Unit-1
Introduction to design
The process for design of a machine elements are as follows:
Sometimes design begins when an engineer recognizes a need and decides to do something about it.
2. Definition of the problem:
It must include all the specifications for the thing that is to be designed. The specifications are the input and output quantities, the characteristics and dimensions of the space (the thing must occupy) and all the limitations on these quantities.
3. Synthesis:
Select the possible mechanism or group of mechanism which will give the desired motion.
4. Analysis and optimization:
Find the different forces acting on each member of the machine, and analyze different mechanism.
Comparison of different mechanism helps to choose most competitive product in market.
5. Material Selection:
Select the material best suited for each member of the machine.
6. Design of element (size and stress):
Find the size of each member of the machine by considering the forces acting on the member and the permissible stress for the material used.
7. Modification:
Modify the size of the member to agree with the past experience and judgement to facilitate manufacture.
8. Detailed drawing:
Draw the detailed drawing of each component and the assembly of the machine with complete specification for the manufacturing processes suggested.
9. Production:
The product as per the drawing is manufactured.
Key takeaways
Select the possible mechanism or group of mechanism which will give the desired motion. Modify the size of the member to agree with the past experience and judgement to facilitate manufacture.
Following are the design factors:
Selection of proper material for the machine component in one of the most important steps in the process of machine design. The best material is one which will serve the desired objectives at minimum cost. Following are the factors should be considered while selecting the materials:
The manufacturing process is as casting, rolling, forging, extrusion, welding, and machine govern the selection of material.
Key takeaways:
Availability, cost, manufacturing are important practical consideration.
Selection of material in mechanical design depends on the following parameters:
2. User’s requirements
3. Efficiency
4. Mass production
5. Accuracy
6. Aesthetics
7. Noise
8. Safety
9. Functional requirement for reliable performance
10. Weight
11. Other objectives
12. Times
13. Delivery
14. Life
In the mechanics of materials, the strength of a material is its ability to withstand an applied load without failure or plastic deformation. The field of strength of materials deals with forces and deformations that result from their acting on a material.
An impact is an extreme force or shock applied over a short time period. As when two or more objects collide. This type of acceleration or force has a greater effect than a lower force applied over a longer period of time.
The formula for calculating impact load is mentioned as E=mgh. In the equation, m is the mass of the object, E is the energy, g is the acceleration due to gravity constant (9.81 m s−2 or 9.81 meters per second squared), and h is the height the object falls from.
Key takeaways:
E=mgh is the important equation in impact load where, m is the mass of the object, E is the energy, g is the acceleration due to gravity constant (9.81 m s−2 or 9.81 meters per second squared), and h is the height the object falls from.
Shock loading refers to a sudden and drastic increase of load, similar to a “hammering” effect. The most common occurrence is when a load is dropped onto a ball transfer unit from a height or when ball units travel over an uneven surface, causing an uneven distribution of load.
Shock load = load x [1 + (1 + (2 x FD x A x E)/(load x L))^1/2].
Where, FD= falling distance, A=area factor, E=modulus of elasticity, and L=length of chord.
Key takeaways:
Shock load = load x [1 + (1 + (2 x FD x A x E)/(load x L))^1/2].
Where, FD= falling distance, A=area factor, E=modulus of elasticity, and L=length of chord.
The behaviour of a material under cyclic and reverse loading or stresses is called fatigue. Fatigue properties of a material is determines the behaviour when subjected to cyclic load in which maximum stress developed in each cyclic load in which maximum stress developed in each cycle is within the elastic range of the material. About 90% of the total mechanical fractures are due to fatigue.
Aeroplane wings, leaf springs, turbine engines, connecting rod in I.C. Engine are subjected to a fluctuating or cyclic load.
For many engineering applications, the finish on a surface can have a big effect on the performance and durability of parts. Rough surfaces generally wear more rapidly and have greater friction coefficients than smooth surfaces.
In engineering design, many times the designer has to specify the size of 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 and width and volume or weight of product.
These parameters are expressed numerically like 5kN, 10 kW or 1000 rpm.
Often the product is manufactured in different size or models, for instance a company may manufacture seven different models of electric motors, ranging from 0.5 kW to 50 kW to fulfil the need of different customer. Preferred number is used to specify the ‘sizes’ of the product in these cases.
Increasing the temperature lowers the activation energy of a reaction. Increasing the temperature results in a higher rate of collision between particles. Increasing temperature produces more effective collisions with enough energy for a reaction to occur.
A solid is composed of molecules that are tightly packed together, thereby giving the object a rigid structure that is resistant to change. As temperature rises, the kinetic energy of the molecules within the solid begin to vibrate, which decreases the attraction of these molecules.
A stress concentration (also called a stress raiser or a stress riser) is a location in an object where the stress is significantly greater than the surrounding region. Stress concentrations occur when there are irregularities in the geometry or material of a structural component that cause an interruption to the flow of stress. This arises from such details as holes, grooves, notches and fillets. Stress concentrations may also occur from accidental damage such as nicks and scratches.
Whenever a machine component changes the shape of its cross section, the simple stress distribution no longer holds good and the neighbourhood of the discontinuity is different. This irregularity in the stress distribution caused by abrupt changes of form is called 'stress concentration'.
The maximum stress is calculated as σmax = Kt σnom, where Kt is the stress concentration factor.
Key takeaways:
This irregularity in the stress distribution caused by abrupt changes of form is called 'stress concentration'
Creep is the slow and progressive deformation of a material under steady load (stress) with time is called creep. In other words, it is permanent deformation under static loading of material over long period of time. It is also defined as time dependent strain occurring under steady load. After application of steady stress in the material at very low temperature, it is termed as cold creep.
When the load is steady it is having the progressive deformation at high temperature, it is called as hot creep.
Key takeaways:
Creep is the slow and progressive deformation of a material under steady load (stress) with time is called creep.
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