In most shaft seals, the elastomeric portion is chemically bonded to a stamped metal case (also known as a shell). Non-elastomeric members (made of materials such as PTFE, which is more difficult to bond) may be mechanically clamped in place inside the case (such as through the use of metal spacers).

Either way, the case does two things for the seal. First, it provides stability, allowing the outside diameter (seal O.D.) to pressfit snugly into a housing bore. Second, the case also provides protection, preventing damage to the lip during installation. The total axial width of the case is the case width (also known as the seal width). Tolerances for seal width are shown in Table 24.

Depending on the needs of the application, a variety of different case configurations are possible. The most common and least expensive design is the L-cup case (see Figure 51). Applications requiring added seal strength may utilize an inner cup (see Figure 52) inserted into the outer case. This inner cup protects both the lip and the spring during handling and installation. Applications in which there is large clearance between the shaft and the bore often utilize a stepped case (see Figure 53). This type of design also offsets the lip if there are parts of the assembly that might otherwise interfere. A reverse channel case (see Figure 54) also helps bridge large shaft-to-bore gaps. Case strength is increased, and a pry-out flange can help with removal. Strength and ease of removal are also helped by a shotgun case (see Figure 55), though this design is costly to make and may trap drawing oils during the metal forming operation. These oils may bleed out and cause bond problems during molding.

This case is typically stamped from strip steel stock using a stamping press. The standard case material is mild carbon steel, an economically priced, general-purpose material. This material is typically designated using a four-digit number, such as 1005, that corresponds to a system developed by the Society of Automotive Engineers (SAE). This system, which mirrors the system of the American Iron and Steel Institute (AISI), assigns a four- or five-digit number to each type of steel. This number is based on the differing levels of carbon and other elements present in the steel. The first digit denotes the primary alloying element (such as a “1” for plain carbon). The second digit indicates the presence of other elements. The last two digits specify the amount of carbon in the steel (in hundredths of a percent). For example, a designation of 1005 indicates plain carbon steel (1) with no alloying elements (0) and a 0.05% carbon content (05). Most shaft seal cases are formed from steel with carbon content in the 0.05% to 0.20% range (SAE 1005 to 1020, AISI C1005 to C1020).

Once the case has been stamped, it is often coated with zinc phosphate to protect against corrosion and to provide an uneven surface to which an adhesive will adhere. This adhesive will facilitate bonding between the metal case and the elastomeric lip during the subsequent molding operation. Grit blasting is also used to prepare the surface for bonding; however, care must be taken because grit blasting can affect the dimensions of the metal case. Once applied, the adhesive is “set” by running the cases through an oven prior to sending them to the production line for molding.

In many applications, the outside of the case is exposed to the elements. Steel cases that have been zinc phosphated and coated with cement will resist these elements in most applications. In highly corrosive environments, a stainless steel case (SAE 30302 to 30304, AISI 302 to 304) may be needed. Shaft seals for use around food or water are also required by the Food and Drug Administration (FDA) to have stainless steel cases so as to prevent development of—and contamination by—rust. Stainless steel cases are more expensive than carbon steel cases. Sometimes the entire metal component is completely encased in rubber to prevent corrosion.

A properly designed seal featuring an undamaged metal case will not leak if installed into a steel housing bore meeting material and finish specifications. However, O.D. leakage can result from use of a seal with a damaged case or from installation into a bore with surfaces that don’t meet specifications. Axial scratches or corrosion on the bore can be particularly troublesome. Differential thermal expansion (due to unlike bore and case materials) can also cause a leak path to develop around the seal O.D. Depending on the specific application, it may be necessary to alter the seal O.D. in order to prevent leakage. For shaft seal O.D.s, there are three basic categories: metal O.D. seals, rubber O.D. seals, and seals in which the O.D. is a combination of metal and rubber.

Metal O.D. seals are economical and well suited for a variety of standard uses, including non-pressure fluid sealing and grease sealing. Metal O.D. seals have proven very effective when placed in steel and cast iron housings. Metal O.D. seals may be treated in various ways to further improve their performance. The entire metal case is usually coated with an adhesive used to bond the seal’s elastomeric member to the case. This coating makes the seal O.D. resistant to corrosion and also assists retention of the seal in the housing bore. Figure 56 shows an example of a metal O.D. seal with a coating of adhesive.

Other possibilities include spraying the O.D. with a polyurethane-based bore sealant to a thickness of .001" to .003". Bore sealants applied to metal O.D. seals are useful when there are (at most) minute scratches or marks on the bore surface. Deep scratches will necessitate use of a secondary adhesive such as Permatex®. The seal shown in Figure 57 is a good example of a metal O.D. seal with a coating of bore sealant.

