Understanding the Principles of Creep, Bernoulli's Equation, and Engine Types, Study notes of Mechanical Engineering

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Questions and Answers in

Mechanical Engineering Part One

Prepared by

Osama Mohammed Elmardi Suleiman

Nile Valley University – Faculty of Engineering & Technology

May 2018

Question One

What are Mechanical Properties of Material which every Mechanical

Engineer Should Know?

Mechanical properties of material: There are mainly two types of materials. First one is metal and other one is nonmetals. Metals are classified into two types: Ferrous metals and Non-ferrous metals. Ferrous metals mainly consist iron with comparatively small addition of other materials. It includes iron and its alloy such as cast iron, steel, HSS etc. Ferrous metals are widely used in mechanical industries for its various advantages. Nonferrous metals contain little or no iron. It includes aluminum, magnesium, copper, zinc etc. Most Mechanical Properties are associated with metals these are- #1. Strength: The ability of material to withstand load without failure is known as strength. If a material can bear more load, it means it has more strength. Strength of any material mainly depends on type of loading and deformation before fracture. According to loading types, strength can be classified into three types.

1. Tensile strength:
2. Compressive strength:
3. Shear strength: According to the deformation before fracture, strength can be classified into three types.
4. Elastic strength:
5. Yield strength:
6. Ultimate strength: #2. Homogeneity:

#8. Brittleness: Brittleness is a property by virtue of which, a material will fail under loading without significant change in dimension. Glass and cast iron are well known brittle materials. #9. Stiffness: The ability of material to resist elastic deformation or deflection during loading, known as stiffness. A material which offers small change in dimension during loading is stiffer. For example, steel is stiffer than aluminum. #10. Hardness: The property of a material to resist penetration is known as hardness. It is an ability to resist scratching, abrasion or cutting. It is also define as an ability to resist fracture under point loading. #11. Toughness: Toughness is defined as an ability to withstand with plastic or elastic deformation without failure. It is defined as the amount of energy absorbed before actual fracture. #12. Malleability: A property by virtue of which a metal can flatten into thin sheets, known as malleability. It is also defined as a property which permits plastic deformation under compression loading. #13. Machinability: A property by virtue of which a material can be cut easily. #14. Damping: The ability of metal to dissipate the energy of vibration or cyclic stress is called damping. Cast iron has good damping property, that’s why most of machines body made by cast iron. #15. Creep: The slow and progressive change in dimension of a material under influence of its safe working stress for long time is known as creep. Creep is mainly depending on time and temperature. The maximum amount of stress under which a material withstand during infinite time is known as creep strength.

#16. Resilience: The amount of energy absorb under elastic limit during loading is called resilience. The maximum amount of the energy absorb under elastic limit is called proof resilience. #17. Fatigue Strength: The failure of a work piece under cyclic load or repeated load below its ultimate limit is known as fatigue. The maximum amount of cyclic load which a work piece can bear for infinite number of cycle is called fatigue strength. Fatigue strength is also depending on work piece shape, geometry, surface finish etc. #18. Embrittlement: The loss of ductility of a metal caused by physical or chemical changes, which make it brittle, is called embrittlement.

An automotive air conditioning system works with the incorporation of parts like

• Condenser
• Compressor
• Evaporator
• Receiver-dehydrator
• Connecting lines including,
• Orifice tube
• Expansion valve
• Absolute valve
• Suction throttle valve
• Evaporation pressure regulator valve
• Thermal sensors
• High pressure cut off switch
• Cycling compressor switch
• Refrigerants (Nowadays, Freon 12 is replaced by the alternative refrigerant R134a) Watch the video on next page to understand the working of the automotive air conditioning system- The working of the automotive air conditioning system is the same as the normal air conditioners. The compressor suppresses the refrigerant vapours at very high pressure coming from the evaporator. The car engine drives the compressor with the belt drive. Hence, the magnetic clutch is responsible for engaging and disengaging the compressor. There is a notable increase in the refrigerant pressure and temperature in the compressor, as a result turns it into vapours. The compressor discharges the high- pressure vapours to the condenser. It much as works like a heat-exchanger and is in front the vehicle. In conclusion, the refrigerant releases the heat and converts it to the liquid form. Because of ram air and the electric driven fan, the temperature of the refrigerant cools down.

The refrigerant at very high pressure moves from the dehydrator and extracts moisture. After extracting moisture, it passes through the expansion valves and expands the refrigerant at low pressure. In result, the expansion process cools down the evaporator. The sensing device also known as the temperature sensing tubes, signals the diaphragm at the expansion valve and it varies the orifice sizing. This entirely depends on the temperature of the evaporator outlet, as a result, helps in automatic temperature control. Most noteworthy, the evaporator is of the similar construction as the condenser.

operating RPM, Horsepower generation, Diameter of pulleys and center distance, take up design, space available for the setup, shock load conditions, issues with static dissipation, the service life of the belt etc. Types of Belt Drives: There are two broad classifications as far as types of belt drives are concerned, they are determined by the amount of power transmission required and arrangement of belts. Belt Drives according to the power transmitted: Light Drives: Used in agriculture machines and small machines. The belt speed generally remains in the range of 10 m/sec. Perfect for applications where small power transfer is required. Medium Drives: Used in industrial and semi-industrial applications the power delivery in this set up is of medium range. Highly utilized in machining and similar applications the belt speed in this type of setup ranges from 10 m/sec to 22m/sec. Large Drives: As the name suggests these are big belt drives used for heavy power delivery. It finds wide application in processes where high transmission power is required. The belt speed in this format of the belt drive is in excess of 22 m/sec. It finds application in running of compressors and similar large machinery.

