The presence of the cab is generally a decisive element in limiting a tractor’s noise exposure. For vibrations, on the other hand, the measures to be implemented may be different and there is no single solution for limiting vibrations within the set limits
Antonio Longo
This is underlined by the contents of the operating manual “Acoustic and vibratory design of machinery and equipment for agricultural use” drawn up by INAIL experts as part of the BRIC Inail ID26 project entitled “Definition of innovative guidelines, based on the current state of the art, for the design, construction, certification and remediation of machinery, equipment and work environments with low risk of noise and vibration exposure for workers”. In the previous September issue of the magazine, the general contents of the aforementioned operating manual were discussed in detail. In the present issue, on the other hand, the specific actions to be taken into consideration to limit the operator’s exposure to vibrations caused by the use of machinery will be identified.
The main causes of vibration in agricultural machinery
In the case of vibrations, in order to limit the operator’s exposure, it is necessary to identify the main sources of vibrations and how they are, naturally, transmitted to the driving position. The main internal sources of vibration are related to the presence on the tractor itself of a thermal engine, transmissions and sometimes hydraulic or pneumatic systems that generate high-frequency vibrations. In the opinion of Inail’s experts, the engine is generally not a problem, unless it is operating at low speeds, as the moving parts are balanced and the engine is fixed to the chassis by anti-vibration mounts.
Based on what emerges from reading the manual, the main external source of vibration is the contact of the wheels or tracks with the ground, especially when the vehicle travels over compact surfaces at high speed. In this case, low-frequency vibrations are produced that are particularly harmful to the operators, when the dynamic properties of the machine amplify the excitation spectrum generated by crossing uneven ground.
The operating machines connected to the tractor also alter the dynamic behaviour of the tractor itself and their operation produces vibrations that propagate to the operator’s station.
Despite enormous progress in both regulations and technology, the problem of vibrations remains critical in Italy because most farms in Italy, especially in the South, are small and generally family-run. In such contexts, in fact, mainly medium-low power machines are used, which are not always equipped with all the devices required to improve the comfort and safety conditions of the operators. Moreover, the tractor market is obsolete, considering that out of a fleet of 1.9 million units, 75% are over 25 years old. This fact is well represented and confirmed by the decline in registrations in recent years.
Techniques to reduce vibrations
The main techniques adopted, for decades now, to reduce vibration levels experienced by the driver include the adoption of passive anti-vibration systems aimed at limiting the transmission of vibrations. As highlighted in the in-depth study conducted by Inail, this is generally done by introducing suspensions on the front axle, the cab and the seat. In addition, tyres for wheeled tractors are an element that can significantly alter the transmission of low-frequency vibrations caused by uneven ground. In recent years there has been a progressive increase in the speed at which work and transfers are carried out, and at the same time a significant evolution in tyre technology. In particular, there is a general tendency to increase tyre width, with a consequent reduction in inflation pressure that allows for better absorption of uneven ground. In addition, manufacturers have recently developed high-flexibility tyres, in which the increase in sidewall flexibility itself allows for greater load capacity at the same inflation pressure and better performance in terms of damping vibrations generated by both pairs of wheels in contact with the ground. This aspect is of particular interest for the rear axle, which is generally not equipped with suspension.
Axle suspension
The front axle suspension is an optional made available by manufacturers for medium-high power tractors. In the beginning, the adoption of suspension on the front axle of the tractor was not motivated by needs related to driver comfort, but by needs for manoeuvrability and safety of the vehicle in fast transport. In fact, as stated in the manual, the increase in the maximum speed of agricultural tractors, which can now exceed even 40 km/h, has led to the emergence of a problem due to the resonance of the entire tractor linked to the stress generated by the imperfect circularity of the tyres. In fact, on even ground, at a high forward speed, the rolling frequency can overlap with one of the resonance frequencies of the entire tractor, which, as a result of the periodic forces caused by the imperfect circularity of the tyres, begins to oscillate in a way that is dangerous to the structural integrity of the vehicle and makes it difficult to manoeuvre. In the absence of front axle suspension, especially when towing trailers, the oscillation of the rear tyres, combined with the thrust in the axial direction exerted by the trailer via the tow hook, triggered dangerous pitching vibrations during transport on public roads. The introduction of suspension devices on the front axle drastically reduced this problem.
