But did you know
that when it snows
My eyes become large
And the light that you shine
can't be seen?

("Kiss From a Rose," by Seal, 1994)

 

The above photo depicts a Bering Seal ... a type of mammal that lives upon the vanishing sea ice in the Bering Strait.

But this informative PTOA Segment #184 is all about Bearing Seals!

PTOA Readers and Students will learn:

• How contaminants invade the Bearing Box Housing of Rotating Equipment.
• How Bearing Seals protect Bearings from the contaminants.
• The form and function of Bearing Seals.
• Lubrication methods used for Bearings.

 

The practical goal of this PTOA Segment #184 is for future PTOA Process Operators to understand the form, function, and jargon related to Bearing Seals so s/he will be somewhat literate when the opportunity arises to view the Bearing Arrangement of the Rotating Equipment that the Process Operator is responsible for.

 

HOW CONTAMINANTS INVADE THE BEARING BOX/HOUSING

PTOA Segment #183 included a pie chart which indicated that a significant percentage of Bearing Failures were attributable to solid particle contamination and liquid moisture contamination. Just super dinky amounts of particulates and water vapor cause major damage to Anti-Friction Bearings (aka Roller-Element Bearings)!

The term "particulate contaminant" includes dust and dirt and metal shavings ... any dinky solid. These particulates cause Abrasive Wear.

"Liquid moisture contamination" is just a phrase that means "drops of water." The drops of water contaminate the Lubrication Oil and start Corrosive Wear.

So how does the water vapor and particulates penetrate the hard shell of the Bearing Box Housing to start contaminating the Lube Oil and Bearings? 

Behold a typical multi stage Centrifugal Pump in the nearby photo and single stage Centrifugal Pump in another nearby photo.

The multi-stage pump is located outside; this pump is exposed to all the weather conditions that Mother Nature can throw at it. Whatever is airborne will impact this Pump.

The single stage Centrifugal Pump is more protected because it is housed in a building.

Thus, the single stage Centrifugal Pump will not be directly impacted by the weather, but will still be impacted by whatever stuff is airborne within the building.

Because ... guess what?

IT'S ALIVE!

Both of these Pumps ... and all Rotating Equipment ... may not have a heartbeat but nevertheless "breathe" the surrounding atmosphere!

 

The "Breathing" of Rotating Equipment

PTOA Readers or Students who have been reading the PTOA Segments in the intended sequential order expertly understand how heat transfer causes the PV Temperature to change.

Once the Rotating Equipment ... like a Centrifugal Pump ... is placed into service, all the hardware components on the rotor start rotating and this movement causes the PV Temperature of the Lubrication Oil and Bearing Box Housing to increase.

This increase in PV Temperature is because the molecules of Lubrication Oil have become excited and start banging into each other...

thus expanding the volume of the liquid.

 

The Lubrication Oil would even spill out of the Bearing Box Housing were it not for Bearing Seals. And it is very important that the lubricant stay in Bearing Box Housing so that it can keep on doing the important jobs of lubrication.

Eventually the Rotating Equipment will cool down again.

The cooling may be caused by a decrease in the surrounding ambient Temperature ...or maybe a change in the Temperature of the Lubrication Oil ... or both!

This cooling causes the Lubrication Oil molecules to become less agitated.

So the oil contracts into a smaller volume, and this 'contraction action' sucks air from the outside environment that surrounds the Rotating Equipment into the Bearing Box Housing.

Any particulate debris concentrated in the ambient air will logically be sucked into the Bearing Box Housing ... and will try to maneuver past the Bearing Seals.

Eventually a dinky concentration of dirt and airborne debris accumulates in the Lubrication Oil and the Rolling Elements of Anti-Friction Bearings. Abrasive Wear eventually causes failure of the Bearing.

Naturally, the "breathing" of the Rotating Equipment also causes water droplets to be sucked into the Bearing Box Housing.

How?

Way back in PTOA Segment #77, brilliant PTOA Readers and Students learned that the amount of water vapor in air ... aka the air's Humidity ... changes with the ambient Temperature that surrounds the Rotating Equipment.

The warmer the air that surrounds the Rotating Equipment, the more vaporized water particles will be concentrated in the air and thence sucked into the Rotating Equipment.

As the PV Temperature inside the Bearing Box continues to decrease, more and more water droplets will condense out of the invading air, contaminating the circulating Lubrication Oil and Bearings, causing Corrosive Wear.

