III/IV B.Tech (Regular) Degree examinations October, 2018 Mechanical Engineering Fifth Semester DESIGN OF MACHINE ELEMENTS-I

SCHEME AND SOLUTION SET

UNIT-I

1. Answer all questions 1 X 12 = 12M a) What is brittle material? Give one example

A material is brittle if, when subjected to stress, it breaks without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Eg: cast iron, concrete, ceramics all types of carbides and oxides.

b) Define Maximum shear stress theory Maximum shear stress theory states that “ the failure of a mechanical component subjected to bi-axial or tri-axial stress occurs when the max. shear stress at any point in the component becomes equal to the maximum shear stress in the standard specimen of the simple tension test, when yielding starts” τmax = Syt/2 or (σ1-σ2)/2 = Syt/2fs

c) What is factor of safety? Factor of safety is defined as the ratio of failure stress to the allowable stress

d) Give two examples for variable loading? Load on engine for different speeds, load on transmission shaft at different speeds

e) What is fatigue stress concentration factor? Fatigue stress concentration factor is defined as the ratio of Endurance limit without stress concentration to Endurance limit with stress concentration.

f) What is the usual taper provided in cotter joint? In general taper provided in cotter joint for cotter is 1 in 24

g) How a rivet is specified? Shank diameter (d)

h) Name the types of riveted joints (i) Lap joint- single riveted, double riveted, chain type and zigzag type (ii) Butt joint- Single strap, double strap, single riveted, double riveted, chain type

and zigzag type. i) What is the cross section of welded joint?

It is the sectional view obtained by cutting the welded joint perpendicular to the weld bead.Eg.,

j) Define pitch of a bolt?

It is the distance between two successive crest or two successive through of the thread measured along the axis of the thread.

k) What do you mean by uniform strength of bolt? Shank of the bolt is turned down to a diameter equal to the core diameter(dc), then the entire boss will be subjected to equal stress. This condition is called the uniform strength of bolt.

l) What is self locking of power screws? If the effort required to lower the load is positive then, the nut will stay at a particular height without sliding down. This condition is called self locking

UNIT-I

2. (a) What are the basic requirements of a machine elements?

The broad objective of designing a machine element is to ensure that it preserves its operating capacity during the stipulated service life with minimum manufacturing and operating cost In order to achieve this objective the machine element should satisfy the following basic requirements 1) Strength 2) Rigidity 3) Wear resistance 4) Minimum dimensions and weights 5) Manufacturability 6) Safety 7) Conformance to standards 8) Reliability 9) Maintainability (3M)

Strength A machine part should not fail under the effect of the forces that act up on it. In should have sufficient strength to avoid failure either due to fracture (or) due to general yielding.

Rigidity A machine components should be rigid that is it should not deflect (or) bend too much due to the forces or moments that act up on it.

Wear resistance

Wear is the main reason that puts the machine parts out of order. It reduces the useful life of the component. There are different types of wear such as a. Abrasive wear b. Corrosive wear c. Pitting Surface hardening can increase the wear resistance of the machine components such as gears and cams.

Minimum dimensions and weights A machine part should be sufficiently strong rigid wear resistance and at the same time with minimum possible dimensions and weights

Manufacturability The shape and material of the machine part should be selected in such a way that it can be produced with minimum labor cost.

Safety The shape and dimensions of machine parts should ensure safety to the operator of machine.

Conformance to standards A machine part should confirm to the national standards covering its profile dimensions grade and material.

Reliability It is the probability that a machine part will perform its intended function. Over a specified period of time that is it should perform its function over its life time.

Maintainability

It is the ease with which a machine part can be serviced or repaired. Minimum life cycle cost. It is the total cost to be paid by the purchaser for purchasing the part and operating it over its life time. (3M)

