Planetary gearboxes are the preferred solution

The role of the planetary gearbox is set to grow and assume even greater importance, says Jon Snaith, Applications Engineering Manager, Brevini UK.

Planetary gears have been in motion for many years.

Their widespread use was pioneered by the American automotive industry in generations of automatic gearboxes.

The system has since been adopted for a broader range of mechanical applications predominantly in mobile and industrial power transmission applications.

Companies such as Brevini founded in Italy in 1960, essentially to produce planetary gearboxes have achieved considerable worldwide success with this technology.

The obvious advantages are the higher torque capacity, smaller size, lower weight and improved efficiency characteristics of a planetary design.

The small size and modular construction of planetary gearboxes also means that they can be assembled in several stages, providing high reduction capability from a highly compact package.

As such, planetary gearboxes are the preferred solution in many areas such as wheel and winch drives and also slewing drives for turning large diameter cogged items that require slow movement at very high loads.

This is typical of their application, in areas where more traditional 'worm and wheel' gear units would simply be too large or too heavy to use.

When compared with some larger worm gear units with high reduction ratios that may be only 60% efficient, the planetary solution is very much more efficient, at 98% efficiency per stage.

This increased efficiency is based on reduced friction; a planetary gearbox is driven by a centre shaft that is connected to a carrier plate carrying three smaller gear wheels.

The gear wheels run inside a toothed outer ring and drive a central gear cut around the central output shaft.

This means that the load is spread among many contact points around each gear.

Many, lighter contact points mean less friction, hence increased efficiency.

The planet wheels are set 120 degrees apart, achieving 'load equilibrium' and resulting in a balanced or 'zero force' within the geared components, which results in very low wear characteristics and high shock loading capacity.

The vital difference between a worm gear unit and a Brevini planetary is that a worm gear unit relies on a turning surface across the driven wheel's teeth to achieve motion, this inevitably causes wear and higher friction.

Planet gears themselves are floating on two rows of needle bearings and so have a very low coefficient of friction.

Less energy is required to turn them and so more of the driving force is converted into useful turning force after being geared down.

Because a planetary gearbox is smaller and lighter, up to half the size and 60% lighter than conventional heavy engineered gearboxes, it is tempting to suppose that it is not as strong.

The torque distribution internally between the gear components however, makes the unit far more resistant to shock loading and in most instances as strong as very much larger examples of alternative designs.

The situation is comparable to Formula 1 motor racing, for example, where a whole host of technological developments have proved that 'light and compact' can also mean 'strong', as a result machine builders from automated mobile bridges to conveyors and high-speed plant machinery are embracing planetary technology and enjoying the benefits.

Other industries such as water processing and materials handling are also using this type of gearbox to great effect because of its reduction capability and high reliability.

To assemble a worm gear, both screw and gear components have to be matched, ie a certain amount of tolerance is built in to suit the application.

This means that each unit has to be manufactured to match the application and is subject to the vagaries of the engineer who is making them.

A planetary gearbox is modular and so all components are manufactured to very finely controlled and consistent manufacturing tolerances.

Hence with Brevini planetary gearboxes there is less room for error as they are of a consistent standard.

It also means that lead times are very much lower on planetary units, hours rather than weeks.

At Brevini, planetary gearboxes are specified to suit each individual application with the aid of bespoke selection software developed with years of engineering experience.

The programme is used to effectively build up a picture of what is the gearbox doing, the demands on it and the expected performance.

The duty cycle is entered in first and can be any combination of torque, speed and time.

Environmental conditions are then considered, type of oil, ambient operating temperature, type of lubrication system, radial and axial loads etc.

The prime mover is also considered, ie an air-motor hydraulic motor etc.

The level of reduction is then entered; eg 100:1.

The programme then matches the smallest Brevini unit available that fits the application criteria; the engineer can then scroll upwards in size based on the life requirement of the unit, ie how many hours of operation.

A constant duty application say on a water pump could be ten years, or 100kh.

An intermittent task such as a winch on a crane might be 10kh for example.

The information provided by the computer selection is then added to the knowledge and experience of an engineer.

Elements such as application experience of extreme thermal conditions, difficult lubrication or intermittent shock load can be crucial to making the right choice.

The specification is applied to ISO DP6336 standard calculations for life expectancy.

In technical terms: "The load capacity of the planetary gear sets is calculated with a hertzian pressure and breaking strain in accordance with ISO DP6336 in relation to theoretical life".

Companies are using planetary gears not just for the weight saving, the reduction capability and the compact size, but also, in some cases because the units are weight balanced, ie in instances where a conventional gearbox is used the shaft is not in line with the bulk of the gearbox and its casing, hence there is an overhang and unbalanced weight to deal with.

Because planetary gearboxes operate around a central shaft, they can be used inline with turbines, pumps and wheel drives for example without obstructing other moving parts or fluid flow patterns.

The planetary design has also found great success in the water industry where the high gearing capacity and resistance to variable loading have made them extremely reliable in arduous and often remote locations which is essential in that sector.

The attributes of the planetary design that have led to its success so far, are also key elements for the future, as the irresistible move towards more efficient, lighter and more compact machinery continues, planetary power transmission solutions are increasingly providing the best match.

To illustrate this; recent energy efficiency requirements are now filtering down beyond the electrical components in industrial processes and reaching the mechanical elements, with an efficiency of 98% per gear stage, in certain applications a planetary gear can provide energy savings of up to 38% over alternative gearing designs.

It is fair to assume therefore, that, as energy saving assumes a greater role in our everyday lives, the role of the planetary gearbox will grow accordingly and assume even greater importance.

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