Different Types of Gear Box, their Applications & UtilizationA gearbox consists of a means of transmitting mechanical torque between two shafts with structural support between them. Normally it is contained within a casing which would provide the structural support and also have containment and safety functions.Most gearboxes are designed for speed reduction though some may be suitable for speed increasing duties. Some types are not suitable for reverse driving and the system may require the prevention of 'over-running'.Shafts are usually provided with a means of accepting and delivering torque in the form of a keyway or splines suitable for connecting to a coupling or to another unit. Shafts will have a limited protrusion from the casing.

Figure 3Typical Features of a Gearbox CasingFigure 3 shows typical features of a gearbox CASING which performs several functions: structural support of the shaft bearings and hence the gear loadings; transfer of torque reaction to supporting structure or further drive element; containment of lubricant and exclusion of foreign matter; provision of safety and noise barrier; dissipation of heat generated by friction; unitisation of assembly, thus aiding testing, installation, and maintenance; enhancement of visual qualities; Following shows the shaft orientations available for the gearbox types covered in this Guide.PARALLEL SHAFTSPERPENDICULAR SHAFTSSKEWED SHAFTS

SPURHELICALEPICYCLICBEVELWORMSPIROIDCROSSED HELICAL

1. SpurSpurgearboxes contain spur gears which have teeth cut parallel to the shaft axis and are only suitable for parallel shaft applications. However they facilitate the arrangement of a sliding gear ratio change. Input and output shafts may be arranged on the same side of the casing or opposite sides. For concentric input and output shafts an internal 'layshaft' is needed.

2. HelicalHelicalgearboxes have many characteristics which are identical to spur boxes, but as a result of tooth form their performance is enhanced in terms of power, speed ratio and peripheral speed. Their mechanical efficiency is marginally inferior due to a greater sliding contact at the gear tooth faces but this is rarely a problem. They are not suitable for a sliding gear change.

3. EpicyclicEpicyclicgearboxes are a versatile arrangement of spur or helical gears in which the input and output shafts are concentric and either shaft or the casing may be constrained to be the stationary element, the torque being transmitted between the other two. The three main elements are thus a 'sun' gear, a 'ring' gear and a number of 'planet' gears meshing with both.Wide ranges of speed ratio are obtainable from a given set of elements and very large reductions result from compounding stages. They tend to have high power/weight and power/bulk ratios and are available for a wide range of powers.

4. BevelBevel gearboxes are used for drives where shafts are not parallel but whose axes intersect. The most common intersection angle is 90 but other angles are possible. A right angle drive with a 1:1 speed ratio is sometimes called a 'mitre' box.

5. WormWormgearboxes allow high ratios of speed reduction within a single stage coupled with non-parallel, non-intersecting shafts. Reverse drive is not normally permissible and under some circumstances positive locking of reverse drive results.

6. SpiroidSpiroidgearboxes perform a similar function to worm boxes but the gears have characteristics which combine those of the bevel and worm gears. High powers and speed ratios are possible and mechanical efficiencies higher than worm boxes for equivalent ratios.

Harmonic drives consist of a gearbox which allows two gears with a large number of teeth to rotate such that a third element rotates according to the difference between the numbers of teeth on the gears.Torque capacity is high in relation to bulk and weight, speed ratios range between 60:1 and 250:1 and mechanical efficiency between 70% and 85%. Backlash is very small and can be totally eliminated with special units.