Foundation Isolation Solutions For Equipment & Machines
Foundation Isolation Solutions forEquipment & MachinesFoundation Isolation Solutions forEquipment & Machines
3IntroductionThe purpose of isolation is to control unwanted vibration sothat its adverse effects are kept within acceptable limits.43BackgroundWhen is a foundation (inertia block, reaction mass) required?In certain applications, it is not desirable or feasible to mount amachine directly on vibration isolators. An integral part of manymachine tool and equipment installations is a properly designedand isolated foundation.7Design ServicesOur Engineering group will assist you with design solutions foryour machinery or equipment foundation including; structuraldesign and dynamic analysis, finite element modeling andmodal analysis, if required.711 Vibration IsolatorsA brief discussion regarding isolator natural frequency, staticand dynamic spring rate, damping and transmissibility,including types of isolators and isolator performance.1814 FABSORBTMFabsorb isolation material is an economical approach tofoundation isolation where high frequency vibration control isrequired.2018 Regufoam and Regupol These vibration isolation materials provide low frequency isolation, ease of installation and design flexibility to meet a widerange of applications.20 INFABTMVibration isolators of molded elastomer that offer lowfrequency isolation and high load capacity.2222 Pneumatic Isolators and Air BagsPneumatic isolators provide exceptional low frequency andshock isolation for sensitive machines and equipment. Air bagisolators allow for large displacements (stroke) where solutionsrequire the same.23 Vibration Measurement & AnalysisFabreeka provides Vibration Measurement & Analysis servicesprior to and after installation to determine and/or verify theresultant amplitude and frequency of vibration at your facility.23
IntroductionVibrating, rotating, reciprocating and impacting equipment create machine-induced vibration and/orshock, which is transmitted into their support systems. Rotating machines and equipment that arenot properly balanced produce centrifugal forces creating steady state and random vibration.Machines generating pulses or impacts, such as forging presses, injection molding, impact testers,hammers, centrifugal pumps and compressors are the most predominate sources of vibration andshock.If the equipment requiring isolation is the source ofunwanted vibration (Figure 1), the purpose of isolation is to reduce the vibration transmitted from thesource to its support structure. This vibration producing equipment consists mainly of machines thatapply severe dynamic forces in their supportingstructures.In order to achieve acceptable amplitudes of vibration at the source or recipient, it becomes necessaryto make the support structure independent (isolated)from the rest of the environment. This separationprevents vibration from being transmitted directlythrough the support structure.Conversely, if the equipment requiring isolation isthe recipient of unwanted vibration (Figure 2), thepurpose of isolation is to reduce the vibration transmitted from the support structure to the recipient tomaintain performance. This includes equipment suchas precision machine tools and measuring machineswhere vibrations must be kept within acceptablelimits to achieve the desired surface finish, tolerances or accuracies.Depending on the circumstances, it should be notedthat a machine could be both a source and recipientof unwanted vibration. For example, a surfacegrinder is generally a vibration-sensitive piece ofequipment that needs to be protected from floorvibrations. However, as the surface grinder reversesits heavy table during operation, it produces a largedynamic force, which may disturb other nearby precision equipment.Figure 1Some machine tools of ordinary precision are neithersensitive to vibration nor produce large dynamicforces, and therefore may or may not require isolation.Operating frequencies of rotating/reciprocatingmachines often are very close to the natural frequency of their support structure (floor slab andsoil). Compressors, for example, can generate vibration of substantial magnitudes at low frequenciesthat coincide with the natural frequency of the floorslab, thus creating a resonance (amplification ofvibration) in the floor.Figure 23
