Mechanical Measurements and Metrology

1. Department of Mechanical Engineering
JSS Academy of Technical Education, Bangalore-560060
MECHANICAL MEASUREMENTS AND METROLOGY
(Course Code:18ME36B)

2. TEXT BOOKS
• Mechanical Measurements, Beckwith Marangoni and Lienhard, Pearson Education, 6th Ed., 2006.
• Instrumentation, Measurement and Analysis, B C Nakra, K K Chaudhry, 4th Edition, McGraw Hill.
• Engineering Metrology, R.K. Jain, Khanna Publishers, Delhi, 2009
REFERENCE BOOKS:
• Engineering Metrology and Measurements, N.V.Raghavendra and L.Krishnamurthy, Oxford
University Press..
Further Reference:
 National Programme on Technology Enhanced Learning (NPTEL)
http://nptel.ac.in/courses/112104121/1

3. MECHANICAL MEASUREMENTS AND METROLOGY
CHAPTER 3: Comparators

4. Learning Objectives
Explain the basic principle of construction and operation of different types of
comparator.

5. Explain the basic measurement principles of comparators
Outcomes

6. Module 2
Comparators: Functional requirements & Classification
• Mechanical comparators - Johnson Mikrokator, Sigma comparators, Dial indicator.
• Electrical comparators - LVDT
• Pneumatic comparators - Back pressure, Solex
• Optical comparators - Zeiss ultra optimeter.

7. Direct measurement vs. Comparison
Introduction
Accuracy of direct measurement depends on four factors:
• Accuracy of the standard
• Accuracy of scale
• Least count of the scale
• Accuracy of reading the scale.
• Human element

8. Direct measurement vs. Comparison
Introduction
Accuracy of comparison measurement primarily depends
• Accuracy of the standard used for setting the comparator
• Least count of the standard
• Sensitivity of the comparator
• Accuracy of reading the scale

9. FUNCTIONAL REQUIREMENTS
1. A comparator should have a high degree of accuracy and precision.
2. Scale should be linear and have a wide range.
3. Should have high amplification.
4. Should have good resolution (least possible unit of measurement).
5. Provision for incorporating to compensate for temperature effects.
6. Should be versatile (provisions for different ranges, attachments).

10. CLASSIFICATION OF COMPARATORS
Based on the principle used for amplifying and recording measurements
• Mechanical comparators
• Mechanical–optical comparators
• Electrical and electronic comparators
• Pneumatic comparators
• Projection comparators and multi-check comparators

11. CLASSIFICATION OF COMPARATORS
Mechanical comparators
• Johnson Mikrokator
• Sigma comparators
• Dial indicator
Mechanical Comparators are made up of mechanical means.

12. MECHANICAL COMPARATORS
Dial Indicator / Dial Gauge

13. MECHANICAL COMPARATORS
Dial Indicator / Dial Gauge
The movement of the Plunger(spindle) will be multiplied thru the series of gears
and pinions and indicated on the main scale on the dial by the indicator(Needle).
Principle (Gear & Pinion type)

14. MECHANICAL COMPARATORS
Dial Indicator / Dial Gauge
Principle (Gear & Pinion type)

15. MECHANICAL COMPARATORS
Dial Indicator / Dial Gauge
Principle (Gear & Pinion type)
• The plunger moves linearly wrt. change on the workpiece while taking
measurements.
• The plunger consist of a rack and it meshes with a pinion(P1) on the gear(G1)
• This Gear (G1) mesh with the series pinions and gears to multiply the movement
to increase the accuracy of measurement.
• Final Pinion (P3) is connected to the Indicator(Needle) this indicator will show
the deflection on the main scale.

16. MECHANICAL COMPARATORS
Johnson Mikrokator / Abramson's movement
• Designed by Hugo Abramson in 1938
• Developed by C.F. Johansson.
• Used to obtain mechanical magnification of the difference in
length as compared to a standard.

17. MECHANICAL COMPARATORS
Johnson Mikrokator / Abramson's movement
• It works on the principle of a button spinning on a loop of string.
• A twisted thin metal strip holds a pointer, which shows the reading on a suitable
scale.
• Since there is no friction involved in the transfer of movement from the strip to
the pointer, it is free from backlash.

18. MECHANICAL COMPARATORS
Johnson Mikrokator / Abramson's movement

19. Johnson Mikrokator / Abramson's movement
• A metallic strip is twisted and fixed between two ends.
• Longitudinal movement will cause the central portion of the strip to rotate.
• One end of the strip is fixed to an adjustable cantilever and the other end is fixed to the spring
elbow.
• The spring elbow is connected to a plunger, which moves upwards or downwards.
• The spring elbow consists of flexible strips and stiff diagonal acts as bell crank lever, causing
the twisted strip to change length when there is a movement in the plunger;
• This change in length results in a proportional amount of twist of the metallic strip.
• The magnification can be varied by changing the length of the spring elbow

20. Sigma Comparator

21. Sigma Comparator
Linear displacement of a plunger is translated into the movement of a pointer over a
calibrated scale.
• Plunger is the sensing element that is in contact with the work part.
• It moves on a slit washer, providing frictionless linear movement, also arrests
rotation of the plunger about its axis.
• A knife edge is attached onto the plunger, bears upon the face of the moving
member of a cross-strip hinge.
• This unit comprises a fixed member and moving block, connected by thin flexible
strips at right angles to each other.

