GB1005570A - Improvements in or relating to supporting constructions for a blackboard, projectionscreen or like article - Google Patents

Abstract

1,005,570. Movable boards. H. WOLTERS. May 28, 1962, No. 20368/62. Heading G5C. [Also in Divisions B6 and G2] In an arrangement for counterbalancing a verti-, cally movable blackboard projection screen, notice board or the like, the blackboard &c. is suspended from chains attached to eccentricallymounted rotatable circular segments, and a spring- loaded chain is attached to one of the segments, the arrangement being such that the torque exerted by the weight of the blackboard on the segments is always approximately equal to that exerted by the springs. As shown, the blackboard 1, which has extensions hinged to its sides, is vertically movable on rollers 8 carried by side members 2a of a fixed frame 2 and its weight is taken by chains 14, 15. The chain 14 passes over a pulley 6 mounted on the frame 2 and passes around and is attached at 16b to a circular disc 16 eccentrically mounted on a spindle 17 fixed to a member 2e of the frame 2. The chain 15 passes around and is fixed at 16c to a half disc 16a of the same diameter as the disc 16 and which is also eccentrically mounted on the spindle 17 and fixed to the disc 16, but its eccentricity is 90 degrees displaced with respect to that of the disc 16, i.e. the point 16c is at a maximum distance from the spindle 17 as is the point 16b. Chains 13 are also secured to the disc 16 at 16b, and have attached thereto springs 4 the tension of which is adjustable by screw devices 3 by which their free ends are anchored to one of the side members 2a. In the highest position of the blackboard, the point 16b is at a position corresponding with that of 16c, Fig. 1, whilst in the lowest position of the board, it is diametrically opposite the position of 16c, Fig. 1. With the arrangement, the two ends of the board move up and down at the same speeds, but the turning moment exerted by the chains 14, 15 on the discs 16, 16a increases as the board moves downwardly and vice versa. The tension exerted by the springs 4 increases as the board moves downwardly but because of the eccentricity of the disc 16, the distance from the spindle 17 of the point where the chain 13 makes contact with the disc decreases so that the torque balances that of the chains 14, 15. Stops 19, 20 on the blackboard engage pins 21, 22 on the frame 2 to limit the vertical movement of the board.