Part of the popularity of pneumatic cylinders is their simplicity, long life, cost-efficiency and ease of installation and maintenance. They produce great amounts of force across a broad spectrum of velocities, maintain high speed operation for long periods without overheating, and suffer no damage if they stall. On top of all that, pneumatic cylinders are robust enough to work in extreme conditions such as dirty and dusty environments, high-pressure wash downs, high humidity and explosive areas.
However, this very versatility can make it difficult to pin down exactly the right type of actuator for your particular application; some key considerations must be taken into account.
The first distinction when choosing pneumatic cylinder design is whether to select rodless or piston rod actuation. In a single-action rod type, compressed air entering through a port at one end of the cylinder moves the piston rod, with one stroke moving the load and an exhaust port that lets the air out. How fast the rod moves depends on how fast the air is exhausted. Double-acting cylinders drive the rod back again as well, allowing a push-pull load motion, while single-acting cylinders use a return spring.
Rod-style cylinders can be broken down yet further into different types:
- Compact, for smaller spaces and short-stroke operation. Mostly single-acting.
- Repairable, for rugged duties and longer life
- Disposable, for lighter-duty operations and no repair potential
- Guided, for controlled, precise linear motion and high side loads
- Rack-and-pinion, to convert linear motion into angular rotation
Rodless cylinders can be a bellows type, with a single-acting inflatable elastomer tube that creates high force and bends in any direction; or linear sliders, with a carriage-mounted or cable-and-pulley load. There are also magnetically coupled actuators and guided linear slides. Rodless Pneumatic cylinders are ideal for high moment loads or applications requiring a long stroke, and save space by containing the stroke action within the overall envelope.
Determining the stroke length will also narrow down the type of cylinder required. Stroke length depends on the job, and overlaps may occur, but general classifications are:
- short stroke, for compact cylinders, as little as 1/16"
- intermediate stroke, for light duty automation, up to 3 feet
- long stroke, for (e.g.) automatic doors, 40-99"
- speciality stroke, with cable-and-clamp pulling a piston, 15-25 feet or more
Cable cylinders can be situated remotely, as the cable can be any length that suits the application and mounting requirements.
Another key criterion for choosing your pneumatic cylinder is how much force you need the cylinder to generate in order to run your application. This can be determined for push force by calculating the internal dimension or bore size of the cylinder, and the air pressure. When calculating pull force, the bore size will be reduced by the cross-section of the piston rod.
Theoretical push force is therefore an equation:
Force = (Air Pressure) x (Bore Size)
Power Factor = Force ÷ (Pressure Available)
and should include a margin of safety, for a safety factor of 50% we need to multiply our cylinder power factor by 1.5 and use this result to find the required cylinder bore.
(Bore Size) x 1.5 = π(Bore)² ÷ 4.
A common rule of thumb is that for high-friction and vertical applications, the force required will be twice the load, but sometimes additional force is required to compensate for excessive friction. Calculations can also be complicated by push or pull factors where spring returns are involved, but fortunately, manufacturers' catalogues provide tables for easy reference. Check out our cylinder sizing calculator
Alternatively you can use the force tables provided by manufacturers in their catalogues, for ease of reference Rowse pneumatics also list these values in our product data fields
Speed affects how well your load can be controlled, and the longevity and productivity of the cylinder. The stroke speed of a pneumatic cylinder can be calculated using the formula:
s = 28.8q/A, where
- s = speed, in inches per second
- q = airflow in standard cubic feet per minute
- A = piston area, in square inches
Speed can also be affected by other factors, including the size of ports, hoses or tubing, and the rate of inlet and exhaust flow via control valves. Bottlenecks can sometimes happen, restricting the air flowing into or out of the cylinder, and restricted air pressure will slow the cylinder down.
It's often necessary to calculate the cylinder’s air consumption carefully, to make sure enough air will be available, especially in fast-cycling applications. Make sure that the compressor has the capacity to supply your pneumatic equipment even in worst-case conditions, as air starvation at a crucial moment will adversely affect performance.
Mounting configuration defines how a cylinder is attached to the equipment. Many mountings are produced as standard, both rigid and articulated, which makes it easier to fulfil specific movement requirements for your application. If the style of cylinder you want doesn't have the appropriate hardware to match your desired mounting position, it can be modified, but commissioning unique hardware will cause delays and add to the cost.
Pneumatic cylinders are usually made of brass, steel, stainless steel, aluminium, engineered polymers or a combination of several materials. What material you choose will depend on its suitability for the operational environment. The same goes for seal materials, but alternatives can be specified for operation in hazardous environments.
Sometimes standard components just won't do the job and you need a custom design. This may mean devising a new configuration for standard and/or modified components, or inventing a whole new unit. If you're looking for a high level of efficiency, complex motion control, or components that don't presently fit standard combinations, you might consider this option. These days, intelligent design is much more able to incorporate specialised configurations and will create custom products that precisely fit your requirements.
There are clearly many issues that can affect your decision. Additional factors arise such as load positions, sensors, temperature requirements, and any special movements that may demand special cylinder modifications. Ancillary components include positioning feedback controls for magnetic cylinders, and cushions or shock absorbers for impact and noise reduction.
Choosing a pneumatic cylinder will account for a significant part of your installation budget, so it makes sense to get it right. At the end of the day, it's always best to discuss your specific needs with the manufacturer or a technical sales engineer.