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.
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
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.
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.
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.
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 assumptions made in the theory of simple bending are:
> The material of the beam is homogeneous this implies that it is uniform in density, strength and have isotropic properties meaning possessing same elastic property in all directions.
> Even after bending the cross section of the beam remains constant.
> During the initial stages the beam is straight and unstressed.
> All the stresses in the beam are within the elastic limit of its material.
> The layers of the beam are free to contract and expand longitudinally and laterally
> On any cross section the perpendicular resultant force of the beam is zero.
> Compared to the cross-sectional dimension of the beam the radius of curvature is very large.
The following types of stresses are prevalent in shafts:
> At the outermost surface of the shaft the max shear stress occurs on the cross-section of the shaft.
> At the surface of the shaft on the longitudinal planes through the axis of the shaft the maximum longitudinal shear stress occurs.
> At 45 degrees to the maximum shearing stress planes at the surface of the shafts the major principal stress occurs. It equals the max shear stress on the cross section of the shaft.
> For certain materials where the tensile and compressive strengths are lower in measure as compared to the shear strength, then the shaft designing should be carried out for the lowest strengths.
> All these stresses are of significance as they play a role in governing the failure of the shaft. All theses stresses get generated simultaneously and hence should be considered for designing purposes
The Hooke`s coupling is used to connect two shafts whose axes intersect at a small angle. The two shafts are inclined at an angle and is constant. During motion it varies as the movement is transferred from one shaft to another. One of the major areas of application of this coupling is in gear boxes where the coupling is used to drive the rear wheels of trucks and other vehicles. In such usage scenarios two couplings are used each at the two ends of the coupling shaft. they are also used to transfer power for multiple drilling machines. The Hooke`s coupling is also known as the Universal coupling. The torque transmitted by the shafts is given by :
T= (pie/16) x t x (d) cube
Where T = torque, t = shear stress for the shaft material and d the diameter of the shaft.
Some of the qualities that should be present in materials for shafts are as follows:
> The material should have a high index of strength.
> Also it should have a high level of machinability.
> The material should possess a low notch sensitivity factor.
> The material must also have wear resistant properties.
> Good heat treatment properties should also be present
The common material used to creates shafts of high strengths an alloy of steel like nickel is used. The shafts are manufactured by hot rolling processes and then the shaft is finished using drawing or grinding processes.