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HOW A SHAFT SEAL WORKS

Developing microasperities is the key to success.

by Rick Hudson with Les Horve

How a shaft seal works A shaft seal is a mechanical device that blocks the clearance gap between a bore and a moving shaft. A shaft seal typically retains fluid (such as lubricant) or pressure and may also exclude contaminants. In our previous issue, I examined the parts of a shaft seal. This time around I’d like to explore how those parts actually function to form an effective and reliable barrier against leakage. Since the mechanics of shaft sealing are complex, I’ve enlisted the help of a true expert: Dr. Les Horve. Among his many accomplishments, Les is the author of Shaft Seals for Dynamic Applications [1].

SEALING SURFACES

Rick Hudson: So, Les, let’s start with basic functionality. Once installed, a typical shaft seal is defined by two sealing surfaces. The first is a tight static seal formed between the seal’s outside diameter—O.D. —and the housing bore. The result of designed-in interference, correct?

Les Horve: That’s right. The seal O.D. is designed to be slightly larger than the bore, typically .004” to .008” larger for metal O.D. seals and .006” to .012” larger for rubber-covered O.D.s. The exact amount of interference depends on the bore size. The interference ensures a tight pressfit that leaves no room for leakage around the O.D. The tightness of this fit also helps keep the seal from becoming dislodged while in use.

Rick: So the seal won’t “back out” or “walk.” Now, the second sealing surface is the one most people really think about, the one between the elastomeric lip and the rotating, reciprocating, or oscillating shaft.

Les: This sealing surface is also caused by an interference fit, here between the lip and the shaft.

Rick: Using a seal whose inner lip diameter is slightly smaller than the shaft diameter ensures that the sealing lip will be expanded—stretched outward—by the shaft upon installation.

Les: Exactly. That outward stretching is known as hoop force. The interaction of that hoop force with the lip’s inherent beam force generates a total radial force—also known as load—between the lip and the shaft.

Rick: Maintaining the right amount of load is critical. That’s where a garter spring can help.

Les: Right. A garter spring designed into the lip can help maintain the desired amount of load even if the lip material undergoes heat- or chemical-induced changes…

Rick: Such as swell…

Les: Such as swell. Having a garter spring in place can also help if the seal is subjected to a high amount of wear while in use. Keep in mind that the optimal radial force is the smallest amount that maintains an effective seal. Any force greater than this contributes unnecessarily to friction.

Rick: Which in turn increases heat and wear.

Les: Precisely.

Rick: Let’s talk more specifically about pressure distribution on the shaft. There’s a steeper angle on the oil side of the lip, and this steeper angle results in a greater pressure gradient on the oil side than on the air side, correct?

Les: Correct. Tests have shown that this angular disparity has a lot to do with the effectiveness of a seal. Here’s how it all seems to work: The shaft surface is plunge ground to a surface finish of 10 to 20 microinches Ra, as recommended by the SAE. The ground shaft surface will abrade away a very thin layer of rubber from the seal tip that is contacting the rotating shaft. If the shaft finish is too smooth, then lip abrasion will not occur. If the shaft finish is too rough, then the seal lip will experience excessive wear.

Rick: But some lip abrasion is desirable, and it’s possible because of rubber compounding.

Les: Right. The seal lip material must be properly formulated to ensure the formation of microscopic pores—known as microasperities—on the seal lip’s wear path. If the material has not been formulated properly, microasperities will not form and the seal wear path will appear relatively smooth when viewed with a high-powered microscope. Once formed, these microasperities are advantageous because they serve as reservoirs to hold lubrication that prevents further lip wear.

Rick: But it’s not just the lip that undergoes a change, is it? The surface of the shaft underneath the lip also changes.

Les: You bet. Even as microasperities are being formed on the lip’s wear path, the plunge ground surface on the shaft that is under the seal lip is worn smooth, creating a smooth path or wear band around the shaft in the circumferential direction. When the shaft rotates, the contact point of the lip is sheared in the circumferential direction.

Rick: And this shearing pulls the microasperities on the lip, right?

Les: Right. The microasperities are pulled so that they are directionally oriented at an angle to the shaft, and they are elongated, thus creating “faux” helices. Because the oil side angle of the seal lip is larger than the air side angle, the faux helices on the oil side of the contact band are shorter with a larger helix angle and a larger pressure gradient than the faux helices on the air side of the contact band. Because of these geometrical features, the pumping activity of the faux helices on the air side is greater than the pumping activity of the faux helices on the oil side of the contact band.

Rick: And the net result is an in-pumping effect that prevents oil leakage from the oil reservoir, or sump.

Les: Exactly. Of course, microasperities are only created and provide a pumping action when the shaft rotates, so all this applies to rotating shafts only. It’s also important to note that pump rate increases as shaft speed increases, and this improves seal reliability.

Rick: What you’ve just described involves development of “faux” helices, but pumping action can also be enhanced through the addition of artificial pumping aids molded onto the air side of the seal lip to create a hydrodynamic seal.

Les: Helical ribs are often molded on the air side of a seal, but they can be used only if the shaft rotates in one direction, thus creating a uni-directional hydrodynamic seal. If the shaft can rotate in both directions, then special pads or triangles are molded on the air side to provide a bi-rotational hydrodynamic seal.

Rick: But enhanced or not, the pumping action of most shaft seals is very directional; in other words, back toward the seal’s fluid side. That’s why it’s imperative that shaft seals be installed in the proper direction.

Les: Right. Installing a seal backward, such that the air side angle is facing the fluid, will result in immediate leakage due to the pumping of fluid out of the sump.

MAINTAINING BOUNDARIES

Rick: Let’s talk a bit about the air-fluid boundary beneath the sealing lip.

Les: As a result of the immense surface tension present between the air, the fluid, and the shaft, a curved meniscus develops at the meeting point between air and fluid, on the air side of the sealing lip. Studies have been conducted to attempt to numerically model the placement of this meniscus in various operating conditions [2]. These studies indicate that the precise location of the meniscus can be a function of shaft speed.

Rick: So the meniscus can shift inward—be ingested—if speeds increase.

Les: Seemingly. Hydrodynamic theory predicts that the underlip film thickness increases even as in-pumping continues.

How a shaft seal works exaggerated view

RL Hudson can assist you in selecting or designing the right shaft seal for your application.

[1] L. Horve, Shaft Seals for Dynamic Applications, Marcel Dekker, Inc., New York, 1996.

[2] R.F. Salant, Modelling rotary lip seals, Wear