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.
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 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.
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.
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.
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