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COMPARATORS
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Comparators
Comparators are used as laboratory standards
from which working or inspection gauges are set
and co-related.
They are also used as working gauges to preventwork spoilage and to maintain required tolerance
at all-important stages of manufacture.
Comparators indicate only the difference in sizebetween the nominal dimension set on the
comparator by slip gauges and the components
being measured.
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They can built to have a magnification as high
as x50000.
They are so designed to allow smallest scale
division of 0.00025mm.
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(i)In mass production, where components are to bechecked at a very fast rate.
(ii)As laboratory standards from which working orinspection gauges are set and correlated.
(iii)For inspecting newly purchased gauges.(iv)Attached with some machines, comparators can be
used as working gauges to prevent work spoilage andto maintain required tolerances at all stages of
manufacturing.(v)In selective assembly of parts, where parts are gradedin three or more groups depending upon theirtolerances.
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(1)Mechanical comparators
(2)Optical comparators
(3)Electrical and Electronic comparators(4)Pneumatic comparators
(5)Fluid displacement comparators
(6)Projection comparators
(7)Multi-check comparators
(8)Automatic gauging machines
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Application of Comparators
Used as laboratory standards from which working
or inspections gauges are set and correlated.
Used, as working gauges to prevent work spoilage
and to maintain required tolerance at all-
important stages of manufacture.
Used as final inspection gauges where selective
assembly of production parts is necessary. Used as receiving inspection gauges for checking
parts received from outside sources.
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Advantages
Not much skill is required on the part of
operation.
The calibration of instrument over full range is
not required since comparison is done with a
standard end length.
Zero error existing in comparator also does not
lead to any problem. High magnification resulting into great accuracy is
possible.
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Mechanical comparators use mechanicalmethods of amplifying the movement of the
contact plunger and their manufacture
requires high degree of accuracy. Usual magnification of the mechanisms ranges
from about 250 to 1,000.
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Rack and pinion Cam and gear trainLever with toothed gear.
Compound levers Twisted taut strip Lever combined
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The necessary high magnification ratio m this
instrument is achieved by the use of gears and
pinions arrangement in a small area, by arrangingthe gear train in a manner similar to a clock
movement.
There is a plunger which is a perfect sliding fit in
its own bearings.
This carries a rack which accurately meshes with
a pinion A.
The rotation of the plunger about its own axis is
prevented by a pin attached to it, which is located
in a slot in a rack guide G.
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In order to keep the plunger in an extended or
normal position a light coil spring S is employed.
The spring exerts a pressure of approximately 6oz.
A small movement of the contact point causes
the rack to turn the pinion A with which it ismeshed.
A large gear B is attached to the same spindle as
pinion A.
The gear B is further meshed with a pinion C,
which thus magnifies the movement of pinion A.
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Attached to the second pinion C is another gear D
which meshes with a third pinion E mounted on
the same spindle as the indicator pointer. This magnification is further enlarged at the tip of
the pointer by an amount dependent upon its
length.
The overall magnification for any dial gauge may
be thus calculated by measuring the distance
between division on the scale and dividing this
dimension by the equivalent movement of themeasuring plunger.
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In order to take up the backlash, a light hair
spring H is always fitted to the dial gauge gear
trains by meshing a gear F to pinion E andattaching spring if with F.
In order to prevent gear backlash, the gears are
precision cut and the movement assemblyresembles that of high-grade watch.
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Sigma comparator is the most widely used for
higher precision work. Magnification ranges
from 300 to 5000.
Figure shows the details of the magnifying
system of the comparator.
Plunger mounted on a pair of slit diaphragms
obtains the frictionless linear motion.
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ELECTRICAL COMPARATOR
An electrical comparator consists of the
following three major part such as
1) Transducer
2) Display device as meter
3) Amplifier
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Transducer:
An iron armature is provided in between two coils
held by a lea spring at one end. The other end is
supported against a plunger.
The two coils act as two arms of an A.C. wheat stone
bridge circuit.
Amplifier:
The amplifier is nothing but a device which amplifies
the give input signal frequency into magnified output
Display device or meter: The amplified input signal is displayed on some
terminal stage instruments. Here, the terminal
instrument is a meter.
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Working Principle
If the armature is centrally located between the coils. the
inductance of both coils will be equal but in opposite
direction with the sign change.
Due to this, the bridge circuit of A.C. wheat stone bridgeis balanced. Therefore, the meter will read zero value.
But practically, it is not possible. In real cases, the
armature may be lifted up or lowered down by the
plunger during the measurement. This would upset the balance of the wheat stone bridge
circuit. Due to this effect, the change in current or
potential will be induced correspondingly.
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On that time, the meter will indicate some
value as displacement. This indicated value
may be either for larger or smaller
components.
As this induced current is too small, it should
be suitably amplified before being displayed in
the meter.
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Checking of accuracy
To check the accuracy of a given specimen or work, first a
standard specimen is placed under the plunger.
After this, the resistance of wheat stone bridge is
adjusted so that the scale reading shows zero. Then the
specimen is removed. Now, the work is introduced under the plunger. If height
variation of work presents, it will move the plunger up or
down.
The corresponding movement of the plunger is firstamplified by the amplifier then it is transmitted to the
meter to show the variations.
The least count of this electrical comparator is 0.001mm
(one microns).
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Pneumatic Comparator
The term pneumatic associates with
pressurised air.
The pressurized air is used as the working
medium in pneumatic comparator.
Based on the physical phenomena, the
pneumatic comparators are classified into two
types.
1) Flow or Velocity type.
2) Back pressure type.
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Flow Type
The float height is essentially proportional tothe air that escapes from the gauge head
Master gauges are used to find calibration
points on the scales The input pressure is regulated to allow
magnification adjustment
A pressure bleed off valve allows changes tothe base level for offset
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Back Pressure System
The Soloflex Back Pressure System uses an orifice withthe venturi effect to measure air flow.
If the gas is not moving, the pressure on both sides of the
orifice will be equal.
If the flow is moving quickly, the air pressure on the
downstream side of the orifice will be at a lower
pressure.
A Differential Back Pressure system uses a split flow
channel, one flow goes to the gauge head, the other goes
to a zero offset valve.
A meter measures the difference in pressures, and thus
gives the differences in pressure.
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