Moisture can also enter the Bearing Box Housing through vents and even from the Lubrication Bottles.

Your Mentor thinks PTOA Readers and Students should realize that the PV Pressure inside the Bearing Box Housing is significantly greater than the Atmospheric Pressure that surrounds Rotating Equipment.

 

The repeated cycling of the hot and cold ambient environment provides the sucking action that results in the invasion of contaminants ... otherwise there would be too much PV Pressure for the contaminants to flow into the Bearing Box Housing!

The "breathing cycle" of Rotating Equipment explains why Bearing Seals are needed to prevent the invasion of water and particulate contaminants into the Bearing Box Housing. Choosing the best combination of Bearing Seal and Lubrication Method  for the Bearing Arrangement is crucially  important for Rotating Equipment.

 

THE FUNCTIONS OF BEARING SEALS

All PTOA Readers and Students now understand why the functions of Bearing Seals can be summarized as the following:

• Keep Lubrication Oil from leaking OUT OF the Bearing Box Housing.
• Prevent solid and water contaminants from GETTING INTO the Bearing Box Housing.

 

At this point in the PTOA process, there simply cannot be one single PTOA Reader or Student that does not know Friction happens anytime two surfaces rub on each other and this Friction causes Wear.

Bearing Seals must accomplish the above tasks at the operating Temperature within the Bearing Box Housing and at the rotating speed of the Shaft without adding:

• More Friction.
• Increased Temperature
• More Wear.

 

 

BEARING SEAL CLASSIFICATIONS

There are two main categories of Bearing Seals: 

Integral Bearing Seals are found only on Anti-Friction/Roller Bearing hardware. As would be expected, the Seal is integrated into the Bearing design.

and ...

External Bearing Seals ... logically named because the sealing hardware is separate from the Bearing hardware; both the Bearing and the Bearing Seal must be designed into the Bearing Arrangement.

Truth be known, the function of Bearing Seals is visually more easy to understand when viewing pictures of Integrated Bearing Seals because External Bearing Seals just look like random component hardware in the vicinity of the Bearings.

 

Two Types of External Bearing Seals

And ... of course ...

there's two subsets of External Bearing Seals ...

(External) Non-Rubbing Bearing Seals are typically called "Gap Seals." Gap Seals leave a gap between the rotating Shaft and the Bearing Box Housing. Thus, Gap Seals create negligible Friction,  Temperature Increase, and Wear.

(External) Rubbing Bearing Seals typically fall into the category of Lip Type Seals." The 'sealing action' is created by a plastic "lip" which is attached to the rotating Shaft and extends to contact the surrounding stationary surface ...  which is typically the inner bore of the Bearing Box Housing. The physical barrier makes it difficult for particulates and moisture to invade. However, the superior 'sealing action' of a Lip Seal is offset by the generation of Friction, increased PV Temperature, and Wear. Thus all Rubbing Bearing Seals must be lubricated and the Shaft must be highly polished and limited to a specified maximum rotational speed.

 

INTEGRAL BEARING SEALS

Bearing Seals that fall into the category of Integral Bearing Seals logically have the seal integrated into the housing of the Bearing.

Integral Bearing Seals are only found in Anti-Friction/Roller-Element Bearings. The sealing hardware is attached to one or both sides of the Anti-Friction/Roller-Element Bearing.

Integral Bearing Seals are supplied already greased and are relatively maintenance free. These seals work well for Rotating Equipment that is expected to be on-line for a long duration between maintenance intervals and where moisture and particulate contaminant is moderate.

The PTOA does not support or endorse any particular manufacturer of bearing with integral seals. That being stated, SKF has a reputation for manufacturing sealed bearings for :

• Deep Groove Ball Bearings.
• Self-Aligning Ball Bearings.
• Double Row Angular Contact Ball Bearings.
• Cylindrical Roller Bearings.
• Needle Roller Bearings.
• Spherical Roller Bearings.
• Track Runner Bearings.

 

 

(EXTERNAL) NON-RUBBING GAP SEALS

Bearing Arrangements may use (External) Non-Rubbing Seals (aka Gap Seals) when the following operating conditions are known to be present.

• The chance of particulate contamination is not great.
• There is no danger of water or steam coming into contact with the Bearing.
• Minimal Friction and Wear is a priority because of the high operating speed of the Shaft and Temperature within the Bearing Box Housing.

