Here in my on-again, off-again style
Here in my on-again, off-again smile
Here in my off-again, on-again grin
Here in my off-again, on-again skin

("Waiting For You," by Gordon Lightfoot, 1993)

FRICTION, WEAR, AND LUBRICATION

PTOA Readers and Students just learned the form and function of hardware associated with "The Pumping Side" of a Centrifugal Pump.

PTOA Readers and Students will soon be learning the form and function of the hardware components that play important roles in "The "Power Side" of a Centrifugal Pump. A list of these pump parts can be found in PTOA Segment #174.

The "Power Side" of the Centrifugal Pump begins where the shaft pierces the Pump-side Casing and enters The Stuffing Box which is also known as a Packing Box.

Naturally, keeping the pumped fluid from leaking out becomes a priority at that interface.

Besides sealing the pumped fluid into the  Pumping Side of the Pump, the component hardware within the Bearing Housing limits radial and axial vibrations.

The hardware components in the Stuffing/Packing Box and Bearing Housing will be sliding or rolling or rotating by another piece of hardware.

Otherwise stated ...

The Rotating Equipment that is used to generate the PV Pressure contains many surfaces that are sliding or rolling/rotating by each other in an on-again, off again relationship ... repeated over and over again ... hundreds and even thousands of times per minute.

Now is the time for all PTOA Readers and Students to start noticing that ...

Whenever two surfaces are rubbing or rolling by each other the force called Friction is generated and ...

Friction causes Wear.

 

The minimization of Friction and Wear is accomplished with Lubrication Oil and in some cases, Grease.

The Outside Process Operator is held accountable for insuring that that the hardware components in "The Power Side" of a Centrifugal Pump will always be properly lubricated.

This PTOA Segment #177 focuses on the causes, types, and laws of Friction that modern Process Operators should be aware of.

Once the theory behind Friction is understood, PTOA Readers and Students will be better prepared to understand how Friction causes Wear, a subject that is featured in PTOA Segment #178.

Then PTOA Readers and Students will learn in PTOA Segment #179 how proper Lubrication prevents both Friction and Wear.

 

PTOA READERS HAVE ALREADY BEEN INTRODUCED TO  "FRICTION"

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order have encountered the concept of "Friction" many times while reading the PTOA PV Pressure Focus Study Area:

• PTOA Segment #159 described how the Friction created between the interior wall of a pipe and the fluid flowing right next to the wall forms a "Velocity Profile" that can be seen as separate radial layers of flow with a protruding "cone head."
• PTOA Segment #161 was the first mention that the duties of the Process Operator include oversight of Lubrication systems so that something called Friction would be reduced.
• PTOA Segment #165 described how the Velocity Profile created by Friction is observed in the real world as "Pressure Drop" (aka ... the loss of line Pressure). Fluids flowing through pipe fittings such as Check Valves also lose the PV Pressure because of Friction.
• PTOA Segment #166 began the PTOA instructional series related to deciphering Centrifugal Pump Performance Charts which featured step by step instructions on how to determine the Total Dynamic Head (TDH) of a Pump. This tedious process required accounting for Friction Head. PTOA Readers and Students learned that "Friction Factors" have been assigned to various types of pipe and piping fittings and are used to determine the Friction Head on the Suction and Discharge sides of a Centrifugal Pump.
• PTOA Segment #168 described how a Centrifugal Pump's Efficiency is reduced by Friction Losses in "The Pumping Side" of the pump as well as the Packing/Stuffing Box and Bearing Box located within "The Power Side" of the pump.
• PTOA Segment #168 also described how the Brake Horsepower for a Centrifugal Pump (BHP) is directly related to the Efficiency of a Centrifugal Pump; PTOA Readers and Students learned that the  BHP of any machine must be sufficient to get the process liquid where it needs to be as well as overcome the Friction Losses that diminish the machine's Efficiency.

 

Heck yeah, all of the above sounds familiar!

Now, what exactly is this thing called Friction?

 

FRICTION DEFINED

Friction is the "Resisting Force" that Opposes Sliding & Rolling

Who amongst the brilliant PTOA Readers and Students is surprised to learn that ...

Friction is created whenever two things slide or roll by one another.

To be boring yet more accurate,

Friction is the force that resists the movement of two things that are sliding or rolling by each other.

Say, what?

Consider the diagram showing the green stationary surface of a table and a purple block being pushed toward the right.

 

A type of Friction force is created when block is pushed toward the right after resting on the surface of the table and ...

the Friction force will always be in the opposite direction of the movement.

