Chapter 17 Couplings and
Introduction to Straight-Edge
Alignment
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“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
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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
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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
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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.
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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.
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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
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Horizontal Angular
◼ To correct the horizontal angular misalignment, loosen
the motor foot mount and slightly turn it in the direction
that corrects the misalignment.
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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.
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Perform Straight-Edge Alignment
◼ 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.
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Perform Straight-Edge Alignment
◼ Step 2 – Perform Vertical Angular Alignment
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◼ 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.
Perform Straight-Edge Alignment
◼ Step 3 – Perform Vertical Parallel Alignment
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◼ 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.
Perform Straight-Edge Alignment
◼ Step 4 –Perform Horiz. Angular Alignment and Set the Coupling Gap
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◼ Rotate coupling halves to 90
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.
◼ Rotate coupling halves to 270
deg. and use steel rule or feeler
gage to measure the gap.
Perform Straight-Edge Alignment
◼ Step 4 –Perform Horiz. Angular Alignment and Set the Coupling Gap
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◼ Adjust the position of the motor
so the gap is the same at both
90 and 270 degrees.
Perform Straight-Edge Alignment
◼ Step 6 – Perform Horizontal Parallel Alignment
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◼ 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.
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Four Categories of Mechanical Couplings
◼ There are four general categories of mechanical
couplings:
❑ Rigid Couplings
❑ Flexible Couplings
❑ Universal Joints
❑ Clutches
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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.
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◼ 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.
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Flexible Metallic Couplings
◼ Beam, Helical, Bellows Coupling
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❑ 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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Flexible Metallic Couplings
◼ 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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Flexible Metallic Couplings
◼ 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.
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Flexible Jaw Coupling
◼ This belongs to the elastomeric family of couplings – use
a rubber-like elements to separate the two coupling
halves.
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◼ 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.
Flexible Metallic Couplings
◼ 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.
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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.
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◼ 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.
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Flexible Elastomeric Couplings
◼ Elastomeric Sleeve Coupling
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Flexible Elastomeric Couplings
◼ Tire/Donut Coupling
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Internal Elastomer Coupling
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External Elastomer Coupling
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Flexible Elastomeric Couplings
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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.
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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
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