Module 5. Properties of material, failures and factor of safety

Lesson 22


22.1 Introduction

The subject Machine Design is the creation of new and better machines and improving the existing ones. A new or better machine is one which is more economical in the overall cost of production and operation. The process of design is a long and time consuming one.

From the study of existing ideas, a new idea has to be conceived. The idea is then studied keeping in mind its commercial success and given shape and form in the form of drawings. In the preparation of these drawings, care must be taken of the availability of resources in money, in men and in materials required for the successful completion of the new idea into an actual reality.

In designing a machine component, it is necessary to have a good knowledge of many subjects such as Mathematics, Engineering Mechanics, Strength of Materials, Theory of Machines, and Engineering Drawing.

22.2 Classifications of Machine Design

The machine design may be classified as follows:

22.2.1 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 alternation or modification in the existing designs of the product.

22.2.2 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 adopting a new material or different method of manufacture. In this case, though the designer starts from the existing design, but the final product may differ quite markedly from the original product.

22.2.3 New design

This type of design needs lot of research, technical ability and creative thinking. Only those designers who have personal qualities of a sufficiently high order can take up the work of a new design. The designs, depending upon the methods used, may be classified as follows:

  • Rational design: This type of design depends upon mathematical formulae of principle of mechanics.
  • Empirical design: This type of design depends upon empirical formulae based on the practice and past experience.
  • Industrial design: This type of design depends upon the production aspects to manufacture any machine component in the industry.
  • 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.
  • System design: It is the design of any complex mechanical system like a motor car.
  • Element design: It is the design of any element of the mechanical system like piston, crankshaft, connecting rod, etc.
  • 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.

22.3 Design Procedure and Specifications

The general procedure for design is widely available in the literature .The following procedure is representative of those found in the literature.

  • Identification of need
  • Problem statement or definition of goal
  • Research
  • Development of specifications
  • Generation of ideas
  • Creation of concepts based on the ideas
  • Analysis of alternative concepts
  • Prototype and laboratory testing
  • Selection and specification of best concept
  • Production
  • Marketing
  • Maintenance and repairs

22.4 General Procedure in Machine Design

In designing a machine component, there is no rigid rule. The problem may be attempted in several ways. However, the general procedure to solve a design problem is as follows:

  • Recognition of need. First of all, make a complete statement of the problem, indicating the need, aim or purpose for which the machine is to be designed.
  • Synthesis (Mechanisms). Select the possible mechanism or group of mechanisms which will give the desired motion.
  • Analysis of forces. Find the forces acting on each member of the machine and the energy transmitted by each member.
  • Material selection. Select the material best suited for each member of the machine.
  • Design of elements (Size and Stresses). Find the size of each member of the machine by considering the force acting on the member and the permissible stresses for the material used. It should be kept in mind that each member should not deflect or deform more than the permissible limit.
  • Modification. Modify the size of the member to agree with the past experience and judgement to facilitate manufacture. The modification may also be necessary by consideration of manufacturing to reduce overall cost.
  • Detailed drawing. Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing processes suggested.
  • Production. The component, as per the drawing, is to be manufactured in the workshop.

22.5 Factor of Safety

It is defined, in general, as the ratio of the maximum stress to the working stress. Mathematically,

Factor of safety =Maximum stress/Working or design stress

In case of ductile materials e.g. mild steel, where the yield point is clearly defined, the factor of Safety is based upon the yield point stress. In such cases,

Factor of safety =Yield point stress/Working or design stress

In case of brittle materials e.g. cast iron, the yield point is not well defined as for ductile materials.

Therefore, the factor of safety for brittle materials is based on ultimate stress.

Factor of safety =Ultimate stress/Working or design stress.

22.6 Selection of Factor of Safety

The selection of a proper factor of safety to be used in designing any machine component depends upon a number of considerations, such as the material, mode of manufacture, type of stress, general service conditions and shape of the parts. Before selecting a proper factor of safety, a design engineer should consider the following points:

  • The reliability of the properties of the material and change of these properties during service.
  • The reliability of test results and accuracy of application of these results to actual machine parts.
  • The reliability of applied load.
  • The certainty as to exact mode of failure.
  • The extent of simplifying assumptions.
  • The extent of localized stresses.
  • The extent of initial stresses set up during manufacture.
  • The extent of loss of life if failure occurs.
  • The extent of loss of property if failure occurs.

Each of the above factors must be carefully considered and evaluated. The high factor of safety results in unnecessary risk of failure. The values of factor are safety based on ultimate strength for different materials and type of load.

Last modified: Thursday, 18 October 2012, 9:35 AM