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How to Build a Better Pneumatic Cylinder

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Here are the details on how to engineer a rodless pneumatic cylinder.

A rodless cylinder is a pneumatic component capable of moving a load in a linear path with compressed air. The traditional pneumatic cylinder uses a rod to push or pull the load from the piston, but a rodless cylinder moves the load alongside the piston. This offers the advantages of having the same stroke length in less space, no rod buckling to worry about with high loads or long strokes, and it delivers the same force in both directions. The rodless cylinder is commonly used for applications like material handling, loading, lifting, web cutting, etc. Figure 1 shows an example of a rodless cylinder.

We turned to Gary Rosengren, director of engineering for Tolomatic, to get instructions on how to build an effective pneumatic cylinder.

Design News: What is the purpose of the cylinder?

Gary Rosengren: Pneumatic cylinders have been used in the industrial marketplace for a very long time. Applications such as lifting, pushing, pressing, and counterbalancing are all common use cases for pneumatic cylinders. While rod-type cylinders are more common in numbers, rodless pneumatic cylinders fill an interesting and valuable niche. Rodless pneumatic cylinders offer space savings and, in some cases, the ability to guide and support a load. Rodless cylinders are characterized by containing their stroke or travel within their overall length. This type of cylinder is therefore useful in applications where space is a constraint or where it is advantageous to use the cylinder's capability to carry a load thereby eliminating the need for guidance systems built into the machine structure.

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The sealing strip must be capable of sealing the slot on the rodless cylinder, but also flexible enough to allow it to be passed through the piston.

DN: What is the process of designing it?

Gary Rosengren: Designing a rodless pneumatic cylinder will begin with targeting a use case or a capability which then provides insights into the size, force, and sometimes load-carrying capability. Force is a function of the bore diameter and effective area. Load-carrying capability is a function of cylinder barrel structure and bearing elements.  

The design of the rodless pneumatic cylinder begins with the operational concept. The three main operational concepts are cable type, magnetically coupled type, and slotted type. The most common of these three is the slotted type. In this configuration, the engineer will consider how to produce the cylinder barrel or tube as the first part of the process. In the slotted type rodless pneumatic cylinder, the cylinder barrel or tube is formed with a slot running the entire length of the cylinder.

This slot is ultimately sealed off by a flexible sealing element such as polyurethane or steel strip or band forming the pressure vessel. This sealing strip must be capable of sealing the slot but also flexible enough to allow it to be passed through the piston. The sealing strip also needs to interact precisely with the cylinder barrel for effective sealing and long-term performance. Most rodless pneumatic cylinders of this type will also include a sealing strip on the outer surface of the slot to help keep environmental contaminants from entering the interior of the cylinder potentially accelerating wear and causing excess leakage.

The customer's mounting point is attached directly to the cylinder piston via a bracket to the yoke that passes through the slot in the cylinder barrel. Since a common method of forming the cylinder barrel is aluminum extrusion, the engineer must have a good working understanding of the extrusion process and its capabilities. Surface finishes and surface coatings will influence the performance and longevity of the cylinder and need to be designed and specified appropriately.     

DN: What type of engineering is involved?

Gary Rosengren: Mechanical design engineering disciplines are required for determining the structure and the various interfaces within the cylinder. The engineer will be responsible for determining the strength of the various components to prevent structural failures and to prevent air leakage. If load carrying is a requirement, the engineer will select appropriately sized bearing elements as dictated by the targeted use case or application. Since pneumatic rodless cylinders are frequently used in high duty cycle applications, the design engineer will need to understand and properly select materials for the wearable element such as piston seals and associated materials to assure longevity.

The study of lubrication (tribology) may also become valuable if the use case or application suggests extreme or unusual environmental conditions such as very warm or very cold ambient conditions. Manufacturing engineering disciplines are deployed to assure the factory can reliably and repeatedly produce the design. Performance and life tests are a valuable part of the engineering process to assure specifications are being achieved as well as to correct potential or unforeseen issues.  

DN: What are the manufacturing steps?

Gary Rosengren: At the core of the rodless pneumatic cylinder is the cylinder barrel or cylinder tube. Many times, this main component is formed using the aluminum extrusion process. The extrusion process yields a very long part that is then plated or anodized for corrosion and wear resistance. Once plated or anodized the part needs to be cut to length for each particular stroke. In the case of the rodless pneumatic cylinder, stroke lengths are only limited by the length of the extruded cylinder barrel. The sealing strip is constructed of plastic that may be extruded. If the sealing strip is metal, slitting and or grinding processes are used to manufacture it from the appropriate material.

The sealing strip and the cylinder piston containing the piston seals are installed into the barrel along with the specified lubricant. End caps seal off each end as well as provide inlet and exhaust ports, which means to anchor the sealing strip and sometimes mounting surfaces get fastened to the cylinder barrel. End caps may be cast or machined from the billet. Most rodless pneumatic cylinders use end caps made from aluminum. If the cylinder configuration requires integral bearing elements these are typically mounted directly to the cylinder barrel and linked or attached to the piston. Final testing assures that the cylinder has been built to specification and operated correctly upon the application of compressed air.   

DN: What are the quality assurances along the way?

Gary Rosengren: To assure conformance to specifications, each major component or process must go through several stages of analysis using tools such as FMEA’s and or 6 sigma methodology. These tools help identify the critical quality features and process capabilities. Data from the use of these tools and methods are shared with the suppliers of the main components helping them monitor and improve their processes.

For the main cylinder barrel component, an inspection of critical features such as surface finish and coating thickness will occur on every lot of material. Depending upon the manufacturing environment, other components may get inspections of critical parameters as they are produced. Final assembly testing evaluates the overall conformance, general performance, and functionality of any additional or optional accessories. Final assembly testing will typically include verification of leakage, smoothness of travel, and function of accessories for each cylinder assuring overall conformance to the specifications.    

Rob Spiegel has covered manufacturing for 19 years, 17 of them for Design News. Other topics he has covered include automation, supply chain technology, alternative energy, and cybersecurity. For 10 years, he was the owner and publisher of the food magazine Chile Pepper.

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