He goes crazy these nights
Watching heartbeats go by...
And they whisper -
We belong together
We belong together

("We Belong Together," by Rikki Lee Jones, 1981)

 

THE PURPOSE OF SHAFT COUPLINGS

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order can already identify the form and function of the Centrifugal Pump hardware featured in the nearby photo:

• The Pump, which encloses the Impeller(s)
• The Packing Box/Stuffing Box with fiber Packing and/or Mechanical Seal
• The Bearing Box, with its Seals and Lubrication Method.

 

What comes next?

This PTOA Segment # 185 is about the Shaft Coupling ... the hardware component which is surrounded by Plexiglas in the nearby photo of a totally red Centrifugal Pump-Coupling-Electric Motor Driver/Prime Mover.

Guess what?

The Prime Mover/Driver (aka Electric Motor in the above photo) was made by a company that has expertise manufacturing Prime Movers/Drivers.

Steam Turbines and Gas Turbines can also be used as Prime Movers/Drivers.

 

On the other hand ...

The red Centrifugal Pump in the nearby photo was made by a company that has expertise manufacturing Centrifugal Pumps.

Positive Displacement Pumps, Centrifugal and Positive Displacement Compressors, and Axial Compressors are other examples of Rotating Equipment.

The Shaft that protrudes from the Prime Mover/Driver will "drive" the Shaft that protrudes from the Rotating Equipment ... such as a Centrifugal Pump.

The Shaft Coupling is a mechanical device that connects the Driver's "Driving Shaft" to the "Driven Shaft" of the Rotating Equipment.

This is what is meant by the word "drive:"

The Shaft Coupling transmits the rotary power necessary to rotate the "Driven Shaft" of the Rotating Equipment.

As would be expected, the type of Prime Mover/Driver chosen to drive the Rotating Equipment impacts the selection of the optimal type of Shaft Coupling.

 

The optimal Shaft Coupling for the Prime Mover/Driver-Rotating Equipment combo also depends upon the time interval that each of them are expected to operate between shutdowns (aka the "Service Factor" of the Prime Mover/Driver and the Rotating Equipment.)

The selection of the optimal Shaft Coupling must also consider ...

• The Rotational Speed
• Horsepower
• Torque
• Shaft Diameters
• and space limitations

 

of the Driver-Rotating Equipment combo.

For a given horsepower, the speed (rpm) decreases and torque increases.  This relationship is the most important consideration in sizing Shaft Couplings.

And, of course ...

The Alignment between the two Shafts within the architecture of the Shaft Coupling must make it possible for Prime Mover/Driver to efficiently drive the Rotating Equipment.

 

 

Both the Shaft of the Prime Mover/Driver and the Shaft of the Rotating Equipment have a Centerline of Rotation.

The greater the Shaft Misalignment between the two Centerlines of Rotation, the greater the rate of Wear and likelihood of premature failure of the machines.

Furthermore ...

The greater the Shaft Misalignment between the two Centerlines of Rotation, the greater the loss of efficiency and increased power consumption of the machines.

 

 

SHAFT ALIGNMENT AND SHAFT MISALIGNMENT DEFINED

To reiterate ...

Each Prime Mover/Driver has an imaginary Centerline of Rotation in the smack dab middle of the Shaft around which the Shaft of the Prime Mover/Driver rotates.

Likewise ...  each piece of Rotating Equipment has an imaginary Centerline of Rotation in the smack dab middle of the Shaft around which the Shaft of the Rotating Equipment (and all the other rotating components on the Shaft) rotate.

Doesn't it just make sense that the goal of a Shaft Coupling is to align the Shaft of the Prime Mover/Driver with the Shaft of the Rotating Equipment to form one straight line?

Any Shaft Misalignment between the Centerlines of Rotation of the Prime Mover/Driver and the Rotating Equipment can result in Vibration.

Vibration generates premature Wear and even catastrophic failure of the machine's Bearings, Seals, and all the other rotating components on the Rotor, or even the Shaft Coupling itself.

 

Shaft Misalignment is not the only source of Vibration ...

Heck, mechanical looseness that originates in a nearby operating machine can impact a well-aligned machine through the foundation or even piping!

However, when excessive Vibration due to Shaft Misalignment at the Coupling is present, major damage to the machinery will occur.

For this reason Shaft Coupling and Alignment is performed by experts.

Yep! Alignment Technician is yet another type of Process Industry job!

 

TWO TYPES OF SHAFT MISALIGNMENT 

Definition of Alignment "Tolerance"

Absolute perfection of Shaft Alignment is not realistically attainable because the process of aligning the Shafts occurs while the machinery is non-functioning and cold yet the machinery thermally expands while attaining the hotter operating temperatures.

Ergo,

Properly Shaft Alignment requires the expertise of professional Mechanics/Mechanic Techs who know how to:

• Accurately measure the cold position of the two Shafts and
• Accurately project how much the machinery will move while attaining the hot operating condition.

