Pump CouplingsBy carefully selecting, installing and maintainingcouplings, you can save substantial dollars inreduced maintenance costs and downtime.

Avariety of flexible couplingsare used on pumps. Yet, onlya small number of specifica-tions and requirements exist

for these coupling types. TheAmerican Petroleum Industry (API)addresses a few of these require-ments in one of its specifications,API 610, Centrifugal Pumps forGeneral Refinery Service (SeventhEdition). This specification states thatcouplings must have an alignmentdiameter concentric to the bore with-in 0.003 TIR” to help with alignment,and that couplings must meet bal-ance requirements of AGMA 515,Class 8. There are not many otherindustry standards for pump applica-tions that specify additional require-m e n t s f o r c o u p l i n g s . Moreimportantly, these specifications andrequirements do not explain howcouplings work or help in the selec-tion process.

FUNCTIONS OF FLEXIBLE COUPLINGS Flexible couplings join two

pieces of rotating equipment whilepermitting some degree of misalign-ment or end movement or both. Thethree basic functions of a flexible cou-pling are to (Figure 1):

1. Transmit power

2. Accommodate misalignment

3 . Compensate for end movement

In general, the power lost by aflexible coupling is small, although

Mechanical element coupling: grid type, horizontal split cover

I :

54 MARCH1994 PUMPS AND SYSTEMS MAGAZINE

donut, block, jaw, urethane tire,corded tire, shear donut, disc anddiaphragm.

GEAR COUPLINGS

Gear type couplings consist oftwo hubs with external teeth thatengage internal teeth on a two- orone-piece sleeve. The teeth may bestraight or curved (crowned). For anapplication requiring over l/4 degree,curved teeth may be better. Thesecouplings obtain their flexibilitybecause of the looseness (backlash)between the mating teeth. Gear cou-plings are used for medium and largepump applications, and are probablythe most power dense type available.They require periodic lubrication,every l-2 years, depending on dutyand type of lubrication. If properlymaintained (good lubrication and rea-sonable alignment), these couplingshave a service life of 3-5 years, and inmany cases 5-10 years.

Some gear couplings have sleevesthat are made of plastic (nylon, orhigh molecular plastic) and do notrequire lubrication. These have muchlower torque capacities than the all-steel couplings and are used mainlyon small pumps.

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Mechanical element coupling: gear type,crowned teeth

GRID COUPLINGSGrid type couplings are similar to

gear couplings. Composed of allmetal, they have some degree ofresilience. These couplings can damp-en vibration and reduce peak orshock loads by 10-30%. They havetwo hubs with serrations (grooves)rather than teeth. The grooves areconnected by a steel grid. The covers,which keep the lubrication in, areeither vertically or horizontally split.These couplings do not transmit asmuch power (per the same outsidediameter) as gear couplings but areusually less costly. Grid couplings areused for medium and small pumpapplications.

These couplings were originallydesigned for torsional dampings (soft-ness). They are often misapplied inpumps because of their low cost, andwill fail immediately with loss ofpacked lubricants.

COMPRESSION DONUT COUPLINGSCompression donut couplings

have a precompressed elastomericdonut. Screws force the donut to asmaller diameter. All legs of thedonut are in compression before theload is applied. Low torsional and lat-

Mechanical element coupling; gear type, straight teeth

era1 stiffness is achieved with thisdesign. Medium and some smallpumps incorporate these couplmgs.

BLOCK COUPLINGSBlock couplings use rubber in

compression. The rubber blocks areinstalled in cavities formed by inter-nal sleeve blades, external hub bladesand two end plates. This type isunique among couplings due to its“fail-safe” feature. If the elastomericfails, the coupling may run for some-time on the metal blades. These cou-plings can provide up to l/2 degree ofmisalignment and parallel off set capa-bilities of 1/64-1/4 inch. Largepumps use these type couplings toreduce vibratory torques or becauseit is torsionally soft and is used totune a system. Sometimes mediumand small pumps will use these cou-plings because if properly aligned,they require no maintenance (exceptfor replacement of blocks every 3-5years).

JAW COUPLINGSJaw couplings have their elas-

tomers in compression. The flex ele-ment can be one piece or split tofacilitate replacement. They also have

MARCH1994 59

a “fail-safe” feature. Flex elements aremade of many types of elastomericmaterials, such as, rubber and ure-thane. The properties (that is, hard-ness, resilience, etc.) can be varied tosuit required loads. These couplingsare used primarily to accommodatemisalignment and transmit power.Small and medium pumps employthese couplings. Be careful, sincesometimes these couplings are usedto absorb energy and dampen loads,particularly when equipment isengine driven. Also be carefulbecause they are torsionally soft andin the past were used to tune a sys-tem out of a torsional critical mode.

