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11/09/2007 - Gears

Gears are mechanical components widely use for power transmission between different axles, parallel and not parallel. The coupling between two toothed wheels is named gear. Between the various systems for power transmission, gears are characterised from higher efficiency (until 98%), higher reliability and duration, and more compact dimensions.
Using wheels of different dimensions allow to have a speed reductions and then an increase in the transmitted torque or, vice versa, a speed increase and a torque decrease,
This effect is a consequence of the fact that the peripheral speed of the wheels, in their contact point, must be the same (with no sliding), condition that leads to:



Where W are the angular speeds, T the transmitted torques and n the teeth number of each gear,
There are several types of gears (Figure 1): spur gears, helicoidal gears, worm gears, etc. For racing applications, spur gears are usually adopted, because they have higher transmission efficiency (but also higher noise level, but this is of very little interest in racing).
Toothed wheels geometry is very complex (Figure 2). Teeth are shaped in such a way to reproduce a circumference involute, that is the curve obtained by the rolling of a straight line on a circumference (known as base circumference). This shape offers some advantage like the easiness of shaping, the possibility to tolerate an assembly error (then functioning axles different from design axles) and the constancy of the transmission ratio.
Concerning toothed wheels manufacture, these can be produced by means of milling or by envelope cut.
In the first case, circular mills are used, and this mills are constructed in such a way that every section made with a plane passing for mill axle, reproduce the space between a tooth and the next one. In this way, the mill can be sharped again without having modifies in tooth geometry. The principal disadvantage of using mills for wheels shaping is that there is the need for a different mill for each wheel module and for each number of teeth. However, if high tolerances are not needed, a mill can be used to cut even wheels with a larger teeth number.
Envelope cut is instead the method that allow to achieve the greatest manufacturing precisions. There are several kind of machines that can perform an envelope cut and the cutter can be a rack (Figura 3) or a Fellows cutter (Figura 4). In the first case, the rack has a reciprocating motion, while the wheel has a composite motion of rotation and translation, in order to reproduce future meshing conditions. Fellows cutter is instead a pinion with a diameter smaller than that of wheel, that cuts the wheel by slotting.
Again, wheels can be shaped using a hob (Figura 5), that is a particular kind of helicoidal mill.
A large number of materials can be used for wheel construction, ranging from plastic, for simple applications (i.e. household appliances) to steel, usually thermally treated (cemented, quenched, etc.), to cast iron.

Figura 5: A typical hob for wheels shaping.

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