 

Fortunately ...

Absolute perfection of Shaft Alignment is not necessary and Mechanics are more interested in ...

"How much Misalignment of the Shaft Centerlines can be tolerated?"

The answer to this question defines "The Tolerance" of Shaft Misalignment ... and is typically expressed in millimeters.

Otherwise stated ...

The incremental millimeters that offset one Shaft from the other is called The Tolerance.

 

"Offset Misalignment" aka "Parallel Misalignment"

Always remember and never forget that the goal of the Shaft Alignment process is to have a single invisible Centerline of Rotation that extends the length of both Shafts.

The Offset Tolerance is the maximum separation (expressed in millimeters) that can exist between Centerline of Rotation of the Driving Shaft and the Centerline of Rotation for the Driven Shaft.

In the nearby photo, the Offset Tolerance that defines the magnitude of the Offset Misalignment would be the measurement of sigma (the symbol σ in the upper left of the nearby graphic).

Note that the nearby graphic refers to Offset Misalignment as "Parallel Misalignment." Both descriptors are found in the literature.

Typically the "Offset Tolerance" is determined by measuring at the center of the Coupling.

 

Angular Misalignment

Angular Tolerance describes the magnitude of Angular Misalignment which can be tolerated between the two Shafts.

Angular Misalignment is "the rate at which the gap between the Shaft Centerlines changes as the distance from The Coupling increases down the axes of the Shaft, thus creating an angle."

In the nearby photo, the Angular Misalignment is defined by the angle theta (θ) in the upper right graphic.

The Coupling will never perfectly align the Centerlines of the Driving and Driven Shafts; even the most precise procedures used for Shaft Alignment will still result in a degree of Offset/Parallel Misalignment (σ) and Angular Misalignment (θ) as is depicted in the bottom photo of the nearby graphic.

The tools of the trade used for Shaft Alignment include dial indicators, taper gauges, feeler gauges, and straight edges.

Wherever precision is demanded, the far more accurate laser is used for Shaft Alignment.

 

THE FLEXIBLE COUPLING

There are many styles of Shaft Couplings. The choice of which type of Coupling to use depends on a variety of criteria which were presented earlier in this PTOA Segment.

The Flexible Coupling design incorporates flexible material that accommodates the transition from the cold, non-operating condition to the higher temperatures and Vibration experienced when the Prime Mover/Driver-Rotating Equipment combo is in operation.

Good quality Flexible Couplings are typically built to tolerate an excess of Shaft Misalignment via Vibration than the amount of Vibration the Prime Mover/Driver and Rotating Equipment combo are designed to withstand.

 

 

TAKE HOME MESSAGES: The Shaft Coupling is a mechanical device that connects the Shaft of the Prime Mover/Driver to the Shaft of the "driven" Rotating Equipment.

The term "drive" means "to transmit the rotary power" that makes it possible to rotate the Shaft of the Rotating Equipment, thus enabling a pump to pump a liquid or enabling a compressor to compress a gas ... etc. etc. 

The optimal choice for a Coupling depends upon:

• The selected Prime Mover/Driver and Rotating Equipment
• The Service Factor of the Prime Mover/Driver-Rotating Equipment combo.
• The Rotational Speed and Diameter of the Shafts
• The Torque 
• The space limitations of the Prime Mover/Driver-Rotating Equipment combo.

 

The sizing of the Coupling depends upon the hp, rotational speed, and torque relationship because at a given hp, the speed of the shaft and torque have an inverse relationship.

Proper, precise alignment of the Shafts reduces the generation of Vibration that would otherwise generate Wear and potentially catastrophic failure of Bearings, Seals, and even the Shaft Coupling.

Shaft Misalignment also causes inefficient operations which wastes money 24/7.

Shaft Alignment and Shaft Misalignment are quantified by how much the Centerlines of the "Driving" and "Driven" Shafts are out of sync, meaning do not form a single, invisible line.

Perfect Shaft Alignment is not realistic because the Prime Mover/Driver and Rotating Equipment are aligned when the hardware is cold and the hardware thermally expands and is exposed to Vibration from other sources while functioning at operating temperature and pressure. 

Misalignment Tolerance is defined as the amount in millimeters that the Shafts can be misaligned, yet still allow the Prime Mover/Driver and Rotating Equipment to operate with a good Service Factor.

The two types of Shaft Misalignment are:

• Offset/Parallel Misalignment.
• Angular Misalignment.

 

There are many types of Shaft Couplings.

The Flexible Coupling tolerates Offset/Parallel and Angular Misalignment without creating excess Vibration that would result in the catastrophic failure of the Prime Mover/Driver-Rotating Equipment hardware.

©2018 PTOA Segment 0185
PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Mover/Driver of Rotating Equipment Extension Study

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