5 The invention relates to an impeller for an axial fan, having an impeller body, which has an outer impeller shell and a carrier disc having a hub for connection for conjoint rotation to a drive shaft, wherein a multiplicity of radially outr.rard-pointing blades, which 10 each have a blade root and a vane, is arranged on the carrier disc. Axial ventilators are widely used in the industry. In many cases, an impeller for an axial ventilator 15 comprises an impeller body, a plurality of radially outward-pointing blades and elements for fastening the blades on the impeller body. Moreover, an inlet shroud fastened on the impeller body is often provided to reduce turbulence. Among the components of the impeller 20 body are an outer impeller casing to guide the flow, a carrier disc and a hub for coupling for conjoint rotation to a drive shaft. The impeller body can be df integral construction and, in this case, can be produced by welding, casting or forging, for example. 25 Moreover, the impeller body can also be of multi-part construction, having individual components fastened by screws, for example. As components with a primarily aerodynamic action, the blades generally comprise a vane and a blade root for fastening on the impeller 30 body. A very wide variety of production methods, e.g. forging, casting, pressing or even milling, may also be considered for the blades. An impeller for an axial ventilator or an axial fan is 35 known from DE 10 2006 001 909 84. The impeller of the axial ventilator comprises a cylindrical carrier disc and a multiplicity of blades, which each have a blade root and a vane. A pair of clamping jaws is provided for the mechanically fixed clamping of each blade. For this purpose, the clamping jaws of each pair of clamping jaws are arranged on both sides of the carrier disc and are screwed to the 5 latter. As regards their configuration, all the clamping jaws are identical in as much as they are completely interchangeable. In its radially outwardoriented region, each clamping jaw merges into a radially outward-facing skirt. On each side of the 10 impeller, the partially overlapping skirts jointly form an encircling hub ring to optimize flow guidance. During the assembly of this previously known impeller, the blades can furthermore be turned about their longitudinal axis before being firmly clamped in a 15 receiving nest defined by the clamping jaws by tightening through-bolts and nuts, thus allowing the angle of attack of each blade to be freely defined. Two distance pieces and a clamping sleeve are furthermore mounted on. each +G.hzough-bolt. 20 A disadvantage with this embodiment is that the clamping jaws are embodied as pressings, castings or forgings with a relatively complex three-dimensional geometry. 25 It is therefore the object of the invention to indicate an impeller for an axial fan which can be manufactured at low cost while using primarily standard components of simple shape. 30 Disclosure of the invention A disclosure is made of an impeller for an axial fan, having an impeller body, which has an outer impeller 35 shell and a carrier disc having a hub for connection for conjoint rotation to a drive shaft, wherein a multiplicity of radially outward-pointing blades, which each have a blade root and a vane, is arranged on the carrier disc. According to the invention, each blade root has a 5 fastening portion, which is accommodated with positive engagement between two trapeziform, triangular or rectangular fastening segments each arranged opposite one another on both sides of the carrier disc, and the in each case two opposite fastening segments and the 10 carrier disc each have at least two through-holes for accommodating a short sleeve and a long sleeve, and the fastening portion can be clamped firmly between in each case two opposite fastening segments with the aid of screw bolts, in each case inserted through the sleeves, 15 and nuts, wherein each blade root is guided in a respective recess of the carrier disc. By virtue of this design configuration, the impeller can be fmanuf.aot.ured using components which are 20 relatively simple and are therefore less expensive to produce. Moreover, manufacture of the blade roots is simpler, in particular. The invention furthermore makes it possible to reduce the number of different components by largely using common parts, even in the 25 case of different sizes of impeller. Moreover, a significant reduction in weight is obtained in comparison with conventional designs. The sleeves serve inter alia to introduce the high radial forces into the carrier disc, which does not necessarily have to be 30 embodied as a disc but can also be embodied as a spoked structure or the like, for example. The recesses each have a rectangular shape. Accordingly, the carrier disc has a radially outward-oriented, trapeziform material region or peripheral contour similar to that of a 35 gearwheel between each pair of adjacent blade roots. In the case of an advantageous development, it is envisaged that there is at least one axial gap between in each case two opposite fastening segments and the carrier disc. According to a development, the sleeves each have a collar at one end. These are preferably a short sleeve and a sleeve of longer design, which each 5 have a collar at one end. The short and long sleeves are inserted positively into the fastening elements and the carrier disc, but the collar of the short and the long sleeve is not inserted positively in its centering. 10 Axial delimitation of the position of the sleeve in the through-holes is thereby obtained, at at least one end. Moreover, only two additional parts have to be installed for each through-hole or each screw bolt with 15 nut. In an advantageous embodiment, each blade root is guided in a hole in the impeller shell. . .. ... . ~ . ~..,, 20 The fixing of the position of the blades on the impeller body is thereby improved. In the case of another embodiment, there is an annular gap between shanks of each long sleeve and each short 25 sleeve. It is thereby possible to compensate for tolerances which arise in the course of manufacture. If appropriate, the axial gap between the carrier disc and the fastening segment can be reduced almost to zero. 30 According to another advantageous embodiment, a shank length of each first sleeve is significantly greater than a shank length of each second sleeve. As a result, a simple visual check on the installation 35 position of the sleeve from one side of the carrier disc is possible, at least in the case of the longer sleeve, and there is furthermore uniform transmission of the centrifugal forces on the blades to both fastening segments. In a technically advantageous development, in each case 5 two opposite fastening segments cover in each case one recess of the carrier disc with a fastening portion, in each case guided therein, of a blade. In combination with the sleeves, particularly reliable fixing of the position of the blades on the carrier disc in a manner 10 which is resistant to high mechanical loads is thereby possible. In an advantageous development, each fastening portion of a blade root is a cylinder having at least one 15 annular groove running around the circumference. As a result, it is possible during installation to turn the blades initially by any angle about their longitudinal axis before final clamping, thus enabling 20 the angle of attack of the blades and hence the flow rate of a medium through the axial fan to be influenced. Moreover, such a cylinder geometry is easy to manufacture. 25 Brief description of the drawings The invention will be described in greater detail below by means of the drawing, in which: 30 Figure 1 shows a plan view of a detail of an impeller according to the invention for an axial fan; Figure 2 shor