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Selection criteria in gearbox design

Selection criteria in gearbox design

Selecting the correct design of gearbox for your application can be a critical decision that will affect performance, efficiency, reliability and cost. So a good understanding of the principles and applications is crucial. Dave Brown of Brevini offers some technical guidance.

Gears have been in use since the times of Archimedes and Aristotle, and they continue to play an essential role in mechanical systems across the world. While the basic principles remain the same, the technology has advanced considerably, with different gear arrangements offering a variety of benefits for the right application.

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 1500rpm, and a driven machine at 500rpm, the ratio is as follows:

Input:Output = 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 in order to achieve the desired output in terms of speed, torque, efficiency, size, noise, lifetime and maintenance requirements. The type of gear arrangement is defined by the design of the gear teeth and how they mesh together.

The most basic type is the spur, or straight cut, gear which has teeth that are parallel to the axis of rotation. This design offers economical performance and is equally 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. However, the straight cut design means that the point at which the gears mesh occurs along one tooth at a time which can cause increased wear and noise, especially at higher speeds. The noise is caused by the single point of contact between the drive and driven gears at the start of gear mesh. This is in contrast to the rolling or sliding type of contact associated with other gear technologies.

A refinement of the spur gear 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. This design has a greater tooth contact area, which increases the amount of torque that can be transmitted by 10-15%, while still maintaining very good 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 as well as assemble and so will carry a price premium compared to the spur gear.

So far, the gear arrangements have transmitted power in parallel axes, but a common requirement is to redirect the rotational axis by 90° which usually involves bevel gear sets or worm drives. The latter have seen significant improvements in efficiency, especially for reduced torque applications at lower ratios and they 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. The most common styles are 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. The final main group of gear arrangements is the planetary gearbox which 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 three), the torque capacity of a planetary gearbox is very favourable against other solutions. Additionally, the symmetry of the design means that most 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 significantly more 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 enables the advantages and benefits of the different technologies to be combined to optimise the solution for the demands of a specific application. This type of compact gearbox is common in many of the heavier industries where reliability, efficiency and total cost of ownership are important factors.

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. Specific information regarding the reduction ratios, input speed and torque are critical. However, it is also important to define the real duty cycle for the equipment, defining frequency and details of the start/stops, variations in the running torque and speed, and so on. Only with a full duty cycle can you accurately select and design a solution that will perform for the required life of the machine.

The location of the application is also 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, which is the space between two meshing gears may need to be specified for particular applications. Every gearbox will have some backlash designed into it, to allow lubrication of the gears and prevent the gearbox from locking up. Finally, there are the more specific design characteristics such as shaft alignment, efficiency and expected lifespan, which can influence certain design choices.

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