This article takes an in depth look at pneumatic cylinders
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Pneumatic cylinders are mechanical devices that generate force through pressurized air. They typically include a piston, piston rod, and a cylinder. As air enters one side of the cylinder, the internal pressure increases, causing the piston to move in a specific direction. The piston rod then transfers the resulting force to the object being 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 play a crucial role in automating machinery and industrial processes. They provide force and motion for various functions, including clamping, ejecting, blocking, and lifting. In manufacturing environments, they are often employed for repetitive tasks such as transferring objects to and from machines or equipment. Additionally, pneumatic cylinders are utilized in piping systems to control valve operations.
Here are the main components of pneumatic cylinders:
Pneumatic cylinders can be classified as either single-acting or double-acting:
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.
In double-acting cylinders, the extension force typically exceeds the retraction force due to the larger surface area on the piston side near the rear-end cap, assuming equal air pressure on both sides of the piston. Additionally, the retraction speed is generally faster than the extension speed because the presence of the rod reduces the effective volume, allowing the chamber to fill with compressed air more quickly.
Double-acting cylinders are ideal for applications like gate and valve operations, where high speed and force are required. They offer more consistent and powerful output and can handle longer strokes, necessitating a robust cushioning system. The dual-action mechanism provides quicker and more precise control of the piston and rod assembly. However, these cylinders consume more compressed air and tend to be more costly. Additionally, in the event of sudden pressure or power loss, the piston’s position may become indeterminate.
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.
Single-acting cylinders can be classified as either push-type or pull-type. In a push-type cylinder, pressurized air extends the piston rod outward from the cylinder. Conversely, in a pull-type cylinder, the pressurized air retracts the piston rod into the cylinder.
Rodless pneumatic cylinders are designed to move loads using a piston that is propelled by compressed air. The piston is linked to a carrier that supports the load, and it travels in a linear path. The movement of the piston always occurs towards the side of the chamber with lower air pressure.
Rodless pneumatic cylinders can achieve strokes that are proportional to their overall dimensions, and they do so at higher speeds. This makes them ideal for situations where space constraints require a compact design. Proper end cushioning is essential to avoid harsh impacts on the piston when it reaches the end of its stroke.
In some less conventional single-acting cylinders, the retraction process is driven by an external force or gravity instead of an internal mechanism.
Single-acting cylinders are valued for their straightforward design and cost-effectiveness, as they consume less air. They are well-suited for tasks requiring force in one direction, such as in clamping, punching, and positioning applications. They are also used in pumps and rams. However, their output force can be limited by the counteracting spring force, which also restricts the stroke length. Additionally, the performance of the piston strokes can become uneven with extended use of the spring.
Rodless pneumatic cylinders are available in three distinct types:
Band cylinders feature a carrier linked to the piston through two sealing bands that run parallel to the direction of the stroke. These bands can be constructed from materials like plastic or stainless steel. The outer band is positioned on the exterior of the cylinder bore slot, attached to the carrier, while the inner band is situated inside the cylinder bore, connected to the piston. As the carrier moves, it causes the sealing band in the direction of the stroke to open, while the band behind the carrier remains closed.
In cable cylinders, a cable connects the piston to the carrier, with the cable passing through pulleys located on each end cap. As the piston moves, it pushes the cable, which in turn shifts the carrier. Cable cylinders are cost-effective and feature a straightforward design, but they can suffer from issues such as cable wear, which may lead to imprecise carrier positioning and potential leakage.
Magnetically coupled cylinders operate without a direct mechanical link between the piston and the carrier. Instead, a powerful magnetic field transfers motion from the piston to the carrier, ensuring a sealed cylinder and preventing air leakage. However, there is a risk that the carrier may become detached from the magnetic coupling and can be sensitive to moment loads.
Other types of pneumatic cylinders include:
Air hydraulic cylinders use air instead of oil to drive their mechanism. When activated, the piston in one chamber moves linearly, triggering the air-powered system. Air enters the piston chamber, and as the pressure increases, the piston moves back to compress the oil in the working area. This compression forces the working piston to perform a power stroke. After the stroke is finished, the air is released, and the components return to their initial positions.
Multi Force Pneumatic cylinders feature a series of cylinders with matching profiles, each connected via an internal air passage. These cylinders share a common piston rod that supports multiple pistons. The total output force increases with the number of pistons, which can range from 2 to 4, thereby producing 2 to 4 times the force 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 feature one or more air chambers equipped with pistons. They deliver significant force relative to their compact size and are suitable for use in challenging conditions. Like other pneumatic cylinders, they are self-contained, which shields their components from contaminants and harsh environments. Their single-piece construction also reduces the need for maintenance.
A tandem pneumatic cylinder, or combination cylinder, operates similarly to a multi-force cylinder, featuring two pistons linked by a single rod to provide double the force. Essentially, it consists of two separate double-acting cylinders arranged in series. Tandem cylinders are ideal for applications where space is restricted but a higher force output is required.
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.
Through rod cylinders feature a piston rod that extends on both sides of the piston. This design ensures that the output force and speeds during both extension and retraction strokes are equal.
