26 November 1999

Case goes on-the-button with new variable drives

Fendt has one, John Deere is

reputed to be developing

one, and now Case has one.

Andy Collings takes a look

at the latest continuously

variable transmission system

to be introduced

CASE has now launched its continuously variable transmission system and, with it, the 130hp CVX130 and 170hp CVX170 tractors.

The unit is also available in the companys Steyr range which has been designated the CVT range – the CVT130 and CVT 170. Two other models with 120hp and 150hp ratings, both in Case and Steyr builds, have also been produced but only Steyr will have all four models available in the UK.

In any event, beneath the bonnet, both the Case and Steyr models are identical.

So, what does CVT offer – and how does it work?

Continuously variable

Developed over a number of years by Steyr-Daimler-Puch, the transmission has the ability to provide continuously variable speed control from 0-50kph without any interruption in tractive force.

As with other similar transmissions, the CVT uses a combination of hydrostatic and mechanical drive. It comprises four main components – a hydrostatic unit, compound planetary gear, mechanical 4-stage transmission, and transmission and vehicle electronics.

Bearing in mind that hydrostatic transmission is not as efficient as mechanical, the CVT designers have endeavoured to ensure that the proportion of mechanical drive employed remains as high as possible.

Power from the engine can take two routes. One takes the hydrostatic route, the other the mechanical.

Taking the hydrostatic route first, the engines output shaft drives a variable displacement pump connected to a fixed displacement motor. By adjusting the output from the pump, the angle of a swash plate is altered, both the speed and the rotational direction of the motor can be changed.

Output from the hydraulic motor then meets up with the mechanical drive in a compound planetary gear unit where the superposition of the two power sources takes place.

The mechanical portion of the gearing is directly driven by the engine and always rotates in the same direction, at speeds governed by the engine. The sun gear receives its drive from the hydrostatic motor – its speed and direction dictated as previously described.

Drive then enters a speed range planetary gear which supplies four mechanical speed ranges. The system is designed to make maximum use of these speed ranges to keep the hydrostatic input as low as possible.

Change over from one gear range to another is achieved through use of hydraulically activated claw clutches.

System key

Key to the whole system is a Transmission Control Unit which activates clutches and power ratios within the transmission to achieve the continuously variable speed ratio.

What might be considered to be the actual brain of the tractor though, is the Vehicle Control Unit which receives and processes signals from the tractors controls and activates the interplay between engine and transmission.

So what does all this mean for the operator?

After the engine has been started and the handbrake released, the travelling direction is selected using a shuttle lever on the left of the steering wheel. The tractor is then accelerated using either the foot pedal or pressing the +button on the Multicontroller unit – a unit which, as will be seen, also offers access to several other functions.

It is worth noting that the throttle pedal is not responsible for increasing engine speed – just the speed of the vehicle.

The ratio between engine and transmission speeds is adjusted automatically to produce the required speed with the required gear shifts made smoothly.

Instructions

Removing the foot from the accelerator or pressing the -button instructs the tractor to stop – braking has priority over all transmission functions.

Once stopped the tractor remains stationary irrespective of whether or not the hand brake is applied.

When the engine is switched off the transmissions parking brake is automatically applied. It is also applied if the engine is running and the tractor has not moved for more than 10 seconds.

Other features of the system include a cruise control which, like those fitted to cars, allows a selected speed – up to 50kph forwards and 20kph in reverse – to be automatically maintained. Buttons on the Multicontroller unit allow a speed to be increased or decreased.

Interestingly, the lowest speed attainable using cruise control is 100m/hour.

The new tractor models are expected to be available during the first half of next year. &#42