Week 4 – Jss 2 Third Term Basic Technology Lesson Notes

Week 4. gears A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape.^[1] Two or more meshing gears, working in a sequence, are called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack, thereby producing translation instead of rotation.
The gears in a transmission are analogous to the wheels in a crossed, belt pulley system. An advantage of gears is that the teeth of a gear prevent slippage.
When two gears mesh, if one gear is bigger than the other, a mechanical advantage is produced, with the rotational speeds, and the torques, of the two gears differing in proportion to their diameters.
In transmissions with multiple gear ratios—such as bicycles, motorcycles, and cars—the term “gear” as in “first gear” refers to a gear ratio rather than an actual physical gear. The term describes similar devices, even when the gear ratio is continuous rather than discrete, or when the device does not actually contain gears, as in a continuously variable transmission.^[2]

Gear types

Spur GearThe most common and easy to produce parallel shaft cylindrical gears. Of a pair of gears, the larger one is called a gear and the smaller one a pinion.

Helical GearQuiet and able to transmit larger torque than spur gears. Cylindrical gears with spiral shaped tooth trace.

Gear RackChanges rotary motion to linear motion. A set consisting of rectangular or circular rod shaped gear with mating small gear.

Bevel GearCone shaped gears used in intersecting shaft applications. There are also bevel gears with spiral shaped tooth trace called spiral bevel gears.

Spiral Bevel GearCone shaped gears used in intersecting shaft applications. There are also bevel gears with straight shaped tooth trace called straight bevel gears.

Screw GearUsed in offset shaft application. Shape wise, they are the same as helical gears.

Miter GearType of bevel gears in which the pair is made of same number of teeth and used where speed reduction or increase is not needed.

Worm GearUsed when a large speed reduction is needed. Worm and worm gear set. Normally, different materials are used for worm and worm gear.

Internal gearGear teeth are cut on the inside surface of hollow cylindrical forms and used in planetary gear systems. The gear teeth are cut using gear shaper machines.