Comparators and its type

Types of Comparators used in Metrology

1. Comparators
By
Prof N D Sadaphal
Assistant Professor

Sanjivani College of Engineering,
Kopargaon (Maharashtra State) 423601

Mechanical Engineering

2. 1
Comparator

A comparator is a precision instrument employed to
compare the dimension of a given component with a
working standard (usually slip gauges).

It thus does not measure the actual dimension but
indicate how much it differs from the basic dimension.

In mass production identical component parts are
produced on a very large scale.

Micrometer and Vernier caliper are not more feasible
because skill involve and time required to measure the
dimension .

In such case comparator is more suitable for the
measurement.
Prof N D Sadaphal
Classification:
1. Mechanical Comparator: It works on gears pinions, linkages,
levers, springs etc.
2. Pneumatic Comparator: Pneumatic comparator works by using
high pressure air, valves , back pressure etc.
3. Optical Comparator: Optical comparator works by using lens,
mirrors, light source etc.
4. Electrical Comparator: Works by using step up, step down
transformers.
5. Electronic Comparator: It works by using amplifier, digital signal
etc.
6. Combined Comparator: The combination of any two of the above
types can give the best result.
Prof N D Sadaphal

3. 2
Prof N D Sadaphal
Mechanical Comparator
MECHANICAL COMPARATOR

Some of the comparator which belong to this class are:
1.
Dial indicator
2.
Reed type comparator
3.
Sigma Comparator
4.
Johnson Mikrokrator
Prof N D Sadaphal

4. 3
Dial Indicator:

It operates on the principle, that a very slight upward pressure on the
spindle at the contact point is multiplied through a system of gears
and levers.

It is indicated on the face of the dial by a dial finger.

Dial indicators basically consists of a body with a round graduated
dial and a contact point connected with a spiral or gear train so that
hand on the dial face indicates the amount of movement of the
contact point.

The indicator is set to zero by the use of slip gauges representing the
basic size of part.
Prof N D Sadaphal
DIAGRAM
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5. 4
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Dial Indicator: (different uses)
Prof N D Sadaphal

6. 5
Applications:
1.
Comparing two heights or distances between narrow limits.
2.
To determine the errors in geometrical form such as ovality, roundness
and taper.
1.
To check the alignment of lathe centers by using suitable accurate bar
between the centers.
2.
To check the parallelism of two surfaces.
Prof N D Sadaphal
Johansson Mikrokator :
Prof N D Sadaphal

7. 6
Principle:

ItworksontheprincipleofaButtonspring,spinningonaloopofstring
likeinthecaseofChildren’stoys.

ThemethodofmechanicalmagnificationisshowninFigure.

Itemploysatwistedmetalstrip.Anypullonthestripcausesthecenterof
thestriptorotate.

Averylightpointermadeofglasstubeisattachedtothecenterofthe
twistedmetalstrip.

Themeasuringplungerisontheslitwasherandtransmitsitsmotion
throughthebellcranklevertothetwistedmetalstrip.

Theotherendofthetwistedmetalstripisfastenedtothecantileverstrip.
Theoverhanginglengthofthecantileverstripcanbevariedtoadjustthe
magnificationoftheinstrument.

Thelongerthelengthofthecantilever,themoreitwilldeflectunderthe
pullofthetwistedmetalstripandlessrotationofthepointerisobtained.
Prof N D Sadaphal

8. 7
Reed Type Mechanical Comparator
Prof N D Sadaphal
HOW IT WORKS

Thereedmechanismisfrictionlessdeviceformagnifyingsmall
motionsofspindle.

ItconsistsofafixedblockAwhichisrigidlyfastenedtothegauge
headcase.

ThefloatingblockBcarriesthegaugingspindleandisconnected
horizontallytothefixedblockbyreedsC.

Averticalreedisattachedtoeachblockwithupperendsjoined
together,asshowninthefigure.Beyondthisjointextendsapointeror
target.

Alinearmotionofthespindlemovesthefreeblockverticallycausing
theverticalreedtoslide.

