DESIGN OF MACHINE ELEMENTS UNIT-1

Transcription

1 DESIGN OF MACHINE ELEMENTS UNIT-1 PART-1 DEPARTMENT OF MECHANICAL ENGINEERING K.L.UNIVERSITY GUNTUR DESIGN:

2 It is essentially decision making process. If we have the problem we will find the solution. In other words to formulate a plan to satisfy a particular need and to create something with physical quantity MACHINE DESIGN: Generally machine is a combination of various elements arranged to work together to serve specific purpose. Machine design includes designing the elements and arranging the elements optimally to serve the need. TYPES OF DESIGN: Adaptive Design Development Design New Design ADAPTIVE DESIGN: In most cases, the designer s work is concerned with adaptation of existing designs. This type of design needs no special knowledge or skill and can be attempted by designers or ordinary technical training. The designer only makes minor alternation or modification in the existing designs of the product EXAMPLE: the commonly used standard-model car is manufactured in different models to obtain high speed, style and various sizes. Similarly the different models of watches, clocks, televisions etc. In this adaptive design the initial product and final product are basically similar in their structures. DEVELOPMENT DESIGN: This type of design needs considerable scientific training and design ability in order to modify the existing designs into a new idea by adapting a new material or different method of manufacture. EXAMPLE: For example, by imposing I.C Engine principle to a cycle, motor cycle is invented. Similarly by combining the properties of some electronic goods, electronic watches are designed, then the motor cycles and electronic watches are developed designers. The final product in developed design may differ quite markedly from the initial product. NEW DESIGN: The type of design needs lot of research, technical ability and creative thinking. Here, whatever be the product which has been designed in the first time is coming under new design. 2

3 EXAMPLE: inventions of cycle, airplane etc. was all considered as new products (i.e. new designs) in their beginning period. Similarly, the invention of any new product in future may also be considered as new design. PHASES OF DESIGN: Recognition of need Definition of Problem Synthesis Analysis of Optimization Evaluation Figure-1 Presentation RECOGNITION OF NEED: Consumption (or use) of the product usually decides the characteristics for the need. To get the information concerning the market or consumer characteristic Research of the past, Present and future consumption requirements is accomplished to get the realistic estimates of the future demand. The final translation of need into statement of 3

4 project goal or objective takes account of the constraints such as ability to solve, the resources the time available and the risk factors. DEFINITION OF THE PROBLEM: Make a written statement of the problem as completely and as clearly as possible and also of the purpose for which the machine is to be designed. It must include all the specifications are the input and output quantities, the characteristics and dimensions, cost, range, reliability. SYNTHESIS (mechanisms): Select the possible mechanism or group of mechanisms which will give the desired motion. ANALYSIS: Find the size of each member of the machine by considering the forces/stresses acting on the member of the machine and the energy transmitted by each member. Select the material best suited for each member of the machine. The analysis may reveal that the system is not an optimum one. If the design fails, the synthesis procedure must begin again. EVALUATION: It is the final proof of a successful phase of the total design process and usually involves the testing of a prototype in the laboratory. It is economical to manufacture and to use. PRESENTATION: This is the final step of design process which refers to the proper communication of design details like correct dimensions, machining process, tolerance details and other working conditions to other engineers through written materials or orally for further actions. COMMUNICATION: These are the written the oral and the graphical forms. Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing drawings with tolerances. 4

5 ACCORDING TO THE METHODS USED, THE DESIGN MAY BE CLASSIFIED AS FOLLOWS: (1) Empirical design: This type of design depends upon empirical formulae based on the existing practice and past experience. (2) Rational design: This type of design depends upon mathematical formulae of principle of mechanic. (3) Industrial design: This type of design depends upon the production aspects to manufacture any machine component in the industry. (4) Optimum design: It is the best design for the given objective function under the specified constraints. It may be achieved by minimizing the undesirable effects. (5) Element design: It is the design of any element of the mechanical system like a piston, crank shaft, connecting rod etc.. (6) System design: It is the design of any complex mechanical system like a motor car. (7) Computer aided design: This type of design depends upon the use of computer systems to assist in the creation, modification, analysis and optimization of a design. GENERAL CONSIDERATIONS IN DESIGNING A MACHINE COMPONENT: 1. Type of load and stresses caused by the load: The load or force is an external agent which, when applied on a machine part produce or tends to produce/destroy motion. Generally the machine members are subjected to various external forces due to 1. Self weight of the machine. 2. Inertia due to reciprocating parts. 3. Power transmission. 4. Change of nature like temp and other 5. Frictional forces. The load is classified with respect to its nature of application as 1. Steady or static or dead load: Whose magnitude and direction will not change with respect to time. 5

