Power Transmission: Belts, Pulleys and Gears
Overview
Power transmission systems transfer mechanical energy from one shaft to another using belts, pulleys, chains, or gears. In ITI Fitter trade (Semester 2), this chapter covers types of belts, pulley arrangements, velocity ratio calculations, gear terminology, and gear trains. Understanding these systems is essential for maintaining industrial machinery, lathes, drilling machines, and conveyors. NCVT papers regularly test velocity ratio formulas, belt types, gear tooth profiles, and crowning of pulleys.
Typed Notes
Belt Drives — Types and Applications
Belt drives transmit power between two shafts using a flexible belt. Flat belts are used for long-distance, high-speed drives (e.g., line shafts). V-belts fit into grooved pulleys and transmit more power without slipping — most common in motors and compressors. Round belts are used in light-duty machines like sewing machines. Timing (toothed) belts give positive drive with no slip, used in CNC machines. Belt material can be leather, rubber, canvas, or synthetic. Slip reduces velocity ratio and causes power loss. Creep is a slow, continuous slip due to elasticity of belt.
Pulleys — Construction and Crowning
Pulleys are wheels mounted on shafts to carry belts. A flat pulley has a slightly convex (crowned) face — the centre is 1/96 of pulley width higher than edges — this keeps the belt centred and prevents it from running off. Step pulleys (cone pulleys) give multiple speed options on a lathe headstock. Fast-and-loose pulley arrangement allows the machine to start or stop without stopping the power source. Jockey pulley (idler) increases the angle of wrap on the driver pulley, improving grip and power transmission without changing the velocity ratio.
Velocity Ratio, Slip and Length of Belt
Velocity Ratio (VR) = N1/N2 = D2/D1, where N1 = driver speed (RPM), N2 = driven speed, D1 = driver diameter, D2 = driven diameter. With slip (S%): N2 = N1 × (D1/D2) × (1 − S/100). Length of open belt: L = π(R+r) + 2C + (R−r)²/C. Length of cross belt: L = π(R+r) + 2C + (R+r)²/C, where C = centre distance, R and r = radii of larger and smaller pulleys. Compound belt drives multiply velocity ratios of individual stages.
Gears — Terminology and Types
Gears are toothed wheels that transmit power positively (no slip). Key terms: Pitch Circle Diameter (PCD) — imaginary circle where gears mesh; Module (m) = PCD/Number of teeth; Addendum = 1×m (tooth above pitch circle); Dedendum = 1.157×m (tooth below pitch circle); Clearance = 0.157×m; Pressure angle is usually 20° (full depth) or 14.5° (older standard). Types: Spur gears (parallel shafts, most common), Helical gears (quieter, angled teeth), Bevel gears (intersecting shafts, used in hand drills), Worm and worm wheel (large speed reduction, right-angle drive, self-locking).
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Belt Drives
- Types: Flat, V-belt, Round, Timing belt
- Open belt vs Cross belt drive direction
- Slip and Creep — causes and effects on VR
- Belt materials: leather, rubber, canvas, synthetic
A motor pulley of 150 mm drives a machine pulley of 300 mm at 1440 RPM input — the machine shaft runs at 720 RPM (VR = 2:1). This is a typical lathe back-gear arrangement.
02Pulleys
- Crowning: centre raised by 1/96 of pulley width
- Step (cone) pulley for variable speeds on lathe
- Fast-and-loose pulley for clutch-less start/stop
- Jockey (idler) pulley to increase angle of wrap
A lathe cone pulley with 4 steps gives 4 different spindle speeds (e.g., 80, 160, 320, 640 RPM) without changing the motor speed — used in bench lathes in ITI workshops.
03Velocity Ratio and Belt Length Calculations
- VR formula: N1/N2 = D2/D1
- Effect of slip: N2 = N1×(D1/D2)×(1−S/100)
- Open belt length formula using centre distance
- Compound drives: overall VR = product of individual VRs
Driver pulley D1=200 mm at 900 RPM, driven D2=400 mm; VR=2, driven speed=450 RPM. With 2% slip: N2 = 450×(1−0.02) = 441 RPM — a common NCVT numerical.
04Gear Terminology and Module
- Module m = PCD / Number of teeth (unit: mm)
- Addendum = 1m, Dedendum = 1.157m, Clearance = 0.157m
- Pressure angle: 20° (standard full depth) or 14.5°
- Pitch Circle Diameter = Module × Number of teeth
A spur gear with 40 teeth and module 2 mm has PCD = 2×40 = 80 mm, addendum = 2 mm, dedendum = 2.314 mm — this type of calculation appears directly in NCVT Fitter papers.
05Types of Gears and Gear Trains
- Spur gear: parallel shafts, straight teeth, most common
- Bevel gear: intersecting shafts (e.g., hand drilling machine)
- Worm and worm wheel: right-angle, large reduction, self-locking
- Simple and compound gear trains — calculating overall gear ratio
A worm with 1 start driving a worm wheel of 40 teeth gives a gear ratio of 40:1 — used in screw jacks and hand winches where self-locking is needed for safety.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Velocity Ratio (VR) = Speed of Driver / Speed of Driven = Diameter of Driven / Diameter of Driver
- 02Open belt drive — shafts rotate in SAME direction; Cross belt drive — shafts rotate in OPPOSITE direction
- 03Crowning on pulleys prevents belt from slipping off the pulley face during operation
- 04Module of a gear = Pitch Circle Diameter (mm) / Number of Teeth; standard unit is mm
- 05Gear Ratio = Teeth on Driven Gear / Teeth on Driver Gear; for speed reduction, driven gear has MORE teeth
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the velocity ratio of a belt drive where the driver pulley diameter is 250 mm running at 1200 RPM and the driven pulley diameter is 500 mm? Also state the driven shaft speed.
Define 'Module' of a gear. A spur gear has 50 teeth and a pitch circle diameter of 100 mm. Find its module.
Differentiate between open belt drive and cross belt drive with respect to shaft rotation direction and a practical limitation of cross belt drive.
What is the purpose of crowning on a flat pulley? State the standard amount of crowning.
In a compound gear train, a driver gear has 20 teeth meshing with a 60-tooth gear on a compound shaft. The second gear on the compound shaft has 25 teeth meshing with a 75-tooth output gear. Find the overall velocity ratio.
Belt drives (flat, V, timing) and pulley systems transmit power with flexibility; velocity ratio depends on pulley diameters and belt slip. Crowning keeps belts centred. Gears provide positive, slip-free transmission; module is the key design parameter. Spur, helical, bevel, and worm gears serve different shaft arrangements. NCVT Fitter Semester 2 papers test VR formula, module calculation, gear types, and belt length formulas regularly.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which type of belt drive causes the driven pulley to rotate in the opposite direction to the driver pulley?
Q2.The module of a gear is 3 mm and it has 30 teeth. What is its Pitch Circle Diameter?
Q3.Which of the following gear types is used when power is to be transmitted between two shafts at RIGHT ANGLES with a very large speed reduction and self-locking property?
Q4.A jockey (idler) pulley is used in a belt drive system. What is its PRIMARY purpose?
Mark karo — dashboard par progress dikhega.