MECHANICAL THEORY

Basics of Mechanical Engineering- Concepts Useful for Interviews

Basics of Mechanical Engineering: All the basic concepts of Mechanical Engineering which will be useful to you in the case of interviews will be presented in this article. Apart from that, it is also a subject in the field of Mechanical Engineering. As all the concepts will be furnished below and with the help of Tabular column you can navigate the concepts by clicking on the links in the Table.

 Basics of Mechanical Engineering
Basics of Mechanical Engineering

Tabular Column for Basics of Mechanical Engineering:

1.Principal Stresses

2.Principal Planes

3.Maximum Shear Stress (in 2D plane stress system)

4.Maximum Shear Stress Planes

5.Extensive Property

6.Intensive Property

7.Resilience

8.Punch Die Clearance in Design

9.Quasistatic Process

10.SI Unit of Pressure

11. What is Strength?

12. Mechanical Properties of Metals

13.Properties of Ceramics

14. Compare Mass and Weight

15.The ratio of Specific heat at Constant Volume and Constant Pressure

16.Post Heating in Welding

17.Preheating in Welding Process

18.Divider Caliper

19.Outside Caliper

20.Inside Caliper

21.Surface Plate

22.Stepper Motor

23.Difference between Specific gravity and Specific Volume

24. What is Thermocouple?

25. What is the Seebeck Effect

26. What are Pyrometers?

27. What is Pressure?

28. What is Absolute Pressure?

29. What is Vaccum Pressure?

30.Numerical based on vaccum and atmosphere

31.Difference between Engineering Mechanics and Strength of Materials

Principal Stresses, Principal planes, Maximum Shear stress, Maximum Shear Stress Planes with Formulas: When you are going for an interview or writing any competitive exam then a compulsory question will be asked on either of the above terms in terms of a Definition or a Numerical. If it is a numerical question, it generally carries 2 marks and if it is a theory question, it may be 1 mark or 2 marks. So, I am focussing on the definitions as well as formulas related to those terms. The Definitions and their Representation were presented below either in the form of text or in the form of Images. If you want formulas, then try to copy from the images.

1.Principal Stresses:

  • The maximum or minimum normal stress is the principal stress.
  • These principal stresses are used in the design.
Principal stress - Basics of Mechanical Engineering
Principal stress – Basics of Mechanical Engineering

 2.Principal Planes:

  • The plane on which principal stresses will be acting is called the principal plane.
  • On the principal plane, shear stress must be zero.
  • Any plane in the material without shear stress is by default principal plane.
  • In a 2-D system, there are two principal planes separated by 90 degrees. On these planes, shear stress is zero.
principal planes-Basics of Mechanical Engineering
principal planes-Basics of Mechanical Engineering

 3.Maximum Shear stress(in 2D Plane stress system):  The Maximum Shear Stress in the 2D Plane stress system is presented below.

 

max shear stress and max shear stress planes-Basics of Mechanical Engineering
max shear stress and max shear stress planes-Basics of Mechanical Engineering

 

 4.Maximum Shear Stress Planes: The plane on which maximum shear stress is acting called as maximum shear stress plane and the formula of  Maximum Shear Stress Planes will be collected from the above image placed in the maximum shear stress section.

  • In a 2D system, there are two (Tow max) planes separated by 90 degrees.
  • The angle between any one principal plane and the nearest (Tow max) plane is 45°.
  • On (Tow max) plane, there will be normal stress and which is in the middle of (Sigma 1) and (Sigma 2) by magnitude.
  • The magnitude of shear stress on mutually perpendicular planes must be the same.
  • The sum of normal stresses on mutually perpendicular planes must be constant.

This is the complete explanation of Principal Stresses, Principal planes, Maximum Shear stress, Maximum Shear Stress Planes with Formulas in a detailed manner. If you have any doubts, then you can ask us from the comments section.


5.Extensive property:

Any property  that is dependent on mass is called as Extensive property

Ex:Volume,Enthalpy,Internal energy,Entropy,Kinetic energy,Potential energy etc.


6.Intensive property:

Any property that is independent of mass is called an Intensive property.

Ex: Pressure, Temperature, and Density, etc.


7.Resilience:

resilience-Basics of Mechanical Engineering
resilience-Basics of Mechanical Engineering

The ability of the material that can absorb energy without undergoing any shape change is called Resilience. Determination of Resilience:

  • The resilience of the material will be determined by calculating the area under stress v/s strain curve up to an elastic point.
  • Resilience says about the strength and ductility properties of the material.
  • If a material possesses higher the resilience means it will absorb more energy against shape change.

Examples of Resilience are shown below.

  • Steel

Retains the shape and does not fail immediately.


8.Punch Die Clearance in Design:

The die opening must be sufficiently larger than the punch to permit a clean fracture of metal. This difference in dimension between the mating members of die set is called clearance.

