# iv/iv b.tech (regular/supplementary) degree examination ... b.tech (regular/supplementary) degree...

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14ME703/ME413 Hall Ticket Number:

IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION

November, 2017 Mechanical Engineering

Seventh Semester Engineering Metrology &Mechanical Measurements Time: Three Hours Maximum : 60 Marks

Answer Question No.1 compulsorily. (1X12 = 12 Marks)

Answer ONE question from each unit. (4X12=48 Marks)

1. Answer all questions (1X12=12 Marks)

a) Differentiate between unilateral tolerance and bilateral tolerance with examples.

Unilateral System

In this system, the dimension of a part is allowed to vary only on one side of the basic size. i.e., tolerance lies

wholly on one side of the basic size either above or below it.

Examples of unilateral tolerance are:

25+0.02+0.01 , 25

+0.020.00 , 25

0.010.02 , 25

+0.000.02 etc

Bilateral System

In this system, the dimension of the part is allowed to vary on both the sides of the basic size. i.e., the limits

of tolerance lie on either side of the basic size; but may not be necessarily equally disposed about it.

Examples of bilateral tolerance are; 250.02 , 25+0.020.01

b) Sketch the different limit gauges used in industry for quality checking. Limit gauges are very widely used in industries. As there are two permissible limits of the dimension of a part,

high and low, two gauges are needed to check each dimension of the part, one corresponding to low limit of

size and other to the high limit of size of that dimension. These are known as GO and NOGO gauges.

The difference between the sizes of these two gauges is equal to the tolerance on the work piece. GO gauges check the Maximum Metal Limit (MML) and NO-GO gauge checks the Least or Minimum Metal Limit (LML).

In the case of a hole, maximum metal limit is when the hole is as small as possible, that is, it is the low limit of size. In case of hole, therefore, GO gauge corresponds to the low limit of size, while NO-GO gauge corresponds to high limit of size.

For a shaft, the maximum metal limit is when the shaft is on the high limit of size. Thus in the case of a shaft GO gauge corresponds to the high limit of size and NO-GO gauge corresponds to the low limit of size.

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c) Describe the working principle of Autocollimator.

Auto collimator is an optical instrument used for the measurement of small angular differences, changes or

deflection, plane surface inspection. It is also used to determine straightness and flatness.

The behaviour of a reflected light can be reviewed here to understand the working principle of Auto

collimator. If a beam of light strikes a flat reflecting surface it is reflected and if the surface is perpendicular

to the ray, it is turned back along its original path.

When the surface is tilted at any other angle, (say ) the total angle through which the light deflected is twice

the angle, 2.

Assume a converging lens with a point of source of light O at its principal focus. When a beam of light

strikes a flat reflecting surface it is reflected and if the surface is perpendicular to the ray it is turned back

along its original path. When the surface is tilted at an angle, the total angle through which the light is

deflected is twice the angle through which the reflector is tilted and is brought to a focus in the same plane

as the light source but shifted through some distance as shown in figure.

On examination of the triangle formed by the ray passing through the geometric centre of the lenses and the

focal length f shows that;

tan 2 =

=

= tan 2

2 = 2

Where, f is the focal length of the lens.

d) What are the requirements of good comparators? 1. Robust design and construction: The design and construction of the comparator should be robust so

that it can withstand the effects of ordinary uses without affecting its measuring accuracy.

2. Linear characteristics of scale: Recording or measuring scale should be linear and uniform (straight

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line characteristic) and its indications should be clear.

3. High magnification: The magnification of the comparator should be such that a smallest deviation in

size of components can be easily detected.

4. Quick in results: The indicating system should be such that the readings are obtained in least

possible time.

5. Versatility: Instruments should be designed that it can be used for wide range of measurements.

6. Minimum wear of contact point. The measuring plunger should have hardened steel contact or

diamond to minimize wear effects. Further the contact pressure should be low and uniform.

7. Free from oscillations: The pointer should come rapidly to rest and should be free from oscillations.

8. Free from back lash: System should be free from back lash and unnecessary friction and it should

have minimum inertia.

9. Quick insertion of work piece: Means should be provided for lifting the plunger for quick insertion of

work.

10. Adjustable Table: The table of the instrument should, preferably, be adjustable in a vertical sense.

11. Compensation from temperature effects: The indicator should be provided with maximum

compensation for temperature effects.

12. Means to prevent damage: Suitable means should be provided for preventing damage of the

instrument in the event of the plunger moving through a greater distance than that corresponding

to the range of its measuring scale.

e) What are the various features to be measured on threaded components?

Major Diameter, Minor diameter, Effective diameter, Pitch, Flank angle and Thread form

f) Name the various instruments required for performing the alignment tests on machine tools.

Dial gauges, Test mandrels, Straight edges and squares, Spirit levels, Autocollimator and Waviness meter

g) List out the elements of measuring system.

Detector transducer element, signal conditioning element and output or readout element.

h) How to measure the velocity of flow using pitot tube?

Pitot tube used to measure the velocity of flow at a point is a tube bent at right angles and placed facing the

direction of flow as shown in figure.

At the tip, the fluid is brought to rest. That is velocity becomes zero and kinetic energy gets converted in to

pressure energy.

Applying Bernoullis equation between a point in the free stream and another at the tip of the Pitot tube,

+

2

2=

1

+ 0 1

=

+

2

2

Where p is the static pressure, v is the free stream velocity and the density of flowing fluid. P1/g is the

total head or impact pressure head or stagnation head. Thus the Pitot tube (1) measures the sum of static

pressure head and velocity head.

Above equation assumes that the flow is steady, incompressible and frictionless.

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If another tube (2) is fixed normal to the direction of flow, it will measure only a static pressure head p/g

and is called a Piezometer tube.

A U-tube manometer connected across the 1 and 2 will directly give the velocity head h.

=2

1

=2

2

= 2

Above expression gives theoretical velocity.

Actual velocity = 2

Where Cv = Coefficient of pitot tube. h in the above relation is expressed in terms of the column of flowing

fluid.

i) Differentiate between sensitivity and resolution.

Sensitivity:

It is defined as the ratio of the magnitude of output signal to the magnitude of input signal.

=

A 1mv recorder might have a 10 cm scale length. Assuming a linear scale, its sensitivity would be 10 cm/mv.

The sensitivity is constant in a linear instrument and usually it is required to be high.

Resolution:

This is the smallest change in input signal or measured value which can be detected by the instrument. The

least count of an instrument can be taken as the resolution of an instrument.

j) List out different types of mechanical pressure gauges. Dead weight tester, piezometer, manometer, McLeod gauge, Bourdon tube, Elastic diaphragms, Bellows and Bridgman gauge.

k) How to measure the temperature using Bi-metallic thermometers?

Principle When two metal strips having different coefficients of expansion are bonded together, an increase in temperature causes the deflection of the free end of the strip as shown in figure.

The deflection with temperature is nearly linear. Invar (iron, nickel alloy) is commonly used as a low expansion material. Brass or other materials are used as high expansion material.

Bimetal strip is commonly used as temperature sensing and control device called as thermostat (on-off type) in home applications such as geysers and ovens, etc.

The bimetallic strip has the advantages of low-cost, negligible maintenance expense, and stable operation over extended periods of time.

For temperature measurement, the sensitivity of bimetal is increased by coiling it in a helical form (see figure). As the temperature increases, the bimetal expands and the helical bimetal rotates at its free end.

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l) What is a load cell?

Strain gauge load cells are most often constructed of a metal, and have a shape such that the range of forces

to be measured results

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