The axles of wheeled agricultural machinery
If tyres are the "shoes," axles can be considered the "legs" of the tractor and other wheeled agricultural machinery. Technical progress has led to the implementation of axles with state-of-the-art performance steering and suspension systems, all to the benefit of vehicle handling and driver’s comfort. And axles equipped with full-electric drive have already appeared on the horizon
The architecture of agricultural vehicles equipped with pneumatic wheels, whether they are driving, directing or simply supporting or rolling, is based on the presence of axles, i.e. These devices are not only connecting a pair of wheels (sometimes twinned), but in the case must also provide adequate transmission of motion, which is important in the tractor for example for traction and, increasingly, also of suspension of the machine body. This plurality of features has made axles progressively more complex to meet best the required multiple functions. In the agricultural sphere, technical solutions already introduced in the automotive sector or the earthmoving and quarry-yard sectors have often been borrowed, duly revised to take into account both the considerable severity of the working environment, as well as the considerable stresses due to the significant loads borne and the inevitable irregularities of the agricultural soil surface.
It is unsurprising that a good group of specialized manufacturers have established themselves in the market, exclusively (or almost exclusively) producing axles for agricultural tractors and operating machinery.
Axles with suspension
Suspension of the front axle of agricultural tractors has been a reality for several decades now. This component, initially installed to counteract vehicle pitching in high-speed driving, has gradually evolved to improve the driver's comfort as well, in a "package" that also includes cab and seat suspension.
In this case, starting with centrally hinged rigid axles, the evolution has involved the appearance of independent semi-axle suspensions with a four-bar linkage scheme. Since this device has a significantly higher cost than rigid axles, its appearance in the tractor market initially concerned high-power machinery. Among other things, the independent-wheel solution has a significantly smaller footprint than the classic one: this allows the steering angle to remain very wide so as to improve maneuverability, especially for high-tonnage tractors. For example, John Deere and Fendt have been adopting this architecture on high-end models for some time now. In detail, the Fendt 1000 Vario is fitted with four-bar linkage suspension whose travel can be set manually, depending on the load, so that comfort and safety in driving the vehicle remain optimal at all times, up to the maximum construction speed of the machine.
In the area of specialized tractors, a turning point was marked by the introduction on Same Deutz-Fahr's Frutteto model of the Active Drive system, i.e., automotive-derived independent four-bar linkage suspension with automatic electronic damping control, defined based on sensor signals that monitor tractor speed, steering angle, as well as forward speed and hydraulic cylinder position. This configuration also includes an anti-sagging effect on the front end when braking (anti-dive) and an anti-roll action (anti-roll), which is important, especially when negotiating high-speed curves, automatically adjusting the system's stiffness to optimize stability and grip. In both cases, better stability of the vehicle-equipment assembly is ensured, and ultimately, a higher driving safety.
For farm trailers, undoubtedly, the most advanced technology suspension is air suspension, similar to what has been the case for trucks for a long time now. Furthermore, independent-wheel suspensions are again available.
ADR of Uboldo (VA) manufactures axles for both these features: compared to mechanical and hydraulic suspensions, air suspensions allow the ride height of the trailer body to be adjusted with a self-leveling function, regardless of the load size. Moreover, braking dynamics can be compensated for, thus providing the best grip on the ground at all times.
Still from ADR, Hydro Evo, on the other hand, features a modular hydraulic solution that enables correct wheel travel on the same axle, even for considerably uneven profiles. In this way, in "gyratory" driving, i.e., with a cross slope, there is the possibility of keeping the flatbed horizontal, which benefits the vehicle's stability and, thus, driving safety.
Steered axles
No longer just two steered wheels (hence only one axle), but more than two, and sometimes all of them. This is a new frontier, both for tractors and equipment, which makes it possible to improve the maneuverability of vehicles, which is all the more important, taking into account that their overall dimensions are increasing more and more. At the same time, the roads (and more generally, the maneuvering spaces) remain basically the same, unchanged over the decades.
Four-wheel steering on tractors is a reality. Again Same Deutz-Fahr, on the specialized Frutteto model, offers Active Steer, with the rear axle wheels steering up to 15° concurrently with the front ones, thus significantly reducing the turning radius while also being able to combine crab steer to advance and move sideways in a single solution, an opportunity that proves useful both for maneuvering in tight farm spaces and also to counteract the vehicle's downhill slide in the case of turn-by-turn (i.e., along level lines) travel.
