The present invention relates to a stabilization system for stabilizing the lateral oscillation of a cabin of a vehicle, especially of a forest tractor comprising a front and rear frame connected to one another, and in which the cabin is preferably located in the front frame, whereby the front frame is arranged onto the back frame with a joint of at least one degree of freedom that allows the rotation of the frames essentially with respect to the longitudinal axle of the vehicle, and that the front frame has at least one axle suspended so that it swings essentially with respect to the longitudinal axle of the vehicle.

Because of the lateral oscillation of the vehicle due to the roughness of the terrain, the driver is subjected to a low-frequency (0.5 to 10 Hz) vibration with an occasionally variable amplitude, which is found to be detrimental to the driver's health. The ill-effects of this vibration have been found to be exhaustion, lowered work efficiency and straining on the joints and the spinal column of the driver which in the long run will cause back disorders, for example. The effect of vibrations on human health has been widely discussed by Griffin (1990) and Depruis & Zerlett (1986) .

The lateral oscillation of a vehicle also has an effect on the comfort of the driver and especially in a forestry equipment on how terrain friendly and stable the equipment is. If the lateral oscillation can be absorbed, the driving speed can be correspondingly increased, which has a direct effect on the output of the equipment . The disadvantages mentioned above have been

solved in different ways in forestry equipments at present. A simple way is to replace a two-wheel axle by a boggie that due to leverage slightly evens out the vibration. The acceleration on the driver can also be reduced by active suspension developed by automobile industry, but this will not remove low-frequency lateral oscillation of the cabin which is, according to studies, most detrimental to human health.

Most forestry equipments are of a two-frame structure, whereby the front and rear parts are connected to one another with a joint that enables the control of the equipment and allows the rotation of these parts with respect to each other if the axles of both parts are secured unmoveably onto the frame parts . Both ordinary axles and boggies are generally used in mechanical transmission. The boggies are normally fixedly mounted onto the frame. A boggie distributes the traction force and the surface pressure more evenly onto the terrain and slightly evens out the vibrations. Ordinary two-wheel axles can be mounted both fixedly and jointedly onto the frame, as a so- called swing axle. Commonly one axle is fixed, while the other may swing.

It is also known to place wheels onto separate shafts that can be moved by a regulating unit with respect to each other and the frame of the vehicle and thus even out the roughness of the terrain.

By using prior art solutions, a problem is how to eliminate the lateral oscillation of the vehicle actively because then the whole mass of the vehicle has to be controlled by the selected lateral oscillation removal arrangement . Depending on the frequency of the swinging, it causes a great need for absorption power which will lead to great and expensive structural solutions.

The lateral oscillation of the cabin has also been reduced by suspending the cabin onto lever arms that are actively controlled by a regulating unit.

However, a problem is the complexity of the mechanism and the great need for space of the lever structure.

The object of the present invention is to eliminate the disadvantages of the prior art and provide a new solution with which detrimental lateral oscillation on the driver of a vehicle can be eliminated in a simple and inexpensive manner.

The invention is based on the idea that in order that the need for hydraulic power is not unreasonably great, the mass to be moved or the speed has to be relatively small. The object of the invention is achieved in such a manner that the stabilization system has the characteristics specified in the claims in accordance with the present invention. More exactly, this equipment according to the invention is mainly characterized in that regulating units are arranged onto the front frame jointed to the axle for controlling the axle so that lateral oscillation caused by the roughness of the terrain will be diminished or eliminated. The solution according to the invention relates to combining a frame joint allowing the mutual rotation of the front and rear frame and a swinging axle of a vehicle, a forest tractor, in particular, so that when driving on rough terrain, the suspension together with the regulating units using them and the system controlling them actively eliminate lateral oscillation of the cabin caused by the roughness of the terrain.

It is not previously known to combine, as shown in the present invention, the swing axle of a

vehicle and a frame joint allowing movement in the parallel direction, and to drive actively the part of the frame of the vehicle secured onto said joints.

The elimination of the lateral oscillation of the vehicle has considerable advantages. The work comfort of the driver will get better and the vehicle can drive faster, whereby its productivity will improve. The surface pressures of tyres are distributed evenly, whereby damage to the terrain and the roots will diminish and it will be easier to move on soft terrain. The solution according to the invention makes it possible to have narrower logging roads and diminishes damages to the bodies of trees left standing. Additionally, the straining on the frame and axles of the vehicle will decrease, which will reduce the need for maintenance and repairs .

In the following, the invention will be illustrated by reference to Figure 1 which shows a schematic view of a frame structure and the suspension of axles of a forestry equipment.

The vehicle comprises two frame parts, a front frame 1 and a rear frame 2 that are jointed together by a joint 3 of at least one degree of freedom enabling the rotation of the frame parts of the vehicle in view of each other according to movement with respect to the longitudinal axle. The vehicle further comprises a cabin 4 that is connected to the front frame 1 by a passive oscillation absorber. The vehicle also comprises a boggie or an axle 5 that is secured fixedly onto the rear frame, and an axle 6 that is arranged jointedly onto the front frame 1 with a joint 7. The axle 6 jointed onto the front frame 1 can be, when needed, braced to be immoveable with cylinders 8 and 9, for example. The front frame 1 jointed onto the axle 6 can be moved with cylinders 8 and 9, as with hydraulic

cylinders, with respect to the axle.

When the vehicle is moving, the cylinders 8 and 9 are active and almost pressureless, whereby the axle 6 attached to the front frame 1 with the joint 7 will not shift the movement around the longitudinal axle of the vehicle to the front frame . The movement around the longitudinal axle of the vehicle is not transmitted from the rear frame 2 to the front frame 1 as the joint 3 is in between. For controlling its regulating units the vehicle preferably comprises a hydraulic system that can be implemented with valves 10 and 11, for example. The system controls the cylinders 8 and 9 so that there is as little pressure and pressure difference as possible in the cylinders. In that case the front frame

I tries to retain such a position during driving where the cylinder powers are as small as possible, that is, it is in equilibrium. The lateral oscillation of the vehicle is eliminated if the roughness remains within the perimeter of movement of the cylinders. The mass centre of the cabin 4 will remain essentially at the centre line of the vehicle.

The hydraulic system controlling the cylinders 8 and 9 may be as shown in the figure, comprising the valves 10 and 11 or some other equilibrium sensing system known per se, such as a proportional valve system controlled by sensors and a computer. The system according to the figure operates so that when the terrain descends at the side of the cylinder 8, the mass centre of the vehicle will shift onto the same side causing the pressure to increase in a line 12 connecting the regulating unit to the hydraulic system which line will connect the left section of the valve

II for operating via a pilot operation line 13 in the system. In that case the pressure of the hydraulic

system of the vehicle is directed to the cylinder 8 that will initiate a thrust motion. At the same time the oil from the cylinder 9 enters the tank in the system via lines 14 and 15 and through the left section of the valve 11. When the mass centre passes over the centre line of the vehicle, the pressure in the cylinder 8 and in the line 12 disappears, whereby the valve 11 closes and the thrust motion terminates. It is important for the operation to dimension the components of the system so that the response time of the control system is short enough and the regulation stable. The hydraulic system also comprises valves 16 and 17 the object of which is to brace the system by rendering the axle unmoveable with respect to the frame 1. The invention is not restricted to the embodiment above, but various modifications are possible within the scope of the inventive idea specified in the appended claims .