interview Question Set 4

Question Set 4

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.

25 . What is mechanism?

A mechanism is an assembly of different parts which perform a complete motion and is often part of a machine.

26 . State Newton's three laws of Motion.

- The law of inertia: Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
- Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object). Force=Mass times acceleration.
- For every action there is an equal and opposite reaction.

27 . State the laws of thermodynamics and its importance of in Mechanical Engineering.

Thermodynamics is a physical science which studies the interrelation between heat, work and the internal energy of any system. Thermodynamics helps study all the systems of mechanical engineering. There are three laws of thermodynamics.

First Law: Energy can be neither created nor destroyed. It can only change forms. In any process in an isolated system, the total energy remains the same.

Eg: turning on a light would seem to produce energy; however, it is electrical energy that is converted.

Second Law: The second law of thermodynamics states that the entropy of any isolated system not in thermal equilibrium almost always increases.

Eg: A car that has run out of gas will not run again until you walk 10 miles to a gas station and refuel the car.

Third Law: As temperature approaches absolute zero, the entropy of a system approaches a minimum.

Eg: Water in gas form has molecules that can move around very freely. Water vapor has very high entropy (randomness). As the gas cools, it becomes liquid which can still move around but not as freely. At this point they have lost some entropy. On cooling further it becomes solid ice where molecules can no longer move freely but can only vibrate within the ice crystals. The entropy is now very low. As the water nears absolute zero, the vibration of the molecules diminishes. If the solid water reaches absolute zero, all molecular motion would stop completely. And at this point the water would have no entropy at all.