The document discusses different types of comparators used to measure small dimensional differences between a workpiece and a standard. It describes mechanical, optical, pneumatic, and electric comparators. Mechanical comparators use lever systems or gears to magnify movements, while optical comparators further amplify mechanical movements using projected images. Pneumatic comparators detect pressure differences caused by air flowing through an orifice and measure workpieces without physical contact. Each type has advantages like accuracy, non-contact measurement, or portability, and disadvantages like required power sources, moving parts affecting precision, or being more complex.

1. COMPARATORS
2017
SUBMITTED BY:
SAJAL GUPTA
2K16/PIE/16

2. CONTENTS:
1. Introduction
1.1 Need for comparator
1.2 Essential characteristics of a comparator
2. Types of comparator
3. Mechanical comparator
3.1 Dial indicator (Dial Gauge)
3.2 Reed Comparator
3.3 Johansson Mikrokator
3.4 Sigma Comparator
3.5 Advantages
3.6 Disadvantages
4. Mechanical optical comparators
4.1 Advantages
4.2 Disadvantages
5. Pneumatic Comparator
5.1 Principle of pneumatic comparator
5.2 Characteristics
5.3 Working principle of pneumatic comparator
5.4 Advantage
5.5 Disadvantage
6. Electric comparator
6.1 Advantage
6.2 Disadvantage

3. 1. INTRODUCTION:
The general principle ofcomparatoris to indicate the differences in size between the
standard and the work being measured by means of some pointer on a scale with
sufficient magnification. It thus does notmeasure the actual dimension but indicates
how much it differs from the basic dimension.
All comparators consist of three basic features:
1) A sensing device
• which faithfully senses the input signal
2) A Magnifying or amplifying system
• To increase the signal to suitable magnitude. Mechanical, Optical, Pneumatic,
hydraulic and electronic methods are used for this purpose.
3) A display system (usually a scale and pointer)
• Which utilizes the amplified signal to provide a suitable readout.
1.1 NEED FOR COMPARATOR:
• In mass production, where components are to be checked at a very fast rate.
• As laboratory standards from which working or inspection gauges are set and
correlated.
• For inspecting newly purchased gauges.
• Attached with some machines, comparators can be used as working gauges to
prevent work spoilage and to maintain required tolerances at all stages of
manufacturing.
• In selective assembly of parts, where parts are graded in three or more groups
depending upon their tolerances.

4. 1.3 Essential characteristics of a comparator:
1. Robust design and construction
2. Linear characteristics of scale
3. High magnification
4. Quick in Results
5. Versatility
6. Minimum Wear of contact Point
7. Free from Oscillations
8. Free from backlash
9. Quick insertion of work piece
10.Adjustable table
11.Compensation from temperature effects
12.Means to prevent damage
2. TYPES OF COMPARATOR:
(1) Mechanical comparators
(2) Optical comparators
(3) Electrical and Electronic comparators
(4) Pneumatic comparators
(5) Fluid displacement comparators
(8) Automatic gauging machines

5. 3. MECHANICAL COMPARATOR:
A mechanical comparator employee’s mechanical means for magnifying the small
movement of the stylus, the magnification of the small stylus movement is obtained
by means of levers, gear trains, rack and pinion or a combination. The usual
magnification obtained by these comparators ranges from about 250 to 1000.
Types of Mechanical Comparators:
1. Dial indicator (Dial Gauge)
2. Read Type
3. Johansson Mikrokator
4. Sigma Comparator
3.1 Dial Indicator:

6. • The simplest type of mechanical comparator
• It consists of a base with a rigid column rising from its rear
• An arm mounted on this column and it carries a dial gauge at its outer end
• The indicator is set at zero by the use of slip gauges
• The part to be checked is placed below the plunger
• The linear movement of the plunger is magnified by means of mechanical
means to a sizable rotation of the pointer
• This type is generally used for inspection of small precision machined parts
• The dial gauge with various attachments are commonly used
• This comparator is ideal for the checking of components with a tolerance of +
0.005 mm
3.2 Johansson Mikrokator:
Figure 2

