Machine Design -I
1) INTRODUCTION TO DESIGN
THEORY QUESTIONS AND ANSWERS
1) State the general design consideration in machine design.
General Considerations in Machine Design
Following are the general considerations in designing a machine component :
1. Type of load and stresses caused by the load.
2. Motion of the parts or kinematics of the machine.
3. Selection of materials.
4. Form and size of the parts.
5. Frictional resistance and lubrication.
6. Convenient and economical features.
7. Use of standard parts.
8. Safety of operation.
9. Workshop facilities.
10. Number of machines to be manufactured.
11. Cost of construction.
2) What are the steps involved in general design procedure.
Machine design is a very vast subject and there are no rigid hard and fast rules to design a component. As the designers skill level goes on increasing he may choose a better and faster methods to design a component, and he may follow his own steps to reach the final design, but for the general consideration, following steps are followed.
1. Recognition of need:
In this state the complete problem is specified in terms of numerical values. A complete statement is prepared giving what exactly needed and how it is to be achieved.
2. Synthesis (Mechanisms):
In this stage the various alternatives to attain the objective stated in the previous step are evaluated on the basis of feasibility. The best alternative is choosen from the available based on primary criterion such as cost,ease,weight etc.
3. Analysis of forces :
After selection of the mechanism or system the next step is to find the forces acting on the system. The various seen and unseen situations are considered and forces are calculated. These forces form the basis of further calculations.
4. Material selection : This is the most crucial stage in the design procedure, during this stage various alternative material are evaluated and opimum selection of material is done.
5. Design of elements (Size and Stresses) :
This is the step of actual calculations of the stresses and sizes of various mechanical components, during this the simple stress formula,Flexural formula, torsional equation etc are used to find the dimensions
6. Modification :
During this staget the size of component is modified keeping the commercial availability of the material and other assembling considerations.
7. Detailed drawing :
Next stage is to prepare the assembly and details drawing of the each component and sub assembly. At this stage the geometrical tolerances and surface finish requirements are finalized.
8. Production :
The final step is to produce the component as per the detailed specification obtained in the previous step.
3) Explain in brief different loads a machine component is subjected to.
Ans : Following are the types of load that a machine member is subjected to ,
- Static load : It is a load which remains constant at its magnitude, directiona and location. According to this definition a hanging weight on a bar is an example of this type of load. The load may be a point load or udl or uvm, as long as it doesn’t changes its value with time it is called static load. Design against such load is easy as compared to other types of loads.
- Dynamic Loads : A dynamic load is one which shows variation in magnitude or direction or point of application. The best example is the wind force, which is never a constant force/load. Since dynamic loada are quite uncertain regarding their variations, hence sufficient reserve strength must be kept while dealing with them.
- Cyclic load : Cyclic loads are the dynamic loads which exhibit a specific pattern at their variations. Example is the inertia load on the connecting rod of engine. Such loads cause the fatigue failure in the component they act on, so while designing for cyclic loads sufficient endurance strenth must be ensured. Such components are designed to withstand ‘Endurance limit stress’ rather than mere ultimate stress. Design should be designed such that it will sustain infinite cycles of such load.
- Impact loads : Instantenous large load produces the impact load, it may be due to collision, falling or suddern jerk. These forces are very large and instantaneous , so while considering in design they must be dealt with sufficient reserved strength (by taking higher factor of safety ).
4) Explain the use of STANDARDS in design or Write a note on Standardization in design. State Advantages of Standardization.
Ans : Standardization are the NORMS which are obligatory to follow and to which every product should conform. The standards include the materials, dimensions,shapes or procedures to be followed. Related to Machine design following area are standardized.
1) Material standards : These include the chemical compositions, mechanical properties and heat treatment methods. Some material standards are for a particular country and some are followed worldwide.
2) Shapes and Dimensions : These include the standard steel sections and other components like bearings,belts,ropes and chains.
3) Standards for Engg drawings : These standards specifty the methods to be adopted as well as the symbols to be used for drawing and drafting, so that uniformity in drawing is achived all over the country/world.
4) Standards for Fits,tolerances and finish ; These are related to the dimensional and geometrical tolerances ,which are standardized so as to achieve the interchangability of the parts in mass production.
5) Standards related to TESTING of products : In order to compare the different products manufactured by different manufacturers, it is necessary that both products are tested by same procedure and against same conditions, this is achieved by standardization of the testing procedures.
Following are the advantages of the standardization :
1) Mass production : standardization makes the mass production easy, which is an important factor in cost reduction.
2) Reduction in Design time : Due to standardization the design process becomes easy, in some cases it reduces to just selection from the manufacturer's catalogue.
3) Easy maintenance : Due to standardization the parts are easily available at all places, which makes the maintenance quick and easy.
4) Improved Quality : Due to specialization of a particular company in a component, the customers get better and better products and at competitive rates.
5) Define Factor of safety. State the factors which decide an appropriate factor of safety.
Ans : Factor of safety is defined as the "ratio of critical load to safe load". When related to stresses it is defined as the "ratio of Ultimate stress to design stress"
The factor of safety provides the margin by which there is safety even when unseen situations arise. If we design with the exact breaking stress then there is risk that the component will fail as soon as any undesirable situation results in highes stress than that of breaking stress. On the other hand choosing higher factor of safety will result in heavier design and more material cost. So wise selection of the factor of safety is necessary so as to reach the golden point between economy and safety.
Following are the factors which are mainly considered while deciding the appropriate value of factor of safety.
1) Type/Nature of load: Different types of loads demand different values of factor of safety. When the load is certain and not changing we may go for lower value of factor of safety, but for the impact or suddenly applied load or the load which fluctuates more, we need to have higher value of FOS.
2) Certainity of load analysis : If the given load is expected to have very less variations, though uncertain, the recommended FOS can be a low value. But if the uncertainity is of larger magnitude and exact load analysis is not known it is better to go for the higher fos.
3) Expected overloading : engineering objects are always subjected to overload during their use. Such overload cannot be ignored, so sufficient strength must be kept reserved for such situations.
4) Type of material used : certain materials like rolled Steel bars, forged materials etc are very reliable in their strength. but some materials like wood, stone, cast iron are not reliable in their strength. for lesser reliable materials higher factor of safety is choosen.
5) Type of manufacturing process : certain manufacturing processes does not hamper the strength prediction of material, but some manufacturing processes like welding alter the strength of material. Hence in such situations where the manufacturing process impacts the strength of material, It is wise to choose higher factor of safety.
6) Risk of life : if the failure of the component can result in loss of human life damage to the property it is always preferred to have how a factor of safety.
7) Economical consideration : as we go for higher factor of safety the design goes on becoming more and more bulky, as well as no cost is involved, so from economy point of view we always try to keep the value of factor of safety to an optimum level.