In fully hydrostatic drive, each wheel being differentiated with a distributor valve or equipped with its own pump for driving independently of each other, one or more wheels losing contact with the ground only decreases the driving power on its own part. If all drive motors are in a common circuit, the loss of contact by one motor stops the advancement completely. Oftentimes, the vehicles are provided with bogies or trucks on one or both axles. One of the features of a bogie is that, in a driving condition, the foremost wheel of the bogie tends to lose weight or come off the ground. Upon rising off the ground, the foremost wheel is running loose, the pressure"escaping"from the foremost wheel's pressure line, while the distributor valve makes sure that the pressure in the rearmost wheel's pressure line does not disappear anywhere. As for this particular bogie, however, the driving power drops to a half. On flat terrain, the demand of driving power being low, there are no special problems, but in snow, treacherous ground, and climbs "the tipping"of a bogie is undesirable.

Hence, an object of the present invention is to provide a bogie control system for eliminating the above drawback. In order to achieve this object, a bogie control system of the invention is characterized by what is set forth in the characterizing clause of claim 1.

The invention will now be described in more detail with reference made to the accompanying drawings, in which : fig. 1 shows schematically one bogie equipped with a bogie control system of the invention, and fig. 2 shows one example of a hydraulic diagram for a bogie control system of the invention.

In fig. 1, reference numeral 9 designates the foremost wheel of a bogie and reference numeral 10 the rearmost wheel of a bogie. Reference numeral 2 in turn indicates a cylinder included in a bogie control system of the invention, which can be used for delivering pressure on the wheel 9 foremost in the driving direction so as to maintain the driving power as high as possible. The cylinder 2 can be a separate or pre-existing bogie control cylinder. In the depicted embodiment, the cylinder 2 is supplied with pressure from the pressure line of a motor 8b (fig. 2) serving the rearmost bogie wheel 10, whereby an increase in the driving pressure increases also the counter-moment produced by the cylinder. The cylinder can be designed for pulling or pushing.

Additionally in fig. 1, reference numeral M1 indicates a moment produced by a foremost motor 8a, reference numeral M2 a moment produced by the rearmost motor 8b, reference numeral M3 a moment produced by the moments M1 + M2 on a bogie, and reference numeral M4 a counter-moment from the cylinder on the bogie.

As the pressure increases in the pressure line of a motor in a driving situation, the moment M1 + M2 produced by the motor results in a moment M3, which tips the bogie and raises the bogie wheel 9 foremost in the driving direction. By delivering the pressure from the pressure line of the rearmost bogie motor 8b to the bogie control cylinder for producing a moment M4, the foremost bogie wheel 9 can be returned to drive. If M4 strives to become sufficiently high to lift off the rearmost bogie wheel 10, the pressure in the pressure line of the rearmost wheel motor will drop and the cylinder pressure will also drop and the moment M4 will go down. At this point, the bogie returns again to driving with both wheels.

The relationships between moments must be worked out in such a way that M4 is higher than M1 or M2 but lower than M1 + M2.

Fig. 2 illustrates a hydraulic diagram for a bogie control system of the invention.

The hydraulic diagram is depicted in the condition, wherein the bogie pressurizing automatics is not in use. Thus, the bogie is controlled by means of a valve block

or it is in a"floating"condition. In the diagram, reference numeral 8a designates a motor for the foremost bogie wheel 9 and reference numeral 8b a motor for the rearmost bogie wheel 10. When the bogie control automatics is switched on electrically, valves 4 and 5 assume a position towards an electric coil. A pump 1 delivers a driving pressure which progresses to a gate A in a pole motor. From the same line is extracted through a throttle and a counter-valve 3 a small (less than 20 I/min) volume flow to the bogie control cylinder 2 for pressing the foremost wheel 9 onto the ground. The volume flow is worked out in such a way that, in all situations, the closed-system feed pump has an output sufficient for maintaining the system in working order. If such a large amount of oil is required by the cylinder that there is not enough time for the throttle to fill the cylinder, the cylinder will receive its filling through the valve 5, said valve 5 being connected to a tank of hydraulic fluid.

As the bogie starts climbing without any obstacles, there is oil flowing from the rod side of the cylinder 2 towards the system. The valve 3 does not admit the return oil into a closed system and, as the pressure has grown to be about 20 bar higher than the driving pressure, a valve 6 will open and the return oil discharges into a hydraulic fluid tank through an appropriate valve (not shown). As the obstacle or terrain changes in such a way that the pressure in the cylinder 2 falls down to the driving pressure, the valve 6 closes. Upon stopping the drive, the cylinder will retain a pressure of about 20 bar for pressing the front bogie wheel 9 against the ground until the valves 4 and 5 are released electrically. At that point, the bogies are in a floating condition or individually controllable by means of a valve block.

The relationship between a moment produced by the drive and a counter-moment produced by the cylinder 2 can be worked out either by dimensioning the diameters of the bogie control cylinders for a correct ratio or by using a relief valve 7.

The bogie control system of the invention can be used for providing automatic bogie control over a bogie, which automatically provides a maximal grip for both wheels of a bogie on the base thereof. The system is applicable to all fully

hydrostatic transmissions equipped with a bogie. The system can be designed to be functional for driving in both directions.

In the depicted embodiment, the pressure required by the cylinder 2 has been extracted from the pressure line of the rearward motor 8b, but the invention can also be implemented by delivering the pressure required by the cylinder 2 from a pressure line external of the pressure line of the motors 8a, 8b, the pressure increase in the pressure line of the rear motor 8b in a driving situation being used for producing a necessary control signal for using an external pressure line.