Unit-1-me331

MEE-331 DESIGN OF MACHINE ELEMENTS UNIT I FUNDAMENTALS OF DESIGN

UNIT I FUNDAMENTALS OF DESIGN INTRODUCTION

Engineering is mostly concerned about satisfying human needs. A designer may have a picture of a particular item or component in his mind. To make that idea into a real product, certain activities need to be carried out. These activities are collectively known as the design process. Design may be defined as follows. Definition of Design: Design is nothing but a series of activities to gather all the information necessary to realize the designer s idea as a real product.

1. THE DESIGN PROCESS Thc entire Design process can be summarized in to different phases as shown below.

1. Recognition of need: This refers to the reasoning why the new product has to be designed. In other words, it refers to the condition that has made us to go for a new design. 2. Definition of problem: In this stage, the problem is clearly defined by its specifications. Specifications arc input and output quantities, characteristics and dimensions of the space required, feed and speed to be given etc. 3. Synthesis: Synthesis is a creative process present in every design. Once all the elements are defined for a particular problem, the relationship between them has to be established. Synthesis is the process of taking the elements of the concept and arranging them in proper order, sized and dimensioned in a proper way. 4. Analysis and Optimization: Analysis refers to find out whether the system satisfies the requirements. By optimization, we see that the system performs in the best possible way. If the present system is not giving optimized performance, the synthesis part has to be again carried out till the optimum performance is achieved. 5. Evaluation and presentation: Evaluation is the final proof that the system is successfully designed. This usually involves testing in laboratory or real conditions. Presentation refers to communicating the design to others. This should be done in such a way that the system s components and working are clearly understood by all. COMPUTER AIDED DESIGN - CAD As it was seen earlier, designing consists of various phases. Computer Aided Design (CAD) refers to the use of a computer in the design process so that we get an optimum design at a lesser time. Basically, various tools are used in CAD. Use of these tools in the CAD process is shown in a diagram given below.

CAD TOOLS As shown in the fig, the basic concepts of geometric modeling and computer graphics are to be applied to our advantage the design process. Computer graphics concepts: These include the basic theory of computer graphics, representation of curves and other geometric entities mathematically and their presentation. Geometric modeling: This includes surface, wire frame and solid modeling of parts. Various packages are used for geometric modeling. Design tools: Beside the above design tools form a constituent of CAD. Those are used for analysis and evaluation purposes and include codes, computer programs etc. Advantages of CAD: 1. Improved productivity. 2. Effective communication and data handling. 3. Reduced lead-time. 4. Better quality of products. 5. Reduced wastages.

OPTIMUM DESIGN Engineering design is an iterative process, aiming at reaching best possible result. If the first design is not satisfactory, further modifications are to be carried out till the best performance is obtained. This is 1mo as the process of optimization. Definition of Optimization : Optimization is the process of maximizing a desired quantity Minimizing an undesired one. Generally, in design optimization, the function or quantity to maximized (or) minimized is mathematically modeled. The constraints related to the objective function are also incorporated. Different techniques are used to work with the model so developed and the process is carried on till the optimal design is obtained. Some optimization methods are given below. 1. Optimization by evolution: This refers to the optimization by an attempt to improve upon existing similar design. 2 Optimization by intuition: Intuition is knowing what to do but without knowing the reason of doing it. it s a kind of flash in mind . By intuition also, optimization can be obtained. 3. Optimization by trial and error modeling: This is the usual situation in modem engineering design. Here, it is assumed that the first feasible design is not necessarily the best. The design model is exercised for a few iterations in the hope of finding an improved design. 4. Optimization by numerical algorithm: In this type, mathematical ideas are employed for optimization. MECHANICAL PROPERTIES OF MATERIALS Various properties of materials assume significance depending on the condition of service. Some of the important properties are given below. Strength:

This refers to the ability of a material to resist external load without breaking. 2 Toughness: Property of a material to resist fracture due to high impact loads. 3. Stiffness: The ability of a material to resist deformation under loading is called stiffness. 4. Hardness: This means the ability of the material to resist scratching, indentation etc. S. Elasticity: Properly of the material to regain its original shape alter deformation when external forces are removed. 6. Plasticity: The property of material, which retains deformation produced under load permanently. 7. Malleability: Ability of the material to be rolled into sheets is known as malleability. 8. Resilience: Property of the material to absorb energy and to resist shock and impact loads. 9. Creep: This refers to a slow and permanent deformation when a part is subjected a constant stress for a long duration. 10. Ductility: Ductility refers to the ability of the material to be drawn into thin wires.

