pressure in pounds per linear inch times the radius of curvature inches of the plenum open end as a new felt is being used it tends to stretch or creep and it is necessary to take up the slack to maintain constant felt tension accordingly i provide an emery load cell 62 fig6 or a strain gauge at a felt roll 22 journal to detect any change in felt tension the load cell cooperates with a movable stretch roll 64 through an actuating diaphragm 66 to restore desired felt tension as shown in fig6 load cell 62 detects felt tension and signals a differential pot 68 which compares the signal to a reference value for felt tension if the felt tension is below a desired value the differential pot will actuate an air valve 70 admitting compressed air to the diaphragm 66 which moves slidably mounted stretch roll 64 to restore the tension of felt 20 to the desired value a bleed valve 72 allows for reducing diaphragm pressure should it be necessary to reduce felt tension in an operating emergency a press felt normally accumulates embedded dirt in the course of its useful life which cannot be removed resulting in decreased permeability of.