Pneumatic actuators

Pneumatic actuators are simple and cost-effective mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear or rotational path. That motion can be inany form, such as blocking, clamping or ejecting. Unlike their hydraulic alternatives, the operating fluid in apneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas.

There are many styles of pneumatic actuators including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders.

CYLINDERS

The most popular style of pneumatic actuator consists of a piston androd moving inside a closed cylinder. Thisactuator style can be sub-divided into twotypes based on the operating principle:single-acting and double-acting.

Single-acting cylinders use one air port to allow compressed air to enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed.

Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high-pressure air.

Image courtesy of Peninsular Cylinder Co.

In a typical application, the actuator body is connected to a support frame, and the end of the rod is connected to a machine element that is to be moved. A directional control valve is used to provide a path of compressed air to the extend port while allowing the exhaust air to escape through the valve to the atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston. less than the resultant force, the piston and rod will extend and move the machine element. Reversing the directional control valve will provide compressed air to the retract port, allowing exhaust to escape the extend port, and the cylinder will return back to its home position.

Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on-off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings.

Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all.

WHY DO AIR CYLINDERS LEAK?

While a leaking pneumatic cylinder does not represent the environmental and safety catastrophe that a leaking hydraulic cylinder does, it’s still a serious situation that you should pay attention to. In addition to the fact that air leakage means wasted energy, it’s also a sign that either the system was designed improperly (say, with high side loads that are damaging the cylinder upon extension), or more likely, that the cylinder is nearing the end of its useful life and has to be replaced.

Image courtesy of BIMBA

An air cylinder will generally leak the most at the shaft, at the point where the rod moves in and out — the location of the rod seal. Some technicians recommend putting a small amount of soapy solution (bubbles) in this area to better see if this is the source of the leak. Other areas include welded seams or at the air connection points — where the air lines enter the cylinder body.

Air cylinders generally leak because their seals have worn out, sometimes exacerbated by internal rusting of the metal components. If a piston or rod seal is the culprit, these can be replaced, and seal kits are widely available. Some cylinders are the nonrepairable type, and if this is what you are dealing with, the entire cylinder will have to be replaced.

COMPACT CYLINDERS

Engineers are sometimes confronted with situations where space andweight are limited, but high force is still arequirement. In these types of circumstances,compact cylinders may be best suited infor the job. Common applications includemedical devices, robotics, packaging, andsemiconductor, among others. These low profile components have been shortened relative to standard pneumatic cylinders. They may be up to 50-60% shorter than the normal cylinder, but still maintain the capacity to exert the same force as their larger counterparts. Important parameters for the proper selectionof a compact cylinder can be broken up intogeneral, dimensional, performance, materialand features.

Originally called the “Pancake cylinder,” these miniature cylinders were first invented in 1958 by Al Schmidt, to fill a need for force in a tight, enclosed space. The basic intent was to get the most stroke in a short overall length using common machined parts and seals. Over the years, this design has been further developed, with many additional features and options to satisfy a variety of customer applications. This round body cylinder has a smooth, clean outside diameter for ease of machinery cleaning.

Even though initially used for strokes less than 1 in., manufacturing methods have allowed increased strokes to as much as 4 in. Non-metallic rod bushings and piston bearings can accommodate extreme or unforeseen loads for long-term durability.

Other compact or mini cylinders vary quite a bit. They can be rectangular or square shaped, offer numerous mounting features and can be placed with adjacent cylinders at a close center-to-center dimension. Piston bearings, materials, hard anodized bore and chrome plated rods can enhance cylinder capability for unexpected side loads and long-term durability. Up to 6-in. strokes can be accomplished with extruded body material. Other features may include metric dimensions, extruded sensor mounting and non-rotating styles. They are available in single-acting and double-acting versions.

COMPACT CYLINDER REDUCES DOWNTIME, MAINTENANCE FOR BUILDING MATERIALS MANUFACTURER

A national building materials manufacturing company was needing to replace cylinders on a shingle palletizing machine three times per year. To make sure the shingles are straight and lined up, the side-pusher cylinders are activated as a set of shingles is added to a pallet. These cylinders had a male thread rod end that would corrode the bolt and break, making the cylinder economically unrepairable. Also, while the load on the cylinders was within specifications, high temperatures and contaminants attributed to only a 4-month life span.

