Gearing up13 November 2014

Choice of gearbox type, not just sizing, is critical to the performance, efficiency, reliability and cost of any application. Brian Tinham takes advice from Brevini sales manager Dave Brown

Choosing the right gearbox is critical to optimising the performance, efficiency, reliability and lifecycle cost of any industrial application. An obvious statement, perhaps, but how many of us put in the time and effort? Are we even up to speed with what's available, and the pros and cons?

A selection process should not be only about determining the primary functions – reduction ratio (the relationship between input and output speed), input speed and torque. As industrial gearbox specialist Brevini's Dave Brown says, what matters just as much is understanding the whole duty cycle.

"Only by defining the frequency and details of start/stops, variations in running torque and speed, etc, can you accurately design a solution that will perform for the life of a machine," he advises. He also points to the importance of considering the location, and factors such as ambient and environmental conditions, space restrictions, mounting arrangements, weight limitations, noise issues and maintenance requirements.

That's before you get into specifics such as shaft alignment, efficiency goals, permitted backlash (due to the essential tolerance between meshing gears) and the required lifespan – all of which continue the process of refining your choice, at least in terms of a preference of certain gearbox types over others.

So what's out there? Well, the most basic remain spur (straight cut) gears, with teeth parallel to the axis of rotation. These are still economical and equally good for both high- and low-ratio applications. And, as Brown points out, they can also be used in multiple stages, to achieve high gear ratios. "However, the straight cut design means the gears mesh along one tooth at a time, which can cause increased wear and noise, especially at higher speeds," he warns – in contrast to the rolling or sliding contact associated with other technologies.

Slanting the teeth offers a refinement that enables multiple teeth to mesh simultaneously in the gearbox. The result is smoother motion, reduced noise (primarily because of the increased contact area) and increased torque transmission, typically by around 10–15%.

However, a helical gear also induces axial thrust, which has to be accommodated either by installing thrust bearings or incorporating twin helix stages to counteract the forces – or a double helix gear with the teeth set in a herringbone format. The latter design involves more complex manufacturing, though, so carries a price premium.

Rotating the axis
So far, so good, but, with these designs, transmitted power remains in parallel axes. For the many applications that need the axis to rotate through 90°, as Brown says, we look conventionally to bevel gear sets or worm drives.

Teeth in bevel gears can be straight cut, but most are spiral cut for the design's reduced noise and higher efficiency. Most common are Gleason and Klingenberg types, with spiral bevel gears most attractive when the application requires more than 7.5kW with a reduction ratio exceeding 20:1. Worm gears, meanwhile, have enjoyed poor press because of their low efficiency. However, it's worth noting that there have been significant improvements of late, making them well worth considering, particularly for reduced torque applications at lower reduction ratios.

That said, planetary gearboxes – which take their name from the arrangement of a central sun with (typically three) orbiting planet gears and the outer ring gear (annulus) – offer a more elegant solution. Splitting loads across multiple planet gears enables greater torque capacity for a given size, while their design symmetry also cancels out gear separation thrusts.

Brown also makes the point that planetary gearbox designs can be combined with a bevel or helical gear system, which then offers the best of both worlds. And while this type of gearbox is currently common in heavy industrial applications – where reliability, efficiency and total cost of ownership are important – it is increasingly also worth considering them for lighter duties.

Brian Tinham

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