## 1 Introduction  
In everyday life, vibrations generated in the surrounding environment may be unpleasant for people and generate temporary discomfort [1, 2, 3, 4]. On the other hand, in some working sectors, daily exposure to vibrations can irreversibly compromise, in the long term, the health of the operator [5, 6, 7]. To protect workers, regulations have been introduced that limit the level of the exposure to vibrations during daily work. Agricultural machine operators, when driving vehicles and mobile work machines, are typically subjected to mechanical vibrations transmitted to the whole body in a seated position [8, 9]. The ISO 2631-1:1997 standard [10] shows that the human body in a seated position is particularly sensitive to vibrations in the frequency band 0.4-100 Hz. In particular, for frequencies between 1 and 2 Hz, vibrations can produce effects of temporary discomfort, such as car sickness; for frequencies between 2 and 20 Hz they can give rise to lesions of the lumbar tract and trauma to the spine (this last effect has a critical frequency of between 3 and 10 Hz) [11]. The driving of tractors determines a postural overload due not only to a prolonged seated position but also to the frequent rotations of the lumbar tract for carrying out particular operations [12]. It is, therefore, necessary that the designers, since the initial phase of the project, focus on the dynamic behavior of the machine to limit the vibration exposure of the operator [13]. The activities carried out in agriculture are varied and of different nature and require the use of specialized machines to carry out individual operations. Typically, an agricultural machine is composed of a driving machine (tractor) connected to one or more operating machines. In fact, the tractor is the most widespread mobile agricultural machine because, thanks to the coupling systems with which it is equipped, it can be connected to different operating machines, allowing several operations to be carried out. Depending on the activity and therefore on the machine used, the vibration exposure level of the tractor driver can vary considerably. Focusing on the tractor, the main internal vibration sources are the engine, the gear changes, and sometimes hydraulic or pneumatic systems that generate high-frequency (over 10 Hz) vibrations. The main external source of vibrations is represented by the contact of the wheels with the ground, especially when the vehicle travels on compact surfaces at high speed [13]. As stated previously, these low-frequency (below 10 Hz) vibrations are particularly dangerous for the operators. The main techniques adopted to reduce the vibration levels of the driver include the adoption of passive anti-vibration systems (suspension systems for the front axle, the cab, and the seat) designed to limit the transmission of vibrations to the operator [14]. A tractor is connected to an operating machine with a standard coupling system that allows the connection of different operating machines to the same tractor. Towed machinery is connected to the tractor by the towing hook. A detailed description of the necessary mechanical connection between the towed and towing vehicles is provided in the five parts of the standard ISO 6489 [15; 16; 17; 18; 19]; a detailed description of the mechanical connection between towed vehicles is provided in the three parts of standard ISO 5692 [20; 21; 22]; and the hitch rings and the attachment to the tractor draw bar are described in the standards ISO 21244 [23] and ISO 20019 [24]. Front and rear-mounted or semi-mounted machinery is connected to the tractor by the front or rear three-point linkages, respectively. A general description of the rear-mounted three-point linkage is provided by the standard ISO 730 [25]. Detailed descriptions of different kinds of hitch couplers are provided in the four parts of the standard ISO 11011 [26; 27; 28; 29], and details about the clearance zones around the operating machines are provided in the standard ISO 2332 [30].