Air cylinders are offered in a variety of shapes, sizes, and types — not to mention the multitude of standard options available. At first glance, the number of permutations can be a bit overwhelming. The good news is that each actuator type and configuration has a place in today’s motion-centric automation environment.
Pneumatic actuators are selected by their ability to do work, and proper actuator selection is vital. To ensure maximum production, actuator application life is often predestined long before the component is ever applied.
We asked a cross-section of air cylinder manufacturers for their suggestions on how to best select and apply air cylinders. Following are 10 critical cylinder selection practices that should be considered when selecting an air cylinder for your next project.
1. Start with the right design
To meet your application’s needs, you must know a little about the parts and options available on air cylinders. Here are some design considerations to keep in mind.
Rod bearings — Usually the first component to require maintenance is the rod bearing, where both debris and wear from use will necessitate seal replacement. Choose a cylinder that offers easy seal or bearing replacement without having to completely disassembling the cylinder. Many cylinder manufacturers now offer bearings with a friction coating and in a non-rigid, floating style to extend life and prevent binding.
Cushions — End-of-stroke cushions reduce noise from end-of-stroke banging by trapping air that decelerates the piston before it impacts the end cap. They can be adjustable or fixed, which is a less costly option.
Piston bearings — Usually a lowfriction strip wrapped around the piston, piston bearings help reduce wear between the piston and tube ID. They are especially effective in harsh applications where a side load may be put on the piston rod.
Coatings vs. stainless steel or brass — Epoxy or chemical coatings on a cylinder are inexpensive alternatives to stainless steel or brass cylinders. Many cylinder manufacturers also offer an option of adding a stainless steel piston rod, bearing, tie rods, etc. to an aluminum cylinder. Certain applications such as food processing, may require a chemical washdown that necessitates an all-stainless steel cylinder.
Specials — Not every application can use a standard NFPA cylinder. Cylinder manufacturers often provide specially designed products to solve specific customer applications. Non-standard mounting brackets, rod or thread extensions, and special seals are examples of options that manufacturers may offer. Don’t hesitate asking for a product designed specifically for your equipment, instead of designing your equipment around a standard air cylinder.
2. Size the piston rod correctly
Properly designed and well-built cylinders offer several piston rod diameters as standard options. You should determine the proper diameter size for the piston rod, for a given application, to prevent cylinder rod buckling or breaking of the cylinder rod. This is easily done by providing application information to the cylinder manufacturer. Or perform the calculations yourself using reference materials provided by the cylinder manufacturer.
3. Higher pressures aren’t necessarily desirable
In an effort to increase the speed of a machine with pneumatic cylinders, an operator may increase the air pressure to a machine, thus increasing pressure and stress on the cylinders but not necessarily increasing cycle speeds.
A better way to increase cycle speed is to install quick exhaust valves on the ports of the cylinder. This allows the air exhausting from the cylinder to go straight to atmosphere at the cylinder site instead of through the exhaust port of the valve, which could be located far from the actuator itself. The quicker the air can escape from the cylinder, the quicker the cylinder will react, thus increasing cylinder speed. If quick exhaust valves are not practical in an application, another way to increase cylinder response time is to locate the control valve as close to the cylinder as possible. Reducing the tubing length between the cylinder and the valve normally improves the cycle time of the cylinder.
Plus, reducing pressures can reduce costs. Many people operate air cylinders at the same pressure for extension and retraction. Typically, the cylinder is only performing work in one direction. This is the direction where maximum force is required. When returning the cylinder in the opposite direction, maximum force is supplied but not required. Operating the cylinder of the machine with a two-pressure setup uses high pressure for the working stroke, and a lower pressure for the return stroke. This reduces the amount of compressed air used for the return stroke and will reduce demand on the air compressor. This also reduces the costs of operating the compressor, lowering operating expenses and extending the life of the compressor.
4. Select the right mounting configuration
Be sure to account for any offcentered loads that may be applied during cylinder operation. Once you determine whether the load is moved vertically, horizontally, or through an arc, that will drive selection of the proper mounting style.
