What is machine design ?

 DEFINITION OF MACHINE DESIGN

● The term design can be defined as the formulation of a plan for the satisfaction of human need. 

● Design means to create something new or arrange existing things in a new order to satisfy recognized need of society.

● Mechanical engineering design essentially means the design of the parts, products and systems of mechanical nature. It deals with all the disciplines of mechanical engineering, such as, machine design, thermal engineering, fluid power engineering, refrigeration and air conditioning, etc.

● Machine design deals with the design of machines, mechanisms and their elements.  The design of machines or mechanisms ultimately comes to the design of their individual elements.  

● Machine design is the process of selection of the materials, shapes, sizes and arrangements of mechanical elements so that the resultant machine will perform the prescribed task. 

● Design of machine element can be defined as the selection of material and the values ​​for independent geometrical parameters so that the element satisfies its functional requirements and undesirable effects and kept within the permissible limits.  

● The concept of machine design is illustrated in Figure.

Figure: Concept of Machine Design

● For example, the process of design of a belt drive consists of: 

   1. Selection of arrangement of mechanical elements like: pulleys, belts, shafts, keys, bearings, etc;  

   2. Selection of shapes of these mechanical elements;

   3. Selection of materials for these mechanical elements, and 

   4. Selection of sizes of these mechanical elements.  

● Most of the problems in mechanical engineering design or specifically in machine design, do not have a unique right answer.  There are nearly endless number of workable designs, none of which could be called an incorrect answer.  But of the correct answers, some are obviously better than others.

DESIGN PROCEDURE

Figure: Design Procedure

The general procedure that is followed in machine design is illustrated in Figure. It consists of Step:

Step 1: Definition of Problem: 

  Define the design problem giving all input parameters, output parameters, and constraints.  

Step 2: Synthesis:

  Once the problem is defined, the next step is synthesis.  Synthesis is the process of selecting or creating the mechanism for the machine and the shapes of the mechanical elements so as to get the desired output with given input.  

Step 3: Analysis of Forces:

  Draw the force body diagram of each element of the machine.  Find out the forces (including moments and torque) acting on cach element by force analysis. 

Step 4: Selection of Material: 

  Select the suitable material for each clement.  Four basic factors that are to be considered while selecting the material are availability, cost, mechanical properties, and manufacturing considerations.  

Step 5: Determination of Mode of Failure:

  Before finding out the dimensions of the element, it is necessary to know the type of failure which the clement will fail when put into the use.  

Step 6: Selection of Factor of Safety: 

  Based on the application, select the factor of safety.  Knowing factor of safety and material strength, determine the permissible or design stresses.

Step 7: Determination of dimensions: 

  Find the dimensions of each element of the machine by considering the forces acting on the element and the permissible stresses.

Step 8: Modification of Dimensions: 

  Modify the dimensions of the elements on the higher side, if required, based on the following considerations:

(i)   Selection of standard parts  available in the market;  

(ii)  Convenience of assembly, and 

(iii) Convenience of manufacturing.  

Step 9: Preparation of Drawings: 

● Prepare working drawing of each element or component with minimum two views showing the following details: 

   (i)   Dimensions;

   (ii)  Dimensional tolerances,

   (iii) Surface finish;  

   (iv) Geometrical tolerances; and

   (v)  Special production requirements like heat treatment. 

● Prepare assembly drawing giving part numbers, overall dimensions, and part list.  ● The component drawing is supplied to the shop floor for manufacturing purpose, while assembly drawing is supplied to the assembly shop 

Step 10: Preparation of Design Report:

  Prepare design report containing details about step I to step 8.

DESIGN CONSIDERATION

● Design considerations are the characteristics which influence the design of the element or, perhaps, the entire system. Normally, a number of such characteristics have to be considered in any design problem.

● In a given design problem, the design engineer should identify the various design considerations and include them in the design process in their order of importance.  

● For example, in the design of a spring, the two most important design considerations are: strength and stiffness.  

● Some of the important design considerations are as follows:

The various design consideration