notes on comparators

8/18/2019 Notes on Comparators

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ComparatorsComparators

Engr. Zaheer Ahmad

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COMPARATORS

Comparator is an instrument used for

comparing the dimensions of a component with a standard of length

Purpose of a comparator, in general, is to detect& display the small differences b/w the

unknown linear dimension & length of thestandard

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COMPARATORS

Essential Parts of Comparators:1. Fixed surface from which all measurements are taken

2. A very sensitive indicator which will show themovement 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/w the length or diameter of a component & a standardof length, usually made up of slip gauges

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COMPARATORSDesirable Features of a Comparator:1. It should at least be able to record variations of 0.0025mm

2. Recording scale should be linear3. Maximum rigidity

4. Maximum compensation of temperature effect

5. Neither lag nor backlash in the movement of the plunger &recording mechanism

6. Indicator should be constant in its return to zero7. Measurement pressure should be low & constant

8. Instrument should be capable of a wide measuring range

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COMPARATORS

Types of Comparators:Most common commercially available comparators can

be classified into the following types:

1. Mechanical comparators

2. Optical comparators

3. Electrical & Electronic comparators4. Pneumatic comparators

5. Fluid displacement comparators

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COMPARATORSTypes of Comparators:

A combination of magnifying principles has lead to

the development of a few more types of comparators, which are:

6. Mechanical-Optical comparators

7. Electro-mechanical comparators

8. Multi-check comparators

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MECHANICAL COMPARATOR

The mechanical comparators utilize mechanical

methods of magnifying, the movement of the contactplunger; their manufacture requires a high degree of skill & accuracy.

The usual magnifications of the mechanisms range

from about 250 to 1000. The least account of mechanical comparator is .01mm.

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Some of the comparator which belong to this class are:

1. Dial indicator

2. Reed type comparator or Dial Gauge

3. Sigma Comparator

4. Johnson Mikrokrator

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1. Dial Indicator:

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MECHANICAL COMPARATOR1. Dial Indicator: (different uses)

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MECHANICAL COMPARATOR1. Dial Indicator: (Various Uses)

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Advantages and Disadvantages of

various Types of Comparators: Advantages of Mechanical Comparators:

These are usually cheaper in comparison to other devices of

amplifying. These do not require any external supply such as electricity

or air and as such the variations in outside supplies do notaffect the accuracy.

Usually the mechanical comparators have linear scale which is easily understood.

These are usually compact and easy to handle.

For ordinary workshop conditions, these are suitable andbeing portable can be issued from a store.


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Advantages of Optical Comparators:

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

In the optical comparators, the scale can be made to

move past a datum line and thus have high range andno parallax errors.

It has very high magnification.

Optical lever is weightless.


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Advantages of Electrical Comparators:

the electrical comparators have got small number of moving 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 various

ranges. The mechanism carrying the pointer is very light and not

sensitive to vibrations. As the instrument is usually operated on A.C. supply, the

cyclic 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.


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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 then with high magnifications a very small range is

obtained. This is usually more expensive than mechanical

instrument.


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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 less

friction and less inertia. Measuring pressure is very small and the jet of air helps in

cleaning 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 thetaperness of the circular bores.


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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 highmagnification is necessary in order to avoid themeniscus errors.

4. The apparatus is not easily portable and is rather

elaborate for many industrial applications.5. Different gauging heads are required for different

dimensions.

notes on comparators

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