Patent GB-736239-A (GB)
Title: Improvements in or relating to strain gages
IPC: G01L1/22
Filed: 1953-02-10
Abstract: 736,239. Resistance strain gauges. STATHAM LABORATORIES, Inc. Feb. 10, 1953 [May 19, 1952], No. 3695/53. Class 40 (1). An unbonded electrical resistance wire strain gauge comprises a movable member hingedly connected to a frame, the member and the frame each having a gauge-supporting leg, and a resistance wire between wire supports on the member and frame respectively. The gaugesuppoiting leg of the member intersects the axis of the hinge and is perpendicular to that axis. Construction. The frame 2, which is U- shaped with legs 10, 11 (Fig. 2) and has a transverse slot 12 (Fig. 1), is rigidly screwed to the case 1. The movable member consists of an upper member 14 joined by means of legs 16 (Fig. 6) to a lower member 15 having an L- shaped extension 23<SP>1</SP>. A flat spring 24 (Figs. 1, 5) is secured horizontally to the underface of frame 2 and to the underface of member 14 by clamping block 25 and sciews 26, and clamping block 28 and screw 29, respectively. Two vertical springs 30 (Fig. 5) are secured between clamping blocks 27, 28, and clamping block 25 and frame 2 by other screws (not shown). The springs 24 and 30 constitute the hinge connecting the movable member to the frame. Wire-supporting pins 20 and 32 are mounted on the movable member and block 28 secured thereto respectively, and similar pins 34 and 33 are mounted on the frame 2 intermediate the legs 10, 11, such that the lines joining pins 20 and 34 and 32 and 33 are perpendicular to the plane of springs 30 and parallel to that of spring 24. Strain wires 40 and 42 are wound in loops on pins 20, 34 and 32, 33 respectively and the ends are terminated on terminal pins 35 and 35<SP>1</SP>, and 36 and 36<SP>1</SP> (not shown) respectively, from which connections lead away via plug 41. At the rear end of the frame are mounted two legs 45 (Figs. 1, 2) having knife edges 47 at right angles to the lines joining the pins, and at the forward end of the lower member 15 is mounted a similar leg 49 with a knife edge 50 parallel to those of the legs 45. The axis of leg 49 lies in the central plane of springs 30 and on the centre line of the instrument midway between the legs 45. Auxiliary legs 54, 55<SP>1</SP>, having knife edges 58, are mounted on the frame 2 and the extension 23<SP>1</SP> of the lower member 15 respectively. Stop screws 59 (Fig. 1) serve to limit the movement of the member 14 with respect to the frame 2. The lower member 15 is provided with a side extension 60 (Figs. 2, 6) which carries a U-shaped member 61. A shaft 63 mounted in the wall of the casing 1 carries a rectangular cam 64 which rides within the legs of the U-shaped member 61. Opeiation. The shaft 63 is rotated by means of knob 68 (Fig. 2) until the corners 67 (Fig. 6) of cam 64 contact the member 61, and is held by detent lock 69 (Fig. 2). The springs 24 and 30 are then unstressed and the zeio reading of the gauge is taken. The knife edges 50 and 47 (or 58) are then set on the member to be stressed and the knob 68 is rotated to release the movable member 14, 15. The test member is then stressed and the relative movement of the knife  edges which results produces relative movement of the pins 20, 32 with respect to pins 34, 33, the amplitude of which depends upon the relative distances of pins 20 and 32, and knife edge 50 respectively from the hinge axis. The tension in one of the wires 40, 42 will be reduced and that in the other increased. The two wires 40, 42 may be replaced by four wires (two loops on each pair of pins) and the four wires may be connected as a Wheatstone bridge. To determine whether the strain over the length between the knife edges 47 and 50 is uniform, the strain may be re-measured over the shorter distance between knife edges 58 and the results obtained compared. Modifications. Embodiments are described having only the legs 45, 49 or the legs 54, 551 in which the spring 24 or the springs 30, respectively, are dispensed with. The distance of the knife edges from the axis of the hinge may also be made adjustable by mounting the legs on pins which fit into the frame and movable member respectively, allowing vertical adjustment of the legs upon the pins to be made.