Boy, you gotta carry that weight
Carry that weight a long time
Boy, you gonna carry that weight
Carry that weight a long time

("Carry That Weight," by The Beatles, 1969)

 

IMAGINE WHAT A BEARING WOULD LOOK LIKE

By the end of this PTOA Segment #182, PTOA Readers and Students will literally "get" their Bearings!

Otherwise stated ...

PTOA Readers and Students will be introduced to the  hardware components called "Bearings" that are logically housed in the "Bearing Box" of Centrifugal Pumps.

In PTOA Segment #174 PTOA Readers and Students learned that a Bearing Box is the part of a Centrifugal Pump that is located between The Stuffing Box and The Coupling.

 

The Bearing Box in the nearby photo of a Centrifugal Pump is contained within the blue Casing/Housing that is just in front of the orange-covered Coupling/Housing.

Note that the Bearing Box Casing/Housing has 'feet' that connect to the supporting frame of the Pump.

 

Before Your Mentor describes in detail what a Bearing is ... try this exercise:

Imagine what you would think the hardware components called "Bearings" would look like once you were made aware that their functions included:

• Providing the physical support of the Shaft while it rotates several thousand times per minute. The "fancy phrase" for this duty translates into nouns and verbs like "a Bearing carries The Load of the spinning Shaft."
• Restricting the Radial Movement of the Shaft (aka restricts the "upward and downward movement" created by the spinning Shaft).
• Restricting the Axial Movement of the Shaft (aka restricts the "thrust" or backward and forward movement created by the spinning Shaft).
• Absorbing the Radial and Axial Movement of the Spinning Shaft (aka Load) as described above and transmitting these movements  into the stationary Casing/Housing of the Pump.

 

Ideally, Bearings would complete all of the above functions while being fabricated in a  form which reduces the Friction that would be naturally be created by a spinning Shaft (aka "Load") sliding or rolling by another surface.

Brilliant PTOA Readers and Students already learned in the 4-part PTOA Tribology Focus Study that reducing Friction is necessary to reduce the Wear that causes hardware components to fail.

Yes, indeedo!

All of the above statements regarding the functions of a Bearing are accurate as stated and sound important.

Now allow Your Mentor to "break it all down to something more simple"...

Bearings are the hardware components that keep industrial Rotating Equipment like Pumps, Compressors, and Gas/Steam Turbines from vibrating into pieces due to the constant back and forth sliding motion (aka "Axial Motion" or "Thrust") or up and down motion (aka "Radial Movement").

This PTOA Segment #182 reveals the form and function of the various types of Bearings found in a typical Bearing Box.

The next PTOA Segment #183 explains how Bearings are the diagnostic heart of any piece of Rotating Equipment and also reveals the major reasons that cause Bearings ... and thus Rotating Equipment ... to fail.

 

WHAT IS INSIDE OF A BEARING BOX?

There is a Bearing Box on the right side of the nearby graphic of a Pump Cutaway with a red Casing. The Bearing Assembly is nestled within the Casing/Housing of The Bearing Box.

The Bearing Assembly appears as a silver cylinder (a Journal Bearing)  held in place between what PTOA Readers and Students will soon learn are a  Locating Bearing and a Non-Locating Bearing.

The Outside Process Operator has the crucially important jobs of:

• Maintaining the proper lubrication for the sliding and rolling Bearing components. Lubrication also performs a sealing function that prevents contaminants ... like water in air ... from entering the Bearing Assembly.
• Being constantly vigilant for abnormal sounds or changes in vibration that hint at an impending Bearing failure.

 

Hey, there's another nearby cutaway graphic of a Centrifugal Pump!

Once again notice the bolt holes on the exterior of the The Bearing Box Casing/Housing. The Pump will be physically bolted to the (not shown) Pump foundation.

This construction makes it possible for the Bearings to transmit the Radial and Axial Movements of the spinning Shaft into the frame that supports the Pump. Outside Process Operators will become accustom to the sound of "normal" operation made by the spinning shaft and thus be able to detect the sound of "abnormal" operation.

 

PLAIN BEARINGS

Plain Bearings (Journal Bearings and Plain Thrust Bearings)

Just like their name suggests, Plain Bearings are not fancy. Plain Bearings are mechanically simple to fabricate and are made out of a variety of metallic and even plastic materials.

