I think a change,
(A change would do you good)
Would do you good.
(A change would do you good)

("A Change Would Do You Good," by Sheryl Crow, J. Trott, B. MacLeod, 1996)

 

EXACTLY HOW DOES A LUBRICANT WORK?

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order know that Lubrication Oil reduces Friction and Wear because the lubricant is situated between two moving surfaces that are rubbing or rolling by each other.

 

But how exactly does Lubrication Oil perform the many functions that were listed in PTOA Segment #179?

For example, how exactly does a liquid like Lubrication Oil perform the basic function of 'lubrication'?  How does it separate the Load (labelled "surface b" in the nearby graphic) from the "surface a" below it? What is holding the two surfaces apart?

Spoiler Alert!

The fancy phrase  "Hydrodynamic Lubrication" that appears as a label in the nearby graphic is a hint.

"Hydrodynamic Lubrication" means the liquid Lubrication Oil has changed into a media that is capable of making the Load hydroplane over the surface below it ...  just like the water in the nearby photo of a Slip N Slide makes it possible for the person on an inner tube to slide over a plastic surface.

This PTOA Segment #180 explains how Lubrication Oils systematically change from a Boundary Layer Lubrication Regime  ... on the left side of the nearby graphic ...

into the Full-Film Lubrication Regime shown on the right side of the nearby graphic.

The Full-Film Lubrication Regime is capable of hydroplaning the Load of a sliding/rotating surface!

No joke! Your Mentor is telling it like it is!

Lubrication Oils change regimes until they are capable of bearing the weight of a sliding or rolling Load. The separation of the two sliding/moving metal surfaces reduces the Friction that is generated ... and thus reduces the Wear caused by Friction.

So what the heck are Lubrication Oils made from that they have this magical power to change from a Boundary Layer Film into a layer of Fully Developed Film that can keep a Load hydroplaned above a surface?

Stay tuned!

 

By the time PTOA Readers and Students have finished this PTOA Segment #180 they will be aware of:

• The Three Regime Changes of Lubrication Oil.
• How Lubrication Oils and Greases are Made.
• Why Lubrication Oils are classified by Viscosity .... aka "the resistance to flow."

 

Modern Process Operators who possess the knowledge of how Lubrication Oils work are better prepared to keep Lube Oil Systems functioning ... and thus keep industrial Rotating Equipment up and running.

 

'HYDRODYNAMIC LUBRICATION' JUST MEANS "HYDROPLANING"

In PTOA Segment #179, PTOA Readers and Students learned that lubricants can be made of solids, liquids, and even high pressured gases.

Only Lubrication Oils ... a liquid state of lubrication ... are capable of creating Hydrodynamic Lubrication.

Don't stress! While learning how a Lubrication Oil works and does its thing ... just  remember that the ten-dollar nerdy phrase for "hydroplaning" is "Hydrodynamic Lubrication:"

"Full-Film Lubrication" = "Hydrodynamic Lubrication" = hydroplaning

There are two factors that make it possible for Lubricating Oils to keep the moving surfaces apart:

• How fast the two surfaces are sliding by each other.
• The Viscosity of the specially made Lubrication Oils that are intentionally selected and matched to their process service.

 

Hey! That's worth repeating!

The generation of a sufficient PV Pressure that can lift and separate moving surfaces depends upon

• The rate of the sliding motion and
• The Viscosity of the Lube Oil.

It is worth emphasizing because:

The Process Operator ...

• Indirectly impacts the rate of sliding motion via his/her operation of the Driver and
• Directly impacts the Viscosity of the Lubrication Oil by making certain that it stays within the desired operating temperature range!

Hey! Since we're at it ... let's get super nerdy with Lubrication jargon!

Hydrodynamic Lubrication is called Elasto-hydrodynamic Lubrication when one of the surfaces is rolling rather than sliding.

Instead of developing lubrication "films," the jargon changes into developing "wedges of lubrication" when the Load is rolling instead of sliding.

In the nearby graphic, the developed "wedge of lubrication" separates the surface of the rolling Load from the surface beneath the "wedge of lubrication."

Hey, that's the same kind of "wedge of lubrication " that makes it possible for water skiers to skim the surface of a lake and causes cars to skid on wet roads!

The next part of PTOA Segment #180 describes how to identify the three Lubrication Regimes that ultimately develop "the wedge" or "fluid-film" that separate moving surfaces.

 

THE THREE LUBRICATION REGIMES

Regime #1: Boundary Lubrication 

The Boundary Lubrication Regime is the starting phase of the lubrication process. The two sliding surfaces are separated by just a little bit of lubricant that is applied on both surfaces.

During the Boundary Lubrication Regime, the sliding surface of the Load  is being supported almost entirely by the bottom sliding surface.

