Design Of Machine important Questions

302043- Design of Machine-DME/DOM-Important questions 

 Spring is defined as elastic body or elastic machine element which deflects under the action of the load and recovers its original shape when load is removed.

Applications: Vehicle suspension springs

Solid Length (Ls): When the compression spring is compressed until the coils come in contact with each other, then the spring is said to be solid and the length of such a solid spring is known as solid length.

Free Length (LF): The free length of a compression spring is the

Length of the spring in the free or compressed condition. 

• Spring Index ©: It is defined as ratio of mean coil diameter to the wire diameter.

• Spring Stiffness (K) It is defined as load required per unit deflection of the spring. It is also called as spring rate or spring constant.

• Pitch (p): It is defined as axial distance between the coils in

Ucompressed condition.

When one helical spring is placed inside the another helical spring, then that combination is called as concentric springs or nested or composite springs.

In order to consider the effect of both direct shear as well as curvature of the wire, a Wahl’s shear stress factor introduced by A.M. Wahl is used.

• Wahl's shear stress factor or Wahl's correction factor is given by,

Kw 4C-1 0.615 + 4C-4 C

8 WD 8 WD Resultant shear stress, t = -xKw= -xKw

Helical torsion springs may be made from round, square or rectangular wire.These wires are wound in a similar fashion as helical tension or helical compression springs but the ends are shaped to transmit torque.

These springs are widely used for transmitting small torques in door hinges, brush holders in motors, automobile starters, rolling shutters, etc.

 Leaf springs or laminated springs or flat springs are semi-elliptical form and commonly used in automobiles.

• Applications: They are widely used in automobiles such as heavy duty vehicles like trucks, buses and in rail - road suspensions.

The process of pre-stressing the spring by providing different radius of curvature before the assembly of leaves is called as nipping of leaf springs.

• Write a short note on ‘Bolt of uniform strength

In ordinary bolt, the cross-sectional area is minimum at the root of the

threads. It means, the stress in the threaded part of the bolt will be higher than that in the shank. The impact or strain energy absorbed at any point in a body is directly proportional to the square of the stress at that point. Strain energy.

Therefore, a large amount of the impact energy will be absorbed in the threaded part and relatively small amount of the energy is absorbed by the shank.

This non-uniform distribution of the impact energy may lead to fracture of the bolt in the threaded part

If the shank of the bolt is reduced to a diameter equal to or even slightly less than the core diameter of the thread (d, then the stress become same throughout the length of the bolt.

It means, a shank will absorb a large amount of the energy thus relieving the material at the sections near the thread.

In this way, the bolt becomes stronger and lighter. It also increases the shock absorbing capacity of the bolt because of an increased modulus of resilience. This type of bolt is called as bolt of uniform strength.

In this method, an axial hole is drilled through the head upto threaded portion such that the area of the shank becomes equal to the threaded or root area.

• What is preloading of bolts ? State is advantages and applications.

Ans. Preload or Preloading of bolts: When the nut is tightened initially, the bolt is subjected to an initial tension which is called as preload or initial tensile force.

Due to preload the connected members (cylinder flange, gasket and cylinder head) are in compression, even the external tensile force is acting.

It is commonly used in the design of pressure vessels, hydraulic press, accumulators, etc.

2. Tap bolts:

Tap bolt is screwed into a tapped hole of one of the parts to be fastened without. Using a nut.

It is used when there is no space for the accomodation of The nut

• Through bolts:

A through bolt or simply bolt is a cylindrical bar with threads for the nut at one end and head at another end.

A bolt is passed through drilled holes in the two parts to be fastened together and clamped them securely as the nut is screwed

Through bolts are made with either hexagonal

According to use of through bolts, they are also called as machine bolts, eye bolts, automobile bolts, etc.

•  State the use of coarse threads and fine threads.

Ans. : 

1. Coarse Threads: Coarse threads are suitable for general

Industrial applications which are free from vibrations and where assembly and disassembly of connected parts is frequent. 2. Fine Threads: Fine threads are suitable for applications where vibrations tend to loosen the nut like in automobile components

• Explain the procedure in the design of bolts for eccentrically loaded bolted joints in shear.

Ans. When a force is acting in the plane of bolts, it will produce two effects. Direct shear stress Shear due to moment set up at C.G.

In this case, the line of action of the force does not pass through the C.G. of the bolt system.

• Enlist the materials used for the following:bolt

Ans.: i) Lightly loaded small bolts, studs and nuts: The ‘free cutting steels’ are used to prepare these bolts, nuts and studs.

ii) High strength bolts: The material often fail in fatigue condition, hence plain carbon steels like 40C8, alloy steels like 35Mn6M06 are used.

iii) Threaded fasteners where corrosion resistance is required: Stainless

steel is used as a thread fastener.

• State the various forms of screw thread.

Ans. Following are the various forms of screw threads in threades

Joints. • British Standard Whitworth (B.S.W.) thread

British Association (B.A.) thread

British Standard Fine (B.S.F.) thread American National Standard (A.N.S.) t1)

Goodman line (Goodman criterion) :

• A straight line joining on the ordinate and on the abscissa is called as Goodman line.

 Soderberg line (Soderberg criterion) :

• A straight line joining Se on the ordinate and S, on the abscissa is called as Soderberg line.

Gerber parabola (Gerber criterion) :

• A parabolic curve joining on the ordinate and on the abscissa is called as Gerber parabola.

• Practically the components are subjected to uniaxial stress and shear

Stress or biaxial stress and shear stress.

In such complicated cases, various theories of failure are used to determine resultant stresses.