0262 1762/05 © 2005 Elsevier Ltd. All rights reserved WORLD PUMPS June 200544

Like any other energy-expendingmechanical device, pumps needa power source to drive them.

They require mechanical energy inorder to carry out the task of movingliquids from one place to another andthat energy is provided either by aninternal combustion engine or anelectric motor. The power sourcedrives the pump through a powertransmission system, which is usuallymade up of an arrangement ofgearboxes, drive-shafts and couplings.That might all seem ratherstraightforward, but there are inherentdemons in the system that need to betamed if the pump is to be drivensafely and efficiently. The first of thesedemons takes the form of a ratherinsidious phenomenon known astorsional vibration. Unless it is dealtwith effectively torsional vibrationcan wreck a power transmissionsystem and the pump it is driving toboot. It can literally shatter solid steeldrive shafts, cause pumps to wrenchthemselves loose from their mountingsand possibly even pose an injury threatto anyone in the vicinity. So what istorsional vibration and how do wedeal with it?

In the early part of the twentiethcentury torsional vibration was a

power transmission engineer’s worstnightmare. It was responsible forplane crashes, for cars breaking downand for an epidemic of failures indrive systems across industry. It iscaused by the almost imperceptiblepulses in torque that are inherent toan internal combustion engine. Eachtime the compressed fuel-and-airmixture ignites it rapidly expandsdriving a piston down a cylinder onits power stroke. Each power strokeproduces a pulse or peak in torquethat causes an indiscernible twistingof the drive shaft.

One would think that something asstrong as a steel drive shaft would nottwist significantly but any piece ofmetal will deflect when a force isapplied, and, when large amounts ofpower are generated the forcesinvolved can be colossal. What'smore, the effects of torsional vibrationcan be amplified by another demon inthe system, a related phenomenonknown as torsional resonance.

If we consider the dynamics of a dieseldriven pumping system each dieselengine has its own natural resonance,a bit like the note of a ringing bell orthe sound of a vibrating guitar string.If this torsional resonance coincideswith the natural frequency of thepump then the results can becatastrophic or, at the very least, willsignificantly reduce the lifetime of thesystem and necessitate an increasedlevel of maintenance.

During the first quarter of thetwentieth century this torsionalvibration was unobservable to thecrude instrumentation that wasavailable at the time because the

twisting vibration of the drive shaftwas superposed over the steadyaverage rate of rotation. Anobservation made by Professor EdwardMiller, one time head of theMechanical Engineering departmentat MIT, demonstrated the sheerdestructive power of torsionalvibration. During the initial testing ofa pump driven by a diesel engine henoted that the shaft connecting theengine to the pump began glowing‘cherry red’ before it failed completely.This phenomenal amount ofdestructive energy is simply the resultof torsional vibration that at the timewas virtually undetectable until failureoccurred.

Engineering knowledge has advancedconsiderably since then and torsionalvibration is no longer the mystery itonce was. Manufacturers providedetailed information on the torsionalresonance of each engine and designengineers are able to ensure that levelsof torsional vibration in a systemremain within acceptable limits. Themodern solution is to fit a flexible, ortorsionally soft, coupling in-betweenthe diesel engine and the drive shaft toisolate the engine’s harmonics fromthe rest of the system. There are twotypes of flexible coupling that arecommonly used for this purpose, oneknown as rubber-in-compression andthe other as rubber-in-shear.

It was Louis Croset, while working forW C Holmes, who invented rubber-in-compression couplings as a way ofproviding the damping and tuningproperties necessary to protect thecomponents of a diesel driven powertransmission system. Croset took the‘Hol’ from Holmes and the ‘set’ from

Taming vibration demons withflexible couplings Unless it is dealt with effectively, torsional vibration can wreck a power transmission systemand the pump it is driving. Alan Dean of Renold Hi-Tec Couplings examins the problem andhighlights the advantages of of rubber-in-compression couplings in providing a solution.

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Figure 1. Anexploded view of arubber-in-compressioncoupling showing itsconstruction and therubber blocks that fitin-between themetal paddles.

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his own name and founded HolsetEngineering in 1952. The company’srubber-in-compression couplings werean immediate success and it later wenton to develop a range ofturbochargers. Its coupling division

was eventually acquired by Renold in1996 and since then the company hastraded under the name of Renold Hi-Tec Couplings (RHC) and hasbenefited significantly from newinvestment.

The basic principle behind Croset’sinnovation is essentially the sametoday as it was then, if perhaps just alittle more sophisticated. Essentially,two round, metal sections fit oneinside the other with what looks likethe paddles of a paddle steamerprojecting inwards from the outersection and outwards from the inner.Rubber blocks are placed in the spacesin-between the paddles, and, as theouter section is turned by the engine,it drives the inner section through therubber blocks. As this happens therubber is compressed, hence the term‘rubber-in-compression’.