6 2. Live or variable or dynamic load: Whose magnitude and direction change time to time. 3. Suddenly or shock load: Initial velocity is zero. Ex: Designing a lathe bed require cast iron which is more hard and high compressive strength, for making dial gauge, glass may be employed. 4. Impact load: This is suddenly applied with some velocity. Ex: blows of hammer, rough road reactions to the wheels and axles of motor cars. In general the components to be designed or dynamic and impact load should be stronger and bigger than that for steady load. The load on a machine component may act in several ways due to which the internal stresses are set up: Tensile, compressive, shear, Torsional, Bending, Combined stresses, Thermal stresses. Load Static or dead or steady load, Live or variable load, Suddenly applied or shock load, Impact load. Stresses Tensile, compressive, shear, Torsion, Bending, Combination of any stresses due to Change in temp (Thermal stress) 2. Motion of the parts: Depending upon the given specification the suitable prescribed motion of the part is to be evaluated. The motion of the parts are (1) Rectilinear motion.(reciprocating) (2) Curvilinear motion. (rotary) (3) Constant speed. (4) Constant or variable acceleration 3. Selection of materials: The selection of material for a part depends upon the forces that are acting on that part and stresses developed on that part. The design engineer should have a thorough or complete knowledge of the mechanical behavior of materials under different loads. 4. Form and size of the parts: Based upon the stresses acting on the part, the size and form shape (appearance) of a component is to be designed. In the process of reducing the size of the machine or existing component the design engineer should always check for the capability of that part to resist the stresses. The size is inversely proportional to material strength if the load is kept constant. 6

7 5. Lubrication: There is always a lot of heat is dissipated between movable parts (rotating, sliding or rolling bearings),a design engineer should always provide a better means of lubrication between the parts. 6. Operational features: The designer should always consider the operational features of the machine. For example the start button, controlling levers, stop button should be designed based upon the convenient handling of the operator. 7. Use of standard parts: using the existing standard parts like bolts, nuts, washers, Gears and pulleys etc Reduces the cost of a machine and also it simplifies the manufacturing process. The designer should always go for selection of available parts of standard sizes; however, if the design requires a new part, then designer has to suggest a new manufacturing process. 8. Safety of operation: Some machines are dangerous to operate at maximum speed. It is necessary that a designer should always provide safety devices for the safety of the operator. The safety appliances should in no way interference with operation of the machine Ex: Electrical main-switch. 9. Work shop facilities: The designer has to always design the part based upon his employer s work shop facilities available to him. Sometimes it is necessary to plan and supervise the work shop operations and to draft methods for casting, handling and machining special parts. 10. Number of components to be manufactured: Based upon the number of parts to be manufactured, the designer has flexibility in designing the part. If the number of components to be manufactured are less, the designer should always look at using the standard shapes and sizes of parts available to him. However if the number of components to be manufactured are more, he can go for a new product (design) of the part. 11. Cost of construction: The designer has to always try to minimize the cost of construction of a machine. Use of standard parts and using the manufacturing process available to him can reduce the cost of construction. 12. Assembling: Based upon the local conditions at errection of the machine, the designer should design the different components of a machine. STANDARDIZATION IN DESIGN: 7

8 For easy identification of materials, further improvement of machine elements, and easy replacement of worn-out parts and for quick and easy manufacturing, the parameters of machine elements are standardized. In design, the aim is to use as many standard components as possible for a given mechanism. Standardization is defined as obligatory norms or standards to which various characteristics of a product should conform. The characteristics include materials, dimensions and quality of the product, method of testing and method of marking, packing and storing of the product. Standardization becomes a global activity to cover all economical, technical and material aspects of engineering products. The work of standardization is accomplished by national or international organizations. The following standards are used in mechanical engineering design: 1. Standards for materials, their chemical compositions, mechanical properties and heat treatment. 2. Shapes and dimensions of commonly used machine elements such as bolts, screws and nuts, rivets, belts and chains, bearings, wire ropes, keys, gears etc.. 3. Standards for fits, tolerances and surface finish of components. 4. Standards for testing of products such as pressure vessels, boilers, overhead traveling cranes. 5. Standards for engineering drawing of components. There are three types of standards used in the design: International standards organization (I.S.O) National standards, such as I.S (Bureau of Indian Standards), D.I.N (Dutch International Number, German), A.I.S.I or S.A.E (American Iron and Steel Institute, USA), B.S (U.K) standards and J.I.S (Japanese Standards). Company standards for use in a particular company or a group of sister concerns like air-craft and ship-building industries manufacture their products with their own standard without adopting the general standard parts. This type of individual plant standardization is known as Normalization. The objective of standardization is to make the mass production of components easier. Interchangeability of components is possible due to standards. Repair and maintenance of machines simple because the worn out or damaged parts can be easily replaced by standard ones. It aims at reduction of design, manufacturing, inventory 8

