Question Set 132

· What are the different types of fits? Explain?

On the basis of Indian standards fits can mainly be categorized into three groups:
> Clearance Fit: These types of fits are characterized by the occurrence of a clearance between the two mating parts. The difference between the minimum size of the hole and the maximum size of the shaft is called the minimum clearance, the difference between the maximum size of the hole and the minimum size of the shaft is known as maximum clearance.
> Interference Fit: In these types of fits the size of the mating parts are predefined so that interference between them always occurs. The tolerance zone of the hole is completely below the tolerance zone of the shaft.
> Transition Fit: As the name suggests these type of fit has its mating parts sized limited to allow either clearance or interference. The tolerance zone of the hole and the shaft overlaps in case of such fits.

For a shaft designated as 40 H8/f7, calculate the tolerances.

Given: Shaft designation = 40 H8/f7
The shaft designation 40 H8/f 7 means that the basic size is 40 mm and the tolerance grade for
the hole is 8 ( i. e. I T 8) and for the shaft is 7 ( i. e. I T 7).
Since 40 mm lies in the diameter steps of 30 to 50 mm, therefore the geometric mean diameter,
D = Square root of (30 x 50) = 38.73 mm
We know that standard tolerance unit,
i = 0.45 x Cube root of (D) + 0.001 D
i = 0.45 × 3.38 + 0.03873 = 1.559 73 or 1.56 microns
i = 1.56 × 0.001 = 0.001 56 mm ...(1 micron = 0.001 mm)
The standard tolerance for the hole of grade 8 (IT8)
= 25 i = 25 × 0.001 56 = 0.039 mm 
The standard tolerance for the shaft of grade 7 (IT7)
= 16 i = 16 × 0.001 56 = 0.025 mm

· What are the factors that can affect the Factor of safety selection?

The factor of safety is used in designing a machine component. Prior to selecting the correct factor of safety certain points must be taken into consideration such as:
> The properties of the material used for the machine and the changes in its intrinsic properties over the time period of service.
> The accuracy and authenticity of test results to the actual machine parts.
> The applied load reliability.
> The limit of stresses (localized).
> The loss of property and life in case of failures.
> The limit of initial stresses at the time period of manufacture.
> The extent to which the assumptions can be simplified.

The factor of safety also depends on numerous other considerations such as the material, the method of manufacturing , the various types of stress, the part shapes etc.

· What is heat treatment and why is it done?

Heat treatment can be defined as a combination of processes or operations in which the heating and cooling of a metal or alloy is done in order to obtain desirable characteristics without changing the compositions. Some of the motives or purpose of heat treatment are as follows:
> In order to improve the hardness of metals.
> For the softening of the metal.
> In order to improve the machinability of the metal.
> To change the grain size.
> To provide better resistance to heat, corrosion, wear etc.

Heat treatment is generally performed in the following ways:
> Normalizing
> Annealing
> Spheroidising
> Hardening
> Tempering
> Surface or case hardening

· What are the rules that must be kept in mind while designing castings?

Some of the points that must be kept in mind during the process of cast designing are as follows:
> To avoid the concentration of stresses sharp corners and frequent use of fillets should be avoided.
> Section thicknesses should be uniform as much as possible. For variations it must be done gradually.
> Abrupt changes in the thickness should be avoided at all costs.
> Simplicity is the key, the casting should be designed as simple as possible.
> It is difficult to create true large spaces and henceforth large flat surfaces must be avoided.
> Webs and ribs used for stiffening in castings should as minimal as possible.
> Curved shapes can be used in order to improve the stress handling of the cast.

· What are the points that should be kept in mind during forging design?

Some of the points that should be followed while forging design are:
> A radial flow of grains or fibers must be achieved in the forged components.
> The forged items such as drop and press forgings should have a parting line that should divide the forging into two equal halves.
> The ribs in a forging should not be high or thin.
> In order to avoid increased die wear the pockets and recesses in forgings should be minimum.
> In forgings the parting line of it should lie as far as possible in a single plane.
> For ease of forging and easy removal of forgings the surfaces of the metal should contain sufficient drafts.

· Describe briefly the different cold drawing processes.

Some of the important cold drawing processes are as follows:
> Bar and Rod Drawing: In the case of bar drawing the hot drawn bars are at first pickled, washed and coated to prevent oxidation. Once this is done a draw bench is used for the process of cold drawing. In order to make an end possible to enter a drawing die the diameter of the rod is reduced by the swaging operation. This end is fastened by chains to the draw bench and the end is gripped by the jaws of the carriage. In this method a high surface finish and accuracy dimensionally is obtained. The products of this process can be used directly without any further machining.
> Wire Drawing: Similar to the above process the bars are first pickled, washed and coated to prevent any oxidation. After this the rods are passed through several dies of decreasing diameter to provide a desired reduction in the size ( diameter ). The dies used for the reduction process is generally made up of carbide materials.
>Tube Drawing: This type of drawing is very similar to the bar drawing process and in majority of cases it is accomplished by the use of a draw bench.

· What are the different theories of failure under static load, explain briefly?

The main theories of failure of a member subjected to bi-axial stress are as follows:
> Maximum principal stress theory ( Rankine’s theory): This theory states that failure occurs at a point in member where the maximum principal or normal stress in a bi-axial system reaches the maximum strength in a simple tension test.
> Maximum shear stress theory ( Guest’s or Tresca’s theory): This theory states that failure occurs when the biaxial stress reaches a value equal to the shear stress at yield point in a simple tension test.
> Maximum principal strain theory ( Saint Venant theory): This theory states that failure occurs when bi-axial stress reaches the limiting value of strain.
> Maximum strain energy theory ( Haigh’s theory): This theory states that failure occurs when strain energy per unit volume of the stress system reaches the limiting strain energy point.
> Maximum distortion energy theory ( Hencky and Von Mises theory): This theory states that failure occurs when strain energy per unit volume reaches the limiting distortion energy.