Gearing up:  Gearbox design matters

Gears have been in use since the times of Archimedes and Aristotle, and continue to play an essential role in mechanical systems. While the basic principles remain the same, technology has advanced considerably.

 

A gearbox is most commonly designed to provide a reduction in speed from a prime mover, such as an electric motor, and deliver the necessary speed and torque for a particular application. The gear ratio is the relationship between input and output speed, with output always defined as unity. For example, if we consider an electric motor as the prime mover – with a speed of 1500 rpm – and a driven machine at 500 rpm, the ratio is as follows: Input : Output or 1500 : 500  =  3.00 : 1. In simple terms, three revolutions of the input shaft will produce one revolution of the output shaft.

 

Gearboxes may use a variety of gear arrangements to achieve the desired output in terms of speed, torque, efficiency, size, noise, lifetime and maintenance requirements. The arrangement is defined by the design of the gear teeth and how they mesh together.

 

Straight cut gears

The most basic type is the spur, or straight cut, gear which has teeth parallel to the axis of rotation. It offers economical performance and is good for both high and low ratio applications. The spur gear can also be used in combination, or multiple stages, to achieve high gear ratios.

 

The straight cut design means the point at which the gears mesh occurs along one tooth at a time can cause increased wear and noise, especially at higher speeds. This is in contrast to the rolling or sliding type of contact associated with other gear technologies.

 

A refinement is to slant the teeth in relation to the axis of rotation, which allows a more gradual engagement of the meshing teeth for multiple teeth to be engaged simultaneously. This provides a smoother motion with reduced noise. The greater tooth contact area increases the amount of torque that can be transmitted by 10-15%, while maintaining efficiency.

However, the design of a helical gear induces axial thrust in the gearbox that has to be accommodated either by installing thrust bearings or changing the gear design to incorporate twin helix stages, which will counteract the axial forces, or a more complicated double helix gear. This is a gear with the teeth set in a herringbone arrangement - but this design of gear is more complicated to manufacture and assemble and so will carry a price premium.

 

Bevel gear arrangements

A common requirement is to redirect the rotational axis by 90° which usually involves bevel gear sets or worm drives. The latter have seen improvements in efficiency, especially for reduced torque applications at lower ratios and can still represent good cost efficiency in some applications.

 

The orientation of the teeth in bevel gear sets can be straight cut, but the more common style is a spiral cut gear which offers improved noise levels and efficiency. Common styles include Gleason or Klingenberg and, in terms of costs, the spiral bevel gear option becomes more attractive when the application requires more than 7.5kW with a ratio above 20:1.

 

Planetary designs

The planetary gearbox takes its name from the normal gear arrangement consisting of a central sun gear, the orbiting planet gears and the outer ring gear, or annulus.

By splitting the loads through multiple contacts between the planet gears (typically 3), the torque capacity of a planetary gearbox is very favourable against other solutions.  Additionally, the symmetry of the design means many of the gear separation loads associated with other solutions are self cancelling in the planetary design. These factors combined mean that the planetary solution can be compact and more cost effective in many applications.

 

The benefits of the planetary gearbox can be combined with a bevel or helical gear system which then offers the benefits of both designs. This type of compact gearbox is common in many of the heavier industries where reliability, efficiency and total cost of ownership are important factors.

 

Crucial duty cycles

Many factors influence the design of a gearbox for a particular application. The key is determining as many of the crucial factors as possible before the selection process begins.

 

Location of the application is important as the design may have to consider ambient and environmental conditions, space requirements, mounting arrangements, weight, noise and maintenance requirements. In addition, the backlash (the space between two meshing gears) may need to be specified.

 

Finally, more specific design characteristics such as shaft alignment, efficiency and expected lifespan, can influence choice. Having gained a basic understanding of the more common gearbox designs, it is possible to appreciate the alternatives for a particular application. The next step is to discuss more specific requirements with engineers who specialise in this field.