7. A very thin metal strip at the center carries a light pointer made up of glass. One end
of the strip is connected to the adjustable cantilever strip and the other end is to the
spring elbow, in turn connected to the plunger. The slight movements of the plunger
will make the bell crank lever to rotate. This rotation will create tension in the strip
and causes the strip to rotate thereby the strip start to untwist resulting in the
movement of the point. The spring ensures that the plunger returns when the
contact is removed. The length of the cantilever can be varied to adjust the
magnification. The mechanical magnification is obtained by “AbramsonMovement.
3.3 Reed Type Comparator:
Figure 3

8. The gauging head is usually a sensitive, high quality, dial indicator mounted on a
base supported by a sturdy column. The reed mechanism is frictionless device for
magnifying small motions of spindle. It consists of a fixed block A which is rigidly
fastened to the gauge head case, and floating block B, which carries the gauging
spindle and is connected horizontally to the fixed block by reeds C
Vertical reed is attached to each block with upper ends joined together. These
vertical reeds are shown in the figure by letter D. Beyond this joint extends a pointer
or target. A linear motion of the spindle moves the free block vertically causing the
vertical reed on the floating block to slide past the vertical reed on the fixed block.
However, as these vertical reeds are joined at the upper end, instead of slipping,
the movement causes both reeds swing through an arc and as the target is merely an
extension of the vertical reeds, it swings through a much wider arc. The amount of
target swing is proportional to the distance the floating block has moved but of
course very much magnified.
The scale may be calibrated by means of gauge block (slip gauges) to indicate any
deviation from an initial setting. Sensitivities of the order of 0.25 micron per scale
division. The mechanical amplification is usually less than 100, but it is multiplied
by the optical lens system. It is available in amplifications ranging from x 500 to x
1000.
3.4 Sigma Comparator:
The plunger is mounted on a pair of slit diaphragms in order to have frictionless
linear movement. A knife edge is mounted on it and bears upon the face of the
moving member of a cross strip hinge. The cross strip hinge consists of the moving
component and a fixed member which are connected by thin flexible strips
alternately at right angle to each other. Thus if an external force is applied to the

9. moving member; it will pivot, as would a hinge, about the line of intersection of the
strips.
To the moving member an arm of Y shape and having effective length I is attached.
Figure 4
3.5 Advantages of mechanical comparators:
• Cheaper than all the other type of comparators
• Does not require any external source of power or air supply
• These comparators use a linear scale that can be easily understood.
• Usually these comparators are robustand compactbut are very easy to handle
• Theseare small in size and canare portable from oneplace to other very easily
without much difficulty.
3.6 Disadvantages of mechanical comparators:

10. • Contains more number of moving parts so there develops friction which in
turn reduces the accuracy
• Slackness in the moving parts reduces accuracy very drastically
• These have more inertia so the instrument is prone to vibrational effects
• Limited range ofthe instrument is another drawback as the pointer moves over
a fixed scale
• Parallax error may also arise when proper scale is used.
4. Mechanical optical comparators:
Figure 5
In mechanical optical comparators small displacements ofthe measuring plunger are
amplified first by a mechanical system consisting of pivoted levers. The amplified
mechanical movement is further amplified by a simple optical system involving the
projection ofan image. The usual arrangement employed is such that the mechanical

11. system causes a plane reflector to tilt about an axis and the image of an index is
projected on a scale on the inner surface of a ground-glass screen. Optical
magnification provides high degree of measuring precision due to reduction of
moving members and better wear resistance qualities. Optical magnification is also
free from friction, bending, wear etc. The whole system could be explained
diagrammatically by Fig. below, which gives very simple arrangement and explains
the principle of above comparator.
In this system,
• Mechanical amplification= l2/l1
• Optical amplification = l4/l3 * 2.
• It is multiplied by 2, becauseif mirror is tilted by an angle δθ, then image will
be tilted by 2 * δθ.
• Thus overall magnification of this system=2*l2/l1 * l4/l3.
4.1 Advantages of optical comparators:
1. It has small number of moving parts and hence a higher accuracy.
2. In the optical comparators, the scale can be made to move past a datum line
and thus have high range and no parallax errors.
3. It has very high magnification.
4. Optical lever is weightless
4.2 Disadvantages optical comparators:
1. As the instrument has high magnification, heat from the lamp, transformer etc.
may cause the setting to drift.

