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.

Best Materials for Spring Energized Seals | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Seal Gland Design for Spring-Energized PTFE Seals

Seal gland design is an important aspect of effective spring-energized PTFE seals and is also the source of most leaks. Such seals can only perform as well as the seal gland supporting them, and even a small geometry error can lead to issues with extrusion, rapid wear, low contact stress, and spiral leakage.

This blog post reviews what makes PTFE spring-energized seals different from other sealing solutions, and then discusses key aspects of gland design, including the anatomy of a basic spring-energized seal and design aspects of geometry and surface finish that have the greatest impact on seal reliability. 

Anatomy of a Spring-Energized Seal

A short review of spring-energized seals is in order. 

The gland is the machined groove in the metal hardware where the seal sits. In the diagram, you can see the E-gap, or extrusion gap. This is a small clearing between the moving and stationary metal parts of the seal. This gap must be kept tight to prevent the seal from extruding or flowing into it under pressure.

The seal jacket is the U-shaped outer body, which, in PTFE spring-energized seals, is manufactured from PTFE. It provides the main sealing interface against the hardware, and the lips of the jack are the points that actually touch the mating surface.

The spring energizer is contained within the jacket, and it provides constant pressure against the jacket lips and provides pressure outward against the gland walls. This pressure, or sealing force, is maintained by the spring-energizer within the jacket and exists even when there is low to no system pressure. As system pressure increases, the jacket enters the U-cup and further augments the spring’s force and the sealing force.

Practical Seal Gland Design Geometry

PTFE is not flexible like elastomers, and that must be taken into account when specifying the gland geometry. First, proper gland width provides the seal with enough clearance to float, and the gland depth strictly controls the interference, and both are critical for providing sufficient axial and radial freedom instead of just simply squeezing the seal into place. 

When the design creates over-compression, the seal will not be able to flex as it should. This leads to increased friction, accelerated jacket wear, and a potential locked condition where the spring cannot compensate for shaft misalignment. Under-compression brings its own set of problems because it will not be able to fully utilize the spring energy. Leakage at low pressures and a delayed response during pressure spikes severely impact the performance of the seal. 

The goal with the design then becomes the achievement of a uniform load distribution that can conform to surface irregularities and maintain the integrity of the seal.

Seal Gland Design: Corner Radii and Lead-In Geometry

PTFE is softer than steel. If you try to shove this plastic seal into a metal bore that has a sharp 90° corner, that corner acts like a knife. It will slice a layer off your seal before the machine even turns on. This can lead to micro-damage that will later present itself as problematic leaks.

Sharp edges are one of the main causes of damage during PTFE seal installation, and this differs significantly from elastomeric seals. PTFE is stiffer, which makes it more likely to suffer a cut from insertion into a metal bore at a 90° angle. The solution is a shallow lead-in chamfer or on-ramp that is around 15° to 20°, and longer than the seal itself. The length is necessary to ensure the seal is fully compressed before it enters the main bore. In addition, the ramp corners must be rounded, and no sharp edges are allowed.

Seal Gland Design: Surface Finish and Hardware Interaction

Because these seals are constantly rubbing against metal, the surface finish of the metal is of utmost importance. If the surface is too rough, it will abrade the seal and lead directly to leakage. On the other hand, if the surface is too smooth (e.g., mirror polish), it will not be able to hold, and the seal will wipe the sealing surface dry. In addition, because PTFE is self-lubricating and forms a transfer film on the sealing surface, the surface cannot be so smooth that the transfer film does not stick. 2 RMS to 16 RMS, depending on what your application is. 

For gases and liquids at cryogenic temperatures, you want a smoother finish with an RMS between 2 and 4. For gases at non-cryogenic temperatures, the recommended surface finish is between 6 and 12 RMS. For liquids, a surface finish between 8 and 16 RMS is sufficient.

In addition, it is important to specify a plunge-ground finish (0° lead) to prevent spiral tool marks from acting as a screw pump that leaks fluid regardless of seal tightness. 

Seal Gland Design: Squeeze, Clearance, and Extrusion Gap Control

Under extremely high pressures, the behavior of PTFE is significantly different from that of elastomers. At extremely high pressures, PTFE is more likely to flow like a liquid to an area of lower pressure. There will always be a small gap between your piston and cylinder wall to prevent them from grinding against each other. This small gap is known as the extrusion gap, or E-gap. If the gap is too large, the pressure will push the heel of the seal into the gap, shredding the seal. This is where careful specifications come in: when specifying the diameter, include a small enough tolerance to ensure that the gap remains small (e.g., smaller than a human hair) even when the parts move.

Conclusion

Seal gland design for PTFE spring-energized seals is a little different from elastomeric seals and involves more focus on aspects such as geometry, lead-in geometry, surface finish, and extrusion gap control. 

If you need help with a PTFE spring-energized seal, our engineers at Advanced EMC are here and ready to help. Contact us today!