Detailed list of physical properties of various materials may be got from tables of PSGDB (Page 1.1 1.20) 1.6. TYPES OF LOADS Load refers to any external force acting on a machine part. Depending on the nature and value, load may be classified as follows. 1. Steady load (or) Dead load: If a load does not change in its magnitude or direction, it is considered as steady load. 2. Live load (or) Variable load: If the load changes either in magnitude or direction or both, it is called as live or variable load. I Shock load: This refers to the load applied or removed suddenly. 4. Impact load: If some initial velocity is. applied to the load, it is called as impact 1.7. STRESSES If a load acts on a body, the body will produce resisting force equal in magnitude and opposite in direction. The ratio of this resisting force to the cross sectional area of the body is known as stress .

Types of stresses: Stresses may fall into various types. They arc explained in this section. Static stress: This refers to the stress, which does not change in magnitude or in direction. Varying stress: Varying or alternate stress refers to the stress in which magnitude or direction or both are changing. Following are the types of variable stress.

(i) Completely reversed or cyclic stresses: Stresses which change from one value of tension to the same value of compression is known as completely reversed or cyclic stresses. (ii) Fluctuating stresses: Stresses which vary from a minimum value to a maximum of same nature (Compressive or tensile) are called as fluctuating stresses. (ii Repeated stress: This refers to a stress, which varies fromzero to a maximum value of same nature. (iv) Alternating stress: Stress varying from a minimum value to a maximum value of the opposite nature [ a minimum compressive to maximum tensile] is known as alternating stress.

On common axis, these stresses may be plotted as given below

This is the maximum value of completely reversing stress that the standard specimen can sustain for an infinite number of cycles without failure. Approximate values of endurance limit for some metals are given Endurance limit stress

S-N curve S-N curve shows the relation between the fatigue stress and number of loading cycles. From the S-N curve, it is possible to find the number of cycles corresponding to a particular value of fatigue stress and vice- versa. For example, in the given S-N curve corresponding to stress, Number of cycles is N and for 02, it is N corresponds to infinite number of cycles (10 power 6 cycles) and this is called endurance limit From the S-N curve, it is seen that as the value of stress is increased, number of cycles (life) is reduced. Low cycle and high cycle fatigue: Fatigue within 10 power 3 cycles is known as low cycle fatigue and at more cycles is known as high cycle fatigue. Basquin equation: The straight-line position of S-N curve is approximated by an equation of Basquin which is given as,

SOLVED PROBLEMS ON STATIC, VARYING AND COMBINED STRESSES Problem . A steel column, square cross-section of 80mm it carries a load of 750 KN at an eccentricity of 10mm in a plane bisecting the thickness. Find the maximum and minimum intensities of stress in the section. Given data:

Given data:

Problem . A mild steel bracket is shown in figure. It is subjected to a pull of 5000N acting at 450 to the horizontal axis. The bracket has a rectangular section whose depth is twice the thickness Find the cross sectional dimensions of the bracket if the permissible stress in the material is 50N/mm2.

IMPACT STRESSES

The stresses produced due to a momentary or falling load is known as impact stress. if time of load application is less than one third 4 the lowest natural period of vibration of the part it is called an impact load. Let us assume that a bar of diameter d and length falls from a height of on to the collar. l is the deformation of the bar. Due to

has a collar its base. A load W

the falling load, energy is gained. But at the same time as the weight falls down, potential energy is lost. Equating these two, Kinetic energy gained = Potential energy lost.

SOLVED PROBLEMS ON IMPACT STRESSES Problem. An unknown weight falls from a distance of 15mm on to a collar rigidly attached to the lower end of a vertical bar 2.5mm long and 500mm cross section. The maximum instantaneous extension is 2mm. Find the corresponding stress and the value of the weigh/falling. E=2x105/mm2

THERMAL STRESSES

Member is free to expansion contract due to temperature, no stress and strain will be induced in the member. But hen the member is rigidly fixed at both ends so that the change u length is prevented due to change in temperature, then stress will be induced in the member. Such stress is called temperature stress or thermal stress. The corresponding strain is called temperature strain or thermal strain. Due to rise in temperature, a member tends to expand. But when it is fixed at its ends, this expansion is prevented and thus the member is under compression .similarly due to fall of temperature, the member tends to contract. When this is prevented, tension is induced . Determination of thermal stress and strain Consider a bar is heated to a certain temperature, as shown in fig Let, L = Original length of the bar. T = Rise or fall in temperature. a = Coefficient of linear expansion E = Young s modulus of the material of the bar. M = Change in length. If the bar is free to expand, then the expanded length of the bar,

Change in length,

TEMPERATURE STRESS IN BARS OF VARYING SECTION Consider a bar ABC of vaxying section as shown in fig 1.21.Fixed at A and C and subjected to variation (increase or decrease ) in temperature.

SOLVED PROBLEMS ON THERMAL STRESSES Problem. A rod 2m long is at a temperature of 10°C Find the expansion of the rod if the temperature is raised to 80°C If t expansion is prevented, find the stress in the material Take E 2x1 05N/mm2 .

Problem A steel rod 1m long is fixed at the ends and subjected to a pull of 9KN.determine residual stress due to an increase of temperature 20°C. Diameter of bar is 12mm, E = 200 KN/mm2, =16x10-6/oC.

