Mechanical Engineering interview questions and answers

Mechanical Engineering interview questions and answers 1.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 2.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. 3.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 4.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. 5.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. 6.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. 7.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. 8. What are the assumptions made in simple theory of bending? 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. 9. Why is stress considered important in a shaft? 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 10. What do you understand by the Hooke`s Coupling what are its 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. 11. What kind of materials should be used for shafts manufacturing? 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. 12. Why should a chain drive be used over a belt or rope driven drive? State pro`s and con`s? The advantages of using a chain drives are: > In a chain drive no slip occurrence takes place. > The chains take less space as compared to rope or belts as they are made of metal and offer much strength. > The chain drives can be used at both short and long ranges and they offer a high level of transmission efficiency. > Chain drives can transmit more load and power as compared to belts. > A very high speed ratio can be maintained in one step of chain drives. Some of the cons of using a chain drive are: > The cost of producing chain drives is higher as compared to that of belts. > The chain drives must be serviced and maintained at regular intervals and henceforth their cost of ownership is high comparatively. 13. What are the different types of springs and explain them briefly? Springs can be broadly classified into the following types: > Helical Springs: These springs as their name suggests are in coil form and are in the shape of helix. The primary purpose of such springs are to handle compressive and tensile loads. They can be further classified into two types: compression helical spring and tension helical spring each having their own unique areas of application./ > Conical and volute springs: Both these spring types have specialized areas of usage where springs with adaptable rate according to the load is required. In case of conical springs they are wound so as to have a uniform pitch while on the other hand volute springs are wound in a slight manner of a parabloid. > Torsion Springs: The characteristics of such springs is that they tend to wind up by the load. They can be either helical or spiral in shape. These types of springs are used in circuit breaker mechanisms. > Leaf springs: These types of springs are comprised of metal plates of different lengths held together with the help of bolts and clamps. Commonly seen being used as suspensions for vehicles. > Disc Springs: As the name suggests such types of springs are comprised of conical discs held together by a bolt or tube. > Special Purpose Springs: These springs are all together made of different materials such as air and water. 14. During the design of a friction clutch what are the considerations that should be made? In order to design a friction clutch the following points must be kept in mind: > The material for the contact surfaces must be carefully selected. > For high speed devices to minimize the inertia load of the clutch, low weight moving parts must be selected. > The contact of the friction surfaces must be maintained at all the times without the application of any external forces. > Provisions for the facilitation of repairs must be there. > In order to increase safety the projecting parts of a clutch must be covered. > A provision to take up the wearing of the contact surfaces must be present. > Heat dissipaters to take away the heat from the point of contacting surfaces must be there. 15. What are the different types of brakes and explain them briefly? Brakes can be classified on the basis of their medium used to brake, they are as follows: > Hydraulic Brakes: These brakes as their name suggest use a fluid medium to push or repel the brake pads for braking. > Electric Brakes: These brakes use electrical energy to deplete or create a braking force. Both the above types of breaks are used primarily for applications where a large amount of energy is to be transformed. > Mechanical Brakes: They can be further classified on the basis of the direction of their acting force: Radial Brakes: As their names suggests the force that acts on the brakes is of radial direction. They can further be classified into internal and external blades. Axial Brakes: In these types of brakes the braking force is acting in an axial direction as compared to radial brakes. 16. On what basis can sliding contact bearings be classified? Explain? Sliding contact bearings can be classified on the basis of the thickness of the lubricating agent layer between the bearing and the journal. They can be classified as follows: > Thick film bearings: These type of bearings have their working surface separated by a layer of the lubricant. They are also known as hydrodynamic lubricated bearings. > Thin film bearings: In this type of bearings the surfaces are partially in direct contact with each other even after the presence of a lubricant. The other name for such type of bearings is boundary lubricated bearings. > Zero Film Bearings: These type of bearings as their name suggests have no lubricant present between the contact layers. > Externally or hydrostatically pressurized lubricated bearings: These bearings are able to without any relative motion support steady loads 17. What are the basis on which the best material for Sliding Contact Bearings manufacturing? Some of the important properties to lookout for in the material for sliding contact bearings are as follows: > Compressive Strength: In order to prevent the permanent deformation and intrusion of the bearing the material selected should be possess a high compressive strength to bear the max bearing pressure. > Fatigue Strength: the material selected for the bearing should be able to withstand loads without any surface fatigue cracks getting created. This is only possible if the material has a high level of fatigue strength. > Comfortability: The material should be able to adjust or accommodate bearing inaccuracies and deflections without much wear and heating. > Embeddability: The material should allow the embedding of small particles without effecting the material of the journal. > Bondability: The bearings may be created by bringing together ( bonding ) multiple layers of the material. Due to the above reason the bondability of the material should be sufficiently high. > Thermal conductivity and corrosion resistance: Thermal conductivity is an essential property for bearing materials as it can help in quickly dissipating the generated heat. Also the material should have a level of corrosion resistance against the lubricant. 