IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION ... B.Tech (Regular/Supplementary) DEGREE EXAMINATION ... Engineering Metrology Mechanical Measurements Time: Three Hours Maximum : 60 Marks Answer Question No.1

Download IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION ...  B.Tech (Regular/Supplementary) DEGREE EXAMINATION ... Engineering Metrology Mechanical Measurements Time: Three Hours Maximum : 60 Marks Answer Question No.1

Post on 28-Apr-2018




5 download

Embed Size (px)


<ul><li><p>1 </p><p>14ME703/ME413 Hall Ticket Number: </p><p>IV/IV B.Tech (Regular/Supplementary) DEGREE EXAMINATION </p><p>November, 2017 Mechanical Engineering </p><p>Seventh Semester Engineering Metrology &amp;Mechanical Measurements Time: Three Hours Maximum : 60 Marks </p><p>Answer Question No.1 compulsorily. (1X12 = 12 Marks) </p><p>Answer ONE question from each unit. (4X12=48 Marks) </p><p>1. Answer all questions (1X12=12 Marks) </p><p> a) Differentiate between unilateral tolerance and bilateral tolerance with examples. </p><p>Unilateral System </p><p>In this system, the dimension of a part is allowed to vary only on one side of the basic size. i.e., tolerance lies </p><p>wholly on one side of the basic size either above or below it. </p><p> Examples of unilateral tolerance are: </p><p>25+0.02+0.01 , 25</p><p>+0.020.00 , 25</p><p>0.010.02 , 25</p><p>+0.000.02 etc </p><p>Bilateral System </p><p>In this system, the dimension of the part is allowed to vary on both the sides of the basic size. i.e., the limits </p><p>of tolerance lie on either side of the basic size; but may not be necessarily equally disposed about it. </p><p>Examples of bilateral tolerance are; 250.02 , 25+0.020.01 </p><p> 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, </p><p>high and low, two gauges are needed to check each dimension of the part, one corresponding to low limit of </p><p>size and other to the high limit of size of that dimension. These are known as GO and NOGO gauges. </p><p>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). </p><p>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. </p><p>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. </p></li><li><p>2 </p><p> c) Describe the working principle of Autocollimator. </p><p>Auto collimator is an optical instrument used for the measurement of small angular differences, changes or </p><p>deflection, plane surface inspection. It is also used to determine straightness and flatness. </p><p>The behaviour of a reflected light can be reviewed here to understand the working principle of Auto </p><p>collimator. If a beam of light strikes a flat reflecting surface it is reflected and if the surface is perpendicular </p><p>to the ray, it is turned back along its original path. </p><p>When the surface is tilted at any other angle, (say ) the total angle through which the light deflected is twice </p><p>the angle, 2. </p><p>Assume a converging lens with a point of source of light O at its principal focus. When a beam of light </p><p>strikes a flat reflecting surface it is reflected and if the surface is perpendicular to the ray it is turned back </p><p>along its original path. When the surface is tilted at an angle, the total angle through which the light is </p><p>deflected is twice the angle through which the reflector is tilted and is brought to a focus in the same plane </p><p>as the light source but shifted through some distance as shown in figure. </p><p>On examination of the triangle formed by the ray passing through the geometric centre of the lenses and the </p><p>focal length f shows that; </p><p>tan 2 =</p><p>=</p><p> = tan 2 </p><p> 2 = 2 </p><p>Where, f is the focal length of the lens. </p><p> d) What are the requirements of good comparators? 1. Robust design and construction: The design and construction of the comparator should be robust so </p><p>that it can withstand the effects of ordinary uses without affecting its measuring accuracy. </p><p>2. Linear characteristics of scale: Recording or measuring scale should be linear and uniform (straight </p></li><li><p>3 </p><p>line characteristic) and its indications should be clear. </p><p>3. High magnification: The magnification of the comparator should be such that a smallest deviation in </p><p>size of components can be easily detected. </p><p>4. Quick in results: The indicating system should be such that the readings are obtained in least </p><p>possible time. </p><p>5. Versatility: Instruments should be designed that it can be used for wide range of measurements. </p><p>6. Minimum wear of contact point. The measuring plunger should have hardened steel contact or </p><p>diamond to minimize wear effects. Further the contact pressure should be low and uniform. </p><p>7. Free from oscillations: The pointer should come rapidly to rest and should be free from oscillations. </p><p>8. Free from back lash: System should be free from back lash and unnecessary friction and it should </p><p>have minimum inertia. </p><p>9. Quick insertion of work piece: Means should be provided for lifting the plunger