A Close Look At The Mighty PTFE Lip Seal
A Sealing “Superhero” in Rotary Applications
By Jim Goodman (August 2021)
Sealing against dynamic surfaces is a challenge that has been around for decades, if not centuries. It’s a challenge that has only grown in complexity with the early introduction and advancement of airplanes and complex machinery. It’s one that thermoplastics like polytetrafluoroethylene (PTFE) rotary lip seals (also known as rotary shaft seals) are increasingly relied upon to address today.
Read on to discover what makes this PTFE rotary lip seal essential and unique and its evolution through the years – we are taking a closer look at the life of the mighty PTFE rotary lip seal.
Origin of PTFE Rotary Lip Seals: The Rise of a Sealing “Superhero”
Every “superhero” has an origin story, and PTFE rotary lip seals are no different. Back in the old days, pioneers used rope, rawhide, or leather straps with fat on wheel axles as some of the first seals or sealing elements, which were prone to leakage and needed regular maintenance. In fact, many elastomeric seal companies that are operating today were once tanneries due to these humble beginnings.
In the late 1920s, the first rotary lip seal was created from leather and a metal fastener case. In the late 1940s, synthetic rubber started to replace leather. Forty years later, several manufacturers began to assess the entire sealing system, often building sealing surfaces into seal assembly and/or incorporating multiple lips with vertical and horizontal contact points.
One of these manufacturers was the Fluorocarbon Company, who in 1982 purchased SealComp, a small family-owned lip seal business in Michigan and relocated it to a South Carolina plant that produced metallic seals for the nuclear and petrochemical markets. The newly acquired lip seal business focused on high pressure hydraulic pumps and motors, military alternators, and other commercial products such as aftermarket diesel truck crankshafts seals and thermostats. The Fluorocarbon Company changed its name to Furon in the mid 1990s, and in 2001, acquired Furon (adding to its already robust spring-energized seals business that had its own founding in 1955).
In 1995, Omniseal Solutions™ added an elastomer band to the lip seal outer diameter to eliminate the metal-to-metal press fit and ensure tight sealing between the seal and the customer's housing gland. Later, additional features were added for seal removal and positive stops to position the seal and prevent incorrect installation.
The Unique Anatomy of PTFE Lip Seals: Their “Super Powers” & Strengths
What makes PTFE rotary lip seals unique? How are they different from elastomer rubber lip seals? When comparing elastomer rubber lip seals to Omniseal® PTFE lip seal solutions, you will find many similarities but also significant differences. Both are similar in their overall construction as they use a metal casing for structure, a press fit into a stationary housing gland, a wear lip material to rub on the rotating shaft, and take up similar amount of space.
Elastomer lip seals are the most common shaft seal product on the market, directly molded to a metal casing providing the needed rigidity. Most elastomer rubber lip seals use an extension spring to provide the loading mechanism for tight sealing. The spring is typically located directly above the seal to shaft interface contact point. This provides the needed loads to pinch off the oil film.
Omniseal® PTFE lip seals in most cases do not use extension springs for sealing. Instead they respond on the loads applied to the stretching of the sealing lip and the bend radius created by the metal casings. PTFE lip seals use a much wider lip to shaft contact pattern as compared to elastomer lip seals and use a lighter unit loading but wider footprint. The reasoning behind this design is to reduce the wear rate by lowering the unit loading also known as PV.
PTFE lip seals are specifically used for sealing rotating shafts with a focus on high speed and are an excellent alternative to elastomer rubber lip seals if the conditions are challenging and exceed their capabilities. Essentially, they were designed to bridge the gap between conventional elastomer lip seals and mechanical carbon face seals. Since they can run at higher pressures and velocities compared to most elastomer lip seals, they become an excellent alternative. They perform well in hostile environments with extreme temperatures, aggressive media, high surface speeds, high pressure or lack of lubrication. An example of their high-performance “super power” is with industrial air compressors where seals are configured to provide in excess of 40,000 hours of maintenance free service.
A common misconception about PTFE lip seals pertains to the fabrication process. Elastomer rubber lip seals mold rubber directly to a metal casing. The metal casing provides the needed rigidity while the elastomer does the working portion of the seal. PTFE lips seals cannot be molded directly to metal casings. PTFE materials never transition to a liquid state or a state that allows the material to flow. Instead, PTFE lip seals are fabricated by machining the sealing members then assembled into a metal casing followed by mechanical clamping.
