By Josh Cosford, Contributing Editor
Seals used for fluid power applications are highly varied and specific to each component and application. From simply sealing pressurized air from atmospheric air for pneumatic valves to preventing hydraulic oil from leaking into waterways from the rudder cylinder on watercraft, there are few conditions in which a fluid power seal doesn’t operate.
It goes without saying that the primary role of a seal is to contain fluid (yes, air is a fluid). Although an engineered amount of leakage is occasionally required, such as for lubrication purposes, leakage is generally considered a failure mode. Fluid leakage not only leads to operational inefficiencies but can also result in safety hazards and environmental concerns. We must understand the importance of proper sealing design and selection in preventing and controlling leakage in hydraulics and pneumatics systems, focusing on the design considerations for the seals and the pockets or cavities into which they’re installed.
Leakage in hydraulic and pneumatic systems can occur due to various factors, including wear and tear, temperature fluctuations, pressure differentials, incompatible media, and poor sealing design. The consequences of leakage extend beyond mere inefficiencies, encompassing increased energy consumption, decreased system performance, and, in extreme cases, equipment failure.
In hydraulics, where liquid is used to transmit power, even a tiny leak can result in a significant loss of fluid over time. This not only raises operational costs but also poses environmental challenges. Pneumatic systems, which use compressed air, face similar issues with leakage affecting overall efficiency. To address these challenges, engineers must focus on sealing solutions that are not only effective and durable but also tailored to the specific needs of each system.
I’ll first discuss the application side of sealing and the four points you must consider to select seals best suited for your pump, valve, actuator or other component. Then, I’ll move along to specialty seals that improve specific applications and the benefits of each. Finally, I’ll wrap it up with practical tips on improving the reliability of any seal used in your application.
Application and Engineering
1. Material selection: The choice of sealing material is critical in designing effective seals for hydraulic and pneumatic systems. Different applications demand different materials based on chemical compatibility, temperature resistance, and wear characteristics. Common sealing materials include nitrile rubber, polyurethane, fluorocarbon, and PTFE. Understanding the operating environment and the nature of the fluids involved is essential in selecting the most suitable material for a given application.
For both hydraulics and pneumatics, Buna nitrile offers a great all-around seal polymer suitable for most applications, although other technologies are preferred when extreme conditions present themselves. Urethanes excel in high-pressure applications, while Viton (fluorocarbon) is a top choice for high-temperature environments.
2. Geometry and profile: The design and geometry of a seal play a vital role in its performance. Engineers must consider factors such as groove dimensions, seal cross-section, and contact pressure distribution. The choice between dynamic and static seals depends on the specific application requirements. Dynamic seals, which experience relative motion between mating surfaces, are commonly used in hydraulic and pneumatic cylinders, while static seals are employed in areas with no relative motion.
Even when dynamic seals are required, manufacturers offer myriad choices for the same application, such as piston seals, for example. A piston seal can range from a simple O-ring to exotically-shaped lip seals. Depending on application and designer preference, a piston may employ energized U-seals, U-seals with bonded backup, T-seals, multiple buffer seals, vee packings, piston cups, crown seals, or multi-piece assemblies.
3. Pressure and temperature ratings:
Hydraulic and pneumatic systems operate under varying pressure and temperature conditions. Seals must be capable of withstanding these extremes to ensure longevity and reliability, and how a seal performs in extremes comes down to many factors. Understanding the pressure and temperature ratings of seals helps with selecting components that can withstand the demanding conditions of industrial applications.
Although fluorocarbon generally handles hot ambient conditions up to 400°F, it’s not like any hydraulic oil can withstand such extreme conditions. When you factor in the reduced pressure rating limitation of the softer Viton, perhaps a urethane with a meaty cross section would be suitable, especially with high-temperature polyurethane rated for 250°F.
4. Friction and lubrication: Friction between sealing surfaces can lead to wear and heat generation, impacting the overall efficiency of a system. Proper lubrication is essential to reduce friction and extend the life of seals. Engineers must consider the compatibility of lubricants with sealing materials and ensure that the lubrication method aligns with the application’s specific requirements.
Aside from lubrication, which is obviously favorable for hydraulic components bathed in oil, the construction style also affects friction, subsequently influencing velocity limitations. A pneumatic cylinder with a loaded T-seal provides excellent leakage protection but, when dry, will be subject to high friction, where repeated high-velocity cycles may quickly soften and wear the seal, ironically leading to leakage. Choosing a softer U-cup for high-velocity applications may give up some load-holding capacity. Still, with less friction, the seal will be maintained within its ideal operating temperature, leading to leak-free reliability.