Explanation about the Punch Die Clearance in Design:

  • When the hole has to be held to size i.e.the hole in the sheet metal is to be accurate, the punch is made to the size of the hole and dies opening size is obtained by adding clearance to punch size.
  • In blanking operation, where the size of the blank is to be maintained accurately, die opening size is exactly equal to blank size and clearance is to be subtracted from die opening size to get punch size.

9.Quasistatic Process:

A system is made to undergo a series of change of states such that it is in thermodynamic equilibrium at each and every state. Then the locus of all these states is called a quasistatic process.

  • It is an ideal and very slow process.

Example of Quasistatic Process: If sand grains are allowed to fall one after the other on a piston-cylinder mechanism, then the movement of such a piston is called a quasistatic process.

Quasistatic process and list the assumptions of Ideal gas-resilience-Basics of Mechanical Engineering
Quasistatic process and list the assumptions of Ideal gas-resilience-Basics of Mechanical Engineering

Assumptions of Ideal gas:

  • Molecular separation is very high.
  • Molecules do not occupy any volume.
  • The molecular collisions are elastic in nature.
  • There are no inter-molecular forces of attraction are present.

The equation of Ideal gas is given as:          

P*V=m*R*T

Where,          

P       -Pressure(pascals)          

V       -Volume(m^3)          

m       -mass(Kg)          

R       -Gas Constant(J/KgK)          

T       -Temp(K)


10.SI Unit of Pressure:

SI Unit of Pressure: The unit of pressure in the SI system is the pascal (Pa), defined as a force of one Newton per square meter.

SI Unit of Pressure-Units of Pressure:

  • 1 atm    =101.325KPa

                    =760mm of Hg Column                     =10.33 m of H2O Column

  • 1bar       =10^5 Pascals

                     =100Kpa                      =751.6 mm of Hg Column

  • 1torr      =1mm of Hg

                     =133.33 Pascals

  • For High Pressures, go for ‘Hg’ manometers
  • For Low Pressures, go for ‘water’ manometers.

  • Pabsolute=Patm+Pguage
  • Pabsolute=Patm-Pvaccum

Note: For calculations part, Vaccum and Guage pressures are not used. Therefore, Absolute pressure comes into the picture.


Basics of Mechanical Engineering:

11. What is Strength? The definition of strength is as follows…  The ability of the body to resist external loads without failure is called Strength.

  • Strength is a material property.
  • It is constant for the material.
  • Any design is a strength-based design only.

This is the definition of strength with its features.


12.Mechanical Properties of Metals:

When you want to apply any type of load on a particular member, you need to know its material properties, density, etc. For that, in this article, I am exploring the necessary properties of metals in a detailed manner.

11 Properties of Metals:

The Properties of Metals which are essential for a mechanical engineer are as follows.

  1. Bond: It possesses a metallic bond.
  2. Strength: The strength of the metal is very high when compared to plastics and ceramics.
  3. Binding Energy: The distance between the atoms is less and thereby the Binding energy is high.
  4. Ductility: It possesses good ductility also.
  5. Conductivity: The metals possess good electrical conductivity because of the presence of free electrons.
  6. Thermal Conductivity: The metals possess good thermal conductivity.
  7. Density: The metals possess high density which means that the weight of the metal is more and this is one of the disadvantages of metals.
  8. Corrosiveness: The metals are highly corrosive to the environment.
  9. Servicing Temperature: The servicing temperature of the metal is 800̊ C-1000̊ C.
  • Hazardous: It is not hazardous and is environmentally friendly.
  • Recyclability: The metals are partially recyclable in nature.

13.Properties of Ceramics:

The mixture of metal oxide powders like the Powder of Aluminum oxide, Powder of Silicon Oxide and the Powder of Zirconium oxide are collectively called as ceramics. The various Properties of Ceramics are shown below in a detailed manner.

Do you want to know the Top 11 properties of Ceramics.
Do you want to know the Top 11 properties of Ceramics.

Top 11 Properties of Ceramics:

The Properties of Ceramics which plays a crucial role in the field of mechanical engineering  are as follows:

  1. Bond: The ceramics possess both Ionic and Covalent bonds.
  2. Strength: The strength of the metal is greater than the strength of Ceramics and that is greater than the strength of Polymer.
  3. Distance between atoms: The distance between the atoms of metal is less than the Ceramics and is less than the Polymers.
  4. Brittleness: The ceramics are brittle in nature.
  5. Conductivity: The ceramics possess bad electrical conductivity because of the absence of free electrons.
  6. Thermal Conductivity: The Ceramics possess bad thermal conductivity but they can sustain at high temperatures easily.
  7. Density: The Density of Metal is greater than Density of Ceramics and that is greater than the Density of Polymers.
  8. Corrosiveness: The Ceramics are Anti-Corrosive to the environment because they are already in corrosive form.
  9. Servicing Temperature: The servicing temperature of the metal is 3000̊ C.
  10. Hazardous: It is not hazardous to the environment.
  11. Recyclability: The ceramics are non-recyclable in nature.