To select the precision sealing solution for your rotary application, you should consider such factors as shaft speed, surface velocity, operating temperatures, media to be sealed, and system pressures. There are many other operating conditions that could also contribute to your decision but the primary conditions are noted above.
PTFE Lip Seals To Your Rescue: Going Against “Archenemies” & Challenges
With great power comes great responsibility. Over time, Saint-Gobain Seals’ focus has shifted to higher demanding applications for their PTFE lip seals. A major benefit of the lip seals is their ability to address challenging environments in rotating applications in industrial, automotive, and aerospace applications.
PTFE lip seals can run at higher pressures and velocities on rotating shafts over elastomer lip seals. Additional benefits of these seals from Saint-Gobain Seals include:
Tight sealing, even under high pressure in excess of 35 BAR
Ability to run at temperatures well above and below elastomer rubber lip seals (typical temperature ranges from -53°C to 232°C)
Elastomer coatings on the seal’s outer diameter for easy installation without damaging mating hardware
Custom designs and wide range of sizes and materials
Inert to most chemicals
Withstands high speed in excess of 35 m/s
Low friction and ability to address rotating equipment and vibration for longer life
Compatible with most lubricants and able to run in dry or abrasive media
PTFE Lip Seals Unite: OmniSeal® & DynaLip® Sealing Solutions
Two common types of lip seals from Omniseal Solutions™ include Omniseal® PTFE metal-cased rotary lip seals and DynaLip® polymer seals. These two types of seals can be designed to be interchangeable. The major difference is in the seal construction. Metal-cased seals use sheet metal to form seal casings. The sealing lips are then assembled and then the seal is mechanically clamped.
Created in the early 1970s, Omniseal® rotary lip seals can withstand hostile environments, with temperature ranges from -53°C to 232°C, and demanding conditions in chemical environments, dry and abrasive media. They are used in dynamic rotary sealing applications such as turbine engines, pumps, compressors, robotics, diesel engines, pharmaceutical processing, food processing, blowers, mixers, manufacturing, and more.
Created about a decade after Omniseal® rotary lip seals, DynaLip® seals were born out of necessity when Omniseal Solutions™ was working on a military application for blending and mixing highly explosive materials. Metal-cased lip seals would most certainly have disastrous effects if they came in contact with rotating shafts mixing explosives; therefore, our design engineers came up with a lip seal without metal but still had the same key benefits.
With DynaLip® seals, they eliminated the need for metal components, with the entire seal instead machined from the same polymer material. In most cases, an elastomer O-ring is used between the seal outer diameter and the mating housing bore. The O-ring provides tight static sealing and anti-rotation. Omniseal® lip seals, conversely, may be composed of three or more materials enclosed in a metal casing.
Today many different versions of DynaLip® seals have grown from the original design. These seals are also ideal for field installation (since they do not require special tools to install) and applications that require seal removal for cleaning. Because of their simpler design, they are often more cost-effective.
How are Omniseal® PTFE lip seals, DynaLip® polymer seals and other lip seals going beyond the boundaries of possible?
When You Need Speed
PTFE lip seals can offer extraordinary sealing functionality and low friction in dry or abrasive media. As a result, they often address challenging applications where speed is a must.
In industrial – A good example of where a PTFE lip seal replaces both elastomer and carbon face seals is the air compressor market. Omniseal Solutions™ started working with most major air compressor companies in the mid 1980s, replacing rubber lip and carbon face seals prone to leakage. Original designs were based on conventional high pressure lip seals. Over time the demands increased to zero leakage and extended seal life. New technologies were developed to more than triple the seal life while maintaining tight leakage control. As a result, our PTFE lip seals are looked to as the industry standard with the ability to offer over 40,000 hours of maintenance-free service.
In screw compressors worldwide, they offer tight leakage control and the ability to run at 1,000 to 6,000 RPM with many types of lubricants and over extended periods (15,000 hours) to reduce warranty claims. Saint-Gobain Seals produces shaft seals ranging from 0.500 inch to 6.000 inch (13mm to 150mm) in diameter for this industry.
Another example of seal customization in this industry is in mixers. Our customers in this space require seals to handle shaft deflection and wobble of up to 0.300 inches, (7.62mm) – a massive amount of dynamic shaft runout. To handle this, plus the operating rotational speed, Omniseal Solutions™ offers a patented floating lip seal design.