Belt Drives according to the arrangement of belt: Open Belt Drive: Open belt drive In this type of belt drive, the assembly of shafts is parallel and rotates in the same direction. The size of the shaft varies and has a large shaft connected to a small shaft. The power is transmitted from the larger shaft to the smaller shaft, the lower side is known as the tighter side. Cross belt drive: In this type of belt drive, the shafts are parallel to each other just like in open belt drive but the belts are in cross configuration and moving opposite to in each other in direction. In this configuration, the same layout of one shaft is bigger than the other is applied. Crossed belt drive has more tension on the side which is acting as the driver i.e. the direction in which the belt is being moved. The side being pulled in known as the tight side and the other one is known as the slack side.

Question Four Explain Bernoulli’s Equation & Applications Bernoulli’s Equation & Applications of Bernoulli’s Equation? Bernoulli’s Equation is one of the most versatile equation ever. This is an important principle involving the movement of a fluid through a pressure difference. Suppose a fluid is moving in a horizontal direction and encounters a pressure difference. This pressure difference will result in a net force, which by Newton’s 2nd law will cause an acceleration of the fluid. The fundamental relation, in this situation can be written as- which furthermore can be expressed as In other words, This principle is generally known as the conservation of energy principle and states that the total energy of an isolated system remains constant — it is said to be conserved over time. This is equivalent to the First Law of Thermodynamics , which is used to develop the general energy equation in thermodynamics. This principle can be used in the analysis of flowing fluids and this principle is expressed mathematically by the following equation: where h is enthalpy, k is the thermal conductivity of the fluid, T is temperature, and Φ is the viscous dissipation function.

Bernoulli’s Equation- ENGINEERING Bernoulli’s Equation & Applications of Bernoulli’s Equation The Bernoulli’s equation can be considered to be a statement of the conservation of energy principle appropriate for flowing fluids. It is one of the most important/useful equations in fluid mechanics. It puts into a relation pressure and velocity in an inviscid incompressible flow. Bernoulli’s equation has some restrictions in its applicability, they summarized in following points:

• steady flow system,
• density is constant (which also means the fluid is incompressible),
• no work is done on or by the fluid,
• no heat is transferred to or from the fluid,
• no change occurs in the internal energy,
• the equation relates the states at two points along a single streamline (not conditions on two different streamlines) Under these conditions, the general energy equation is simplified to:

Question Five Explain Centrifugal Pump: Principle, Parts, Working, Types, Advantages, Disadvantages with its Application? Centrifugal pump is a type of turbomachinery which is dynamically axisymmetric and work absorbing in nature. In more simpler terms it’s a pump which is used to lift liquids from a lower area to a higher area. Its most widely used in industries where sensitive fluids as in chemical industries are required to be moved. The basic principle of centrifugal pumps is the conversion of rotational kinetic energy to hydrodynamic energy of fluid movement. The fluid enters through the pump impeller near the rotating axis and gets accelerated reaching the desired destination. Parts of a centrifugal pump: Rotating Parts: Impeller: It is the heart and soul of a centrifugal pump it has following subtypes. Open Impeller: This impeller does not have a crown and base plate, it finds wide application where physical impurities in the liquid to be pumped has to be kept at bay.

Closed Impeller: It is completely covered with no scope of any foreign body entering. Widely used for pumping water. Semi-Open Impeller: Lacks a crown plate and is suited for fluids which might have charged debris in them. Shaft: This is the component which is responsible for the rotation of the impeller. It also transmits torque to the impeller and keeps it in sync with other components of the centrifugal pump. Shaft Sleeve : It's a covering for the shaft assembly and protects the unit from corrosion. Its open from one end. Casings: Casings used in a centrifugal pump are of two types: the volute casings and vortex casings. Volute casings are funnel- shaped and are designed to reduce the overall pressure of the fluid on the shaft of a centrifugal pump. It acts as a safety measure

Multistage centrifugal pumps : It's the most complex type of centrifugal pump and has a unique configuration. In this type of pump, the impellers can be mounted on a single shaft or on multiple shafts depending upon the use. This pump also has many stages of fluid movement. In every stage, the fluid is moved to center before getting discharged. In case of higher pressures, the impellers are connected in series and for the higher output, they are connected in parallel. Advantages of centrifugal pump:

• No leakage issues.
• Can be used for pumping sensitive fluids like petrol and diesel.
• No loss of power due to friction.

• Economical to operate Disadvantages:
• At times clogging of pipes may occur
• Any external vibration can damage the pump
• Low flow may cause overheating
• Risk of cavitation is high