Here's two logical conclusions to draw related to Friction:

• Naturally, it would be easier to push the purple block without the opposing Force of Friction.
• Naturally, it takes more energy to push the block and overcome Friction than it would if Friction did not exist in the first place or could be reduced some how.

 

Here's a less obvious observation:

• Even if the two surfaces appear to be perfectly smooth, two sliding surfaces are interacting at the molecular level.

 

Spoiler Alert! The molecular-level interaction is what causes Wear!

An industrial example of two surfaces moving by each other is shown in the nearby picture of a Bearing that is preventing the rotating Shaft from moving up and down.

 

 

In this Shaft-Bearing example, Friction would be generated

• Wherever the surfaces of the Shaft are surrounded by the Inner Race of the Bearing.
• Wherever each of the ball bearings roll by the Inner and Outer Races.

 

The sliding and rolling surfaces of the Bearing generate an opposing Friction force just like the nearby sand blocks do when they are intentionally rubbed together to make a rhythmic, scratchy sound.

And that brings up a good point ...

Friction-Generated Noise and Heat

Friction generates:

• Noise and
• Heat

 

The noise that Rotating Equipment makes is just one of the ways Friction makes its presence known.

The very best Outside Process Operators will:

• Always protect their hearing with proper ear protection.
• Know the "normal sounds" that Rotating Equipment makes so that s/he can notice when the Rotating Equipment is generating an abnormal noise.

 

 

Friction-generated heat must be removed or cooled down.

Overheating the hardware components found in Rotating Equipment can cause thermal degradation up to the point of fusing metal.

Fred is wondering why Friction didn't appear in the PTOA PV Temperature Focus Study Area since it generates heat.

Great observation, Fred!

Friction generates a localized type of heat that is definitely associated with the generation of the PV Pressure, not the PV Temperature as will be futher explained below.

 

Examples of "Good" Friction

A lot of the time, Friction is a friend.

"Good Friction" is known as "traction."

We could not hold anything, walk, pile a stack of wood, or even make a sand castle without if  Friction forces did not exist.

 

Everybody is totally dependent upon Friction to be created whenever they use a  paper clip, nail, screw, brake, or clutch.

The price of automobile tires is based on how good their "traction" with a road will be under various weather conditions ...not to mention  questionable  parking practices.

The SUV in the nearby photo is totally dependent upon the force of Friction to stay put.

And yes  ...

 

 

 

This would be the appropriate moment to acknowledge and appreciate "the gift of being alive!" that Friction generates when skin comes in contact with skin.

 

 

 

Uh oh!  Fred's blushing!

Your Mentor apologizes to Fred.

The point was to make certain PTOA Readers and Students understood Friction can be a good thing before learning how challenging Friction is for Rotating Equipment!

 

Examples of "Unhelpful Friction"

In the vast majority of industrial applications, Friction isn't helping at all because it reduces the ease of sliding or rolling/rotating between two surfaces.

Friction is "a waste of the energy it takes to create the PV Pressure."

Truth be known, if Friction wasn't generated anytime two surfaces slid or rolled by each other, the multimillion dollar Lubricant and Grease manufacturing industry would go out of business. But that won't ever happen because ...

 

The generation of the PV Pressure and the generation of Friction are linked together! Here's why:

• Building up the PV Pressure with Rotating Equipment requires a repetitive sliding or rolling/rotating motion between two surfaces.
• Friction is generated any time two surfaces are sliding or rolling by each other.
• Since the force of Friction acts in a direction that opposes the sliding/rolling/rotating movement, Friction wastes some of the effort and expense that is used to build up the PV Pressure!
• The Lubrication & Grease Industries make products that reduce Friction and thus increase the Efficiency and run life of Rotating Equipment!

 

Otherwise stated ...

 

Friction literally burns up some of the money that is spent on the getting the Driver to spin or rotate a shaft while in the process of generating the PV Pressure!

 

FRICTION LAWS

There are two somewhat interesting laws that Friction must obey.

First: The amount of Friction generated by two sliding and rolling surfaces DOES NOT depend on the area of contact!

That purple block being pushed to the right on the green surface of a table would generate the same amount of Friction even if the shorter side was on the table.

Hey that's kind of interesting if you think about it!

Second: The amount of Friction generated by sliding DOES DEPEND on the weight of the sliding object.  

Example, please!

