NOVELTY - The hollow shaft for agitator useful in the fermenter of biogas plant, comprises an inner tube or bulk material, a propeller blade (7), and a tap intended at a first end of the hollow shaft and a cross-section with multi edge or nibs for torque proof fixation of the propeller blade on the hollow shaft. The interior tube is connected free from play with the hollow shaft and pressed into the hollow shaft. The hollow shaft is formed as rectangular tube. The hollow shaft and the inner tube are welded area-wise. The hollow shaft at its second end has the tap.    USE - Hollow shaft for agitator (claimed) for use in a fermenter of biogas plant.    ADVANTAGE - The hollow shaft highly withstands the corrosion attack of the surrounding media and partly reduces the stress on it, thus the durability of the shaft is increased.    DETAILED DESCRIPTION - The hollow shaft for agitator useful in the fermenter of biogas plant, comprises an inner tube or bulk material, a propeller blade (7), and a tap intended at a first end of the hollow shaft and a cross-section with multi edge or nibs for torque proof fixation of the propeller blade on the hollow shaft. The interior tube is connected free from play with the hollow shaft and pressed into the hollow shaft. The hollow shaft is formed as rectangular tube. The hollow shaft and the inner tube are welded area-wise. The hollow shaft at its second end has the tap. At an end of the tap, in the interior tube, a first flange is intended, and that an outside diameter of the first flange corresponds to the inside diameter of the interior tube. The first flange is welded with the tap. The first flange with the interior tube forms a press fit and/or is welded with the interior tube. At an end of the interior tube a second flange is intended, and the outer contour of the second flange corresponds to the profile of the hollow shaft. A groove is intended in the second flange. The second flange is welded with the tap, the interior tube and/or the hollow shaft. The second flange is welded over the total circumference with the hollow shaft. The second flange is welded over the total circumference with the interior tube. The hollow shaft in the contact zone is welded between the hollow shaft and the interior tube. The interior tube is welded in parts with the first flange. The taps serve for the storage of the hollow shaft. The tap at the first end of the hollow shaft for the drive or the propeller blade is angular momentum-coupled in the hollow shaft and in the interior tube. The propeller blade is torque-coupled into the hollow shaft and the interior tube or over the tap at the first end of the hollow shaft. A part of the interior area included by the hollow shaft is filled with oil, and an opening for an engine oil dipstick is intended in the first tap in the first end of the hollow shaft. The space is oil-immersed between the hollow shaft and the interior tube or the bulk material. At the opening a tube line is connected, which ends in the oil-immersed part of the hollow shaft. The hollow shaft at their second end exceeds over the interior tube or the bulk material.    DESCRIPTION OF DRAWING(S) - The drawing shows a hollow shaft for agitator useful in a fermenter of biogas plant.    Drive unit (1)    Reference symbol (5)    Propeller blade. (7)