piston area and the extend forces obtainable from various air pressures.
Pulling force, calculated when the cylinder retracts, brings the rod diameter into account. As shown below, air pressure can act only on part of the piston in retract mode because the rod blocks the center portion of the piston.
Thus the retract power factor is calculated as an annular ring – piston area minus rod area.
Effective piston areas
Again, helpful tables from cylinder catalogs give rod areas to speed your calculations of “pulling” power factors. (Shown in the sample Sizing Guide "B" below.
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, so there is generally a balance that must be achieved.
(3) The cylinder stroke
Stroke is usually a given in most applications. Decide on it early so you can determine how much of a package size you'll have when it comes time to address the mounting style.
Okay, so what stroke do you need? Pulling a load might require a 15 foot stroke if you’re closing the door to a large oven. Lifting a stop gate on a conveyor could require only a 2” motion whereas pushing a load off of the conveyor might require 30” or more. Staking a rivet wouldn’t require much stroke at all – most likely only a fraction of an inch.
Whatever your task, knowing the stroke starts to define the type of cylinder you’ll need and the envelope size required for mounting it. For purposes of discussion here we classify and show examples of four cylinder types by stroke as follows: (a) Short stroke, (b) Intermediate stroke, (c) Long stroke, and (d) Specialty stroke cylinders. Note that some cylinders may overlap all of the categories.
• Short stroke, compact cylinders
These come in a variety of body styles and have strokes as short as 1/16” with bores down to 1/2”.
• Intermediate stroke – strokes to 36”
We define these as cylinders having strokes to 36”. After World War II, fast acting, light duty automation applications gave rise to compressed air as an alternative to hydraulics. The then popular tie-rod cylinder construction was copied using aluminum wherever possible to reduce weight and cut manufacturing costs.
• Long stroke – strokes to 99”
We are showing NFPA interchangeable cylinders in this category because their upper stroke limit is 99”.
But, they are well adapted to applications calling for strokes from 4” and up.
Magnetically coupled (rodless) cylinder has strokes to 1,000 mm.
• Specialty stroke – strokes to over 99”
Cable cylinders, made by several manufactures, are one example of specialty cylinders. As can be seen from the basic drawing, a clamp can be pulled left or right by a cable attached to the cylinder’s piston.
A 15 foot stroke cable cylinder could be used to control the 15 foot oven door we mentioned earlier. Because the cable can be any length we want, the cylinder can be mounted anywhere that is convenient for your design - directly on the oven or across the room from it if need be.
Cable cylinders can have strokes over 25 feet and can be located remotely from the workload.
Mounting can be a challenging issue. At this point it's time to determine the way you would like to mount the unit and then decide if you have the proper structure and space to do so. This seems like a very basic concept, but failure to plan here can demolish your plans quickly.
For example, if you would like to flange mount a unit but there is a fitting in the way of your mounting surface, it may force a redesign to a bottom mount where an additional plate may need to be added. Okay then, here's a question:
• Will the cylinder be pushing a load linearly, or turning a crank arm? (The answer determines the type of mount we want for the cylinder.)
• Rigid Mounting
For pushing, pulling or lifting along a straight line, we want the cylinder to be rigidly mounted. We could bolt the cylinder to our equipment either by bottom tapped holes or stand it on end and run bolts into sleeve nut mounts in the end cap. Or we could choose any of the other standard rigid mounts from the Quick Reference to the Interchangeable NFPA Mounts (inset below).
• Rigid thru-hole mounting
Thru-hole mounting is available on many short stroke cylinder models. It provides counter-bored holes drilled through the cylinder body for easy mounting with socket head cap screws.
If turning a crank arm, the cylinder would need to pivot. As you can see below, a rear clevis mount attached to the cylinder would allow it to pivot but restrain lateral motion.
Lastly, by attaching the crank to the piston rod with a flexible coupling, we can secure our desired motion.
Flexible Mounting – short stroke cylinders
Short stroke cylinders have many of the same mounts as the big boys. Eye mounts, for example, are shown below on 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.
Flexible Mounting – non tie rod cylinders
Most cylinder styles have mounts similar to the NFPA mounts. 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.
5) room availability
Finally, at this point you have figured out how much room your cylinder will require and compared it to the room you have available. You either know exactly what you need, or you know the better part of what you need.
(6) Specialty cylinder applications
Specialty cylinder considerations begin now that you have addressed the preliminary questions and found no standard cylinder to meet your specific requirements. You are left with a few more questions.
• What type of cylinder best suits my needs?
When designing in a custom cylinder, it's easier if you start with a particular family of cylinders, i.e., • NFPA Interchangeable
• Compact • Non-repairable, etc., that will be conducive to your application.
Questions: Is the application very heavy duty? Is there plenty of room available? Answer: If yes, then you may wish to start with an NFPA style cylinder.
Questions: Is real estate at a premium? Answer: If yes, then you may be forced into starting with a compact style.
Questions: Is price the main concern? Answer: 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. But, often times you find that mounting will require a special bolt pattern or mounting style that is non-standard. You'll have to ask how the style of cylinder you have selected can be modified to fit the pattern for a minimum cost. Will it require that special parts be manufactured? Will it need unique mounting hardware such as plates, flanges, or brackets?
In cases where the mount is built into or uniquely attached to your cylinder, costs will increase along with manufacturing lead times. So beware.
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?
(7) 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.
• Must the load stop at any intermediate position?
You’re in luck. You can get three or more rod positions from a single cylinder! Many cylinders styles are offered with 3-position options.
The short stroke tie rod cylinder shown below is 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. Nice!
More luck! You can also get numerous cylinder styles in back-to-back configurations that enable positioning at up to 4 four end-points. As the name implies, two single rod cylinders are assembled with their back end caps attached. As shown below, by anchoring one rod end and allowing the cylinder body to “float”, four distinct end points can be obtained.
• Can the load be allowed to rotate slightly?
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.
The drawing below shows 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°. 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, physical damage, and 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 like the Pancake® Series shown above.
External non-rotating options
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°.
Twin rod, non-rotating options
Above twin piston rods are 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 insures 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 counter-bored mounting holes.
Stroke adjust styles may also be needed when the stroke can change either on the extension or the retraction of the unit.
Adjustable Retract Stroke
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.
Adjustable Extend Stroke
It is 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!
Sensing can often change the cylinder based on the type of sensing needed. Standard electronic switching will require magnets to be added to the piston. Proximity switching may require internal or external changes to the cylinder so that the sensing probes will have targets which 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.
• Will the cylinder have strong side loading or heavy overhung loading?
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 (“d”) 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, in order 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.
Double rod cylinders
If you have room available, a double rod cylinder gives you the best piston rod assembly support.
You’ll have rod bearings in both end caps reducing the load on the piston. And you’ll have maximum distance between support points.
(8) 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, etc., might have to be made of stainless steel or a unique type of plastic. If not material changes, then a chemical coating process may be required in order to use the cylinder in a hostile environment.
Heat may be another external issue that demands materials changes.
Internal issues address fluids being used to operate the cylinder, or contaminants that could enter into the cylinder.
If fluids other than air are required to run the cylinder, then seal changes may be needed.
When aggressive items are trying to enter into the unit, rod wipers or scrapers may be need. 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 your cylinder manufacturer with the 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, and/or designed. When you are looking at your next design project, we 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.
Good luck now with your future air cylinder application!