Title: Smarter Seal Solutions Achieving Reduced Machinery Maintenance Requirements; Special Situations Require Special Seals
Author(s): Roger Sanderson, Magnetic Seal Corp
Abstract:
Conventional seals cannot accommodate the tolerances and extreme range of operating conditions critical systems must endure leading to unscheduled maintenance, shut down, and system failure.
The following is a brief overview of conventional seal design limitations and how specialty seal solutions overcome these limitations to improve critical system reliability and performance.
Lip Seals
Lip seals offer a simple and light weight seal often found in pumps and low-speed gearboxes; however, the direct seal-shaft contact causes rapid seal wear and shaft fretting aggravated by high shaft speed and shaft run out which preclude lip seals from many critical applications.
Labyrinth Seals
Two-part labyrinth and face seals employ a stator and rotor seal interface to minimize shaft wear and improve seal performance.
Labyrinth seals feature a non-contacting channel between the stator and rotor. This is a non-hermetic seal. During operation, heat and positive pressure breathes out through the channel. At rest the housing cools creating a negative pressure, which pulls contaminants in when operation resumes. Labyrinth seals can be found in diverse applications from propulsion shaft seals at the hull penetration in surface ships to some variants of gas turbine engines.
Mechanical Face Seals
In contrast, mechanical face seals provide a hermetic seal. During operation, lubricant forms a thin dense hydrodynamic film between seal faces while a mechanical load maintains the seal. This provides excellent leak and contamination protection; however, the mechanical load and seal interface present engineering challenges.
Most mechanical face seals use a solid wave or coil spring housed in a stamped cup. Due to a combination of spring and operating length tolerances, the mechanical spring produces a large variation of the face load. When compressed, spring unevenness can transfer through thin washers and distort the seal face. Furthermore, oil trapped in the spring cavity can turn into coke, compromising the spring effectiveness and retaining heat. High load, face unevenness, and coking increases power consumption and heat generation, accelerating seal failure.
One effort to mitigate heat generation is “lift-off” technology, which forms an air cushion between the faces, but this only works in limited environments. Particles in the air can contaminate the liftoff surface, and variations in air density – such as from altitude – compromise the liftoff capability resulting in face contact.
Finally, all spring seals have limitations beyond face friction: system vibration can resonate with and weaken the spring leading to seal face separation, the mechanical drive design transfers vibration to the carbon face which can cause chipping, and the higher axial length requirement can make lip seal replacement difficult.
Magnetic Seals
An alternative is to use magnetics to provide the mechanical load. This has several advantages including a lower and more even load on the seal face, vastly improved performance in high shaft speed and high vibration applications, and smaller axial length requirements for lip seal replacement.
The Magseal is a rotary shaft face seal featuring an alnico 5 permanent magnet stator and a rotor with a small section of carbon press fit into a precision machined steel housing. Unlike the highly variable spring load, the magnet places an even load across the entire seal face calibrated by a precise air gap and the magnetizing process of the Alnico 5. The seal faces are also lapped to industry leading flatness of 2 helium light bands. The low load and seal face flatness reduce friction, power consumption, and heat generation while the small section of carbon and thermal conductive housing accelerates heat dissipation. This increases seal life and makes the Magseal an ideal solution for high shaft speed applications. Furthermore, unlike a spring seal’s mechanical drive, which transmits vibration and system liberation, the Magseal’s O-ring drive dampens vibrations maintaining seal performance. The O-ring drive and low weight rotor also proved the only solution capable of withstanding extreme accelerations and decelerations in critical aerospace applications. Finally, the Magseal’s small axial length requirement allows for simple replacement of lip seals and can eliminate the costly need to replace a fretted shaft.
The Magseal improves critical system performance and mean time between failure. In a mission critical US Air Force program, the Magseal more than doubled service life relative to the conventional seal it replaced.
Unlike conventional seals, which are not designed for special conditions, Magseal leverages innovative design and materials to offer specialty seals optimized to the specific conditions, operating environments, and service life requirements of critical systems.