 

How Gap Seals Work

As stated above, Gap Seals leave a teeny, dinky gap between the Shaft and the interior of the Bearing Box Housing.

Hey! That doesn't sound good to the ear!

Wouldn't a gap between the Shaft and the Bearing Box Housing allow outward leakage of Lubrication Oil and inward invading contamination or particulates and moisture?

Righteeo!

The nearby graphic attempts to illustrate how contaminants that appear as red arrows can invade the gap between the rotating Shaft and the interior of the Bearing Box Housing ... especially after the metal has thermally expanded during operation.

The effectiveness of Gap Seals ...

and all categories of (External) Non-Rubbing Seals ...

is therefore highly dependent upon the 'sealing action' that is created between multiple narrow gaps placed between the rotating component (aka, the Shaft) and the stationary component (aka, the internal side of the Bearing Box/Housing).

This staggered set of gaps keeps oil from flowing out of Bearing Box. Additionally, one or more concentric or helical grooves are masterfully machined into the  Bearing Box Housing which returns the otherwise escaping Lubrication Oil back to where it should be ...  available to lube the Bearings!

The 'sealing action' of Gap Seals can be improved by cramming Grease into the gaps, thus further preventing the invasion of contaminants. Still, it is best to rely on Gap Seals only where the Rotating Equipment will be surrounded by a  dry and dust free environment.

PTOA Readers and Students should pause a moment to consider how much Process Technology success is in the details! For example, the application of a Grease with just-the-right firmness and Temperature range and a masterfully cut groove that makes oil flow back to where it belongs are both examples of "Industrial Process Technology."

Because Gap Seals create zero Friction, Temperature Increase, and Wear they are the best choice for Bearing Arrangements that anticipate high rotating speeds and high PV Temperatures within the Bearing Box/Housing.

 

Simple Gap Seal: Pressed Steel Washers

The most simple and cheap Gap Seal is created with the Shaft threaded through an inexpensive pressed steel Washer just before the Shaft protrudes through the Bearing Box Housing.

As described above, PTOA Readers and Students like Fred are wondering ...

Where's the gap?

The whirling speed of the Shaft lifts the Washer away from direct contact with the Shaft, creating the dinky gap.

Sealing efficiency can be greatly improved by adding multiple Washers. Lubrication Oil trying to leak out of the Bearing Box Housing and contaminants trying to invade the Bearing Box Housing might get by one Washer ... but it is much harder to get through successive barriers!

An alternative descriptor for a "Multiple-Washer Gap Seal" is "Simple and Cheap Labyrinth Seal."

 

Labyrinth Seals

On the other end of the investment spectrum, the true Labyrinth Seal is the most effective and efficient Gap Seal, yet is quite expensive.

Fred is wondering what the fancy word "labyrinth" means.

The official definitions are:

"a complicated irregular network of passages or paths in which it is difficult to find one's way."

with the synonyms listed as:

"a maze" or "entanglement."

Imagine being a particulate contaminant or a speck of moisture trying to navigate the pathways in the above maze ... while spinning around and flung radially outward!

So who is surprised to learn that a Labyrinth Seal is manufactured to force the potentially leaking lubricant or invading contaminant to flow through a succession of multiple dinky pathways fabricated from successive Labyrinth Blades and Labyrinth Cavities  ... all the while being flung radially outward due to being whirled around?

To make it even harder to flow in or out of the Bearing Box Housing, the teeth of the Labyrinth Seal are filled with Grease.

The nearby graphic of a Labyrinth Seal illustrates how the red particulate and moisture contaminants cannot easily invade the Bearing Box Housing whilst the green lube oil likewise cannot escape from the Bearing Box Housing.

The oil and the particulate are caught in the teeth of the Labyrinth Seal and intentionally directed away from the Bearings.

Labyrinth Seals are the best way to prevent moisture contamination and can be designed to work in flooded or submerged environments.

 

Spring Washers

The (External Non-Rubbing) Gap Seal known as a Spring Washer provides simple, inexpensive and spacing saving seal action for greased Anti-Friction Bearings like Deep Groove Ball Bearings.

Your Mentor could not find a picture of an (External Non-Rubbing) Spring Washer Seal that would be appreciated by PTOA Readers and Students.

The nearby blow up graphic shows an Anti-Friction Ball Bearing protected by a Spring Washer which is labelled "retainer."

Spring Washers have a spring that clamps the washer against either the outer ring or the inner ring.

