PTFE rotary shaft seals behave very differently from their elastomeric counterparts. Because one of their primary mechanisms is transfer film, they have different requirements related to the mating surface to achieve a successful solution. This blog post looks at three key factors that impact the performance of PTFE rotary shaft seals: surface finish, hardness, and coating.
Surface Finish
For PTFE rotary shaft seals, surface finish is extremely important. To achieve the least possible friction with a PTFE seal, the mating surface needs a specific texture. The mating surface must be rough enough to abrade a microscopic amount of PTFE to form a transfer film during the break-in period. This transfer film achieves a PTFE-on-PTFE effect, resulting in extremely low friction.
If the surface finish is too smooth, on the order of <2µm Ra, the transfer film will not adhere. To make matters worse, the seal lip will hydroplane, experience stick-slip friction, and generate significant heat that can char the lip.The surface finish can be too rough, as well. If the surface is > 4µm Ra, the shaft will act like a file, abrading the seal lip faster than the transfer film can form. This damages the seal itself and causes leakage.And while Ra is key, Rs (Skewness) is also important. The goal is to achieve negative skew so the surface has plateaus and valleys rather than sharp peaks that can slice the seal.
In addition, if the shaft is finished using a standard turning process, it may look perfect, but result in mysterious leaks. During standard turning, microscopic helical grooves are left in the shaft material. The grooves are like the threads of a screw, and during rotation they can pump oil under the seal through this micropump effect. The industry standard for PTFE is a plunge-ground finish, which ensures that marks from turning and grinding are circumferential, eliminating the pumping effect.
Hardness
PTFE is a soft material that normally would not damage a metal surface, but virgin PTFE is rarely used for a rotary shaft seal. In such cases, PTFE is filled with glass fibers, bronze, carbon, or graphite — all abrasive fillers — to improve structural integrity and sealing performance. If the shaft is softer than these fillers, the seal will wear a groove into the shaft and leak. To prevent this, experts recommend a mating surface with a hardness of 55-65 HRC (Rockwell C).
Surface Coatings
Surface coatings on the mating surface are often used to achieve the required hardness or to repair a worn shaft, but this can lead to issues if not done correctly. PTFE is an excellent thermal insulator, and PTFE rotary shaft seals depend on the shaft to conduct away the heat generated by friction. Some ceramic coatings are also thermal insulators, and when used they can trap heat at the seal interface. This can lead to a rise in temperature that softens the PTFE and leads to seal failure.
For such reasons, many engineers will use hard chrome as the shaft coating because it is both hard and thermally conductive. Another option is DLC (Diamond-Like Carbon), which has sufficient hardness to prevent grooving and an extremely low coefficient of friction that significantly reduces heat buildup at the lip of the PTFE seal.
Conclusion
Because PTFE rotary shaft seals are fundamentally different from their elastomeric counterparts, they have different requirements for the mating surface. For a successful sealing solution, engineers must consider the surface finish, hardness, and coatings or run the risk of leaks. If you need a dynamic sealing solution, consider PTFE rotary shaft seals. Contact us today to learn more about your options and how Advanced EMC can support you design needs.
When to Use PTFE Rotary Shaft Seals: Key Benefits and Industry Applications. Rotary shaft seals are critical in a wide range of industries, particularly in environments that require reliable sealing solutions in harsh conditions or adherence to strict regulatory standards like the FDA’s. Among the materials used for rotary shaft seals, PTFE (Polytetrafluoroethylene) stands out for its exceptional properties. But the question is, when should PTFE be the material of choice for rotary shaft seals?
In this blog, we’ll dive into the industries that rely on PTFE rotary shaft seals, explore the specific scenarios where PTFE seals shine, and look at the unique characteristics of PTFE that make it an essential material for high-performance applications.
Industries That Rely on PTFE Rotary Shaft Seals
PTFE rotary shaft seals are used in a variety of demanding industries. These industries require seals that can withstand harsh environments, high temperatures, aggressive chemicals, and strict regulatory requirements. Here are some of the key sectors that depend on PTFE seals:
Oil and Gas: The extreme temperatures and pressures in oil and gas drilling and refining make PTFE seals a critical component.
Transportation (including motorsports): High-speed applications and exposure to harsh conditions like heat and friction make PTFE essential in vehicles, especially in motorsports.
Semiconductor: In semiconductor manufacturing, where cleanliness and chemical resistance are key, PTFE’s non-reactive nature is indispensable.
Aerospace and Defense: PTFE seals are used in applications that must withstand extreme temperatures and low outgassing for high-altitude or space environments.
Medical Devices: In the medical field, where both chemical resistance and FDA compliance are critical, PTFE rotary shaft seals are widely used.
Food and Dairy: PTFE is valued for its compliance with strict health and safety regulations in food processing and dairy systems.
Fluid and Air Handling: PTFE’s resistance to various chemicals and ability to handle a wide range of temperatures makes it ideal for valves and pumps.
Manufacturing: Across different sectors of manufacturing, PTFE seals are used where aggressive media and high-speed rotary applications are involved.
Renewable Energy: Wind turbines, solar power systems, and other renewable energy technologies utilize PTFE for its low maintenance and durability in varying conditions.
