Technical Field The present invention relates to a stepless gear device comprising a rotational axis, around which power transmission means are rotably and radially movable. Further the radius of the gear device is variable by a circumferential path, which interact with the power transmission part, like for example a belt, chain or other transmission means.

Background Art Gear devices with variable gear ratio by change of the radius of a wheel are known for use at for example driving machines. Gear devices are today used in a number of different applications where it is often advantageous to use a variable gear to control the torque and the number of revolutions to be transmitted at for example vehicles, machines and other devices where power is transmitted to a supplier. There are many different kind of gear with variable gear ratio on the market. Nevertheless there are limitations with this kind of gear regarding the possibility to transmit large forces and at the same time achieve a high efficiency.

Summary of the Invention One objective of the present invention is to achieve a cost efficient gear device that can take large load with high coefficient of efficiency. The above objects and other objects that will be evident from the following description are achieved by a device and system according to the appended claims . The fundamental idea of the invention is based on the understanding that a gear device with variable radius can be achieved by means of discs carrying and supporting transmission means. According to one aspect of the present invention a stepless variable gear device is provided comprising power transmission elements, which are rotable around a rotational axis and radially movable, with variable radius of a circumferential path for with the gear device interacting transmission elements. Said power transmission elements have a disc shaped extension and are positioned in planes in the circumferential direction, which coincide with said rotational axis. The power transmission elements are radially displaceable in the holder part, at least one operating part is provided for displacement of said power transmission element in a radial direction. The peripheral located edge parts of the power transmission elements forms a circumferential path for said transmission element, i.e. the edge parts of the power transmission elements will support the transmission element. The edge parts will, during change of the gear ratio, move the transmission element such that it runs either on a circumferential path with greater or smaller radius. The transmission element on the other hand can constitute of a belt, chain or similar means . Preferably the power transmission parts consist of discs, which are flexible in a tangential direction in a vicinity of the engagement of the disc with the transmission part. The expression discs means thin disc shaped elements. The tangential flexibility is achieved for example by bending of the disc at the U-shaped section. The discs can also be formed with a profile in the plane, i.e. material has been removed to achieve a certain geometry, but the discs are still thin and has extension essentially in plane. Further the peripherally positioned edge parts of the power transmission elements are flexible in radial direction. This can be provided by the power transmission parts, for example in the form of discs, having a geometry or material that provides the flexibility. Alternatively the power transmitting discs can be provided with a certain flexibility to give the gear device desired characteristics . By using the disc- shaped form of the power transmission part a number of discs can be positioned around the gear device, which gives a number of supporting edges for the transmission part. Preferably the operating means has, in the radial plane, an angled contact surface in relation to the rotational axis, which contact surface is provided for transmission of an axial operating movement of the operating means to a radial movement of said power transmission part. This angled contact surface has preferably a conical geometry at the operating means that affects the radial position when the operating means is displaced in axial direction. The operating means and the power transmission part is preferably interacting with a claw coupling, like for example a finger within a fork or similar. This kind of coupling has the advantage that it can be formed so that it affects the discs, the power transmission parts, both inward and outward in radial direction. The claw coupling can be formed to constitute the angled contact surfaces for transmission of an axial operating movement of the operation means to a radial movement of said power transmission part. The disc is for example formed with extensions on both sides of a "finger" of the operation part. In this way the disc is affected both outwards and inwards in radial direction of the finger of the operation part. This finger can be of a finger-like geometry alternatively with a shape extending into a claw shaped geometry of the disc. Preferably the