2(b) What are the general considerations in designing a machine components

01)Type of Load and Stresses caused by the Load:- The load on the Machine Component,may act in several ways due to which the Internal Stresses are set up. 02)Motion of Parts:- The successful operation of any Machine depends largely upon the simplest arrangements of the Parts,which will give the required motion.The Motion of the Part may be A)Rectilinear Motion,which includes Unidirectional and Reciprocating Motion. B)Curvilinear Motion,which includes Rotary,Oscillatory Simple Hormonic. C)Constant Velocity. D)Constant or Variable Acceleration. 03)Selection of Material:- Every Machine Design Engineer should have a thorough knowledge of the Properties of Material and their behaviour under working conditions. 04)Form and Size of the Parts:- In order to design any Machine Part for form and size,it is necessary to know the Forces which the Part must sustain.Any suddenly applied or impact load must be taken into consideration,which may cause failure.The smallest Practicable Cross-Section may be used,but it may be checked that the Stresses induced in the Designed Cross-Section are reasonably safe. 05)Frictional Resistance and Lubrication:- There is always a Loss of Power due to Frictional Resistance. Careful attention must be given to the matter of Lubrication of all surfaces which moves in contact with others. 06)Safety of Operator:- A Machine Designer should always provide safety device for the safety of the operator. The Safety Appliances should in no way interfere with the operation of the Machine. 07)Use of Standard Parts:-

The use of Standard Parts are closely related to the Cost of Machine, because the Cost of Standard Parts is only a fraction of the cost of similar parts made to order. 08)Convenient and Economical Features:- The operating feature of the Machine should be carefully studied. The Starting, Controlling and Stopping Levers should be located on the basis of convenient handling. 09)Workshop Facilities:- A Design Engineer should be familiar with limitation of his Employer's Workshop, in order to avoid the necessity of having work-done in some other Workshop. 10)Assembling:- Every Machine must be Assembled as a unit before it can function. The final Location of any Machine is important and the Design Engineer must anticipate the exact location and the local facilities for erection. Above considerations are most important in machine design engineering. (6M)

3 (a) What do you understand by preferred numbers? What is their significant in design?

In industrial design, preferred numbers (also called preferred values or preferred series) are standard guidelines for choosing exact product dimensions within a given set of constraints. Product developers must choose numerous lengths, distances, diameters, volumes, and other characteristic quantities. While all of these choices are constrained by considerations of functionality, usability, compatibility, safety or cost, there usually remains considerable leeway in the exact choice for many dimensions. (2M)

Preferred numbers serve two purposes:

1. Using them increases the probability of compatibility between objects designed at different times by different people. In other words, it is one tactic among many in standardization, whether within a company or within an industry, and it is usually desirable in industrial contexts (unless the goal is vendor lock-in or planned obsolescence)

2. They are chosen such that when a product is manufactured in many different sizes, these will end up roughly equally spaced on a logarithmic scale. They therefore help to minimize the number of different sizes that need to be manufactured or kept in stock.

(2M)

(2M)

4(a) What are the factors to be considered while designing machine parts to avoid fatigue failures

Factors to be considered to avoid the fatigue failures are

(a) Surface finish factor (Ka) : such as polished, ground, machined as-forged, corroded, etc. surface is perhaps the most important influence on fatigue life.

(b) Size factor (Kb): this factor accounts for changes which occur when the actual size of the part of the cross section differes from that of the test specimens

(c) Load Factor (Kc): this factor accounts for differences in loading (bending, axial, torsional) between the actual part and the test specimens

(d) Temperature (Kd): this factor accounts for reductions in fatigue life which occur when the operating temperature of the part differs from room temperature

(e) Reliability factor (Ke): this factor accounts for the scatter of the test data. For example, an 8% std deviation in the test data requires a Ke value of 0.888 for 95% reliability and 0.753 for 99.9% reliability

(f) Miscellaneous factors (Kf): this factor accounts for reductions from all other effects, including residual stressed, corrosion, plating, metal spraying, fretting and others.

(OR)

UNIT-III

6 (a) Explain the failures of riveted joints with neat sketches.

(OR)

7(a) what is reinforcement in weld? What are its advantages and disadvantages?

The depth of the weldment penetrates below a straight line draw at the weld surface of the plates is called reinforcement of weld.

Advantages:

1. Joint strength is increases 2. Material properties becomes same at the joint

Disadvantages:

1. Thermal stresses will induce 2. Surface becomes hard

UNIT-IV

8. (a) Explain the method of determining the size of the bolt when the bracket carries an eccentric load perpendicular to the axis of the bolt.

(OR)

Prepared By Approved by S.VIJAY (Prof. T. Nancharaiah) Asst. Prof. HOD MED MED