46 and a back chamber 44 by diaphragm 36 a return spring 43 is positioned against diaphragm 36 in front chamber 46 the back chamber or pressure chamber 44 is in communication with a source of pressurized air through valve 50 pressure regulator 52 three way valve 54 and restricter 56 located adjacent to the loading device 40 all serially connected in pipeline 58 restricter 56 limits the flow of air to or from back chamber 44 and is in the form of a sintered metal plug which can be a pressure snubber no 25s supplied by chemiquip products co inc gages 57 and 59 are tied into pipeline 50 to indicate 1 pressure supplied to chamber 44 and 2 the pressure between the chamber 44 and the restricter 56 respectively in operation when a short section of larger denier tow passes between rolls 22 and 24 roll 24 is moved away from roll 22 compressing the air in chamber 44 to a higher pressure than indicated by gage 57 via the linkage of arm 27 and rod 34 air immediately begins to flow through restricter 56 into supply line 58 but at a slow rate when the larger denier section of tow has passed rolls 22 24 the immediate need is to restore roll 24 to its just previous equilibrium position the higher pressure developed in chamber 44 by the displacement of roll.