If two purple blocks were stacked on top of each other and pushed to the right ... Then twice as much Friction force opposing the movement would be generated compared to a single block.

 

Three blocks stacked on top of each other and slid to the right would generate three times more Friction force opposing the movement compared to a single block ...

and so on and so on.

That just makes sense if you think about it!

 

Friction is "resistance to movement" ...

 

so more weight would make it harder for the two surfaces to slide or roll/rotate by each other ... hence more Friction would be generated!

 

STATIC AND DYNAMIC FRICTION

By now all PTOA Readers should know:

• 'Static' is the ten-dollar word that means "not moving" and "standing still." For example, the speed skaters in the nearby photo are not yet moving. Their "starting stance" may look strange but is a strategy to quickly overcome the Static Friction which will oppose their first glide forward.

• 'Dynamic' and 'Kinetic' are the ten-dollar words that means "in motion." The race is underway for the speed skaters in the nearby photo. Their forward motion is hindered by the Kinetic Friction that they must overcome with the blade-ice surface interface.

Otherwise stated:

Static Friction is the Friction force that is generated the very first time two surfaces slide/roll/rotate by each other.

Kinetic Friction is the Friction Force that is generated after the two surfaces are repeatedly sliding/rolling/rotating by each other.

Guess what?

More Friction force (aka resistance to moving) is generated the first time two surfaces slide or roll/rotate by each other (aka "Static Friction") than is generated after the two surfaces keep repetitively sliding/rolling/rotating by each other (aka Kinetic Friction).

For the speed skaters, the Static Friction force that must be overcome at the beginning of the race is 70% more than the Dynamic Friction force that must be overcome during the race.

Wow!

The difference between the magnitudes of  Static Friction and Kinetic Friction explains the big difference in gas mileage observed when driving in stop-and-go city traffic versus uninterrupted highway driving!

Hey!

If the Friction Laws are applied to the speed skaters that must mean:

The skaters must overcome the same amount of Friction to start skating whether using the tip of the skate blade or the entire blade length to begin the race!

And ...

A heavy-set skater with more mass ... ergo, weight ... will have to overcome a greater amount of Friction to start skating than the typical skater would have to overcome.

How do the concepts of Friction Laws, Static Friction, and Kinetic Friction reveal themselves in Process Industry Rotating Equipment?

The design of Rotating Equipment takes the Friction Laws into account.

Start Up Procedures for Rotating Equipment will also have steps that Outside Process Operators must take to reduce that initial jolt of Static Friction that the expensive equipment will bear upon being energized.

 

Rolling Friction

What happens when the sliding surface is replaced with round objects ... like ball bearings?

Rolling Friction is generated when a wheel, ball or cylinder rolls freely over a surface.

Rolling Friction is also in the opposing direction of rotation and must be overcome.

 

Just forming a rolling side on a solid surface decreases the Friction that will be generated by a factor of 100 to 1000 compared with trying to slide straight-sided surfaces by each other!

That fact explains why The Wheel was invented!

 

Very soon PTOA Readers and Students will be learning about component parts called Radial Bearings ... which have ball bearings inside them.

 

 

Smart PTOA Readers and Students will instantly understand that this style of Bearing was chosen because it generates much less Friction than sliding surfaces would.

 

TAKE HOME MESSAGES: Friction is a force that is created wherever two surfaces slide or roll or rotate by each other.

Friction always opposes the sliding/rolling/rotating movement, so Friction burns up some of the energy that must be used to make the two surfaces move.

For Rotating Equipment, Friction is a waste of the energy it takes to create the PV Pressure.

Friction can be heard as noise and felt as heat.

Friction causes surface Wear.

Friction Laws:

• The amount of Friction generated by the sliding/rolling/rotating movement IS NOT dependent upon how much of the surface area between the two surfaces is in contact.
• The amount of Friction generated by sliding IS dependent upon the weight of the object. 

 

Types of Friction:

• Static Friction is the amount of Friction generated when two surfaces first begin to slide/roll/rotate by each other.
• Kinetic Friction is the amount of Friction generated after two surfaces are in motion sliding/rolling/rotating by each other and is less than Static Friction.
• Rolling Friction is the amount of Friction generated when one or more of the two surfaces is rounded and Rolling Friction is always less than Kinetic Friction by factors of 100 to 1000.

 

Reducing the impacts of Friction on Rotating Equipment are factored into:

• The design of Rotating Equipment
• The Start Up Procedures that Outside Process Operators follow.

 

©2017 PTOA Segment 0177
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study

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