By Larry J. Castleman
Larry J. Castleman is Technical Director, Product Development for Trelleborg Sealing Solutions Americas
While pneumatic cylinders are often a preferred option for linear actuation in many applications, all portions of the cylinder design must be balanced and work together for efficient and reliable function. One of the more critical components that rely on good design is the sealing system. To achieve the necessary sealing system effectiveness, several engineering disciplines need to work together to reach the right balance for each application. A good place to start is to take a closer look at linear actuator output.
Good linear actuator design includes many performance measures. Past practices are well defined for identifying such variables as load capacity, stroke length, positional accuracy, and cylinder life. Today’s technology actuation choices are wider, with subsequently more variables to manage during the design process. Application parameters now typically include such features as:
• Acceleration and deceleration requirements
• Vibration of the linear actuator load
• Impact of environment variables around the linear actuator such as thermal effects, contamination, noise, and hardware growth or distortion
• Linear actuator repair and reuse
• End of life disposal requirements
• Initial and sustaining system costs
For efficient and reliable function, pneumatic cylinders rely on a well designed sealing system. To achieve the necessary sealing system effectiveness, several engineering disciplines must work together to reach the right balance for an application.
All of these factors require examination of all components to insure the necessary balance for maximum system operation. To understand how to achieve this balance, let’s look at the role of the sealing system in cylinder function.
The role of the sealing system
The sealing system is a critical element of cylinder design. All of its performance measurements can be grouped in one of the following four areas:
1. Excessive leakage of the internal fluid out or the external fluid in
2. Friction levels that fall outside the desired specifications
3. Total system cost that exceeds the maximum levels
4. Too short a working life for the sealing system, which can result from any one or more of the above factors falling outside acceptable limits in less than the desired time frame.
These areas 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.
Failure of the sealing system often masks or hides larger system issues, as seals tend to be one of the areas that fail most often in a pneumatic cylinder. The sealing function involves a few basic critical elements – a seal material(s), a seal surface, and internal and external fluids.
The basis mechanics of a seal are to:
• use the position of the sealing material to create the necessary clearance with the mating surface to reduce 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 generate required pressure differential.
In many cases, seal design includes an initial stress placed on the seal material through interference and compression of elastomeric materials, springs, and other loading elements.
Positioning the seal material for tight clearances controls leakage, while using designs that use available sources of energy like system fluid pressure, thermal effects, and motion help assist in loading the seal material.
The seal material(s), seal surface, and internal and external fluids are critical elements to the sealing function.
How environmental factors affect effectiveness
For the required clearances to be effective at sealing, it isimportant to consider many environmental variables 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 the above factors eventually affect the clearances required for desired pneumatic cylinder performance, especially where there is a significant pressure differential between the internal and external fluids.
There are four main measures of sealing system performance: leakage, friction, life, and system cost. There are also three main areas to adjust when working with other engineering disciplines to optimize sealing systems: materials, geometries, and processes.
To better understand how materials, geometries, and processes are selected in the design process, an overview of the impact 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 influence final system design. Several phenomena can occur when designing pneumatic cylinders for various pressures using different internal fluids to control the entrance of external fluids:
• The seal material can degrade chemically (swell, hardening, cracking, and so on.)
• Foreign material or fluid will permeate through the seal
• Leakage can occur through the mating surfaces’ microstructure
Other behavior is related to the pressure rate and path of the gases involved. Some examples 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 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 that control clearances for good fluid flow control, they are subject to 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 temperature, fluid temperatures, and mating surface heat flow, all combine to affect the necessary clearance levels required to maintain an effective seal. Here are some common behaviors associated with thermal changes:
• Softening or hardening of the seal material and mating surface materials, which affects 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 seal position
• Accelerated chemical degradation
• Growth or shrinking of components from thermal contraction or expansion
As discussed previously, fluid composition and the resulting chemical degradation of the seal material alter all four critical elements in the basic sealing process. Other factors are introduced during cylinder operation that involve one or more of these materials, 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 affects sealing clearances. Some 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
Proper assembly assures the sealing system begins life in the best possible condition. Some common issues involving assembly include:
• Appropriate hardware design (radii, chamfers, removing burrs, and so on) for 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 influence 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.
Tips for addressing environmental factors
Because the basic function of a seal is influenced by a range of environmental factors, it’s imperative for the design team to communicate to insure the proper balance of measurables. The following process will achieve that goal:
• Identify all parties involved in the design of the components of a pneumatic cylinder
• Address leakage, friction, system cost, and life, and assure that all parties know how these measures are calculated
• Identify various design options and determine the best for design, test and validation areas. Some examples include: 3D assembly, process mapping, Finite Element Analysis (FEA), surface finish analysis, materials testing, product validation, Failure Mode Effects Analysis (FMEA)
This approach allows a robust, speedy design process. Effective pneumatic cylinder function requires proper sealing system function, which necessitates close examination of the basic sealing function so one can examine all of the environmental factors in conjunction with other pneumatic cylinder engineering functions.
A closer look at designing a new range of pneumatic sealing systems
Trelleborg Sealing Solutions has been working with customers to develop a new line of pneumatic sealing systems. Demands in the market indicated the need for a more robust, longer life, and more cost effective sealing system for pneumatic cylinders. After gathering market data, the best value for cylinder performance turned out to be 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 4000 miles
To achieve this balance, the design team addressed
materials, designs, and process improvements concurrently. The result was a focus on three areas:
• Develop suitable Zurcon® polyurethane materials that would offer excellent wear and abrasion resistance with the right strength, chemical compatibility, and friction characteristics required to achieve long life, excellent sealing, low friction, and appropriate total system cost.
• Develop appropriate geometries with rounded contact area at the seal lip to maintain grease film, small lip thickness for low radial force and friction, air channels to allow proper pressure activation, and other features available through the use of the low hardness Zurcon material. FEA and product testing validated and confirmed the performance of these features.
• Develop and validate the use of an injection molding process that limits cost and provides the required material properties.
Tests were done to ensure that proper balance was achieved between leakage, friction, system cost, and lifetime measurables. The graph on the right shows an
endurance test conducted to determine the rod seal leakage during the course 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.
To study the effects of leakage, friction, and wear on the geometry and material, this FEA analysis shows a sealing profile at 29 psi and 145 psi.
An endurance test for rod seal leakage showed leakage trends at 29 psi and 145 psi.
Trelleborg Sealing Solutions R&D
: Design World :