Stephenson:Neal:Quicksilver:271:Daniel saw gears, cranks, shafts. (Trismegis2)

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Trismegis2 writes: ...and you thought Neal was making this up about Leibniz.

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Wikipedia: Gears

A gear is a toothed wheel designed to transmit torque to another gear or toothed component. The teeth of a gear are shaped to minimize wear, vibration and noise, and to maximize the efficiency of power transmission.
Spur gears found
on a piece of
farm equipment

Different-sized gears are often used in pairs, allowing the torque of the driving gear to produce a larger torque in the driven gear at lower speed, or a smaller torque at higher speed. The larger gear is known as a wheel and the smaller as a pinion. This is the principle of the automobile gearbox. As a gearbox is not an amplifier or a servo system, the amount of power delivered by the output gear or shaft will never exceed the power applied to the input gear, regardless of the gear ratio. There is actually a loss of power due to friction.

The most common type of gear wheel, spur gears, are flat and have teeth projecting radially and in the plane of the wheel. These gears can be fitted only to parallel axles. Helical gears offer a refinement over spur gears. The teeth are cut at an angle, allowing for more gradual, hence smoother meshing between gear wheels. A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accomodated by appropriate thrust bearings. Double helical gears overcome this problem by having teeth that are 'V' shaped. They can be directly interchanged with spur gears without any need for different bearings. Beveled gears have angled teeth, allowing torque to be transmitted between non-parallel but intersecting axles. If the axles are skewed, i.e. non-intersecting, then a worm gear can be used. This is a gear that resembles a screw, with parallel helical teeth, and mates with a normal spur gear. The worm gear can achieve a higher gear ratio than spur gears of a comparable size.

Torque can be converted to linear force by a rack and pinion. The pinion is a spur gear, and mates with a serrated bar that can be thought of as a spur gear with an infinitely large radius of curvature. Such a mechanism is used in automobiles to convert the rotation of the steering wheel into the left-to-right motion of the tie rod(s).

A crown gear is a special form of bevel gear which has teeth at right angles to the plane of the wheel, allowing it to drive axles at right angles to its own. A variation of this mechanism is used in the differential gear, a complex arrangement of gears that transmits power to two axles moving at variable speeds, such as those on a cornering automobile.

A contrate wheel has teeth at a right angle to the axis and meshes with a straight cut spur gear or pinion (used in clocks, some instrumentation, wind-up toys and Meccano).
A crown gear

Simple gears suffer from backlash, which is the error in motion that occurs when gears change direction. When moving forwards, the front face of the drive gear tooth pushes on the rear face of the driven gear. When the drive gear changes direction, its rear face is now pushing on the front face of the driven gear. There is slight 'slop' in any gearing where briefly neither face of the driving gear is pushing the driven gear. This means that input motion briefly causes no output motion. Assorted schemes exist to minimize or avoid problems this creates.

In some machines it is necessary to change the gear ratio to suit the task. There are several ways of doing this. For example:

manual gearbox ('stick shift' in the US)
automatic gearbox
derailleur gears
hub gears (also called epicyclic gearing or sun-and-planet gears)
continuously variable transmission
infinitely variable transmission

The tooth form used for most applications is involute but there are other tooth forms such as cycloidal (used in mechanical clocks) or rack (used in automobile steering).

Wikipedia: Crank (mechanism)

A crank is a bent portion of an axle, or shaft, or an arm keyed at right angles to the end of a shaft, by which motion is imparted to or received from it; also used to change circular into reciprocating motion, or reciprocating into circular motion. Familiar examples of a crank for manual use include the crank on a manual pencil sharpener and the cranks on the pedals that drive a bicycle.

Cranks were formerly common on some machines in the early 20th century; for example almost all phonographs before the 1930s were powered by clockwork motors wound with cranks, and internal combustion engines of automobiles were usually started with cranks before electric starters came into general use.

Wikipedia: Crankshaft or shaft

A crankshaft

The crankshaft is that part of an engine which translates linear piston motion into rotation. Generally more than one piston is attached to the crank to provide a smoother delivery of power to the rotating part, though many small engines, such as those found in mopeds or garden machinery, use only a single piston.

The configuration and number of pistons in relation to each other and the crank leads to straight, V or flat engines.

Some (outdated) aircraft engines had a fixed crankshaft and rotating cylinders in a star configuration - rotary engine.

In the Wankel engine, the rotors drive the eccentic shaft, which can be considered the equivalent of the crankshaft in a piston engine.