claw coupling is formed as a fork in the disc-shaped power transmission part, which part is in connecting interaction with the operation means extending into said fork for operation of the radial position of the power transmission parts. When the operation means is displaced in axial direction the power transmission part is forced to move in radial direction due to the interaction of the angled fork with the operation means. Further the power transmission parts are formed as discs, which are radially displaceable in the holder part. The holder part consist of a circular drum with at least one gable, which is provided with guiding tracks for the discs. The holder part and the discs will keep their axial position when the operation means is displaced in an axial direction. The holder part, power transmission part and operation means will follow with the rotation of the gear device. When two or more holder parts are used at the same gear device the holder parts are also mutually coupled to achieve an simultaneous rotation, whereby bending and displacement of the discs between the holder parts is avoided. The circular lateral area of said holder part is preferably provided with slots across the rotational direction for guiding the power transmission parts that can move radially by said track. These tracks, at the lateral area of the holder part, are supporting the power transmission parts, which is also done by the tracks provided at the circular gables of the holder part. Preferably the power transmission parts is formed with recesses in the peripherally located edge portions for engagement with said transmission parts. By this design it is possible to achieve a gear device where the radius for engagement with the transmission part provides desired gear ratio. Advantageously the above mentioned recesses at the edge portions of the power transmission parts are formed by a U-shape at the power transmission parts. The U-shape has a crank-shaped design, which cause that it is flexible in the radial direction. The flexibility is achieved by bending and twisting in the material of the power transmission part in tangential direction, this flexibility can be increased or be reduced by changing the design of the crank-shaped portion. This flexibility is used when the gear ratio is changed since the power transmission part in that case is moving in radial direction, the distance between the edge portions of the power transmission parts endeavour to change. Advantageously the above described gear device is provided in a system comprising a first and second power interacting gear device, which are coupled for simultaneous operation so that when the radius of the first gear device is increasing, the radius of the second gear device will decrease. In this way a constant length of the transmission part, transferring power between the first and second gear devices, can be maintained. Since the operation means of the first and second gear devices are facing in opposite directions the simultaneous operation is performed by displacing of both first and second operation means in the same direction, in axial direction. In an alternative embodiment of the above mentioned system the first and second gear devices are power interacting by direct interaction between the power transmission parts of the gear devices, i.e. the discs are engaging each other. Also in this case the first and second gear device can remain at a constant distance from each other and be operated simultaneous by displacement of the operation means in the axial direction. The disc-shaped form of the power transmission parts result in that the outer edge portions form engaging edges for the transmission part, this is advantageous for avoiding slipping. Further the thin thickness of the disc result in that the radial space can be used by a number of discs. Since the gear device can be provided with a number of discs it is not necessary with any supporting belt around the gear device to span the distances between the discs. Furthermore the limited thickness, non-rigid and flexibility of the discs for example in the radial direction, which admit change of the gear ratio even though for example a number of discs are engaged with a transmission part. Preferably the angled contact surface of the operation means is constituted of the lateral area a cylindrical cone around which the fork-shaped portion of the discs engage to transmit the axial movement of the operation means to radial movement of the discs.

Brief Description of the Drawings The invention will now be described with reference to the accompanying drawings, which for the purpose of exemplification will show present preferred embodiments of the invention. Figure 1 shows a perspective view, partly in section, of a gear device in a position with small radius according to the invention. Figure 2 shows a perspective view partly in section of a gear device in a position with great radius according to the invention. Figure 3 shows in section a gear system with a first and second gear device respectively according to the invention.

