Planetary wheel drives offer an off-road edge

The demands to reduce size and weight, and yet achieve improved price/performance ratios, are familiar to designers of mobile equipment, worldwide. In their attempts to meet these demands, designers have focused on the area of drive systems, seeking simpler and more efficient alternatives to the conventional axle drive systems that are still used widely. Increasingly, engineers are finding the answers to their questions in this area with planetary wheel and track drives.

The reason for this lies with the planetary design.

Comprising several smaller planetary gears around an input pinion (instead of one larger gear running to one side), this arrangement offers a number of major advantages including: high efficiency - usually 97-98% per stage; reduced size - planetaries facilitate multiple reduction stages in a very compact space; and increased reliability - the result of a design that reduces radial loads.

In addition, because the torque being transmitted at any time in a planetary is shared between multiple sets of teeth on the primary drive pinion, torque capability is greatly increased.

Brevini has developed the planetary principle, proven in its range of industrial gearboxes, into highly compact wheel and track drives.

The wheel drives are used widely on such vehicles as agricultural machines, access platforms, fork lift trucks, sweepers and low speed wheeled vehicles; and the track drive units are specifically designed for use on tracked vehicles, such as excavators and earthmoving machines, weighing from 1.5 to 40t.

Both the wheel and track drive units permit a much neater and more compact design solution than axle drives and provide greater clearance for rough terrain vehicles.

The wheel drives offer the ability to accept directly mounted standard wheel rims and packaged hydraulic motors; and the track units are designed to mount inside the track sprockets of vehicles.

The wheel drive units employ the highly efficient combination of a planetary gear unit and high-speed hydraulic motor.

This combination is much more economical than using large slow-speed hydraulic motors on their own, and offers significant operating advantages.

In the event of hydraulic fluid contamination only the inexpensive package motor need be replaced, and this can be removed and refitted in minutes.

And the gearbox/ motor combination is capable of the very low speeds required when, for example, inching vehicles into position.

The wide range of ratios available with the wheel drive units makes it easy for designers of mobile plant to adapt the reduction unit to the most suitable hydraulic transmission.

In addition, further flexibility is provided by the ability of the wheel drives to accommodate the mounting of all types of orbit, geared and axial piston type hydraulic motors into their universal input.

Also included in the comprehensive design specification is a mechanical disconnect device.

This is manually operated from the hub end-plate and enables a vehicle to be towed in an emergency.

Complementing this device is the additional option of an integral multiple-disc negative parking brake, which is spring applied and hydraulically released.

This functions as an emergency fail-to-safe brake should the hydraulic pressure fail and is an important safeguard for access vehicles, as they are sometimes manoeuvred with the platform fully raised.

Where the nature of the application is more heavy duty, wheel drives give way to track units.

These, in common with wheel drives, are rotary-case-type planetary units that use multiple gear stages to reduce size.

They are available with integrated hydraulic motors, which are either axial piston or orbital types, and can be fitted with options such as brakes and a disconnect device to aid towing.

Thanks to the inherent capacity of planetary gear trains to deliver high reduction ratios in small packages and to transmit several times the torque of similarly sized, conventional gear units, the integrated track drive package is remarkably compact and lightweight, and requires little installation space.

In addition, the planetary drive is around 98% efficient and, critically, is able to provide extremely low speeds without any loss of efficiency.

A recent application highlighting the operational benefits of planetary wheel drives is in beet harvesting.

Dennis Van Arkel, Sales Manager at Brevini Benelux explains: 'Beet harvesting is a tough, arduous and generally very dirty process; environmental conditions are rarely optimal, with harvesting machines having to work either in rain-sodden, muddy fields, or in extremely dry conditions where dust poses a constant problem to equipment'.

'Obviously, these conditions place a premium on the performance of power transmission equipment used in the operation of the harvesters'.

'Yet despite all the rigours, Brevini has established a supply partnership with manufacturers of beet harvesters, based on quality, reliability and after sales service, which goes back nearly two decades'.

Brevini supplies Agrifac/WKM and Vervaet, the market leaders in Holland, with wheel drives for use on their beet harvesters.

Both companies use the drives for several reasons.

First, because they are wheel mounted in a package with a hydraulic motor, they permit a much simpler design solution than axle drives, and provide improved clearance over rough ground.

Secondly, the absence of an axle means that manufacturers have the flexibility to fit ultrawide terra tyres to spread loading over a wider area.

Thirdly, with no standard axle to consider, the space under the harvester is free and can be used to transport beets from front to back of the equipment.

Forthly, the flexibility as regards brakes: the wheel drives can be supplied fully equipped with dynamic disc and parking brakes enabling the harvester manufacturer to conform easily to the specific laws in each country where the equipment is sold.

In another application of mobile planetary technology, Richard Larrington is making its giant dumper trucks lighter and more manoeuvrable using Brevini track drives.

The Brevini units enable the track-driven Predator vehicles to turn on the spot and transport 20t of peat across steep and unstable terrain at speeds in excess of 8km/h.

Thanks to an innovative design, the Predator can transport peat up and down 20-degree inclines and boggy terrain with loads in excess of 20t - more than four times that of conventional tractors.

The secret of the Predator's superior performance is two large rubberised tracks that spread the load over a large surface area and allow the vehicle to turn 360 degrees on the spot.

The tracks are driven by a pair of Brevini two-stage reduction drive systems, which provide the levels of torque control required to power and position the machine that can weigh in excess of 43t when fully loaded.

Jon Snaith, Sales Manager at Brevini UK explains: 'The steep and boggy terrain on which the vehicle was designed to operate, required the drive system to be track rather than wheel based as traditional tractors would often 'dig in', even when carrying less than a quarter of the load'.

'Also, in order to maximise the load carrying capacity of the Predator, the machine itself was designed to be as light as possible without compromising strength, making a two stage reduction wheel drive system a vast improvement over heavier axel based or larger direct drive systems'.

The drive units themselves consist of hydraulic motors integrated with Brevini planetary gearboxes, making them small enough to fit inside the Predator's tracks.

The compact motors produce rotational speeds of up to 2750rev/min, which is geared down to ratio of 40:1 by the Brevini boxes, providing levels of torque that can reach 38,000Nm.

Despite the high gearing, the Predator is capable of an impressive 13km/h unloaded and 8km/h at full carrying capacity.

As the Brevini track units are designed for use on large farm machinery such as crop harvesters, a dual braking function is provided as standard, thus enabling the Larrington Predator to be stopped hydrostatically using fluid pressure, and dynamically, via a disc brake inside the gearbox, which holds the vehicle while stationary.

This is an especially important feature when the machines are exported to countries where legislation requires dual braking in order for the vehicle to be road legal.