Listen
(Doo-wah-doo)
Do you want to know a secret?
(Doo-wah-doo)
Closer
(Doo-wah-doo)
Let me whisper in your ear ...

("Do You Want to Know a Secret?," by the Beatles, 1963)

 

BEARINGS: THE DIAGNOSTIC HEART OF ROTATING EQUIPMENT

PTOA Readers and Students who have been reading the PTOA Segments in the intended sequential order can expertly explain how the sliding/rolling surfaces of a Bearing and the spinning surface of a Shaft make an interface between moving parts.

 

No matter how small or large the Rotating Equipment is, the Bearings can sense if there is a potential problem at this crucial interface.

If the Bearing is Misaligned, unbalanced, or loose ... these problems are telegraphed through the Bearings.

The Outside Process Operator can detect a pending Bearing failure by noticing ...

• An increased in the PV Temperature detected by TIs.
• Increased noise.
• Increased vibration sensed through the Casing/Housing or by vibration monitoring instrumentation.
• The presence of insoluble particles clogging up filters and strainers.

 

For all of these reasons ...

Bearings are the diagnostic heart of Rotating Equipment and are constantly emitting messages which predict the internal state of the equipment.

 

THE TYPICAL BEARING ARRANGEMENT

Locating and Non-Locating Bearings

The term "Bearing Arrangement" refers to how all the hardware of the Shaft and Bearing Assembly are situated in the Bearing Box Housing/Casing.

 

The Bearing Arrangement must also take into consideration:

• How all the rotating/sliding interfaces will be lubricated.
• How the Lubrication Oil will be prevented from leaking out of the Casing/Housing and...
• How the Bearing Arrangement will be protected from dirt and humidity entering into the Bearing Box.

 

Spoiler Alert! The last two items above are determined by the Bearing Seals chosen for the Bearing Arrangement.

The typical Bearing Arrangement has the Shaft supported by a  Locating Bearing on one end and a Non-Locating Bearing on the opposite end.

This Bearing Arrangement centers the Shaft to counteract up and down Radial Movement and back and forth Axial Movement.

Otherwise stated, the Bearing Arrangement is supposed to prevent the spinning Shaft from contacting the surfaces of the Housing/Casing walls ... which are in close proximity.

The Locating Bearing will be physically attached to the internal surface of the Bearing Box Housing/Casing as well as have contact with the Shaft.

Ergo ...

Locating Bearings must be capable of counteracting some Radial Movement as well as guiding the forward and backward Axial Movement of the Shaft.

In fancy words ...

A successfully installed Locating Bearing provides "Axial Movement Guidance" for the Non-Locating Bearing.

The important job of the Non-Locating Bearing is to allow back and forth Axial Movement to occur.

Otherwise, the Bearings would mutually stress each other when the Shaft thermally expands or contracts during startup and shutdown.

 

 

CAUSES OF BEARING FAILURE

Bearing Failure will occur when a Bearing is no longer able to support the Shaft or one of its hardware components is unable to rotate/roll.

In the absence of an entire backup piece of Rotating Equipment ...

Bearing Failure → Rotating Equipment Failure → Emergency Shutdown!

Brilliant PTOA Readers and Students have figured out that understanding the root causes of Bearing Failure will decrease the chance of an emergency shutdown of Rotating Equipment.

This PTOA Segment #183 explores the root causes of  Bearing Failures and explains how the work of "woke" Mechanics, Mechanic Techs and Process Operators reduces Bearing Failures.

The list of Bearing Failure Root Causes includes:

• Lubrication Problems.
• Water and Particulate Contamination Problems.
• Misalignment.
• Fatigue/Overload.

 

 

 

 

BEARING FAILURE CAUSED BY LUBRICATION PROBLEMS

The nearby Bearing Failure Chart declares that the majority of Pump Bearing Failures are due to Lubrication Problems.

These Lubrication Problems include:

• Incorrectly Specified Lubricant (aka Unsuitable Lubricant).
• Insufficient and/or Inadequately Applied Lubrication.
• Failure to Replace Lubricant (aka Aged Lubricant).

 

Note that the root-cause of these problems is "Human Error."

 

Incorrectly Specified Lubricant

The Mechanics and Mechanic Techs are responsible for choosing the appropriate lubricant for the respective Rotating Equipment service.

The Mechanics/Mechanic Techs may choose a Grease for lubrication.

The Grease will be selected for the following properties:

• Viscosity and consistency of the Grease's base oil.
• The Grease must work within the operating Temperature range of the spinning Shaft ... the higher the Temperature, the thicker the Grease!