Detailed Description of Preferred Embodiments The invention will now for the purpose of exempli¬ fication be described in more detail by means of examples and with reference to the accompanying drawings. According to a embodiment in figure 1 of a gear device according to the invention the axially displaceable operation means 5 is shown, which in this embodiment is constituted of two conically formed cones . The operation element is constituted of two conical operation means, which are connected for common operation of the radial position of the power transmitting discs. The conical operation means is constituted of the envelope surface of a cone, except the basis, with a small thickness of material. One disc is affected by the conical operation means by having a tongue of material both on the outside and the inside of the conical operation means. The contact surface of the disc towards the outer envelope surface of the cone is formed so that the disc has a radius equal or larger than the largest diameter of the cone. The contact surface of the disc towards the inner envelope surface is equal or smaller than the smallest radius of the cone. By displacing the operation means 5 in figure 1 to the right the power transmission element 2 is forced to move outwards in radial direction. In this way the radius of the contact surface towards the radial edge parts of the power transmission means, which is positioned between both holder parts 4 increase. Both holder parts 4 are kept in position in relation to each other, in an axial direction, by two gear wheels which are hold together by an axis. These gear wheels will engage with teeth positioned in guide tracks at the peripheral circumferential of the holder parts. Further said gear wheels transmit power from one holder part to another holder part. The gear wheels will not rotate with the gear device. There is a plurality of power transmitting discs and they are further positioned around a center axis, shown in figure 1 and 2, due to that the discs are thin they can be positioned tight together and provides a good support surface for a power transmission part 3 also in the position when the device has its greatest operative radius. The holder part 4 will further control the power transmitting discs 2 so that they will not follow the operation cones 5 in the axial movement, but only moves in an radial direction when the device is operated. The holder parts 4 are provided with tracks on the inside of the circularly outer gables for guiding the power transmission parts in radial direction. The inner positioned gables of the holder elements can either, as shown in figure 1, be continuously slotted for guiding the discs in radial direction, or the inner gables can be provided with a larger center hole and track on the inside of the gable, in the same way as for the outer gable. In figure 2 the position is shown in which the gear device 1 is operated to the outer position of the discs 2 i.e. the edge portions of the discs form their maximal radius. During operation of the gear device 1 the holder parts 4, the discs 2 and operation cones 5 are rotating with the same speed, in this way there is no friction due to rotation between the parts. The operation part 5 is further rotary carried by the bearing 6 in one end. The discs are formed with a U-shaped portion for carrying the transmission element 3. Due to the U-shaped portion the transmission element can be provided close to the center axis and the smallest diameter of the gear device can be minimized. By this minimizing of the smallest active radius, the gear ratio for the gear device is maximized. Further the U-shaped portion causes, during use of toothed chain/link/belt, the disc at the U- shaped portion to bend in tangential direction and gives the teeth of the transmission element greater possibility to go down deeper between the edge portions of the discs. In this way a greater power transmission is achieved and at the same time the steps between the teeth will be less abrupt. The edge portions of the discs are partly locked in the transmission element and an increased bending of the U-shaped portion of the disc in tangential direction cause a minor increase in gear speed. The above described flexibility of the U-shaped portion of the disc provides a smooth operation and easy changing between different gear ratio. Further flexibility is achieved by the disc 2 from the claw-formed fork that is cooperating with the operation cones 5. The claw-formed fork means aims at the portion of the disc, with two extending parts, that capture the operation element 5. The left fork (figure 2) connect to the U-shaped portion of the disc with a long, narrow portion of material. The right U-shaped portion connect to the remaining disc with a portion of material that connects at the middle of the part to the right of the U-shaped portion. These portion of material provides a certain flexibility in radial direction and also for the structure of the disc 2 when the operational movement is transmitted from the cone 5 to the disc 2. Gear wheels 7 connects both holder parts 4, by means of these gear wheels 7 synchronized rotations are achieved for both holder parts. Furthermore the gear wheels 7 contribute to achieve a constant axial distance between the holder elements 4. Figure 3 shows in section a system of two cooperating gear devices that are connected by a transmission part 3 consisting of a belt, chain or other element with constant length. First and second operation element respectively are connected (not shown) for simultaneous operation of both gear devices . By the orientation of both gear devices in different directions the operation cones 5 will during operation increase the radius of one gear device at the same time as the radius of the second gear device decrease, in this way the length of the transmission part 3 remain constant. In an alternative embodiment only one surface of the operation cone is used, for example the outer surface of the cone can be used for moving the discs outward and hence increase the radius of the circle. To move the discs 2 inwards the discs can for example be spring loaded towards the operating cone, i.e. towards a position with less radius for the circle. In a further alternative embodiment the disc is formed in a inverted way, i.e. the finger-like portion is provided on the disc and engages with a fork like formation of the operation part. Different kind of transmission parts can be used, for example a chain with teeth can be used for power transmission. Further a toothed belt, flat belt or other kind of belt can be used. In a further alternative a gear wheel is used acting direct towards the discs of the gear device. This gear wheel then has to be movable in radial direction to adapt to the varying radius of the gear device. It is also possible to achieve a gear device according to the invention that has one operation cone instead of two, as described above.