SEED Guides Unit Selection - Gearbox < >

A 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 3 Typical Features of a Gearbox Casing

Figure 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;

Casings are generally cast or fabricated in ferrous or light alloys and an important consideration in casing design is shaft orientation. Although gearboxes are usually designed to be as compact as possible the overall dimensions and location of mounting positions will vary widely between manufacturers.

Casings must be accurately located and securely fastened to their foundations in order to maintain rigidity in support and safety and reliability in operation. They must also be suitably ventilated to allow dissipation of heat.

The main PERFORMANCE AND GEOMETRICAL factors concerning gearboxes covered in this Guide are shown in Figure 4.

Spur gearboxes 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.

Spur boxes may have single or compound ratios but for each stage the speed reduction is limited to about 6A. The highest peripheral speed of a spur gear is also limited, because of noise generation, to about 20 m/s. This limits the input rotational speed according to gear size.

Helical gearboxes 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.

Epicyclic gearboxes 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.

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.

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

Either straight cut or spiral bevel gears may be specified depending on the power, speed and speed ratio. It is possible to specify a variety of shaft arrangements with more than one output shaft if geometry demands and permits.

Worm gearboxes 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.

The high proportion of tooth sliding results in heat generated so casings are often provided with 'fins' to enhance heat dissipation. There is a consequential reduction in mechanical efficiency.

Spiroid gearboxes 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.

Crossed helical gearboxes are the general case of which worm boxes are a special case. They contain helical gears which are designed to mesh between shafts which are skewed but do not intersect. The speed ratio depends on the helix angle of both gears, which may be different, and within the range 0š to 90š. The sum of the individual helix angles must be equal to the shaft skew angle.

Power capacity of crossed helical gears is severely limited and they are often found on auxiliary drive trains. Mechanical efficiency is also significantly inferior to helical boxes so heat generation can be a problem.