Core concept
Design of Gears
Gear teeth are checked for beam (bending) strength and surface wear/pitting.
29 min13 Interview12 GATEGATEInterview
Level
Advanced
Topic tags
Design of gears, Machine Design, Vb Bhandari Design
Useful for
GATE favourite, Interview ask
Source
Design of Machine Elements
What you'll learn in this topic
- Spur gears: force at pitch point splits into tangential (torque) and radial
- Face width b often ~ 10–12 module as a starting guide (check code)
- Design against bending breakage and surface pitting
- Helical gears run quieter with higher contact ratio; thrust appears
Key Formulas
Important equations & their meanings
Worked Examples
Step-by-step solved problems
- 1.Tangential force
- 2.Conceptual check — Design of Gears
Common Mistakes
Avoid these errors in exams & interviews
- Mixing module and diametral pitch units
- Using only wear or only strength when both are asked
- Forgetting radial force Fr = Ft tan φ
Practice Questions
Strengthen your concepts with questions
25+
Practice Questions
Interview Questions
Most asked interview problems
13
Interview Questions
GATE Questions
Previous year GATE questions
12
GATE Questions
Applications in real world
Shafts
Power transmission design
Gears
Tooth strength & wear checks
Joints
Bolted and welded connections
Bearings
Life and lubrication selection
Visual concept
Load Case
Stress Check
Factor of Safety
Sized Part
Design size from stress with a clear factor of safety.
Recommended Book
Design of Machine Elements
Design of Machine Elements
VB Bhandari
Read: Syllabus unit
Related Topics
What should I learn next?
Nearby topics on your path
University exams
Important Topic
Industry relevance
High
Concept difficulty
Hard
Average time
29 min
See how this topic connects to real mechanical engineering careers
Learn from engineers who use this concept in their daily work.
Introduction
Focus on spur gears for exams: given power and speed, find forces, choose m and b, check Lewis strength and a wear criterion when data allow. Define module, addendum, dedendum, and pitch circle.
Scope in B.Tech and GATE syllabus
Mention why cast iron vs hardened steel changes the wear limit. Helical/bevel appear as short theory.
Key relations & formulas
module:
velocity factor / dynamic load forms vary by code (e.g. Barth)
Lewis beam strength:
wear/Hertz limits surface durability (Buckingham / AGMA-style ideas)
velocity factor / dynamic load forms vary by code (e.g. Barth)
Lewis beam strength:
(tooth as cantilever)
wear/Hertz limits surface durability (Buckingham / AGMA-style ideas)
Notation and sign conventions
Symbol and sign-convention guide for the equations listed under Key relations & formulas.
Keep SI units consistent end-to-end (do not mix mm with m, or N with kN, in one substitution).
Symbol guide:
• — governing quantity for this relation
• — tooth as cantilever
• — governing quantity for this relation
• — tooth as cantilever
Sign convention: lock the textbook’s positive sense (force, moment, rotation, heat, or flow) before substituting. A correct symbolic setup still earns method marks in most Indian university papers even if arithmetic slips.
Write relations with symbols exactly as in Design of Machine Elements — VB Bhandari before substituting numbers.
Fundamentals and definitions
The tooth is a short cantilever (Lewis). Dynamic load rises with speed — velocity factors inflate effective force. Surface fatigue (pitting) depends on contact stress and hardness; case hardening raises wear capacity.
Governing relations in practice
Shigley frames gear design as system design (shafts, bearings, lubrication). Bhandari supplies the classic Indian numerical path. Always state assumptions (20° full-depth, cast tooth form factor y, etc.).
Assumptions and validity limits
State assumptions explicitly before using any relation for design of gears — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.
Wrong assumptions invalidate the entire solution even when the formula is correct. In Machine Design viva and GATE descriptive questions, listing valid assumptions often earns separate marks.
Step-by-step problem approach
1. Read the question and list given data with SI units (common in Machine Design papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to design of gears.
4. Use equation 1:
5. Use equation 2: velocity factor / dynamic load forms vary by code (e.g. Barth).
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to design of gears.
4. Use equation 1:
.
5. Use equation 2: velocity factor / dynamic load forms vary by code (e.g. Barth).
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.
Applications & exam relevance
Design of Gears appears in shafts, keys, bearings, springs, gears, and fasteners. In Indian mechanical curricula this topic is tested because it connects theory to safe sizing of mechanical components.
GATE and semester exams often combine design of gears with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use design of gears?" — answer with a lab, mini-project, or plant visit example if possible.
Common mistakes in exams
• Mixing module and diametral pitch units
• Using only wear or only strength when both are asked
• Forgetting radial force
• Ignoring dynamic/velocity effects at high speed
• Using only wear or only strength when both are asked
• Forgetting radial force
• Ignoring dynamic/velocity effects at high speed
Quick revision checklist
Before attempting design of gears problems, confirm you can:
1. Spur gears: force at pitch point splits into tangential (torque) and radial
2. Face width b often ~ 10–12 module as a starting guide (check code)
3. Design against bending breakage and surface pitting
4. Helical gears run quieter with higher contact ratio; thrust appears
2. Face width b often ~ 10–12 module as a starting guide (check code)
3. Design against bending breakage and surface pitting
4. Helical gears run quieter with higher contact ratio; thrust appears
Revise the solved examples in Design of Machine Elements — VB Bhandari and one previous-year GATE or university paper for this unit.