That must mean that the sliding motion and Viscosity of the Lubrication Oil are not yet able to keep the surfaces apart! Righteeo!

The Friction-Modifier chemicals that have been mixed into the lubricant are crucially important during the Boundary Lubrication Regime.

The Friction-Modifiers will reduce the Friction that will be automatically generated between the rubbing surfaces ... which will inevitably cause Wear.

In PTOA Segment #177, PTOA Readers and Students were introduced to a table of static and kinetic "Friction Coefficients" which compared the Friction generated between sliding surfaces made from different materials.

As the Y axis on the nearby graphic shows, the "Friction Coefficient" generated by the sliding surfaces is greatest during the Boundary Lubrication Regime.

So, the Boundary Lubrication Regime does not depend upon the Viscosity of the Lubrication Oil but rather the Friction-reducing additives in the Lubrication Oil to keep the industrial machinery running.

 

Regime #2: Mixed-Film Lubrication

The Mixed-Film Lubrication Regime is an in-between state characterized by the Load being partly carried by a film of Lubrication Oil and partly by direct contact between the sliding surfaces.

The motion of sliding surfaces begins to generate a thickness ... or liquid film ... that is capable of generating a little bit of separation between the two surfaces.

Albeit thin, this thin film is critically important because it prevents damage to the surfaces.

The Anti-Wear Additives that have been mixed into the base Lubrication Oil are doing their job during the Mixed-Film Lubrication regime.

When the Load is major ... like the hypoid gears shown in the nearby photo ...

Extreme Pressure Additives are mixed into the base Lubrication Oil.

Friction Modifier Additives are still important during the Mixed-Film Lubrication Regime.

As the nearby photo shows, the Friction Coefficient generated between the moving surfaces reduces during the "regime change" between the Boundary Film and Mixed-Film Regimes.

 

Regime #3: Full-Film aka "Hydrodynamic Lubrication"

During the third and final Full-Film Lubrication Regime (aka "Hydrodynamic Lubrication"), the fast, repeated sliding motion of one surface over the lubricant on another surface causes sufficient film thickness to develop and separate the surfaces.

Successfully achieving the Full-Film Lubrication Regime involves 3 things:

• Design details of the Load ... because reducing the Load reduces the Full-Film thickness.
• The operating speed of the Driver for the Rotating Equipment (pump, compressor, gas or steam generator, etc) ... because increasing the rpms increases the Full-Film thickness.
• The Viscosity of the Lubrication Oil ... because increasing the Viscosity of the oil increases the Full-Film thickness.

 

Once the Full-Film Lubrication Regime has developed and the surfaces are separated, the Friction between the two surfaces lines out to a magnitude that is greatly reduced from the initial Boundary Film Regime.

The Friction that is generated during the final Full-Film Lubrication Regime is between two fluid-to-metal surfaces and is therefore significantly less than the Friction that is generated between the two metal-to-metal interfaces during the initial Boundary Film Regime.

Recall that Elasto-hydrodynamic Lubrication impacts the rolling parts of bearings, and cams, and certain types of gears.

Elasto-hydrodynamic Lubrication can create high pressure "wedges"...that can become very viscous and elastically deform and separate surfaces. The Load and the shape of the surface below it squeeze the high PV Pressure film toward the rolling surface contact.

To prevent the deformation caused by Elasto-hydrodynamic Lubrication, the Viscosity of the Lubrication Oil must be matched to the service of the Rotating Equipment.

 

HOW A LUBRICATION OIL IS MADE

Guess what?

The manufacture of industrial Lubrication Oils and Lubrication Additives are  multi-billion dollar Process Industries that employ Process Operators!

The nearby "Process Flow Diagram of Lube Oil" shows the processing steps that convert the raw product of "Atmospheric Crude Tower Bottoms" (aka "Atm. Bottom")  into 5 kinds of Lubrication Oil that are  distinguished by the type and amount of Additives mixed into the Base Oil during the Blending Process.

Petroleum-Derived Lubricant Base Oils

Mineral Oils

Petroleum-Derived Lubrication Oil (aka "Mineral Oil") is made via the conventional crude oil refining processes that distill naturally occurring hydrocarbons from the crude.

Find the "Lubricating Oil" product flowing from the Distillation Tower in the nearby graphic; it's on the right side of the tower.

This is the raw material product that flows into the "FRU Unit" that appeared on the "Process Flow Diagram of  Lube Oil."

 

Higher Performance (More Expensive) Lubricant Base Oils

Higher Performance Lubricants are made by chemically rearranging the distilled Lubricating Oil process stream with hydrogen in a Reactor ... like the hydrocracking reaction shown in the nearby graphic (and which was featured in PTOA Segment #28).