for quick insertion of </p><p>work. </p><p>10. Adjustable Table: The table of the instrument should, preferably, be adjustable in a vertical sense. </p><p>11. Compensation from temperature effects: The indicator should be provided with maximum </p><p>compensation for temperature effects. </p><p>12. Means to prevent damage: Suitable means should be provided for preventing damage of the </p><p>instrument in the event of the plunger moving through a greater distance than that corresponding </p><p>to the range of its measuring scale. </p><p> e) What are the various features to be measured on threaded components? </p><p> Major Diameter, Minor diameter, Effective diameter, Pitch, Flank angle and Thread form </p><p> f) Name the various instruments required for performing the alignment tests on machine tools. </p><p>Dial gauges, Test mandrels, Straight edges and squares, Spirit levels, Autocollimator and Waviness meter </p><p> g) List out the elements of measuring system. </p><p> Detector transducer element, signal conditioning element and output or readout element. </p><p> h) How to measure the velocity of flow using pitot tube? </p><p>Pitot tube used to measure the velocity of flow at a point is a tube bent at right angles and placed facing the </p><p>direction of flow as shown in figure. </p><p>At the tip, the fluid is brought to rest. That is velocity becomes zero and kinetic energy gets converted in to </p><p>pressure energy. </p><p>Applying Bernoullis equation between a point in the free stream and another at the tip of the Pitot tube, </p><p>+</p><p>2</p><p>2=</p><p>1</p><p>+ 0 1</p><p>=</p><p>+</p><p>2</p><p>2 </p><p>Where p is the static pressure, v is the free stream velocity and the density of flowing fluid. P1/g is the </p><p>total head or impact pressure head or stagnation head. Thus the Pitot tube (1) measures the sum of static </p><p>pressure head and velocity head. </p><p>Above equation assumes that the flow is steady, incompressible and frictionless. </p></li><li><p>4 </p><p>If another tube (2) is fixed normal to the direction of flow, it will measure only a static pressure head p/g </p><p>and is called a Piezometer tube. </p><p>A U-tube manometer connected across the 1 and 2 will directly give the velocity head h. </p><p> =2</p><p>1</p><p>=2</p><p>2 </p><p> = 2 </p><p>Above expression gives theoretical velocity. </p><p>Actual velocity = 2 </p><p>Where Cv = Coefficient of pitot tube. h in the above relation is expressed in terms of the column of flowing </p><p>fluid. </p><p> i) Differentiate between sensitivity and resolution. </p><p>Sensitivity: </p><p>It is defined as the ratio of the magnitude of output signal to the magnitude of input signal. </p><p> = </p><p>A 1mv recorder might have a 10 cm scale length. Assuming a linear scale, its sensitivity would be 10 cm/mv. </p><p>The sensitivity is constant in a linear instrument and usually it is required to be high. </p><p>Resolution: </p><p>This is the smallest change in input signal or measured value which can be detected by the instrument. The </p><p>least count of an instrument can be taken as the resolution of an instrument. </p><p> j) List out different types of mechanical pressure gauges. Dead weight tester, piezometer, manometer, McLeod gauge, Bourdon tube, Elastic diaphragms, Bellows and Bridgman gauge. </p><p> k) How to measure the temperature using Bi-metallic thermometers? </p><p>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. </p><p>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. </p><p>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. </p><p>The bimetallic strip has the advantages of low-cost, negligible maintenance expense, and stable operation over extended periods of time. </p><p>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. </p></li><li><p>5 </p><p> l) What is a load cell? </p><p>Strain gauge load cells are most often constructed of a metal, and have a shape such that the range of forces </p><p>to be measured results in a measurable output voltage over the desired operating range. The shape of the </p><p>linearly elastic member is designed to meet the following goals: (1) provide an appropriate range of force-</p><p>measuring capability with necessary accuracy, (2) provide sensitivity to forces in a particular direction, and </p><p>(3) have low sensitivity to force components in other directions. </p><p>A load cell comprises four strain gauges; two of these are used for measuring the longitudinal strain while the </p><p>other two for measuring the transverse strain. The four strain gauges are mounted at 90 to each other, as </p><p>shown in below figure. </p><p>Two gauges experience tensile stresses while the other two are subjected to compressive stresses. At the no-</p><p>load condition, resistance in all the four gauges will be same. The potential across the two terminals B and D </p><p>are same. </p><p>The Wheatstone bridge is now balanced and hence output voltage is zero. When the specimen is stressed </p><p>due to the applied force, the strain induced is measured by the gauges. </p><p>UNIT