Friday, April 29, 2011

Vibration Isolator, a solution to reduce acoustic vibration

Finally, I solved a mystery that has pounded me for the past 4 years. It is sometime in my head because it exist everywhere but the engineers at the place I work in never give an exact answer. One just shun away from the question while other never think of this question before. I still remember one reply saying, "this is nothing important, dunno why they make this thing, waste of money.""Don't bother about it." But I never stop looking for a solution to it.

Inertia Block, this is one item that is commonly found in the plant room everywhere no matter which industry is involved. In plant room inertia block exist in term of concrete and in an Air Handling Units, it exists in form of steel structure. I have been wondering why don’t people just put the equipment on the vibration isolator instead of putting it on a block of concrete as what everywhere does. It does have hard-standing or perforated standing. The solution is brought by Paul N.Cheremisnoff, D.A. Buies & CH Harman in Industrial Noise Control and Engineering Noise Control respectively. The most common found is steel spring but through the explanation, it explains that other than steel springs, there are two other types of isolators such as cork materials and rubber materials. Of course, as mentioned in the article, if the design is within the fatigue limit, steel isolator can have infinite life span providing there is not destruction on the steel properties itself.
Below is some of the extract of Vibration Isolators

Inertia Block

These are usually made of concrete poured into a structural steel frame with reinforcing bars, brackets for the attachment of vibration isolators and mounting bolts for the equipment. Although they are generally referred to as inertia blocks and are normally installed to reduce the movement of the mounted equipment, in most cases they are used for other reasons.

To increase stability of the system

The concrete inertia block provides mean of widening the support and a more stable geometry.

Lowering the center of gravity or centroid

Mounting equipment on a substantial concrete base has an effect on lowering the centroid of the complete assembly. This adds to the improvement of the stability provided by extending the width of the base, and also has the effect of reducing the likelihood of rocking motion.

To give a more even weight distribution

Equipment items are very much heavier at one end than the other. This means that if they are mounted directly on vibration isolator, very different arrangements are needed at opposite ends of the equipment to cope with the uneven weight distribution. If the equipment is mounted on concrete block, the weight distribution will be more even and, providing the block is heavy enough, it may enable a symmetrical mounting block to be used.

To minimize the effect of external forces

Although the use of an inertia block does not improve then transmissibility for a given static deflection, it does means that very much stiffer isolators can be used for the same static deflection, i.e. if the mass of the equipment is doubled, the stiffness of the isolator necessary to support it is also doubled. This means that the equipment is far less susceptible to the effects of external forces such as fan reaction pressure and transient torques due to changes in speed and load.

To provide or replace rigidity

An inertia base can be used to provide rigidity for the mounted equipment in the same way that a steel base is used.

To reduce problems due to coupled modes

The higher of the two rocking sideways coupled movement for a tall item of equipment may occur at two to three times the frequency of the basic vertical frequency. This can lead to resonance problems. Adding an inertia base has the effect of lowering the rocking natural frequency which helps to avoid the problem.

To minimize the effects of errors in estimated positions in the equipment’s center of gravity

When vibration isolators are being selected, it is necessary to calculate the total load on each isolator so that the appropriate isolator can be chosen. This normally has to be done before the equipment is available and estimated positions of the centers of gravity of each item have to be used. If this information is inaccurate, the estimated loads may be considerably different from te ones which occur in practice. This may lead to vibration isolators being grossly under or overload, or to the equipment sitting at an unacceptable tilt. The latter problem becomes increasingly likely as vibration isolators with high static deflections are used. If a concrete inertia base is used, the center of gravity of this is normally made known accurately and if the mass of the base is comparable with the mass of the rest of the equipment, it means that even if the equipment information is not accurate, the possible inaccuracies in the final estimated center of gravity are small. This reduces the possible errors in isolator loading and reduces the likelihood of a tilted installation. The probability of a tilted installation is also further reduced because of the stiffer springs that will be used to carry the additional weight of the inertia base.

To act as a local acoustical barrier

When very noisy equipment is mounted directly on the floor of an equipment room, the floor directly under the equipment may be subject to very high sound pressure levels in the immediate vicinity of the equipment. This local area where the floor is exposed to these high levels may cause problems of noise transmission into the room below. A concrete inertia base acts as an effective barrier, protecting the vulnerable area of the floor.

It should be noted that reduction of transmissibility is not listed as a reason for installing an inertia block. This is because of the transmissibility determined by the static deflection of the vibration isolators, regardless of the presence or absence of an inertia base. The effect on the vibration process on the inertia block is to reduce amplitude of motion in proportion to the increase in mass, i.e. if the mass of the equipment is doubled by the additional of the block, the movement will be reduced one-half.

Unless special conditions dictate a structural analysis for the concrete inertia block, a general rule of thumb currently applied for block thickness is one-twelfth the longest dimension of the equipment supported, and in no case less than 6 inches thick. Concrete blocks need not be thicker than 12 inches unless specifically recommended.

