chapter 17 couplings and introduction to straight-edge

Chapter 17 Couplings and

Introduction to Straight-Edge

Alignment

1MMC-140 Mechanical Drive Maintenance

“This coupling together of science with

international peace is, I think, particularly

significant.”

- Irving Langmuir (stupid commie)

Function of a Coupling

◼ A shaft is used to transmit

power between two points.

◼ A coupling is used to

connect two shafts.

◼ Couplings connect two

shafts or more shafts to

create one long shaft.

◼ Couplings reduce the

damage to equipment

when their respective

shafts are misaligned


Shaft Alignment

◼ You MUST also consider operating temperature, so

called “hot alignment”, when performing shaft alignments

◼ There are four main types of procedures for shaft

alignment:

❑ Straight Edge

❑ Dial Indicator

❑ Reverse Dial Indicator

❑ Laser

MMC-140 Mechanical Drive Maintenance 3

Shaft Alignment

◼ There are five checks and corrections for shaft

alignment:

❑ Vertical Parallel

❑ Horizontal Parallel

❑ Vertical Angular

❑ Horizontal Angular

❑ Coupling Gap

◼ The two vertical alignment checks are done first because

they require shims to be added to the motor's feet. Doing

this last would upset the horizontal alignments.

◼ Alignment is normally done by moving the driver

equipment and not the driven equipment


Vertical Parallel

◼ Vertical parallel alignment means to make the height of

the two shafts the same.

◼ To correct for vertical parallel misalignment, raise or

lower the entire motor. Do this by adding or removing

shims equally on all four motor feet.


Horizontal Parallel

◼ To correct for the horizontal parallel misalignment, loosen

the motor foot mounts and move all four feet an equal

amount.

◼ Correcting horizontal parallel alignment often upsets the

horizontal angular alignment, so repeat horizontal angular

alignment and horizontal parallel alignment procedures

until the measurements are within the tolerances before

tightening the bolts.


Vertical Angular

◼ To correct the vertical angular misalignment, add shims

to the front or back of the motor, depending on the

location of the misalignment


Horizontal Angular

◼ To correct the horizontal angular misalignment, loosen

the motor foot mount and slightly turn it in the direction

that corrects the misalignment.


Perform Straight-Edge Alignment

◼ Step 1 – Pre-Alignment Steps

◼ Determine which device is going to be moved and

which will remain stationary. Normally the driver device

is the Machine to be Moved (MTBM).

◼ Check both devices for initial soft foot.

◼ Mount driver and driven equipment and tighten bolts.

◼ Check for final soft foot.

◼ Check for run-out and end float.

◼ Level both shafts.



◼ Step 1 – Pre-Alignment Steps (cont’d)

◼ Clean the coupling of dirt or grease and mount on

shaft.

◼ Move the couplings to where the coupling gap is the

amount recommended by the manufacturer.

◼ Tighten the mounting bolts of both machines.



◼ Step 2 – Perform Vertical Angular Alignment


◼ Mark coupling halves with

chalk or ink. Rotate to 0

deg. Measure gap with

feeler gage.

◼ Rotate to 180 deg.

Measure gap with feeler

gage.

◼ Shim as necessary to

obtain equal gap.


◼ Step 3 – Perform Vertical Parallel Alignment


◼ Rotate coupling halves to 0

deg. Measure offset with

straight edge and feeler gage.

◼ Rotate to 180 deg. Measure

offset with straight edge and

feeler gage.

◼ If offsets are same, shim all

feet equally.

◼ If offsets are different, take the

average and shim using this

value.


◼ Step 4 –Perform Horiz. Angular Alignment and Set the Coupling Gap



deg. (as you look down the

driver shaft) and use steel rule

or feeler gage to measure the

gap.

◼ Loosen MTBM mounting bolts

and move to adjust the gap to

proper specification.


deg. and use steel rule or feeler

gage to measure the gap.


◼ Step 4 –Perform Horiz. Angular Alignment and Set the Coupling Gap


◼ Adjust the position of the motor

so the gap is the same at both

90 and 270 degrees.


◼ Step 6 – Perform Horizontal Parallel Alignment


◼ Use straight edge and feeler

gage to measure the

misalignment at the 90 & 270

degree positions.

◼ Carefully bump or move the

side of the motor without

losing the angular alignment

until the measurements at 90

& 270 are the same or zero.

Coupling Gap

◼ The coupling gap is the distance between the two

coupling hubs. Set the coupling gap to the coupling

manufacturer's specification.

❑ This specification is designed to permit the coupling to assemble

correctly.

❑ Simply move the motor forward or back.


Four Categories of Mechanical Couplings

◼ There are four general categories of mechanical

couplings:

❑ Rigid Couplings

❑ Flexible Couplings

❑ Universal Joints

❑ Clutches


Rigid Couplings

◼ Rigid couplings couple two shafts together rigidly so that

the shafts act as a single continuous assembly. They

extend the length of a shaft in applications that need

very long shaft lengths.


