The present invention relates to a self-cleaning cutting head and a lawnmower adapted with such head, more particularly a low consumption robotic lawnmower powered by an electric motor.

Lawnmowers with rechargeable batteries are designed in order to run with a minimum power. The cutting device must be adapted to this primary need. It is more particularly the case for mowers which are entirely robotic, with unattended operation, such as the ones described in the international patent application PCT/BE91/00068.

Traditional cutting systems perform the cutting operation under a cover, the cut grass being carried along by the rotating system. The ensuing result is that an important part of the energy is wasted during the cutting operation in order to transfer a kinetic energy to the cut grass which is dragged away by the cutting blade.

The cutting head usually used in robotic machines with a rechargeable battery is most often made of a lower plate, essentially circular, assembled to an upper plate by attaching flanges in such a way that a narrow space is left between the two plates.

A cutting disk is located between such plates and fixedly attached to a vertically extending output shaft from an electric motor. The cutting disk comprises preferably at least two retractable blades on its periphery, each of them pivoting around a shaft located on said periphery and capable of returning between said plates.

When the cutting disk starts to rotate, driven by the electric motor, the blades extend due to the centrifugal force and protrude out from both fixed plates in order to perform the grass cutting operation.

The rotating part in contact with the grass is then dramatically reduced and this arrangement decreases substantially the friction on the surface to be mowed, the carrying along or drawing away of the grass by the rotating system and the aerodynamic turbulences.

The use of such lower static plate entails however some problems.

In spite of the centrifugal effect due to the rotating disk, grass debris may on the long run and more particularly in wet weather, adhere to the internal face of the lower plate resulting in a braking effect with concomitant increase of energy consumption.

A low energy consumption is a vital criterion for an efficient performance of an electric lawnmower, more particularly a robotic mower with rechargeable battery.

In particular, for such an autonomous lawnmower with a battery which is rechargeable by induction at a fixed station, it is important than the search for this fixed station according to a predetermined algorithm (such as disclosed in patent application PCT/BE95/00013) be performed with a minimum energy consumption. For this purpose, the rotating cutting disk is disabled and will not operate during the searching step.

The lower plate acts then as a crumb scoop and scrapes the surface to be mowed with ensuing penetration of earth, twigs or dead leave debris between the plates. This may create a problem when the cutting disk has to be restarted.

The purpose of the present invention is to overcome the above mentioned disadvantages.

According to the invention, there is provided a cutting head comprising a cutting disk and at least one lower circular plate , preferably of similar dimension, wherein the static lower plate of the prior art is replaced by a mobile plate freely fixed at the end of the rotating shaft of the driving motor, for example by using a rolling bearing such a ball bearing a sliding bearing or a roller bearing.

The lower plate, in normal operation, which replaces the static lower plate of the prior art, is in contact with the grass of the surface to be mowed and do not rotate, or only slightly.

This construction provides a simultaneous absence of friction between the rotating cutting disk and to decrease the aerodynamic friction.

if debris becomes stuck between the lower plate and the cutting disk, the cutting disk will drag along the lower plate when the rotation starts at the beginning of a grass cutting operation, together with the debris which are then ejected by the rapid rotating movement. The rotating speed (for example 2,500 rotations per minute) is indeed of such an amplitude that no debris will remain between the cutting disk and the lower plate. The cutting head performs therefore as a self-cleaning cutting head.

The accompanying drawing given by way of example is a view in cross-sectional elevation of a device according to the invention,

The device comprises a motor 1 fixed to the chassis (not shown), possibly through an upper supporting plate 2. The motor is equipped with a shaft 3 at the end of which a cutting disk with retractable blades 5 is fixedly mounted.

At the free end of the shaft, a lower circular plate 7 is mounted, which may freely rotate, through the use of a ball bearing 8. The plate 7 is parallel to the cutting disk and separated by a distance of p.e. 5 to 10 mm.

Advantageously, the cutting head may be mounted slightly inclined forward in relation to the direction of movement of the mower such that there is a tilt angle of a few degrees between the spin axis and the perpendicular to the

horizontal plane (the plane of the grass to be cut) . Preferably the angle is between 4 and 10°, still preferably about 7°. The rear of the plates and cutting disk are therefore upswept and elevated over the mowed surface with no contact with the grass. According to this particular arrangement, the cutting blades will not touch again the grass at the rear of the cutting head, that is to say at a point opposite to the cutting operation, avoiding in this manner an additional energy consumption of poor efficiency.

This feature of the invention may also be applied independently to a cutting head of the prior art mower machine, with or without a lower plate.

The foregoing is considered as illustrative only of the principles of the invention. Since numerous modifications and changes will readily occur to those skilled in the art it is not desired to limit the invention tro the exact construction znd operation shown and described and accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the invention.