Chemical Processing: PTFE’s unmatched chemical compatibility makes it a staple in industries handling corrosive substances.
In these industries, some key characteristics stand out: exposure to corrosive media, extreme temperatures, the need for low friction, reliability in dynamic movements, and strict compliance with regulations. PTFE excels in each of these areas, making it the go-to material for many rotary shaft seal applications.
When Should PTFE Rotary Shaft Seals Be Used?
Certain conditions make PTFE the superior choice for rotary shaft seals. Let’s take a look at the specific circumstances under which you should consider using PTFE:
1. When There’s Aggressive Media Involved
In industries such as chemical processing, oil and gas, and even food and beverage, seals are often exposed to harsh, corrosive media. PTFE is considered the most chemically compatible material available for seal lips, making it a perfect fit for aggressive environments. While rare compounds like certain alkali metals or fluorinated substances can affect PTFE, it is generally resistant to chemicals such as:
PTFE’s ability to withstand these harsh chemicals without degrading makes it a standout choice for applications where chemical exposure is a daily occurrence.
2. When Low Friction or Dry Running Is Needed
PTFE is one of the most low-friction materials in existence, with a coefficient of friction as low as 0.04 for virgin PTFE, making it unmatched in reducing friction. PTFE is also self-lubricating, eliminating the need for external lubricants in many applications. In rotary shaft seal applications, this property is especially beneficial in situations involving:
High-speed applications
Dynamic movements that generate friction
Seals where breakout torque must be minimized
Moreover, the lack of stick-slip behavior in PTFE rotary shaft seals helps to significantly reduce breakout torque, improving the overall efficiency of systems that rely on these seals.
3. When Applications Involve High Speeds
In high-speed applications, PTFE rotary shaft seals truly excel. They can handle shaft surface speeds of up to 35 m/s—much faster than many other seal materials. PTFE’s low friction and heat-resistance properties also make it ideal for seals exposed to high-speed rotation, where the interaction between the rotating shaft and seal lip could otherwise generate excessive heat and cause premature seal failure.
In motorsports, aerospace, and transportation industries, where high rotational speeds are common, PTFE is often the material of choice for seals.
4. When Thermal Stability Is Critical
PTFE’s thermal stability is one of its defining features. It can function in extreme temperatures, making it suitable for both high-heat and cryogenic applications.
High temperatures: PTFE can withstand temperatures of up to 500°F, making it a popular choice in the oil and gas industry, where thermal extremes are frequent.
Cryogenic conditions: At the other end of the spectrum, PTFE can also handle temperatures as low as -459°F, making it a suitable material for cryogenic applications where most other materials would fail.
Moreover, PTFE seals have a low coefficient of thermal expansion, meaning they maintain their dimensional stability even when exposed to significant temperature fluctuations. This is particularly useful in environments where seals must be able to adapt to both rapid heating and cooling without losing their shape or effectiveness.
5. When FDA/USDA Compliance Is Necessary
In industries like food and dairy or medical devices, where strict regulatory standards apply, PTFE is highly valued for its compliance with standards such as:
PTFE also stands up well to the rigorous cleaning and sanitation processes required in these industries, including exposure to steam, hot water, and aggressive cleaning chemicals. In addition, PTFE is hydrophilic, which means it resists moisture and prevents the buildup of water around the seal during cleaning.
Grades of PTFE Used in Rotary Shaft Seals
Several grades of PTFE are used in rotary shaft seals, each offering specific benefits:
Virgin PTFE: Ideal for slow, low-duty applications.
Glass-Filled PTFE: Enhanced strength and wear resistance; should only be used with high-hardness shafts.
Glass MoS2-Filled PTFE: Increases wear resistance and strength without the abrasiveness of standard glass-filled PTFE.
MoS2-Filled PTFE: Extends seal life and wear resistance, best suited for higher-duty applications.
Carbon-Filled PTFE: Adds wear resistance without significantly increasing the coefficient of friction.
Carbon and MoS2-Filled PTFE: Improves wear resistance, performance at high temperatures, and dry-running capabilities.
These different grades of PTFE allow engineers and manufacturers to select the perfect material for their specific rotary shaft sealing requirements.
Other Benefits of PTFE Rotary Shaft Seals
There are several other benefits to using PTFE for rotary shaft seals:
Wider Temperature Range: PTFE performs well in a broader temperature range compared to elastomeric seals.
Longer Lifespan: PTFE seals generally have a longer operational lifespan, reducing the need for frequent maintenance and replacements.
Low Outgassing: In applications like aerospace, where outgassing is a concern, PTFE is valued for its minimal emissions.
Inert to Most Chemicals: PTFE’s chemical inertness makes it compatible with most lubricants and aggressive fluids, and it is also self-lubricating, which improves longevity and reduces maintenance.
Why PTFE Rotary Shaft Seals Matter
The versatility, durability, and unique properties of PTFE make it an ideal choice for rotary shaft seals in a wide range of demanding applications. Whether it’s resisting harsh chemicals in the oil and gas industry, ensuring compliance with food safety regulations, or operating in high-speed motorsports environments, PTFE is a material that excels where others might fail.
If you’re considering PTFE rotary shaft seals for your application, contact Advanced EMC Technologies today. Our team of experts is ready to help you find the perfect sealing solution for your needs.
