TECHNICAL FIELD 
     The invention relates to a flexible line guide for an industrial robot according to the introduction to claim 1. 
     Such guides are commonly used for carrying an outer line in the form of a welding cable or a pressurised fluid line, for example, between the foot and the rotating upper section of an industrial robot. 
     PRIOR ART 
     Guides of this type are known, for example, from U.S. Pat. No. 5,694,813 and EP-A-0 552 688. A disadvantage of such known guides is that the outer peripheral side of the loop, especially at the bend, must be guided by the inside of a stationary cylindrical wall or a cylindrical wall that rotates together upper section of the robot, in order not to deviate outwards from the path of movement around the axis, which causes undesirable friction and wear on the covering of the line. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to achieve a guide of the type stated in the introduction that significantly reduces wear and thus increases the working life of the covering of the line. 
     A second objective is to obtain a guide that makes it easy to replace the line. 
     These are achieved by means of the features that are stated in the following claims. 
     According to an aspect of the invention, a separate groove for every strand is the only engagement provided between the cable loop and the parts that rotate relative to one another. In this way, none of the strands nor the bend come into sliding contact with a wall, as is the case in the prior art, so that essentially all the relative movement between the covering of the line and the parts that rotate relative to one another that causes wear is eliminated. A further advantage with this arrangement is that thanks to the resulting free space between the grooves, the line is easier to remove from the guide and be put back there when it needs to be replaced. 
     According to one preferred embodiment of the invention, the grooves have different mutual radii so that the bend will be angled relative to the common axis of rotation. In this way, the radius of curvature of the bend can also be increased advantageously at a pre-determined axial distance between the grooves. This ensures partly that the insignificant relative movement that does nevertheless occur between the line and the groove takes place primarily only at the transition between the strand and the bend of the groove with the smaller radius and partly that the bend has less of a tendency to deviate at a tangent outwards from the path of rotation since it leans inwards from the largest radius. 
     If the grooves are formed in groove elements that can be mounted on the outsides of the respective parts in a removable fashion, an existing robot can later easily have an additional guide fitted according to the invention. 
     Other features and advantages of the invention are evident from the claims and the following detailed description. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention is described by way of example in detail below with reference to the enclosed drawings, where 
     FIG. 1 is a diagrammatic view from behind of an industrial robot with some parts removed fitted with a guide according to the invention and; 
     FIG. 2 is a simplified diagrammatic view of the guide according to FIG. 1 seen from above at an oblique angle; 
     FIG. 3 is a view partly in cross-section and with parts removed of one preferred embodiment of a groove element according to the invention; 
     FIG. 4 is a cross-sectional view of an alternative embodiment of a groove element according to the invention; 
     FIG. 5 is a view seen from the front and from above at an oblique angle of a foot and the frame element of an industrial robot equipped with a groove element according FIG. 3; and 
     FIG. 6 is a side view of a guide according to the invention provided with a support for an upper part of the guided line. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the diagrammatic representation according to FIG. 1, an industrial robot is generally designated  10  and includes a lower section  12  in the form of a stationary foot and a rotating upper section  14  in the form of a frame unit that rotates relative to the foot around a vertical axis  18 . The lower section  12  and the upper section  14  are joined with one another via a bearing arrangement  16  that is not shown in any greater detail. 
     A flexible, continuous line  40  with a covering that is not shown in detail extends between the lower section  12  and the upper section  14 . This covering can typically enclose a welding cable, but also other types of lines for, for example, transport of fluids or other media. The covering can also be a separate outer covering for that part of the line that extends between parts  12  and  14  (not shown). In order for the upper part  14  to rotate without hinder for approximately 180° clock-wise and 180° counter-clockwise from the mid-point of rotation in FIG. 1, a slack of the line  40  to accommodate the movement of rotation extends in a loop round about 90° of the space between parts  12  and  14 . More specifically, the loop has a lower partly circular strand  42 , an upper partly circular strand  44  and a bend or nose  46  of line  40  that joins the strands. When the upper part  14  rotates counter-clockwise, the upper strand  44  is transferred to the lower strand  42  via the bend  46  “rolling forwards”. When part  14  rotates clockwise, the lower strand  42  is transferred to the upper strand  44  in the reversed manner via the bend  46  “rolling backwards” 
     According to the invention, strands  42 ,  44  are guided by their engagement with their own groove  22  and  32  respectively. In the examples of the embodiments shown, grooves  22 ,  32  both extend in a circular arch around a front of the robot  10  when this is positioned in the middle of its field of rotation. 
