ComparatorsComparators

Engr. Zaheer Ahmad

1

COMPARATORSComparator is an instrument used for

comparing the dimensions of a componentwith a standard of length

Purpose of a comparator, in general, is to detect& display the small differences b/w theunknown linear dimension & length of thestandard

2

3

COMPARATORSEssential Parts of Comparators:1. Fixed surface from which all measurements are taken2. A very sensitive indicator which will show the

movement of a sliding piece usually terminating inan anvil with curved surface

3. Some means of setting the curved face Arrangementof above parts is used to measure the difference b/wthe length or diameter of a component & a standardof length, usually made up of slip gauges

4

COMPARATORSDesirable Features of a Comparator:1. It should at least be able to record variations of 0.0025mm2. Recording scale should be linear3. Maximum rigidity4. Maximum compensation of temperature effect5. Neither lag nor backlash in the movement of the plunger &

recording mechanism6. Indicator should be constant in its return to zero7. Measurement pressure should be low & constant8. Instrument should be capable of a wide measuring range

5

COMPARATORSTypes of Comparators:Most common commercially available comparators can

be classified into the following types:1. Mechanical comparators2. Optical comparators3. Electrical & Electronic comparators4. Pneumatic comparators5. Fluid displacement comparators

6

7

COMPARATORSTypes of Comparators:

A combination of magnifying principles has lead tothe development of a few more types of comparators,which are:

6. Mechanical-Optical comparators7. Electro-mechanical comparators8. Multi-check comparators

8

MECHANICAL COMPARATOR

The mechanical comparators utilize mechanicalmethods of magnifying, the movement of the contactplunger; their manufacture requires a high degree ofskill & accuracy.

The usual magnifications of the mechanisms rangefrom about 250 to 1000.

The least account of mechanical comparator is .01mm.

9

MECHANICAL COMPARATOR

Some of the comparator which belong to this class are:1. Dial indicator2. Reed type comparator or Dial Gauge3. Sigma Comparator4. Johnson Mikrokrator

10

MECHANICAL COMPARATOR1. Dial Indicator:

11

MECHANICAL COMPARATOR1. Dial Indicator: (different uses)

12

MECHANICAL COMPARATOR1. Dial Indicator: (Various Uses)

13

Johansson MikrokatorThe Johansson Mikrokatorused a twisted strip with apointer attached, as theplunger is depressed, itcauses the strip to stretch.As the twisted strip isstretched, it changes theangle of the pointer, and thusthe indicated deflection.

14

Advantages and Disadvantages of various Types of Comparators:

Advantages of Mechanical Comparators:

These are usually cheaper in comparison to other devices ofamplifying.

These do not require any external supply such as electricityor air and as such the variations in outside supplies do notaffect the accuracy.

Usually the mechanical comparators have linear scalewhich is easily understood.

These are usually compact and easy to handle. For ordinary workshop conditions, these are suitable and

being portable can be issued from a store.

Disadvantages:

The mechanical comparators have got more movingparts than other types. Due to more moving parts, thefriction is more and ultimately the accuracy is less.

Any slackness in moving parts reduces the accuracyconsiderably.

The mechanism has more inertia and this may causethe instruments to be sensitive to vibration.

The range of the instrument is limited as the pointermoves over a fixed scale.

Advantages of Optical Comparators:

it has small number of moving parts and hence ahigher accuracy.

In the optical comparators, the scale can be made tomove past a datum line and thus have high range andno parallax errors.

It has very high magnification. Optical lever is weightless.

Disadvantages:

As the instrument has high magnification, heat fromthe lamp, transformer etc. may cause the setting todrift.

An electrical supply is necessary. The apparatus is usually large and expensive. When the scale is projected on a screen, then it is

essential to use the instrument to a dark room in orderto take the readings easily.

The instruments in which the scale is viewed throughthe eyepiece of a microscope are not convenient forcontinuous use.

Advantages of Electrical Comparators:

the electrical comparators have got small number ofmoving parts.

It is possible to have a very high magnification and thesame instrument may have two or more magnifications.Thus the same instrument can be used for variousranges.

The mechanism carrying the pointer is very light and notsensitive to vibrations.

As the instrument is usually operated on A.C. supply, thecyclic vibration substantially reduces errors due tosliding friction.

The measuring unit can be made very small and it is notnecessary that the indicating instrument be close to themeasuring unit, rather it can be remote also.

Disadvantages:

It requires an external agency to operate i.e., the A.C.electrical supply. Thus the vibrations in voltage orfrequency of electric supply may affect the accuracy.

Heating of coils in the measuring unit may cause zerodrift and alter the calibration.

If only a fixed scale is used with a moving pointer thenwith high magnifications a very small range isobtained.

This is usually more expensive than mechanicalinstrument.

Advantages of Pneumatic Comparators:

The gauging member does not come into contact with thepart to be measured and hence practically no wear takesplace on the gauging member.

It has usually very small number of moving parts and insome cases none. Thus the accuracy is more due t lessfriction and less inertia.

Measuring pressure is very small and the jet of air helps incleaning the dust, if any, from the part to be measured.

It is possible to have very high magnification. The indicating instrument can be remote from the

measuring unit. It is very suitable device for measuring diameter of holes

where the diameter is small compared with the length. It is probably the best method for determining the

taperness of the circular bores.

Disadvantages:

1. It require elaborate auxiliary equipment such asaccurate pressure regulator.

2. The scale is generally not uniform.3. When indicating device is the glass tube, then high

magnification is necessary in order to avoid themeniscus errors.

4. The apparatus is not easily portable and is ratherelaborate for many industrial applications.

5. Different gauging heads are required for differentdimensions.