Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Brush and bearing replacement
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Brush and bearing replacement
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Subject: Auto Electrical Works
Class: Senior Secondary 3
Term: 3rd Term
Week: 2
Theme: Charging System
This page supports the lesson note with a companion video and a short classroom-ready summary.
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Identify the sign for brush replacementneed. Identify the needfor bearing failure. Punch out and replace brushes and bearing. Re-in stall the alternator. Register appropriatedrive-belt tension.
Charging System Identification of brushes, slip rings/commutator, and bearings. Visual inspection for wear on brushes and signs of bearing failure (e.g., roughness when spinning the shaft if possible, or visible damage). Step-by-step procedure for removing worn brushes (desoldering/unclipping) and installing new ones. Step-by-step procedure for removing old bearings (using a puller or carefully tapping out) and installing new ones (using a driver/press). Emphasize safety precautions with tools. Briefly demonstrate reassembly of the unit. Demonstrate correct drive-belt tensioning on an actual engine or an engine mock-up, showing the use of a pry bar or tensioner tool, and explaining the 'feel' method and the use of a tension gauge.
Guided Practice Introduction (10 minutes): Divide students into small groups (e.g., 3-4 students per group). Distribute an alternator/starter (or the components) to each group, along with appropriate tools. Provide clear instructions for the hands-on practical session. Supervision and Feedback (Throughout practical activities): Circulate among groups, providing guidance, correcting techniques, and answering questions. Ensure students adhere to safety protocols (e.g., wearing safety glasses, proper tool handling). Provide constructive feedback on their practical skills.
Conclusion (5 minutes): Summarize key takeaways from the lesson. Address any remaining questions. Assign independent practice tasks. 3.
2. Student Activities Active Listening and Note-taking: Students will listen attentively to explanations and take comprehensive notes.
Observation: Students will observe the teacher's demonstration keenly, paying attention to tool usage, safety, and procedural steps.
Hands-on Practical (30-45 minutes): In groups, students will: Disassemble a given alternator/starter motor (if available). Identify and inspect the brushes, slip rings/commutator, and bearings for signs of wear or failure. Practice removing and installing brushes (simulating replacement if new brushes are not available for every group). Practice removing and installing bearings (simulating replacement if new bearings are not available for every group). Practice reassembling the component. On an engine mock-up, practice re-installing the component and adjusting drive-belt tension.
Question and Answer: Students will ask clarifying questions during the lesson and participate in discussions.
Group Discussion: Students will discuss observations and procedures within their groups.
Problem Solving: Students will identify hypothetical faults based on presented symptoms.
4. Guided Practice (With Solutions)
Scenario: A customer drives a Toyota Corolla to your workshop in Ikeja, complaining of a "battery light" staying on and sometimes a "squealing noise" from the engine bay, especially on cold mornings. Question 1 (Targeting Performance Objectives 1 & 2): Based on the customer's complaints, what specific components within the alternator might be failing, and what are the tell-tale signs for each?
Solution 1: Component 1: Alternator Brushes Tell-tale signs: The "battery light" (which is typically the charge warning light) staying on indicates the alternator is not charging the battery effectively. Worn brushes can lead to intermittent or complete loss of contact with the slip rings, preventing the rotor from being excited and thus no charging output.
Component 2: Alternator Bearings * Tell-tale signs: The "squealing noise," especially on cold mornings, is a classic symptom of worn or dry bearings within the alternator. As the bearings degrade, they generate friction and noise. If the belt itself is squealing due to insufficient tension, it could also be due to a stiff or partially seized bearing putting excessive drag on the belt. Question 2 (Targeting Performance Objective 3): If upon inspection, you confirm both the brushes and bearings need replacement, outline the sequence of steps a technician would take to remove the worn bearings from the alternator shaft and housing.
Solution 2:
1. Safety First: Disconnect the battery and ensure the vehicle is safely supported.
2. Remove Alternator: Unbolt and disconnect all electrical wiring and the drive belt, then remove the alternator from the engine bay.
