Optimizing Radial Shaft Seal Performance Enhances Machine Reliability

EP Editorial Staff | July 1, 2006

Careful evaluation of your application is part of a holistic approach to seal specification. This is no time to take shortcuts.

Radial shaft seals retain lubricants in and exclude contaminants from bearing housings, ensuring the reliable operation of mechanical power transmission devices, such as gearboxes, pumps and motors. More than a quarter of mechanical failures in these kinds of machinery are attributable to bearing malfunctions, 80% of which are caused by contamination of the bearing housing. A methodical approach to seal selection and application assessment will eliminate the risk of contamination and optimize the service life of machinery.

Design selection based on application
In selecting a seal, its general design must be suitable for its application and operating conditions. There are several application criteria to consider, including surface speed, misalignment, temperature, pressure and fluid compatibility.

Surface speed. . .
Radial seals offer three types of lip design—plain, wave or helix—to accommodate different surface speed ranges. The operator or maintenance technician should consult the manufacturer’s data to determine which design is suitable for the application’s surface speed. If the surface speed exceeds the limit for which the seal is designed, the lip will fail to control fluid.

Most rubber radial seals range from 3,500 to 5,000 feet per minute (fpm), or 17.78 to 25.40 meters per second (m/s). In general, as surface speed increases, other capabilities, such as run-out allowance, decrease. Additionally, friction from oil shear and other factors increase underlip temperature as speed increases.An increase in shaft speed of 800 fpm (4.06 m/s), for example, can increase underlip temperature by 25 F (40 C). For an optimum balance of capability and seal life, always evaluate speed in context to the other operating conditions.

0706_equipmentreliability_img2Misalignment. . .
Radial seal designs tolerate a limited range of shaft deviation from the true center, or shaft to bore misalignment (STBM). Also, radial seal designs can tolerate some dynamic run-out (DRO), expressed as total indicated reading (TIR), the degree to which the shaft diameter does not rotate in a true center. The manufacturer determines limits on both STBM and DRO. Beyond these limits, however, excessive STBM misalignment can crush the seal lip and cause rapid wear or create a gap through which fluids may leak. High DRO typically causes a uniform, but wide, wear band, ultimately resulting in premature failure.

Temperature. . .
Each seal material has an optimum temperature range. Exceeding the range creates thermal stress that may harden or degrade the seal material. Heat aging is a more common cause of ultimate failure than wear among seals composed of nitrile (NBR) rubber. Typical evidence of this kind of failure includes radial cracks. Ambient heat, surface speed, sump temperature and lubricant viscosity all contribute to the overall thermal load for the application.

With NBR, an increase of underlip temperature of 25 F (40 C) can reduce seal life by half. Seals made of fluoropolymers or polytetrafluoroethylene (PTFE) offer a higher thermal limit and are used for many high-temperature applications. But even these materials have practical limits. (And these limits must be observed.)

Pressure. . .
Normal system conditions or faults within a system, such as a plugged vent, may cause pressure loading. This will mechanically distort the lip profile, accelerating seal wear and failure. In general, pressure capability and surface speed are inversely proportional. The design of standard radial seals typically permits about seven pounds per square inch (psi) or .05 mega Pascal (MPa). Specially engineered profiles and materials offer solutions to compensate for pressure and, in some cases, achieve a pressure X velocity (PV) of 300,000. For contacting radial lip seals, though, such extreme values are only possible at lower surface speeds.

Fluid compatibility. . .
If the seal lip material is not compatible with the fluid being retained or excluded, the fluid may chemically attack the seal. Swelling or softening is often an indication of media incompatibility. Additives used to improve fluids and lubricating oils may be incompatible with seal materials. Disulfide-type additives, for example, reduce wear on mechanical components, but also may advance the cure state of the seal element, resulting in accelerated hardening.

Nitrile rubber compounds. . . NBR, a synthetic co-polymer of acrylonitrile (ACN) and butadiene, is the most common elastomer compound. It is economical and has an elastic recovery, resiliency and pliability similar to that of natural rubber, but is more oil and abrasion resistant and offers longer service life and higher reliability. Its temperature range is minus 65 F to 250 F (minus 54 C to 121 C).Use with a polar solvent such as acetone, however, will result in catastrophic swell, which will soften and eventually destroy the seal. NBR also does not withstand weather aging very well; extended ozone or ultraviolet light exposure results in surface cracking and hardening.

