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1. Unit No. 5 Vibration and Strain
Measurement
Babasaheb Phadtare Polytechnic, Kalamb-Walchandnagar
Subject- Mechanical Engineering
Measurement

2. Unit No. 5
Vibration and Strain
Measurements
Course Outcome (CO):
Select relevant Instruments for
measurement of Vibration and
Strain

3. 1. Free Vibration
If a system after an initial disturbance is left to vibrate
on its own, the resulting vibration is known as free
vibration.
2. Force Vibration
When the body vibrates under the influence of external
force, the body is said to be under forced vibrations.
Velocity Pickup (Vibration Measurement)
The relative motion between the permanent magnet
and the coil generates a voltage that is
proportional to vibration. As the test object attached
to the pickup vibrates, the inside the
pickup forced the magnetic field by
vibratory motion.

4. Accelerometer
The basic principle of accelerometer is that it consists
piezo electric material which produces electrical charge
whenever stressed by a vibrating force.
The acceleration of the test structure is transmitted to a
seismic mass which generates a proportional force on the
piezoelectric crystal which generates voltage or charge on
its terminals.
This charge is proportional to the applied
force or proportional to acceleration if the
mass is constant. The charge is connected
into voltage using charge amplifiers
and associated signal conditioning circuit.

5. Inductive Type Transducer
The variation in air gap is often caused by the
displacement of the armature, resulting in the change of
inductance. This change of inductance can be measured
by AC bridge circuit or by a change of current in the
circuit.
Advantages: 1. No direct physical contact with measured.
2. It gives linear characteristics over wide range of output.
3. It is free from maintenance.
Disadvantages:
1. They have short sensing range.
2. It works for sensing certain types of
metal objects.
Applications: 1. Flow measurement.
2. Displacement measurement.
3. Speed measurement.

6. Capacitive Transducers
the capacitance of capacitor is varies inversely with the
distance between the plates. Any change in the distance
'd' between the two plates causes a corresponding
variation in the capacitance. Here capacitance becomes
the function of spacing between to plate. This principle is
used to measure the displacement.

7. Stroboscope
The main components of stroboscope are the high
intensity lamp called strobe and an electronic circuit to
adjust or vary the frequency of flashing the lamp.
When stroboscope flashing frequency is synchronized
with the vibrating element of machine or structure , the
vibrating component appears to be stationary, it is called
as stop motion effect.

8. Introduction to FFT Analyzer
FFT is a method of analysis, based on vibration waveform.
Generally waveforms are complicated and difficult to
analyze. In FFT analyzer breaks input waveforms down
into a series of discrete sine waves and evaluate each
individually.
FFT analyzer converts the time domain waveform into
frequency domain. FFT calculates the spectral
components of waveform and the spectrum in required
format displayed by the display device.
Application: 1. For analysis of sound and vibrations.
2. Vibration analysis of various drives, machines.

9. Strain Measurement
Requirements of an Ideal Strain Gauge
(1) Extremely small in size.
(2) It should have insignificant mass.
(3) Easy to attach to the member being analyzed.
(4) Highly sensitive to strain
(5) Capable of indicating both static and dynamic strains.
(6) Capable of remote indication and recording.
(7) Inexpensive. (cost factor).
(8) Unaffected by temperature, vibration humidity or
other ambient conditions likely to be encountered in
testing machine parts under service loads.

10. Terms Related to Strain Gauges
(1) Cross sensitivity :
The portion of the grid senses the strain in the transverse
direction and the output on account of that is
superimposed on the longitudinal output. This is called
cross sensitivity.
(2) Creep :
When the measurement are carried out over a long
duration of time creep is considered only in static strain
gauge testing. When the gauge is subjected to stress only
once and loading cycle is not repeated or it is subjected
to slow varying loading cycles. Creep exists in gauge.

11. (3) Fatigue :
Strain gauge may fail because of fatigue
loading. The fatigue sensitive area is the discontinuity
formed at the junction at the grid and the lead wire
where the connections are made usually. Isoelastic is
better than constantan carrier material is an important
factor in determining the life at gauge under fatigue
conditions.
(4) Post yield gauge :
Post yield gauges are those whose usable range extended
to approximately 10 to 20 percent. A very ductile grid
material is used. This is known as development of elastic
plastic grid.

12. Classification of Strain gauge
(A) Unbounded
(a) Metallic wire.
(B) Bonded.
(a) Semiconductor.
(b) Metallic.
(i) Helical wrap wound on a flattened grid
(ii) Etched foil gauge
1. Single element.
2. Multiple element.
(iii) Wire type flat grid.
1. Single element.
2. Multiple element.
(iv) Wire type woven grid.

13. Strain gauge Material
(i) Constantan - Nickel 45%, Chromium 55%. (ii) Advance -
Copper 57%, Nickel 43%. (iii) Isoelastic - Iron 52%,
Nickel 36%, Chromium 8%, Molybdinum 0.5%
(iv) Nichrome - Nickel 80%, Chromium 20%.
Desirable characteristics of grid material
1. It should have high specific resistance.
2. It should have low temperature sensitivity.
3. It should have low hysteresis.
4. It should have good workability, solderability and weldability.

14. Unbounded Strain gauge:
Fine wire filaments that is the resistance
wires are stretched around rigid and
electrically insulated pins on two frames A
and B which can move relative to each other.
When the frame A moves relative to the
frame B the wire filaments are strained.
Wire type strain gauge:
- Desirable characteristics of bonded type
resistance strain gauges.
- Small size and very low mass.
- Fully bonded to basic spring structure.
- Excellent linearity over wide range of
strains. -Small surface area
- Low leakage. - High isolation

15. Calibration of bonded strain gauges:
Calibration can be performed as follows resistance strain
gauges are manufactured under controlled and the
gauge factor is provided to the manufacturer with a
tolerance at about # 0.2 %. By knowing the gauge factor
and the gauge resistance it is possible to calibrate a
strain gauge.
In this system response is studied by introducing a
known small resistance change at the gauge and
calculating as equivalent strain.
The resistance change is introduced by shunting a
relatively high value precision resistance across the
gauge.

16. Metal foil gauge:
The gauge is produced by printed circuit technique and
consists of a foil grid on plastic backing.
The desired grid pattern is first printed on a thin sheet of
metal-alloy foil with an acid resistant ink and then the
unprinted portion is etched away. This construction
allows the use of varying sections throughout the grid
length; larger area can be provided at the ends where
lead connections are made.

17. Semi-conductor or piezoresistive gauge:
These gauges are produced from silicon or germanium
crystals in which exact amount of special impurities such
as boron have been added to impart certain desirable
characteristics.
The process is called doping and the crystals are known
as doped crystals. The semi-conductor gauges are
classified into :
1. Negative or n-type whose resistance decreases in
response tensile strain.
2.Positive or p-type whose
resistance increases in response
to tensile strain.