BackgroundThe separation method of cutting the existing floorslab or even creating trenches around machines toreduce the vibration being transmitted by the soilbeneath the floor slab is experimental at best andoften not a practical solution. A thorough understanding of the machine, the support structure(floor) and the soil is required. The effectiveness ofthis approach relies heavily on the soil mechanics,magnitude and frequency of the vibration amplitudes to be reduced. To be an effective solution,trenches and slab cuts can be up to 6 feet deep and10 inches wide, which requires the soil to beextremely stable and can also cause safety issues.Additionally, the natural frequency of soil canincrease if the input vibration amplitudes are smalland can decrease when the input vibration amplitudes are larger.The damping property of most soils decreases as thepressure beneath the foundation increases and alsowhen amplitudes of vibration are small. The largerthe vibration input and the contact area of thefoundation, the larger the damping value of thesoil, and as a result, the lower the amplification ofvibration at the soil's natural frequency.Soil MechanicsNatural Frequencies of Soils*When installing machinery or equipment on a support foundation that rests directly on soil as themeans of providing isolation, the soil conditionsmust be taken into account. Poorly designed andinstalled foundations may amplify vibration orworse, may settle unevenly and sink. Interactionbetween the soil and the foundation is equally asimportant as the interaction between the machineand the foundation.Any static and dynamic forces exerted on the foundation also are exerted on the soil, and the loadbearing capacity of the soil is a key factor in determining the size of the foundation.If soil alone is to be used as the means of isolation,it is necessary to know the characteristics of theenergy dissipative properties of the soil. Establishingthese properties depends not only on the type ofsoil, but also on the physical design of the foundation; in particular, the depth, the ratio betweenlength and width and the material and density ofthe backfill.It is difficult to take into account the influence of allthese factors on the value of the energy dissipativeproperties of the soil. Therefore, the natural frequency and damping properties of the soil cannotbe clearly defined based on the soil type alone.(Estimated values for soil natural frequency are listedin Table 1.)4Ground or StructurePeatFrequency (Hz)7Suspended concrete floor10 - 15Ground floor12 - 34Soft clay12Medium clay15Stiff clay19Loose fill19Dense medium grain sand24Very dense mixed grain sand24Uniform coarse sand26Pea gravel28Limestone30Hard sandstone34Table 1*Assumes soil is homogeneous. Values do not account foramplitude of vibration input or foundation geometry.The determination of a soil's dynamic properties(spring rate, damping) can be highly indeterminate.In many cases, the calculations are complex andmany assumptions are made. Energy dissipationdoes occur in soil; however, the rate of damping andthe natural frequency are a function of the magnitude of the vibration input and foundation geometry.
In many cases, manufacturing and quality controlmust co-exist in workcells or in close proximity toone another. For certain machines, the permissibleamplitudes of machine foundation vibrations in amanufacturing environment are very low. It often isvery difficult to decrease or isolate vibration amplitudes by properly selecting the contact area wherethe foundation meets the soil. It also may not bepossible to increase the stiffness (rigidity) of themachine support structure (floor) itself to avoid resonance or amplification of vibration. In these cases,unacceptable vibration amplitudes can be significantly reduced by using vibration isolators.Foundations Requiring Vibration IsolatorsIn certain applications, it is not desirable or feasibleto mount a machine directly on vibration isolators.Direct installation of vibration isolators on a machinewhose frame/bed stiffness is marginal or inadequateand requires a stiff connection can cause bending,relative displacement and other problems, evenwhen the floor is sufficiently rigid. For smallermachines, this can be remedied by securing theframe/bed to a rigid plate, thereby creating a rigidsupport structure, and then installing the isolatorsbetween the plate and the floor. For largermachines, the frame/bed is attached to a properlydesigned concrete foundation, which is then supported on the appropriate isolators for the application.A concrete support structure (foundation, inertiablock, reaction mass) is used to satisfy one or moreof the following conditions:1) Provide/improve structural stiffness for themachine/equipment being isolated.Some types of equipment do not operate properlyunless supported by a rigid structure. This applies tocertain types of machine tools that are not inherently rigid and therefore need a rigid support to maintain the prescribed accuracy. In other types ofmachinery (such as printing presses) consisting ofarticulated components, a rigid support may beneeded to maintain the proper alignment of working parts.Dual horizontal arm coordinatemeasuring machine with separateworkpiece table. The foundationmakes a rigid connectionbetween the measuring arms andthe workpiece. Pneumatic isolators (installed in the pockets atthe base) support and isolate thefoundation.5
