Patent GB-1021702-A (GB)
Title: Improvements in or relating to grinding apparatus for treatment of fibrous material
IPC: D21D1/30
Filed: 1962-09-21
Abstract: 1,021,702. Disc mills. DEFIBRATOR A.B. Sept. 21, 1962 [Sept. 22, 1961], No. 35961/62. Heading B2A. [Also in Division G3] A grinding apparatus for treatment of fibrous material comprising both stationary and rotatable grinding members 30, 28, Fig. 1, carried by an axially-displaceable shaft 12 movable by a fluid-actuable servomotor 38 whose piston 42 is actuable from both sides to adjust the spacing between the grinding members and produce grinding pressure therebetween, is characterized in that the piston is actuated by members which cause a relative displacement of the grinding members responsive to compression and tension in the pressure transmitting parts of the apparatus thereby maintaining the gap between the grinding members at a predetermined value. In the embodiment shown in Fig. 1, the piston of the servomotor 38 movably surrounds the shaft 12 and a pilot valve 45 is rigidly secured to the casing of the servomotor. Oil of constant pressure is applied through a pipe 58 to the central chamber 52 of the valve, and when the piston 46 is in its central position is equally distributed to both chambers of the servomotor, movement of the piston 46 producing a corresponding opposite movement of the servomotor piston 42. Thus a pressure is maintained between the grinding discs dependent upon the position of the piston 46, which is determined by a set screw 76 and the reaction against the piston of a spring 78. If the pressure between the grinding discs then increases due e.g. to accumulation of material, the servomotor piston 42 is displaced to the left together with the set screw which is secured thereto. Oil pressure is then increased in the space 69 of the pilot valve and in chamber 70 of the servomotor. Corresponding changes take place when pressure between the discs is reduced. When the apparatus is grinding the pressure transmitting parts are compressed and the parts of the base 10 between the grinding disc 30 and the casing 40 of the servomotor are stretched, causing left-hand displacement of the pilot valve and a right-hand displacement of the set screw. To compensate for this, a movable wedge or cam member 92 between the set screw 76 and the pilot valve piston is automatically adjusted by movement of the wedge by a further piston 82 operated by the pilot valve. In a further embodiment, stress compensation is effected by a mechanical linkage comprising a bar at one end pivotally secured to a bracket attached to the casing of the grinder, and at the other end connected to a pivot 1 on a link 2 which is pivotally secured at one end to a bracket on the servomotor casing and at the other end to the casing of the pilot valve. The bar is of a length corresponding to the length of the base likely to be elongated in operation. When the apparatus is under stress, the link turns clockwise relative to the bracket and the casing is pulled to the right, causing adjustment of the pilot valve piston and corresponding adjustment of the servomotor. The pivoted arrangement of the linkage is arranged to take account of the compression in those parts transmitting grinding pressure from the servomotor piston to the casing of the grinder and causing displacement of the set screw 78. In a further embodiment, Fig. 3 (not shown), the piston 42 of the servomotor co-operates with a stop ring 110 screwed on to the piston to limit approach movement of the grinding members. The stop ring 110 is toothed on its external face and driven by a gear 122. Stress impulses from a transducer 128 mounted on the grinder casing 32 are amplified 130 and applied to a motor 126 to adjust the gear and stop ring and thereby compensate for elongation or compression. The chambers of the servomotor are controlled by a valve 116 in a known hydraulic system.