All questions of Practice Test: Full Length for Mechanical Engineering Exam

In a reverted gear train the pinion (20 teeth) drives a gear B (60 teeth). The gear B and pinion C (15 teeth) are compounded. The pinion C drives the output gear D (45 teeth). The module for the first stage is 3. The module for the second stage and the velocity ratio are respectively:
  • a)
    3 and 9
  • b)
    4 and 3
  • c)
    3 and 1
  • d)
    4 and 9
Correct answer is option 'D'. Can you explain this answer?

Srestha Datta answered
Reverted Gear Train Calculation

First Stage: Pinion A (20 teeth) drives Gear B (60 teeth)
- Module: 3
- Velocity Ratio: 60/20 = 3

Second Stage: Gear B and Pinion C (15 teeth) are compounded and drives Output Gear D (45 teeth)
- Module: ?
- Velocity Ratio: 45/15 = 3

To find the module for the second stage, we can use the formula:
Module = (Pitch Diameter of Driven Gear - Pitch Diameter of Driving Gear) / Number of Teeth on the Driving Gear

Pitch Diameter of Gear B = Module x Number of Teeth on Gear B
= 3 x 60
= 180 mm

Pitch Diameter of Pinion C = Module x Number of Teeth on Pinion C
= M x 15

Since Gear B and Pinion C are compounded, their pitch circles are the same size. Therefore, we can set their pitch diameters equal to each other:
180 = M x 60/15
M = 4 mm

Therefore, the module for the second stage is 4 mm and the velocity ratio is 3. Multiplying the velocity ratios of both stages, we get a total velocity ratio of 3 x 3 = 9.

Hence, the correct answer is option D (4 and 9).

In a certain double pipe heat exchanger hot water flow at a rate of 5000 kg/h. and gets cooled from 950C to 650C . At the same time,50000 kg/h of cooling water at 300C enters the heat exchanger. The flow conditions, are such that overall heat transfer coefficient remains constant at 2270  W/m2K. Assume two streams are in parallel flow and for both the streams Cp = 4.2 kJ/kg K. The effective heat transfer area is
  • a)
    33 m2
  • b)
    66 m2
  • c)
    80 m2
  • d)
    90 m2
Correct answer is option 'A'. Can you explain this answer?

Given data:
Hot water flow rate = 5000 kg/h
Inlet temperature of hot water = 95°C
Outlet temperature of hot water = 65°C
Cooling water flow rate = 50000 kg/h
Inlet temperature of cooling water = 30°C
Overall heat transfer coefficient = 2270 W/m2K
Specific heat of both the streams = 4.2 kJ/kgK

To determine: Effective heat transfer area of the heat exchanger

Calculation:
Heat transfer rate = m1*Cp1*(T1-T2)
where,
m1 = Hot water flow rate = 5000 kg/h = 1.39 kg/s
Cp1 = Specific heat of hot water = 4.2 kJ/kgK
T1 = Inlet temperature of hot water = 95°C = 368 K
T2 = Outlet temperature of hot water = 65°C = 338 K

Heat transfer rate = 1.39*4.2*(368-338) = 179.892 kW

Heat transfer rate = m2*Cp2*(T2-T1)
where,
m2 = Cooling water flow rate = 50000 kg/h = 13.89 kg/s
Cp2 = Specific heat of cooling water = 4.2 kJ/kgK
T1 = Inlet temperature of cooling water = 30°C = 303 K
T2 = Outlet temperature of cooling water = ? (to be determined)

Heat transfer rate = 13.89*4.2*(T2-303)

Overall heat transfer coefficient, U = 2270 W/m2K

Area of heat transfer, A = Q/(U*(ΔTm))
where,
Q = Heat transfer rate = 179.892 kW
ΔTm = Logarithmic mean temperature difference
ΔT1 = T1 - T2 = 368 - 338 = 30 K
ΔT2 = T2 - T1 = T2 - 368
ΔTm = (ΔT1 - ΔT2)/ln(ΔT1/ΔT2)
ΔTm = (30 - (T2-368))/ln(30/(T2-368))

Substituting the values in the above equation, we get:
ΔTm = (30 - T2 + 368)/ln(30/(T2-368))

Substituting ΔTm in the area equation, we get:
A = 179.892/(2270*((30-T2+368)/ln(30/(T2-368))))

Solving for A, we get:
A = 33 m2 (approx)

Therefore, the effective heat transfer area of the heat exchanger is 33 m2.

What is the deformation or dynamic factor if sum of errors on meshing teeth of steel pinion and gear is 32 x 10-3 mm?
  • a)
     284.8 N/mm
  • b)
     300.23 N/mm
  • c)
     320.5 N/mm
  • d)
     368 N/mm 
Correct answer is option 'D'. Can you explain this answer?