2) Increase stability on the vibration isolators by limiting dynamic deflection.If a machine (such as a diesel engine, forging hammer or electro-dynamic shaker) generates relativelylarge forces during its operation, the overall movement of the machine on its isolation system tends tobecome excessive unless its effective mass is substantially increased. This increase in effective masscan be achieved by attaching the machine rigidly toan inertia block and mounting the inertia block(reaction mass) on isolators.3) Isolate the equipment/machine from the environment when installing isolators directly beneath theunit would compromise the conditions above.In applications in which the frequency of excitationis low, the natural frequency of the isolation systemmust be very low to provide low transmissibility andtherefore good vibration isolation. A problem oftenarises with a machine intended to be mounted onlyat its base, because a low-stiffness base-mountedsystem tends to be unstable and will allow excessivemotion to take over.Effective isolation may therefore be difficult toachieve. A mounting arrangement where the isolators are relocated may be used to move the isolationsystem's elastic center closer to the center of gravityof the machine. This will reduce the effect of "rocking," improve the vibration isolation and reducemotion on the isolators. In most applications, it ismore feasible to attach the machine rigidly to afoundation (to lower the center of gravity of themachine and foundation together) and to suspendthe foundation on isolators located in the same horizontal plane as the center of gravity.A foundation or mass designed to meet the requirements outlined previously may be installed eitherabove floor level or in a pit below floor level.Isolators used to support the foundation may bemade of rubber, mat material, steel springs, airsprings or other suitable, resilient material. Therequired size of the foundation depends on the reason for its use, the type and size of equipment andthe type of isolation required.Inglis forging hammer installed on concrete reaction masssupported by coil spring isolators.6The desired natural frequency (stiffness) and damping for the isolation system is usually established bythe operating characteristics of the mounted equipment (source) and/or the isolation required (recipient). The design basis for the support foundationnatural frequency assumes that the foundation is arigid body with a stiffness much greater than theisolators. Similarly, the pit base also should be stifferthan the soil supporting it.
Design ServicesFoundation DesignThe function of a foundation is not only to supportthe weight of the machine/equipment, but also tokeep the vibration levels and dynamic displacementof the isolation system within acceptable limits.Designing foundations supporting machines that canproduce static and dynamic loads requires soundengineering procedures for a reliable result. Anincorrectly designed foundation is extremely difficultto correct once installed.Engineering disciplines involved in the proper designprocedures for isolated support foundations includetheory of vibrations, geotechnical engineering (soilcharacteristics), structural analysis, and in someapplications, dynamic analysis.The design conditions and requirements can beclassified into three groups: machine properties,including unbalanced forces, operating speeds;weight, center of gravity and allowable deflection;soil parameters, including load bearing capacity, andenvironmental requirements - What degree of isolation is required and at what frequencies?SoilThe machine/equipment, foundation, isolators andpit ultimately all are supported by the soil beneaththem. Geotechnical recommendations andevaluation of the soil (soils analysis) should be madeand must be part of the design. This analysisincludes soil characteristics, including load-bearingcapacity, shear modulus, density, soil type and thecomposition of the soil at various depths. In thestructural design of the support foundation, pilesmay be required depending on the load bearingcapacity of the soil, high water table or generallypoor soil conditions that indicate unacceptable permanent settling of the foundation will occur.Settling, if any, should be uniform and kept to aminimum, especially when designing support foundations for equipment providing large dynamicloads/forces. If the foundation supported by isolatorsis used to enhance the machine frame/bed stiffnessor is used as an integral part of the structural sup-port of the machine (i.e. gantry CMM, turbine, rollgrinder), then the dimensions of the foundation aredefined by the machine geometry. The weight andtype of machine along with a preliminary foundationsize will give an indication of the soil's supportrequirements.The traditional rules observed in the past of makingthe foundation 3 to 5 or even 10 to 12 times theweight of the equipment/machine it supports areapplicable only when the foundation will be isolatedby the soil and where the soil dynamic properties areknown.Structural Design and StiffnessTo be acceptable, the proposed design of a foundation or any support structure must provide a reliablestructural configuration that also meets the staticand dynamic criteria for the structure. Deflections inthe foundation caused by static loads or by dynamicforces/inputs should be within acceptable limits. Thisdesign approach sometimes requires modeling ofthe foundation, so that the real structure behavior ispredetermined and errors are minimized.The calculations for the stiffness of a foundationyield the static and dynamic behavior and stress concentration points that occur. Stresses are related tothe geometry of the foundation and the distributionof loads and forces acting upon it. A stress analysiswill indicate the magnitude of stress imposed bystatic and dynamic loading (Figure 3).Figure 3 - Foundation stress analysis.7
