be described in detail later it will thus be seen that if the rpm of the feed roll 3 falls below a predetermined value set at the desired value transmitter 24 the control device 16 causes the conduit 14 to be pressurized whereupon the setting device 13 effects an inward movement of the piston rod 23 connected to the lever 20 as a result the lever 20 and the arm 18 secured thereto move clockwise as viewed in fig2 causing an opening movement of the web guide portion 11a that is a motion away from the crush rolls 6 and 7 conversely if the rpm of the feed roll 3 exceeds a predetermined value the web guide portion 11a by virtue of the pressurization of the conduit 15 as commanded by the control device 16 moves into its closed position that is towards the crush rolls 6 and 7 details of the control device 16 and the setting device 13 connected thereto are shown in fig3 the measured value transducer 17 which is a voltage generator dynamo is driven by the feed roll 3 of the carding machine the desired value transmitter 24 is a relay the input of which is connected to the output of the voltage generator the output of the relay 24.