Matthias Keck provides insights into the critical aspects of pneumatic cylinder design and performance Air cylinders provide a suitable option for linear actuation in diverse industrial applications. However, to achieve a high level of cylinder performance at optimal costs, the design aspects must be balanced. Air or pneumatic cylinders have a large number of engineered components which need precisiondesigning to perform properly. One of the critical engineering areas which must be focused upon to achieve optimal performance is sealing system design. To achieve optimal sealing system effectiveness, engineers need to work on several design parameters.The output of a linear actuator is a critical factor in understanding the factors that must be balanced for optimal cylinder performance. The Role of Linear Actuators Ideally, the design for a linear actuator must incorporate all the measurable parameters of performance. While design practices are standardised for identifying typical cylinder design variables like load capacity, stroke length, positional accuracy and cylinder life, contemporary engineers have a wider range of actuation technologies to choose from, as well as an expanded range of linear actuator performance variables to optimise during the design process. A few of the application parameters typically considered during the design process are listed below:
Acceleration and deceleration performance Vibration of the linear actuator load Impact of environment variables around the linear actuator—thermal effects, contamination around the linear actuator, noise impact and hardware growth or distortion Linear actuator repair and reuse End-of-life disposal requirements Initial and sustaining system costs. Besides these performance parameters, the role of the sealing system also needs to be understood to achieve a balanced design.  Sealing System: Role in Cylinder Performance The sealing system is a critical element of cylinder design. When the sealing system performance does not reach the desired level, a diagnostic examination can be performed on the basis of the parameters listed below. The sealing system should be checked for the following: Excessive leakage of the internal fluid out or the external fluid Friction levels that fall outside of the desired specifications Total system cost exceeding the maximum levels Too short a working lifetime for the sealing system.This can be due to any combination of the three factors—leakage, friction, and system costfalling outside the acceptable limits in less than the desired time frame. For successful sealing system design,it must be understood that the above four areas used to meansure sealing system performance are interdependent, and often require careful examination and balance. Good system design requires tradeoffs—an improvement in one area often means a sacrifice in one or more of the other three areas. One of the consequences of playing such a critical role in cylinder performance is that sealing system failure often masks or hides bigger system issues, as seals tend to be one of the areas that fail most often in an air cylinder. To understand the reason behind this the basics of sealing function needs to be understood in greater detail. The mechanics of a seal is basically the use of the position of the sealing material to create the necessary clearance with the mating surface in a way that reduces the flow of the internal fluid outward and the flow of the external fluid inward. This keeps the clearances to a low enough level to keep the fluids appropriately separated and/or generate the required pressure differential. In many cases, seal designs incorporate an initial stress in the seal material through interference and compression of elastomeric materials, springs and other loading elements, in order to position the seal material. Positioning the sealing material for tight clearances will help achieve the necessary leakage control, while utilising designs that use available sources of energy—systemfluid pressure, thermal effects, and hardware motion—to help in loading the seal material. Impact of Environmental Factors With the required clearances for effective sealing,many environmental variables become important issues as they affect the basic seal function. Some of these environmental factors include the following:
Fluid flow and pressure profiles Thermal changes Media changes Hardware motions Assembly processes Time All these environmental factors eventually affect the clearance required to achieve the desired performance, especially in applications like air cylinders, where there is a significant pressure differential between the internal and external fluids. Modern sealing systems have three main areas—materials, shapes and processes—to adjust when working with other engineering disciplines. To understand how materials, geometries, and processes are selected in the design process, an overview of the impact of environmental factors on sealing systems is necessary. Note that these factors are not independent,and there are many different dependencies between each of these factors. Fluid flow and pressure profile The fluids involved in the sealing function play a critical role in determining the final system design. As air cylinders can commonly be designed for different pressures—using various internal fluids while trying to keep out external fluids—several common phenomena can occur:
Chemical degradation of the seal material (swell, hardening, cracking, etc.) Permeation through the seal material Leakage through the mating surfaces’microstructure. Other behaviours are related to the pressure rate and path of the gases involved. Some examples of this behaviour are: Explosive decompression: Cracking and blistering of the material caused by rapid release of pressure and subsequent tearing of the permeated gas as it rapidly escapes from the seal materials Extrusion of seal materials accelerated by high pressure or pressure spikes Erosion of materials by fluid jetting Directional sealing performance dependent on whether the sealing system is pressurised from one side or both sides Thermal changes Because seals are contact elements which control clearances for optimal fluid flow, there is a natural increase in temperature associated with the frictional heating from any dynamic contact of the seal material with the mating surface.This frictional heating, in combination with other environmental changes like external environmental temperature, fluid temperatures, and mating surface heat flow, all combine to impact the necessary clearance levels required to maintain effective seal system performance. A few common behaviour patters associated with thermal changes are:
Softening or hardening of the seal material and/or mating surface material, which effects the depth of penetration of the seal material into the mating surface, ultimately affects friction,wear, and leakage control Softening or hardening of bearing materials near the seal that affect the positioning of the seal Accelerated chemical degradation Growth or shrinking of components from thermal contraction or expansion.  Material changes Fluid composition and the resulting chemical degradation of the seal material is an important factor in understanding material changes in all four basic critical elements in the sealing process. Many other factors are introduced during the operation of the cylinder.A few of the changes that occur during operation are:
Wear or corrosion of the seal material or mating surface Contamination within the internal or external fluids Degradation of the lubricating qualities of the internal or external fluids. Hardware motion The dynamics of moving, pressurised components always affect the critical clearances for the basic sealing function. Some types of hardware motion that affect the sealing function include:
Offset, side loading, angular misalignment, or cocking Excessive tolerance stackups Ballooning, which is the growth of hardware under pressure Vibrations or dithering (which is high-frequency cycles at a short stroke). Assembly processes Assembly is a critical element in any successful sealing system, as the process is critical in ensuring that the sealing system begins life in the best possible condition. Some common assembly issues that must be examined include:
Appropriate hardware design (radii, chamfers, removing burrs, etc.) to facilitate good installation Use of integrated piston/wear ring assemblies, rod cartridges and other designs which are easily assembled The use of mandrels, loading cones, and other tools to facilitate manual or automatic assembly.  Time
All systems change over time, so it is no surprise that time-based activities are a significant environmental factor in sealing system performance. Some examples of time-related factors include:
Creep, stress relaxation, compression set, breakdown or chemical degradation of materials Increased variation of hardware dynamics caused by wear Fatigue, stress softening, and other duty-cycle-related phenomena Various special events like storage or long-term holding and positioning operations. These factors are often overlooked, so it’s good practice for all parties to understand the various steps of the duty cycle and the timing involved. Addressing Environmental Factors To insure successful sealing system performance for air cylinders, it is necessary to ensure effective communication among the various system component engineers to balance the measurable parameters. To insure that all factorsare considered in the design process for sealing systems, it is recommended that designers follow the process below. Identify all parties involved in the design of the components of a pneumatic cylinder Address the measures of success for each of the four main areas of sealing system performance—leakage,friction,system cost and life—and insure that all parties know how these measures are calculated Identify various options and determine the best option using various methods to design, test and validate system performance. Some examples of design methods to employ include: 3D assembly, process mapping, Finite Element Analysis (FEA), surface finish analysis, etc. Effective air cylinder performance requires effective sealing system performance. Achieving a successful performance requires a close examination of the basic sealing function to successfully examine all of the environmental factors in conjunction with other air cylinder engineering functions. In doing so,one can successfully optimise materials, designs and processes to provide world-class cylinder performance. The author is Manager, Product Development, Trelleborg Sealing Solutions |
