Vibration Isolation Group Project

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DYNAMICS II (TIE 22O6)

LECTURER: W. TUMBUDZUKU

GROUP ASSIGNMENT: VIBRATION ISOLATION

NAMES: BURAWA MELUSI - N0107411RMAGAYA DUNCAN - N0107451RMANJORO JEROME - N0107216ZMAPAKO PATIENCE - N0107388PMAPURANGA CLAUDIA - N0109062DNDUNA TARIRO - N0107406BNYASHA TENDAI - N0107530H


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ContentsVIBRATION ISOLATION .................................................................................................................................. 1

Introduction .............................................................................................................................................. 1

PASSIVE ISOLATION ....................................................................................................................................... 1

Transmissibility Curves for Passive Isolation ............................................................................................ 3

ACTIVE VIBRATION .................................................................................................................................... 3

Transmissibility Curves for Active Vibration ............................................................................................. 4

TYPES OF VIBRATION ISOLATION .................................................................................................................. 5

EXAMPLES VIBRATION ISOLATORS ............................................................................................................... 5

Neoprene or rubber isolators ................................................................................................................... 5

Spring isolators .......................................................................................................................................... 5

CONSIDERATIONS WHEN SELECTING A VIBRATION ISOLATOR .................................................................... 8

Mathematics of Isolator Selection ............................................................................................................ 9

Isolation Theory ...................................................................................................................................... 10

VIBRATION CALIBRATION ............................................................................................................................ 11

VIBRATION ISOLATION AS A CONTROL TECHNIQUE ................................................................................... 11

Conclusion ................................................................................................................................................ 12

REFERENCES ................................................................................................................................................ 12


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VIBRATION ISOLATION

Introduction

1) Vibration isolation is the process of isolating an object, such as a piece of equipmentfrom the source ofvibrations. (www.wikipedia.org)

2) Vibration isolation is the isolation in structures, of those vibrations or motions that areclassified as mechanical vibrations involves the control of the supporting structure, the

placement and arrangement of isolators, and control of internal construction of the

equipment to be protected. (dictionary of engineering 2nd

Edition)

Isolation - It refers to imbedding the transmission of troublesome noise. Isolation can be used to

prevent harmful energy from entering into a system and disturbing it.

Damping - It is the reduction of amplitude of a resonance. There are two general types; a tuned

mass damper is designed to damp specific resonance in a structure. A dashpot is used in

automobile shocks as an example of a tuned mass damper.

Vibration isolation can be present in two main forms that are

Passive isolation Active isolation

PASSIVE ISOLATION

Passive vibration isolation systems consist essentially of a mass, spring and damper (dash-pot).

An example of a suspension bracket of the automobile will be used to explain the vibration

isolation system. In any suspension bracket there are elastic elements, which soften pushes and

impacts of the road. The shock-absorber is intended to terminate the excited oscillation.

Functions of the suspension bracket are directly connected to maintenance of contact of the

wheels with the road.

Too hard suspension system of a car results in throwing of the car on unevenness of the road,

while too soft suspension system will swing the car, which results in loss of the contact between


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the wheels and the road. On the other hand too strong damping also has the negative

consequences.

Figure 1: Basic Passive Vibration Isolation System

Motions of a suspension bracket caused by roughnesss of the road are of vary from individual

pushes to periodic oscillation. For example, on a wavy road the resonance oscillation can be

excited and dash-pots have to provide the maximal damping to keep contact of the wheels to

road. At unitary sharp pushes the damping should be minimal to soften them as much as

possible.

Two types of passive vibration control:

(i) Vibration isolation and(ii) Vibration absorption.

Vibration isolation requires tuning the natural frequency and damping ratio of a single-D.O.F

system to reduce the "transmissibility ratio" between input and output.

Vibration absorption is a method of adding a tuned mass-spring absorber to a system to create

anti-resonance at a resonance of the original system.

Passive IsolatorConsists of a resilient member (stiffness and an energy dissipater (damping))

Example. Metal springs, felt, pneumatic


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Transmissibility Curves for Passive Isolation

Figure shows transmissibility of the passive vibration isolation system for three damping

coefficients related as 1 (blue), 3 (green) and10 (red).

In the Figure when the value of damping is big the vibration isolation properties of the system

are practically vanishes (red line), while when the damping is week the considerable resonance

peak is observed (blue line). The optimum value of damping corresponds to the case when the

amplitude of oscillation increases only insignificantly near to resonant frequency (green line).

ACTIVE VIBRATION

Figure 3: Active Vibration Model (Adapted from; http://www.jrs-si.com/)

Figure 2: Transmission Curves of Passive Vibration Isolation System


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In active vibration isolation system the spring is the feedback circuit and consists of a

piezoelectric accelerometer, an analog control circuit, and an electromagnetic transducer.

The spring supports the weight of the table top and the device which is mounted on the table.

The piezoelectric accelerometer detects the motion of the table consisting of a mass resting on it.