Journal Bearings 

A Journal Bearing is a type of Plain Bearing.

Guess what?

Another name for a "Shaft" is a "Journal."

So when Your Mentor states that a "Journal Bearing is a type of Plain Bearing," PTOA Readers and Students can translate that statement into the following realization:

A Journal Bearing is a Bearing that supports a Shaft.

Thus PTOA Readers and Students who Imagined a Plain Bearing as a cylinder (or part of a cylinder) surrounding the Shaft are right on!

The nearby photo shows a Shaft supported by a Journal Bearing that looks a lot like a Shaft Sleeve.

Yes indeedo!

A Shaft Sleeve is a type of Plain Bearing because it supports the Shaft.

The term Journal Bearing specifically refers to the lower part of the Shaft Sleeve ... the lower half of the cylinder that is supporting the weight of the spinning Shaft.

The  Journal Bearing  bears the constant Load of the spinning Shaft and has to "carry that weight a long time."

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already know the crucially important role that the wedge of lubricating oil shown in the nearby graphic plays with respect to lifting the spinning Shaft so that there is no direct contact between the spinning Shaft and the Journal Bearing.

 

 Split Journal Bearings

Industrial-sized Shafts tend to have large diameters. They are also heavy because they are fabricated from durable metal.

Split Journal Bearings are easier to install around large diameter Shafts ... and most industrial sized Rotating Equipment have large diameter Shafts.

So industrial Journal Bearings are typically split cylindrical shells like the one shown in the nearby photo. The grooves in the Split Journal Bearing help channel Lubrication Oil to flow where it is needed.

Typically, the outer part of the cylindrical shell is fabricated from hard, strong metal and is held in a rigid support.

The inner part of the cylindrical shell  ... the surface that supports the Shaft/Journal ... is fabricated from a soft metal.

Why?

 

Cladding a softer metal interior onto a stronger metal exterior makes it possible for the Journal Bearing to forgive Misalignment Errors that are a result of poor Bearing installation or Shaft deflection.

 

Bushings

Process Operators will hear some Mechanics and Mechanic Techs refer to a "Bushing" in the Bearing Assembly.

A Bushing is a type of  Plain Journal Bearing/Sleeve Bearing that extends completely around the Shaft ...

 

So a Bushing is not split like a Split Journal Bearing would be. A Bushing is also intentionally easy to replace.

As shown in the nearby photo, some Bushings have holes that help guide lubrication oil to flow where it is needed.

 

Plain Thrust Bearings

The phrase "Axial Thrust" describes the natural tendency of a rotating Shaft with an attached spinning Impeller to lurch forward.

PTOA Readers and Students learned in PTOA Segment #176 that "Axial Thrust" is the same pulling-forward action pilots depend upon to fly airplanes.

However, in Rotating Equipment, this forward thrusting Axial Movement of the Shaft (aka Axial Thrust) is not desirable and absolutely must be equally counteracted.

 

 

The "fancy words" that describes the job of Plain Thrust Bearings would be something like ...

"Thrust Bearings carry/bear Loads that are parallel to the Shaft ..." (yawn) ...

The normal speak translation would be:

A Thrust Bearing keeps the Shaft from moving forward or backward.

A Plain Thrust Bearing looks like a plate at the end of a Shaft. The simple construction of the Plain Thrust Bearing hides its important job of bearing the continual push of Axial Movement.

Your Mentor could not find a picture of an industrial-sized Centrifugal Pump with Plain Thrust Bearings. That's because Anti-Friction Bearings are now available to limit both Axial and Radial Movement and are the more popular choice for the Bearing Assembly of an industrial-sized pump.

 

ROLLING-ELEMENT BEARINGS (aka ANTI-FRICTION BEARINGS)

Plain Bearings like Journal Bearings endure repeated sliding of one surface over another surface.

Once again, brilliant PTOA Readers and Students who have completed the PTOA Tribology Focus Study already know that surfaces which roll rather than slide by each other reduce the generation of Friction by a factor of 1000 ... which greatly reduces Wear.

Rolling-Element Bearings (aka Anti-Friction Bearings) counteract the Axial and Radial Load of the Shaft by rolling rather than sliding.

All Rolling-Element Bearings are characterized by:

• A shape of rolling hardware that may or may not be ...
• Caged in an angular grooved track ... which is called
• A race.