• The ability of the Grease to carry/counteract the Load of the Spinning Shaft.
• Rust inhibiting ability.

 

Lubrication Oil will be used instead of Grease when:

• Operating Temperatures exceed 200 °F (93 °C). Over 250 °F (121 °C) will definitely result in Bearing problems.
• The Shaft spins at a high speed.
• The continuously generated heat of Friction must be transferred into Lubrication Oil so that this heat will be continuously removed from the Bearing Box.
• When other hardware components in the vicinity must be lubricated with Lube Oil. For example, the Bearings in many pieces of Rotating Equipment share a common Lube Oil system which has been chosen to be compatible for gears.  Any Lube Oil selected for gear application will likely be just fine for Bearings.

 

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already know that the Lubrication Oil selected by the Mechanics/Mechanic Techs will primarily be chosen for its Viscosity ... because Viscosity determines the Oil's ability to form films.

Next the Mechanics/Mechanic Techs verify that the Lubrication Oil candidate will also be compatible with the rotational speed of the spinning Shaft and Bearings ... because that combination of factors will determine the Temperature that will be created by Friction.

A couple "rules of thumb" Mechanic/Mechanic Techs use when choosing Lubrication Oil are:

• The lower the speed of spinning Bearings, the thicker (more Viscous) the Lube Oil should be.
• Oil with a Viscosity of 70 SUS/SSU will be used for Cylindrical Roller Bearings and all Ball Bearings.
• Somewhat heavier (more Viscous) Lube Oil will be selected for other types of Roller Bearings.

 

Insufficient Lubrication

Guess what?

Over-lubrication can result in high temperatures and excessive loss of lubricant.

However, over-lubrication is a much less serious condition than Insufficient Lubrication ... which is the most common root cause of  premature Bearing Failure.

A properly lubricated bearing has the best chance of attaining maximum service life. 

 

How Mechanics/Mechanic Techs Contribute to Insufficient Lubrication

The Mechanics and Mechanic Techs are responsible for adequately lubricating the Bearings during installation of the Bearing Assembly.

Bearings are not easy to access while the Rotating Equipment is operating, so it is not possible to correct the initial application of lubricant if it is insufficient prior to startup.

 

How Process Operators Contribute to Insufficient Lubrication 

Any card-carrying PTOA Reader or Student who has read the PTOA 4-part Tribology Focus Study knows why the moving parts in Rotating Equipment must be lubricated and the many functions of lubricants.

Once the Rotating Equipment is ready for start up and continuous operation, the Outside Process Operator is  responsible for continuously providing filtered, cooled lubricant and being alert as to when it is time to replace the lubricant.

Hey!

Find the Oiler in the nearby photograph!

Many Outside Process Operators passed by the empty Oiler shown in the nearby photo. The lack of lubricant caused the Bearings to fail, hence expensive repairs and downtime of the Rotating Equipment.

The Friction generated by the hot, under-lubricated metal surfaces caused the surfaces to fuse together and seize up.

No PTOA Reader or Student would allow a pump to run dry of lubrication!

 

Bearing Failure Due to Failure to Replace Aged Lubricant

Process Operators and Mechanics/Mechanic Techs have a shared responsibility to monitor the condition of the lubricant and replace it when necessary.

Changing out the filtering media in the Lube Oil Filters will help extend the usable life of the inventory of circulating Lube Oil.

The nearby picture shows a sample of Lubrication Oil before and after filtration.

 

Even with regular filter changes, the Lubrication Oil circulating within the Lubrication Oil Auxiliary System will eventually become darker, which means the useful life of the lubricant has been spent.

As a lubricant becomes more spent, its Viscosity increases.

Alert Process Operators will notice the need to change out filters more frequently as the generation of sludge and insoluble particulates increase in the Lube Oil System.

The Lube Oil color change, increase in Viscosity, and the inability to remove contaminants with simple filtration hint that the Lube Oil has become contaminated with reactions products from:

• Water vapor entrained in air.
• Metal particles from abrasive wear and leaked process fluids.
• Oxygen from entrained air interacting with ...
• Heat generated by Friction.

 

The nearby graphic attempts to illustrate how an increase in acids is generated due to the chemical reactions between metals, heat, and oxygen. Naturally, the increased acidity will also increase the corrosiveness of the Lube Oil.

Outside Process Operators can monitor the Total Acid Number (TAN) and the Viscosity of the Lubrication Oil by submitting samples to the Facility QA/QC Laboratory  ... or asking the Lubricant Vendor to supply this service.

