Assembly and Dismantling of Machines
Overview
Assembly and Dismantling of Machines is a core Semester 3 chapter for ITI Fitter (NCVT). It covers systematic procedures for joining and separating machine components using correct tools, sequences, and safety practices. Topics include types of fits, use of pullers and presses, bearing assembly, key and keyway fitting, gasket and seal replacement, and torque tightening. Mastery ensures accurate machine maintenance, reduced downtime, and prevention of component damage during workshop operations.
Typed Notes
Principles of Assembly and Dismantling
Assembly is the process of joining machine parts in a definite sequence to form a complete unit. Dismantling (disassembly) is the reverse — separating parts without damage for inspection or repair. The golden rule: always follow the manufacturer's service manual. Before dismantling, clean the machine, drain fluids, disconnect power, and tag/mark parts for correct reassembly. Use correct tools for each operation. Improper dismantling causes burrs, bent shafts, or cracked housings. Organised part storage (trays, labels) prevents loss and mix-up of small components like washers, circlips, and shims.
Fits, Tolerances and Interference in Assembly
A fit describes the relationship between mating parts. Three types: Clearance fit (shaft smaller than hole — used in bearings with sliding motion), Transition fit (may be clearance or interference — used in gears, pulleys), and Interference fit (shaft larger than hole — used for press-fit bearings and bushes). Interference fit assembly requires a hydraulic press or arbor press; heating the outer part (expansion) or cooling the inner part (shrinkage using dry ice or liquid nitrogen) eases assembly. Example: a 50 mm bearing pressed into a housing with 0.02 mm interference. Correct fit selection prevents vibration and wear.
Bearing Assembly and Dismantling
Bearings are precision components. During assembly, force must be applied only on the ring being pressed (inner ring on shaft, outer ring in housing). Tools used: bearing puller (for removal), arbor press, hydraulic press, and bearing heater (induction type, heats to 80–100 °C max). Never strike a bearing with a steel hammer — use a brass drift or bearing fitting tool. Check bearing number stamped on the ring (e.g., 6205 — deep groove ball bearing, 62-series, 05 = 25 mm bore). After fitting, check for smooth rotation and correct axial/radial clearance. Lubricate as specified.
Keys, Keyways, Gaskets and Seals
Keys transmit torque between shaft and hub. Common types: Sunk key (rectangular/square), Woodruff key (semicircular, self-aligning, used in tapered shafts), Feather key (allows axial sliding), and Gib-head key (tapered, easy removal). Keyway is machined with an end mill or broach. Fit key with a light mallet; check for side clearance (zero) and top clearance (0.1–0.4 mm for sunk key). Gaskets (paper, cork, copper, rubber) seal static joints; O-rings seal dynamic or static joints. Always replace gaskets during reassembly. Apply gasket sealant (e.g., Loctite 515) on metal-to-metal joints. Oil seals (lip seals) must be pressed in flush, lip facing the oil side.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Assembly and Dismantling Procedures
- Pre-dismantling steps: isolation, draining, cleaning, marking
- Correct dismantling sequence (reverse of assembly)
- Part storage, tagging and documentation
- Safety precautions during machine work
Before dismantling a lathe gearbox, the fitter drains the gear oil, tags each gear with a numbered wire tag, and photographs the assembly — ensuring correct reassembly later.
02Fits and Tolerances in Machine Assembly
- Clearance fit — sliding and running applications
- Interference fit — press-fit bearings and bushes
- Transition fit — gears and pulleys
- Thermal methods: heating/cooling for interference assembly
A 6205 bearing (25 mm bore) with 0.02 mm interference is assembled by heating the bearing to 90 °C in an induction heater, then sliding it onto the shaft without pressing.
03Bearing Assembly and Dismantling
- Types of bearings: ball, roller, taper roller, thrust
- Bearing designation system (e.g., 6205, 6305)
- Tools: bearing puller, arbor press, induction heater, brass drift
- Lubrication and clearance checking after fitting
A 6305 deep groove ball bearing (25 mm bore, 62 mm OD) is removed from a motor shaft using a two-jaw bearing puller, pulling on the inner race only to avoid damage.
04Keys, Keyways and Splines
- Types: sunk, Woodruff, feather, gib-head key
- Keyway machining: end mill cutter or broach
- Fit tolerances: zero side clearance, 0.1–0.4 mm top clearance
- Splines for heavy torque and axial movement applications
A Woodruff key (6 × 9 mm) is fitted on a tapered milling machine spindle; its semicircular shape allows it to self-align as the pulley hub is tightened onto the taper.
05Gaskets, Seals and Torque Tightening
- Gasket types: paper, cork, copper, rubber, composite
- O-ring and lip seal (oil seal) installation rules
- Torque wrench use and torque values (N·m)
- Bolt tightening sequence: cross/star pattern for flanges
Cylinder head bolts on a diesel engine are tightened in a cross pattern to 100 N·m using a torque wrench — preventing warping of the head gasket and ensuring uniform clamping.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Assembly sequence must follow manufacturer's manual; dismantling is always done in reverse order of assembly.
- 02Interference fit components (e.g., bearings) require a hydraulic press or bearing puller — never use a hammer directly on the race.
- 03Torque wrench is used to tighten bolts to specified torque (e.g., cylinder head bolts: 80–120 N·m depending on engine).
- 04Woodruff key and feather key are common keys used to transmit torque between shaft and hub.
- 05Gaskets and O-rings must always be replaced with new ones during reassembly to prevent leakage.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the correct procedure for assembling a bearing with an interference fit onto a shaft?
Differentiate between a feather key and a Woodruff key with one application each.
Why must gaskets and O-rings always be replaced during reassembly, and what sealant is used on metal-to-metal joints?
What is the star (cross) pattern for bolt tightening, and why is it used on flanges and cylinder heads?
Explain the bearing designation system with the example '6205'.
Assembly and Dismantling of Machines requires correct sequencing, proper tools (pullers, presses, torque wrenches), and knowledge of fits and tolerances. Bearings need induction heaters or presses; keys transmit torque; gaskets and seals must be renewed every time. Following manufacturer torque values and reassembly sequences prevents machine failure and ensures safe, efficient workshop practice for ITI Fitter trade.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which tool is used to remove a bearing from a shaft without damaging it?
Q2.A Woodruff key is shaped like a:
Q3.What does the last two digits '08' in bearing number 6308 indicate?
Q4.Which type of fit is used for a bearing pressed into a housing?
Mark karo — dashboard par progress dikhega.