Comparator Types - HKK

8/11/2019 Comparator Types - HKK

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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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Comparator Types - HKK