VIBRATION ANALYSIS FOR CBMBASIC CONCEPTS – UNIT 1

BY DIBYENDU DE

ALL MACHINES VIBRATE

Because of: Inertia

Stiffness Imposed forces

VIBRATION MONITORING

What causes Vibration ?

INDUCED FORCE

&

FREEDOM FOR MOVEMENT

VIBRATION MONITORING

Types of Forces ?

Inertia ForcesAcceleration & Deceleration

Looseness & Impacts

MACHINE VIBRATION IS COMPLEX

• Behaviour of each component creates their own frequencies

• Frequency tells us about a possible problem. • Amplitude tells us about the severity of the

problem. • Multiple frequencies combine to produce the

overall vibration• Vibration from 1 Hz to 1000 Hz captures

overall amplitude

ONE CYCLE OF VIBRATION

• Movement of weight from neutral position to upper limit

• Upper limit back through neutral position to lower limit

• Lower limit to neutral position• The movement of the weight

plotted against time is a sine wave

•

FREE & FORCED VIBRATION

• With sudden impulse the system will vibrate at its natural frequency

• Free vibrations die out with time if the system is stable. (Bump test)

• Forced vibration are periodically imposed on the system (like motor). The response is also periodic

FORCE, MOBILITY & VIBRATION

• Response to forced vibration is not uniform at all frequencies.

• Response depends on the mobility of the structure

• Mobility varies with frequencies. High at resonance. Low where damping is present.

MEASUREMENT OF AMPLITUDE & FREQUENCYPeak for velocity and acceleration (used for algorithms)

Peak to Peak Displacement

RMS Velocity & acceleration (Europe) Energy content (only for sine wave)

Average Not used in vibration analysis

Frequency = 1/Period (Hz/CPM)

MEASUREMENT OF AMPLITUDE

Peak for velocity and acceleration

Peak to Peak Displacement

RMS Velocity & acceleration (Europe) Energy content (only for sine wave)

Average Not used in vibration analysis

DISPLACEMENT, VELOCITY & ACCELERATION

What?• Displacement = d = x = A sin (wt)

• Velocity = v = dx / dt = Aw cos wt = Aw sin (wt + 90o)

• Acceleration = a = dv /dt = - Aw2 sin wt = Aw2 sin (wt + 180o)

Why?• Strain Energy ∝ to d2

• Kinetic Energy ∝ to v2

• Acoustic Pressure ∝ to a2

• Force

VIBRATION - CAR ANALOGY

Low FrequencyLong period (T)High displacementAverage VelocityLow Acceleration

Medium FrequencyMedium period (T)Average displacementAverage VelocityAverage Acceleration

High FrequencyShort period (T)Low displacementAverage VelocityHigh Acceleration

THANK YOU FOR COMPLETING UNIT 1 CONTACT: FOR FACILITATED TRAINING

DIBYENDU DE, EMAIL ID: DDE337@GMAIL.COM