Rotary shaft seal applications, ranging from wind energy to automotive seals, require reliable performance in often challenging environments. To meet the challenges, filled-PTFE is often the material of choice for the seal lip. But which type of PTFE will work best for your application? In this week’s blog post, we will go over the various types of PTFE used in rotary shaft seals, their pros and cons, and what type of PTFE is best for you.
Pros and Cons of Virgin PTFE
Looking at the pros and cons of virgin PTFE reveals where there is a need for filler materials when used in rotary shaft seals.
Benefits of virgin PTFE:
- Extremely low friction
- Low breakout friction
- Generates very little torque
- Dry running and no stick-slip behavior
- FDA and USDA approved
- Excellent chemical compatibility
- Can handle pressures up to 500 psi
- Excellent performance in the presence of extreme temperatures
- Works well for large diameter seals
Virgin PTFE contains no additives or fillers and is ideal for food, dairy, and pharmaceutical applications as long the pressure is extremely low. It works extremely well in cryogenic environments but is otherwise limited to light-duty, slow rotary applications. It is recommended for use in applications that involve soft shaft surfaces.
There are also areas where virgin PTFE can be improved for use as a rotary shaft seal. It has very limited wear resistance, stiffness, and compressive strength. In addition, it does not last long when used in conjunction with hard surfaces. The answer to these limitations for PTFE rotary shaft seals is the addition of fillers.
Fillers Available for Use with PTFE
PTFE is available with different fillers that can enhance its natural properties. In the context of rotary shaft seals, these are the most commonly used fillers:
- MoS2-Filled PTFE
- Carbon-filled PTFE
- Carbon and Graphite-filled PTFE
- Carbon and MoS2-filled PTFE
- Glass-Filled PTFE
- Glass and MoS2-filled PTFE
- Polyimide-filled PTFE
Molybdenum Disulfide-Filled PTFE
MoS2-filled (Molybdenum Disulfide) is a mineral that is only added in small quantities and usually in conjunction with materials such as carbon or glass (although it can also work as a stand-alone filler).
MoS2 makes for a harder, slippery material. Because it acts as a lubricant, it can be used to increase the wear resistance, surface hardness, and compressive strength of PTFE without significantly increasing the coefficient of friction as much as, say, carbon or glass filler.
There are two different forms that carbon filler can take ( powder and fiber) and the carbon can be either natural or synthetic. Sometimes the term graphite is used synonymously with carbon, and while both are made from carbon, graphite is actually an allotrope of carbon in an extremely stable form. In addition, carbon comes in the form of short fibers or powder while graphite takes the form of flakes when used as a filler.
Because carbon serves as a natural lubricant (as does graphite), it increases the wear resistance of a PTFE rotary shaft seal without serious impact on the low coefficient of friction exhibited by PTFE. In addition, carbon is extremely neutral in a chemical sense and can handle extreme environments. As a filler, carbon also contributes to the compressive strength of PTFE, enhances its thermal conductivity, and aids in the dissipation of static electricity (because of its natural conductivity).
In short, carbon-filled PTFE rotary shaft seals work well in aggressive environments that may involve extreme temperatures, high pressures, corrosive chemicals, and exposure to steam. Because carbon is not nearly as abrasive as glass fibers, carbon-filled PTFE works on a wider range of shaft surfaces.
Carbon and Graphite-Filled PTFE
Usually provided in a mixture of 23% carbon and 2% graphite (by volume), this particular grade of PTFE is a good choice for general-purpose rotary shaft seal applications. The combination of carbon and graphite also makes it ideal for situations where extrusion and deformation are problematic because of the additional stiffness provided by both carbon and graphite. The addition of graphite also increases the lubricity of the carbon-filled PTFE and provides self-lubricating properties that may be lost due to the use of carbon, which allows it to work well on softer shaft surfaces.
Carbon and Molybdenum Disulfide-Filled PTFE
There are also PTFE grades that combine carbon and MoS2. The result is increased wear resistance and better high-temperature performance while still maintaining the capability of dry running.
Glass, usually in the form of glass fibers (i.e., fiberglass) and added at 25% (by volume), is very effective at enhancing both the wear resistance and strength of PTFE. It also provides additional stiffness, does not compromise the chemical neutrality of PTFE, is lightweight, and can contribute to the high-temperature performance of PTFE. Because of its abrasiveness, however, it should only be used on shafts with a high hardness value (surface hardness greater than 62 Rockwell C).
Note that the milled glass fibers are extremely small, with an average nominal diameter of 13µm and a length of about 0.8 mm. E glass, also known as electrical glass because of its insulating properties, is the only type of glass used for reinforcing PTFE and composite materials in general.
Glass and Molybdenum Disulfide-filled PTFE
To reduce the abrasiveness of glass without losing the benefit of the added strength, stiffness, and wear resistance, one option is to combine glass with MoS2. The typical percentages, by volume, are 15% glass and 5% MoS2. This particular grade works well in high-speed rotary applications, vacuum pressures, or when used with inert gases.
The final filler under discussion is Polyimide (PI), a synthetic resin known for extremely low friction (the lowest of all PTFE-fillers discussed) and non-abrasive behavior. It works well when used with very soft shaft materials such as aluminum or other plastics. PI-filled PTFE is also capable of dry running and works extremely well in start-stop applications.
PTFE is available with various fillers to enhance critical properties such as compressive strength, stiffness, and wear resistance. This has made it a common choice for many rotary shaft applications where it has been found to perform reliably.
If you are considering the use of a PTFE rotary shaft seal, contact the sealing group here at Advanced EMC. They will be able to help you select not just the correct type of seal but the ideal material for your design.