3. Initial Disassembly: Remove the pulley nut, then use a pulley puller to safely remove the alternator pulley. Remove the through-bolts holding the alternator housing halves together.
4. Separate Housing Halves: Carefully pry apart the front and rear housings. This will expose the rotor (with the front bearing usually pressed onto its shaft) and the stator Brush and bearing replacement Term: 3rd Term Week: 9 ---
1. Overview and Learning Objectives This topic introduces students to the critical components of automotive rotating electrical machines, specifically alternators and starter motors: brushes and bearings. Understanding the function, signs of failure, and the correct procedures for replacing these components is fundamental for aspiring auto electricians in Nigeria. These parts are vital for the proper operation of a vehicle's charging and starting systems, directly impacting vehicle reliability and safety on Nigerian roads. Mastery of this topic equips students with essential diagnostic and repair skills, making them competent for employment in vehicle repair workshops or for establishing their own auto electrical service centres across Nigeria. Upon completion of this lesson, students will be able to: Recognise the indicators that signal the need for brush replacement in an alternator or starter motor. Identify the symptoms and causes associated with bearing failure in automotive rotating electrical components. Demonstrate the correct procedure for safely removing worn brushes and bearings and installing new ones. Correctly reinstall a serviced alternator or starter motor back into a vehicle. Properly adjust the drive-belt tension for optimal performance of the re-installed alternator.
2. Key Concepts and Explanations 2.
1. Introduction to Rotating Electrical Machines (Alternators and Starter Motors) Automotive electrical systems rely heavily on rotating electrical machines.
Alternator: Generates electrical power to run the vehicle's electrical components and recharge the battery. It converts mechanical energy (from the engine's crankshaft via a drive belt) into electrical energy.
Starter Motor: Converts electrical energy (from the battery) into mechanical energy to crank the engine for starting. Both components contain brushes and bearings, which are subject to wear and require periodic replacement. 2.
2. Brushes: Function, Failure, and Replacement Function of Brushes: Brushes are typically small, carbon-graphite blocks that conduct electrical current between stationary and rotating parts of an electrical machine. In alternators, brushes make contact with slip rings on the rotor shaft to transfer current for exciting the rotor windings, thus creating a magnetic field. In DC starter motors (older types) and DC generators, brushes make contact with a commutator to transfer current to/from the armature windings. They are held against the rotating surface by springs to maintain continuous electrical contact. Signs of Brush Wear/Replacement Need (Performance Objective 1): Reduced Charging System Output (Alternator): Dimming headlights, slow battery charging, battery warning light illuminated on the dashboard. This is due to poor or intermittent contact between worn brushes and slip rings, leading to insufficient current flow to the rotor.
Engine Cranking Issues (Starter Motor): Slow cranking, no-crank condition (clicking sound), or intermittent starting. Worn brushes can prevent sufficient current from reaching the armature windings.
Visible Wear: Brushes are significantly shorter than new ones, or worn down to their wear limit (often marked on the brush or housing).
Grooving/Pitting of Slip Rings/Commutator: Excessive brush wear can also indicate wear on the mating surface, though the primary sign for brush replacement is the brush itself.
Causes of Brush Wear: Friction: Constant contact and friction against the rotating slip rings or commutator naturally wears down the carbon material over time.
Heat: High operating temperatures can accelerate wear.
Contamination: Dust, dirt, or oil can create abrasive conditions.
Excessive Current: Overloading the system can cause arcing and rapid wear. * Replacement Procedure (General Steps for Brushes - Performance Objective 3):
1. Disconnection and Removal: Disconnect battery, remove the alternator/starter from the vehicle.
2. Disassembly: Carefully remove the outer casing, voltage regulator (for alternators), or solenoid (for starters) to access the brush holder assembly.
3. Inspection: Visually inspect the brushes. Note their length, condition, and the tension of the springs. Also, inspect the slip rings/commutator for excessive wear, burning, or grooving. If the slip rings/commutator are severely worn, resurfacing or rotor replacement might be necessary.
4. Removal of Old Brushes: The brush holder assembly typically holds the brushes. In some designs, brushes are soldered, while in others, they are simply pulled out after removing a retaining clip or screw.