22. Sigma Comparator
• When the plunger moves up or down, the knife edge drives the moving member
of the cross-strip hinge assembly, deflecting an arm, which divides into ‘Y’ form.
• Extreme ends of Y-arm is connected to a driving drum by means of phosphor-
bronze strips.
• The movement of the Y-arm rotates the driving drum, in turn, the pointer spindle.
Working

23. CLASSIFICATION OF COMPARATORS
Electrical comparators
• LVDT
• Electrical comparator
• Electrical comparators convert the linear movement of the plunger into
electrical signals and these signals.
• They rely on a Wheatstone bridge circuit for measurement.

24. LVDT (Linear Variable Differential Transformer)
• Electromechanical transducer, convert the rectilinear motion of an object to
into a electrical signal.

25. LVDT (Linear Variable Differential Transformer)
• The main components of an LVDT are transformer and a core.
• The transformer consists of three coils - a primary and two secondaries wound on
a hollow cylindrical tube.
• The primary coil is located between the two secondary coils.
• The ferro magnetic core moves freely inside the cylindrical tube.
• Non-ferromagnetic shaft / push rod, is coupled to the core and connects to the
object being measured.

26. Electrical comparator

27. Electrical comparator
• The plunger is the sensing element, the movement of which displaces an
armature inside a pair of coils.
• Movement of the armature causes change in inductance in the two coils,
resulting in a net change in inductance.
• This change causes an imbalance in the bridge circuit, resulting in an output.
• The output display device (analog or digital) is calibrated to show the readings in
units of length, i.e. linear displacement.

28. CLASSIFICATION OF COMPARATORS
Pneumatic comparators
• Operated by the gas or air under pressure.
• Pneumatic comparators enables two approaches for measuring the
Deflection in measurement;
1. Measuring the Air pressure
2. Measuring the Velocity of Airflow

29. Back Pressure comparator
• It works on the principle of pressure difference generated by the air flow.
• Air is supplied at constant pressure through the orifice and the air escapes in
the form of jets through a restricted space which exerts a back pressure.
• The variation in the back pressure is used to find the dimensions of a
component.
Principle

30. Back Pressure comparator
Principle

31. Back Pressure comparator
Principle
• The system uses a two-orifice, Orifice O1 (control orifice) & Orifice O2 (measuring
orifice).
• The measuring head gets compressed air supply at a constant pressure P (source
pressure).
• It passes through the control orifice into an intermediate chamber.
• Air exits the through the measuring orifice.
• Size of the control orifice remains constant.
• Effective size of the measuring orifice varies, as the gap d between the measuring
orifice and the work surface.
• Depending on the gap d, the back pressure Pb changes, providing a means for
measuring dimension d.

32. Solex comparator
Principle

33. Solex comparator
• The instrument is produced commercially by Solex air gauges Ltd. USA
• Designed for internal measurement
• Consists of a tank, water is filled up to a certain level (H)
• Dip tube is immersed.
• Air is passed at high pressure, some air will leak out from the dip tube in the
form of air bubbles.
Principle

34. Solex comparator
• This ensures the air moving towards control orifice at a constant pressure.
• The air at a reduced pressure then passes through the control orifice.
• Then air escapes from the measuring orifice in the measuring head.
• Based on the clearance between work part and measuring orifice, back
pressure is created, resulting in the head of water being displaced in the
manometer tube.
Principle

35. Optical comparators
• Device that applies the principles of optics to the inspection of parts.
• In optical comparator, a light source and a reflecting surface(Mirror) are used as
the optical means.
• An incident ray will hit the mirror & gets reflected, this ray is projected on to the
scale.
Principle
Incident angle = Reflecting angle

36. Zeiss Ultra-optimeter
1.Light Source
2.Green Filter
3.Condenser
4.Two Reflecting Mirrors (M1, M2)
5.Plunger
6.Objective Lens
7.Graticule Screen
Principle

37. Principle
• The Light from the source passes through the green filter and condenser.
• The function of the Green filter is to allow only green light
• The filtered beam light passes through the Condenser.
• The function of the condenser is to focus the light to the movable mirror (M1).
• There are two mirrors, Mirror 1 (M1) and Mirror 2 (M2).
• Mirror 1 is a movable mirror, operated by the Plunger & Mirror 2 is fixed.
• The plunger is the measuring tip (Contacts to the workpiece to take the deviation.
• Once the focused light from the condenser falls on the mirror M and get reflected on to the
Mirror 2 and reflected back to the Mirror 1.
• The objective lens will collect this image and projects on to the graticule.
• This image on the Graticule is observed by the Eye Piece.

38. End of Module