Design of Machine Elements - short answer questions from AMIE exams (Winter 2018)

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Explain the following in brief (20 marks)

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.

Hardness and Toughness

Toughness
  • The toughness of a material is its ability to withstand both plastic and elastic deformations. It is a highly desirable quality for structural and machine parts to withstand shock and vibration. Manganese steel, wrought iron, mild steels are tough materials.
  • Example: If a load is suddenly applied to a piece of mild steel and then to a piece of glass the mild steel will absorb much more energy before failure occurs. Thus, mild steel is said to be much tougher than glass.
  • Toughness is a measure of the amount of energy a material can absorb before acmal fracture or failure takes place. "The work or energy a material absorbs is called modulus of toughness”
  • Toughness is also called resistance to shock loading. 
  • It is measured by a special test on Impact Testing Machine.
Hardness
  • Hardness is closely related to strength. It is the ability of a material to resist scratching, abrasion, indentation, or penetration.
  • It is directly proportional to tensile strength and is measured on special hardness testing machines by measuring the resistance of the material against penetration of an indentor of special shape and material under a given load. 
  • The different scales of hardness are Brinell hardness. Rockwell hardness. Vicker's hardness, etc.
  • The hardness of a metal does not directly relate to the hardenability of the metal. Hardenability is indicative of the degree of hardness that the metal can acquire through the hardening process, i.e. heating or quenching.

The factor of safety and allowable stress

While designing a component, it is necessary to provide sufficient reserve strength in case of an accident. This is achieved by taking a suitable factor of safety (fs). The factor of safety is defined as fs = failure stress/allowable stress or  fs = failure load/working load

Euler’s formula in columns

Euler formula is

{W_{cr}} = \frac{{n\pi EA}}{{{{(l/k)}^2}}}

where 
E = Modulus of elasticity or Young’s modulus for the material of the column,
A = Area of cross-section,
k = Least radius of gyration of the cross-section,
l = Length of the column, and
n = Constant, representing the end conditions of the column or end fixity coefficient.

The values of n are as follows:


Effect of shot peening on fatigue strength 

In a large number of applications, the external force acting on the spring fluctuates with respect to time resulting in fatigue failure. Due to the poor surface finish of the spring wire, the fatigue crack usually begins with some surface irregularity and propagates due to tensile stresses. It has been observed that the propagation of fatigue crack is always due to tensile stresses. 

In order to reduce the chances of crack propagation, a layer of residual compressive stress is induced on the surface of the spring wire. One of the methods of creating such a layer is shot peening. In this process, small steel balls arc impinged on the wire surface with high velocities cither by an air blast or by centrifugal action. The balls strike against the wire surface and induce residual compressive stresses.

Construction of leaf springs

The laminated or leaf spring (also known as flat spring or carriage spring) consists of a number of flat plates (known as leaves) of varying lengths held together by means of clamps and bolts, as shown in the figure. 

Leaf spring

These are mostly used in automobiles. The major stresses produced in leaf springs are tensile and compressive stresses.

SKF bearings 

AB SKF (Swedish: Svenska Kullagerfabriken; 'Swedish Ball Bearing Factory') is a Swedish bearing and seal manufacturing company founded in Gothenburg, Sweden, in 1907. The company manufactures and supplies bearings, seals, lubrication and lubrication systems, maintenance products, mechatronics products, power transmission products, condition monitoring systems and related services globally.

Gear trains

A gear train consists of two or more gears transmitting power from the driving shaft to the driven shaft. The gear trains are classified into the following categories:
  • Simple gear train
  • Compound gear train
  • Reverted gear train
  • Epicyclic gear train
A simple gear train

A simple gear train is one in which each shaft carries only one gear. In this type of train, the velocity ratio is equal to the number of teeth on the last driven gear to the number of teeth on the first driving gear.

Design of keys

Forces acting the key are shown below

P = \frac{{{M_t}}}{{(d/2)}} = \frac{{2{m_t}}}{d}

where 
Mt = transmitted torque (N-mm)
d = shaft diameter (mm)
P = force on key (N)

The design of square or flat key is based on two criteria, viz., failure due to shear stress and failure due to compressive stress. The shear stress 𝛕 in the plane AB is given by,

\tau = \frac{{2{M_t}}}{{dbl}}

The failure due to compressive stress will occur on surfaces AC or DB.

The compressive stress σc in the key is given by,

{\sigma _c} = \frac{{4{M_t}}}{{dhl}}

For square key,
h = b

Internal and External Shoe brake

Internally Expanding Brake
In this braking system, the brake shoes are mounted inside of the brake drum and the lower ends of brake shoes are connected with help of anchor pins and brake carrier plate. Based on this the movement of the brake shoe is positioned. In between the upper ends of brake shoes, one brake cam is mounted. The brake pedal is located at the right side corner of the driver’s leg. When the brake pedal is pressed, a long rod that is connected with the brake cams in the four brake carrier plates simultaneously is expanded inside the brake drum which causes friction that stops the rotating wheel. When the brake pedal is released, the brake cam is brought to its original position. The brake shoes are also brought to their original positions at the same time without touching the brake drum due to the spring’s compressive force. Hence the wheels on the brake drum rotate immediately without any friction.

Internal expanding shoe brake

Externally contracting type brake
This type of braking system is mostly used in cars as a hand brake. In this system, the outer layer of the brake drum has a brake lining that is in contact with the brake band. The brake band is operated by a brake lever and the linkages to stop the rotating brake drum. By pulling the hand brake lever, the brake band is forced to press the brake drum tightly. Hence the rotation of the brake drum is stopped and the vehicle is stopped. When the hand lever is released, the brake band is loosened and the force acting on the brake drum is released. Due to this action, the wheels connected with the brake drums are started to rotate without any difficulties.

External expanding shoe brake

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