(SEM VI) THEORY EXAMINATION 2017-18 MACHINE DESIGN II

Machine Design–II (NME-602)

Complete Section-Wise Explanation – B.Tech Semester VI

Introduction to the Subject

Machine Design–II is an advanced mechanical engineering subject that focuses on the design of machine elements subjected to complex loads and real operating conditions. While Machine Design–I covers basics like shafts, keys, and couplings, this subject goes deeper into gears, bearings, 
crankshafts, pistons, connecting rods, and lubrication systems.

This subject is highly numerical and application-oriented, requiring:

Use of standard design data books

Proper assumptions where data is missing

Step-by-step design methodology

Sound understanding of failure theories and service conditions

The question paper is divided into three sections: A, B, and C.

SECTION A – Fundamental Concepts & Definitions

Pattern:
Attempt all questions
10 questions × 2 marks = 20 marks

Nature of Section A

Section A checks your conceptual clarity. Answers must be short, accurate, and technically correct. These are direct theory questions frequently repeated in exams.

Explanation of Section A Questions

Interference in Involute Gears
Interference occurs when the tip of one gear tooth interferes with the non-involute portion of the mating tooth, leading to improper contact and increased wear. It usually happens when the number of teeth is too small.

Condition for Constant Velocity Ratio in Spur Gears
For constant velocity ratio, the common normal at the point of contact of gear teeth must always pass through a fixed point on the line of centers. This is known as the law of gearing.

Tredgold’s Approximation for Bevel Gears
Tredgold stated that bevel gear teeth can be designed by assuming them equivalent to spur gears with a formative number of teeth, simplifying strength calculations.

Working Drawing of Bevel Gears
This requires a neat sketch showing pitch cones, addendum, dedendum, face width, and gear mesh alignment.

Herringbone Gear
A herringbone gear consists of two helical gears of opposite helix angles placed side by side. It eliminates axial thrust and is used in heavy-duty gearboxes.

Helix Angle and Normal Pitch
Helix angle is the angle between the helix and the gear axis.
Normal pitch is the distance between corresponding points measured normal to the tooth.

Efficiency of Worm Gear
Efficiency depends on lead angle, friction coefficient, and pressure angle. Maximum efficiency occurs at an optimum lead angle where friction losses are minimum.

Hydrodynamic Lubrication
In hydrodynamic lubrication, a full oil film separates bearing surfaces, preventing metal-to-metal contact and reducing wear.

Bearing Characteristic Number & Bearing Modulus
These dimensionless parameters describe bearing performance, lubrication regime, and heat generation characteristics.

Maximum Twisting Moment in Crankshaft
Maximum twisting moment occurs slightly after the top dead center when gas pressure and crank angle produce maximum torque.

SECTION B – Design Theory & Numerical Applications

Pattern:
Attempt any three questions
3 × 10 marks = 30 marks

Nature of Section B

This section requires detailed explanations and numerical solutions. Clear assumptions, formulas, and logical steps are essential to score well.

Explanation of Section B Questions

Hydrodynamic Journal Bearing Terms

This question tests understanding of bearing geometry and lubrication theory. You must explain terms like journal diameter, bearing length, clearance, eccentricity ratio, oil film pressure, attitude angle, and Sommerfeld number with a neat sketch.

Journal Bearing Numerical (Load, Length & Heat Removal)

Here, bearing length is calculated using allowable bearing pressure. Heat generated due to viscous friction is equated with heat carried away by lubricant to find heat removal per minute. Knowledge of viscosity, speed, clearance, and temperature is essential.

Ball Bearing Life & Dynamic Load Rating

This problem applies bearing life theory. Equivalent dynamic load is calculated using load variation percentages. The required basic dynamic load rating is determined for a given life and reliability (90% survival).

Helical Gear Design (Strength & Wear)

This is a core design problem.
First, tangential tooth load is calculated from power and speed.
Then module is selected based on bending strength using Lewis equation.
Finally, wear strength is checked to ensure safe operation. Face width is based on module.

Crankshaft Design Procedure

This theory question explains the step-by-step design of a crankshaft, including determination of gas load, inertia forces, bending moment, twisting moment, combined stress, and selection of shaft dimensions using failure theories.

SECTION C – Advanced Design Problems

Pattern:
Attempt any one part from each question
5 questions × 10 marks = 50 marks

This section carries the maximum marks and largely determines final results.

Question 3 – Gear Design

Spur Gear Strength Design
This numerical involves calculating power transmitted using Lewis equation, velocity factor, face width, and allowable stress. The weaker member governs the design.

Bevel Gear and Shaft Design
This is a comprehensive design problem involving:     Bevel gear strength calculation

Formative teeth concept                                               Velocity factor

Shaft bending and torsion                                            Bearing load consideration

It tests combined knowledge of gears and shaft design.

Question 4 – Worm Gear / IC Engine Design

Worm Gear Forces & Efficiency
You calculate tangential force, axial thrust, separating force, and efficiency using friction and lead angle relations.

Diesel Engine Cylinder Design
This problem includes calculation of cylinder bore from power and pressure, thickness of cylinder head using strength considerations, and sizing of studs based on tensile stress.

Question 5 – Bearings & Helical Gear Reducer

Sliding Contact Bearing
This question tests hydrodynamic lubrication theory and safe bearing load calculation using viscosity, clearance, speed, and pressure.

Helical Gear Reducer Design
The gears are designed for bending and wear with overload consideration. Incremental dynamic load is included to find safe transmitted power.

Question 6 – Connecting Rod / Piston Design

Connecting Rod Design
This is a high-level design problem involving gas force, inertia force, buckling, and stress analysis under maximum explosion pressure and overspeed condition.

Cast Iron Piston Design
This problem covers piston head thickness, ring dimensions, skirt length, heat balance, and material strength assumptions.

Question 7 – Gear Theory / Spur Gear Drive Design

Cycloidal vs Involute Teeth & Gear Failures
This theory question explains tooth profiles, advantages of involute teeth, and causes of gear failure such as bending fatigue, pitting, scoring, and wear.

Spur Gear Drive Design
A complete spur gear design including power transmission, module selection, gear dimensions, shaft and key design, and starting torque consideration.