The customer turned to Motion Industries, Tampa, Fla., for a solution to help prevent unplanned downtime and expensive maintenance costs. Motion Industries and SMC reviewed the application and recommended switching to the MGPM50TN-50AZ compact guide cylinder, which has a shorter rod and thinner plate, resulting in a weight reduction. Designed for high side-load applications found in material handling, lifting and stopping, the cylinder utilizes an ultra-compact design by incorporating the cylinder body as part of the guide body. As the stroke length increases, so does the bearing length, thus enhancing the cylinders’ load capacity. With a female threaded rod end, there is no breakage.

Image courtesy of SMC

The customer has three cylinders which have been in service for over a year without any issues, saving the customer expensive downtime and maintenance. Not only has the MGPM50TN-50AZ outperformed the original cylinder by three times, it also is a more economical product replacement part, saving over $475 per cylinder for a total of $4,288.50 per year.

Motion Industries | motionindustries.com

SMC Corp. | smcusa.com

PNEUMATIC RODLESS & CYLINDER SLIDES

When an application calls for power and linear motion while also supportingside loads, pneumatic rodless and cylinderslides (also known as guided cylinders) areup to the task. Unlike standard pneumaticcylinders, which are unable to hold theposition of the piston rod, pneumatic slidescan stabilize and hold a load because there isno rotating rod to cause side loads. Becausethey feature a non-rotating platform to mountother actuators and tooling, they are idealfor automation applications where there isrepeated pick-and-place of parts.

Image courtesy of Festo

Rodless cylinder slides are different from standard pneumatic cylinders because they have no piston rod that extends outside of the cylinder body. This makes them ideal for long-stroke applications or where space is limited. The rodless design eliminates common problems from side loads, such as rod bending and overhang, among others. Instead of the rod, a magnetic or mechanical coupling system connects an internal piston to an external carriage.

They are also popular choices when longer distances of travel are required, or when the overall length must be minimized due to space constraints.

Be aware of several considerations when selecting the best type of pneumatic cylinder slides. These include:

• Load capacity required. Total payload must be calculated to start the selection process.

• Life required from the slide. The bearing system selected will have an impact on the expected life of the unit along with the required speed and payload.

• Speed required. The slide speed is a critical component including the ability of the slide to handle the kinetic energy as the load stops at the end of travel. Cylinder shock pads, cylinder cushions or shock absorbers may be required based on the load and speed of the slide.

• Deflection needed. The amount of deflection will vary based on the bearing system and the payload being carried. This deflection will affect the positional accuracy of the slide.

Image courtesy of BIMBA

Specification and sizing software allows users to select the proper slide required for various applications.

The idea of applying a load to a linear actuator is common, and there are a number of types of cylinder slides that can be used for these applications. The first basic style of powered slide is commonly known as a “thruster” or cantilever type unit. This type of guided slide is typically powered by a rodstyle pneumatic cylinder, which is attached to the body of the slide, or may be integral to the slide. In either case, the cylinder piston rod is attached to a tool plate providing power and motion. The tool plate is supported by a bearing mechanism, and together they are able to carry any loads that are attached, rather than transferring the load to the cylinder rod. This type of slide is designed to carry an overhung load known as a cantilevered load.

Gantry slides can handle heavier loads while traveling longer distances with a higher thrust than most other designs. Similar to thruster slides, they use a moving or reciprocating carriage between two fixed bars for their motion.

Image courtesy of Emerson

The second basic type of cylinder slide is called a saddle slide or base slide. The pneumatic cylinder is attached to a saddle that supports the bearing system on each end of the slide’s travel. This type of powered slide can be used for longer travels with less deflection based on the bearing system being supported on each end. Like the thruster style slide, the saddle carries the load versus the cylinder’s piston rod.

Another type of slide is a rodless slide. In this case, the bearing system is attached to the rodless cylinder directly on one or both sides of the cylinder. The cylinder’s piston is linked to a carriage mounted upon the bearing system, offering load carrying capability as well as resistance to side loads. Rodless slides offer the most space savings as the cylinder’s travel is contained within its own overall length.

Other pneumatic cylinder slides use profile rails with reciprocating ball carriage bearings. The profile rail bearing systems provide long life with minimum deflection. These can be incorporated in both thruster and saddle type slides.

Image courtesy of AutomationDirect