The analog control circuit and amplifier process the acceleration signal which is fed to the

electromagnetic.

As a result of such feedback system a considerably stronger suppression of vibrations as

compared to ordinary damping is achieved.

From Figure 3 two accelerometers and electromagnetic transducers are shown as well as the

bottom part which shows the record of the noise displacement of a vibrating platform. This

system allows considerable reduction of amplitude of the table oscillation to be achieved,

especially in high-frequency region.

Transmissibility Curves for Active Vibration

The figure shows the transmissibility of active damping systems. The signal of accelerometer is

integrated, so that the feedback signal applied to electromagnetic actuator is proportional to

velocity of the table top.

Figure 4: Transmissibility Curves for Active Vibration Isolation Systems


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Red curve corresponds to the case when feedback was switched off. We can see the resonance

pick at frequency of about 0.6 Hz. Green curve shows the case when weak feedback was

switched on. This weak feedback removed the resonance pick, while the transmissibility at low

and high frequencies is about the same. And, finally the blue curve shows the influence of the

strong feedback signal. The residual vibrations are considerably suppressed from low frequencies

up to about 10 Hz.

Maximal advantage of active vibration isolation system can be achieved in the middle frequency

region, near resonance, which is very important for most of practical applications.

Active Isolator: Consists of a servo mechanism with a sensor, signal processor and an actuator.

TYPES OF VIBRATION ISOLATION

Vibrations in most cases are undesirable and the examples includes vibration of cars and

carriages, motors and machine tools, oil and gas platforms, buildings and constructions in a zone

of seismic activity, undesirable vibrations of laboratory tables, etc. In all these cases an object

has to be isolated from the source of vibrations. Despite of all constructional distinctions the

essence of vibration isolation systems is identical.

EXAMPLES VIBRATION ISOLATORS

Neoprene or rubber isolators

These are used between the sets base and pad and also to isolate generator components, such

as controls. Frequently, Neoprene integral mounts are fitted by the manufacturer between the

engine-generator assembly and the skid. They provide as much as 90 percent isolation

efficiency, which is sufficient for most installations at or below grade level.

Spring isolators

These isolators provide up to 98 percent vibration isolation and are suitable for all applications.

They are required when the generator set is installed above grade. When choosing a spring

type, be sure the model matches the weight of the generator, to avoid overly compressing the

springs. The designer should consult local codes to determine if spring isolators are required.

Spring types are mounted between the generator skid and the mounting surface.


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(

Figure 5: Front Pictorial View of Spring Isolator (Adapted: Indian Institute of Technology; Harmonics Lecture Notes)

Spring type with sub base tank: When spring isolators are mounted between the concrete pad

and a sub-tank, special consideration must be given to the spring isolators selection to

compensate for the variable weight of the package that will occur because of the amount of

fuel in the tank.

Another solution would be to specify that the spring isolators will be mounted between the

generator base and the sub-base fuel tank. However, while eliminating weight considerations

this solution may be less pleasing aesthetically.


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Unusual or exacting code requirements: Two types of isolators can be used when an

installation is planned in an area where state and local codes specify seismic or earthquake

proof mounts, or where the installation is powering an application that is extremely sensitive to

vibrations.

Seismic prone area: Spring isolators, sized to the weight of the generator system that are

supporting and mounted between the generator skid and the concrete mounting pad can be

used in seismic-prone areas.

Figure 6: Spring Isolators used in Seismic -Prone areas

Bulk isolators: Bulk isolators are used in the most complex and expensive of all mounting

systems, but bulk isolation is also the most effective when limiting vibration is critical. Bulk

isolation is achieved by mounting the generator set to a solid, massive inertia block, then

surrounding that block with fiberglass, cork or other motion-absorbing material to separate it

from adjacent structures.


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CONSIDERATIONS WHEN SELECTING A VIBRATION ISOLATOR

The objective of installing a machine on vibration-isolating mounts is to reduce its impulse and

sinusoidal vibration. In particular, it is the amplitude of the elastically-mounted machines

movement that is to be held within certain constraints. In choosing a vibration isolator, it is

therefore necessary to provide for sufficient damping capacity of which the following measures

can be taken:

When building or correcting a design, the machine under investigation and the elementthat it drives should both rest on a common base.

Always design the isolators to protect against low frequency that can be generated by themachine.

Design the system so that its natural frequency will be less than one third of the lowestforcing frequency present.

The isolation device should also reduce the transmissibility at every frequency containedin the Fourier spectrum of the forcing function.

1) Machine Location

As far away from sensitive areas as possible And on as rigid a foundation as possible (on grade is best)

2) Proper sizing of isolator units

Correct stiffness (specified by the static deflection, more flexible is generallybetter)

Sufficient travel to prevent bottoming out during shock loads, or during systemstartup and shutdown

3) Location of isolators isolators should be equally loaded, and the machine should be level.