Design the application with the correct cylinder mounting style to ensure proper operation and life expectancy of an air cylinder. Foot or flange mounted cylinders may be easy to design and install. Consider if there is any chance for an off centered load on the piston rod. If so, a rear clevis mount or trunion mount may give the cylinder the alignment flexibility it needs to prevent the side load stress on the rod of the cylinder, reducing wear and increasing cylinder life.
5. Know your loads
Unless the load is being lifted vertically, it can be fairly difficult to determine the actual load in question. Calculating force loss due to sliding friction can be a daunting task as well. If you are sizing for an existing application, it is imperative that you measure the required force. If you are sizing for a completely new application, perform as much physical experimentation as possible to validate the calculations.
Designs should usually minimize moment loading, which can cause premature cylinder failure. Some types of cylinders can handle higher moment loads than others. For those requiring greater capabilities, you may need to incorporate a stop tube to extend the life of the cylinder.
6. Decelerate the load
All cylinders can accelerate the load, however, not all can decelerate the load effectively. The load can be stopped by impact of the piston to the end cap, or decelerated with air cushions, impact damping seals, and external shocks.
As mentioned above integral air cylinder cushions effectively reduce the speed of the piston at the end of the cylinder stroke and reduce the destructive impact on machinery. Size the cushions with respect to velocity load, drive speed, and orientation.
7. Don’t forget the air
Clean, dry, and sometimes lubricated compressed air is required for optimal cylinder performance and life. But unlike hydraulic fluids, air is less predictable in dynamic movement of cylinders, so size with margin in mind. There must be adequate extra force designed into the solution to account for this less predictable medium.
8. Are you being safe?
Some applications require that an air cylinder moves to a safe position upon loss of air pressure. Rather than stopping in mid-stroke upon loss of air pressure, some situations require that the cylinder move to a fully extended or fully retracted position upon the loss of air pressure in the system.
This can easily be accomplished with a spring extended or spring retracted air cylinder. Designed to normally operate as a double acting air cylinder, a spring cylinder will move to a fully extended or fully retracted position if air pressure is lost. Special consideration must be given to overcoming opposing forces upon pressure loss and spring compression during normal operation. A qualified cylinder manufacturer can help you determine the best design for an application.
Cylinders with integral piston rod locking devices can prevent load movement either due to gravity or release of stored energy in the system. With the absence of air pressure, the piston rod is clamped and held in position, helping to reduce the possibility of injury to personnel.
9. Get (proportional) control
Most air cylinders are used for only two positions: fully extended or fully retracted. In these situations, to change the distance moved, the cylinder itself must be replaced with one of a different stroke or one with adjustable hardstops that are manually set.
Many designers don’t know that a continuous position feedback sensor and a closed-loop valve controller can turn a basic cylinder into a variable positioning system. These systems provide the function of linear electric drives, but with the advantages of pneumatics. This adds great flexibility, such as:
- a cylinder can be commanded to follow a motion profile or index to several electronically adjustable locations,
- a single design can be used for many different configurations or applications
- cylinder start and stop points can be updated by computer during quick changeovers (avoids maintenance crew effort and error), and
- by commanding start and stop points of a motion to be less than the full stroke of a cylinder, the life of a cylinder can be greatly improved.
Many ways exist to add position feedback, including buying a cylinder with a sensor already built in, or retrofitting an existing cylinder with an external sensor. Although these systems involve sensors and other electronics, the initially higher cost is often outweighed by very compelling additional features, flexibility, hand lower total cost of ownership.
10. Test it!
Not testing while trying to size an air cylinder for a specific application can be a great mistake. Once application hardware is in place, it can be extremely costly to make adaptations for a larger diameter air cylinder. Often times, the entire application must be completely redesigned. Unless you are against strict time restrictions or you like to redesign your application, it is always a good rule of thumb to oversize your cylinder slightly.
The following companies contributed to this article: Cylinders & Couplers Inc., Enfield Technologies, Numatics, Hydra-Dynamics Inc., Parker Hannifin Corp.’s Industrial Cylinder Div., Norgren, Clippard Instrument Laboratory, and Lehigh Fluid Power Inc.