Bearings, Lubrication and Maintenance
Overview
Bearings reduce friction between moving parts and support rotating shafts. Lubrication forms a protective film between surfaces to minimise wear, heat, and corrosion. Maintenance practices ensure long service life of machines. In ITI Fitter Semester 2, candidates must know bearing types, fits, lubrication methods, lubricant properties, and scheduled maintenance procedures. These topics appear regularly in NCVT theory and practical papers with questions on classification, selection, and fault diagnosis.
Typed Notes
Types of Bearings
Bearings are classified into two main groups. Plain (sliding) bearings — also called journal bearings or bush bearings — have a shaft sliding inside a bush; they suit heavy loads at moderate speeds (e.g., engine crankshaft). Rolling contact bearings use balls or rollers between inner and outer races, giving less friction and higher speed capacity. Sub-types: (1) Deep-groove ball bearing — most common, handles radial and axial loads. (2) Cylindrical roller bearing — heavy radial loads. (3) Taper roller bearing — combined radial and thrust loads (e.g., vehicle wheel hub). (4) Needle roller bearing — thin section, high radial load (e.g., rocker arm). Thrust bearings handle only axial (end) loads.
Bearing Fits, Mounting and Removal
Correct fit is critical for bearing life. Shaft tolerance uses interference (press) fit — the bearing inner race is slightly smaller than the shaft diameter so it grips firmly; typical interference is 0.01–0.03 mm. Housing bore uses transition or clearance fit so the outer race can be removed without damage. Mounting methods: (a) Cold press using a bearing press or drift tube — force applied only on the race being fitted. (b) Hot mounting — bearing heated in oil bath to 80–100 °C (never exceed 120 °C) to expand it before sliding onto shaft. Removal uses a bearing puller to avoid damage to shaft and races. Never strike the bearing directly with a hammer.
Lubrication — Types, Properties and Methods
Lubricants reduce friction, dissipate heat, prevent corrosion, and flush away contaminants. Types: (1) Oil — mineral or synthetic; used where heat removal is needed; applied by splash, drip, wick, ring, or forced (pressure) feed systems. (2) Grease — oil thickened with soap (lithium soap grease is most common); used in sealed bearings, vertical shafts, and dusty environments; grease nipple (grease gun) is used for application. (3) Solid lubricants — graphite, MoS₂ (molybdenum disulphide) — used at extreme temperatures. Key properties: viscosity (resistance to flow), viscosity index (VI — higher VI means less change with temperature), flash point, pour point, and oiliness. SAE (Society of Automotive Engineers) grades oil viscosity; higher number = thicker oil.
Maintenance Practices and Bearing Failures
Preventive Maintenance (PM) is planned, time-based servicing — checking oil levels, greasing, alignment checks, belt tension — done to prevent unexpected breakdown. Predictive Maintenance uses condition monitoring tools (vibration analyser, thermometer) to detect faults early. Breakdown Maintenance is corrective action after failure. Common bearing failures and causes: (1) Flaking/spalling — fatigue due to overloading. (2) Scoring/scuffing — insufficient lubrication. (3) Fretting corrosion — micro-movement between mating surfaces. (4) Brinelling — false brinelling from vibration when stationary; true brinelling from shock load. (5) Seizure — complete lubrication failure causing welding of surfaces. Bearing life is expressed as L10 life (hours at which 10% of bearings fail under given load).
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Classification of Bearings
- Plain (journal/bush) bearings — sliding contact
- Rolling contact bearings — ball, roller, needle, taper
- Thrust bearings — axial load only
- Selection criteria — load type, speed, space
A deep-groove ball bearing 6205 (25 mm bore) is used on a 1450 RPM single-phase motor shaft in a domestic water pump.
02Bearing Fits and Mounting
- Interference (press) fit on shaft — 0.01–0.03 mm
- Clearance/transition fit in housing bore
- Hot mounting in oil bath at 80–100 °C (max 120 °C)
- Bearing puller for safe removal
A fitter heats a 6308 bearing in an oil bath to 90 °C for 15 minutes, then slides it onto a 40 mm shaft without using a hammer.
03Lubrication — Lubricants and Methods
- Oil lubrication — splash, drip, ring, forced feed
- Grease lubrication — grease gun via nipple
- Solid lubricants — graphite, MoS₂ for extreme conditions
- Lubricant properties — viscosity, VI, flash point, pour point
SAE 40 engine oil is used in a lathe headstock gearbox operating at 60 °C ambient temperature, while lithium-based grease (NLGI Grade 2) is packed in the spindle bearing housing.
04Bearing Failures and Diagnosis
- Spalling/flaking — fatigue from overload
- Scoring — inadequate lubrication
- Brinelling (true and false) — shock load or vibration
- Seizure — complete lube failure; fretting corrosion — micro-movement
A grinding machine spindle bearing shows blue discolouration and seizure marks — investigation reveals the oil feed pipe was blocked, starving the bearing of lubrication for 2 hours.
05Maintenance Practices
- Preventive maintenance (PM) — scheduled, time-based
- Predictive maintenance — vibration and temperature monitoring
- Breakdown (corrective) maintenance — post-failure
- L10 bearing life concept and maintenance records
A workshop follows a PM schedule: grease all conveyor roller bearings every 500 operating hours using a grease gun, and replace bearings at 8000 hours regardless of condition.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Rolling contact bearings (ball, roller, needle, taper) reduce friction more than plain/sliding bearings and are preferred for high-speed applications.
- 02Viscosity is the most important property of a lubricant; SAE 40 oil is thicker (higher viscosity) than SAE 20 oil.
- 03Interference fit (press fit) is used to mount bearings on shafts; clearance fit is used in housing bores for easy removal.
- 04Grease lubrication is preferred for sealed bearings, slow-speed heavy loads, and where frequent re-lubrication is difficult.
- 05Preventive maintenance (PM) is scheduled maintenance done before breakdown; breakdown maintenance is done after failure occurs.
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
What is the difference between a ball bearing and a roller bearing? When would you select a taper roller bearing over a deep-groove ball bearing?
Explain interference fit and clearance fit as applied to bearing mounting. Why must force never be applied through the rolling elements?
What is viscosity and viscosity index (VI)? Compare SAE 20 and SAE 40 oils for a slow heavy-duty machine vs a high-speed light machine.
List four common bearing failures, their causes, and one prevention method for each.
Differentiate between Preventive Maintenance and Breakdown Maintenance. Why is Preventive Maintenance preferred in industry?
Bearings support rotating shafts and reduce friction; rolling bearings suit high speeds while plain bearings suit heavy loads. Correct interference fit on shaft and clearance fit in housing ensures long bearing life. Lubrication — oil, grease, or solid — prevents wear and corrosion; viscosity and SAE grade are key selection criteria. Preventive maintenance schedules minimise downtime and extend machine life. Bearing failures like spalling, scoring, and brinelling are directly linked to wrong lubrication or overloading.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which type of bearing is most suitable for handling BOTH radial and axial (thrust) loads simultaneously?
Q2.The maximum temperature to which a bearing should be heated in an oil bath during hot mounting is:
Q3.Which lubricant property is described as 'resistance of oil to flow at a given temperature'?
Q4.Grease lubrication is preferred over oil lubrication in which of the following situations?
Mark karo — dashboard par progress dikhega.