Making today’s pneumatic cylinders work effectively
Pneumatic cylinders continue to provide a valuable and preferred option for linear actuation in many applications. To achieve these unprecedented levels of effectiveness, all portions of the cylinder design must be balanced to reach the desired performance and cost ratio.
As you can see from the cutaway cylinder in Figure 1, pneumatic cylinders have a large number engineered components working together. One of the critical areas of engineering required for achieving this performance is good sealing system design. To achieve the necessary sealing system effectiveness, engineers for a variety of components involved in a pneumatic cylinder need to work together to reach the right balance for each application. Looking at the output of linear actuator is a great place to start to begin to understand the factors to be balanced for an effective linear actuator.
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Typical Pneumatic Cylinder Design
Understanding the output for a linear actuator
Good linear actuator design incorporates all types of measurables of performance for the linear actuator application. While past practices are well defined for identifying typical cylinder design variables like load capacity, stroke length, positional accuracy, and cylinder useful life, today’s engineers have a wider range of actuation technologies to choose from, and an expanded range of linear actuator performance variables to optimize during the design process. Application parameters now typically include these features:
- Acceleration and deceleration performance
- Vibration of the linear actuator load
- Impact of environment variables around the linear actuator such as 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
All of these factors lead to today’s cylinder designs now requiring a close examination of all components to insure that we achieve the necessary balance for proper optimization of system performance. To understand how to achieve this balance, let’s look at the role of the sealing system in cylinder function.
The sealing system’s role in cylinder performance
The sealing system is a critical element of cylinder design when striving to achieve optimum performance. When sealing system performance does not reach required targets, any examination of sealing system performance is often diagnosed and discussed using many different terms, but all performance measurables can be grouped in one of the following four areas:
- Having excessive leakage of the internal fluid out or the external fluid in
- Having friction levels that fall outside of the desired specifications
- Having total system cost exceed the maximum levels
- Having too short a working lifetime for the sealing system, which is any single or combination of the three factors above (leakage, friction, and system cost) falling outside the acceptable limits in less than the desired time frame
To achieve success for sealing systems, all parties involved need to recognize that the above four areas for measuring sealing system performance are mutually dependent, and often require careful examination and balance. Good system design requires tradeoffs where improvement in one area often means a sacrifice in one or more of other three areas.
One of the consequences of playing such a critical role in the 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 a pneumatic cylinder. To understand why this phenomenon exists, we’ll examine the basic sealing function in greater detail. As shown in Figure 2, sealing function involves a few basic critical elements – a seal material(s), a seal surface, and internal and external fluids.
Basic seal function.
The basis mechanics of a seal, as shown in Figure 2, are to use 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. To keep the clearances to a low enough level to appropriately keep the fluids separated and/or generate 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 to position the seal material. Positioning the seal material for tight clearances achieves necessary leakage control, while utilizing designs that use available sources of energy like system fluid pressure, thermal effects, and hardware motion to help assist in loading the seal material.
As one can imagine with the required clearances shown in Figure 2 for effective sealing, many environmental variables become important to consider, as they affect the basic seal function. Some of these environmental factors include:
- Fluid flow and pressure profiles
- Thermal changes
- Media changes
- Hardware motions
- Assembly processes
All of these environmental factors eventually affect in some way the clearances required for desired performance, especially in applications like pneumatic cylinders where there is a significant pressure differential between the internal and external fluids. Let’s look more closely at what these factors to understand what sealing system engineers are doing to better achieve success.
What environmental factors do to sealing system effectiveness
Good sealing system engineers work toward understanding the impact of each environmental factor to maximize any benefits and minimize any negative impact as they seek to achieve the necessary balance between the four main measures of sealing system performance: leakage, friction, life and system cost.
Today’s sealing system engineers have three main areas to adjust when effectively working with other engineering disciplines to optimize sealing system products: materials, shapes, and processes. To better understand how materials, geometries, and processes are selected in the design process, an overview of the affect of the environmental factors involved is helpful. 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 sealing function play a critical role in determining final system design. As pneumatic cylinders can commonly be designed for various pressures using various internal fluids while trying to keep out external fluids, we have several common phenomena that occur:
- Chemical degradation of the seal material (swell, hardening, cracking, etc.)
