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Troubleshooting Power Transmission Couplings

Introduction

Power transmission couplings are used to connect two shafts that turn in the same

direction on the same centerline. There are three principle types of couplings; rigid,

flexible and special purpose.

Rigid couplings are used in applications where misalignment is not a factor,

where flexibility is not required and where the coupling is not required to absorb load

shocks or torque changes. Rigid couplings connect shafts using bolted flanges, keyed

sleeves or ribbed clamps bolted together over the shaft ends with keyways. Rigid

couplings are used primarily for vertical drive systems. Lubrication is not required, but

larger couplings, or those running at high speeds, may require balancing to reduce

vibration.

Flexible couplings also connect two rotating shafts, but are designed to dampen

vibration, absorb some shock loading and provide some axial movement or end float of

the shafts, as well as compensate for minor misalignment.

There are three fundamental categories of flexible couplings; “mechanically

flexible,” such as gear and chain couplings, “material flexible,” such as disc, spring,

diaphragm, elastomeric and bellows and “combination,” such as metallic grid couplings,

that provide a combination of mechanical and material flexibility.

Special purpose couplings include such devices as mechanically flexible “U-

joints” and “constant velocity joints” used for automobile applications, “magnetic

couplings,” such as magnet to magnet and eddy current couplings and “fluid couplings,”

such as liquid, silicone and shot filled types. Magnet and fluid couplings provide “no

contact” between drive and driven elements, offer low maintenance and both are capable

of absorbing shock loads.

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A unique special purpose type, the “Schmidt or offset” coupling is designed to

handle large parallel shaft offsets of up to 18 inches and 1,000,000 in-lbs of torque. (SEE

FIGURE 1.)

FIGURE 1. The Schmidt or offset coupling.

There are over 80 styles of flexible couplings used today in approximately 90

percent of all industrial applications. The most common are briefly described below

under their respective categories.

1. Mechanically flexible couplings, such as gear and chain couplings, provide a

flexible connection by permitting coupling components to move or slide over

each other. Some minor misalignment is provided by the clearances between gear

teeth and chain and sprocket teeth respectively, however shafts should be aligned

to less than 0.002 inches to ensure long coupling life. These coupling types

require lubrication using recommended coupling grease or EP oil. As might be

expected, approximately 75% of gear and chain coupling failures are caused by

misalignment or improper or insufficient lubrication. Some newer types of gear

and chain couplings contain nylon gear sleeves or nylon chains respectively and

these types do not require lubrication. (SEE FIGURES 2. & 3.)

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FIGURE 2.

FIGURE 3. Chain coupling – steel. Chain coupling – nylon.

2. Material flexible couplings, as the name implies, provide flexibility by

incorporating elements that accommodate a certain amount of bending or flexing.

The flexing materials that provide the connection between the coupling drive and

driven components include laminated discs, bellows, diaphrams and elastomeric

materials that may include rubber or plastics, such as neoprene and urethane.

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Generally speaking these coupling types require little maintenance other than

alignment and their service life is limited by the fatigue limit of the flexing

material itself. (SEE FIGURES 4. & 5.)

FIGURE 4. Laminated disc coupling undergoing laser alignment.

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FIGURE 5. Typical flexible coupling with elastomeric material which might be natural rubber, urethane or neoprene.

3. Combination mechanical/material flexible couplings include the very popular

grid coupling, which is a compact unit capable of transmitting high torque at

speeds up to 6,000 rpm. The construction of the coupling consists of two flanged

hubs each with specially designed grooved slots cut axially on the outer edges of

the flanges.

The flanges are connected by using a serpentine spring grid that fits the grooved

slots. The flexibility of this grid provides torsional resilience, can provide some

misalignment and end float, dampen vibration and may reduce peak or shock

loads by up to 25%. Grid type couplings require lubrication using good quality

coupling grease in the areas of the grooved slots and serpentine spring. (SEE

FIGURES 6 & 7.)

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FIGURE 6. Grid coupling. (The serpentine spring has been removed so that it does not interfere with the alignment procedure).

FIGURE 7. The wear areas of the grid coupling are the grooved slots and the serpentine spring.