Failure to Replace Lubricant will inevitably lead to a classification of Bearing Failure due to Contamination of Bearings.

 

CONTAMINATION OF BEARINGS

The nearby graphic claims that 52% of all Bearing Failures are due to Particulate Contamination and corrosion caused by Water Contamination.

The Bearing Failure Chart for just Pumps presented earlier in this PTOA Segment claimed that 20% Particulate Contamination and 5% Water Contamination (aka "Liquid based Contamination") caused pumps to fail.

The appropriate conclusion from both charts is that Water and Particulate Contamination cause a significant amount at Bearing Failures ... and that's why Bearings need Seals ... which will be featured in PTOA Segment #184.

The Ball Bearings in the nearby photo will be protected from contamination once they are rotated back into the protective outer race, which also performs the function of a Seal.

As stated above and restated here for redundancy ...

Lube Oil contamination changes the Lube Oil Viscosity.

Because Viscosity is the most important physical property of the lubricant, Water and Particulate Contamination will impact the reliability of the Rotating Equipment.

 

Water Contamination of Bearings

Super dinky amounts of water contamination are particularly troublesome to Bearings.

The nearby graphic shows how just the step-wise addition of 200 parts per million (ppm) Water changes the physical appearance of Lube Oil from clear/opaque (on the left side) to cloudy on the right side.

No kidding! Certain Bearings can lose 75% of their useful life  due to Water Contamination ... even before the water content is visible as cloudiness!

When the Lube oil is derived from mineral oils or synthetic oil (refer to PTOA Segment #180), super dinky concentrations of water will form a very stable emulsified mixture.

The stable water/oil mixture increases the Viscosity of the Lube Oil and that directly impacts the Lube Oil's ability to form a strong film that keeps metal surfaces apart.

The nearby graphic illustrates that a 300 ppm increase in water concentration ...

from an expected value of 100 ppm to 400 ppm ...

decreases the expected life of a Bearing by a whopping 60%!

Furthermore, the super dinky concentration of water forms acids, sludge, and varnishes which also changes the Viscosity of the Lube Oil.

Even worse ...

Water Contamination also dilutes the additives in the Lube Oil. The additive particles just prefer to hang with the water droplets instead of doing their job in the Lube Oil.

That means diluted Corrosion Inhibitors cannot do their job preventing corrosion at the exact same time acids are being formed in the Lube Oil precisely because of the presence of dinky amounts of water!

An endless cycle that ends in doom!

And, Guess What?

Corrosion of Bearings due to Water (Vapor) Contamination can easily begin when the Rotating Equipment is at rest. And once the off-line corrosion starts, Bearing Failure will eventually occur!

Who remembers what pump Cavitation is?

Don't remember?

Re-read PTOA Segment #171.

Guess what?

Water Contamination is the leading cause of hydraulic pump Cavitation!

Even worse than Cavitation ... water particles that flow through hot frictional surfaces can expand and then explode ... causing metal fracture!

Who amongst the brilliant PTOA Readers and Students is wondering how the heck dinky amounts of water get into a Bearing Box in the first place?

That question will be answered in the next PTOA Segment!

 

Particulate Contamination of Bearings 

PTOA Readers and Students who have read the 4-Part Tribology Focus Study Area understand the nuances between the following types of Wear caused by Friction:

• Galling: The tearing away of one sliding surface over another which generates metal particles.
• Pitting: The metal particles removed when many, many holes are made in the Bearing surface.
• Abrasive Wear: Fine particles in Lube Oil wearing down sliding/rolling surfaces.

 

All of the above types of Wear result in metal contaminants released into the Bearings and circulating Lube Oil.

These are the metal particles that react with oxygen and heat to generate the acids and/or just float around as insoluble contaminants in the Bearings and Lube Oil.

 

The remedies to preventing Bearing Failure due to both Water Contamination and Particulate Contamination are:

• The selected technique used for Bearing lubrication.
• The selected Bearing Seals.

 

These topics are featured in the next PTOA Segment.

 

BEARING FAILURE CAUSED BY MISALIGNMENT

Misalignment of the Shaft and Bearings
Within The Bearing Box Casing/Housing

Once again, the nearby cutaway of a Multi-Stage Centrifugal Pump illustrates a Bearing Arrangement.

The Bearing Arrangement fits snugly into the interior of the Bearing Box Casing/Housing.

Hey!

Take the time to notice the Oiler Bottle on the exterior of the Casing!

A Bearing Misalignment will result in a Bearing making unintentional contact with the interior of the Bearing Box Casing/Housing ...

which translates into a short run life for this pump and expensive repairs!