◼ They do not allow for

misalignment.

◼ Rigid couplings

sometimes connect

motors to pumps.

❑ This is not recommended

because any misalignment

will cause bearings and

seals to wear out quickly.Flange coupling

Flexible Couplings

◼ Flexible couplings connect two shafts together and allow

for some misalignment.

◼ In general, flexible couplings consist of two hubs and

some type of flexible component that connects the two

hubs together.

◼ Flexible couplings are used in applications that require

two independently supported coaxial shafts to be

coupled together.

◼ The flexing component of the couplings may be

elastomeric or metallic.


Flexible Metallic Couplings

◼ Beam, Helical, Bellows Coupling


❑ Used for smaller power

applications where angular

flexibility is needed but it is

important to have no

torsional flexibility.

Grid Coupling

◼ Grid Coupling

❑ The grid coupling uses a metal grid that is inserted into grooves

in the two coupling halves.

❑ It is a low-cost metal coupling that is torsionally soft, allowing it to

accept shock loads and dampen vibration.


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◼ Chain Coupling

❑ The chain coupling uses a double strand roller chain to connect

the two hubs.

❑ It and the grid coupling are the two lowest cost metal couplings.

❑ The chain coupling is more torsionally stiff than the grid coupling.


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◼ Gear Coupling

❑ The gear teeth that are coupled together by an internally geared

sleeve.

❑ The metallic version coupling is more expensive than either the

grid or the chain coupling but it is available in much larger power

sizes.

❑ Gear couplings have better balance for high speed operation and

are torsionally rigid.

❑ They require internal lubrication

◼ Except for the nylon sleeve coupling


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Flexible Jaw Coupling

◼ The flexible jaw coupling is a flexible coupling which

uses a rubber-like insert called a spider to connect two

hubs.

◼ Each hub has jaws that mesh with the spider. As the

driver coupling half begins to rotate, the jaws press on

the spider which, in turn, presses on the jaws of the

driven coupling half.


Flexible Jaw Coupling

◼ This belongs to the elastomeric family of couplings – use

a rubber-like elements to separate the two coupling

halves.


◼ Hubs are made of

aluminum, cast iron, or

steel. Mounted with a key

or bushing.

◼ Spider made of rubber,

urethane, or metal.

◼ Allows for some misalignment due to rubber spider. Low

or medium power applications.


◼ Disc Coupling

❑ The metal disc coupling uses a stack of thin

metal discs as a connecting element

between the two coupling halves.

❑ It is the most expensive of the metal

couplings but it can handle even higher

loads and speeds

❑ The disc coupling is torsionally very rigid

but has angular flexibility.

❑ Another advantage of a disc coupling over

some metallic couplings discussed is it

does not require lubrication.


Flexible Elastomeric Couplings

◼ The flexible jaw coupling is a type of flexible coupling

that uses a rubber-like insert called a spider to connect

the two hubs.

◼ The hubs of a jaw coupling are constructed of aluminum,

cast iron, or steel, depending on the power rating.

◼ They can be mounted with either a key fastener or

bushing.

27

◼ The advantage of this type

of coupling is that it allows

more misalignment than

most flexible couplings

because of the elastic

properties of the spider.

MMC-140 Mechanical Drive Maintenance

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◼ Elastomeric Sleeve Coupling



◼ Tire/Donut Coupling


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Internal Elastomer Coupling


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External Elastomer Coupling


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Schmidt Coupling


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Universal / Cardan Joint Couplings


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Constant Velocity Joint Couplings


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Clutches

◼ Clutches are designed to allow two turning shafts to

connect and disconnect from each other.

◼ Clutches are used to start machines in an, unloaded

condition, prevent reverse rotation, and act as a safety

device if the shaft torque overloads.


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What To Consider When Selecting Pump Couplings

◼ Horsepower & operating speed - HP and speed are

used to calculate torque. Every coupling will have a

maximum torque rating and max HP/100 RPM rating.

These maximum ratings will depend on the

manufacturer and type of coupling. HP and speed are

required to calculate these values and size the coupling.

◼ Ambient Temperature - All materials used in couplings

will have some sort of min/max temperature limits. As

long as your selection is within that materials limits, you

should be fine, but you should consider that thermal

expansion/contraction on the components that you are

coupling together can occasionally play a factor.


What To Consider When Selecting Pump Couplings

◼ Space Limitations - Each coupling will be able to accept

a specific range of distance between shaft ends

(“DBSE”). Depending on the pump (or other equipment)

design, it may be beneficial (or required) to increase or

decrease that distance.

◼ Angular / Offset Misalignment & Axial Travel - Each

coupling will have limitations on how much misalignment

it can take (based on type, mfg., etc.) Too much

misalignment will cause premature coupling failure


chapter 17 couplings and introduction to straight-edge

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