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This article takes an in depth look at pneumatic cylinders
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Pneumatic cylinders are mechanical devices that produce force by using energy from pressurized air. These devices consist of a piston, piston rod, and cylinder. The pressure inside the cylinder rises as air enters on one side of the cylinder. The rise in internal pressure causes the piston to move in a specific direction. The piston rod transmits the developed force to the object to be moved.
The working fluid in pneumatic cylinders is compressed air. Hence, pneumatic cylinders are desirable for environments requiring a high level of cleanliness, as the fluid will not contaminate the surroundings in case of leakage. Pneumatic cylinders operate quietly and do not require large storage tanks for the working fluid.
Pneumatic cylinders are used in the automation of machines and industrial processes. The force and motion produced by pneumatic cylinders can be used in mechanisms such as clamping, ejecting, blocking, and lifting. In factories, they are used in repetitive pick-up and placement of objects into a machine or equipment. In piping systems, they are used in operating valves.
The following are the components of pneumatic cylinders:
A pneumatic cylinder may be a single-acting or a double-acting cylinder:
In double-acting cylinders, compressed air can be introduced on both sides of the piston. The piston and rod assembly will move toward the side of the chamber with less internal pressure. Hence, the piston and rod assembly can perform both extension and retraction strokes. The piston and rod assembly returns to its original position by supplying pressurized air on the other side of the cylinder.
The extension force of double-acting cylinders is greater than the retraction force because the area is greater on the side of the piston near the rear-end cap. This is true only when pressurized air supplied on both sides of the piston is equal. Moreover, the retraction speed is faster than the extension speed because the rod decreases the effective volume which makes the chamber filled with compressed air quicker.
Double-acting cylinders are useful in gate and valve opening and closing. They are used for applications requiring high speed and high force. They have stronger and more constant output force and longer strokes. Hence, they require a stronger cushioning system. The movement of the piston and rod assembly is quicker and more controlled since pressurized air moves it in both ways. However, double-acting cylinders have higher compressed air consumption and are more expensive. The piston’s position cannot be determined in case of a sudden pressure or power loss.
In single-acting pneumatic cylinders, a spring is fitted around the piston rod, which aids in the retraction of the piston and rod assembly. Compressed air enters through one of the cylinder caps and fills only one side of the chamber. This causes the piston and rod assembly to move linearly and extend in one direction while compressing or stretching the spring. Once the piston rod makes its maximum thrust, the spring returns to its original position together with the piston and rod assembly. Air is released on the vent port in one of the caps. In case of pressure loss or power interruption, the piston will simply return to its base position.
A single-acting cylinder can be a push-type or a pull-type cylinder. In a push-type cylinder, the pressurized air pushes the piston rod out of the cylinder (out stroke or extension). In a pull-type cylinder, the pressurized air pulls the piston rod inside of the cylinder (in-stroke or retraction).
Rodless pneumatic cylinders move loads along with a piston driven by compressed air. The piston is attached to a carrier where the load is mounted. The piston moves the carrier in a straight line. The direction of the piston movement is always to the side of the chamber with lower internal pressure.
Rodless pneumatic cylinders offer strokes comparable to their assembly size at faster speeds. Hence, they are suitable if the overall length must be minimized due to limited space. End cushioning is necessary to prevent hard impact on the piston after full-length travel at the end caps.
In less common types of single-acting cylinders, the retraction mechanism is accomplished by an external load or gravity.
Single-acting cylinders have a simple construction and are cost-efficient due to less air consumption. They are ideal for the application of force in a single direction, such as clamping, punching, and positioning. They are also found in pumps and rams. However, the output force is limited due to the opposing spring force. The size of the spring limits the stroke length. Piston strokes become inconsistent with prolonged usage of the spring.
There are three types of rodless pneumatic cylinders:
In band cylinders, the carrier is connected to the piston by two sealing bands that run parallel to the stroke direction. The sealing bands can be made from plastic or stainless steel. The outer band is located on top of the cylinder bore slot, which is connected to the carrier. Meanwhile, the inner band is located inside the cylinder bore, which is connected to the piston. As the carrier moves to either end, it opens the sealing band towards its stroke direction while closing the band behind the moving carrier.
