Disc harrows and cultivators typically need to be worked relatively fast, in order to do their job.
This is because the discs need to be afforded the opportunity to throw up enough soil to create a viable seed-bed, particularly if being used as a primary cultivation tool.
That means putting plenty of horsepower up front and forging ahead as quickly as possible – within reason, of course.
When John Deere – as a marketing gimmick – put a set of discs behind one of its combine harvesters, it certainly nailed the horsepower requirement. Alas, the combine’s hydrostatic transmission was not quite the ticket for heavy-duty pulling.
The science bit…
That’s because hydrostatic transmissions, which rely on oil being circulated through hydraulic pumps and motors, are relatively inefficient – from the perspective of converting engine flywheel torque into useful traction at the drive wheels. Mechanical transmissions, in which torque is transmitted through meshing gears, do better in this regard.
Of course, the benefit of hydrostatic transmissions – hence explaining why they are used on combine harvesters and self-propelled forage harvesters – is that they allow forward (travel) speed to be smoothly altered irrespective of engine rpm (and without having to dip a clutch or interrupt ‘power’ delivery to the wheels).
Of course, the advent of CVTs (Continuously Variable Transmissions) promises the benefits of both – by combining the driver convenience of a hydrostatic with efficiency approaching that of a mechanical box (at least at some working speeds, depending on the way the internals are configured).
Modern CVTs – certainly those used in agriculture – typically employ a combination of hydrostatic and mechanical elements. The hydrostatic elements are principally a swash-plate (variable-displacement) pump and accompanying motor. The mechanical portion typically comprises planetary (epicyclic) or other gear-sets.
T670i called ‘Tamina’
Technicalities aside, the video (below) shows a 449hp John Deere T670i combine – nicknamed ‘Tamina’ – towing a set of 6m-wide discs. It was filmed in Sweden.
The combine, which was harvesting 9.1m in each pass, was cutting triticale.
The implement was attached using two chains – to leave enough distance between the rear of the harvester and the discs to ensure that the chopped straw settled in front of the harrow.
While acknowledging that a combine harvester might not be the optimum choice to pull a cultivator, John Deere did say that the T670i’s ProDrive transmission, front tracks and anti-slip management system helped in its efforts to tug the discs along.
Alas, it is doubtful that the combine’s forward speed (while harvesting and discing simultaneously, which can be seen close to the end of the video) would be sufficient to enable the discs to work effectively. Hats off, nonetheless, to John Deere for showing the muscle that its T Series combines can muster.