9 and handling costs as well as the efforts to achieve uniformity, efficiency and a specified quality. PREFERRED NUMBERS:- In engineering design many times the designer has to specify the size of the product. The size of the product is a general term which includes different parameters like power transmitting capacity, load carrying capacity, speed, dimensions of the component such as height, length, width and weight of the product. These parameters are expressed numerically e.g.: 5 KW, 10 KN, 1000 rpm. Often the product is manufacturing in different sizes or models for instance, a company may be manufacturing different models of electric motors ranging from 0.5 KW to 50 KW to cater to the need of different customers. Preferred numbers are used to specify the sizes of the products in these cases. Preferred numbers were first introduced by Charles Renard. These are nothing but a series of numbers in a geometric progression (G.P) specially selected to be used for standardization in preference to any other random numbers. They are written as integral powers of 10. The first four are called basic series and other are called derived series, (denoted as R5, R10, R20, R40 and R80 series) which increase in steps of 58%, 26%, 12%, 6%, and 3% respectively. Each series has its own series factor. The series factors are as follows: R5 series 5 10 = 1.58 R10 series = 1.26 R20 series = 1.12 R40 series = 1.06 R80 series = 1.03 The resultant numbers are rounded as per international standards and shown in Design Data Hand Book K. Mahadevan Page No: 402, Table 23.1, 23.2 Ex: Hydraulic cylinder capacities are in R5 series Hoisting capacities (in tones) of cranes are in R10 series Wire diameters of helical spring are in R40series Hydraulic cylinder diameters are in R20 series USES:- Preferred numbers are an important tool, which minimize unnecessary variation in sizes. They assist the designer in avoiding selection of sizes in an arbitrary manner. The complete range is covered by minimum number of sizes which is advantageous to producer and consumer. They are unlimited towards the lower as well as higher numbers. 9

10 MATERIALS AND THEIR PROPERTIES Selection of a proper material for the machine component is one of the most important steps in the process of machine design. The best material is one which will serve the designed objective at minimum cost.selection of material by trial and error method. While selecting the material to follow the factors (1) Availability (2) Cost (3). Mechanical properties (4) Manufacturing considerations. Availability: The material should be readily available in the market, in large enough quantities to meet the requirement. Cost: For every application, there is a limiting cost beyond which the designer cannot go. When this limit is exceeded the designee has to consider other alternative materials. In cost analysis there are two factors, namely cost of material and the cost of processing the material into finished goods. It is likely that the cost of material might be low, but the processing may involve costly machining operations.\ Mechanical properties: These properties govern the selection of materials. Depending upon the service conditions and the functional requirement, different mechanical properties are considered and a suitable material is selected. Ex: connecting rod of I.C-Withstand fluctuating stresses due to combination of fuel- Endurance strength criterion of design. Piston rings resist wear- Surface hardness criterion of design. Bearing material have low coefficient of friction. Clutch or brake lining -has high coefficient of friction. Manufacturing considerations: The manufacturing processes such as casting, rolling, forging, extrusion, welding and machining govern the selection of the material. Machine ability of material is an important consideration in selection. Sometimes, an expensive material is more economical than a low priced one. Which is difficult to machine. Where the product is of a complex shape, casting properties are important. Past experience is a good guide for the selection of material. Iron and its alloys, Cast iron, carbon steels, Alloy steels. Cu, Zn, Mg, Ni, Silica, Ag Various Properties of materials are classified as follows: 1. Physical properties: They include density, porosity, structure, Fusibility, Shape and size. 10

11 2. Mechanical properties: They include strength, stiffness, elasticity, plasticity, ductility, brittleness, creep, fatigue, hardness etc 3. Magnetic properties: They include thermal permeability and hysterics. 4. Thermal properties: They include thermal conductivity, specific heat, latent heat and thermal stresses. 5. Electrical properties: They include dielectric strength, conductivity and resistively. 6. Chemical properties: They include chemical composition, corrosion resistance, acidity and alkalinity. Mechanical properties of materials: The mechanical properties of the metals are those which are associated with the ability of the material to resist mechanical forces and load. Which undergo any changes in shape and structure during the application of force on these elements. Ex: if a rod is subjected to a tensile load, its length can be increased and soon. 1. Strength: It is the ability of a material to resist the externally applied forces without breaking or yielding. Static load: Ultimate tensile Strength or tensile yield strength. Fluctuating load: Endurance strength. 2. Elasticity: It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is disinable for materials used in tools and machines. It may be noted that steel is more elastic than rubber. 3. Plasticity: It is the property of a material which retain the deformation produced under load permanently. This property of the material is necessary for forging, in stamping images on coins and in or nonmetal work. 4. Ductility: It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductility is usually measured by the terms percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice are mild steel, copper, aluminum, nickel, zinc, tin and lead. 5. Brittleness: It is the property of a material opposite to ductility it is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads snap off without giving any sensible elongation. Cast iron is a brittle material. 11

12 6. Toughness: It is the property o a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. This property is desirable in parts subjected to shock and impact loads. 7. Malleability: It is a special case of ductility which permits materials to be rolled or hammered into thin sheets due to compressive force. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice are lead, soft steel, Wrought iron, copper and aluminum. 8. Creep: When a part is subjected to a constant stress at high temp for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing I.C engine, boilers and turbines. 9. Fatigue: When a material is subjected to repeated stresses it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of progressive crack formations which are usually fine and microscopic size. This property is considered in designing shafts, connecting rod, springs, gears etc. 12