Lesson Notes By Weeks and Term v3 - Junior Secondary 2
Gears
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Gears
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Subject: Basic Technology
Class: Junior Secondary 2
Term: 3rd Term
Week: 4
Theme: Tools, Machines And Processes
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Identify differenttypes of gears State the uses of the various typesof gears in amechanicalsystem Determine gearratios Describe the relationshipbetween gearratio and speed of rotation State the functions of lubricants in gears Construct and usegears
Visual Aids: Utilize a wide range of diagrams, actual gear samples, videos, and 3D models to cater to different learning styles.
Group Work: Encourage collaborative learning where students can share ideas and help each other.
Spur Gears: Description: These are the most common and simplest type of gears. They have straight teeth cut parallel to the axis of rotation. They are mounted on parallel shafts.
Characteristics: Simple to manufacture, highly efficient, but can be noisy at high speeds due to sudden impact of teeth.
Uses: Widely used in general machinery where noise is not a major concern and shafts are parallel. Examples include simple gearboxes, washing machine agitators, clock mechanisms, and some early automobile transmissions. In Nigerian context, they are found in hand grinders and some simple farm machinery.
Helical Gears: Description: Helical gears have teeth cut at an angle (helix) to the axis of rotation. When two helical gears mesh, the teeth engage gradually, starting at one end and progressing to the other.
Characteristics: Smoother and quieter operation than spur gears, greater load-carrying capacity due to more teeth in contact, but they produce an axial thrust (force along the shaft) which needs to be accommodated by bearings.
Uses: Preferred for high-speed and high-load applications where quietness is important. Common in automotive transmissions (car gearboxes), power tools, and industrial machinery.
Bevel Gears: Description: Bevel gears are cone-shaped and have teeth cut on a conical surface. They are used to transmit power between intersecting shafts, typically at right angles (90 degrees).
Characteristics: Allow for changes in the direction of power transmission.
Uses: Found in differential drives of automobiles (allowing wheels to turn at different speeds when cornering), hand drills, printing presses, and right-angle drive systems in some agricultural machinery.
Worm and Worm Wheel (or Worm Gear): Description: A worm gear consists of a worm (a screw-like shaft with a helical thread) and a worm wheel (a gear resembling a helical gear, but with a specially cut profile to mesh with the worm). The worm usually drives the worm wheel.
Characteristics: Provides very large speed reductions in a single stage, self-locking (the worm cannot be turned by the worm wheel), compact design.
Uses: Used where large speed reduction and high torque are required, and where self-locking is an advantage. Examples include lifting equipment (hoists, elevators), tuning mechanisms, conveyor belts, and some steering gears. In local context, they can be found in some industrial mixers or processing machines.
Rack and Pinion: Description: A rack is a straight bar with teeth cut along its length, resembling an uncoiled gear. A pinion is a small spur gear that meshes with the rack.
Characteristics: Converts rotary motion into linear motion or vice versa.
Uses: Primarily used in steering systems of vehicles (converting the steering wheel's rotary motion into the linear motion of the steering linkage), machine tools (e.g., drilling machines, lathes for moving the carriage), and some lifting mechanisms.
Driver Gear (Input Gear): The gear that receives power from the motor or prime mover and transmits it to another gear.
Driven Gear (Output Gear): The gear that receives power from the driver gear and transmits it to the output shaft or next component.
Teeth (T): The projections on the gear that mesh with the teeth of another gear.
Pitch Circle Diameter (PCD): An imaginary circle on a gear where the teeth engage. It's used for calculating gear ratios. The gear ratio (GR) is a comparison of the number of teeth on two meshing gears, or a comparison of their speeds. It indicates the mechanical advantage gained by the gear system.
Formulae: Based on number of teeth: GR = (Number of teeth on driven gear, T_driven) / (Number of teeth on driver gear, T_driver)
Based on speed of rotation: GR = (Speed of driver gear, N_driver) / (Speed of driven gear, N_driven) (Note the inverse relationship for speed compared to teeth)
Worked Example 1: Calculating Gear Ratio (Teeth) A driver gear has 20 teeth, and it meshes with a driven gear that has 60 teeth. Calculate the gear ratio.
Solution: T_driver = 20 teeth T_driven = 60 teeth GR = T_driven / T_driver GR = 60 / 20 GR = 3 This means for every 3 rotations of the driven gear, the driver gear rotates 1 time.
Correction based on speed relation: GR = T_driven / T_driver = 60/20 =
3. This means the driven gear will rotate 1/3 as fast as the driver gear. The driver has to rotate 3 times for the driven to rotate once. To achieve an output speed of 1/3 of the input speed, the gear ratio (N_driver/N_driven) would be
3. Worked Example 2: Calculating Driven Speed If the driver gear from Example 1 (20 teeth) rotates at 300 revolutions per minute (rpm), what is the speed of the driven gear (60 teeth)?
Solution: N_driver = 300 rpm T_driver = 20 teeth T_driven = 60 teeth Using the relationship: (T_driven / T_driver) = (N_driver / N_driven) 60 / 20 = 300 / N_driven 3 = 300 / N_driven N_driven = 300 / 3 N_driven = 100 rpm The driven gear rotates at 100 rpm.
Bicycles and Motorcycles: Gears are prominently used in bicycles to allow riders to adjust the effort needed to pedal, especially when climbing hills or accelerating. Different gear combinations (front chainrings and rear sprockets) provide varying gear ratios for speed or power. In motorcycles, the gearbox uses gears to transmit power from the engine to the wheels, allowing the rider to select appropriate speeds for different riding conditions on Nigerian roads.
Local Grinding Machines/Milling Machines: Many small-scale businesses in Nigeria rely on grinding machines for processing food items like maize, beans, palm kernels, or cassava. These machines often employ gearboxes to reduce the high-speed output of the electric motor to a slower, more powerful rotation required for the grinding mechanism. Understanding gear ratios helps in appreciating why these machines can crush tough materials. Vehicle Transmissions (Cars, Lorries, Tractors): The most complex gear systems are found in vehicle transmissions. From changing gears in a car to drive at different speeds, to the differential gears that allow the rear wheels of a truck to turn at different speeds when cornering, gears are indispensable. This directly relates to the vast number of vehicles on Nigerian roads and their maintenance. Agricultural tractors also heavily rely on robust gear systems to deliver power efficiently to ploughs and other farm implements in fields across the country.