In cable cylinders, the piston is connected to the carrier by a cable that passes through a pulley on each end cap. The cable is pushed by the piston in order to move the carrier. Cable cylinders are inexpensive and have a simple construction. However, cable wear causes inaccurate carrier positioning and leakage.
In magnetically coupled cylinders, the piston is not mechanically attached to the carrier. Instead, the carrier is moved by the piston through a strong magnetic field. Hence, air leakage is prevented since the cylinder is fully enclosed. However, the carrier may disengage from coupling and is reactive to moment loads.
The other types of pneumatic cylinders are the following:
Air hydraulic cylinders replace the driving force of oil with air. When activated, the piston in one chamber moves linearly until it is stopped which activates the air power system. Air flows into the piston chamber. As the air pressure builds, the piston in the chamber moves linearly in reverse to compress the oil in the working area. The oil forces the working piston to produce a power stroke. Once the stroke is completed, the air is vented and the components return to their start positions.
Multi Force Pneumatic cylinders have cylinders with identical profiles placed in a series with an internal air passage for all pistons. They have a common piston rod with multiple pistons attached. The output force is increased in accordance with the number of pistons, which can be 2, 3, or 4 pistons with a force output of 2, 3, or 4 times the force output of a single piston pneumatic cylinder.
Rotary cylinders, or pneumatic rotary actuators, are used to convert energy from compressed air into an output torque. They use rotary motion to drive devices in tight spaces and are small double acting cylinders that exert force in a clockwise and counterclockwise direction. The piston rod has a rotary profile against a worm wheel to provide linear movement.
Pneumatic rotary actuators have one or more air chambers with a piston. They provide high force relative to their size and can be used in hazardous conditions. As with all pneumatic cylinders, they are a self contained mechanism, which protects their parts from contamination and aggressive harmful environments. Their one piece design eliminates the need for maintenance.
A tandem pneumatic cylinder, also known as a combination cylinder, is similar to a multi force multiplying cylinder and has two pistons connected by a single rod that supply twice the force. The two components of a tandem cylinder are separate double acting cylinders connected in a series. They are used in limited space applications where higher force is needed.
Telescopic cylinders have a series of segmented tubes that extend when compressed air fills the cylinder. These tubes progressively decrease in diameter. The tube with the smallest diameter is referred to as the piston rod. Telescopic cylinders have exceptionally long strokes. The tubes consume small space when they are nested together or when the cylinder is in a retracted position. These cylinders are available in single and double acting modes. The telescopic design is more common in hydraulic cylinders than in pneumatic cylinders.
In through rod cylinders, the piston rod extends on both sides of the piston. This makes the output force and speeds from extension and retraction strokes equal.
There are two common types of rotary cylinders:
A rack-and-pinion cylinder consists of a circular gear (pinion) engaged in a linear gear (rack). The pinion is moved linearly by the piston when actuated, which rotates the pinion. A shaft is connected to the center of the pinion. Two sets of rack and piston are usually installed on the opposite sides of the pinion to double the output torque.
In vane actuators, the shaft is connected to the center of a movable vane. When pressurized air is supplied on one side of the vane, the differential pressure causes the vane and its shaft to rotate towards the other side of the chamber. Air is simultaneously released on the port of the other side of the chamber. Vane actuators can have a single or double vane configuration.
Welded cylinders have their end caps directly welded to the pneumatic cylinder bore and do not have a tie rod. They are more compact, durable, and can withstand higher internal pressures. However, they are difficult to repair and dismantle due to their welded construction.
The following are the types of pneumatic cylinders based on their mounting style. The mounting style is an important consideration which affects the performance, strength, and reliability of the pneumatic cylinder.