Figure 4 - Mode shapes of a support foundation.Data on forces, such as axial, shear, torques andmoments for maximum loading at each support orattachment location of the machine are necessary topredict the load conditions on the foundation. Theseloads are used to determine the longitudinal and/ortransverse (width) reinforcement and concretestrength required, which relates directly to anydeflection.The modulus of elasticity is a key design factor inthe strength of concrete. (See Figure 6.) Limits onthe differential deflection allowed from one point toanother on a foundation are set to avoid possibledamage or misalignment of conduit and other connections. The depth of a foundation is determinedby the bearing strength of the soil, the machine support requirements (structural stiffness) and in criticaldesigns, the dynamic stiffness, which includes thefoundation's natural frequency and bending modes.Mode shapes (stiffness of a structure in each axis)identify the physical direction of each frequencymode and any deformations, such as bending ortwisting. In general, a structure's modes indicate therelative degree of structural stiffness among variouspoints on that structure (Figure 4).Examining mode shapes in a vibrating structure is avaluable step in adjusting vibration amplitudes atcritical points by varying the stiffness, mass anddamping in a structure.Forces imposed by the supported machine caninduce a high enough vibration amplitude at thenatural frequency (or one of the response modes) ofthe foundation to cause resonance or amplificationof the vibration. The single most important factor inany successful design where machine induced vibration is involved (source) is to avoid resonancebetween the machine and the foundation.Geometry and mass are important considerations inthe dynamic design of foundations. However, thefoundation-to-equipment mass ratios that are sometimes recommended, do little in preventing foundation vibration unless the dynamic response of thefoundation is known.A finite element analysis will define and model themode shapes and response frequencies of the foundation, as well as the response of the isolation system and foundation to machine induced inputsand/or environmental inputs (Figure 5).Figure 58
Amplification at the point of resonance should beaddressed for environmentally induced, random orsteady state vibration, although the vibration isolators supporting the foundation should provide sufficient isolation at the foundation's natural frequencyto avoid amplification.During startup or shutdown of a machine, a temporary resonance condition may be tolerated, wherethe support structure or even the vibration isolatorsare in resonance with the machine's operating frequency, especially if significant damping is available.Data on the operating speed and forces generatedby a machine, or the measured vibration amplitudesand frequencies at which they occur for a machinesensitive to vibration, are therefore required in adynamic analysis in order to check for possible resonances.Figure 6ConcreteAn important part of a foundation's structure andstiffness is the specified concrete strength used inthe design.A specified concrete strength is easy to obtain and isoften used as the only criteria. However, shrinkagecontrol can be one of the most important factors inproviding a successful project. The following aremajor factors controlling shrinkage:Shrinkage is simply the reduction in volume thattakes place when the concrete dries from its originalwet condition down to a point where its moisturecondition reaches equilibrium with the humidity inthe air. Unrestrained shrinkage does not developcracks.1) Water/cement ratio (slump) of delivered concrete2) Aggregate proportioning and size3) Water reducing additives4) Site conditions, such as hot, dry climate5) Curing6) Control joints and reinforcingEach of these six factors needs consideration. Slumpis controlled by controlling the total water per cubicyard of concrete, while strength is governed by thethickness or consistency. This thickness is determinedby the ratio of the weight of water to the weight ofcement.Concrete sample and slump measurement ofconcrete mix before pouring foundation.9