In the field tractor sector, Austria's Lindner has for some time now been marketing the Lintrac 90 with a 75 kW engine, a 4-wheel steer model extremely suitable for mountain farming and municipal use. In this case, spheroidal graphite joint housings are used. As a matter of fact, other than the front axle characterized by a maximum steering angle of 52°, the rear axle can also steer up to 20°, including crab mode.
Of particular use on the subject are trailer axles equipped with steering devices, which are usually hydraulic. It is a valuable function, especially for dump trucks with high load capacities and, therefore, a large footprint. To properly manage one or more of a dumper's axles, ADR of Uboldo (VA) manufactures the SmartDrive system, which determines steering based on the relative angle between the tractor and trailer. The system automatically locks the axles in the center position above a predefined speed, typically for fast travel on public roads. The device relies on a series of sensors, speed and angle type, on the drawbar of the trailer and each of the steered axles. Obviously, it also has some safety functions, such as pressure relief valves on each active axle and free auto-steer mode in case of a lack of hydraulic pressure in the circuit.
The modes of operation are "road," when the electronic control steers the wheels according to the angle of the drawbar; "lock," which is the locking of all wheels in an aligned position (at a predefined speed); "man-lock," where the same action is triggered manually; and finally "manual," to manually control the steering of the axles. Everything is managed by the operator on board through a control panel with immediate intuitiveness.
Electric axles
Among the most recent innovations, the electric solution is undoubtedly the most challenging solution, the boundaries of which, however, are expanding daily. There are several solutions, ranging from a "mild hybrid" powertrain, i.e., an electric boost to reduce diesel consumption and optimize footprint by integrating the internal combustion engine and an electric motor, to a full-electric axle, which can minimize power dissipation, even for high input speeds.
More specifically, there are currently several ways in which an axle (or two semi-axles) can be operated with the support or contribution of an electrical power source. A key question in this regard concerns which mechanical, i.e., conventional, parts are integrated or, conversely, replaced tout-court by electrical components. The electric engine, in fact, can drive a classic mechanical transmission upstream (central geared system), be mounted directly on the differential (central direct drive system), or move the entire axle directly (e-axle), or the axle shafts can be driven by two separate electric motors (e-hub drive).
Carraro Agritalia from Rovigo has provided a valuable product overview, starting with an implementation that is well suited to tractors, namely an electric motor powered by a 20 kW low voltage (48 V) battery pack that, when combined with, for example, a 50-60 kW diesel engine, guarantees substantial performance equivalence of a classic 100 Hp endothermic engine, however, with a tangible reduction in diesel consumption, and related gaseous pollutant emissions.
Basically, a "smart" electric boost is installed, which provides for peak loads, allowing the diesel engine to work almost constantly in a condition of maximum efficiency.
One alternative, again from Carraro Agritalia for tractors, is the classic hybrid solution, extremely suitable for specialized tractors, that is, a parallel architecture between the traditional endothermic and electric systems, in which a 56 kW diesel is combined with a 20 kW continuous electric motor, enslaved by a 25 kWh capacity LiFePo4 battery.
In this way, power delivery can be optimized as needed: it can work in full electric, inside enclosed structures, greenhouses, stables or for uses in municipalities, or with the diesel engine alone, in road transfers with extremely heavy towing, or hybrid mode, in heavy PTO work. A further rationalization involves mounting dedicated electric motors for auxiliary services.
Overall, the hybrid solution reduces fuel consumption by zeroing out emissions when operating in full electric mode, can contribute to greater productivity because the power available from the two powertrains can be added together, can decrease the maintenance costs of the endothermic engine due to its lower wear and tear, and undoubtedly reduces overall noise and vibration levels perceived by the driver.
But undoubtedly, the implementation, for the moment, most oriented for machinery intended for load handling, such as forklifts, compact wheel loaders or dump trucks, is the axle with complete integration in its central body of an electric motor, with scalable power sizes.
For this sector, Carraro Agritalia offers axles with electric powers up to 45 kW, peak torque at the wheel of about 220 Nm and a maximum dynamic load of 5,600 kg.