Specialty seal options
5. Polymeric seals with enhanced properties: Recent advancements in material science have led to the development of polymeric seals with enhanced properties, such as improved wear resistance, flexibility, and chemical compatibility. These advanced materials contribute to longer seal life and better overall system performance. For instance, polytetrafluoroethylene (PTFE) seals are known for their low friction, high-temperature resistance, and chemical inertness, making them suitable for demanding applications in possibly corrosive and caustic conditions. Thermoplastic elastomers (such as Hytrel) offer sealing, pressure capacity, and friction characteristics similar to other choices but with a broader temperature range.
Even within the realm of traditional seal materials, such as urethane, advancements have resulted in sophisticated compounds offering superior pressure capacity, wider operating temperature operation range and even capacity to work with exotic fluids previously incompatible with older technology.
6.Self-lubricating seals:Self-lubricating seals incorporate materials that inherently reduce friction, eliminating the need for external lubrication. They are particularly beneficial where frequent maintenance is challenging or costly. Self-lubricating seals contribute to energy efficiency and extend the intervals between maintenance cycles by minimizing friction. Common self-lubricating compounds are bronze-filled PTFE or carbon/graphite-impregnated rubber.
7. Smart seals with monitoring capabilities:Integrating sensors and monitoring technologies into seals allows for real-time performance analysis and is on the cutting edge of Industry 4.0 technology. Smart seals can provide data on factors such as temperature, pressure, and wear, enabling predictive maintenance strategies. By identifying potential issues before they escalate, smart seals increase system reliability and reduce downtime.
8. Seals with low-compression set: Compression set refers to the permanent deformation of a seal after prolonged compression. Seals with low compression set characteristics maintain their shape and sealing effectiveness over extended periods, even under high pressures. This property is particularly crucial in hydraulic and pneumatic systems where seals are subject to continuous compression and decompression cycles. A compressed seal will not correctly fill out the groove or pocket, leading to leakage.
Practical tips
9. Regular maintenance and inspection: Implementing a routine maintenance schedule is essential for preventing and identifying potential leakage issues. Regular inspections allow operators to detect wear, damage, or misalignment of seals early, enabling timely replacement and avoiding more extensive damage. Although many leaks are internal (think piston seals), these failures can be monitored or observed, such as when a cylinder drifts or fails to hold a load stationary. It goes without saying that visual leaks are the most apparent form of leakage.
10. Proper installation techniques: Seals must be installed with precision to ensure optimal performance and leak prevention. Proper installation involves attention to correct alignment, lubrication, and torque specifications. Sometimes, a seal will roll inside its machined pocket after it’s installed, especially when it isn’t pre-lubricated, such as with rod seals subjected to dry friction of the chromed surface. As well, end seals should be prevented from interference with potential pinching surfaces, burrs or sharp edges. A pinch seal will result in immediate leakage during operation in most cases.
11. Pressure and temperature control: Operating hydraulic and pneumatic systems within specified pressure and temperature ranges are crucial for seal integrity, as exceeding these limits can lead to seal failure, leakage, and, ultimately, system breakdown. Implementing pressure and temperature control measures, such as efficient circuit design and cooling systems, contributes to the longevity of seals. Also, heat from fluid, friction or ambient conditions softens the seal, making it more likely to extrude through even the smallest gaps.
12. Training and education: Adequate training for operators and maintenance personnel is crucial for effective system management. Training programs should cover proper handling, installation, and troubleshooting of seals. A well-informed team is better equipped to promptly identify and address potential leakage issues, not only once equipment is installed, but to help prevent the previous two examples from occurring.
Leakage prevention and control in hydraulic and pneumatic systems are paramount for efficient and reliable industrial operations. Advances in sealing technology, coupled with best practices in design and maintenance, contribute to the longevity and performance of seals in these critical systems. As industries continue to evolve, integrating smart seals and innovative materials will play a pivotal role in enhancing overall system efficiency and reducing the environmental impact of fluid leakage. Engineers and operators must remain vigilant in their approach to sealing design and selection, embracing the latest advancements to ensure the seamless operation of hydraulic and pneumatic systems in future years.
Filed Under: Components Oil Coolers, Engineering Basics, Sealing, Sealing & Contamination Control Tips, Technologies, Tube & Hose
