Agricultural tractor anti-vibration systems, a technological challenge

A lot of time has gone by since the only device for attenuating tractor vibrations was, aside from the tires, a spartan seat built as a sheet steel shell supported by a simple U spring. Today the seats, cabs, axles, suspensions and new generation tires are combined to improve comfort and safety to levels not previously imaginable

Tractors and other self-propelled machinery are complicated, built for widely diversified operations in the field or off-road work, sometimes performed simultaneously, so it is not easy to identify the causes of vibrations and the parameters to make the task of minimizing these disturbances often very difficult. The main sources of high frequency vibrations on tractors are the engine, the gear box and the moving parts run off the hydraulics as well as, when present, the tires. On the other hand, the impact of the driving parts, tires or track, on the ground often on compacted terrain at high speed generates low frequency vibrations which far and away produce greater disturbance and damage for the driver. In these cases, the vibrations depend on the stress the machine is subjected to, the extent of which is correlated to the speed and the uneven surface of the ground. Not to be ignored is the contribution coming from mounted or trailed implements which can strongly condition the degree of vibration comfort of the tractor to which they are hitched.

It must also be kept in mind that on self-propelled machinery – combine harvesters, mower loaders, grape, tomato, sugar beet, potato harvesters and the like – leaving aside the case for tractors, the work of the harvesting components, rotors, beaters, fans, augers and the like, often significantly influence the overall levels of vibrations.

The regulatory problem

The arrival of the European Union Directive 2002/44/EU, which established maximum value of vibrations workers are exposed to, including those in the agricultural sector, has forced the manufacturers of agricultural machinery, providing the principle sources of vibrations, to face a problem which has still not yet been fully settled in that the generation levels at the work location, the drivers seat, often strongly exceed the limits prescribed.

The technical inadequacies are difficult to top up in light of the especially harsh limits stipulated which are quite frankly not fully justified in that they take account of rather restrictive criteria as regards the health of the operator. These points mean consideration must be given to the objective difficulty of putting these regulatory measures into practice, measures typically for daily exposure which are certainly valid in the industrial environment but certainly not for farming, which is usually associated with seasonal work with an annual rhythm of operations.

Various operational solutions

The noise parameter for modern agricultural machinery can now be described as relatively comfortable and, except for some well identified cases, the current limitations are generally respected. But this cannot be said for vibrations because of the reasons cited above. So the technical progress must be advanced further to reach compliance with the norms stipulated.

The active remedies in this setting, that is, those which act to reduce the source of the vibrations generated, are the systematic applications providing a significant reduction to vibration exposure for the agricultural worker. Leaving these aside, the efforts of designers have been undoubtedly focused over the decades to the passive reduction of vibration levels experienced by the driver by working on the combination of seats, the cab and front axle for equipping these components with suspension systems.

 

The role played by tires 

It may seem to be taken for granted but tires can act in a very positive, or negative, way regarding the transmission of low frequency vibrations to the drivers seat. Facing the potential of a greater input of the vibrations present, caused by the increased speed of the vehicle in field operations and on the road, fortunately tread technology has evolved to bring in widespread benefits.

In detail, expanding the tread width for high power tractors even beyond 7-800 mm and lowering normal inflation pressure by 1 bar or less have substantially increased tire contact with the soil with the benefit of dampening rugged conditions and irregularities of the land surface.

Further advantages have been brought in with the introduction of tires with very flexible sidewalls, labeled IF, for Increased Flexion, and especially VF, Very High Flexion, thanks to the greater elasticity of these sidewalls which strongly decrease the transmission of low frequency vibrations by the tractor travelling on rugged terrain.

