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VIBRATION & BALANCING

 

VIBRATION : Vibration is defined as the response of a system to an

internal or external stimulus causing it to oscillate. Components in a

vibrating system have three properties and they are Mass, Elasticity and

damping. The property of mass causes an object to resist acceleration and

also enables an object to store energy in the form of velocity (kinetic). The

property of elasticity enables an object to store energy in the form of

deflection i.e. you must spend energy to deflect a spring. The property of

damping enables an object to dissipate energy.

The above three properties define the resonant frequency or natural

frequency of a system which is very important for knowing the

phenomena of vibrations.

 

VIBRATION DISPLACEMENT :- The total distance traveled by the

vibrating system from peak to peak is the peak to peak vibration

displacement and generally expressed in mils where 1 mils is equal to one

- thousandth (1/1000) of an inch. It is also measured in microns. One

micron is equal to 1/1000 of a millimeter.

 

VIBRATION FREQUENCY :- Vibrations are in general in form of a

sinusoidal wave. Thus the vibration frequency is the number of complete

sine waves per minute/second C.P.M./CPS which is denoted by Hz (hertz)

and its unit is in general CPS (Cycles per second).

 

PERIOD :- The total time require to complete one cycle is called the

period of the Vibration wave.

So frequency  = 1 / period    or         [CPM=Hertz × 60]

 VIBRATION VELOCITY :- Vibrating objects moves at some velocity. It is the rate of change of displacement of the object. In a sinusoidal wave, at the top of the curve the velocity of the object becomes zero and it is maximum at the Centre line of the curve. As velocity of the motion is continuosly changing throughout the Cycle so ‘peak’ velocity is measured to know the severity of the vibration. It is measured in inches per second peak or millimeters per second peak.VIBRATION ACCELERATION: Vibration acceleration is the rate of change of velocity of the vibrating object. The acceleration in the vibrating objects

also change continuosly and it is inversely proportional to velocity. As the velocity increases acceleration  decreases and thus the vibration acceleration is greater at the highest peak where velocity is zero and minimum at the center line where velocity is maximum It is expressed as  ‘gs’ peak where g   =  acceleration due to gravity.

 

CAUSE OF VIBRATION :- Some common factors which causes vibration

in a system are listed below.

-        Unbalance of rotating parts

-        Mis-alignment

-        Bend Shafts

-        Worn out rotary parts (gears, couplings etc.) or eccentricity

-                     Failure of bearings or bad bearings

-                     Excessive clearances in the bearings

-        Mechanical looseness

-        Rubbing

-        Resonance due to system

-                     Oil whirl

-                     Bad vane flow pattern

-                     Piping tension.

 

CRITICAL SPEED :- Critical speed (RPM) of the machine is the speed at

which the vibration coincides with the natural frequency of the machine.

There may be more than one critical speed for rotary components.

 

Vibration Monitoring:

Vibrations are measured in form of displacement, velocity or acceleration

of the vibrating part. Analysis of vibration needs measurement of its

frequency also. These days lot of modern monitoring equipments are

available in the market which may gives direct diagnosis of the problem.

 Measurement of displacement and frequency can indicate the vibration

severity. Vibration velocity takes into account both displacement and

frequency and so vibration velocity is a direct measure of vibration

severity.

          If the vibration frequency is below 600 CPM then vibration

displacement may be the best indicator of vibration severity. At low RPM

machine failure may occur due to stress (displacement) rather than

fatigue (velocity). Vibration occurring in the range of 600 to 60,000 CPM

frequency range is the medium range and in this range vibrations

measured using vibration velocity for vibration severity gives good results.

          Acceleration is closely related to force and large forces can occur at

very high frequencies even the displacement and velocity may be small in

such cases. At higher frequency failure will most likely occur due to

excessive force. Thus for high frequency cases (more them 60,000 CPM)

acceleration measurement may be the best indicator of vibration severity.

 

VIBRATION PHASE :- Phase is defined as the position of a vibrating part

at a given instance with reference to a fixed point or another vibrating

part. On a rotating part the phase is defined in angle.

 

PHASE MEASUREMENT :- Phase measurement is very essential part in

vibration analysis to diagnose the specific machinery problems. In general

the phase is measure in angles w.r.t. a fix point on the rotary part.

Generally vibrations are directly related to running RPM of the machine.

The same amount vibration displacement or frequency or velocity may be

good for low RPM machine but severe for high RPM machine. Generally

OEM specifies the normal / max. vibration limits for specific machines.

For instance 2 mils peak to peak of vibration at 3600 RPM machine are

much more destructive  than at 300 RPM machine.

 BALANCING AND DETERMINATION OF BALANCING QUALITY LIMITS :-  Unbalance occurs in a rotating machine when the center line of the mass and the Geometric center line do not coincide with each other (e.g. Fan blade assembly of an FD or ID fan, Impeller of Centrifugal Pumps or Compressor, Blade rotor assembly of a Turbine or Crankshaft of a Compressor)..

 

Unbalance amount is expressed as U= m × r

where m = Unbalance Mass and

 r = Distance of unbalance mass to shaft / Rotor centreline.

 

The unbalance force generating the vibration is expressed as

F = m × r × w2               

Where                                                F = force (N),

m = Unbalance Mass

r = Radius (m) and

w = speed (rad/sec)

 

or F (kg) = 0.01 m × (g) × r (cm) × (RPM/1000)2

 From the equation it is clear that more the radius or RPM of the rotating part for a same unbalance mass, more will be the vibration force. The unbalance response depends on dynamic stiffness of the shaft, type of bearings, quality of foundation etc. Combining all these factors will results in the unbalance force and resulting vibrations.There are various international vibration standards for rotating machine, which are internationally accepted throughout the industry. The standards define the accepting vibration levels, balancing quality, acceptance, vibration monitoring and analysis, vibration severity. API has made wide range of standards, which are used for testing and acceptance of the new and rebuilt machinery in shop floor.          For reference some ISO  and API standards are given below. 1.       ISO-7919 Series      -        Mechanical Vibration of non-reciprocating machines - measurement  of rotating shafts and evaluation criteria.2.       ISO-10816 Sencu    -        Machanical vibration - Evaluation of machine vibration by           measurement on non - rotating parts.3.       ISO-10816 Sem (Part6)-    Mechanical vibration of Reciprocating machine above 100 Kw.4.       ISO/TC108/SC2      -        Guidelines for selecting vibration evaluations methods by          measurement on the rotating shaft/non-rotating parts - to select appropriate vibration standards for a specific machinery classification.5.       ISO-13373-1:2001  -        Condition monitoring and diagnostic of machines - General           procedures provides general guidelines for the measurement of machinery vibration for conditon monitoring.6.       ISO-1940-1, 2        -Mechanical vibration - Balance qualtiy requirement of rigid rotors balance tolerence and balance errors.