I </p><p>2. a) Sketch three main types of fits and name them. </p><p>Clearance Fit: In this type of fit shaft is always smaller than the hole. i.e., the largest permissible shaft </p><p>diameter is smaller than the diameter of the smallest hole. So that, the shaft can rotates or slide through </p><p>with different degrees of freedom according to the purpose of mating part. </p><p>Interference Fit: In this of type of fit the minimum permissible diameter of the shaft is larger than the </p><p>maximum allowable diameter of the hole. Thus the shaft and the hole members are intended to be attached </p><p>permanently and used as a solid component. </p><p>Elastic strains developed on the mating surfaces during the process of assembly prevent relative movement </p><p>of the mating parts. For example steel tyres on railway car wheels, gears on intermediate shafts of trucks, </p><p>bearing in the gear of a lathe head stock, drill bush in jig plate, cylinder linear in block, steel ring on a wooden </p><p>bullock cart wheels etc. </p><p>4M </p></li><li><p>6 </p><p>Transition Fit: Transition fit lies mid way between clearance and interference fit. In this type the size limits of </p><p>mating parts (shaft and hole) are so selected that either clearance or indifference may occur depending upon </p><p>the actual sizes of the parts. Push fit and wringing fit are the examples of this type of fit. </p><p> b) Calculate the limits of hole and shaft in the hole and shaft pair designated by 40H7d 8. </p><p>Assume: (i) 40 lies in the diameter step of 30 and 50mm (ii) The standard tolerance unit, i </p><p>(microns) = 0.45D1/3</p><p> + 0.001D where D is the geometric mean of the lower and upper limits </p><p>if diameter step in which the diameter consideration lies, D is in mm, (iii) The fundamental </p><p>deviation for shaft d= -16D0.44</p><p> microns (iv) The standard tolerance grade IT7 = 16 i , (v)The </p><p>standard tolerance grade IT8 = 25 i </p><p> Solution: </p><p> = 30 50 = 38.73 Fundamental deviation for hole H is zero. Fundamental deviation for shaft = 160.44 </p><p>= 16 38.730.44 = 79.957 80 . 0.080 </p><p> The tolerance unit </p><p> = 0.45 3</p><p>+ 0.001 = 0.45 38.733</p><p>+ 0.001 38.73 = 1.5612 Tolerance grade IT7=16i </p><p>= 16 1.5612 = 24.9782 = 0.025 Tolerance grade IT8=25i </p><p>= 25 1.5612 = 39.03 = 0.039 HOLE Low limit=Basic size fundamental deviation=40.00+0.000=40.00 mm High limit= Low limit tolerance =40.00+0.025= 40.025 mm. SHAFT High limit=Basic size-fundamental deviation of shaft=40.00-0.080=39.92 mm Low limit= High limit-tolerance=39.92-0.039=39.881 mm. </p><p>8M </p><p>(OR) 3. a) Discuss the design procedure for plug and ring gauges as per Taylors principle of gauging. </p><p>Taylors Principle of Gauge Design </p><p>(1) It states that GO gauges should be designed to check the maximum material limit (MML), while the NO-GO gauges should be designed to check the minimum or Lowest Material Limit (LML). </p><p> Figure: GO gauge to check MML and NOGO to check LML </p><p>The difference in size between the GO and NO-GO plug gauges &amp; Snap gauges is approximately equal to the tolerance of the tested hole or shaft as may be the case. </p><p>6M </p></li><li><p>7 </p><p>(2) GO gauges should check all the related dimensions (roundness, size, location etc) simultaneously, where as NO-GO gauge should check only one element of the dimension at a time. According to this rule, GO plug gauge should have a full circular section and be of full length of the hole it has to check. This ensures that any lack of straightness, or roundness of the hole will prevent the entry of full length of GO plug gauge. </p><p>For example, suppose the bush to be inspected has a curved axis and a short GO plug gauge is used to check it. The short plug gauge will pass through all the curves of the bent bushing. This will lead to a wrong result that the work piece (hole) is within the prescribed limits. Actually such a bushing with a curved hole will not mate properly with its mating part and thus defective. A GO plug gauge with adequate length will not pass through a curved bushing and the error will be detected. A long plug gauge will thus check the cylindrical surface not in one direction, but in a number of sections simultaneously. The length of the GO plug gauge should not be less than 1.5 times the diameter of the hole to be checked. </p><p>Now suppose, the hole to be checked has an oval shape. While checking it with the cylindrical NOT-GO gauge the hole under inspection will overlap (hatched portion) the plug and thus will not enter the hole. This will again lead to wrong conclusion that the part is within the prescribed limits. It will be therefore more appropriate to make the NOT-GO gauge in the form of a pin as shown in below figure. </p><p>Figure : checking the oval shaped hole with NOT-GO gauge </p><p> b) (i) What are slip gauges? What are their uses? Slip gauges or gauge blocks are universally accepted end standard of length in industry. These were </p><p>introduced by Johanso...</p></li></ul>


View more >