Comfortable driving: reducing vibrations

From the simple U spring under the seat to independent suspensions for the rear axle, miracles have been performed in technical progress as regards agricultural tractor vibrations all to the advantage of the driver's comfort and health. Shock absorber systems for vibrations work at the level of the seat, the cab and the front axle

Great attention to safety and comfort in agricultural work, as regards mechanical engineering, is focused especially on the tractor and the remarkable technical evolution of the drivers seat. The best solutions have been introduced first for high power tractors for economic reasons (lower percentage of cost impact of these best seats) as well as size (difficulties often caused by installation on small tractors not strictly requiring them). These seats were then progressively extended to the medium and low-power categories due in part to the recent enactment of norms imposing strict limitations.

From the point of view of comfort – as well as safety in that the two are closely interconnected – the main risk is undoubtedly made up of two so-called physical agents, vibration and noise. Though the latter problem has been practically solved, at least for tractors equipped with cabs, vibrations transmitted to the driver are still considerable and often beyond the established limits (see box).

 

Seat suspension

In light of the typical lack of suspension devices mounted directly for the wheels, the common practice for limiting the vibration load experienced by the drivers of agricultural tractors, also on older tractors, is to install a seat equipped with suspension. The first seats of this type were equipped with elementary devices such as the classic U spring, that is an X parallelogram completed with an internal spiral spring regulated by its precompression to absorb a part of the vibrations transmitted by the machine.

The most modern seats, on the other hand are normally equipped with mechanical-hydraulic or pneumatic suspensions. In both cases, the parallelogram is used, also for raising or lowering the seat. The difference is in the shock absorber device. In the first case, this is a hydraulic cylinder regulated by a small manual pump. However, for comfort at the top end there is the pneumatic seat capable of reducing vibrations transmitted to the seat cushion to the minimum.

What is fundamental in all cases is that the regulation of pressure or the stiffness of the spring according to the weight of the driver. For this purpose there is often an indicator which discloses this weight in kilograms, which allows the driver to adjust everything on the ground on this basis, or a semafor requiring the driver to be seated to take the indicator to green.

Also in this case technical evolution has recently been speeded up with the arrival on the market of seats with automatic regulation for height of the seat and the stiffness of the suspension as calculated by the weight of the driver, seat warming, partially rotating seats, those with pneumatic lumbar support, with variable inclination as well as those with cushioned backrests but especially seats with semi-active suspension capable of the most effective cushioning of vibrations.

 

Cab suspension system

The cab, or the platform alone, mounted on the tractor has always been an object drawing great attention. The high level of high frequency vibrations can cause excessive stress on the structural load and lead to fatigue fractures. To curb this problem, and avoid allowing the cab to become a sounding board, for the past ten years tractor cabs have been mounted on rubber components known as silent block bushes. More recently, to further reduce vibrations and especially those of low frequency, cabs are also installed on shock absorber devices. The most simple ones are mechanical, robust springs with associated dampers. As for the adoption of hydraulic cylinders and nitrogen charged accumulators for the front axle, technical developments have brought in the benefit of easily regulating them for stiffness, also from the drivers seat. In this case, some manufacturers mount alternative pneumatic suspensions.

 

Devices for the front axle

Other than the help offered by tires and seat and cab suspensions for improving the comfort and dampening vibrations reaching drivers, for some time suspentions have been added to the front axle, initially mounted to drastically reduce pitch, the jumping motion typical when the tractor is traveling at high speed to sometimes make it difficult to control the speed. This phenomenon is favored by wheel rims and tires which are not perfectly round; when rolling frequency is approached by the resonance of the entire tractor, often at speeds of 38-42 km/h, and when small imperfections of the road surface are amplified pitch has a significant impact on the safety of driving. From the point of view of concept, the suspension is simple, the axle is coupled to the body of the machine with one or two hydraulic cylinders. The hydraulic circuit is equipped with one or more gas accumulators capable of absorbing shock and limiting piston rod travel by varying the volume of gas in the chamber. Actually, the construction of the circuit can be very complicated.

For work operations, it is usually possible to block suspension. In this case, almost all manufacturers provide the full withdrawal of the cylinders to bring the front axle into direct contact with the body of the tractor for the advantages of greater stability and the decidedly better control of the machine. Other construction solutions call for mounting a single cylinder which can be located laterally or centrally. In this case, the construction is often an articulated quadrilateral to increase the precision of the control device.

The most interesting recent technical development is front axis wheels equipped with independent shock absorbers derived from automobiles. This system involves two independent semi-axles with each wheel linked to the body of the tractor by an articulated parallelogram and a hydraulic cylinder. Among the various obvious advantages of this configuration is the steering angle. With a traditional axle, even suspended, the joint is a central point so during a normal roll a wheel approaches the engine compartment reducing play in the steering angle to avoid functional problems. On the other hand, dividing the axle physically into two parts allows almost vertical travel of the wheel while the steering angle and driving conditions do not vary. Moreover, the suspension works at four points to improve the stability of the tractor thanks to the hydro-pneumatic plant which acts against roll because of the variability of the spring modulated by the oil pressure.

Some manufacturers have opted instead for the application of pneumatic suspensions fully similar to those mounted on trucks with functions like those of hydraulic suspensions but featuring better performance at low temperatures. This is because in these conditions, the viscosity of the oil substantially reduces the capacity of absorbing oscillations.

Vibration regulation

The general reference is now Legislative Decree 81/08 of the Single Text on On the Job Safety. A series of measures on the protection of workers listed under the VIII heading refer to the physical risks which include noise levels and vibrations. For the later, two areas of work are named, hand-arm and entire body. For agricultural machinery operated manually, mowers, chainsaws, electrical equipment and the like, attention is trained on hand-arm, and then on the entire body due to the generalized impact of self-propelled machines, tractors in primis because of the drivers seat. 

Legislative Decree 81/08 cites the ISO 5349-1 and 2631-1 norms as the technical reference for evaluating vibration levels. In detail, vibrations experienced by the entire body are measured with sensors and accelormeters usually located on the seat cushion.

The data, detected through three orthogonal coordinates, once acquired are calculated on the basis of frequency to the extent arriving from the three horizontal coordinates, to which 40% is added to take into account the greater sensitivity of the human body. The maximum value limit, with reference to the equivalent of 8 hours of continuous acceleration, is 1.0 m/s² but already at the activity level of more than 0.5 m/s² the employer is required to enact measures aimed at lowering the exposure of the operator.