Solutions > Archives > Tech Sessions > THE SHAFT SEAL OD: THINK "OPTIMAL DESIGN," NOT "OBSTINATE DILEMMA."
THE SHAFT SEAL OD: THINK "OPTIMAL DESIGN," NOT "OBSTINATE DILEMMA."
by Rick Hudson
I recently received a phone call about a particularly troublesome
shaft seal application. Repeated failure of the seal to stay in place in
the housing bore had become (in the words of a weary colleague) an "obstinate
dilemma." My
advice was simple. "When it comes to selecting a shaft seal O.D.," I said, "think ‘optimal
design.’" Here’s what I meant:
At its core, a shaft seal is but one component in a three-part system comprised of a shaft, a housing, and the seal. When considering the multitude of different standard and non-standard seal designs, keep in mind that shaft seals are defined by their two sealing points. In addition to the dynamic seal formed between the inner seal lip and the moving shaft, there is also a static seal formed between the seal’s outside diameter (O.D.) and the housing bore into which it fits. In order for the seal to perform successfully, both the dynamic and static elements must function properly. In your efforts to achieve the perfect lip-shaft interface, do not ignore the equally important interface between the housing bore and the seal O.D.
For example, let’s say you’re trying to find just the right shaft seal O.D. to operate in a steel housing. You have three standard choices: a metal O.D., a rubber coated O.D., or a combination of metal and rubber. Let’s take a closer look at each type of design.
METAL O.D. Seals featuring a metal O.D. are economical and well suited for a variety of standard uses, including non-pressure fluid sealing and severe grease sealing. Steel O.D. seals have proven very effective when placed in steel and cast iron housings. The "SBY" design shown in Figure 1 is a good example of a metal O.D. seal.
Metal O.D. seals may be treated in
various ways to further improve their performance. The first of these treatments
- I’ll call it Option "P" -
involves painting the O.D. with a polyurethane-based bore sealant to a thickness
of .001" to .003". Painted metal O.D.s 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®.
A second treatment - Option "C" - involves coating the metal O.D. with the
same adhesive used to bond the seal’s inner elastomer to the outer
metal case. This coating makes the seal resistant to corrosion and assists
its retention in the housing bore.
A third treatment - Option "G" - is a ground metal O.D. The chief advantage here is that the seal has high retention strength. If pressed into a bore with a good surface finish (i.e. of no more than 80 RMS), a precision ground O.D. can be very effective. If the bore surface is rough, however, a secondary adhesive / sealant will be needed.
RUBBER COATED O.D. But 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 Option "R" - a rubberized coating on the seal O.D. 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. The rubber coating is capable of a tight, "reactive" fit during thermal expansion (and later contraction) of the aluminum housing. The fact that rubber and aluminum have comparable expansion rates also helps maintain the O.D. sealing line, thus preventing leakage. The standard "SC" design shown in Figure 2 is a good example of a rubber coated O.D. seal.
Though it offers many advantages, a rubber coated O.D. seal is not without its drawbacks. The rubber portion can be damaged during installation if the housing surface is not sufficiently smooth (it should be no rougher than 125 RMS) or if proper lead-in chamfers are not built into the design. Excessive heat 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. Despite this increased interference, the combination of system vibration and thermal cycling can still sometimes cause a rubber coated O.D. seal to walk out of the housing.
METAL AND RUBBER O.D. For truly troublesome applications, the answer may be a shaft seal O.D. that incorporates both metal and rubber. For example, the standard "SBR" design shown in Figure 3 features a predominantly metal O.D. with a rubber "nose." This type of design only works if the gland into which it is installed has a counterbore (ledge) against which to seal.
The non-standard "TBW" design shown in Figure 4 also features both metal and rubber on the O.D. In this case, 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. Because the metal supports the rubber portion, compression set is less problematic than with purely rubber coated O.D. seals.
RL Hudson is pleased to offer shaft seals in a wide variety of standard and non-standard designs, including metal O.D.s, rubber coated O.D.s, and metal-rubber combinations. If your application requires a shaft seal, please call us at 1-800-722-6766.