The floor plan shape of inertia blocks should be configured to suit each piece of equipment mounted on the block. This is particularly true in case of pumps where it is desired to pick up weight of connected piping on the inertia blocks.
Once a piece of vibrating equipment has been properly isolated, care should continue to be exercise to see that utility connections to the equipment does not bridge or bypass the isolation system.

Isolator Types

Isolators can be conveniently be divided into three types

• Cork material
• Rubber material
• Steel springs.

Cork material

Cork has been used in many fields of industry as a vibration isolation material. A popular type of cork is made of pure granule compressed together and baked under pressure to achieve a controlled density. Cork materials are used mainly under concrete foundations. The cork will remain reasonably durable under exposure to acids, oil and temperature between 0 degree Fahrenheit to 200 degree Fahrenheit; however, it will be affected upon contacted with strong alkaline solutions. Cork will rot however, from repeated wettings and dryings. As a vibration isolator, cork is limited to frequencies above 1800 cycles per minutes. Because of a great degree of damping in cork, the natural frequency cannot be obtained from the static deflection. As an alternative, the natural frequency can be obtained through tests by vibrating the cork under various loads to find the resonance frequency.

Rubber materials

Rubber is useful for frequencies above 1200 cycles per minutes. Alkali solutions or acids will not affect rubber, but degradation problems could arise if it is exposed to sunlight. For natural rubber, the temperature range from 50 degree Fahrenheit to 150 degree Fahrenheit; for neoprene from 0 degree Fahrenheit to 200 degree Fahrenheit. As rubber ages, it gradually loses its resiliency. The useful life span of rubber mounts is approximately five years under impact applications and seven years under non-impact applications, although it will retain its sound-insulating properties for longer period. Individual molded rubber mountings are economical only with medium and lightweight machines because heavier capacity mounting approach the cost of more efficient steel spring isolators. Rubber and other isolators provide maximum deflection on the order of 0.25 inch. Some steel spring isolators can even reach deflection of 10 inches.

Steel Spring

The most common form of steel spring is a simple coil spring. Provided that the right combination of coil diameter, spring height and wire diameter is used, the springs are stable. They are capable of providing resonant frequencies down to the order of 2Hz. When a coil spring is designed within the material fatigue limit, spring can have an infinite life. Coil springs are virtually un-dampened and if damping is required, this has to be provided by auxiliary means. Steel springs have the disadvantage that at high frequencies vibration can travel along the wire of the coil, causing transmission to the structure. This is normally overcome by incorporating neoprene pad in the spring assembly so that there is no metal-to-metal contact.

Calculation for selection of Isolator

Step 1: Required Isolator Efficiency

An Isolator efficiency of 80% is usually specified. Unless otherwise stated in the requirement.

Step 2: Transmissibility

Given equation, Isolator efficiency = 100(1 - T)
Thus Transmissibility, T = 1 – (80/100)
= 0.2

The maximum transmissibility, T of the system which requires efficiency of 80% is 0.2

Step 3: Forcing Frequency

To determine the value of the lowest frequency, f (i.e. the frequency of X excitation)

With the rotational speed of the motor as 1450rpm, it has to be divided by 60 seconds to obtain forcing frequency in Hz (cycles per seconds
Forcing Frequency, f= 1450rpm / 60 (seconds per minutes) = 24.2Hz

Step 4: Natural Frequency

To obtain natural frequency fn of the isolated system (i.e. the mass of the equipment supported on isolators) required to provide a transmissibility, T for a forcing frequency, f.

Given equation, T = 1 / [((f/fn)^2) - 1 ]

With transmissibility, T=0.2 (determined in step 2); forcing frequency, f = 24.2 Hz (determined in step 3)

Thus, Natural frequency, fn = 9.9Hz.

Step 5: Static Deflection

To find the static deflection, sigmast required to provide the required natural frequency, fn.

Given equation, fn = 3.13 / (sigmast)^1/2

With natural frequency, fn = 9.9 Hz (determined in step 4)
Thus, Static deflection, sigmast = 0.1 inch (2.54mm)

Step 6: Stiffness of Isolation System

To find the stiffness, k required to provide the required natural frequency, fn.
Given equation, fn = [(k/1000m)^1/2 / 2pi]

With natural frequency, fn = 9.9Hz (determined in step 4)
Thus, Stiffness, k = 383 N/mm(2188lb/in)

Step 7: Stiffness of Individual Isolator

Since there will be one isolator at each corner of the base of the support mass, the isolators are in parallel, with n = 4.

Thus, Stiffness on individual isolator = 383/4
= 96 N/mm (547lb/in)

Step 8: Total Load to be Supported by all Isolators

The total load of the system on the inertia base is 99kg.

Step 9: Load on individual isolator

Load is divided equally among the four identical isolators
Thus, loading on individual isolator = 99/4 = 24.8kg (54.9lb)

Step 10: Isolator Selection

Isolator having a spring constant of 96 N/mm (547lb/in) and the capacity to support a static weight of approximately 24.8kg (54.9lb)


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