By Larry Castleman, Technical Director, Product Development Trelleborg Sealing Solutions Fort Wayne, Ind.
|Pneumatic cylinders are precision-engineered systems where numerous individual components must work together for the actuator to function properly. Good seals are essential for low leakage and long life. |
Pneumatic cylinders are a valuable and preferred option for many linear-actuation applications, combining long life, low maintenance, and ruggedness in an economical package. On the surface they might appear to be simple devices. In reality, however, pneumatic cylinders are precision systems with a number of engineered components that must work together for the actuator to function properly.
Designing these building blocks isn’t getting any easier. Cylinder design practices might be well defined for identifying traditional variables such as load capacity, stroke length, and positioning accuracy. But today’s engineers must optimize an expanded range of performance factors during the design process. These typically include:
• Acceleration and deceleration capabilities.
• Vibration of the load during actuation.
• Ability to withstand harsh environments, such as extreme temperatures without hardware growth or distortion; or external contamination without corrosion.
• Noise limitations.
• The ability to repair and reuse the cylinder.
• End of life disposal requirements.
• Total lifetime ownership costs. Designers must balance the effects these factors have on system components to satisfy user expectations. One area particularly critical to top-notch performance is a good sealing system. Here’s a closer look at factors that impact sealing systems when optimizing overall cylinder performance.
Seals are critical elements in cylinder design because they must balance performance and life with cost by:
• Preventing unacceptable amounts of internal fluid from leaking out or external fluid from getting in.
• Keeping static and dynamic friction within desired specs. Too little friction can mean not enough force to seal; too much leads to excessive heat, energy loss, and wear.
• Having an acceptable working life.
• Meeting total system cost demands.
|Finite-element analysis is a good tool to predict seal behavior and help optimize leakage, friction, and wear characteristics. These examples show a polyurethane pneumatic-cylinder seal at 29 and 145 psi. |
One consequence of playing such a critical role in cylinder performance is that seal failures often mask bigger system issues. Seal failures are often the first obvious sign of system problems, as many times the root cause of failure lies with bearing, side-load, or misalignment issues.
The basic mechanics surrounding seal operation involves minimizing the clearance gap between mating surfaces, which separates fluids and maintains the required pressure differential. In many cases seals rely on interference and compression of elastomeric materials, springs, and other loading elements to create an initial stress. Designs also use other available sources of energy, such as fluid pressure, thermal effects, and hardware motion that help assist in loading the seal material. The result is the right fluid-film thickness control for optimum leakage, friction, life, and system cost.
As one can imagine, with the minimal clearances required for effective sealing, application and environmental variables can significantly affect how well the seal functions, especially in applications like pneumatic cylinders where there is a significant pressure differential between internal and external fluids. Here’s a closer look at these factors and the problems they create.
Fluid flow — Compatibility of seal materials with the working fluid is normally not a concern in pneumatic cylinders, unless internal lubricants or external caustics, corrosives, and contaminants lead to chemical degradation that causes seal materials to swell, harden, or crack.
External fluid can also, in some cases, permeate through seal materials or leak through the cylinder’s metal-surface microstructure. Another factor is behavior related to pressure profiles, including:
• Explosive decompression. The rapid release of pressure can cause permeated gas to quickly escape from the seal and, in the process, tear, crack, and blister the material.
• Extrusion of seal materials through the gap between mating components, accelerated by high pressures or pressure spikes.
• Erosion of seal materials by fluid jetting.
Thermal changes — Dynamic contact between the seal and mating surface causes frictional heating and a natural increase in temperature. This heating, combined with external ambient conditions and fluid and cylinder temperatures can affect clearances required for effective sealing. Thermal changes can: • Soften or harden seal materials and metal surfaces, which affect how deep the seal material penetrates into the mating surface and, ultimately, friction, wear, and leakage control. • Soften or harden bearing materials near the seal that can alter seal position. • Cause excessive temperatures that accelerate chemical degradation. • Create temperature extremes that expand or contract the seal. Material changes — In addition to chemical degradation of seal materials, fluid incompatibility can corrode and accelerate wear of metal sealing surfaces. And, in addition to the fluid itself, contaminants within the internal or external fluids can physically or chemically attack the seal. Finally, the lubricating qualities of internal and external fluids can degrade over time and accelerate wear.
