data or of the results of monitoring to a line 47 leading to the display 41 so that the corresponding data can be displayed there a selector switch 48 located between the lines 45 46 and 47 enables various outputs of the computer 44 to be displayed selectively the wiper 50 of a potentiometer schematically shown at 49 in fig1 is connected to the pivot axis 24 of the two lever arms 23 25 fig2 this potentiometer emits a signal representing the angular position of the two lever arms 23 25 and thus representing the location of the tensioning roller 22 via a line 51 which is connected to a regulator 52 which contains a command value transducer and is connected via a line 53 the data evaluation and distribution circuit and eventually to the control circuit 38 electric current is supplied to the individual portions of the circuit by means of a power pack 54 the power supply line of which is shown at 55 and the current supply lines of which leading to the individual parts of the circuit are identified as 56 58 the supply of electric current to the data evaluation and distribution circuit 33 is effected via a line 59 and a switch 60 which is mechanically coupled to the tensioning roller 22 two supply lines 61 62 carrying supply voltages of opposite polarity are connected to the switch 60 so that from the polarity of the supply voltage present on the line 59 the data evaluation and distribution circuit 33 can recognize the position of the switch 60 initially it is assumed that the.