5 April 1996

Its déjà vu for half-tracks

By Andy Collings

HALF-TRACKS are back. A system which found favour with a number of tractor manufacturers some 30-plus years ago, has now been reintroduced by Boston-based Richard Larrington.

But unlike its mid-century predecessors, the Larrington design draws on modern rubber track technology – and is used on a four-wheel drive tractor.

"Weve already had experience with the construction of tracked vehicles through our range of trailers, but it was the result of a series of customer enquiries which led us to develop a tractor system," explains Mr Larrington.

The MTS-80 track system has been designed to fit a wide range of tractor models, although, for its first airing it has been fitted to a New Holland 7740.

First point to note is that the all-steel drive wheel is the same size as the tyred wheel it replaces – a situation which means four-wheel drive can continue to be used without any expensive re-gearing.

Tracks run forwards from the drive wheel to a smaller jockey wheel while a third, centrally mounted wheel, keeps the whole length of the track on the ground. Tensioning is achieved through the use of hydraulic rams.

Conversion of the tractor – reckoned to be a factory or dealer-based operation – adds another two tonnes to all-up weight, a beefy subframe contributing in no small way. But, in the case of the 7740 at least, ground pressure is reduced from about 44psi to 9.75psi when 35cm (14in) wide tracks are used. There are options for 60cm and 90cm (24in and 36in) tracks.

The track itself comprises a rubber belt, with a claimed breaking strain of 10,000t, to which strengthened rubber blocks are attached via aluminium channelling. Steel plates, with welded guide lugs are fixed to the inside of the belt.

In operation, the track is driven initially by the friction between the drive wheels own rubber blocks or, if the going gets tough, by these blocks intermeshing with the gaps formed between the blocks attached to the outside of the belt. Mr Larrington describes it as a "semi-friction" drive.

"We are still unsure just how much tension to put on the tracks," he says. "Clearly, there needs to be some degree of slip between drive wheel and track – the tractors transmission could be in danger in some circumstances."

Adjustable track height

Another feature of the design is the ability to hydraulically raise or lower the front jockey wheel slightly. For travelling on the road it is in its highest position to keep a percentage of the track clear of the surface and reduce wear. For dry or normal field operations it is lowered to the level position with all the track in contact with the ground.

But for sticky, difficult conditions, the jockey wheel can be forced even lower, to place added weight on the tracks and create a more positive drive.

Mr Larrington admits he has yet to try his system in the field but is convinced the combination of tracks and front-wheel drive has a lot to offer vegetable and root crop growers who often need to harvest in difficult conditions to meet market demands.

"It is a relatively cheap way of creating a tracked vehicle," he says. "And in some conditions, its performance could actually exceed that of a completely tracked machine. Once the system has been purchased it can be swapped on to a new tractor when the time comes for a replacement."

Price of a 35cm (14in) track system is £16,750, rising to £19,250 for the 90cm (36in) tracks. &#42

Half-track detail:

Note the construction of the track and drive wheel. Part of the units subframe can be seen behind the steps.

Turning is claimed to pose few problems for the Larrington half-track conversion. Although adding another 2t to the weight of the tractor, overall ground pressure is reduced from 44 to 9.75psi.