Hydrogenated nitrile (HNBR) offers increased tensile strength and heat, abrasion, hotoil, ozone,weather and ultraviolet resistance. Its temperature range of minus 40 F to 300 F (minus 40 C to 149 C) is wider than that of NBR. Carboxylated nitrile (XNBR) is another NBR compound that offers greater abrasion and wear resistance than standard NBR, but not higher thermal capability. Both HNBR and XNBR cost more than standard NBR.

Fluoroelastomer compounds. . .
Fluoroelastomer compounds include fluorocarbon (FKM) rubber. FKMs are premium elastomers that offer excellent wear properties, an extended service life and resistance to degradation from chemically aggressive lubricants, corrosive media and weathering effects. They have a temperature range of minus 40 F to above 400 F (minus 40 C to above 200 C).

PTFEs (polytetrafluoroethylene) are in a class of chemically inert plastics. Resistant to a wide range of aggressive media and high levels of contamination, they also offer a very extended service life and tolerate more physical stress, including PV factors in excess of 250,000. Temperature range of PTFEs is minus 400 F to 500 F (minus 240 C to 260 C).Due to their relative stiffness, though, seal lips constructed of PTFEs require extra care in assembly. Furthermore, to deliver their full potential, they need a high-quality countersurface and high shaft hardness values, depending on application conditions. They also typically are made to order and considerably more costly than nitrile seals.

Dimensional data
After identifying the correct seal design for the application, the hardware dimensions must be confirmed. Seals usually are press-fitted into the bore, so the outside diameter of the seal must be larger than the bore diameter. The seal’s dimensional system, however,must be a correct match for the mating hardware.

0706_equipmentreliability_img4English standard vs.metric. . .
In selecting the correct dimensions, use the same system of measurement (English standard or metric) as used for the hardware. Typically, equipment manufactured in the United States still uses English standard (inch) dimensional data (though metric measurements are being introduced), while equipment manufactured elsewhere predominantly uses metric dimensional data.

Ranges for English standard (RMA) bore tolerances include both plus and minus values, but metric standard (ISO, DIN, JIS) tolerances typically have only a plus value, with the minus side being zero. The difference in tolerance ranges can create an improper interference (or press) fit if a metric size is selected as a substitute for an English-standard-sized seal (or English standard for metric). Being
close enough for a specific size may not hold
true in another installation.

The reason is that a metric seal, for example, is not designed for a minus value in the bore, and may work free of the housing during the operation of the equipment or be damaged during installation. This is a potential problem, especially if the housing dimension is undersized and the seal’s outer diameter is at the high end of its tolerance, which will result in excessive interference stress. Shaft tolerances generally are not as critical, but the correct measurement system should be used for optimum seal performance and service life.

The designer or maintenance technician should seek the guidance of the seal manufacturer in selecting the proper English-standard or metric shaft and bore dimensions as compliance with established tolerances for radial seal dimensions is the responsibility of the manufacturer.

Surface finish. . .
Shaft surfaces that appear smooth to the eye are actually textured with peaks and valleys, and an out-of-specification shaft is second only to heat damage as the most common cause of leakage. If a shaft surface is too smooth, the absence of asperities will fail to maintain a lubricant film (typically 0.00001 inch, or 0.25 micron, thick) and the underlip temperature will increase. If the surface is too rough, however, high peaks will project through the lubricating film and abrade the lip.

The seal manufacturer should provide grinding specifications based on industry standards, usually RMA or DIN. A good target is a shaft roughness value of eight to 17 μin Ra (.20 to .43 μm). Electronic tracing instruments can accurately assess surface finishes, including other key roughness parameters. Gauges also measure approximate Ra values.

Even if the roughness is correct, a shaft can have directional lead, which is a spiral pattern created by transverse movement of the cutting tool or grinding wheel during the initial preparation of the surface.An inward lead might be beneficial, but an outward pattern may cause more oil to auger under the lip than the seal’s pumping action can control.

Plunge grinding is often the recommended method for achieving lead-free shafts. The RMA lead standard is less than 0 plus or minus 0.05 degree. If carefully done, a simple string and weight test will confirm the presence of shaft lead. Along with roughness, shaft leads that exceed recommended limits frequently cause lubricant leakage. Housing bore roughness is less critical as 100 μin Ra (2.5 μm) or smoother is acceptable.Here, lead is not critical.