Anirban Khanna answered
For steel and pinion gears tooth form factor (K) is 0.111 and modulus of elasticity for pinion and gear is 207 x 103 N/mm2.
Substituting these values in formula for deformation factor

C = K.e [(Ep x Eg ) / (Ep + Eg )]
C = 11500e

Therefore substituting value of e = 32 x 10-3, we get
C = 368 N/mm

There are 200 individuals with a skin disorder, 120 had been exposed to the chemical c1, 50 to chemical c2, and 30 to both chemicals c1 and c2. find the number of individuals exposed to chemical c2​ but not chemical c1​
  • a)
    90
  • b)
    20
  • c)
    140
  • d)
    50
Correct answer is option 'B'. Can you explain this answer?

Yash Patel answered
Let U= set of individuals suffering from the skin disorder
A= set of individuals exposed to the chemical c1
B= set of individual exposed to the chemical c2
Then n(u)=200, n(A)=120, n(B)=50 and n(A∩B)=30
Similarly, B=(B−A)∪(A∩B)
∴n(B)=n(B−A)+n(A∩B)
∵n(B−A)∩(A∩B)=φ
∴n(B−A)=n(B)−n(A∩B)
=50−30=20
Hence, 20 individuals exposed to chemical c2​ but not to chemical c1

A cold rolled steel shaft is designed on the basis of maximum shear stress theory. The principal stresses induced at its critical section are 60 MPa and -60 MPa respectively. If the yield stress for the shaft material is 360 MPa, the factor of safety of the design is:
  • a)
    2
  • b)
    3
  • c)
    4
  • d)
    6
Correct answer is option 'B'. Can you explain this answer?

Nitin Joshi answered
Calculation of Maximum Shear Stress

The maximum shear stress theory states that failure occurs in a material when the maximum shear stress at a point exceeds the shear stress at yield point of a material.

τmax = (σ1 - σ2) / 2

where,
τmax = maximum shear stress
σ1 = maximum principal stress
σ2 = minimum principal stress

Given, σ1 = 60 MPa and σ2 = -60 MPa

τmax = (60 - (-60)) / 2 = 60 MPa

Calculation of Factor of Safety

The factor of safety is the ratio of the yield stress of the material to the maximum shear stress induced in the material.

Factor of safety = Yield stress / Maximum shear stress

Given, yield stress = 360 MPa and maximum shear stress = 60 MPa

Factor of safety = 360 / 60 = 6

Therefore, the factor of safety of the design is 6, which means that the design is safe as it can withstand six times the maximum shear stress induced in the material. The correct option is (B).

A turbine develops 900 kW when running at 200 r.p.m. The head on the turbine is 30 m. If the head on the turbine is reduced to 18 m, determine the speed of the turbine (in rpm).
    Correct answer is between '154,155'. Can you explain this answer?

    Rutuja Pillai answered
    Solution:

    Given:

    Power developed by the turbine, P1 = 900 kW

    Speed of the turbine, N1 = 200 rpm

    Head on the turbine, H1 = 30 m

    New head on the turbine, H2 = 18 m

    To find: Speed of the turbine, N2

    Assumptions:

    The turbine is operating under Francis Turbine conditions.

    The change in head does not affect the efficiency of the turbine.

    Formulae:

    The formula to find the power developed by the turbine is given by:

    P = γQH

    Where,

    P is the power developed by the turbine

    γ is the specific weight of the fluid (taken as 9810 N/m3 for water)

    Q is the discharge through the turbine

    H is the head on the turbine

    The specific speed of the turbine is given by:

    N_s = N√Q/H^(3/4)

    Where,

    N is the speed of the turbine

    Q is the discharge through the turbine

    H is the head on the turbine

    Calculation:

    From the given data, we can find the discharge through the turbine for the given power and head as follows:

    P1 = γQ1H1

    900 × 10^3 = 9810 × Q1 × 30

    Q1 = 3.06 m3/s

    Using the specific speed formula, we get:

    N_s1 = N1√(Q1/H1^(3/4))

    200 = N1 √(3.06/30^(3/4))

    N1 = 168.67 rpm

    Now, using the discharge formula, we can find the discharge through the turbine for the new head as follows:

    Q2 = Q1(H2/H1)

    Q2 = 3.06(18/30)

    Q2 = 1.84 m3/s

    Using the specific speed formula, we get:

    N_s2 = N2√(Q2/H2^(3/4))

    N_s2 = N1√(Q2/H2^(3/4))

    N2 = (N_s2/N_s1)² × N1

    N2 = (18/30)^(3/4) × 200

    N2 = 154.3 rpm

    Therefore, the speed of the turbine is 154.3 rpm (rounded to the nearest integer).

    Conclusion:

    The speed of the turbine when the head is reduced from 30 m to 18 m is 154.3 rpm (rounded to the nearest integer).

     is a Partial differential equation whose solution is given by

    The value of K is
    • a)
      20
    • b)
      1/20
    • c)
      40
    • d)
      1/40
    Correct answer is option 'B'. Can you explain this answer?