There are two primary types of rotary cylinders:
A rack-and-pinion cylinder features a circular gear (the pinion) that meshes with a linear gear (the rack). When the piston is actuated, it moves the pinion linearly, causing it to rotate. A shaft is attached to the center of the pinion. Typically, two sets of racks and pistons are positioned on either side of the pinion to increase the output torque.
In vane actuators, the shaft is attached to a centrally positioned movable vane. When air pressure is applied to one side of the vane, the resulting differential pressure causes the vane and shaft to rotate toward the opposite side of the chamber. At the same time, air is expelled from the port on the other side of the chamber. Vane actuators can be designed with either a single or double vane arrangement.
Welded cylinders feature end caps that are directly welded to the cylinder bore, eliminating the need for tie rods. This design makes them more compact and robust, allowing them to endure higher internal pressures. However, their welded construction makes them challenging to repair and disassemble.
Here are the types of pneumatic cylinders categorized by their mounting style. The choice of mounting style significantly impacts the cylinder's performance, strength, and overall reliability.
Centerline mounts are designed to support loads aligned with the centerline of the pneumatic cylinder. These mounts are suitable for both push and pull applications, evenly distributing the load across the mounting bolts and minimizing sideloading. The types of pneumatic cylinders that fall under this category include:
Tie rod cylinders are known for their ability to handle high internal pressures and are the most commonly used type of pneumatic cylinder with regard to mounting style. These cylinders feature tie rods that extend across their end caps, enabling them to be securely mounted on surfaces or machinery. They absorb push or pull forces symmetrically along their centerline, which helps to minimize shear stress on the mounting bolts.
Flanged cylinders feature a flange mounted on one of the end caps, providing effective centerline force transfer and stable mounting. Typically, the flange is affixed to the front end cap for pulling applications or to the rear end cap for pushing applications. However, flanged cylinders are less tolerant of misalignment.
Centerline lug cylinders are mounted using pins positioned at the midpoint of each end cap. This design allows the cylinder to uniformly absorb forces along its centerline, making it well-suited for applications requiring straight-line force. For high-pressure or high-impact scenarios, dowel pins should be used to firmly secure the cylinder to its mounting surface.
Side mounts refer to mounting styles where the mounting surface is offset from the cylinder's centerline. The cylinder can be secured either on the side of the end caps (side-mounted cylinders) or on the front and rear of the cylinder (foot-mounted cylinders). This mounting configuration makes the cylinder prone to moment loads, which can cause it to rotate around its mounting bolts. To mitigate this issue, it is important for the stroke length and bore size to be equal. Cylinders with large bore sizes and shorter strokes may experience higher stress on the mounting bolts, leading to rotation and potential misalignment.
Pivot mount cylinders accommodate misalignment within a single plane, allowing for adjustments when the cylinder is not perfectly aligned or when the load follows a non-linear path. These mounts provide rotational freedom to the cylinder. However, excessive misalignment should be avoided to prevent excessive bending stress and deflection of the piston rod.
The actuation angle affects the force transmitted by the pneumatic cylinder, which can be calculated as the product of the generated force and the sine of the actuation angle.
Pivot-mounted pneumatic cylinders come in two distinct types:
Certain pneumatic cylinders adhere to ISO standards, ensuring compatibility with various machine components, connection methods, mountings, and accessories. By following these standards, manufacturers make it easier for customers to find pneumatic cylinders that suit their specific needs.
Here are some of the ISO standards for pneumatic cylinders:
The ISO 15552 standard outlines the essential dimensions for single and double rod pneumatic cylinders, including those with magnetic sensors. It applies to cylinders with a maximum pressure rating of 10 bars and bore sizes between 32 mm and 320 mm.
The ISO 6432 standard pertains to compact single rod pneumatic cylinders, specifically those with bore diameters ranging from 8 mm to 25 mm and a maximum pressure rating of 10 bars.
The ISO 21287 standard applies to single rod compact pneumatic cylinders, both with and without magnetic features, covering bore sizes from 20 mm to 100 mm and a maximum pressure rating of 10 bars. These cylinders do not have adjustable cushioning; instead, they use rubber bumpers for shock absorption. For cylinders with bore sizes between 32 mm and 100 mm, ISO 15552-compliant end mountings can be used.
ISO 8139 defines the mounting dimensions for rod-end spherical eyes used with pneumatic cylinders. These spherical eyes feature male or female threads that allow them to connect to a threaded piston rod. They can also be used for mounting the cylinder to a surface. Typically, they attach to the rear cap of the cylinder, with a male clevis attachment pin inserted through the center, enabling rotational movement around the pin.
Rod-end spherical eyes adhering to ISO 8139 are designed to handle pressures up to 10 bars and are compatible with pneumatic cylinders that meet ISO 15552, ISO 6432, and ISO 6430 standards.
ISO 8140 specifies the mounting dimensions for rod-end clevis attachments used with pneumatic cylinders. These clevis attachments have a fork-like shape, allowing them to be connected to a load or utilized for securing the cylinder to a surface.
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