Hardware motions — The dynamics of moving, pressurized components always impact the critical sealing clearances. Some types of movements that can play havoc with seals include offset, side loading, angular misalignment, and cocking; ballooning, where the cylinder diameter grows under pressure; and high-frequency, shortstroke cycling due to vibrations or dithering.
|Endurance testing tracked rod leakage past the polyurethane seal at 29 and 145 psi operating pressures. Cylinders have a 50-mm diameter, 250-mm stroke length, and 2-kg side load. Speed is 1 m/sec with a cycle time of 0.5 Hz. Testing was discontinued without failure after 36 million cycles, or about 18,000 km of travel. |
Assembly processes — How cylinder components are manufactured and assembled is critical to tools to aid manual installation or automatic assembly. Experts also recommend using appropriate radii and chamfers on glands and other cylinder hardware to ease installation, as well as deburring the parts to minimize chances of nicks or cuts that could accelerate seal failure.
Time — Seals change over time. Some examples of timerelated behavior include creep, stress relaxation, compression set, and chemical degradation of materials, as well as fatigue, stress softening, and other duty-cycle-related phenomena. Seal wear can increase variation in cylinder performance. And don’t forget to consider how seal performance might be impacted by lengthy storage periods or long-term position-andhold operations.
Designing pneumatic seals
|Test cylinders in a temperature chamber experience cycles between –25 and 80°C. |
Because so many external and environmental factors beside basic actuator performance requirements can influence how a seal functions, we recommend the following design process for sealing systems.
• Identify all parties involved in designing components for a pneumatic cylinder.
• Clearly state the benchmarks for successful seal performance in terms of leakage, friction, cost, and life, and make sure everyone involved in the cylinder’s design knows how these measures are calculated.
• Identify seal options, and address how to determine the best one using various methods to design, test, and validate system performance. Design methods might include: 3D assembly, process mapping, finite-element analysis (FEA), surface-finish analysis, materials testing, product validation, and failure mode effects analysis (FMEA). This approach lets all members of the design team contribute to a robust and speedy engineering process.
Seal manufacturers continually develop new seals for pneumatic cylinders. Market demands indicate the need for a more robust, longer lasting, and more cost-effective sealing system. Given this input, we believe the best mix of cylinder performance and value is a sealing system that:
• Works well in oil-free compressed air with minimal lubrication at startup.
• Handles compressed air pressures to 230 psi.
• Covers working speeds to 100 fpm with maximum short-term excursions to 400 fpm.
• Generates low friction and no stick slip during operation.
• Gives lifetime travel of 4000 miles.
To balance these factors, our design team addressed materials, designs, and process improvements concurrently. Results focused on three areas:
• Develop polyurethane materials that balance the high wear and abrasion resistance of polyurethane with the strength, chemical compatibility, and friction characteristics required for long life, excellent sealing performance, low friction, and appropriate total system cost. We call this the Zurcon family of materials.
• Design appropriate seal geometries. This includes a rounded contact area at the seal lip to maintain lubrication film, a thin lip that generates low radial force and low friction, air channels to permit pressure activation, and other features that best make best use of a low-hardness polyurethane. FEA and product testing validated performance of the new design features. The FEA process shown in the accompanying example studies the geometry and material behavior at 29 psi and 145 psi to optimize leakage, friction, and wear characteristics.
• Develop and validate an economical injection-molding process that optimizes the material’s superior properties.
A final battery of product testing gauged leakage, friction, system cost, and life. An endurance test determined rod-seal leakage over time at 29 psi and 145 psi, and other validation tests included breakout friction, low-temperature performance, and high-pressure and burst-pressure tests. The resulting sealing system successfully optimizes materials, designs, and processes and meets or exceeds all critical cylinder-performance goals.
For further information, Trelleborg Sealing Solutions R&D, 2531 Bremer Road, Fort Wayne, IN 46803, (260) 749-9631, www.tss.trelleborg.com.