     According to the invention, grooves  22  and  32  have different radii R 1  and R 2  respectively with regard to their common axis  18 . The lower groove has the larger radius in the example shown, but the opposite case is also possible. It is also possible to have grooves with different radii in the same radial plane with regard to axis  18 , so that the parts of the loops and the bend run in a common horizontal plane around the robot (not shown). 
     The diagrammatic representations in FIGS. 1 and 2 show grooves  22 ;  32  in principal only in the form of thin-walled channels with a flat bottom wall  24 ,  34  and outwardly angled side walls, flanks or flanges  26 ,  28 ;  36 ,  38 . In these cases, the groove can be made of, for example, folded and possibly welded steel plate that can be attached to the associated parts  12 ,  14  respectively by means of screw fittings. However, it is also possible to make the grooves or channels of plastic material. 
     The cross-section of the groove can be varied to achieve an optimal compromise between minimal wear and best guidance, especially between the transition of the line between the strand and the bend in the groove with the smallest radius R 2 . According to one preferred embodiment of the invention shown in FIGS. 3 and 5, such an optimised groove is formed in groove element  21  from extruded and bent light metal, preferably aluminium. 
     In the embodiment shown in FIG. 3, each groove is delimited by an essentially vertical inner flange  28  and an outer flange  26  that is angled outwards from the bottom wall  24 . The inner sides of flanges  26 ,  28  are slightly convex to securely fix the cross-section of line  40  and guide line  40  down towards the bottom of the groove. However, as has already been mentioned, this also facilitates fitting or removing line  40  due to the free space between the grooves. In addition, the free ends of the outer flange  26  have a wider section that provides extra rigidity, that forms a gently rounded finish to reduce the risk of personal injuries, and that forms an end stop that can help prevent line  40  from jumping out from the groove if for some reason—such as due to a foreign object in the groove—it momentarily comes under pressure to be pushed out from the groove. 
     Each groove element  21  (FIG. 3) is attached to the associated part  12  of robot  10  from the outside of the robot by means of, for example, a screw fitting and is suitably positioned at the peripheral edge of associated part  12  via a peripheral corner recess  27  formed on the outside of the bottom  24  of the groove. 
     The angle of opening and the width between the flanks of the groove can also be formed so that the cross-section of the line snaps firmly into the groove. The downwards facing groove especially can be formed in this way to prevent the line falling out from the groove due to its weight. 
     Since an operator or service technician will for various reasons from time to time need to climb up onto the robot and can then need to use the groove elements as foot supports, these are suitably dimensioned to have the size and durability to match this need. 
     So that the upwards facing groove of the guide will not collect liquids and foreign objects, such as welding sparks and similar, this groove preferably has openings running through it. The openings can, in a way not shown, be accommodated in the bottom and in the flanges of the already described embodiments made of steel plate and light metal. 
     In the embodiment shown in FIG. 4, the groove element is made of a welded steel wire construction. In this case, the groove is formed from essentially a half-circular arch-shape of steel wires  50 ,  52  that are carried on steel wires  54  (only one is shown) that are at right angles to these and that are evenly spaced peripherally, and that are shaped to the desired profile of the groove. As a consequence, the desired openings in the groove are formed by the gaps  56  between the steel wires. 
     The groove elements can, however, be formed in other ways. For example, they can be formed by straight sections that are welded together (not shown). The groove need not be continuous either, but can extend around the groove periphery in the form of discrete elements with the gap between them (not shown) forming the openings. 
     FIG. 6 shows another embodiment of a guide according to the invention in the position at the end of its rotation (180° clockwise rotation) where the major portion of line  40  is in the upper strand  44 . To avoid the risk that this might fall down from the upper groove  32 , a lower support  60  is arranged for line  40 . In the preferred embodiment, lower support  60  has the form of a hanging support comprised of a bent tube  62 . Tube  62  extends concentrically to the upper groove  32  in such a way that it captures and holds the upper strand  44  from below when this tends to exit the groove due to its weight. Tube  62  suitably has a smaller radius than the radius of upper groove  32 . If lower groove  22  additionally has a larger radius that upper groove  32 , as is preferable, the bend of line  40  is angled inwards towards the upper groove without the risk of coming in contact with tube  62 .
Guide ( 20) for a loop of a flexible line ( 40) between a pair of rotatable parts ( 12, 14) of an industrial robot ( 10) that rotate relative to one another around an axis ( 18), whereby the loop includes a pair of strands ( 42, 44) of the line joined by a bend ( 46) that extend around axis ( 18) with their outsides engaged with the respective parts so that the loop is caused to perform a movement around the axis ( 18) when the parts rotate relative to one another. To reduce wear on the cover of the line and to facilitate replacing the line, it is suggested that, among other things, the engagement consists of a separate groove ( 22, 32) arranged for each strand ( 42 respectively 44).