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About Lesson

#### 61. What is enthalpy?

A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume. h = u + PV.

#### 62. What is entropy?

Change in entropy is defined as the amount of energy dispersed divided by the absolute temperature.

Zero

#### 64. What is the relationship between COP of heating and cooling?

COP of heating is one (unity) more than COP of cooling.

#### 65. 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 spring 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 lead 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 spring are used in circuit breaker mechanisms.

Leaf spring: 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.

#### 66. Why should a chain drive be used over a belt or rope driven derive? State proâ€™s and conâ€™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 production is high.

: The chain drives must be serviced and maintained at regular intervals and henceforth their cost of ownership is high comparatively.

#### 67. What are different types of fits? Explain?

: 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.

#### 68. 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 (Rankineâ€™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.

: Maximum shear stress theory (Guestâ€™s or Trescaâ€™s theory):- This theory states that failure occurs when the bi-axial stress reached 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 theory (Haighâ€™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.

#### 69. What is stress concentration factor?

Stress concentration factor ( Kt ), is a dimensionless factor which is used to quantify how concentrated the stress is in a material. It is defined as the ratio of the highest stress in the element to the reference stress.

#### 70. What is mechanical advantages?

The ratio of the force produced by a machine to the force applied to it.

If you want to have an output force that is greater than the force you input, you want a machine with a high force mechanical advantage. An example is when you want to lift a weight or load using a lever.Â

The force mechanical advantage equation is

MAF = FL / FE;Â

FL is the resistance force or load, FE is the effort force required to move the object.