Assignment 1Mechanics of Machines

1. What are the different types of vibration isolator and what are the uses or advantages of each of them?

Vibration isolators used to reduce the transmission of noise and vibration from mechanical equipment onto a building structure, support.

There are mainly divided into two types respectively

1. Passive isolators- Passive vibration isolators consist of a spring and damper. The spring acts as the absorber by softening vibrations and pushes. The damper, then, has to end the oscillation. Passive vibration isolators are used every day. One example is the suspension bracket of an automobile. They are of different types like

Mechanical springs - These are heavy-duty isolators used for building systems and industry.

Sometimes they serve as mounts for a concrete block, which provides further isolation.

Pneumatic isolators- These are bladders or canisters of compressed air. A source of compressed air is required to maintain them. Air springs are rubber bladders which provide damping at the same time as isolation and are used in large trucks. Some pneumatic isolators can attain low resonant frequencies and are used for isolating large industrial equipment. Air tables consist of a working surface or optical surface mounted on air legs. These tables provide enough isolation for laboratory instrument under some conditions. Air systems may leak under vacuum conditions. The air container can interfere with isolation of low-amplitude vibration.

Pads or sheets of flexible materials - Used under heavy machinery, under common

household items, in vehicles and even under higher performing audio systems.  Such as

elastomers, rubber, cork, dense foam and laminate materials.

Bungee cord isolators and tennis balls - Bungee cords can be used as a cheap isolation

system which may be effective enough for some applications. The item to be isolated is

suspended from the bungee cords. This is difficult to implement without a danger of the isolated

item falling. Tennis balls cut in half have been used under washing machines and other items

with some success.

Active Isolators: Active vibration isolation systems contain, along with the spring, a feedback circuit which consists of a sensor (for example a piezoelectric accelerometer or a geophone), a controller, and an actuator. The acceleration (vibration) signal is processed by a control circuit and amplifier. Then it feeds the electromagnetic actuator, which amplifies the signal. As a result of such a feedback system, a considerably stronger suppression of vibrations is achieved compared to ordinary damping. A couple of companies produce active isolation products as OEM for research, metrology, lithography and medical systems.

2. What are the properties or parameters used to define or specify a vibration isolator?

Transmissibility and frequency are two factors which are used to define a vibration isolator.

Transmissibility is the ratio of vibration of the isolated surface to that of the source. Vibrations are never

completely eliminated, but they can be greatly reduced. T>1 means amplification and maximum amplification

occurs when forcing frequency (ff) and natural frequency (fn) of the system coincide.

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal

frequency, which emphasizes the contrast to spatial frequency and angular frequency. 

3. What are the calculations needed and how to do them?

a. For calculating vibration:

General: f= 12π √ accelerationdisplacement

Pendulum: f= 12π √ gl where g is gravity and l is length

Springs: f= 12π √ km where K is spring stiffness and m is mass

Shafts: f= 12π √GJILe where G is radius of gyration, J is polar moment of inertia

and I is second moment of inertia and Le is equivalent length

b. For transmissibility:

T =

1

(f df n

)2

−1

c. For isolation effectiveness:

% of isolation = (1-T) X 100

4. What is the acceptable noise and how do you eliminate these noises?

Acceptable noise level (ANL) is defined as the maximum level of background noise that an individual is willing to accept while listening to speech. The type of background noise does not affect ANL results with the possible exception of music.

In industries the acceptable noise level is 85db for a period of 8 hours. With maximum of 140db

In residential areas acceptable noise level is 35db to 45db.

To reduce noise

Avoid metal-on-metal impacts, eg line chutes with abrasion-resistant rubber, and reduce drop heights.

Vibrating machine panels can be a source of noise - add material to reduce vibration ('damping').

Isolate vibrating machinery or components from their surroundings, eg with ant vibration mounts or

flexible couplings.

Fit silencers to air exhausts and blowing nozzles.

Use absorptive materials within the building to reduce reflected sound, eg open cell foam or mineral

wool.

Keep noisy machinery and processes away from quieter areas.

Design the workflow to keep noisy machinery out of areas where people spend most of their time.

Any other consideration:The noise level of jet take off is very high at 150db. It can cause eardrum rupture if a person comes close to jet. To avoid this all staff of airports were given safety equipment like earplugs, headphones to protect from noise.

5. What is the solution for the problem below?Select a standard off the shelf vibration isolator for the new air compressor unit (ABAC Genesis 7508) installed outside block H of MIT. The compressor is currently making a lot of vibration and noise and is affecting the teaching in the H block classroom.

Common causes of the vibrations and noise are unbalance in rotating and reciprocating parts and abnormal aerodynamic forcesTo reduce vibration spring isolators can be used because a spring isolator gives desired natural frequency to obtain phase difference in the frequencies for a rotating machine. They are efficient up to 90%. Since it is placed outdoor there is no need to reduce the vibrations to zero. However for more precise damping viscous dampers can be used. In order to reduce sound the unit can be places inside a small housing whose inside consists of sound absorbing material like a heavy dense foam/rubber material used for sound proofing buildings/floors etc.

References-

https://www.youtube.com/watch?v=DmDzoTcxdfIhttp://www.aircompressorworks.com/blog/index.php?mode=post&id=33http://www.hse.gov.uk/noise/reducenoise.htmhttps://en.wikipedia.org/wiki/Vibration_isolation