RESIDUAL STRESSES Residual stresses are those existing in a part when the part is free from internal forces. They are also called as internal stresses (or) locked-in stresses. They are developed due to the non-uniform plastic deformation of the body. Two types of residual stresses are 1.Tensile residual stress 2. Compressive residual stress. Generally, if the residual stress in the part is compressive, it will improve the endurance limit. An operation such as shot peening, hammering and cold rolling induces compressive stresses in parts and thus improves the endurance limit. 1.8. FACTOR OF SAFETY In all designs, the working or design stress is to be kept lower than the maximum stress. The ratio between maximum stress to working stress is known as factor of safety.

To deciding the factor of safety: 1. Material properties. 2. Nature of load. 3. Presence of localized stresses 4. Presence of initial stresses 5. Mode of failure.

PRINICPAL STRESSES AND THEORIES OF FAILURE .1. Principal Stresses

In a loaded material, there are some planes in which only normal stress. No shear stress is present in these planes. These planes are Principal planes and the stresses in these planes are called principal stress.

.2. Theories of Failure: A given machine member may fail (i.e., it will no longer be able to form its intended function) due to various reasons in various modes. It is necessary to know the various conditions of failure of machine members. Some failure theories are as follows: - (Refer to PSG Data book Page 7.3)

SOLVED PROBLEMS ON PRINCIPAL STRESSES AND THEORIES OF FAILURES Problem The stress state in a machine member is given as

Problem To determine maximum and minimum normal stresses and maximum shear stresses at the crankshaft bearing.

Probmem. To determine the maximum and minimum normal stresses and maximum shear stresses at the crankshaft bearing.

Problem. A bolt is subjected to a tensile load of 25 KN and a shear load of IOKN Determine the diameter of the bolt according to a) Maximum principal stress theory. b) Maximum principal strain theory c) Maximum shear stress theory Assume factor of safely as 2.5, yield point stress in simple tension = 300 N/mm Poisson s ratio = 0.25 Given data:

STRESS CONCENTRATION Whenever there is a rapid change in cross section or discontinuity of a body. stress concentration is present Local stresses at these sections will be more than the nominal stress. This kind of a situation is present in notches, keyways and shoulders. It should always be tned to reduce the stress concentration. Stress concentration factor (Kr): Stress concentration factor K is defined as the ratio of the maximum stress at the change of cross section to

the nominal stress.

Methods of finding stress concentration factors: Some methods of finding stress concentration factors arc: 1. Photo elasticity method. 2. Grid method 3 Brittle coating method 4. Strain gauge method 5. Finite clement techniques.

Stress concentration factor depends on the geometry of the part and it varies with the part shape. Such values of stress concentration factors may be found out using curves given in PSGDB Page 7.8 7.17. Stress concentration factor depends on the geometry of the part and it varies with the part shape. Such values of stress concentration factors may be found out using curves given in PSGDB Page 7.8 7.17. PROBLEMS ON STRESS CONCENTRATION Problem. A rectangular plate 60mm xl 0mm with a hole 12mm diameter is as shown below and subjected to a tensile load of 12000N Find the maximum stress induced.

Problem. Taking stress concentration with account find the maximum stress induced when a tensile load of 2OKN is applied to i) A rectangular plate 80mm wide and 12mm thick with a transverse hole of 16mm diameter. ii A stepped shaft of diameters 60 and 30mm with a filled radius of 6mm.

INTRODUCTION TO FRACTURE MECHANICS If a crack is present in a material, it may propagate through it causing the material to fail. This kind of failure is referred as fracture. The study of fracture is known as fracture mechanics. Fracture mechanics is mainly concerned with the study of whether a crack can sustain growth and lead to failure. Types of fracture: 1. Brittle fracture 2. Ductile fracture 3. 1. Brittle fracture: In this type, crack growth influences the material to only a very little depth. remaining material is not affected by the crack. 2. Ductile fracture: In ductile fracture, large amount of plastic deformation is present to a higher depth.

TWO MARK QUESTIONS. 1. Define Design and explain the design process. 2. What are CAD tools? State the advantages of CAD. 3. What is Optimization ? What are the methods for optimization? 4. Define factor of safety. What factors dictate the selection of factor of safety? 5. Differentiate between hardness and toughness of materials. 6. Explain creep , resilience . 7. List the various types of loads and explain. 8. Distinguish between different types of variable stresses. 9. Explain endurance limit. What factors influence endurance strength? 10. State the significance of S-N curve. PROBLEMS 1.Find the size of a hole that can be punched in a 20mm thick steel plate having ultimate shear strength of 300N/mm2.Maximum permissible the compressive stress in the punch material is 1200N/mm2. [Ans 20 mm] 2. A square ties bar 20x20mm cross-section carries a load; it is attached to a bracket by 6 bolts. Calculate the bolt diagram if

[Ans 43 mm]