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ANATOMY OF A SHAFT SEAL
Understanding design variables can help you optimize performance.
by Rick Hudson
Since their development almost 80 years ago, shaft seals utilizing
an elastomeric lip have become integral to worldwide manufacturing. Also
known as oil seals and radial lip seals, shaft seals are widely used
in conjunction with rotating, reciprocating, and oscillating shafts to
contain fluids and to exclude contaminants. In some designs, shaft seals
may also contain pressure or separate fluids.
Primarily used in rotary applications, shaft seals offer lots of advantages. They’re economical, easy to install, and effective in a wide range of environments. Rarely does a day go by that I don’t learn about a new design or new shaft seal application.
But as popular as shaft seals are, they’re also much more complex in both form and function than O-rings or most other types of seals. Over the course of our next several issues, I’ll take a closer look at the many factors that influence the design of an effective shaft seal, beginning here with a review of the basic purpose and anatomy of the seal itself.
BLOCKING THE GAP Any
mechanical assembly containing fluids must be designed so that these substances
flow only where intended and do not leak into other parts of the assembly
(or out of the assembly entirely). Seals are incorporated into mechanical
designs to prevent such leakage at the points where different parts of an
assembly 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 this clearance gap so that nothing passes through it.
A shaft seal is a mechanical device specifically designed to block a given clearance gap and, in many instances, to also exclude contaminants, contain pressure, and/or separate fluids. A shaft seal is but one part of a three-part system. Part two is the shaft itself, which is in motion. This motion may be rotary (round and round), reciprocating (in and out), or oscillating (rotating back and forth). Part three is the housing within which the shaft is able to move thanks to the presence of the clearance gap. A bore within this housing holds the shaft seal, which in turn blocks the gap and may, depending on the needs of the application, also perform the other functions previously mentioned. Figure 1 shows a shaft seal installed into a housing bore and onto a shaft.
KEEPING IN TOUCH As shown in Figure 2, the cross-section of a typical shaft seal is made up of many variable features. The most important part of the seal is the elastomeric sealing lip. The lip length (also known as beam length) is the axial distance from the thinnest portion of the lip (the flex thickness) to the point at which the lip contacts the shaft. A short lip exerts more force on the shaft (with a corresponding increase in friction and wear), but a short lip also has better resistance to deformation caused by high pressure. A longer lip exerts less force on the shaft (reducing friction and wear). A longer lip is also more flexible and can thus more easily follow any shaft eccentricities, such as shaft-to-bore misalignment (STBM) or dynamic runout (DRO).
Long or short, a lip’s ability to maintain the desired amount of contact with the shaft (also known as interference) is often augmented through use of a garter spring. A garter spring is a helically coiled spring, typically made of carbon steel or stainless steel wire, formed into a ring. If present, the garter spring rests in a radiused opening molded into the sealing lip. The tension supplied by this spring is intended to compensate for any loss of sealing force in the elastomeric lip due to exposure to fluids and/or high temperatures.
The axial distance between the centerline of the garter spring and the contact point is known as the R value. A positive R value means the spring centerline is located toward the air side of the seal relative to the contact point, and this is desirable. A negative R value means the spring centerline is located toward the fluid side, which will result in immediate leakage.
Two other important lip-related variables are the angles that meet at the head of the lip (portion nearest the shaft) to form the contact point. The angle facing the fluid being sealed is known as the oil side (or scraper) angle. The angle facing away from the fluid being sealed is known as the air side (or barrel) angle. In order to provide both a proper point of contact and necessary reverse pumping action, the oil side angle must be greater (steeper) than the air side angle, but the specific ratio between these angles will always be dictated by the needs of an application. The diameter of this sealing lip (measured with the garter spring installed) is referred to as the seal’s lip diameter, or lip I.D.
In addition to the primary sealing lip, many designs also incorporate a smaller, secondary lip to exclude dust, dirt, and other contaminants. Unlike the primary lip, this secondary lip typically faces the application’s air side (since dirt and other unwanted matter may try to migrate in from outside the assembly). If present, a secondary lip generally originates away from the primary lip, at the opposite end of the elastomeric beam (in the area known as the heel, rather than the head). Depending on the needs of the application, a secondary lip can be oriented either radially (facing the shaft; known as a radial dirt lip) or axially (facing away from the shaft; an axial dirt lip).
PROTECTING AND SERVING In most shaft seals, the elastomeric portion is chemically bonded to a stamped metal case. Non-elastomeric members (made of materials such as PTFE, which is more difficult to bond) may be mechanically clamped to the case. Either way, this rigid case (typically made of carbon steel, or for especially corrosive environments, stainless steel) does two things for the seal. First, it provides stability, allowing the outside diameter (seal O.D.) to pressfit snugly into a housing bore. Some applications may call for a rubber-covered O.D. (see Figure 1) in which an elastomeric layer is applied to the outside of the case for increased sealability. Second, the case also provides protection, preventing damage to the lip during installation. In some instances, a double-layered case may be used for both added protection and added stability. Figure 3 shows a double case, double lip seal with a garter spring.
Again, each application dictates the design of the seal(s) to be used, so numerous variations are possible. Regardless of the application, however, most of the features outlined here are present in one form or another. In our next issue, I’ll be discussing the nitty-gritty of how a shaft seal actually works with an acknowledged expert, Dr. Les Horve. In the meantime, visit our web site at www.rlhudson.com to download your very own copy of our Shaft Seal Design Guide. Just click on “Knowledge Base” in the menu bar. And, as always, please don’t hesitate to call us at 1-800-722-6766 if we can assist you in selecting or designing the right shaft seal for your application. We’ll be happy to help.