Hardness. . .
Opinions vary concerning the value of shaft hardness. On one hand, shafts with low hardness are cheaper to produce and, under relatively clean conditions, shaft hardness itself does not automatically result in better seal function or life. Still, seals are vulnerable to handling damage and possible wear. Generally, a shaft hardness value of Rockwell C scale (HRC) 30 or higher eliminates the risk of seal failure. Even though particle contamination often is harder than most steels, some seal designers recommend a Rockwell hardness in the contact zone of HRC 45, and 60 in abrasive or high-speed (2362 FPM or 12 M/S) environments.

0706_equipmentreliability_img5Installation and assembly
Both the shaft and bore should have lead-in chamfers of 15 to 30 degrees or a smooth radius. Square corners often cause a rolled lip or bent seal case. All contact surfaces should be free of burrs and nicks that could cut lip elements or score seal cases.

Seal failure often is the result of improper installation. The lubricant should be the same for the seal as it is for the machinery in which the seal is installed. The seal also must be installed square to the bore. When installing the seal with an I.D. lip, direct all force to the outer diameter of the seal case only. Use only an arbor press and a tool specifically designed for seal installation.

A hand tool, such as a hammer, is appropriate only if buffered with a block of wood; hammer strikes directly on the seal case may distort the seal profile and a strike near the inner diameter of the metal case may displace the rubber element.

Additional sealing options
Besides conventional elastomer radial shaft seals, there are specialty and alternative designs to meet the severe demands of many industrial applications or improve the performance of a radial seal.

V-ring seals. . .
Basic V-ring seals are constructed entirely of rubber materials, with nitrile as the standard and fluoroelastomers available in many sizes. The seal mounts directly to the shaft by hand and is pushed axially against a counterface, housing, bearing race or similar surface. Axially contacting V-ring seals function like slingers and exclude particle and fluid contaminants. Offering very high surface speed capabilities, V-ring seals can operate dry or with minimal lubrication. Minimal friction and heat accumulation result in an extended seal life.

V-ring seals are comprised of a body, conical self-adjusting lip and a hinge. The elastic body fits to the rotating shaft, creating a static seal along the shaft plane. The hinge enables the sealing lip to apply very light face contact pressure against the counterface and compensate for some angular and axial movement. Minimal counterface or shaft preparation is necessary and simple turned surfaces usually are sufficient.

The elasticity of its rubber material enables the V-ring seal to stretch to 21/2 times its molded diameter and can be mounted easily without disassembling the shaft, including over flanges, steps on the shaft and other assemblies.Metalclad versions also are available with a metal shell press fit onto the shaft, providing physical protection for the rubber lip element.Applications for these seals include conveyor rollers, transport equipment, rolling mills, agricultural machinery, paper mills, grinding equipment and appliances.

Bearing isolators. . .
Bearing isolators are seals that use a labyrinth internal structure, instead of contact lips, to collect and eject contaminants, such as fluid spray and particles, and prevent their entry into the mechanism. The non-contacting rotor and stator sections are constructed of PTFE. O-rings, used to secure the stator in the bore and drive the rotor, usually are molded of FKM, providing overall chemical and temperature resistance.

Even though they have limited oil retention capability, bearing isolators feature outstanding protection of machine components while their service life rivals that of bearings. Applications include pump power frames, electric motors, fans, blowers, pillow block bearings, conveyors, rollers, turbines, centrifuges, gearboxes and many other kinds of plant equipment with rotating parts.

Selecting radial shaft seals through a methodical consideration of the system requirements, dimensions and operating conditions will ensure that the chosen seals will perform for their intended service life, reducing the frequency of maintenance procedures and the risk of machine failure. In turn, unplanned downtime will be minimized and productivity maximized.

A precise examination of the application parameters is part of a holistic approach to seal specification that takes the seal’s practical and technical limits into consideration.When failure does occur, a systematic approach to a diagnostic analysis will deliver an accurate and timely solution. MT

Glenn Gabryel is a product engineer at SKF Sealing Solutions.He has 30 years in the sealing industry, primarily working with products for industrial applications in plant operations and heavy machinery. E-mail Glenn.E.Gabryel@skf.com




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