- Permeation through the seal material
- Leakage through the mating surfaces’ microstructure
Other behavior is related to the pressure rate and path of the gases involved. Some examples of this behavior include:
- Explosive decompression, which is a cracking and blistering of the material caused by rapid release of pressure and subsequent tearing of the permeated gas as it rapidly escapes out of 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 pressurized from one side or both sides
Because seals are contact elements whose job is to control clearances for good fluid flow control, 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. Here are some common behaviors associated with thermal changes:
- Softening or hardening of the seal material and/or mating surface materials, which effects the depth of penetration of the seal material into the mating surface and ultimately the friction, wear, and leakage control
- Softening or hardening of bearing materials near the seal that impact the positioning of the seal
- Accelerated chemical degradation (see fluid flow and pressure section)
- Growth or shrinking of components from thermal contraction or expansion
As discussed previously, fluid composition and resulting chemical degradation of the seal material is an important factor in understanding material changes in all four basic critical elements in the basic sealing process. Many other factors are introduced during the operation of the cylinder that involves one or more of these materials and the changes in them that occur during operation, including:
- Wear or corrosion of the seal material or mating surface
- Contaminates within the internal or external fluids
- Degradation of the lubricating qualities of the internal or external fluids
The dynamics of moving, pressurized components always impact the critical clearances for the basic sealing function. Some types of hardware motions 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 is a critical element in any successful sealing system, as the process is critical to insure the sealing system begins life in the best possible condition. Some common issues involving assembly that need to 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 easily assemble
- Use of mandrels, loading cones, and other tools to facilitate manual or automatic assembly
All systems change over time, so it’s 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.
How to address environmental factors
As we’ve seen, the basic function of a seal is influenced by a wide range of environmental factors. To insure successful sealing system performance for pneumatic cylinders means that early communication among the various system component engineers is crucial to insure the proper balance of measurables. To insure that all factors are considered in the design process for sealing systems, it’s recommended to follow this type of process:
- Identify all parties involved in the design of the components of a pneumatic cylinder
- Address among all parties the measures of success for each of the four main areas of sealing system performance, which are leakage, friction, system cost, and life, and insure all parties know how these measures are calculated
- Identify various options and address how to determine 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, materials testing, product validation, Failure Mode Effects Analysis (FMEA)
This approach allows all parties to have a robust, speedy design process.
An example of the design process
As an example of the design process in action for a range of pneumatic cylinders requiring cost effective, high performance sealing systems, Trelleborg Sealing Solutions has been working with many customers to develop a new line of pneumatic sealing systems. Demands in the market indicated that we need a more robust, longer life, more cost effective sealing system for pneumatic cylinders requiring a good balance of low cost and long life. After gathering the market input, the best value for cylinder performance is a sealing system that would work well in the following environment:
- In oil-free compressed air with minimal lubrication at startup
- Compressed air conditions of 100 psi, with 230 psi maximum pressure
- A working speed range of 100 fpm, with maximum short term excursions to 400 fpm
- Low friction and no-stick slip during operation
- Lifetime travel of 4,000 miles
To achieve this balance, a Trelleborg Sealing Solutions design team addressed materials, designs and process improvements concurrently. The result was a focus on three areas:
- Developing suitable Zurcon polyurethane materials to achieve the desired level of performance through balancing the excellent wear and abrasion resistance of polyurethane with the right strength, chemical compatibility and friction characteristics required to achieve long life, excellent sealing performance, low friction and appropriate total system cost
- Designing appropriate geometries with rounded contact area at the seal lip to maintain grease film, small lip thickness gives low radial force for low friction, air channels to allow proper pressure activation, and other features that optimize using a low hardness Zurcon polyurethane material. As an example of using available tools, FEA and product testing validated and confirmed performance of these new design features. An example of the FEA process is shown in Figure 3 below, where the geometry and material are studied to see the sealing profile at 29 psi and 145 psi to optimize leakage, friction, and wear characteristics
- Development and validation of the use of an injection molding process that limits cost and provide superior material properties
FEA at various pressures.
To ensure the proper balance was achieved, a final battery of product testing was conducted to insure the leakage, friction, system cost and lifetime measurables were achieved. As an example of robust validation process, Figure 4 shows an endurance test conducted to determine the rod seal leakage during the course of an endurance test to see trends related to leakage at 29 psi and 145 psi. Other validation tests included breakout friction, low temperature performance, high pressure tests, and bursting pressure tests, all of which are designed to provide validation of a critical measurable of performance.
Rod seal leakage during endurance testing.
Effective pneumatic cylinder performance requires effective sealing system performance. As shown in our example with our pneumatic Zurcon sealing system, you can achieve successful performance. It requires close examination of the basic sealing function so one can examine successfully all of the environmental factors in conjunction with other pneumatic cylinder engineering functions. In doing so, we can optimize successfully materials, designs and processes to provide world-class cylinder performance.
Special thanks to the Matthias Keck and the rest of Trelleborg Sealing Solutions R&D worldwide for their support. For more information, contact Trelleborg Sealing Solutions R&D, 2531 Bremer Road, Fort Wayne, IN 46803, (260) 749-9631, firstname.lastname@example.org.
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