Centerline mounts are mounting styles designed to bear loads in the centerline of the pneumatic cylinder. They can be used for push and pull applications. The load is distributed evenly among the mounting bolts. Sideloading is also minimized. The types of pneumatic cylinders under this category are the following:
Tie rod cylinders can withstand the highest internal pressures and are the most common type of pneumatic cylinder based on mounting style. The tie rods of this type of cylinder extend over its end caps which allows them to be mounted on a surface or machine member. Tie rod cylinders absorb push or pull forces in their centerline symmetrically, which prevents shear stress on its mounting bolts.
Flanged cylinders have an installed flange on one of the end caps, which allows good centerline force transfer and secure mounting. The flange is usually attached to the front cap for pulling applications or to the rear cap for pushing applications. However, flanged cylinders have a low tolerance for misalignments.
In centerline lug cylinders, the cylinder is pinned on the midpoints of the caps. This allows the cylinder to absorb forces about its centerline uniformly and makes them ideal for straight-line force applications. When the cylinder is used under high pressure or high impact conditions, dowel pins must be used to secure the cylinder to its mounting surface.
Side mounts are mounting styles when the plane of the mounting surface is offset from the centerline of the cylinder. The cylinder is pinned either on the side of the end caps (side-mounted cylinders) or on the front and rear side of the cylinder (foot-mounted cylinders). This mounting style, however, makes the cylinder reactive to moment loads which causes it to rotate about its mounting bolts. To prevent this, the stroke length and the bore size must be equal. Cylinders with large bore sizes and shorter strokes tend to have increased stress in their mounting bolts and rotate about their mounting surface, which causes misalignment.
Pivot mount cylinders allow changes in misalignment in one plane. They absorb forces in the cylinder’s centerline, should the cylinder be misaligned or when the actuated load travels through a non-linear path. Pivot mounts provide rotational freedom to the cylinder. However, misalignments should be limited to prevent excessive bending stress and deflection on the piston rod.
The actuation angle reduces the transmitted force of the pneumatic cylinder, which is equivalent to the product of the generated force and sine of the actuation angle.
There are two types of pivot-mounted pneumatic cylinders:
Some pneumatic cylinders conform to ISO standards to enable their compatibility with members of different machines, connection methods, mountings, accessories, and others. As manufacturers follow the metrics covered by these standards, customers can easily look for the pneumatic cylinders appropriate for their applications.
Some of the ISO standards for pneumatic cylinders are the following:
The ISO 15552 standard specifies the basic product, mounting, and accessories dimensions for single and double rod pneumatic cylinders, with or without the provision of magnetic sensors. This standard caters to pneumatic cylinders with a maximum pressure rating of 10 bars with bore sizes ranging from 32 mm to 320 mm.
The ISO 6432 standard is applicable to small single rod pneumatic cylinders with bore sizes of 8 mm to 25 mm with a maximum pressure rating of 10 bars.
The ISO 21287 standard is applicable to single rod compact pneumatic cylinders, with and without magnetic function, with bore sizes of 20 mm to 100 mm with a maximum pressure rating of 10 bars. ISO 21287 cylinders have no adjustable cushioning; instead, they utilize rubber bumpers for cushioning. Pneumatic cylinders with bore sizes ranging from 32 mm to 100 mm can use ISO 15552-compliant end mountings.
ISO 8139 establishes the mounting dimensions for rod-end spherical eyes for pneumatic cylinders. Rod-end spherical eyes have male or female threads that enable them to be coupled with a threaded piston rod. They can also be used to mount the cylinder to a surface. They can be attached to the rear cap of the cylinder in which the male clevis attachment pin is inserted on its center, enabling it to rotate about the pin.
ISO 8139 rod-end spherical eyes are designed to withstand pressures of up to 10 bars and are compatible with pneumatic cylinders conforming to ISO 15552, ISO 6432, and ISO 6430 standards.
ISO 8140 establishes the mounting dimensions for rod-end clevis attachments for pneumatic cylinders. Rod-end clevis attachments are components shaped like a fork which can be coupled to a load or used to mount the cylinder to a surface.
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