These are the top 11 Properties of Ceramics which are discussed in a detailed manner.


14.Compare Mass and Weight:

Compare Mass and Weight
Compare Mass and Weight

Mass of the body:

  • Mass of the body does not vary from place to place and it is not affected by the gravitational force.
  • It is represented by “m”.
  • Unit of mass “m”=Grams

Weight of the body:

  • Weight of a body varies from place to place and depends on the acceleration due to gravity existing at that place.
  • It is represented by “W”.
  • The formula for weight of the body is

                           W=m/g

  • If “g” is positive then the sign of “W” cannot changes whereas
  • If “g” is negative then the sign of “W” changes.

So, this is the main difference between mass and weight of anybody.

Unit of weight “W”=mg=Kg*(m/s2)

Therefore, Unit if weight =Newton


15.The ratio of Specific heat at Constant Volume and Constant Pressure:

The ratio of Specific heat: It is defined as the ratio of specific heat at Constant Pressure to Specific heat at Constant Volume. It is represented by Gamma(γ)

(γ or Gamma) = Cp/Cv.

A ratio of Specific heat at Constant Volume and Constant Pressure

Specific heat at Constant Volume and Constant Pressure:

The ‘γ‘ value is different for different gases and is as follows.

  • γ = 1.63 for monoatomic gases.
  • γ = 1.4 for Diatomic Gases.
  • γ = 1.33 for Triatomic gases.

Specific heat at constant volume:

It is defined as the ratio of change in Internal energy to Change in Temperature at Constant Volume.
(Cv) = (du/dt)@Constant Volume

Specific heat at Constant Pressure:

It is defined as the ratio of Change in Enthalpy to Change in temperature at Constant Pressure.
(Cp) = (dh/dt)@Constant Pressure.


16.Post Heating in Welding:

  • Post-heating is done in order to relieve further residual stresses present in the weld pool after heat treatment process also.
  • If post-heating is not done, then residual stresses develop as cracks and propagate throughout, resulting in the failure of the material.
  • Therefore Preheating and Post heating must be done during welding.

17.Postheating in Welding Process:

Need of Preheating in Welding process: Preheating in Welding involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature called as preheating temperature, prior to welding. In this article, I am going to publish about, what is the need of Pre-heating in welding process in a detailed manner.   

Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source.

preheat in welding process

 WhyPre heating in Welding is necessary?

There are four primary reasons to utilize Preheating in Welding

(1) It lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistance to cracking.

 (2) The slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking.

(3) It reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints.

(4) It raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch and toughness.


18.Divider Caliper:

Divider-Measuring Tools used in Engineering Workshop: Divider is used for marking circles, arcs, laying out perpendicular lines, bisecting lines, etc. Divider Calipers are used in the process of marking out suitable workpieces.

Divider-Measuring Tools used in Engineering Workshop
Divider

Explanation of Divider:

  • The Points are sharpened so that they act as a scribe.
  • One leg can then be placed in the dimple or hole created by a center punch and the other leg is pivoted so that it scribes a line on the workpiece surface, thus forming an arc or circle.

19.Outside Caliper:

Outside Caliper-Measuring Tools used in Engineering Workshop: The Outside caliper is used to measure the external size or Outer size of an object. It consists of two legs which are inverted to each other and are connected by means of a screw and nut mechanism. Apart from that, the usage of outside Caliper was also placed in this article.

outside caliper
outside caliper

Uses of Outside Caliper:

  • The same observations and techniques are applied to the outside caliper compared with inside caliper.
  • With some understanding of their limitations and usage, these instruments can provide a high degree of accuracy and repeatability.
  • They are especially useful when measuring over very large distances i.e.for example to measure a large diameter pipe.
  • A vernier caliper does not have the depth capacity to straddle this large diameter while at the same time reach the outermost points of the pipe’s diameter.
  • They are made from high carbon steel.

20.Inside Caliper:

Inside Caliper-Measuring Tools used in Engineering Workshop: The inside caliper is used to measure the internal size of an object. It consists of two legs which are not inverted to each other and are connected by means of a screw and nut mechanism. Apart from that, the usage of Inside Caliper was also placed in this article.

inside caliper
inside caliper

Uses of Inside Caliper:

  • The two legs of the Inside Caliper are adjusted on to the workpiece in such a way that the legs can touch the surface until they will almost pass over the object.
  • The Inside Caliper has an adjusting screw that permits it to be carefully adjusted without removal of the tool from the workpiece.

This is the detailed Explanation of Inside Caliper with the image.


