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Gearing Systems

The invention of pulleys and gearing systems is subject to some mystery, but it is known that by the 1st century Â BCE that the mechanical advantage in using gears was well documented. People such as Archimedes, Vitruvius and Hero of Alexandria all worked at improving these mechanical systems.

A simple spur gear system comprises a pinion A of small diameter having possibly 20 teeth engaging with a larger one B having perhaps 80 teeth. Thus the pinion will rotate at 4 times the speed of the wheel. In spur gears the efficiency may be as high as 98% and therefore next to no power is lost in the transmission. The power P transmitted is proportional to the rotational speed S and the torque or turning action T. Thus P = constant Ã— T Ã— S, P = constant Ã— TA Ã— SA, P=constant Ã— TB Ã— SB. Thus TB=TA SA/SB But SA/SB=Â¼ so TB = 4 Ã— TA This illustrates the use of gears by changing the rotational speed to a lower value a higher torque is transmitted.

The gear box of a motor car has 4 or 5 sets of gears for forward motion and usually one for reverse. The feature of a single pair of gears in mesh is that the shafts turn in opposite directions. Thus to get shafts to turn in the same direction an intermediate shaft is used carrying for instance wheel B and another pinion C which engages with wheel D. In this way pinion A and wheel D rotate in the same direction, and this concept is used to produce forward or reverse movement in the automobile gear box.

In cars of traditional design the drive from the gearbox is transmitted by the propeller shaft to the rear axle, which is perpendicular to it. The special type of pinion and gear wheel is used here called a hypoid drive. Each wheel has teeth which are spiral in form and special oils are needed to reduce friction and wear as the teeth slide over each other.

In large ships driven by steam turbines the economical recovery speed of the turbine is much faster than the economical recovery speed of the propeller. The mismatch in speeds is accommodated by reduction gears where the pinion may be 0.5 metres in diameter and the gear wheel 3 metres in diameter. These gears and those in an automobile require specialist machines for their manufacture since each tooth of the pinion must be identical and match identically the teeth on the gear wheel. Errors in the profiles of the teeth lead to noise and vibration which may break the teeth or fracture the shafts. AA

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