Thismovementcausesthepointerortargettoswingthroughanarc
overthescaleproportionaltothedistancetraveledbythespindleand
ofcourseverymuchmagnified.Comparatorsofthistypemayhave
sensitivitiesoftheorderof0.25micronperscaledivision.
Prof N D Sadaphal

9. 8
Mechanical -Optical Comparator
:
Prof N D Sadaphal
Principle:

In mechanical optical comparator, small variation in
the plunger movement is magnified first by
mechanical system and then by optical system
Prof N D Sadaphal

10. 9
HOW IT WORKS

Inmechanicalopticalcomparator
smalldisplacementofthe
measuringplungerareamplified
firstbymechanicalsystem
consistingofpivotedlavers.

Theamplifiedmechanical
movementisfurtheramplifiedbya
simpleopticalsysteminvolvingthe
projectionofanimage.

Mechanicalsystemcausesaplane
reflectortotiltaboutanaxisand
theimageofanindexisprojected
onascaleontheinnersurfaceofa
groundglassscreen.

Opticalmagnificationprovides
highdegreeofmeasuringprecision.
Prof N D Sadaphal
IMPORTANT

In this system,
Mechanical amplification = x2 / x1
Optical amplification = (x4 / x3 ) * 2

It is multiplied by 2, because if
mirror is tilted by an angle δθ, then
image will be tilted by
2 x δθ.

Thus over magnification of this
system = 2 (x2 / x1) *(x4 / x3)

Advantages of Optical
Comparators:-

1.Ithassmallnumberofmoving
partsandhenceahigheraccuracy.

2.highrangeandnoparallaxerrors.

3.Ithasveryhighmagnification.

4.Opticalleverisweightless.
Prof N D Sadaphal

11. 10
Pneumatic Comparators (Solex Gauge):
Principle:
•Itworksontheprincipleof
pressure difference
generatedbytheairflow.
•Airissuppliedatconstant
pressurethroughtheorifice
andtheairescapesinthe
formofjetsthrougha
restrictedspacewhichexerts
abackpressure.
•Thevariationintheback
pressureisthenusedtofind
thedimensionsofa
component.
Prof N D Sadaphal
How It Works
Working:

AsshowninFigure(a)theairiscompressedinthecompressorat
highpressurewhichisequaltoWaterheadH.

Theexcessairescapesintheformofbubbles.Thenthemetric
amountofairispassedthroughtheorificeattheconstantpressure.

Duetorestrictedarea,atA1position,thebackpressureisgenerated
bytheheadofwaterdisplacedinthemanometertube.

Todeterminetheroundnessofthejob,thejobisrotatedalongthejet
axis,ifnovariationinthepressurereadingisobtainedthenwecan
saythatthejobisperfectlycircularatpositionA1.

ThenthesameprocedureisrepeatedatvariouspositionsA2,A3,A4,
positionandvariationinthepressurereadingisfoundout.

AlsothediameterismeasuredatpositionA1correspondingtothe
portionagainsttwojetsanddiameterisalsomeasuredatvarious
positionalongthelengthofthebore
Prof N D Sadaphal

12. 11
Sigma Comparator:
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13. 12
Prof N D Sadaphal
HOW IT WORKS

The plunger is attached to a bar which is supported between the bending
plates at the top and bottom portion as shown in Figure (a)

The bar is restricted to move in the vertical direction. A knife edge is
fixed to the bar.

The knife edge is attached to the sapphire plate which is attached to the
moving block.

The knife edge extorts a force on the moving block through sapphire
plate. Moving block is attached to the fixed block with the help of
crossed strips as shown in Figure (b).

When the force is applied on the moving block, it will give an angular
deflection.

A Y-arm which is attached to the moving block transmits the rotary
motion to the driving drum of radius r.

This deflects the pointer and then the reading is noted.
Prof N D Sadaphal

14. 13
Advantages of Mechanical Comparator

1. They do not require any external source of energy.

2. These are cheaper and portable.

3. These are of robust construction and compact
design.

4. The simple linear scales are easy to read.

5. These are unaffected by variations due to external
source of energy such air, electricity etc.
Prof N D Sadaphal
Disadvantages:
•
Range is limited as the pointer moves over a fixed
scale.
•
Pointer scale system used can cause parallax error.
•
There are number of moving parts which create
problems due to friction, and ultimately the accuracy
is less.
Prof N D Sadaphal