 

The nearby graphic is a cutaway of an Anti-Friction Bearing that features cylindrical roller-shaped hardware ... aka Roller Bearings ... as "the Rolling Element."

Don't the little rolling cylinders look like little barrels?

The most common and popular Anti-Friction Bearing is the Ball Bearing which logically has spherical shaped balls as "the Rolling Element."

 

Sometimes the Anti-Friction Bearing is held in place by a frame which might be called a Pillow Block or a Plummer Block. The nearby photo illustrates how an Anti-Friction Bearing in a Pillow Block limits the upward and downward Radial Movement of a spinning Shaft.

 

Limitations of Anti-Friction Bearings Compared to Plain Bearings

Anti-Friction Bearings (aka Rolling-Element Bearings) have a few drawbacks when compared to Plain Bearings.

The rotating speed of the Shaft (rpms) for a Bearing Assembly with Anti-Friction Bearings will be less than the rpms that a similarly sized Shaft with similarly sized Plain Bearings can attain.

Furthermore, Anti-Friction/Rolling-Element Bearings are mechanically complex to fabricate compared to Plain Bearings.

For example, the Needle Roller Bearing that appears in the nearby graphic is fabricated from heat-treated specialty steels via highly sophisticated procedures that require using special equipment.

Naturally that means that Anti-Friction Bearings are more expensive than Plain Bearings which can be fabricated in most any local machine shop.

Additionally, Anti-Friction Bearings are very sensitive to environmental contaminants like dirt and water that enter the Bearing Box.

The nearby graph shows how the life of an Anti Friction Bearing significantly reduces when the water vapor increases above just 100 parts per million (ppm)!

This same graph shows the expected life of the Bearing will be twice as long if the moisture content is kept below 25 ppm.

For this reason ...

The Bearing Assembly for Anti-Friction Bearings must include Seals that keep dirt and moisture out of the Bearing Box.

Plain Bearings do not need Seals.

However, as in many things in life, the smart decision to invest in Anti-Friction Bearings over Plain Bearings can be explained with the phrase ...

Invest a bit more money now to buy an Anti-Friction Bearing or burn up a lot more money daily using a Plain Bearing!

 

Advantages of Anti-Friction Bearings Compared to Plain Bearings

The primary advantage of Rolling Element/Anti-Friction Bearings is that they can hold a Shaft in "Precise Alignment."

Wow! There's that world "Alignment" again!

PTOA Readers and Students will learn all about Shaft Alignment in the upcoming PTOA Segment #183.

And ...

If a Rolling-Element Bearing/Anti-Friction Bearing has been correctly installed, well maintained by Mechanics,  and correctly lubricated by Process Operators...

Then Plant Mechanical Engineers can actually calculate the run life of the Bearing and therefore determine exactly when to schedule a shutdown for planned maintenance to replace the Bearings.

Anti-Friction Bearings are also easier to lubricate than Plain Bearings.

When lower Friction is generated between moving parts that means less heat is generated within The Bearing Box.

Typically the Lubrication Oil chosen for any Gear Boxes in the processing facility will also work just fine lubricating Anti-Friction Bearings in The Bearing Box.

And who amongst the brilliant PTOA Readers and Students who have read the 4 part PTOA Tribology Section could logically predict that Anti-Friction Bearings have a much lower Static Friction Factor and therefore experience less Friction and Wear when being started up?

For all of the above reasons, Anti-Friction Bearings have replaced Plain Bearings where feasible in the Rotating Equipment that is used in Process Industries.

 

 

 

COMMON SHAPES OF "ROLLING-ELEMENTS"

BALL BEARINGS

When the Rolling-Element is a sphere the Bearing is the well known Ball Bearing.

Deep Groove Ball Bearings are the most popular type of Bearing used in industry.

The deep grooves make it possible to limit upward and downward Radial Movement at high and even very high Shaft rotating speeds.

They are also popular because they require little maintenance or service.

 

The Double Deep Groove Ball Bearing design can counteract an even greater amount of up and down Radial Movement and can also reduce some forward and backward Axial Thrust Movement.

 

Thrust Ball Bearings

Thrust Ball Bearings are used when a significant forward or backward Axial Thrust Load must be counteracted.

"Single-Direction Thrust Ball Bearings" can be used to counteract either backward OR forward Axial Movement ... but not both directions.