Why Couplings Fail Couplings fail for several reasons, but the primary causes are improper selection

for the particular application, excessive misalignment, improper, inadequate, or

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insufficient lubrication, harsh environmental or operating conditions and excessive

speeds or loads.

1. The application factor. Following is a general guide for coupling application

and a corresponding selection recommendation. (SEE TABLE 1.)

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TABLE 1.

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FIGURE 8. Some manufacturers like Falk, provide a coupling selector guide that carries the service factor for various equipment types and provides corresponding coupling recommendations.

2. The alignment factor. There is a perception that flexible couplings can

accommodate a great deal of parallel offset and/or angular shaft misalignment.

This is untrue. Depending upon the coupling type, flexible couplings can only

accommodate from ¼ to about 2 ½ degrees of misalignment and high speed,

high load drive applications must be aligned to much closer tolerances. (SEE

FIGURE 9.)

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FIGURE 9. Types of Misalignment

Symptoms of misalignment include;

Noise at the coupling,

Powdered rubber particles or leaking lubricant directly below the coupling

(depending upon coupling type),

Process fluid and/or oil leaks at the drive, driven (or both) shafts,

Premature shaft, shaft keyway, or key fatigue or breakage,

Premature or frequent bearing or seal failure at one or both machines,

“Soft Foot” condition at foot bolts,

Broken or constantly loosening of foot bolts at one or both machines,

“Shimmering” of oil on base plates, or near the foot bolts,

High operating temperatures at or near the coupling,

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High vibration conditions, usually at both machines,

Cracked or broken foundation, particularly at or near the foot bolts,

Continuing or intermittent leaks at pipe joints caused by pipe strain,

High energy consumption,

No compensation for “thermal growth” at either drive or driven machine

during initial alignment procedure,

Settling of machine or foundation after installation.

The following chart lists acceptable misalignment tolerances based on

machine speed. (SEE FIGURE 10.)

TABLE 2.

Allowable Misalignment Tolerances

Machine Speed RPM

Parallel Offset

Misalignment (in MILS)

Angular Misalignment

(in MILS)

<500 5 1.5

500 – 1250 4 1

1250 – 2000 3 .5

2000 – 3500 2 .3

3500 – 7000 1 .25

>7000 .5 .2

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FIGURE 10. Gear coupling with spacer undergoing laser alignment. The coupling connects an electric motor driving a hammer mill.

Fluid couplings are referred to as “special purpose,” however they are also rigid

and cannot accommodate misalignment. (SEE FIGURE 11.)

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FIGURE 11. Fluid coupling undergoing a laser alignment procedure.

Alignment can also be determined initially with a dial indicator. To determine

parallel offset, mount the indicator on one hub with the point touching the outside

diameter (OD) of the other flange. Rotate the hub on which the indicator is

mounted while holding the other flange stationary. The parallel misalignment can

be determined by obtaining the minimum and maximum readings and dividing by

two. (Rotating the flange under the indicator point will not produce a measure of

parallel offset).

To determine angular misalignment, move the dial indicator point so that it is

touching the face of the other hub. Again rotate the hub on which the indicator is

mounted while holding the other stationary.

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Again note the minimum and maximum readings. Each 0.016 inch difference

between the two readings for each inch the indicator point is located from the

centerline of the shaft indicates an angular misalignment of about one (1) degree.

Another issue that may affect alignment is end float or axial shaft movement.

There is a tendency for shafts to be pulled closer to their own machine as soon as

they are started. As shafts tend to separate at startup, axial end float tries to pull

the coupling apart and allowable end float should not exceed ¼ inch.

1. The lubrication factor. There are three flexible coupling types that require

lubrication. These are gear and chain “mechanically flexible” couplings and

“combination” mechanical and material flexible grid couplings. The lubricants

used for gear and grid couplings may include; ISO 460 compounded oils with

tackiness additives for low speeds and centrifugal G forces of up to 8000 or

ISO 100 rust and oxidation inhibited anti-wear oils for high speeds, centrifugal

G forces of over 8000 and low temperatures of -20°C (-4°F).

NLGI Grades 1 and 2 coupling greases containing high viscosity oil fortified

with rust and oxidation inhibitors, extreme pressure additives and tackiness

agents to prevent separation at high speeds and high centrifugal G forces, may

be recommended for gear, chain and grid type couplings.