The much more expensive Higher Performance Lubricant Base Oils have the following desirable attributes:

• Do not react with air as easily (aka have "lower oxidation tendency").
• Do not catch fire as easily (aka "have less volatility").
• Can work within higher temperature ranges (aka "have better thermal stability").
• Have a higher Viscosity Index.

 

Uummm, what's a Viscosity Index?

Remember how forming the Full-Film during the Full-Film Boundary Regime depended upon the Lubrication Oil Viscosity and the rate of sliding? It sure would be nice to be able to predict the  Viscosity of the Lubrication Oil and thus be assured that a protective film will be formed.

The Viscosity Index (VI)is the rate of change in the Viscosity of the Lubrication Oil as its PV Temperature is increased (or decreased).

A high VI is preferable as it signifies that the Lube Oil Viscosity will not change as widely when the PV Temperature is changing.

In the nearby graphic, the Viscosity/PV Temperature relationship of the Blue Lubrication Oil (VI=150) does not vary as much as the same relationship described by the Red Lubrication Oil (VI=95).  Another way of describing the compared relationship is "the Red Lubricant has a steeper slope than the Blue Lubricant."

Synthetic Lubrication Oils

Synthetic Lubrication Oils are made via complex Humankind processing technologies targeted to limit the variance in molecules found in Mineral Oil by selectively encouraging production of just the chemical species with the most favorable lubricating properties.

The production requires many processing steps to ultimately generate a chemically uniform final product with superior lubrication performance and durability.

Synthetic Lubrication Oils are used when:

• There is a need for non-reactivity with air (aka "low oxidation tendency") and/or
• There is a need to operate the Rotating Equipment over a wide temperature range (aka "a high degree of thermal stability").

 

The nearby graphic defines the operating temperature range for compared lubricants as a horizontal white bar.

The Mineral Oil lubricant in the nearby graphic (top) has a significantly smaller range of Temperature operation compared to the two Synthetic Lubrication Oils shown lower.

Both Petroleum-derived Mineral Oils and Synthetic Lubrication Oils are used to lubricate gears, bearings, engines, hydraulic systems, compressors, refrigerated system components etc.

Mineral and Synthetic Oils might even be mixed to make a semisynthetic lubricant. Desired attributes to the mixture can also be attained by blending in additives.

 

Non-Petroleum-Derived, Biodegradable Lubricant Base Oils

Vegetable Oils (like Rapeseed oil or Castor plant derived oil) are the Base Oil for lubricants used in hydraulic fluid applications where sensitive environmental concerns cannot tolerate an accidental spill or leakage.

These oils easily biodegrade but cannot withstand a high service temperature and have a much shorter service life.

The Fatty Oils that can be extracted from vegetables, animal, and fish matter are excellent lubricants but interact with air and oxidize rapidly.

However, adding Fatty Oils to Petroleum-Derived Lubricant Base Oils create lubricants that can repel moisture. This handy characteristic is why they are used in steam engines and some worm gear applications.

Grease

Fatty Oils are also added to Petroleum-Derived Base Oils to make Greases.

That's right! Grease is just 70-90% Lubricant Base Oil with thickeners that hold the oil in place. So that explains why a layer of oil can be found on the top of a newly opened jar of Grease!

The specific purposes of Grease are to:

• Seal Lubricating Oil inside while
• Keeping air and process contaminants outside.

 

Greases are rated on "Consistency" or "Firmness" on a scale of 000 to a maximum of 6.

A  soft grease has Consistency/Firmness number of 1 and a medium Grease would have a Consistency/Firmness of 2.  A  "hard" Grease is rated 4 and a very hard Grease is rated 5. A Grease with the Consistency/Firmness of a block has the top rating of 6.

 

DESIRED CHARACTERISTICS OF LUBRICANTS

Generally speaking the industrial Lubrication Oil will be blended to achieve:

• Excellent Lubricity ... Like, Duh!

• Excellent Viscosity Index ... a Lubricating Oil that is less sensitive to PV Temperature changes.
• High PV Temperature Stability.
• Low Volatility (aka "Fire/Ignition Resistant").

 

By now all brilliant PTOA Readers and Students understand that Additives are chemical compounds mixed into the Base Lubrication Oil to modify or enhance the performance of the lubricant.  Here are some of the Additives found in Lubrication Oils.

• Air Release Agents
• Antifoamants
• Antioxidants
• Anti Wear Agents
• Biocides
• Corrosion  Inhibitors
• Demulsifiers
• Detergents
• Dispersants
• Emulsifiers
• Extreme Pressure Agents
• Friction Modifiers
• Metal Deactivators
• Odor Improvers
• Rust Inhibitors
• Tackiness Agents (for Grease)
• Viscosity Index Improvers

 

Viscosity ... aka "Resistance to Flow"

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already fundamentally understand that Viscosity is "resistance to flow" as was defined in PTOA Segment #162.