URETHANE TIRE COUPLINGSCommon on small pumps, ure-

thane tire couplings have their elas-tomeric elements in shear. The tiresare split to enable easy assemblywithout removing hubs. Urethanetire couplings offer a high degree of

flexibility, When thecoupling fails, usuallyonly the elastomericelement is replaced.

CORDED TIRECOUPllNGS

Corded tire cou-plings also have theirelastomeric elementsin shear. They use areinforced element(similar to belted autotires). Because of thereinforcement in theelement, the torquecapacity is greater peroutside diameter thanthe urethane tire cou-pling. Most small andsome medium pump Metallic element coupling; disc type,applications use these continuous disc

SHEAR DONUT COUPLiNGS(UNCLAMPED)

Another typec o m m o n o n s m a l lpumps, shear donutcouplings have theirelastomeric elementsin shear. They pro-vide low torsionalstiffness and low reac-tionary forces. Over-sizing can lead topremature failure bywearing the engagingteeth rather than amaterial failure fromflexing.

OISC COUPllNGS

can and has been optimized for itscharacteristics. These couplings arecomposed of all metal, and do notrequire lubrication. The discs are usu-ally continuous but can be individuallinks. Most disc couplings use multi-ple thin discs rather than one thickdisc/link because the stresses frommisalignment are proportional to Ntsversus T^3. These couplings are usedin medium pump applications. Ifproperly installed and misalignmentis kept under 1/4 degree, this typewill provide a long life with littlemaintenance. If the misalignmentincreases beyond 1/2 degree duringoperation, the flexible element willprobably fail in fatigue.

DlAPHRAGM COUPLlNGSDisc couplings

transmit torque by asimple tensile forcebetween alternating

d r i v i n g a n d d r i v e n bolts on a common bolt circle. Misalign- ment is accomplished from the flexibility that comes from the length of material between the bol ts . Disc couplings have

b e e n a r o u n d f o r H o w T o S e l e c t C o u p l i n g s

Diaphragm couplings transmittorque between the inside and out-side diameters of the flex element.The diaphragm can be constant or ofvariable thickness, and singular ormultiple. The diaphragm shapes areusually modified to increase flexibili-ty (such as, tapered, convoluted, andcut out profiles). These couplings canusually handle up to l/2 degree ofmisalignment. Large, critical pumpapplications rely on these couplings.

HHOWDlAPHRAGM COUPLlNGSTOSELECHTCOUPllNGSyears but with the use When selecting a coupling howof Finite Element do you know which one is best? TheAnalysis, this type chart in Table 1 shows there is no

Elastomeric element coupling; compressiontype, compression donut

62 MARCH1994 PUMPS AND SYSTEMS MAGAZINE

Metallic element coupling: diaphragm type, multiple

6. Stiffness requirements

7 . Bearing load restrictions

8 . Potential excitation or critical fre-quencies

If you change the coupling,make sure that all maintenance per-sonnel understand the requirements.For example, changing from a gearcoupling to a disc coupling willrequire different instructions for set-ting a motor on its magnetic center.Alignment procedures also differ.Review all coupling changes toensure success.

INSTALLATION CONSIDERATIONS

Most coupling failures occur rela-tively soon after installation or after acoupling has been reassembled onthe equipment. Usually failures occurbecause the installer didn’t read andfollow the manufacturers instruc-t ions. To achieve success, theinstaller needs to read and followthose instructions!

Things not to do, or a sure wayto increase failures:

1. The coupling doesn’t fit on theshaft. Get out the torch and heatthe “H” out of the hub, or openup the bore until the couplingslides on.

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2. The keys are too tight. Grindthem until they fall in.

3 . You lost a bolt. Take one fromthe maintenance department’sbolt can-any one will do.

4 . These bolts are snug enough.

5. The pump or motor is unbal-anced. Let’s rebalance and take itall out of the coupling.

6 . Part of the coupling looks bad.Replace the bad part with anoth-er part that you took out lastyear.

7 . Put the same grease in the cou-pling, like you do in your bear-ings.

8. Don’t check alignment. Thepump has been running fine for2 years.

9 . You request a coupling to fit onyour 3” shaft and with one l/2”keyway. You supply no otherinformation.

10. You buy another coupling typebecause it costs 30% less thanthe current one.

continuad