Note: Centrifugal Force “G” may be calculated from the equation:

G = 14.2 X 10 – 6 dN2

Where d = pitch diameter of the coupling in inches

And N = revolutions per minute.

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Generally, oil or grease can be used at coupling speeds of 3600 to 6000 rpm. Oil

is recommended where coupling speeds exceed 6000 rpm.

Gear and grid style couplings incorporate seals or gaskets to contain the lubricant

and prevent the entry of foreign material. These units contain access plugs for re-

lubrication which must be properly torqued after re-lubrication to ensure that

centrifugal force does not loosen the plugs and cause a serious safety hazard.

Re-lubrication and inspection intervals should be considered as preventive

maintenance tasks and carried out at least annually, (or more frequently, if

conditions such as high temperatures call for shorter intervals).

The amount of lubricant used is also important. Gear couplings, if overfilled with

lubricant, may lockup and the coupling will no longer maintain its flexibility.

(Any increase in vibration of the coupling after re-lubrication suggests

excessive lubricant). If a coupling is disassembled for re-lubrication, ensure that

the two flange halves are marked for correct reassembly and be certain to use

specified coupling flange bolts properly torqued. Install new seals or gaskets if

the old seals and gaskets have any damage, nicks or are no longer pliable.

Before reassembly of grid type couplings, carefully inspect the serpentine spring

for damage, wear or fatigue cracks and inspect the grid grooves for excessive

wear or evidence of fatigue. On chain and gear type couplings, inspect the chain,

sprocket and gear teeth for looseness, wear and evidence of fatigue. (SEE FIGURE

12.)

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FIGURE 12. Courtesy American Axle & Manufacturing. Coupling shows failed serpentine spring and grease residue that had undergone oxidation.

The old lubricant removed from the coupling should also be inspected for metal

particles indicating wear and darkening in colour suggesting oxidation.

Finally, after re-lubrication and re-installation, ensure that safety guards or mesh

screens are reinstalled and secured.

2. The operational and environmental factors. Operating conditions and

environmental considerations play a large role in the long life and reliability of

flexible couplings. Operating temperatures will affect not only the lubricant, but

can seriously shorten the life of the materials used in elastomeric couplings.

Generally, seals and coupling components cannot withstand temperatures much

above 104°C (220°F) and group I mineral base oil begins to oxidize at 71°C

(160°F). These guidelines should be understood and applied by machinery

operators and as a result, when high operating temperatures and/or where zero

back lash is required, flexible disc couplings are recommended over elastomeric

types.

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In addition, solvents, caustic process fluids and water washing practices may be

harmful to some elastomeric couplings and disc, grid or gear couplings may be

required. Wherever high loads and speeds are required, disc and gear couplings

may be the best choice.

Finally, if high temperatures are being experienced, remember that an angular

misalignment of as little as 0.001 inches can increase adjacent bearing

temperatures by as much as 18°C (65°F). The lesson is that coupling operating

conditions and environment, lubrication and alignment are all related. (SEE

TABLE 3.)

TABLE 3.

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These troubleshooting guidelines are general in nature and cover only those very

common flexible couplings described in the guide. For uniquely designed or

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special purpose couplings, contact the appropriate manufacturer for selection,

maintenance and troubleshooting recommendations.

Conclusion

Troubleshooting coupling problems is quite simple, so long as the

troubleshooter understands coupling design and application limitations and

remembers that reliable machine trains are dependent upon the couplings that

connect drive to driven equipment.

R e f e r e n c e s

Condition Monitoring Standards, Volume 1, IDCON Inc. 2001, PP 101 – 109. Industrial Machinery Repair, R. Smith and K. Mobley, 2003, Butterworth-Heinemann, PP 215 – 244. Power Transmission Handbook, 4th Edition, Power Transmission Distributors Association, 2006, PP 109–135. Coupling Standards, American Gear Manufacturers Association, AGMA 9000, 9001, 9002, 9003, 9004, 9008, 9009. The Practical Handbook of Machinery Lubrication, 3rd Edition, L. Leugner, PP 106–110, 169–173.