So they already know that the phrase "honey is more viscous than water" means that honey does not flow as easily as water does.

Viscosity is the most important property of a lubricant because the lubricant must be able to form films that are sufficiently thick to minimize Friction and Wear but not so excessive to bind the machinery up and cause efficiency loss.

Some PTOA Readers and Students may be unaware that they already know this!

PTOA Readers and Students who own an automobile are familiar with commercial lubricants which are sold in 'Viscosity ranges.'

 

A chart that cross references ISO (Blue Column), AGMA (Orange Column), and SAE Viscosity ranges (Green and Red Columns) for the services listed (Crankcase, Gear Oil) is shown below.

When an automobile owner purchases a different grade of motor oil depending upon the season of the year, s/he is protecting the engine from Viscosity changes that occur between hotter and colder ambient temperatures!

So the inverse relationship between Lubrication Oil Viscosity and the PV Temperature is summarized:

Viscosity decreases as the PV Temperature increases, and vice versa.

That explains why the Viscosity Index (VI) is an important characteristic of Lubrication Oils!  Lubricants with a high VI have will have less change in Viscosity when the PV Temperature changes  ... and that is important for Rotating Equipment lubrication services that experience a wide variance in temperature!

Lubrication Oil Viscosity also changes directly with the PV Pressure:

Viscosity increases with the PV Pressure ... and vice versa!

The internal PV Pressure of Rotating Equipment can reach many thousands of psi. The rolling parts of bearings, gears, and other machine elements can be negatively impacted by the increase in Viscosity that occurs with an increase in PV Pressure.

The high film pressures generated in these hardware components increase the Viscosity of the lube oil ... and that can harmfully increase Frictional Forces and decrease the Load carrying capacity.

 

PTOA TRIBOLOGY FOCUS SUMMARY

High Five with Your Mentor!

You just completed the PTOA Tribology Focus Section!

All brilliant PTOA Readers and Students now competently understand how:

 

• The Friction generated between the moving surfaces of Rotating Equipment causes Wear ... and
• Lubrication is the front line of defense to prevent Wear and keep expensive Rotating Equipment working!

 

No PTOA Reader or Student would let the site glass on "an oiler" run dry as the many Process Operators on shift did as shown in the nearby photo!

 

TAKE HOME MESSAGES: Lubrication works by developing 'films'  or 'wedges' which separate the sliding/rolling surfaces. The development of the film or wedge takes place over three phases, aka 'regimes.'

Successfully developing the Full-Film Lubrication Regime depends upon:

• The rate of sliding/rolling movement ... which is controlled by the Driver.
• The Viscosity of the Lubricant ... which will change depending on Temperature or Pressure.

 

Process Operators impact the success of Lubrication Oil by:

• Making certain The Driver operates at the design/desired rpm.
• Operating the Lubrication Oil Coolers/Heaters as needed to maintain desired Viscosity.

 

The ten-dollar word for the "Full-Film Lubrication Regime" is "Hydrodynamic Lubrication."

When there is a rolling element involved in the hardware, the Full-Film Lubrication Regime is called "Elasto-hydrodynamic Lubrication."

Lubrication Base Oils made by the industrial distillation of crude oil are called "Mineral Oils." Higher Performance Lubrication Base Oil is made from chemical reactions that produce molecules with improved lubricating properties.

Synthetic Lube Oils are made by isolating uniform chemical species with superior lubricating properties.

Greases are semi-solid lubricants made from Lubricating Oil Bases with added thickening agents. They are graded from 000 (super soft) to 6 (hard).

The purposes of Grease are:

• Keep Lubrication Oils from leaking outward
• Prevent contaminants from entering into gear boxes.

 

Additives are mixed in with Lubrication Base Oil. Some of the important additives are:

• Anti Wear Agents
• Extreme Pressure Agents
• Friction Modifiers
• Viscosity Index (VI) Improvers

 

 Viscosity is the most important characteristic of Lubrication Oils and must be "just right" to optimize the performance of Rotating Equipment. Lubrication Oils are characterized (and sold by) Viscosity;

Lubrication Oil Viscosity decreases as the PV Temperature increases, and vice versa.

A Lubrication Oil with a high VI will have a more constant Viscosity as the PV Temperature changes.

Lubrication Oil Viscosity increases when the PV Pressure increases, and vice versa.

The high PV Pressures generated in the rolling parts of bearings, gears, and other non-sliding machine parts can be impacted by the resulting increased Viscosity of Lubrication Oil.

©2017 PTOA Segment 0180
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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