Once the rotor is spinning, an axial pressure is exerted against the non-clamped ring.

After a certain run-in period, these initially Rubbing Seals become Non Rubbing, very narrow Gap Seals.

 

(EXTERNAL) RUBBING SEALS

(External) Rubbing Seals create 'sealing action' by exerting a pressure directly onto the sealing surface, thus blocking moisture and contaminants from invading the Bearing Box Housing.

The 'sealing surface' is typically the inside bore of the Bearing Box Housing but can be whatever stationary (meaning non moving) interior surface surrounds the spinning Shaft.

Although (External) Rubbing Seals provide reliable sealing for Bearings, they do create Friction, Increased Temperature, and Wear.

Ergo, the rotational speed of the Shaft limits the use of Rubbing Seals.

To minimize the creation of  Friction, Temperature Increase, and Wear, Rubbing Seals are lubricated and the surface of the Shaft is ground to a fine finish.

Furthermore, Rubbing Seals that have been improperly mounted or exposed to particulate contaminants will quickly fail. So the Bearing Arrangement typically incorporates a Non-Rubbing Seal (Gap Seal) in front of a Rubbing Seal to protect it!

Felt Seals

(External Rubbing) Felt Seals are simple, inexpensive, and excel at stopping particulate contaminants from invading the Bearing Box Housing. Unfortunately, they are not successful at blocking moisture, so they can only be used in dry environments.

The nearby photo shows a Felt Seal that is about to be replaced.

Before inserting the Felt Seal into a groove made in the Bearing Box/Housing, the felt rings or strips must be soaked in oil at 176 ° F (80 °C). The surface of the Shaft must also be ground to a fine finish.

Felt Seals are limited to a rotational speed of 13 ft/s (4 m/s) and operating temperatures of 212 ° F (100 °C).

Typically, the 'sealing action' of a Felt Seal is not solely relied upon.

A Felt Seal may be combined with a Simple Labyrinth Seal.

 

(Radial) Lip Seals

A (Radial) Lip Seal is made out of soft rubber and rotates with the Shaft. This type of Rubbing Seal is used for oil lubricated Bearings that operate in the temperature range of -40 to 392 °F (-40 to 200 °C).

The "lip" is a piece of elastomer or rubber that plugs off flow between the Shaft and whatever the sealing surface is ... like the inner bore of the Bearing Box Housing.

The synthetic rubber lip of the seal is pressed into place with a "garter spring," thus "sealing action" is created against the stationary sealing surface.

The Lip Seal will face outward if the main job of the seal is to keep lubricant from leaking. The Lip Seal will face inward if the main job of the seal is to prevent an invasion of contaminants.

The heat-created Friction generated by (Radial) Lip Seals will harden the materials of the Lip Seal. So Lip Seals are not used when the rotational speed of the Shaft exceeds 2000 rpm.

Shaft speeds that exceed 2000 rpm in Bearing Arrangements that use oil for lubricant must use Felt Seals or Brush Seals.

 

Brush Seals and V-Ring Seals

Brush Seals and V-ring Seals are likewise attached to the Shaft and rotate with it. Likewise, the 'sealing action' presses outward typically upon the bore of the Bearing Box Housing ... or whatever stationary (non-moving) surface surrounds the Shaft.

Brush Seals are fabricated from fibers that brush the Shaft surface.

They are used instead of Lip Seals when the Shaft of the Rotating Equipment will rotate at high speeds and the equipment is required to function for long periods of continuous operation.

The V-ring Seal looks like a rotating disc with a V-shaped slash in it. In the nearby graphic the V-ring Seal is yellow.

The V-ring Seal can be used with both Grease and Lubrication Oil. When Grease is the lubricant, the seal will be external to the Bearing. When oil is used as a lubricant the seal will be integrated into the Bearing.

The cool thing about V-Ring Seals is that they work even when a relatively large Shaft Misalignment is expected in the design of the Bearing Arrangement.  Also, the V-Ring Seal body performs the function of a Flinger Ring (discussed below), meaning it flings dirt away from the Bearings.

 

LUBRICATION OF BEARINGS

By now there cannot be one PTOA Readers or Student who does not expertly understand why Grease or Oil lubrication is important to rotating metal components.

PTOA Readers and Students have recently learned that grooves can be strategically cut into the inner bore of a Bearing Box Housing which will return the lube oil trapped by a Labyrinth Seal back to where the oil can do its job.

