🛠️ NPCIL Interview Experience 2025 – Mechanical

📍 Location: Kalpakkam
📅 Date: 10th June 2025

🔧 1. Structural Mechanics & Machine Design

Q1. Why is an I-section (I-beam) widely used in engineering applications?
Key Concepts to Cover:

• High moment of inertia about the bending axis ⇒ greater bending strength
• Material is concentrated away from the neutral axis
• Optimized for bending loads, while saving material and weight
• Used in bridges, building structures, cranes, etc.

Q2. A tank is mounted on an overhead platform and connected to a tap.
a. How would you calculate the time to drain it completely?
b. Which equation applies?
Answer:

• Use Bernoulli’s equation combined with Torricelli’s law: v=2ghv = \sqrt{2gh}v=2gh​
• Then integrate the flow rate to get total time depending on tank shape.

🚆 2. Mechanics of Rolling Stock (Rail Transport)

Q3. Why are railway wheels conical instead of cylindrical?
Key Points:

• Conical shape allows for self-centering
• Helps manage different turning radii of inner and outer wheels on curves
• Minimizes wear and lateral movement
• Ensures stability and smoother ride

🧪 3. Material Science & Strength Testing

Q4. What is Impact Strength?

• The ability of a material to resist shock loading or sudden force
• Measured using Charpy or Izod impact test setups
• Indicates toughness under high strain rate

Q5. What is a Brake? How does it work in practical applications?

• Device used to slow or stop motion using frictional force
• Types:
  • Mechanical Brakes (Drum, Disc)
  • Hydraulic Brakes
  • Electromagnetic or Pneumatic Brakes
• Converts kinetic energy into heat energy

🧱 4. Solid Mechanics – Stress & Strain

Q6. Define Stress and Strain. Illustrate with a diagram.

• Stress (σ): Force per unit area (Tensile, Compressive, Shear)
• Strain (ε): Deformation per unit length
• Use stress-strain curves to explain material behavior

Q7. Under hydrostatic loading, how do you calculate strain in the X-direction?

• Use Generalized Hooke’s Law: εx=1E[σx−ν(σy+σz)]\varepsilon_x = \frac{1}{E}[\sigma_x – \nu(\sigma_y + \sigma_z)]εx​=E1​[σx​−ν(σy​+σz​)]
• For hydrostatic stress, all normal stresses are equal.

🧰 5. Mechanical Systems & Lifting Devices

Q8. What are the different types of Overhauling in lifting systems?

• Overhauling: When the load tends to move downward on its own
• Systems:
  • Screw Jacks
  • Hoists
• Types:
  • Self-locking Screws (no movement without external torque)
  • Overhauling Screws (load moves unless restrained)

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