12. 2. An electrical supply is necessary.
3. The apparatus is usually large and expensive.
4. When the scale is projected on a screen, then it is essential to use the
instrument to a dark room in order to take the readings easily.
5. The instruments in which the scale is viewed through the eyepiece of a
microscope are not convenient for continuous use.
5. PNEUMATIC COMPARATORS:
5.1 Principle of pneumatic 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 then used to find the dimensions of a component.
5.2 CHARACTERSTICS PNEUMATIC COMPARATORS:
• Very high amplifications are possible.
• As no physical contactis made either with the setting gauge or the part being
measured, there is no loss of accuracy because of gauge wear.
• Internal dimensions can bereadily measured notonly with respectto tolerance
boundaries but also geometric form.
• It is independent of operator skill.
• High pressure air gauging can be done with cleansing of the parts which helps
to eliminate errors due to dirt and foreign matter.
• Gauging pressures can be kept sufficiently low to prevent part deflection.

13. • Dimensional variations throughout the length of shaft or cylinder bore can be
explored
• Not only it measures the actual size, but it can also be used to salvage
oversized pieces
• The total life cost of the gauging heads in much less.
• It is accurate, flexible, reliable, universal and speedy device for inspecting
parts in mass production.
• It is best suited for checking multiple dimensions and conditions on a part
simultaneously in least possible time.
5.3 WORKING OF PNEUMATIC COMPARATORS:
Figure 6
The air is compressed in the compressor at high pressure which is equal to Water
head H. The excess air escapes in the form of bubbles. Then the metric amount of
air is passed through the orifice at the constant pressure. Due to restricted area, at
A1 position, the back pressure is generated by the head of water displaced in the

14. manometer tube. To determine the roundness of the job, the job is rotated along the
jet axis, if no variation in the pressure reading is obtained then we can say that the
job is perfectly circular at position A1. Then the same procedure is repeated at
various positions A2, A3, A4, position and variation in the pressurereading is found
out. Also the diameter is measured at position A1 corresponding to the portion
against two jets and diameter is also measured at various position along the length
of the bore.
5.4 ADVANTAGES OF PNEUMATIC COMPARATORS:
• It is cheaper, simple to operate and the cost is low.
• It is free from mechanical hysteresis and wear.
• The magnification can be obtained as high as 10,000 X.
• The gauging member is not in direct contact with the work.
• Indicating and measuring is done at two different places.
• Tapers and ovality can be easily detected.
• The method is self-cleaning due to continuous flow of air through the jets and
this makes the method ideal to be used on shop floor for online controls.
5.5 DISADVANTAGES OF PNEUMATIC COMPARATORS:
• It requires elaborate auxiliary equipment such as accurate pressure regulator.
• The scale is generally not uniform.
• When indicating device is the glass tube, then high magnification is necessary
in order to avoid the meniscus errors.
• The apparatus is not easily portable and is rather elaborate for many industrial
applications.

15. • Different gauging heads are required for different dimensions.
6. ELECTRICAL COMPARATORS:
Figure 7
Electrical comparators have no moving parts, thus having high degree of
reliability. Generally there are two important applications of electrical
comparators:
• Used as measuring heads for precise measurement.
• Used for electrical gauging heads, to provide usual indication to check the
dimensions within the limits laid down. This application is used to check
dimension.
It is indicated with a green light if a dimension is within the limits. A red lamp
indicates an undersized dimension; a yellow lamp indicates an oversize

16. dimension. So the operator is not required to be aware of the actual tolerances on
the dimension.
6.1 ADVANTAGES OF ELECTRICAL COMPARATORS:
• Measuring units can be remote from indicating units.
• Variable sensitivity which can be adjusted as per requirement.
• No moving parts, hence it can retain accuracy over long periods.
• Higher magnification is possible as compared to mechanical comparator.
• Compact sizes of probes arc available.
6.2 DISADVANTAGE OF ELECTRICAL COMPARATORS:
• The accuracy of working of these comparators is likely to be affect due to
temperature and humidity.
• It is not a self-contained unit; it needs stabilized power supply for its
operation.
• Heating of coils can cause zero drifts and it may alter calibration.
• It is more expensive than mechanical comparator.