COMPORATOR
• Linear measurement instruments, such as verniers and micrometers,
in which standards are in-built and calibrated. Hence, these
instruments enable us to directly measure a linear dimension up to the
given degree of accuracy.
• In certain devices the standards are separated from the instrument. It
compares the unknown length with the standard. Such measurement
is known as comparison measurement and the instrument, which
provides such a comparison, is called a comparator.
• Comparators are generally used for linear measurements.
Classification of Comparators
1. Mechanical comparators
2. Mechanical–optical comparators
3. Electrical and electronic comparators
4. Pneumatic comparators
MECHANICAL COMPARTORS
1. SIGMA COMPARATOR
2. Johansson Mikrokator
3. Dial indicator
4. Eden-Rolt Reed system
SIGMA MECHANICAL COMPARATOR
• A linear displacement of a plunger is translated into the movement of a
pointer over a calibrated scale.
• The plunger is the sensing element that is in contact with the work part. It
moves on a slit washer, which provides frictionless linear movement and
also arrests rotation of the plunger about its axis.
• A knife edge is screwed onto the plunger, which bears upon the face of
the moving member of a cross-strip hinge. This unit comprises a fixed
member and a moving block, connected by thin flexible strips at right
angles to each other.
• Whenever the plunger moves up or down, the knife edge drives the
moving member of the cross-strip hinge assembly. This deflects an
arm, which divides into a ‘Y’ arm. The extreme ends of this Y-arm
are connected to a driving drum by means of phosphor-bronze
strips.
• The movement of the Y-arm rotates the driving drum and, in turn,
the pointer spindle. This causes the movement of the pointer over a
calibrated scale.
NOTE - They are mainly used to measure the roughness of the surface
by calculating the dimensional difference between the measured
value and the standard value of the surface material.
• The magnification of the instrument is obtained in two stages.
• In the first stage, if the effective length of Y-arm is L and the
distance from the hinge pivot to the knife edge is x, then
magnification is L/x.
• The second stage of magnification is obtained with respect to the
pointer length R and driving drum radius r. The magnification is
given by R/r.
• Therefore, overall magnification is given by (L/x) × (R/r).
Joh ansson Mikrokator
• The basic element in this type of comparator is a light pointer made of
glass fixed to a thin twisted metal strip.
• The two halves of the thin metal strip, which carries the light pointer, are
twisted in opposite directions.
• Therefore, any pull on the strip will cause the pointer to rotate. While one
end of the strip is fixed to an adjustable cantilever link, the other end is
anchored to a bell crank lever.
• The other end of the bell crank lever is fixed to a plunger. Any linear
motion of the plunger will result in a movement of the bell crank lever,
which exerts either a push or a pull force on the metal strip.
• Accordingly, the glass pointer will rotate either clockwise or anticlockwise,
depending on the direction of plunger movement.
• The comparator is designed in such a fashion that even a minute
movement of the plunger will cause a perceptible rotation of the glass
pointer.
• A calibrated scale is employed with the pointer so that any axial
movement of the plunger can be recorded conveniently.
• We can easily see the relationship of the length and width of the strip with
the degree of amplification.
• Thus, dθ /dl ∝ l/nw2,
• where dθ/dl is the amplification of the mikrokrator, l is the length of the
metal strip measured along the neutral axis, n is the number of turns on
the metal strip, and w is the width of the metal strip.
• It is clear from the preceding equation that magnification varies inversely
with the number of turns and width of the metal strip.
• The lesser the number of turns and thinner the strip, the higher is the
magnification.
• On the other hand, magnification varies directly with the length of the
metal strip.
• These three parameters are varied optimally to get a compact but robust
instrument.
D IAL IND ICATOR
Ed en-Rolt Reed system
Mech anical Optical Comparators
RRB JE PY Q 2019
Q.1 sigma comparator is a type of _________ comparator.
a) Electrical b) Electronic c) Mechanical d) optical
Q.2 in case of a comparator , measurement is done by
a) Displacement method b) Interchange method
c) Direct method d) Parkinson method
Q.3 Dial indicator is a
a) Sensor b) Comparator c) Recording device d) Transducer
Q.4 which comparator is generally used to measure small displacement of
spindles
a) Read type comparator b)sigma comparator
c) optical comparator d) electric comparative
