Gear
Gears
History and Etymology of Gears
– Gears date back to the 4th century BC in China and were preserved at the Luoyang Museum of Henan Province.
– The Antikythera mechanism, built between 150 and 100 BC, is an early example of a complex geared device used to calculate astronomical positions.
– Aristotle mentioned gears around 330 BC, specifically in wheel drives in windlasses.
– Gears were used in water raising devices by Philon of Byzantium in ancient Greece.
– Gears were developed by Archimedes in 3rd-century BC Ptolemaic Egypt and were connected to the mechanics of the Library of Alexandria.
– The word ‘gear’ is derived from Old Norse ‘g√∏rvi,’ meaning apparel or gear.
– The specific mechanical sense of gears as parts by which a motor communicates motion originated in 1814.
– The term ‘cog’ was first used in the 14th century to refer to a tooth on a wheel.
– Historically, cogs were made of wood and were used when large metal gears couldn’t be cut.
– Wooden cogs were often made of maple wood and were used in paper mills and grist mills.
Comparison with Drive Mechanisms
– Gears provide an advantage over other drives in precision machines like watches due to their definite ratio.
– Gears require fewer parts compared to other drives when the driver and follower are proximal.
– However, gears are more expensive to manufacture and have higher lubrication requirements, leading to higher operating costs.
Types of Gears
– External gears have teeth formed on the outer surface of a cylinder or cone.
– Internal gears have teeth formed on the inner surface of a cylinder or cone.
– Bevel gears have an internal gear with a pitch angle exceeding 90 degrees.
– Spur gears, also known as straight-cut gears, are the simplest type of gear.
– Helical gears have teeth that are not straight-sided but have a helix shape.
External versus Internal Gears
– External gears cause output shaft direction reversal.
– Internal gears do not cause output shaft direction reversal.
– Internal gears are used for bevel gears with a pitch angle exceeding 90 degrees.
Helical Gears and Configurations
– Helical gears offer a refinement over spur gears.
– The teeth are set at an angle, forming a segment of a helix.
– Can be meshed in parallel or crossed orientations.
– Engage more gradually, resulting in smoother and quieter operation.
– Used in high-speed applications and for noise abatement.
– Parallel configuration: shafts are parallel to each other, most common orientation for helical gears.
– Crossed configuration: shafts are non-parallel, gears are sometimes known as skew gears.
– Disadvantages of helical gears include resultant thrust along the axis of the gear and sliding friction between meshing teeth.
– Herringbone gears and double helical gears can address these disadvantages.
– Double helical gears overcome axial thrust presented by single helical gears by using a double set of teeth slanted in opposite directions.
A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called cogs), which mesh with another (compatible) toothed part to transmit rotational power. While doing so, they can change the torque and rotational speed being transmitted (in inverse proportion) and also change the rotational axis of the power being transmitted. The teeth on the two meshing gears all have the same shape.
The basic principle behind the operation of gears is analogous to the basic principle of levers. Meshing gears of different diameters produce three changes — (i) a change in torque, creating a mechanical advantage, (ii) an inverse change in rotational speed and (iii) a change in the sense of the rotation, a clockwise rotation becoming an anti-clockwise one and vice-versa. The ratio of the output torque to the input torque is equal to the ratio of the diameter of the output gear to that of the input gear τout⁄τin = diaout⁄diain. This is called the gear ratio. The ratio of the output rotational speed to the input rotational speed is equal to the inverse of the ratio of the diameter of the output gear to that of the input gear ωout⁄ωin = (diaout⁄diain)-1 = diain⁄diaout. The diameters of the gears are measured at a point between the root and tips of the gear teeth called the pitch circle.
A gear may also be known informally as a cog.
Two or more meshing gears, working in a sequence, are called a gear train or a transmission. 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. In transmissions with multiple gear ratios—such as bicycles, motorcycles, and cars—the term "gear" (e.g.,e "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 (CVT). Sometimes a CVT is referred to as an "infinitely variable transmission".
Furthermore, a gear can mesh with a linear toothed part, called a rack, producing movement in a straight line instead of rotation (movement in a circle). See Rack and Pinion for an example.
GD Garage Door Service MN • 651-373-0970