5. Installation of New Brushes: Insert new brushes into the holder, ensuring the springs brush holder assembly.
3. Inspection: Visually inspect the brushes. Note their length, condition, and the tension of the springs. Also, inspect the slip rings/commutator for excessive wear, burning, or grooving. If the slip rings/commutator are severely worn, resurfacing or rotor replacement might be necessary.
4. Removal of Old Brushes: The brush holder assembly typically holds the brushes. In some designs, brushes are soldered, while in others, they are simply pulled out after removing a retaining clip or screw.
5. Installation of New Brushes: Insert new brushes into the holder, ensuring the springs provide adequate tension. If soldered, carefully desolder the old brushes and solder new ones, ensuring clean, strong connections. Reassemble the brush holder.
6. Reassembly and Testing: Reassemble the alternator/starter. A bench test (if available) can verify proper function before re-installation. 2.
3. Bearings: Function, Failure, and Replacement Function of Bearings: Bearings reduce friction and support rotating shafts (e.g., alternator rotor shaft, starter armature shaft). They ensure smooth, precise rotation and prevent wobbling or misalignment. Ball bearings are commonly used in automotive applications due to their ability to handle both radial and axial loads. They consist of an inner race, an outer race, rolling elements (balls), and a cage to space the balls, all packed with grease. Signs of Bearing Failure (Performance Objective 2): Noise: This is the most common indicator. A failing bearing will produce a distinctive sound: Grinding or Growling Noise: Often indicative of worn or pitted bearing races/balls. This noise usually increases with engine speed.
Squealing or Chirping Noise: Can sometimes indicate a dry bearing or one that is about to seize.
Whining Noise: May also suggest a failing bearing.
Excessive Play/Wobble: When the alternator/starter is removed, attempting to move the shaft manually will reveal excessive lateral (side-to-side) movement or wobble if the bearings are failing. This can damage other components like the stator/armature.
Overheating: Friction from a failing bearing generates excessive heat, which can be felt on the component's casing.
Seizure: In severe cases, a completely failed bearing can seize, preventing the shaft from rotating, leading to drive belt squealing (alternator) or a no-crank condition (starter).
Causes of Bearing Failure: Lack of Lubrication: Grease dries out, leaks, or is contaminated over time, leading to metal-on-metal contact.
Contamination: Dirt, moisture, or foreign particles entering the bearing can abrade the surfaces.
Overloading/Impact: Excessive stress or impact (e.g., from an overtightened drive belt or physical damage) can damage the bearing elements.
Age and Fatigue: Material fatigue over years of use is a natural cause.
Heat: Prolonged exposure to high temperatures (e.g., near exhaust manifolds) can degrade lubricants. Replacement Procedure (General Steps for Bearings - Performance Objective 3):
1. Disconnection and Removal: Disconnect battery, remove the alternator/starter from the vehicle.
2. Full Disassembly: Alternator/starter must be completely disassembled to access the bearings, which are typically pressed onto the rotor/armature shaft and into the end housings.
3. Removal of Old Bearings: For bearings pressed onto the shaft: Use a specialized bearing puller to carefully extract the bearing from the shaft without damaging the shaft.
For bearings pressed into the housing: Use a socket or a bearing driver that matches the outer diameter of the bearing and gently tap it out from the opposite side, or use a hydraulic press if available.
4. Cleaning and Inspection: Clean the shaft and housing thoroughly. Inspect for any damage or burrs that could impede new bearing installation.
5. Installation of New Bearings: For bearings onto the shaft: Heat the bearing (e.g., in hot oil or on a hot plate – exercise extreme caution and do not overheat) or chill the shaft (e.g., with dry ice) to aid installation. Alternatively, use a bearing driver and a press or hammer (tapping evenly around the inner race) to gently tap the bearing onto the shaft until fully seated. Never hammer directly on the balls or outer race.
For bearings into the housing: Use a bearing driver or a socket of the correct size to press or gently tap the new bearing into the housing, ensuring it is square and fully