Design of Machine Elements - short answer questions from AMIE exams (Winter 2020)

Answer briefly on the following

Free body diagram
A free-body diagram is a sketch of an object of interest with all the surrounding objects stripped away and all of the forces acting on the body shown. The drawing of a free-body diagram is an important step in the solving of mechanics problems since it helps to visualize all the forces acting on a single object. The net external force acting on the object must be obtained in order to apply Newton's Second Law to the motion of the object.


Different types of loads
Point load
This type of load acts relatively on a smaller area. For example, the force exerted by a  chair or a table leg on the supporting floor or load exerted by a beam on a supporting column are both considered to be concentrated. 



Distributed load
As the name itself implies, a uniformly distributed load is spread over a large area. Its magnitude is designated by its intensity (N/m or kN/m). The water pressure on the bottom slab of a water tank is an example of such loading. If a floor slab is supported by beams, the load of the slab on the beams is certainly uniformly distributed.


Uniformly Varying Load (UVL)
This type of load will be uniformly varying from zero intensity at one end to the designated intensity at the other end. A triangular block of brickwork practically imposes such loading on a beam. The water pressure distribution on the walls of a water tank could be another example.


Concentrated Moment
If for some purpose, a beam is to accommodate a load on a bracket mounted on it, what gets transmitted on the beam is a concentrated moment.


BIS code
BIS is the National Standard Body of India established under the BIS Act 2016 for the harmonious development of the activities of standardization, marking and quality certification of goods and for matters connected therewith or incidental thereto.

Define fatigue and creep
Creep
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses. It can occur as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods and generally increases as they near their melting point.

A creep curve

Fatigue
Fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle. The crack will continue to grow until it reaches a critical size producing rapid propagation and typically complete fracture of the structure.

S-N curve (fatigue)

Notch sensitivity
It is the measure of how sensitive a material is, towards the notches or geometric discontinuities. Numerically it is given by the ratio of increase in actual stress over nominal stress with an increase in theoretical stress over nominal stress. These increases in stress are due to stress concentration.


Define hardness and toughness
  • Hardness: A material’s ability to withstand friction, essentially abrasion resistance, is known as hardness. Diamonds are among the hardest substances known to man, it is incredibly difficult to scratch a diamond. However, while a diamond is hard it is not tough. 
  • Toughness: How well the material can resist fracturing when force is applied. Toughness requires strength as well as ductility, which allows a material to deform before fracturing. Do you consider silly putty to be tough stuff? Under these terms, believe it or not, it actually is relatively tough, as it can stretch and deform rather than break.
State the efficiency of the screw
The efficiency of a lead screw refers to how well a screw converts rotary energy (torque) into linear motion. Rolling contact has a lower coefficient of friction than sliding contact. Lead screws, including acme screws, offer efficiency levels of between 20 per cent and 80 per cent.

Mention different kinds of a rolling contract bearing
Following are the two types of rolling contact bearings:
  • Ball bearings
  • Roller bearings.


The ball and roller bearings consist of an inner race which is mounted on the shaft or journal
and an outer race which is carried by the housing or casing.
The rolling contact bearings, depending upon the load to be carried, are classified as
  • Radial bearings
  • Thrust bearings.
Gear material
The gears may be manufactured from metallic or non-metallic materials. The metallic gears with cut teeth are commercially obtainable in cast iron, steel and bronze. Nonmetallic materials like wood, rawhide, compressed paper and synthetic resins like nylon are used for gears, especially for reducing noise.

Application of key
A key is a piece of mild steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them. It is always inserted parallel to the axis of the shaft. Keys are used as temporary fastenings and are subjected to considerable crushing and shearing stresses. A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key.

Types of keys

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