 

The seat

This component has always been considered basic for improving comfort against tractor vibrations. Various types of suspensions, mechanical, hydraulic and pneumatic, capable of dampening the transmission of low frequency vibrations have been installed under the drivers seat. Other than mechanical support often based on a pantograph or a four-bar linkage mechanism equipped with springs, technical evolution has been focused on components to dampen oscillations. After a long period when mechanical or hydraulic forks were installed, the recent trend has shifted to air springs which are generally cylinders inflated by a small compressor powered by the batteries of the vehicle to vary pressures in relation to the weight of the driver. In this case, the advantage is that they can be regulated, automatically so for some models, for the stiffness of the air spring to optimize dampening. In fact, because the efficiency of attenuating vibrations reaching the seat is strongly influenced by the weight of the driver, these suspension devices must be carefully regulated but unfortunately, this step is generally ignored in real conditions. Thus much of the efficiency of the suspension is nullified to the point that the opposite effect is produced, that is, an increase in the level of vibrations reaching the seat compared to the base. This has been undoubtedly verified in various research projects on this issue. 

The most recent arrival for seat suspensions is the semi-active type ensuring greater efficiency for dampening vibrations according to a reading of their input by producing a counter-phase movement for maintaining the effect on the drivers seat at practically zero. In practice, the usually vertical movement of the seat is monitored by a sensor possibly transmitting to a accelerometer for a microprocessor elaboration of the signals and acting on the suspension. This operation is actually new. The common solution is the use of so-called Rheological (balance achieved in the material deformed by effect of stresses) fluid which changes viscosity in relation to exposure to modulated magnetic field performed by the microprocessor according to the input of vibrations to vary the viscosity and thus the rigidity of the dampener.

All the major manufacturers of work seats for tractors and other self-propelled machines offer semi-active suspension seats able to monitor the input of vibrations with a very high frequency of up to 70 per second and regulate them in real time to provide an excellent response of the suspension for the reduction of disturbances by as much as 40%, according to these producers. 

 

The cab

The technology generally now applied is for the isolation of the cab and, in practice, all cockpits provided with a platform, by inserting ant-vibration devices under them, silent-blocks usually made of high density rubber spacers. These devices are effective for attenuating high frequency vibrations greater than around 20-30 Hz which, because of the broad panels and glass, could be transform the cab into a sound chamber greatly amplifying the level of sound reaching the ears of the driver.

But for real comfort for the driver, for dampening vibrations of less than 15-20 Hz, for a number of years silent blocks have been fully or partially replaced by real suspension systems.

These involve various solutions beginning with the classic combination of mechanical spring dampeners to reach the more recent pneumatic suspensions. In some cases, top range tractors are marketed with one or more combinations to allow the end-user to select the type most suitable for his needs and financial consideration.

 

The front axle

Over time, the tractor front axle has been developed for canceling or at least effectively reducing pitching and the overall level of vibration reaching the tractor seat with the troublesome bouncing sometimes accentuated by a towed implement when travelling at speed on an asphalt surface. Also in these cases, the direction taken was for the integral suspension of the front rigid axle with a classic set of spring dampeners assisted by one or more nitrogen shock absorbers. The technical differences are mainly the fluid used for the shock absorbers which is generally hydraulic oil, sometimes replaced by compressed air for improved operations in harsh climates because there is little change in viscosity with temperature variations. With the fine turning of techniques and the resulting lowered costs, the application of suspension to the front axle was gradually extended, often offered as optional equipment, to all open field models seven or eight years ago and also to specialist tractors for vineyard and fruit orchard operations. Very recently launched for this specialist segment was an interesting front a four-bar linkage and independent semi-axles based on automotive design providing advantages for comfort against vibrations as well as greater stability of the machine in steering and braking.

 

Integral suspension

The integral suspension system for the entire machine on both axles is a solution rarely used on typical agricultural tractors but, from the point of view of comfort against vibrations, the system objectively provides unsurpassed benefits, especially at speeds of greater than 40 km/h, now allowed in other countries, but not in Italy, and in other circumscribed conditions.

In any case, considering the continuous and compelling requests for increasing the operational speed, there are expectations that in the not too distant future agricultural tractors could be marketed with guarantees of road safety for speeds of 50, 60 or more km/h with the vibration comfort needed to satisfy these growing requests ensured by the adoption of effective measures such as integral suspension systems for the entire vehicle.