When designed and cured properly, large foundations result in very low concrete shrinkage while in acontrolled environment. Most of the shrinkageoccurs in the first two months and it is nil in the following months if the ambient environment does notchange. Concrete surface sealants, if required,should be applied after most of the shrinkage hasoccurred.For critical designs or for precision equipment, concrete samples should be taken at least one for each25 cubic yards of concrete placed to check theslump. Test samples should also be taken at 7 and28 days (assuming a 28-day cure) to verify thestrength.Design factors in the dynamic analysisof an isolated support foundation include:s Unbalanced forces applied by supported equipment/machines Center of gravity of machine/equipments Natural frequency (resonance) andresponse modes of foundations Transmissibilitys Displacement on vibration isolators10SummaryA good foundation design requires realistic analysisand supervision during construction. Stiffness indesign is important both structurally and dynamically. Dynamic coupling or amplification at resonancedue to the interaction of all components in the isolated foundation design can be avoided if the natural frequencies of the soil, pit, isolators and supportfoundation are verified.Direct vibration measurements can be made that willrender the actual frequency response of the soil andthe best possible values for analysis. This is particularly important for foundations that are isolatedusing mat materials directly on compacted soil without using a rigid concrete pit or sidewalls.Once the approved foundation has been constructed, the machine/equipment should be attached tothe foundation to makea structurally soundconnection. To achievethis, the connectionshould meet the rigidityand support requirements of the machine.Typical connections,which also offer levelingadjustment are anchorbolts with shims andleveling wedges.Grouting also may be required to provide a solid,load-bearing attachment.
Vibration IsolatorsThe purpose of an isolator is to decrease the amplitudes of forced, random and steady state vibrationsbeing transmitted into a machine or equipment support foundation. Isolators exist in many forms,including rubber, mat materials, metal coils, air bagsand pneumatic isolators. The type of isolator (performance) used as the solution for an applicationdepends on the type of machine to be isolated, static load, dynamic deflection and damping propertiesof the isolator.Where Fd is the disturbing frequency and Fn is thenatural frequency of the isolator. When consideringthe property of damping, the equation is rewrittenas Equation (2).All vibration isolators are essentially springs with anadditional element of damping. In some cases, the"spring" and "damper" are separated, as in thecase of a coil spring isolator used in conjunctionwith a viscous damper. The majority of isolatordesigns however, incorporate the spring and damperinto one integral unit.Where ζ represents the damping ratio of the isolator.Important characteristics of any isolator are its loaddeflection and load-natural frequency properties.The dynamic spring rate and damping of an isolatormostly are determined by the type of material used,while the stiffness (static and dynamic) is a functionof the isolator design (material, shape). Static springrate, dynamic spring rate, creep, natural frequency,damping and load deflection values vary widely frommaterial to material and design to design. Therefore,materials or elements used for vibration isolation arechosen based on the significant differences in theirperformance when used to isolate specific frequencies and amplitudes.The ratio of the vibration transmitted after isolationto the disturbing vibration is described as transmissibility and is expressed in its basic form in Equation(1).(1)T 1-[Fd2/Fn2](2)T (1-[Fd2/Fn2])2 (2ζ[Fd/Fn])2Natural frequency and damping are the basic properties of an isolator that determine the transmissibility of a system designed to provide vibration and/orshock isolation. Additionally, other important factorsmust be considered in the selection of anisolator/isolation material. Two such factors are:w The source and type of the dynamic disturbance causing the vibration / shock.w The response of the isolator to thedynamic disturbance.With an understanding of its properties, the type ofisolator is chosen primarily for the load it will support and the dynamic conditions under which it willoperate.Natural Frequency, Spring RateTransmissibility11 (2ζFd/Fn)2Theoretical,undampedtransmissibilityNot all isolators whose isolation characteristics arebased on mechanical deflection have a linear relationship between load and deflection. A commonmistake is that the following equation [Equation (3)]can be used to calculate the natural frequency for allisolators if the spring rate (k) and weight (w) to support are known.(3)1k2πmFn wwhere mass (m) g11