18. Briefly explain the advantages of Cycloidal and Involute gears? The advantages of the Cycloidal gears are as follows: > Having a wider flank as compared to Involute gears they are considered to have more strength and hence can withstand further load and stress. > The contact in case of cycloidal gears is between the concave surface and the convex flank. This results in less wear and tear. > No interference occurs in these types of gears. The advantages of Involute gears are as follows: > The primary advantage of involute gears is that it allows the changing of the centre distance of a pair without changing the velocity ratio. > The pressure angle remains constant from start to end teeth, this results in less wear and smooth running of the gears. > The involute gears are easier to manufacture as they can be generated in a single curve ( the face and flank ). 19. How can the reaction of support of a frame be evaluated? Generally roller or hinged support are used to support the frames. The conditions of equilibrium are used to determine the reaction support of a frame. The condition of equilibrium takes place when the sum of the horizontal and vertical forces sum equal to zero. The system must form a state of equilibrium even after considering the external loads and the reactions at the supports. For equilibrium to be prevalent in the system the following conditions are required to be in occurrence: > Summation of V = 0. This implies that the summation of all the forces in the vertical direction results to zero. > Summation of H = 0 . This implies that the total of all the forces acting in horizontal direction is also zero. > Summation of M = 0. The sum of all the moment of forces around a point must be zero. 20. Explain in an orderly manner how the force in the member of a truss be detected using the method of joint. The steps required to calculate the force are as follows: > The reaction at the support has to be first calculated. > Once the reaction is calculated the direction of force of the member is made to make it tensile. On getting the result to be negative the direction assumed is wrong and this implies the force being compressive in nature. > A joint needs to be selected whose 2 members are not known. The lami`s theorem is used on the joint on which less than three forces are acting. > After the above process is complete the free body diagrams of the joint needs to be made. Since the system is in equilibrium the condition of Summation of V and H must result in zero. > After the above step the resolution of forces method needs to be used on the joint on which more than 4 forces are acting. 21. In order to derive the torsional formulas what are the assumptions taken? The torsion equation is derived on the basis of following assumptions: > The shaft material is uniform, throughout the shaft. > Even after loading the shaft circular remains circular. > After the application of torques the plain section of a shaft remains plain. > Any twist that occurs in the shaft remains uniform and constant. > After the application of torque the distance between any two cross-sectional references remains constant. > The elastic limit value of a shaft is never exceeded even after the shear stress induced because of torque application. 22. What are Bevel Gears and what are its types? Bevel gears are the type of gears in which the two shafts happen to intersect. The gear faces which are tooth bearing are conical in shape. They are generally mounted on shafts which are 90 degrees apart but they can be made to work at other angles as well. The bevel gears are classified into the following types on the basis of pitch surfaces and shaft angles: > Mitre Gears: These types of gears are similar to each other ie. they have the same pitch angles and contain the same number of teeth. The shaft axes intersect at 90 degrees angle. > Angular bevel gears: When two bevel gears connect at any angle apart from 90 degrees. > Crown bevel gears: When the two shaft axes intersect at an angle greater than 90 and one of the bevel gears have a pitch angle of 90 degrees they are known as crown bevel gears. > Internal bevel gears: In these type of gears the teeth on the gears is cut on the inside area of the pitch cone. 23. What are the different values that need to be determined in order to design a cylinder for an ICE? The following values are needed to be determined: > Thickness of the cylinder wall: The cylinder walls in an engine is made witness to gas pressure and the side thrust of a piston. This results in two types of stresses: longitudinal and circumferential stress. Both the types of stresses are perpendicular to each other and hence it is aimed to reduce the resulting stress as much as possible. > Length and bore of the cylinder: The length of the cylinder and the length of the stroke is calculated on the basis of the formula: length of cylinder L = 1.15 times the length of the stroke (l). L = 1.15(l) > Cylinder flange and studs: The cylinders are always cast integral as a part of the upper crankcase or in some cases attached to it by means of nuts and bolts. The flange is integral to a cylinder and henceforth its thickness should be greater than that of the cylinder wall. The thickness of flange should generally be between 1.2t-1.4t where t is the cylinder thickness. The stud diameter is calculated by equating gas load ( due to max pressure ) to the grand total of all the resisting forces of the studs. 24. What are considerations taken into account while creating a piston head? The piston head is designed on the basis of the following considerations: > The crown should have enough strength to absorb the explosion pressure inside the engine cylinder. > The head must always dissipate the heat of the explosion as quickly as possible to the engine walls. The thickness of the head is calculated on the basis of another formula which takes into consideration the heat flowing through the head, the conductivity factor of the material. The temperature at the center and edges of the head. > The thickness of the piston head is calculated on the basis of the Grashoff`s formula which takes into consideration the maximum gas pressure of an explosion , the permissible bending and the outside diameter of the piston.