 

Misalignment of the Shaft to the Casing/Housing will occur when the Shaft bends due to Radial Movement/Load.

In this situation the Bearings that have been chosen for the Bearing Arrangement were not properly matched to the anticipated weight or the spinning velocity of the Shaft.

Misalignment of the Shaft is also a concern when the Shaft is supported by Bearings in separate Casings/Housings which are far apart.

Nowadays professional Bearing Installation Technicians use laser guidance to align Shafts that are supported by Bearings in separate Casing/Housings.

Professional alignment of Bearings and Shafts has greatly decreased the approximate 16% of Bearing Failures that were historically attributed to poor installation.

Guess what?

Proper installation techniques for Bearings begins way before the Bearing becomes part of the Bearing Arrangement.

The Bearing manufacturer will apply a preservation coating and wrap each Bearing prior to shipment.

The Mechanics and Mechanic Techs must not unwrap the Bearing until the last possible moment; There is no need to wash off the protective coating with solvent unless it has been soiled with dirt some how.

The Mechanics/Mechanic Techs must also verify the smoothness of the Shaft as well as verify that the Shaft size will work as expected with the Bearings.

Some Bearings must be heated prior to installation to make them expand.

When heating is necessary, the Mechanics/Mechanic Techs must heat the Bearing using an electrical source or perhaps an oil bath ... but never, ever by applying a direct flame on the Bearing.

 

Proper installation of the Bearing also involves pressing the Bearing into place ... never pounding it into place ... because steels are hard and brittle and easily fracture.

The nearby photo shows a damaged Bearing race. The race was damaged by using an inappropriate tool during installation.

All of the above cautions must be taken by Mechanics/Mechanic Techs to decrease the occurrence of Misalignment.

 

BEARING FAILURE DUE TO OVERLOAD

The below graphic states that Bearing Failures due to Overload accounts for 4% of Bearing Failures.

The nearby Bearing Failure Chart for Pumps states 5% of Bearing Failures are due to Overload (aka "Consequential Damage").

Some other sources in the literature of Bearing Failure claim that the percentage of premature Bearing Failure due to Overload, Incorrect Servicing, and just plain Neglect increases the percentage of Bearing Failures to 34%.

Whatever the percentage actually is, the Overload of a Bearing results in sudden failure of the Rotating Equipment.

This type of sudden shutdown can be avoided since the Bearings emit early warning signals.

Process Operators must use their eyes and ears ...

(and the vibration and temperature instrumentation) ...

to interpret the condition of the Bearings ... the diagnostic heart of any Rotating Equipment.

 

 

TAKE HOME MESSAGES: Bearings are the "diagnostic heart" of any Rotating Equipment because they telegraph the internal condition of the Bearing Arrangement and thus whether or not the equipment needs to be scheduled for repairs.

Alert Process Operators can determine the condition of the Bearings/Rotating Equipment by monitoring the temperature, noise, vibration, and whether or not the filters are being clogged more frequently.

A Bearing Arrangement includes all the hardware associated with the Shaft and Bearings and also answers how: 

• The sliding/rotating surfaces will be lubricated.
• The Lubricant will be prevented from leaking out of the Bearing Box.
• The Bearing Seals will protect the Bearings from Water and Particulate Contaminants.  

 

The typical Bearing Arrangement has:

• A Locating Bearing on one end of the Shaft that is attached to the Casing and the Shaft.
• A Non-Locating Bearing on the other end of the Shaft that accommodates back and forth Axial Movement.

 

Bearing Failures cause emergency shutdowns of Rotating Equipment.

Common sources of Bearing Failure related to Lubrication are:

• Incorrectly Specified Lubricant.
• Insufficient and/or Inadequately Applied Lubrication.
• Failure to Replace Lubricant.

 

Changes in Lube Oil Viscosity, Color, Total Acid Number, and accumulated particulates that clog filters are signals that the Lube Oil needs to be replaced. 

Additional Common Causes of Bearing Failure include:

• Water Contamination of Bearings.
• Particulate Contamination of Bearings.
• Misalignment of Bearings and other Installation Errors.
• Pre-Installation Errors.
• Bearing Overload.

 

Water and Particulate Contaminants will impact the Viscosity of the Lube Oil and thus change its ability to perform the important duties of a lubricant.

The selected lubrication technique and Bearing Seals protect the hardware within the Bearing Box from contamination.

Misalignment errors greatly decrease when lasers are used during alignment.

©2018 PTOA Segment 0183
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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