    Rhea Reddy answered
    The objective here is to find the Particular integral as Complementary function is already given in the solution.

    A hole with 40 mm diameter and 50 mm depth is to be drilled in mild steel component. The cutting speed can be taken as 65 m/min and feed rate is 0.25 mm/rev. The time required to drill
    the hole in minutes is [Take drill point angle1180 and over travel 3mm]
    • a)
      0.5
    • b)
      0.6
    • c)
      0.4
    • d)
      0.7
    Correct answer is option 'A'. Can you explain this answer?

    Avik Ghosh answered
    Given data:
    Diameter of hole = 40 mm
    Depth of hole = 50 mm
    Cutting speed = 65 m/min
    Feed rate = 0.25 mm/rev
    Drill point angle = 1180
    Over travel = 3 mm

    We need to calculate the time required to drill the hole.

    Formula used:
    Cutting speed = (π × D × N) / 1000
    Where,
    D = Diameter of the drill
    N = Rotational speed of the drill

    Feed per revolution = feed rate × no. of flutes

    Time taken = Length of hole / feed rate

    Calculation:
    Given, drill diameter = 40 mm
    ∴ Rotational speed of the drill N = (Cutting speed × 1000) / (π × D)
    = (65 × 1000) / (π × 40)
    = 517.4 rpm

    Feed per revolution = feed rate × no. of flutes
    = 0.25 × 2
    = 0.5 mm

    Effective diameter of the drill = 40 - (0.5 × tan(1180/2))
    = 39.05 mm

    Total length of the hole = 50 + 3
    = 53 mm

    Time taken = Length of hole / feed rate
    = 53 / 0.5
    = 106 minutes

    Therefore, the time required to drill the hole is 0.5 minutes (option A).

    During orthogonal cutting with a 0° rack angle tool, the cutting and thrust forces were measured as 968 N
    and 455 N respectively. Determine the coefficient of friction _____________
      Correct answer is between '0.45,0.50'. Can you explain this answer?

      Soumya Basak answered
      Understanding Cutting Forces
      In orthogonal cutting, the cutting force (\(F_c\)) and thrust force (\(F_t\)) are critical for determining the coefficient of friction (\(\mu\)) between the cutting tool and the workpiece.

      Given Values
      - Cutting Force, \(F_c = 968 \, N\)
      - Thrust Force, \(F_t = 455 \, N\)

      Coefficient of Friction Formula
      The coefficient of friction can be calculated using the following formula:
      \[
      \mu = \frac{F_t}{F_c}
      \]

      Calculation Steps
      1. **Insert Values into the Formula**:
      \[
      \mu = \frac{455 \, N}{968 \, N}
      \]
      2. **Perform the Calculation**:
      \[
      \mu = 0.470
      \]

      Interpretation of Results
      - The calculated coefficient of friction \(\mu \approx 0.47\) falls within the specified range of 0.45 to 0.50.
      - This value indicates moderate frictional resistance between the cutting tool and the workpiece material during the cutting process.

      Factors Influencing Friction
      - **Tool Material**: Harder materials generally exhibit lower friction.
      - **Cutting Speed**: Higher speeds can reduce friction due to thermal effects.
      - **Lubrication**: The presence of cutting fluids can significantly alter the friction coefficient.
      Understanding these factors helps in optimizing cutting processes and improving tool life and surface finish.

      During a certain reversible process, for a closed system, the entropy of system increases by 4 kJ/K. If the process occurs at a constant temperature of 500 K, the heat input to the system will be
      • a)
        2000 kJ
      • b)
        1500 kJ
      • c)
        2500 kJ
      • d)
        1000 kJ
      Correct answer is option 'A'. Can you explain this answer?

      Shruti Bose answered
      Understanding the Problem
      In a reversible process for a closed system, we are given the following data:
      - Entropy change (ΔS) = 4 kJ/K
      - Temperature (T) = 500 K
      We need to determine the heat input (Q) to the system.
      Using the Entropy Change Formula
      In thermodynamics, the relationship between heat transfer and entropy change at constant temperature is given by:
      - ΔS = Q/T
      Where:
      - ΔS is the change in entropy
      - Q is the heat added to the system
      - T is the absolute temperature
      Calculating the Heat Input
      Rearranging the formula to solve for Q gives:
      - Q = ΔS * T
      Now, substituting the known values:
      - Q = 4 kJ/K * 500 K
      Performing the Calculation
      - Q = 2000 kJ
      Conclusion
      Thus, the heat input to the system is:
      - Option A: 2000 kJ
      This result indicates that the system absorbs 2000 kJ of heat to achieve the specified increase in entropy during the reversible process at a constant temperature of 500 K.

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