Unit No 2 Design Considerations INTRODUCTION TO MACHINE

Unit No. – 2 Design Considerations

INTRODUCTION TO MACHINE DESIGN Definition of Machine: An assembly of interconnected components arranged to transmit or modify force in order to perform useful work. Definition of Machine Design: It is defined as the use of scientific principles, technical information and imagination in the description of a machine or a mechanical system to perform specific functions with maximum economy and efficiency. The description of a mechanical system is in form of drawings, which specify the material selected, shapes and sizes and arrangements of mechanical elements to perform the prescribed task.

NEED OF DESIGN 1. Design for Functional Requirements The product or system is designed to perform a prescribed task satisfactorily. 2. Design for Safety The product or system is designed to ensure no failure during specified life of the product. 3. Design for Cost Reduction The proper designing of the product helps in reducing cost of the product and make it competitive. 4. Design for Optimization The design will help to optimize the geometrical, material and performance parameters of the product or system.

NEED OF DESIGN 5. Design for Compactness Proper designing helps in reducing the size and weight of the product or system. 6. Design for Comfort Proper design will provide more comfort to the user, or it can be more user friendly. 7. Design for Appearance A better appearance can attract customers even if a product with identical parameters exist in the market. 8. Design for Innovations Design & redesign helps in incorporating new ideas into the product.

TYPES OF MACHINE DESIGN 1. System Design It is design of any complex mechanical system, which consist of number of subsystem, which further contain elements. 2. Product Design It is design of a product which is a subsystem of any mechanical system. Ex: Design of Gearbox etc. 3. Element Design It is design of any mechanical element which is a part of mechanical subsystem or product. Ex: Gear, shaft, etc.

TYPES OF MACHINE DESIGN 4. Empirical Design It is design using empirical formulae/relations that are based on past experience and practices. Used when design equations are not available or complex. Ex: Gearbox casing, etc. 5. Optimum Design For any problem number of design solutions are available. From the available solutions, that which minimizes undesirable effects and maximizes useful parameters is called optimum design. 6. Computer Aided Design Computer system is used to assist in the creation, modification, analysis and optimization of a design.

STEPS IN DESIGN PROCESS Definition of Problem Selection of Factor of Safety Determination of Dimensions Synthesis Determining modes of failures Modifications of Dimensions Analysis of Forces Selection of Material Preparation of Drawings Preparation of Design Report

STEPS IN DESIGN PROCESS STEP 1: Definition of Problem The problem is defined giving all input parameters, output parameters and constraints. STEP 2: Synthesis involves selecting/creating the mechanism for the machine and the shape of the mechanical elements so as to get desired output with the given input. STEP 3: Analysis of Forces The forces acting on each element is determined drawing the free body diagram (FBD) of each element of the machine.

STEPS IN DESIGN PROCESS STEP 4: Selection of Material A suitable material is selected considering its availability, cost, mechanical properties and manufacturing considerations. STEP 5: Determination of Mode of Failure Identify the mode of failure of the element if put to use because of the influence of the forces. STEP 6: Selection of Factor of Safety Based on the application, the factor of safety and material strength is selected and hence the permissible stresses determined.

STEPS IN DESIGN PROCESS STEP 7: Determination of Dimensions The dimensions of each element is determined by considering the forces and the permissible stresses. STEP 8: Modification of Dimensions Modify the dimensions of the element on the higher side, if required based on selection of standard parts available in market , convenience of assembly & manufacturing. STEP 9: Preparation of Drawing Working drawing of each element is prepared with minimum two views showing details like dimensions, and its tolerance, surface finish, geometrical tolerance, etc.

STEPS IN DESIGN PROCESS STEP 10: Preparation of Design Report The design report is prepared containing details about step 1 to step 8.

GENERAL DESIGN CONSIDERATIONS 1. Strength Since the machine is subjected to a combination of various loads, it should have sufficient strength to avoid failure due to yielding or fracture (failure) under the loads. 2. Rigidity An element should have sufficient rigid so that its linear as well as angular deflections under loading are within limits. 3. Reliability A machine element should have reasonably good probability so that it can perform its function satisfactorily over its life span.

GENERAL DESIGN CONSIDERATIONS 4. Safety A machine element should be designed such that it ensures safety of the users and machine. 5. Cost The life cycle cost (made up of production cost, operating cost, maintenance cost, disposal cost) should be minimum possible. 6. Weight A machine element should have a minimum possible weight. 7. Ergonomics is a scientific study of man-machine-working environment relationship to solve problems arising from the same. A machine should be designed with ergonomic criteria.

GENERAL DESIGN CONSIDERATIONS 8. Aesthetic deals with appearance of product (a major factor in attracting customers if products have identical parameter). 9. Manufacturing The manufacturing process should be so selected to produce a machine element in minimum cost and available facilities. 10. Conformance to Standards A machine element should conform to national/international standards and codes. 11. Assembly A machine element should minimize assembly cost and time.

GENERAL DESIGN CONSIDERATIONS 12. Life A machine element should be designed for adequate life. 13. Vibration A machine element should be designed to keep vibration levels minimum. 14. Lubrication Due consideration must be given for lubrication of elements, if there is relative sliding or rolling motion between elements. 15. Maintenance A machine element should be such that it can be easily repaired or serviced.