When rubber is loaded in compressionit behaves as an incompressible fluidand is inherently much stronger thanthe alternative technology of rubber-in-shear, or tension, where theslightest scratch in the rubber can leadto catastrophic tears and completecoupling failure. RHC's rubber-in-compression couplings areintrinsically failsafe because even ifthe impossible happened and therubber blocks vaporized, the strongmetal paddles would come together toprovide the drive. In a more likelyscenario where the rubber blocks werecut or torn, perhaps as a result of highshock loading, then the couplingwould continue to operate with littleloss of efficiency. In manyapplications, such as fire fighting,

pump failure is not an option andrubber-in-compression couplings areoften specified because of their failsafequalities.

Rubber-in-compression couplings arealso maintenance free and do notrequire lubrication or adjustment ofany kind, which means that theyprovide the lowest lifetime cost on allpump applications. The onlyserviceable items are the rubberblocks, which in most cases are goodfor over 10 years of service. Rubber-in-compression couplings are alsovery robust and ideally suited to morearduous applications where largeparticulates such as gravel or sand arebeing pumped. Arduous applicationstend to produce stronger vibrationsand the rubber blocks in the couplingdampen the shock loads and reducethe vibratory torque in the drivetrain.

An alternative to diesel-drivensystems is the variable frequencydrive (VFD,) which is becomingmore common in pumpingapplications. The use of a VFDallows more economical use of themotor, but, by its very nature, itproduces a pulsating output, whichagain requires a flexible coupling tocontrol torsional vibration. Therubber in the flexible couplingdampens the pulses and changes thenatural frequency of the drive line sothat it is below the operating speedrange of the pump.

Flexible couplings can also be used toaccept varying degrees ofmisalignment. For applications inside

Figure 2a and Figure 2b..Rubber-in-compression couplings, which are commonly used onpumping systems to solve and prevent problems occuring. The cover has been removedfrom these couplings so that the rubber blocks can be seen in position. When the cover isfitted they are totally enclosed.

Figure. 3. A universal joint coupling for use where there ismisalignment between the pump and the diesel engine.

Figure. 4. A typicaltorsional vibrationchart.

Figure. 5. A rubber-in-compressioncoupling fitted in-between a dieselengine and acentrifugal pump.

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bell housings the coupling only hasto accept the small amounts ofmisalignment caused bymanufacturing tolerances. However,for free-standing applications thedegree of misalignment depends onhow accurately the pump is alignedto the prime mover. Quite frequentlythe pump and prime mover aresecured on anti-vibration mounts, abit like a car's suspension system, andvarying degrees of misalignment willoccur depending on the amount ofmovement on the mounts. Theselevels of misalignment can beaccommodated by using two flexiblecouplings with a shaft between themso that the shaft can articulate as thepump and motor move in relation toeach other.

When very soft anti vibrationmounts are used or when the pumpand prime mover are on differentbase plates there may be as much as50 mm or more of misalignment. Inthese cases it may be necessary touse universal-joint-shaft-couplingsand a universal-joint shaft.Universal-joint shafts put varyingdegrees of sinusoidal loads into thecoupling, depending on the angleof operation, and the higher theangle the larger the load.Universal-joint shaft couplings aredesigned with radial and axialbearings to accept the sinusoidalloads from the universal-jointshaft.

In some pumping applications thedrive train in-between the pumpand the prime mover is so long thatany increase in temperature willcause the drive train to expand inlength and transmit axial forces tothe adjacent equipment. Becausethe rubber elements of a rubber-in-compression coupling are notbonded to the metal parts of thecoupling the inner member willmove relative to the outer afterapproximately 0.5mm ofexpansion. This releases the axialload and therefore the axial forcetransmitted will never be above 0.5times the coupling's axial stiffness.

Every pumping system is differentbut common to all of them is theneed for safe operation, reliabilityand the lowest possible maintenancecosts. Rubber-in-compressioncouplings meet all of thesechallenges and can be found onpumping applications all over theworld. Each system is unique thoughand RHC’s engineers use their ownsoftware to analyse the dynamics of asystem before recommending thecorrect coupling solution. ■

CONTACT

Alan Dean, Renold Hi-Tec Couplings 112 Parkinson Lane Halifax HX1 3QHTel: 01422 255103e-mail: alan.dean@renold.comwww.renold.com

Figure 6. A rubber-in-compression coupling fitted in-between an electric motor and apump.