A third option is to grind the metal case O.D. This process provides a straight wall with a very accurate outside dimension. The seal will have uniform retention strength after installation. If pressed into a bore with a good surface finish (80 to 100 Ra), a precision ground O.D. can be very effective. If the bore surface is rough, however, a secondary adhesive/sealant will be needed. Figure 58 shows a seal with a precision ground O.D.

The O.D. specifications for metal O.D. seals are shown in Table 25. The nominal pressfit is the difference between the seal O.D. nominal dimension and the bore I.D. nominal dimension. Maximum out-of-round (OOR) for the seal O.D. is the maximum deviation from a perfect circle.

Rubber O.D. seals are often used in applications where metal O.D.s will not work. For example, what if the housing in your application is aluminum rather than steel? A metal O.D. seal won’t be your best bet. The reason: differential thermal expansion of the metals in use. When heated, aluminum expands at roughly twice the rate of steel. Progressive expansion as a result of thermal cycling will decrease the interference (retention force) between a steel O.D. and an aluminum bore. Less retention force means the seal will be allowed to “walk” (move) within the housing. Leakage becomes a possibility. In such an instance, you may benefit from a rubberized coating on the seal O.D. The seal shown in Figure 59 is a good example of a rubber coated O.D. seal.

At .010" to .050" thick, this rubber coating encapsulates the seal’s metal case and ensures good contact between the O.D. and the bore. In actuality, a rubber O.D. allows for a higher pressfit than a metal O.D. does, and less force is exerted on the housing. In addition, the rubber coating is capable of maintaining a tighter, more “reactive” fit during thermal expansion (and later contraction) of the aluminum housing. Rubber O.D. seals are also good in corrosive environments; the rubber coating shields the metal case. Figure 60 shows the distribution pattern of the contact pressure between a rubber O.D. and a housing bore.

Though the rubber O.D.s shown in Figures 59 and 60 are straight, it is also possible to mold in small, round ribs along the seal O.D. These ribs can be advantageous because they provide high point-of-contact unit loading to increase sealability and retention. An example of a seal with a ribbed O.D. is shown in Figure 61. The distribution pattern of the contact pressure between a ribbed rubber O.D. and a housing bore is shown in Figure 62.

Though it offers many advantages, a rubber coated O.D. seal does have drawbacks. The rubber portion can be damaged during installation if proper lead-in chamfers are not built into the design. Care must also be taken due to a phenomenon known as springback. Springback is the tendency of a shaft seal with a rubber O.D. to unseat itself slightly following installation due to shearing stresses between the rubber and the housing bore. An exaggerated example of springback is shown in Figure 63.

Even if installation is perfect, excessive heat during service may cause the rubber coating to take a compression set, thus creating a leak path. In order to compensate for rubber’s higher coefficient of thermal expansion (compared to metal) and for the greatly reduced stiffness of the rubber O.D. (again, compared to metal), greater initial interference between the seal and the bore is required than when using metal O.D. seals.

Metal and rubber O.D. seals may be needed for truly tough applications. The metal provides retention while the rubber provides sealability. For example, the design shown in Figure 64 features a metal O.D. with a beaded rubber “heel.” The metal portion protects the rubber portion from installation damage. The metal also assists with accurate alignment in the bore and minimizes seal cocking and/or movement during use. The rubber element allows a tighter elastic fit into the bore than with metal alone.

Another possibility is the “nose” gasket shown in Figure 65. Nose gasket seals are often used when the bottom of the housing bore is not finished properly. The presence of the rubber “nose” helps prevent leakage. Be aware that half-rubber, half-metal O.D. seals are more difficult to manufacture and thus more expensive than the other seal O.D. treatments discussed in this section.

The O.D. specifications for rubber O.D. seals are shown in Table 26. As with Table 25, nominal pressfit is the difference between the seal O.D. nominal dimension and the bore I.D. nominal dimension. Maximum out-of-round (OOR) for the seal O.D. is the maximum deviation from a perfect circle.

There’s one other important issue related to the seal case to consider: the radial wall dimension (RWD). The RWD of a seal is the radial distance between the seal O.D. and the lip I.D. (contact point) as measured on a complete but uninstalled seal (see Figure 66). The extent to which this distance is not consistent is known as the radial wall variation (RWV). Excessive RWV will result in the contact force of the sealing lip on the shaft not being consistent, making leakage more likely. The maximum allowable RWV values are shown in Table 27.

Keep in mind that, though a single shaft seal design can be effective in a wide range of sizes, there does come a point at which the radial wall dimension becomes so small that the seal cannot be manufactured. Please consult R.L. Hudson & Company if you have any concerns about minimum radial wall dimensions.

As relates to the seal’s interior (oil side), there are two main choices: metal or rubber. A rubber coating on the oil side interior is often used if the seal is to face corrosive media that would damage a metal interior.

ANATOMY OF A SHAFT SEAL MAIN PAGE