Rubber tracks for large agricultural machinery
Rubber tracks had a time success record on the market thanks to their several advantages; they can equip both small tractors and large self-propelled agricultural machinery. In the latter case, they are technically quite complex devices
For several years now, rubber tracks, long known in the construction and earthmoving sector, have also become widespread in agriculture, providing an increasingly valid alternative to traditional steel tracks. This is a solution that, in a nutshell, sums up the advantages of classic steel elements and tires, that is to say, high strength but at the same time, considerable elasticity.
The advantages of the mobility provided by rubber tracks, both in the field and on the road, have made it possible to apply them successfully on tractors that are at the extremes of the current offer. On the one hand, the small specialized machines for vineyards and orchards, where stability and respect for the soil are required, and, on the other hand, those of considerable size and power, designed to develop robust traction efforts with equipment of considerable productivity.
In order to allow a sufficiently safe and comfortable ride, the most advanced versions of rubber tracks available today are very complex from a technical point of view, as they have sophisticated suspension devices, and equipped to withstand considerable loads, such as those of combine harvesters with high working capacity.
In detail, the intermediate rollers (in steel or aluminum, with a polyurethane coating, or entirely in polyurethane) often have hydro-pneumatic suspension, while the drive wheel and two tensioning wheels are raised for maximum driving comfort. The full radius curves of the rubber tread ensure greater strength and durability. Optimization is therefore achieved both in the field, thanks to the excellent "flotation" capacity, that is to say, the reduction to a minimum of the specific pressure on the ground (even with heavy loads), and on the road, for the ability to ensure fast, comfortable and safe travels.
This note illustrates those models that are mounted on tractors, but also on numerous self-propelled operating machines, of high power and size, often offered as partial or total alternatives to tires. Manufacturers in the sector have recently developed, in combination with increasingly sophisticated track trolleys, models capable of ensuring excellent performance also from the comfort point of view. In the agricultural sector, rubber elements have initially appeared, designed to replace the traditional steel belt, thus leaving the wheels on each side, the drive side and the idler side connected. Although there is no lack of intermediate solutions, in the meantime, another constructive solution has spread, that is to replace only the rear wheels, or all of them, with an equal number of rubber tracks, almost always with a substantially triangular development. The real advantage of single tracks compared to traditional tracks that join the wheels on each side of the machine lies in the reduced damage to the stand when steering, since they behave very similarly to wheeled vehicles, being equipped with a steered front axle or central articulation. Conversely, vehicles with traditional tracked tracks must necessarily exploit the different peripheral speed of the two propulsion components.
For some time now, all the leading manufacturers of high power and self-propelled tractors have included rubber tracked vehicles in their market offer: sometimes they are derived from wheeled vehicles, with the mere replacement of the latter, and sometimes with the replacement of steel tracks.
Tread belts
In addition to a tangible reduction in compaction, most of the rubber tracks dedicated to high-powered tractors must be fitted with tread belts that are particularly well-treated from the point of view of traction i.e., to ensure that the vehicle on which they are equipped can also develop the highest traction force. This is the reason why, like the most performing tires, they are characterized by ribs of considerable height, conformed for the best contact with any surface, whether rigid, such as asphalt, or soft, such as agricultural soil.
The German company Continental offers Trackman, a tread strip equipped with "Armorlug Ultra" ribs, which contain three layers of fabric to maximize their durability, while the belt body consists of a thick layer of galvanized steel wires (to prevent corrosion), placed side by side approximately every 2 mm.
The advanced tread design contributes to driving comfort, as the manufacturer claims a 50 to 70% reduction in vibration input compared to the standard track. Besides, since the intense friction to which the tread is subjected, especially in fast transfers, can lead to overheating of the material, the Trackman incorporates sensors that monitor the temperature of the track belt and signal any anomalies in the cab, so as to slow down, reducing the degradation of the rubber. The material temperature control is also provided, for example, on the tread strips produced by the Canadian Camso.
For the agricultural sector, the multinational company Bridgestone offers wide width tread belts for balanced load distribution and even wear, with asymmetrical walled ribs and a double angle of development, to promote traction and mud discharge.
Besides, the edge of the ribs is blunted, to encourage progressive steering respectful of the topsoil.
Claas' Terra Trac
Available for combine harvesters, forage harvesters, and tractors, Claas' Terra Trac is a technologically advanced rubber half-track. The motion transmission between the drive wheel and the track is obtained through friction, without teeth and gears, with the advantage of distributing the traction force over the entire available width. The two wheels of the track (drive and steering wheel) are large in diameter, which, combined with the high widths available, results in a huge contact surface area with the ground, which ensures excellent flotation on the ground of reduced bearing capacity and high traction efficiency.
However, the Terra Trac's real added value is the "intelligent" hydro-pneumatic suspension with independent damping of the steering wheel and support rollers. This allows traveling with optimum comfort at up to 40 km/h, also ensuring secure stability during headland turns. The tension of the crawler track is automatically electronically managed with a dedicated hydraulic cylinder. In the event of a drop in tension, the operator is promptly notified via CEBIS, the on-board control, and the monitoring system. Depending on the steering angle (i.e., when turning at the bottom of the field and when driving through particularly tight curves on grassland), the Terra Trac track's traction wheel is hydraulically raised automatically, reducing the track bearing surface by one third, thus limiting damage to the grass turf.
Even with narrower crawler tracks (635 mm, compared to 735 or 890 mm), the average ground pressure is always less than 1.0 bar, winning the comparison with any alternative tire size.
Poluzzi's Ultron TerraDrive
The Italian company Poluzzi from Vicolungo (NO) produces a wide range of iron and especially rubber tracks. The top of the range is Ultron TerraDrive, which includes a carriage with 4 rollers, polyurethane covered wheels, roller bearings on mechanical rocker arms, and the tensioning through a hydraulic cylinder completed by a nitrogen shock absorber. The drive wheel has 23 slits, where the tread belt reliefs are inserted, about 86 cm wide. The distinctive feature is the vertical hydraulic suspension, also in this case made by a hydraulic cylinder integrated by a nitrogen shock absorber.
Friction or gear drive
These are two alternative ways of transmitting movement and traction from the drive wheel to the rubber track.
In the case of the friction drive, the inner surface of the belt of the track is equipped with a series of guide ribs, which are inserted into a drive wheel groove during rotation to ensure the vehicle's directionality. Traction transmission is ensured only by the friction of the smooth inner part of the crawler track, which is maintained at the highest level thanks to a robust hydraulic cylinder that ensures the tension of the belt, and thus its optimal grip on the drive wheel.
On the other hand, as the name suggests, the positive drive is equipped with teeth (or bars), among which the rubber pads on the inner side of the tread belt adhere.
These are technical solutions that involve both values and potential faults: the friction wheel may be subject to slippage concerning the track, which vice versa is not possible with the positive wheel; conversely, the internal traction pads are more exposed to wear.
Moreover, the carriage with the friction wheel is more straightforward to manufacture than the one with the positive wheel, where significant compliance with tolerances is required.
Rubber track regeneration
Can rubber track be retreaded like tires? Indeed, and it can also be certified, with a tangible cost-saving, considering the multiple cycles of use after the initial one. However, to take full advantage of retreading, it is crucial that the crawler tractor is used in such a way that the ribs are not completely worn, and the integrity of the body is not compromised. The fact to watch out for is the correct weight distribution on the axles to avoid excessive slippage. However, the real key element is the wear of the ribs, whose residual height must not fall below 13 mm in order to be able to carry out a profitable regeneration. The Canadian Camso ensures the regeneration of its positive rubber tracks for all width sizes from 18 to 36 inches
