Shops o f t en ove r look v ib ra t ion i s sue s when in -s ta l l ing machine too l s or equ ipment . However,p l a n n i n g f o r v i b r a t i o n c o n t r o l , o p t i m a l l y b e -f o r e c o n s t r u c t i n g a n e w f a c i l i t y o r d u r i n g r e -

de s i gn o f an ex i s t ing one , can save shops a lo t o fm o n e y i n t h e l o n g r u n . T h a t ’s b e c a u s e i s o l a t i n gmach ine s and p roce s s e s f rom unwanted v ib ra t ionreduce s fu tu re p rob l ems such a s bad par t - su r facef in i she s , pa r t s ou t o f to l e rance , and even phys io -logica l damage to shop personnel .

According to Fabreeka International Inc., Stoughton,Mass., all structures, including machine tools and otherequipment, vibrate, or oscillate, when displaced from

MACHINEINSTALLATION

Isolationsystemsoptimizemanufacturingprocesses.

By Leslie Gordon,associate editor

ISOLATING UNWANTED

VIBRATION

Large machines typicallyrequire attaching theirframes or beds to a concretefoundation, often called areaction or inertia mass.

Large machines typicallyrequire attaching theirframes or beds to a concretefoundation, often called areaction or inertia mass.

Reprinted from AMERICAN MACHINIST May 2005

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An example from the Fabreekafiles includes a company with an

industrial fan transmitting vibrationinto its floor support, felt by officepersonnel on the same floor. Thevibration was not severe, butpersonnel found it annoying.

the i r equi l ibr ium ( s ta t i c )pos i t ion and cont inue v i -brating naturally until dis-s ipating al l the energy re-c e i v ed . Vib ra t i on i sexpressed in frequency, ornumber of oscillations/unitof t ime. The un i t —cyc l e s / s e c — i s c a l l ed aHertz (Hz).

Ev e r y phy s i c a l s y s t empossesses a natural vibra-t ion - f r equency proper ty.For some, such as a slab ofsteel, the natural frequencyis high, and for others, suchas rubber, it is low. An iso-lator, which suppresses un-wanted vibration, also hasa damping property that de-creases the frequency am-p l i t ude o f an o s c i l l a t i ngsystem.

Unwantedvibration

Machines and equipmentcan be either the source orthe recipient of unwanted vi-bration and both may requireisolation. For example, rotat-ing, reciprocating, and im-pacting equipment all createunwanted machine-induced vi-bration and shock, a tran-s i en t cond i t ion where an

MACHINEINSTALLATION

After a survey, Fabreeka hadinformation that included:

- Fan weight: 14,000 lb,uniformly distributed.

- Fan speed: 1,800 rpm (30 Hz).- The fan is near a floor column

support, meaning the support isstiff.

- There are no adverseenvironmental conditions.

- The fan is skid-mounted andanchored to the floor.

After calculations, the solutioninvolved placing isolators at theanchor-bolt locations. Sincevibration disturbance was notsevere, only annoying, a 75%reduction would suffice. Fabreeka specialists calculated the transmissibility ratio and used an isolator with a 13.39-Hznatural frequency.

REDUCINGINDUSTRIAL-FANVIBRATION

Equipment requiring isolation is oftenthe source of unwanted vibration...

...or the recipient of it.

applied force suddenly dis-rup t s a s y s t em ’s equ i l i b -rium. This vibration trans-m i t s t o t h e mach in e ’ssupport ing f loor-s lab andthe soil underneath. Here,using isolation reduces thevibration transmitted to thefloor.

On the other hand, preci-s i on mach in e t oo l s andCMMs require protect ionfrom vibration. With them,isolat ion systems keep vi-bration within acceptablelimits to maintain machineper formance and ach ievedesired finishes, tolerances,and accuracies.

Machines can a lso bothc r ea t e and r e c e i v e un -wan t ed v i b r a t i on . Fo r example, a surface grindertypical ly requires protec-t ion from floor vibration.Howeve r, t h e g r i nde r ’sheavy table reversing whilein operation also produceslarge dynamic forces thatcan disturb nearby equip-ment.

Fabreeka says isolation isnot typically required for less-sensitive machines. But pro-tection i s c r i t i ca l when i tcomes to b ig inves tmentssuch as ultra precise and ac-curate equipment , or ma-ch in e s w i th l ong b e d s ,wh ich r equ i r e anchor ingand aligning.

Robert Haley, engineering manager at Fabreeka, ex-plains, “To better understand

A standard pneumatic isolator has anatural frequency of approximately 2.5 Hz. These isolators require a source of clean, dry gas with pressures from 60 to 120 psi.

