by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Surface Finish, Hardness, and Coatings: The “Quiet” Variables That Make PTFE Rotary Seals Live or Die

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.

by Brandon Pascual Brandon Pascual No Comments

In-Depth Look: How PTFE Rotary Shaft Seals Revolutionize Maintenance and Machinery Longevity

How PTFE Rotary Shaft Seals Revolutionize Maintenance and Machinery Longevity

In high-performance industries, the demands placed on machinery are relentless. Whether it’s operating under intense pressure, extreme temperatures, or aggressive chemicals, rotary shaft seals are expected to perform without failure. However, not all seals are created equal, and PTFE rotary shaft seals are increasingly recognized for their exceptional ability to reduce maintenance and extend machinery life.

More than just another material choice, PTFE (Polytetrafluoroethylene) offers unique properties that make it indispensable in industrial applications where reliability, durability, and efficiency are critical. In this comprehensive article, we’ll delve into how PTFE rotary shaft seals reduce the frequency of maintenance, extend equipment longevity, and improve overall operational performance in high-stress environments.


The Science Behind PTFE Rotary Shaft Seals

Understanding the value of PTFE rotary shaft seals starts with appreciating the material’s intrinsic properties. PTFE, a fluoropolymer, has become synonymous with resilience in the face of mechanical stress, chemical attack, and thermal extremes. These qualities come from its molecular structure, where carbon and fluorine bonds are incredibly strong. This bond creates a material that is chemically inert, thermally stable, and capable of enduring dynamic movement.

Let’s break down the key properties that make PTFE rotary shaft seals so effective in reducing maintenance:

  • Extreme Chemical Resistance: PTFE’s non-reactive nature allows it to resist almost all corrosive chemicals, with few exceptions. This property is pivotal in applications like chemical processing, where traditional seals would degrade over time, leading to leaks or system failure.
  • Self-Lubrication: PTFE’s ultra-low coefficient of friction means it’s naturally lubricating, reducing wear on both the seal and the rotary shaft itself. This dramatically lowers the maintenance burden, as there’s less need for external lubricants and fewer parts to replace due to friction damage.
  • Thermal Stability: With a melting point of 620°F (327°C) and functionality down to -459°F (-273°C), PTFE performs in some of the most extreme temperature environments found in industries like aerospace, oil and gas, and cryogenic applications. Thermal expansion is also minimal, ensuring dimensional stability across varying temperature ranges, which is crucial for maintaining seal integrity in fluctuating conditions.
  • Low Outgassing: Especially important in high-vacuum environments like aerospace, PTFE emits very low levels of gas, ensuring that it does not compromise the vacuum integrity of the system. This property is vital for long-term performance in precision engineering sectors, where even minor contaminations can cause significant issues.


How PTFE Rotary Shaft Seals Reduce Maintenance Costs

In most industrial settings, maintenance is a critical but costly factor in operational efficiency. Machinery downtime results in lost productivity, and frequent maintenance increases operational costs. PTFE rotary shaft seals, due to their self-lubricating and wear-resistant nature, drastically cut down on both planned and unplanned maintenance events. Here’s how:

1. Enhanced Resistance to Wear and Tear

One of the main reasons seals fail is due to the mechanical wear and tear caused by continuous operation. Traditional seals, particularly those made from elastomers or rubbers, degrade over time because of friction between the shaft and seal lip. This wear is accelerated when the machinery operates in high-speed or high-pressure conditions.

In contrast, PTFE’s low coefficient of friction significantly reduces the contact stress between the rotating shaft and the seal lip, thus minimizing wear. For example, virgin PTFE has a coefficient of friction as low as 0.04, making it one of the slickest materials available. This means that PTFE rotary shaft seals can run for significantly longer periods without the degradation commonly seen with other materials, leading to fewer replacement cycles.

2. Self-Lubricating, Low Maintenance

Unlike traditional materials that rely on external lubrication to maintain performance, PTFE rotary shaft seals are self-lubricating. This property makes them ideal for applications where regular maintenance isn’t feasible, such as in aerospace or subsea oil drilling. The self-lubricating nature of PTFE reduces the need for regular application of lubricants, significantly lowering maintenance costs and extending the operational life of both the seal and the shaft it interacts with.

3. Handling Harsh Chemical Environments

In industries like chemical processing, semiconductor manufacturing, and food and beverage, seals are regularly exposed to harsh chemicals. Most traditional seals would degrade rapidly under these conditions, leading to increased downtime and replacement costs. However, PTFE’s ability to resist corrosive media is unmatched, allowing it to function effectively even in the presence of aggressive chemicals such as:

  • Hydrochloric acid
  • Sulfuric acid
  • Organic solvents
  • Hydrocarbon-based fuels

Because of this, PTFE rotary shaft seals dramatically lower the need for frequent maintenance in chemical environments, as they maintain their structural integrity far longer than other materials.