If the stiffness or spring rate (k) is not known, theequation can be rewritten [Equation (4)], so that thestatic natural frequency of the isolator is a functionof its static deflection (δs). This results in a determination of the isolator's static natural frequencywhere (g) represents the gravitational constant.(4)1g2πδsFn Theoretical,undampedstaticnatural frequencyHowever, using the static, linear principle inEquation (4), the following is true:1) Large deflections are required for low frequency isolation.2) Damping properties are neglected.3) Only the static natural frequency isobtained.4) The isolator is assumed to have a linearspring rate.The static deflection principle can be used onlywhen the isolator under consideration is both linearand elastic. For example, rubber, felt, fiberglass andcomposite pads tend to be non-linear and exhibit adynamic spring rate, which differs from the staticspring rate.The curves are developed using the known properties of the isolator - dynamic natural frequency anddamping [Equation (2)]. Note that as damping isincreased, the curve of transmissibility is flattened,so that in the region near to resonance, the curve isreduced, but in the region where isolation isrequired, the curve is increased. The curves showthat if there is a significant amount of damping inan isolator, its natural frequency has to be reducedto retain a desired degree of isolation at the frequency ratio of concern.The ideal isolator would have as little damping aspossible in the isolation region and as much as possible at the isolator's natural frequency to reduceamplification at resonance.With an understanding of the basic properties anddynamic characteristics of an isolator, it is possible todesign for and calculate the true transmissibility ofthe isolator as a function of frequency. However,dynamic stiffness (natural frequency vs load) or atransmissibility vs frequency curve with the actualdamping coefficient of the material is required.The natural frequency calculated using the staticdeflection (δs) determined from a static load deflection test of an isolator invariably will give avalue lower than that experienced during vibration(dynamically).Any isolator with a calculated natural frequencybased on static deflections may not behave in thepredicted way because the dynamic spring rate differs from the static spring rate.It is the dynamic natural frequency which has to beused in calculations rather than the static.DampingThe property of damping is neglected in the staticevaluation [Equation (4)], and this can have a significant effect on the isolation efficiency. Damping in anisolator has a beneficial effect because it helps tosuppress vibration, but can also lead to a loss of isolation efficiency. To appreciate the effects of damping, refer to the transmissibility curves in Figure 7.12Figure 7
Figures 8 and 9 show how isolation materials can beused in constructing and isolating a foundationbelow floor level. A concrete pit of the required sizeis lined with the isolation material. Then this materialis covered with plastic sheeting, and the concrete ispoured on the required reinforcing rods to form arigid foundation. The desired natural frequency isobtained by using material of the appropriate thickness and area."Snubbers" or restraints should only be used in seismic designs to prevent motion due to earthquakesand protect the supported equipment. Snubbersused for stability indicate a poorly designed isolationsystem.Finally, external connections of a vibration isolatedobject can detrimentally affect the isolation efficiency. Mechanical attachment of conduits (service lines)including electrical, signal and other connections canaffect the performance of a vibration isolation system, especially when installed under precision equipment being isolated. These connections create agood transmission path (short circuit) for vibration,which can be present at the connection source andtransmitted to the support foundation. All rigid service conduits should be attached via flexible connections and in large loops to reduce stiffness andtransmission.Figure 8To obtain a low natural frequency for the isolatedsystem, a large static deflection is required whenusing rubber or coil spring isolators. However, nostatic deflection is required when using pneumaticisolators (air springs) with low natural frequencies.If the isolators are located substantiallybelow the combined center of gravity of thefoundation/machine, a tendency towardinstability is introduced, an effect whichbecomes more important if the machinegenerates large forces during normal operation, or motion is created due to high acceleration/deceleration of moving parts."Rocking" can be minimized by installing theisolators in positions closer to the upper surface of the foundation, supported on abutments extending inward from the walls ofthe pit. A more refined version of this concept is the T-shaped foundation illustrated inFigure 9. With such a design, it is possible to locatethe isolators in the same horizontal plane as thecombined center of gravity of t
and isolated foundation. Design Services Our Engineering group will assist you with design solutions for your machinery or equipment foundation including; structural design and dynamic analysis, finite element modeling and modal analysis, if required. Vibration Isolators A brie
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