21.Surface Plate:

Surface Plate used in Fitting-Engineering Workshop: The Surface plate is used for testing the flatness of a w/p.It is also used for Precision inspection, marking out layout and tooling setup. The surface plate is a common tool used in industry and is often permanently attached to robotic-type inspection devices such as a coordinate-measuring machine. 

Surface Plate used in Fitting-Engineering Workshop
Surface Plate used in Fitting

Accessories used along with Surface Plate:

The surface plate is used in conjunction with accessories such as a 

  • Dial indicator
  • sine bar
  • angle plate
  • straight edge
  • square
  • parallels,
  • sine plate
  • gauge blocks
  • height gauge etc.

Precision:

  • It is more precise than the marking table.

The material of Surface Plate:

  • The surface plate is made up of Cast Iron(C.I.) or hardened steel or Granite stone

22.Stepper Motor:

Explanation of Stepper Motor along with its Importance: Stepper Motor is widely used in the field of Robotics where high precision is needed. In this article, I will be explaining about the stepper motor and its importance in a detailed way.

Explanation of Stepper Motor:

The stepper motor is taking electrical energy input in the form of pulses and converting into mechanical energy in the form of revolutions. But, by changing no. of pulses of electrical energy to a motor (or) by changing the rate of pulses of electrical energy input to the motor, the speed of the stepper motor can be changed.

What is the Explanation of Stepper Motor along with its Importance
What is the Explanation of Stepper Motor along with its Importance

Because of the presence of inertia effect of moving parts, even though the power supply is stopped to the motor, it is not possible to stop the motor axis immediately. Hence, the tool is traveling some more distance and therefore accuracy of the components produced is poor. i.e. The positional accuracy of the machine tool will become poor.

This is the complete explanation of the stepper motor in detailed. There is an advanced of a stepper motor i.e. servo motor which will work much effectively than it and can be used in various installations.


23.Difference between Specific gravity and Specific Volume:

Specific Volume:

It is the reciprocal of Density which is defined as the ratio of Volume of the body to its unit mass. It is denoted by (ν)

  • Specific Volume(ν)= Volume of the body/unit mass.
  • Units                   =  m3/Kg

Relative density or Specific Gravity:

It is defined as the ratio of the Density of any substance to the Density of water.

Specific Gravity = (Density of any substance/Density of Water).

Units                     = No Units


24. What is Thermocouple?

These are used to measure the temperature at a particular point.

They are manufactured by using two dissimilar materials belonging to thermoelectric series.

  • One is higher order element and another is a lower order element and 2 junctions are made.
  • One junction is kept at Ice Point and Another junction is kept at the place whose temperature is to be measured.
  • Due to the difference in temperature, an EMF is generated and this EMF is related to temperature.

thermocouple


25. What is the Seebeck Effect?

Development of EMF due to the difference in temperature is called a Seebeck effect.


26. What are Pyrometers?

They can function on the principle of radiation. They are used to measure the temperature’s from a distance.


27.What is Pressure?

It is defined as the ratio of Force acting per unit Area of the cross-section.

Pressure (P)=F/A

Measurement of pressure is done by the manometer.

Unit of Pressure in S.I. system is “Pascal”.

The eloborative units were presented above at Point No.10


28.What is Absolute Pressure?

The Sum of Atmospheric pressure and Gauge pressure is called Absolute Pressure. or

The Difference between Atmospheric pressure and Vaccum pressure is also called as Absolute Pressure

Absolute Pressure=Atmospheric pressure+Gauge pressure

Absolute Pressure=Atmospheric pressure-Vaccum Pressure.


29.What is Vaccum Pressure?

It is defined as the difference between Atmospheric Pressure and the Absolute Pressure

 Vaccum Pressure = Atmospheric Pressure-Absolute Pressure.

30.Numerical based on vaccum and atmosphere:

Vaccum and Atmosphere are the two most popular terms used in the thermodynamics. So, let’s do a numerical on it. Therefore, In this article, I am going to explain about the simple Numerical based on vaccum and atmosphere in a detailed manner.

Numerical based on Vaccum

1. A mass of 1Kg falls to a distance of 10m in Vaccum. What is Work done?

Sol:             Work done = Zero

 Numerical based on Atmosphere:

1. A mass of 100N falls through a distance of 10m overcoming drag resistance of 5N. What is the work done?

Sol:

W =F*S

    =FSCos(theta)

    = FCos(theta)*S

    =resisting force*distance

    =5*10

    =50Nm


31.Difference between Engineering Mechanics and Strength of Materials:

This type of questions will be asked in the interview point of view and the answer for this question is well explained in the next paragraph.

Difference:

Engineering Mechanics deals with RIGID Bodies whereas Strength of materials deals with Deformable bodies.

Rigid Body in the sense a body which cannot be deformed by the application of external force also called a Rigid body. It is also called as a real body.

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