"Double-Direction Thrust Ball Bearings" can counteract both backward and forward Axial Movement.

What Thrust Ball Bearings cannot do is counteract any upward or downward Radial Movement.

A different Bearing must be added into the Bearing Assembly to counteract the Radial Movement caused by the spinning Shaft

 

Self-Aligning Ball Bearings

There's that word "Aligning" again!

Self-Aligning Ball Bearings have two rows of balls with a common raceway in the outer ring.

This construction gives the bearings a valuable ability to self-align ...

which means this type of Bearing can compensate for misalignment of the Shaft relative to the Casing/Housing.

Misalignment happens from installation errors and also from deflection of the Shaft during the operation of the Rotating Equipment.

 

OTHER ROLLING-ELEMENT BEARINGS

Cylindrical Roller Bearings

Cylindrical Roller Bearings will be used to counteract the Axial and Radial Movement created by the heavy industrial Shafts that exceed the range that Ball Bearings can support.

Otherwise stated ...

Compared to Ball Bearings ...

Cylindrical Roller Bearings can counteract even greater upward and downward movement caused by a spinning Shaft.

In fact ...

Cylindrical Roller Bearings excel at counteracting significant upward and downward Radial Movement even when the heavy Shaft is spinning at a very high speed.

 

Needle Roller Bearings

Needle Roller Bearings are used when the speed of the Shaft and the weight of the Shaft are moderate.

Their long, slender shaped hardware increases the contact area with the Shaft; the more Needle Rollers per Bearing, the more Load the Bearing can support!

Certain models of Needle Bearings can be fabricated to perform the duties of Journal Bearings ... without having to make modifications to the Shaft or the Bearing Box Casing/Housing!

 

Tapered Roller Bearings

Tapered Roller Bearings are chosen when there is a need to counteract both upward and downward Radial Movement and forward and backward Axial Movement.

They greatly reduce the Friction associated with rolling contact.

An example of a Tapered Roller Bearing is included in the nearby SEMRI Bearing Chart which features common Bearing hardware found in the Bearing Assemblies of Process Industry Rotating Equipment.

Hey!

There was almost Too Much technical Information in this PTOA Segment!

High Five with Your Mentor!

Because you read through it ...

And now ...

You "got" Your Bearings!

 

 

TAKE HOME MESSAGES: Bearings keep Rotating Equipment from vibrating apart due to the constant forward and backward Axial Motion and upward and downward Radial Motion caused by a spinning Shaft.

Bearings are purposely chosen for their ability to limit Axial and Radial Motion and are part of the Bearing Assembly found in the Bearing Box of Rotating Equipment.

Besides limiting Axial and Radial Movement of the Shaft, Bearings support the Load of the Shaft.

A Journal Bearing supports the weight of the Shaft. To be more specific the bottom half of a Journal Bearing supports the weight of the Shaft.

Bearing Boxes transmit the vibrations of the Radial and Axial Movements into the external surface of the Bearing Box Casing/Housing. Outside Process Operators must also be constantly vigilant for abnormal vibrations because they hint a Bearing failure is in progress. Outside Process Operators must also make certain the sliding/rolling surfaces of Bearing hardware are properly lubricated, 

Examples of Plain Bearings are Journal Bearings, Shaft Sleeves, Bushings, and Plain Thrust Bearings.

Anti Friction/Rolling-Element Bearings:

• More precisely align Shafts which keeps the Rotating Equipment optimally operating.
• Greatly reduce the generation of heat and Wear caused by Friction.
• Are easier to lubricate.
• Are extremely sensitive to dirt and moisture. For this reason there will be a Seal in the Bearing Assembly.

 

Nowadays Anti-Friction/Rolling-Element Bearings can be manufactured to limit both Axial and Radial Movement.

The most popular Rolling-Element Bearing is the Ball Bearing. Thrust Ball Bearings can limit Axial Movement and generate far less friction than Plain Thrust Bearings. Ball Bearings can be made to self-align, which means they compensate for misalignment of the Shaft due to poor installation or Shaft deflection.

Other Rolling-Element Bearings include:

• Cylindrical Roller Bearings
• Needle Roller Bearings
• Tapered Roller Bearings

 

©2018 PTOA Segment 0182
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

You need to login or register to bookmark/favorite this content.