Shaft seals are used on rotating, reciprocating, and oscillating shafts to contain oil and grease and exclude contaminants. Known as oil or radial lip seals, shaft seals can also contain pressure or separate fluids. They have several key strengths: They are economical, easy to install, and effective in many environments.
A shaft seal is but one part of a three-part system. Another part is the moving shaft itself. (Its motion can be round-and-round rotation, in-and-out reciprocation, or back-and-forth rotating oscillation.) The third part of a sealing system is the housing into which the seal is installed. Fig. 1 shows a shaft seal installed in a housing bore.
Any mechanical assembly containing fluids must be designed so that these substances flow only where intended and do not leak out of the assembly. Seals incorporated into mechanical designs prevent such leakage at the points where different assembly parts meet; these meeting points are known as mating surfaces, and the space between them is called a clearance gap. The purpose of a seal is to block the clearance gap so that nothing passes through it.
Shaft seals are common in gearbox assemblies, hydraulic pumps and motors, and reciprocating applications, as wiper seals.
Materials are paramount. To illustrate: Automotive air conditioning systems used to rely on R-12, a chlorofluorocarbon (CFC) refrigerant. But because CFCs contribute to ozone depletion, a push was made in the 1990s to replace them with hydrofluorocarbon (HFC) refrigerants. R-12 was replaced by what is known as R-134a. The latter, however, also necessitated the use of different lubrication. This new lubrication, in combination with higher operating temperatures, forced seal designers to seek more resistant materials; now, hydrogenated nitrile (HNBR) is used effectively for air conditioning seals.
Evolution of shaft seals
Technological advances have spurred the development of increasingly sophisticated radial lip seals over the past century. In actuality, the first “shaft seals” (such as those found on the axles of low-speed frontier wagons) were nothing more than leather strips attempting (typically with very limited success) to contain the animal fat used as lubrication.
Later, motorized vehicles replaced wagons, and rope packings made of flax, cotton, and hemp replaced leather strips. Though still relatively crude, such packings worked because lubricants tended to be very viscous, operating speeds were still low, and temperatures never got high enough to degrade the lubricants or seal materials.
In the 1920s, thinner, more environmentally unfriendly lubricants became common, and sealing them adequately became more difficult. Rope packings were superseded by assembled leather seals — chemically treated to improve oil resistance, then clamped into a metallic case to facilitate installation and removal. The metal case allowed for a pressfit seal to prevent bore leakage, and the leather lip rode a region of the shaft ground to a prescribed roughness.
As machinery, vehicles, and road surfaces were refined, shaft speeds and application temperatures increased and new oils were developed to withstand these higher temperatures — but caused swelling and degradation of leather sealing lips. These difficulties were overcome in the 1940s with the development of oil-resistant polymers. Assembled synthetic rubber seals featuring lips made of nitrile (NBR) became the norm.
By the 1950s, technology allowed for the chemical bonding of rubber to metals. This made possible a seal in which the rubber lip was chemically bonded to the case (rather than clamped). Seals of the 1960s began to feature lips made of materials other than nitrile, including silicone and polyacrylate materials for bonded seals. Polytetrafluoroethylene (PTFE) has great chemical and temperature resistance in combination with good low frictional properties. As a result, it was used to replace leather and NBR materials in assembled lip seals. (Methods of bonding PTFE to rubber or metal did not yet exist.) Fluoroelastomer also grew in use in the 1970s. Though all of these alternative materials were useful, they were also more expensive than nitrile, so seal designers sought ways to minimize material usage and reduce costs. This resulted in the production of seals with reduced bonding areas.
Seal designers also began to look beyond the seal for ways to further improve performance and to extend reliability. They turned their attention to the sealing surface itself, and by the 1980s, seals that incorporated running surfaces became common. These cassette seals eliminate the preparation of running surfaces on the shaft.
Now, improved cements allow composite seals of PTFE bonded to rubber. Seal designers combine them with other components from the sealing area, such as filters, reinforcing inserts, and excluders.
Shaft seals are commonly used in gearboxes, which convert high speed input from an electric motor into low speeds that drive various machines or conveyors. A cut-away view of a gearbox assembly is shown in Fig. 2.
Typical gearbox applications have two shaft seal styles: One for the input seal and one for the output seal or seals. The input seal is a higher-speed (usually 1,750 rpm) seal. Because both external and internal contamination can be a problem in gearbox assemblies, new-generation gearbox seals incorporate additional contamination exclusion, both internally and externally. The input seal is often a TC or TCW design made with fluoroelastomer. The input seal shown in Fig. 3 combines a TCW design with an oil-side contamination exclusion lip to prevent dirt and metal particles in the oil from reaching the primary sealing lip.
In addition to the input seal, there are also one (or more) output shaft locations. The output seal usually operates at lower speeds than the input seal. For this reason, different cross-sections — designs that would generate too much friction as input seals — can be used as output seals.
The cross-section of a typical output seal is shown in Fig. 4. Note that it's designed to exclude heavy contamination.
Fluid power seals
Shaft seals used as hydraulic pump and motor seals can encounter both high speeds (8,000 rpm) and pressures (1,500 psi) so special designs are required. One option is a crimped PTFE seal for use in high-pressure hydraulic motors, with two lips held in place within the case by metal spacers. A rubber seal with a support ring incorporated into the cross-section can also be used for high-pressure applications.
Hydraulic and pneumatic applications often make use of special WP shaft seal designs as wiper seals. These designs utilize thickened sealing lips (without garter springs) intended for dust and dirt wiping (or scraping) in reciprocating hydraulic and pneumatic cylinder applications. A wide variety of lip configurations are possible, with ODs of metal or rubber. Wipers are typically made from 90-durometer nitrile or polyurethane.
Washing machine seals
Sealing a washing machine tub is a demanding application. Water must be contained, but the water, bleaches, detergents, and other washing products can easily corrode the metallic portions of the seal — the case and the spring.
What allows a tub seal to function effectively has everything to do with lip design. A comparison between the contact point of a normal seal and that of a tub seal is shown in Figs. 5 a and b. Note that the contact angles and the R value on the sprung lip of the tub seal are reversed. This is what allows a tub seal to prevent water and suds from getting into the machine's grease-filled bearings. Figs. 6 a and b show two different tub seal designs. Even as it keeps water out, the sprung lip also helps keep in grease. The reversed contact angles and R value also place the garter spring on the airside of the application, so water cannot reach it. This helps protect the garter spring from corrosion.
The material in this article is excerpted from the Shaft Seal Design and Materials Guide, published by R.L. Hudson and Co. The book can be read in its entirety, and hard copies purchased, at rlhudson.com.
Topics of discussion:
Gearbox seals: Speedand motion
Special fluid power considerations