March 11th, 2021
Rotary lip seals are critical machine components that can dramatically extend the services lives of bearings and oscillating/rotating shafts by maintaining optimal lubricant levels and mitigating exposure to contaminants. They are known as lip seals because of their characteristic design featuring a flexible inner lip that conforms to contacting interfaces, creating a high-integrity gas-tight seal.
A Brief History of Shaft Seals
Sealing solutions for rotating shafts date back to pre-industrial times when rags coated in grease were tied to the ends of cart wheels. This concept was revolutionized with automation when self-contained bores were added to wheel hubs to retain greased/oiled packings. Higher shaft speeds and operating temperatures drove the need for novel lubricants (pitch, wax, etc.), which required different seal designs, eventually leading to the familiar rotary lip seal that is commonly used today.
Materials Used in Rotary Lip Seals
Early lip seals were comprised of leather washers with bevelled edges and robust metal casings, but these were unable to operate at the high temperatures required of modern applications. Rubber was a suitable alternative for a brief period, but advanced thermoplastics like polytetrafluoroethylene (PTFE) quickly became the go-to material for rotary lip seals, especially for small diameter bearings.
Much like the innovative pre-war, metal-encased rubber shaft seals, modern PTFE rotary lip seals often feature supporting ring materials made of aluminium alloys, low-carbon steels, or stainless steels. Selecting rotary lip seal materials requires an in-depth understanding of typical use conditions, including: bore material; internal pressure; lubricant composition and volume; rotation speed; temperature; and so on.
How Rotary Lip Seals Work
Rotary lip seals are either springless or spring-loaded. Each configuration works by being interposed in the gap between a shaft and bearing, and retaining a thin layer of lubricant between the two. They are different from other sealing solutions such as spring-energized seals or elastomer lip seals. When compared to elastomer lip seals, rotary lip seals use a far wider lip to shaft contact pattern. The design addresses wear rate with the function of lowering unit loading (also known as PV).
Read More: What are Spring-Energized Seals?
Thermoplastic shaft seals can be designed with various contacting patterns to maximise the seal integrity, and a growing choice of PTFE derivatives are available to reduce the degradation induced by friction or high temperatures. Over time, friction between the seal and shaft can create hotspots that accelerate the breakdown of grease/oil, contributing to a loss of lubricity and potential component failure. Mitigating friction and heat around bearings is subsequently a key engineering goal. Fortunately, PTFE-based rotary lip seals can run at high speeds and temperatures with limited lubrication, enabling extended bearing performance far beyond that achievable with any other shaft seal material.
Rotary Lip Seals from Saint-Gobain Seals
At Saint-Gobain Seals, there is a team of experts in the fields of design engineering and manufacturing with specialties in both metal fabrication and various polymeric molding processes. Their rotary lip seals are utilized for bearing applications ranging from aircraft alternators to pharmaceutical mixers. If you would like to learn more, why not contact a member of the team directly today?