19 August 1997

Deere lays down rubber

The 8400T is the biggest of four rubber tracked tractors John Deere plans to bring into the UK next year. Geoff Ashcroft reports

"ITS not simply a matter of throwing away the front axle and fitting some tracks," explains John Deeres tractor specialist Gordon Day, about the new 8000T series rubber tracked tractors.

According to Mr Day, although based on Deeres 8100, 8200, 8300 and 8400 wheeled tractors, over 400 new parts have been used in the development of the 8000T series. The result is a 185-260hp tractor range with a price tag from £103,900 to £121,000.

Despite the £32,000 price premium compared to the wheeled alternatives, Deeres managing director Alec McKee reckons the tracked machines are more than a passing fad.

"I expect tracked models to account for about 40% of the 100 or so wheeled 8000 tractors we sell each year," explains Mr McKee. "We will use this autumn to gauge customer reaction before making the full range available next year."

Making up the range is the 185hp 8100T, 210hp 8200T, 230hp 8300T and 260hp 8400T, and from the front weights back to the rear of the powershift transmission, each tractor is the same as its wheeled sister.

Beyond this, its all change. The mid-chassis section is much beefier to carry each track frame. It is on this, that the entire tractors weight is supported. This centre section also carries the fuel tank and hydraulic pumps.

Running gear extends to front idler, three middle rollers and rear friction drive wheel, all wrapped in one-piece 60cm (24in) wide, moulded rubber tracks.

By adjusting the position of the track carriage on its mid frame, the 8000T series can run at settings of 1.72m, 1.82m, 1.93m and 2.23m (68, 72, 76 and 88in). Deere says a narrower 40cm (16in) track will be available to suit 1.52m (60in) tramlines. Track tensioning is through the use of a hydraulic cylinder with an accumulator, and any reduction in track tension sets off an in-cab alarm. Recharging the system is via the tractors own hydraulic system.

Despite the precariously long nose and heavy front end appearance of the 8000T series, weight distribution is 55% on the front idler and 45% on the rear drive wheel.

Being slightly nose heavy helps to counter some of the torque lift on the tracks caused by heavy draft loads, and it should give the machine a 50:50 weight distribution when in work, reckons Mr Day.

Because the track system uses a smaller diameter drive wheel than the wheeled versions, it means a revised rear end casting and three-point linkage is used. The result is that the rear linkage is now closer to the rear of the tractor (as there are no large tyres to foul an implement when turning) and this adds to stability when carrying heavy, fully mounted equipment.

Steering the machine is simply a matter of turning the steering wheel, but unlike conventional hydrostatic systems, this one uses an electronic over hydraulic system to hydrostatically control steering planetary gears – there is no differential and the system does not interrupt the full mechanical powershift drive.

A position sensor on the steering wheel is used to determine how much of a turn is required. This sends information to an electronic control unit which directs oil flow into two swash plate pumps – one on each planetary unit in the rear axle. By speeding up or slowing these planetaries, track speed is altered and a steering manoeuvre is made.

A spring detent returns the steering wheel to centre should the operator let go and lock-to-lock requires only 1.75 turns of the steering wheel.

"Using an electronic-based steering system also allows proportional steering," adds Mr Day. "The faster the forward speed, the less responsive the system becomes. It makes the machine less twitchy when travelling at 30kph road speed."

"The system also allows the tracks to contra-rotate so we can turn on the spot. Though for safety when the transmission is in park with the engine running, the steering is dead."n

In dry, hard conditions, John Deeres 8000T series tractors run at 2-5% track slip. Should grip be a problem, up to 14 50kg ballast weights can be added where its needed, says Deere – on each track frame.

Weight distribution is 55% on the front idler and 45% on the rear drive wheel. The idea is to give a 50:50 split for heavy duty draft work.


John Deere 8000T series tractors

ModelPowerPrice

8100T185hp£103,900

8200T210hp£109,500

8300T230hp£115,400

8400T260hp£121,000