The introduction of suspension systems also has a significant effect on improving the driver’s vibration comfort. The most common configurations are of the semi-active type, consisting of hydraulic cylinders, which can be classified as single-cylinder damped axle, two-cylinder damped axle, and independent-wheel damped axle.
In the first two configurations, which are the most common, the cylinders are positioned at the front between the axle and the tractor chassis. In the third configuration, borrowed from the automotive sector, the cylinders are, on the other hand, connected directly to the front wheels, which are connected to the chassis by means of an articulated quadrilateral, creating an advantage in terms of both a reduction in overall dimensions and an increase in steering angles and therefore better tractor manoeuvrability. All three configurations are based on the same operating principle. As explained in the manual, the cylinders are connected to a hydraulic nitrogen accumulator, which consists of a container divided into two chambers by an elastic membrane. The upper chamber contains inert gas under pressure, usually nitrogen, while the lower chamber contains oil and is connected to the hydraulic circuit by means of an exhaust and a delivery valve. When the tractor wheel travels over uneven ground, there is a flow of oil from the cylinder chambers to the hydraulic accumulators, which produces a pressure increase in the cylinder chamber that is being emptied and a pressure decrease in the cylinder chamber that is being filled. This tends to bring the system back to the equilibrium position where the force acting on the cylinder rod is balanced by the pressures in the two chambers.
The axle suspension generally has the option of being blocked, effectively making the connection between the front axle and the chassis rigid, since during field work any oscillations and consequent load variations could generate a loss of traction.
The most technologically advanced configurations, on the other hand, are of the active type and aim to maintain constant tractor conditions regardless of external forcing. Specifically, an accelerometer positioned at the front manoeuvres the chassis movements in relation to the irregularity of the terrain. In addition, these configurations also favour comfort when lifting, lowering or manipulating the three-point linkage, allowing temporary stiffening of the suspension in such conditions.
Seat suspension with possible active control
On agricultural tractors, which originally had no suspension systems attached to the wheels, the seat suspension was the first device introduced to improve operator vibration comfort. It is common practice to suspend the driver’s seat by means of a suspension that limits the transmission of low-frequency vibrations to the operator. Over the years, a considerable technological evolution of these devices has been observed, which have gone from very basic configurations to much more articulated configurations exploiting more complex kinematics whose motion is regulated by elastic and damping elements in passive systems or by actuators in the case of active and semi-active suspensions. Currently, the seat suspensions consist of an articulated system that also allows height adjustment and a shock-absorbing device made up of a hydraulic cylinder, which can be adjusted by means of a small manual pump, or an air spring with pressure adjustable via push buttons. Pressure adjustment in the case of air springs modifies the stiffness of the suspension and is necessary to adapt the suspension to the operator’s mass. Incorrect adjustment nullifies the effectiveness of the suspension and in some cases can produce effects opposite to those desired, increasing the vibration level to which the operator is subjected. The INAIL experts point out that this has led manufacturers to introduce aid systems, such as indicators and warning lights for correct adjustment, and to put on the market seats with automatic adjustment of the height and stiffness of the suspension according to the driver’s mass. More recently, the introduction of semi-active systems that are able to dampen seat vibrations more effectively has been observed on the market. In this case, sensors monitor the motion of the seat and a control system modifies the damping of the system by means of magneto-rheological systems, adjusting the dynamic behaviour of the suspension in real time.
The suspension of the cabin
The suspension of the seat is not, however, sufficient to guarantee the correct comfort for the operator, as this has a good effect in reducing low frequency vertical accelerations, but plays a marginal role in reducing higher frequency vibrations that can cause the cab panels themselves to vibrate, transforming the cab into a sounding board. Therefore, anti-vibration plugs were initially inserted to isolate the cab from the tractor chassis, with the main aim of limiting acoustic problems. In order to have an effect on the exposure to lower frequency vibrations to which the operator is subjected, actual suspension systems have been introduced in recent years, consisting of springs combined with dampers made up of hydraulic cylinders with associated nitrogen hydraulic accumulators or alternatively pneumatic suspension. These solutions, as suggested in the Inail manual, can be adopted only on the rear support of the cab or on both supports.