4) Stability sideways motion should be restrained with snubbers. The diameter of the spring

should also be greater than its compressed height. Isolator springs should occupy a wide

footprint for stability.

5) Adjustment springs should have free travel, should not be fully compressed, nor hitting a

mechanical stop


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6) Eliminate vibration short circuits any mechanical connection between machine and

foundation which bypasses the isolators, such as pipes, conduits, binding springs, poorly

adjusted snubbers or mechanical stops

7) Fail safe operation should a spring break or become deflated, you must have mechanical

supports on which the machine can rest without tipping.

Mathematics of Isolator Selection

Isolators are usually specified by their static deflection , or how much they deflect when theweight of the machine is placed on them. This is equivalent to specifying their stiffness and has

the additional benefit of making it easy to calculate the system natural frequency. Coil spring

isolators are available in up to 3 static deflection. If more flexibility is needed, air springs are

used. The natural frequency of the system (assuming a single degree of freedom) can be

calculated by:

Where:

D= static deflection of spring

g = gravitational constant

In the case where vibrations are present due to a constant steady-state oscillation of imbalance

in a machine a precise formula may be applied with reasonable certainty of attaining desired

results. In substance, this formula is based on the ration of the operating frequency of the


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machine or other equipment to be isolated, to the natural frequency of the isolated system.

The disturbing frequencyf dof a machine can be readily determined either by measurement or

by the known operating characteristics of the equipment. Generally the lowest R.P.M. in the

system is used as the disturbing frequency.

The natural frequencyfn of a machine set on resilient material is a function of the static

deflection of the resilient material under the imposed load. For practical purposes the natural

frequencyfn is described by the formula: where d = static deflection

Isolation Theory

The ratio (fd/fn) establishes the efficiency of the isolation from the following formula:

. [1.1]

E = percentage of vibration isolated.

fd = Disturbing frequency of the isolated machine.

fn =Natural frequency of the isolated machine.

The percentage of isolation efficiency attained as a measure of the amount of reduction in the

amplitude of the transmitted mechanical vibration. Refer to figure 'A' to readily select the static

deflection required to attain desired isolation efficiency.

VIBRATION TECHNOLOGY FOR MACHINERY

Reducing both vibration emission and elimination are important objectives in operating

machinery and other equipment. The continuous gains in machine performance achieved during

the past several years have generally provided for increases in rotation speed and cutting speed

as well as in the impact force available for non- shaping. For this reason, the amount of vibration

generated and emitted to the environment has increased, requiring of manufacturers that they

intensify their vibration isolation measures in the context of environmental protection.


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VIBRATION CALIBRATION

The effectiveness of vibration isolation depends to a great extent on the relationship between the

rotational speed of the machine and the natural frequency of the insulator (damping ratio). In

general it is true that the effectiveness of vibration isolation rises as natural frequency of the

insulator drops, that is, as the ratio between the frequency of the vibration (rotational speed of the

machine) and the natural frequency of the insulator rises.

VIBRATION ISOLATION AS A CONTROL TECHNIQUE

In the control of noise three areas are considered: the source, the path and the receiver.

Vibration control may also involve vibration isolation as a technique.

Some of the things to consider when choosing a vibrator isolation system:

The manufacturers specification of allowable vibration for the equipment to beisolated.

The weight of the equipment to be isolated. If the load distribution is not uniform, what isthe load at the heaviest end or corner?

The height of the center of gravity for the equipment to be isolated. The recommendedthe center of gravity height should not exceed 25% of the shortest distance between

isolation supports.

If there is a moving load that you are trying to isolate, you should consider what the loaddistribution at the mounting points would be when the moving load is at its minimum and

maximum displacement. Also, isolator load capacity should be at least double the

capacity of a stationary application.

How the addition of a vibration isolator will change the way you use a system taking intoaccount the ergonomics of the system.

The isolator shouldnt interfere with simple loading or service access. The sensitivity of the instrument and the environment surrounding it should also be

considered.

In all cases of isolation an object is isolated from the source of vibrations.


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The most important factors when choosing vibration isolators are the natural frequency and

isolation efficiency.

Conclusion

Vibration Isolation is thus an important technique in protecting vital equipment subject to shockand vibrations so as to increase their lifespan and maintain its functionality. For best

performance, the weight of a typical load should not be more than 80% of the equipments rated

capacity. Ergonomics is also important, there is no point in eliminating vibrations if the operator

is not comfortable and alert.

REFERENCES1. Harris C (2002). Pier sol Harris Shock and Vibration Handbook 5th Edition2. J.S Lamancusa (2002). Pennsylvania State University Vibration Lecture Notes3. [Vibration Isolation]. Online. Available. htttp://www.wikipedia.org/ [27 March 2012]4. [Vibrations]. Online. Available. http://www.jsr-si.com/[27 March 2012]
http://www.jsr-si.com/http://www.jsr-si.com/http://www.jsr-si.com/

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