Enhancing Machine Longevity with PTFE Seals

Beyond minimizing maintenance, PTFE rotary shaft seals extend the overall operational lifespan of the machinery they’re used in. This impact is not just about reducing downtime but about ensuring that machinery operates at peak efficiency for longer periods. Let’s look at some specific ways PTFE seals enhance machine longevity:

1. Protecting the Shaft from Damage

In many sealing applications, it’s not just the seals that wear out—the rotating shafts they interact with are also prone to damage. Many materials used in traditional seals, particularly those filled with abrasive additives like glass fibers, can cause significant shaft wear over time. Repairing or replacing a worn-out shaft is far more expensive and disruptive than replacing a seal.

PTFE, being naturally non-abrasive, protects the shaft from this wear. Even when filled with reinforcements such as carbon or MoS2, PTFE remains less abrasive than traditional materials, helping to prolong the life of the shaft. This is a particularly critical advantage in high-value equipment such as aerospace engines, turbines, or high-speed manufacturing systems, where shaft integrity is essential to overall system performance.

2. Handling High Speeds Without Overheating

In applications involving high-speed rotation, such as motorsports, aerospace, or manufacturing equipment, heat generation is a significant concern. Traditional seals often fail because of the heat produced by friction at high rotational speeds. As the seals heat up, their material properties change, causing them to lose their sealing effectiveness.

PTFE rotary shaft seals, however, are designed to handle high shaft speeds of up to 35 m/s. PTFE’s low friction properties mean less heat is generated during operation, allowing the seal to maintain its performance even at high speeds. This capability is essential in preventing premature seal failure and extending the life of the machinery it protects.

3. Enduring Extreme Temperatures

Both high-temperature and cryogenic conditions can wreak havoc on seals. In extreme heat, many materials soften, expand, or degrade, losing their ability to maintain a tight seal. At extremely low temperatures, materials can become brittle, crack, or lose elasticity, causing leaks or component failures.

PTFE’s broad temperature range—from 500°F (260°C) down to -459°F (-273°C)—makes it the material of choice for rotary shaft seals in temperature-sensitive environments. Whether in oil and gas refineries where equipment must function in both hot and cold conditions, or in aerospace applications where rapid temperature changes are common, PTFE remains stable. This thermal stability not only extends the life of the seal but also ensures the machinery operates reliably for longer periods.


Real-World Examples of PTFE in Action

Oil and Gas Industry

In oil and gas applications, such as downhole drilling, equipment is subjected to a combination of high temperatures, pressures, and aggressive chemicals. PTFE rotary shaft seals are ideal for this environment, where traditional elastomer seals would break down due to the presence of hydrocarbons and drilling fluids. The ability of PTFE seals to resist these harsh media extends the operational life of drilling equipment, reduces downtime, and cuts the frequency of expensive repairs.

Aerospace Industry

In the aerospace sector, components are expected to operate flawlessly under extreme temperatures and dynamic pressures, especially in high-altitude or space missions. PTFE’s low outgassing properties and ability to handle extreme conditions make it the go-to material for seals used in landing gear systems, turbine engines, and rocket propulsion. These seals reduce the need for maintenance during missions, making aerospace operations more efficient and reliable.

Food and Beverage Processing

Seals used in food processing equipment must not only be FDA-compliant but also resistant to the aggressive cleaning processes used to maintain hygiene. PTFE rotary shaft seals meet both of these requirements. In high-speed mixers or packaging machines, PTFE seals extend the lifespan of the equipment by withstanding harsh cleaning agents and high-pressure washdowns, reducing the frequency of shutdowns for maintenance.


Maximizing Performance and Efficiency with PTFE Rotary Shaft Seals

PTFE rotary shaft seals go beyond reducing maintenance—they fundamentally improve the longevity, efficiency, and reliability of industrial equipment. By providing exceptional chemical resistance, self-lubrication, and thermal stability, PTFE seals allow machinery to operate under extreme conditions for extended periods without failure.

Whether you’re looking to cut maintenance costs, reduce downtime, or extend the operational life of your equipment, PTFE rotary shaft seals provide a proven, high-performance solution.

Contact Advanced EMC Technologies today to find out how PTFE seals can optimize the performance of your machinery and help you achieve greater reliability in your operations.

Links

The Revolutionary Impact of PTFE Rotary Shaft Seals on Industrial Applications
Maximizing the Performance of Your PTFE Rotary Shaft Seals
Why PTFE Rotary Shaft Seals Are Essential for High-Performance Machinery