The design and specification of rotary shaft seals is challenging enough, but things get even more complicated for high-speed seals. High-speed rotary shaft seals pose their own set of wear and heat generation problems that can make it difficult to select an appropriate lip material, but PTFE is up to the challenge.
Rotary Shaft Seals in High-Speed Applications
In the context of rotary shaft seals, high speeds are often defined as those above 3,600 rpms. Such seals can be found in industries such as pulp and paper, wind energy, pumps, gearboxes, steel and aluminum processing, electric motors, medical devices, etc.
High-speed applications, such as those found in turbomachinery, can cause a seal to wear out faster and generate more heat because speed and friction do not get along well together. If the heat generated is sufficient, it can result in higher operating temperatures and changes to the geometry of the seal. And not all high-speed applications are compatible with lubricants, so in some cases, the seal may need to be capable of dry running. It is also key that these seals do not exhibit stick and slip behavior at startup.
Requirements for High-Speed Rotary Shaft Seals
PTFE Rotary Shaft Seals
High speed rotary shaft seal materials, in addition to the normal requirements for seals, must be …
- abrasion and wear-resistant (to reduce wear)
- Dimensionally stable (to prevent changes in geometry due to high temperatures)
- Thermally conductive (to dissipate heat generation)
- High operating temperature (to account for heat generated during use)
- Possess an extremely low coefficient of friction (to reduce heat generation and wear)
- Reduced stick-slip and breakout friction
- Self-lubricating (for when lubricants cannot be used)
While there are several options available for seals that meet these requirements, one in particular stands-out: PTFE, or polytetrafluoroethylene.
PTFE High-Speed Rotary Shaft Seals
PTFE exhibits several key qualities necessary for high-speed rotary shaft seals. It has good abrasion and wear-resistant properties, is dimensionally stable, and has good thermal conductivity. PTFE also has an operating temperature of up to 500°F and a melting point of almost 650°F. It also has the lowest coefficient of friction of any solid currently known to mankind, exhibits reduced stick-slip, has extremely low breakout friction, is self-lubricating, and can continuously operate as a dry-running material.
PTFE also comes in various grades beyond virgin PTFE. It is available fillers such as Molybdenum Disulfide (MoS2) for increasing wear resistance, carbon for increasing wear resistance while keeping friction low, glass for better hardness and wear resistance, or various combinations of these. Keep in mind that there are also FDA approved seals for use in connection with pharmaceuticals and medical applications as well as food and beverage production.
PTFE rotary shaft seals are available in hydrodynamic, plain and multi-lip configuration and for situations where the production volume is low, they can be constructed from machined shells so there are no tooling charges. At the same time, high production volumes can be manufactured from pressed shells to reduce unit costs.
Conclusion
For applications that demand reliable, long-lasting high-speed rotary shaft seals, PTFE is the engineer’s choice for reliable performance. It combines low friction, high operating temperatures, good wear properties, and dry running capabilities that can handle the rigors of high-speed applications.