A variety of additional methods are used to lubricate Bearings in Rotating Equipment.

Wicks

Wicks can be used to supply oil to Bearings.

Wicks can filter particulate contaminants from the oil, but unfortunately can become clogged with those contaminating particles after extended use or if the oil is circulated.

 

 

Slinger Rings

Slinger Rings are brass rings that rotate with the Shaft.

The job of the Slinger Ring is to pick up oil from a reservoir and "sling the oil," thus distributing the gift of lubrication over the Bearings.

Lubricating oil systems that use a Wick may incorporate a rapidly rotating Slinger Ring to spray the oil over the Bearing.

 

Flinger Rings

Flinger Rings are collars that attach to the Shaft between the Gland and the Bearing Box Housing. They may be included in Bearing Arrangements that use Lubrication Oil with Gap Seals.

Flinger Rings prevent process fluids from entering the Bearings Box Housing by "flinging" contaminants away from the Bearings.

Flinger Rings are also designed to "funnel" captured lubrication oil back into a channel which has been masterfully cut into Bearing Box Housing. The channel returns the oil into the oil sump or reservoir where it can be used to continue lubricating moving parts.

 

Mist Oiling Lubrication

Mist Oiling is often the best means of lubricating Bearings that will operate at very high Temperatures.

The Lubrication Oil in an Mist Oiling system has been atomized by compressed air.

The air assists in cooling the Bearing which explains why this is a good choice for cooling Bearings when the  Shaft rotates at high rpms.

Wow!

A lot of Process Technology sure goes into protecting and lubricating Bearings!

PTOA Readers and Students who become Process Operators now understand why so much attention goes into choosing the optimal combination of Bearing Seals and lubrication method for the Bearing Arrangement of each piece of Rotating Equipment!

 

TAKE HOME MESSAGES:  Bearing Seals protect Bearings by

• Keeping lubrication Oil from leaking OUT OF the Bearing Box Housing.
• Preventing  solid and water contaminants from GETTING INTO the Bearing Box Housing.

 

Moisture and Particulate Contamination invade the sturdy Bearing Box Housing because of the successive increase and decrease in ambient temperatures which cause the inanimate equipment to "inhale"... sort of. Moisture contamination causes Corrosion Wear of Bearings. Particulate Contamination causes Abrasive Wear of Bearings.

The design of Bearing Seals prioritizes limiting creation of additional Friction, Increased in Bearing Box Temperature, and additional Wear.

The two main categories of Bearing Seals are Integrated Bearing Seals and External  Bearing Seals. Integrated Bearing Seals are for Anti-Friction/Rolling-Element Bearings only. External Bearing Seals are separate from the Bearings they protect.

The two categories of External Bearing Seals are Non-Rubbing and Rubbing.

Non-Rubbing Seals are called "Gap Seals" because of the gap between the rotating Shaft and the stationary sealing surface ... which is usually the bore of the Bearing Box Housing.

Gap Seals create no additional Friction, do not increase the Bearing Box Temperature, nor increase Wear on moving parts. They are used for high speed rotating shafts and high Bearing Box Temperatures.

Non-Rubbing Seals must still be designed to keep oil from escaping the Bearing Box Housing through the gap.

Typical Non-Rubbing aka Gap Seals are:

• Pressed Steel Washers.
• Simple (aka Cheap) Labyrinth Seals made of several Washers.
• Labyrinth Seals ... which are the best seal stto prevent moisture contamination.
• Spring Washers.

 

Typical (External) Rubbing Seals are:

• Felt Seals
• (Radial) Lip Seals
• Brush Seals
• V-Ring Seals

 

Rubbing Seals have a barrier that physically straddles the gap between the spinning Shaft and the stationary sealing surface ... which is usually the bore of the Bearing Box Housing.

By virtue of having a physical barrier,Rubbing Seals have superior sealing action but create Friction, Increased Bearing Box Temperature, and Wear. For this reason all Rubbing Seals are lubricated and attached to Shafts with fine finishes. Use of Rubbing Seals is also limited by rotational speed of the Shaft and design Bearing Box Temperature.

Hardware components specifically designed to lubricate Bearings include:

• Wicks
• Slinger Rings.
• Flinger Rings.
• Mist Oiling.

 

The optimal Bearing Seal and Lubrication method for the Bearing Arrangement is crucial to the successful operation of Rotating Equipment.

©2018 PTOA Segment 0184
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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