both distances a2 and n2 have become smaller than distances a1 and n1 since n2 however is the desired distance such a change has to be taken into account in advance after an extended period of operation wear appears on the clothings as shown in fig4 c the distance a2 which corresponded to the original desired distance a2 has not changed since the cylinder and the flat bars themselves have not been exposed to wear the clearance n3 however has changed the distance n3 is thus greater than the desired distance n2 and as shown in fig4 d has been corrected by adjustment to assume the value n4 which essentially corresponds to the value n2 at the same time the distance a2 has been changed to a3 the latter thus being less than the original working distance a2 in its simplest embodiment the sensor 17 is inserted into one of the flat bars 14 and may be replaced during the run of the machine with a normal flat bar 14 39 since the travel speed of the flat bar is only about 8 cm per minute such a replacement may be readily carried out as shown in fig5 a the flat bar 14n is provided with a socket 29 which may receive a jack of a cable 28 the cable 28 is connected with a measuring device 24 as shown in fig6 since measuring is carried out only when the sensor surface 18 is oriented towards that is faces the cylinder clothing 20 the cable 28 is plugged into the socket 29 only after the sensor 17 has reached its measuring position in the embodiment according to fig5 a the electric contact has to be interrupted and reestablished several times in case a plurality of measurements has to.