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vibration isolation and damp-ing, picture a car. Its chassisrests on leaf spr ings . I f youtravel down a road withoutshocks and hit a pothole, thewho l e c a r o s c i l l a t e s a t t h ena tu ra l f r equency o f t h esprings for many cycles untildissipating the energy and thecar stops. However, when youpu t shock ab so rbe r s on ,which are really dampers, thecar oscillates at the same fre-quency, but only for maybeone cycle and at a much loweramplitude.”

According to Fabreeka, thebiggest sources of unwantedvibration are machines gener-ating pulses or impacts, suchas injection molders, impacttes ters , hammers , s tampingpresses , centr i fugal pumps,and compressors, which typi-c a l l y s end s e v e r e dynami cforces to the floor.

Achieving isolationShops achieve isolation by

placing an isolator, or elasticelement, between the unit vi-brating and its support struc-ture. Francis J. Andrews, P.E.,explains, “A vibration isola-tor acts as a mechanical filter.Isolator efficiency varies withits natural frequency, which isboth a function of the isolator

MACHINEINSTALLATION

A T-shaped foundation puts the isolatorin the same horizontal plane as thecombined center of gravity of themachine and the foundation to eliminatemotion.

stiffness and the mass beingsupported.”

All vibration isolators areessentially springs with theadded element of damping.In some cases , the spr ingand damper are separate ,such as a coil spring isola-t o r u s ed w i th a v i s cou sdampe r. Mos t i s o l a to r s ,however, incorporate bothin one unit.

I n add i t i on t o sp r i ng s ,other types of isolators in-clude rubber; mats of vari-ous materials such as felt ,r ubbe r, and co rk ; me t a lcoi ls ; air bags; pneumaticc y l i nde r s ; and conc r e t e“floating” foundations. Airi so lators y ie ld the lowestna tu ra l f r equency, w i thsteel springs next, followedby e l a s tomer (na tu ra l o rsynthetic rubber) pads.

Unfortunately, just plac-ing a rubber mat under acompressor, for example ,doe sn ’t a lway s work . I nf a c t , i t c an d rama t i c a l l yamplify vibration transmit-ted to the floor. What hap-pen s i s t h e na tu ra l f r e -quency o f t h e i s o l a torcoincides exactly (resonates)with the compressor’s drivingfrequency . The resu l tantincrease in vibration ampli-tude is limited only by theamount of damping presentin the isolation system.

Th i s i s why Fab r e ekastresses that designing a vi-bration-isolation system is

not a do-it-yourself project.An expert’s skill lies in select-ing the proper system basedon i t s na tu ra l f r equency,which involves determiningapplication variables such astransmissibil ity, or the ratioof output vibration to inputvibration.

An effective isolator for anapplication has a natural fre-quency well below the appli-cation’s input vibration. If thetransmissibility ratio is muchgreater than one, vibration isamplified, whereas if the ratiois less than this, vibration isreduced. Vibration isolationfor any isolator begins at a ra-tio of 1.414.

Small machines are typicallymounted directly to isolators.Larger machines require at-taching their f rames or bedsto proper ly designed founda-t i on s , a l s o c a l l ed r e a c t i onmasses or inertia blocks. Ac-cording to Haley, it takes a lotmore force to move a machinejoined to a concrete block at-tached to springs than it doesto move a machine attachedonly to springs.

Some equipment manufac-turers provide allowable-vi-b r a t i on sp e c i f i c a t i on s f o rth e i r mach in e s , bu t mos tdon’t, which makes choosingan isolator system diff icult .The key is knowing the ampli-

An elastomeric isolator consists of aneoprene-elastomer vulcanize-bondedto steel components, giving it stiffnessin all directions.

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MACHINEINSTALLATION

tudes of vibration frequenciestha t ha rm mach ine ry. Fab -r e eka r epor t s th i s i s wheresavvy companies call in a con-sultant or company specializ-ing in vibration protection.

Such specialists measure vi-bration with highly accurateinstrumentation such as real-t ime signal analyzers. Thesedev i c e s c ap tu r e r aw da t a ,

wi thout b ias , for pos t -pro-cess ing and quanti fy ampli -tude and frequency of vibra-t i on . Wi th t h i s da t a , t h espe c i a l i s t r e commends t h ebest isolation solution. Engi-neers then conduct acceptancetes t measurements a f ter in-stallation to verify amplitudesand the r e s u l t a n t t r a n s -m i t t e d v i b r a t i o n . ■

This housed spring isolator’s sidebolt provides adjustable dampingby applying a compression load toan internal elastomer pad.

Copyright © 2005 by Penton Media, Inc.

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