In freon recovery pumps, Omniseal® lip seals are easy to install, address strict EPA leakage requirements and are compatible with oil and refrigerants, in a small space for the life of the pump.
Additionally, our lip seals address dynamic sealing environments and extreme speeds, pressures, and temperatures in vacuum pumps, blowers, chemical pumps, encoders and alternators, drilling and tapping spindles, hydraulic motors and pumps, air conditioning recovery pumps, and more.
They also are used on equipment requiring FDA cleared materials, with hygienic motors seals in pharmaceutical mixers, pharmaceutical pill compactors, food processing equipment, food saws and mixing equipment. These demanding applications require very low seal frictional drag to keep temperatures cool. In addition to being FDA compliant, seals must be free of cavities to trap materials being sealed and be compatible with acids and bases and cleaners, as well as handle high pressure wash-down and pass IP69K testing.
In aviation – Omniseal Solutions™ lip seals are used in auxiliary power units (APUs), turbine engines, starters, alternators/generators, fuel pumps, Ram Air Turbines (RATs), and flap actuators, one of the biggest markets for lip seals. Did you know that the APU was deployed on US Airways Flight 1549 (Miracle on the Hudson), providing electrical power and helping to land the airplane safely? Our lip and spring-energized seals were installed in this core system. This system is considered critical to flight and must work 100% of the time when deployed.
There are many reasons why aerospace manufacturers depend on these lip seals. Custom-designed Omniseal® lip seals offer a tighter seal and improved performance over elastomer seals and require less space than mechanical carbon face seals in turbine shafts and external gearboxes. They address temperatures ranging from -65°F to 350°F (-53°C to 177°C) and pressures up to 25 PSI (0 to 1.7 BAR) with typical surface velocity of 2,000 to 4,000 ft./min. (10 to 20 m/s). Some of our solutions in this space run at speeds over 20,000 feet per minute (102 meters per second).
Aircraft engine seals are also one of their biggest markets where they provide external gearbox seals for many of the aircraft engine builders. Our lip seals are also used in geared turbofan jet engines. These engines feature a gear system that separates the engine fan from the low pressure compressor and turbine, allowing each module to operate at their optimum speeds. As such, they can offer greater efficiency. With a typical airliner burning about a half-gallon of fuel per mile, these engines are expected to save an average of $1.7 million per airliner per year in operating costs.
Not only are PTFE lip seals supporting in the commercial arena but also in military applications for the Department of Defense, which include fighter jets, aircraft carriers and helicopters. The lip seals are used on various military aircraft in lift fans for vertical lift, helicopter gearbox engine seals (their spring-energized seals are also used in the rotorhead), wing flap and slats actuator seals and in critical equipment in arresting systems to catch the aircraft as they land on the deck. These application are critical and must perform as intended. Imagine if any of this equipment were to fail!
And The PTFE Lip Seal Story Continues …
While these are some of the main places that PTFE lip seals are used, they are also relied upon in demanding applications in life sciences with piston cup seals for oxygen concentrators to help people breathe better and be more mobile, as just one example. These piston cup seals use unique spin forming technologies to minimize seal frictional drag extending the time the unit can be used without re-charging. Custom blended materials help to maximize performance.
Our lip seals are also making an impact in rescue applications such as being used to propel fluids in pump systems as fire fighters spray foam retardant. Most elastomer rubber lip seals would not survive the chemicals used in these systems.
Whatever your needs, Omniseal Solutions™ draws on extensive experience in designing quality, innovative sealing solutions to address specific applications. Through a co-engineering approach, they engage with their customers’ engineers at the outset of a project to develop and meet specific requirements such as optimal sealing jacket material formulations, sealing jacket design configurations, and optimal spring energizer configurations to arrive at the best result.
They are always looking for better ways to design and build their seal products. They thoroughly test products to find the upper limits in speed, pressure, and temperature and continually look to go beyond these perceived limits. Today, their seals on the main turbine shaft of turbine engines run at speeds and temperature previously considered impossible years ago. They also focus on designing in features that offer multiple benefits such as shipping mandrels to help keep sealing lips formed and/or customized mandrel design to be used as a multi-purpose installation tool.
Discover how their design and application engineers can help you with critical PTFE lip seals or other sealing solutions such as their spring-energized seals. Download the PTFE lip seal handbook or make contact one of our lip seal experts today!