on center line 39 each of the sensors 35 and 36 generates an output signal which is utilized to maintain its respective sensor in a predetermined position relative to the adjacent opposed side edge of web 10 as shown in fig3 the signal generated by sensor 35 is fed by a line 40 to an optional amplifier 41 and then to a controller 42 controller 42 compares the input signal from amplifier 41 to a set point signal and generates a difference signal which is discharged from controller 42 through a line 43 to a servo valve unit 44 accordingly there are two or three possibilities for operation of servo valve 44 by one possibility optional the signal fed thereto produces no change no operation of valve 44 in a second mode the signal 43 fed to servo valve 44 causes the valve 44 to admit fluid to double acting control cylinder 45 whereby the piston 46 thereof moves to the right thereby moving the rod 47 to the right rod 47 is terminally connected to an arm 48 which is rigidly connected to the chain 32 hence the upper rim of chain 32 is thus moved to the right by the third possibility the signal fed to valve 44 by the line 43 causes the piston 46 to move to the left and by a similar sequence of steps the chain 32 is moved to the left moving the chain 32 to the right causes the sensor 35 to move away from the associated edge of web 10 and conversely moving the chain 32 to the left causes the sensor 35 to move towards the.