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.
· What happens if gasoline is used in a Diesel engine?Diesel engine will work ?
No, It will not work,as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.
· Which Mechanism is used in Automobile gearing system
Differential mechanism
· Why different types of sound are produced in different bikes though they say run on SI engine
Engine specifications are different in different manufactures like as Bore Diameter(CC), Ignition timing.Also the exhaust passage take more responsible for sound.
· Why entropy decreases with the increase in temperature?
ds=dQ/TEntropy is inversely proportional to the temperature so.as temp. increases,entropy decreases.
· 1 hp how much watt?
746.2Watt
· How to calculate bearing number to diameter of the inner and outer
Divide the shaft diameter size by 5, it will give last two digitof the bearing no. and according to type of load we have tochose the type of bearing and that will give prior no. ofthe bearing.
· Explain Bicycle rear wheel Sprocket working?
Rear wheel sprocket works under the principle of ratchet and pawl.
· Definition of Octane number & Cetane number
Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.
Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.
· Poisons ratio is higher in (rubber/steel/wood)
· What is important to remember about radial bearings?
A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder. These bearings must be accurately aligned to maintain the close clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.
· How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed). The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
· How is a fly ball governor used with a hydraulic control?
As the turbine speeds up, the weights are moved outward by centrifugal force, causing linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a spring-loaded piston. The movement of the piston controls the steam valves.
· What is meant by critical speed?
It is the speed at which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the vibration is caused by critical speed and not by some other trouble).
· How is oil pressure maintained when starting or stopping a medium-sized turbine?
An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.
· Besides lubrication, what are two functions of lubricating oil in some turbines?
In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.
· Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
You should keep the cooling water circulating for about 15 mill or more so that the condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.
· How can the deposits be removed?
- Water soluble deposits may be washed off with condensate or wet steam.
- Water insoluble deposits are removed mechanically after dismantling the turbine.
- Experience shows that water soluble deposits are embedded in layers of water-insoluble deposits. And when the washing process is carried out, water soluble parts of the deposit dissolve away leaving a loose, friable skeleton of water-insoluble deposits which then break loose and wash away.
· How can the fatigue damage on high-pressure blades be corrected?
Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc admission. This can be corrected by switching over to full arc admission technique.
Mechanical Engineering Interview Question Paper Set 5
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