Patent GB-597601-A (GB)
Title: Improvements in or relating to electrical devices sensitive to mechanical strain
IPC: G01L1/14
Filed: 1945-08-23
Abstract: 597,601. Dynamometers. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. Aug. 23, 1945, No. 21598. Convention date, Aug. 25, 1944. [Class 106 (ii)] [Also in Groups XXXVI and XXXVIII] In cases where the usual strain sensitive resistance cannot conveniently be connected directly in the measuring circuit, for example in measuring the torque transmitted by a rotating shaft, the strain sensitive resistance is connected across the secondary, or it may form the short-circuited secondary of a transformer, the stationary primary of which is connectcd in the measuring circuit, the secondary being secured to the shaft. In Fig. 1, the strain sensitive filament 4 is wound upon or in insulation 3 bonded to a rotating shaft 1 between magnetic rings 2 which form narrow air-gaps with the flanges of a stationary concentric magnetic housing 8 carrying the transformer primary 7 forming one arm of the Wheatstone bridge 10. The helical filament 4 is shortcircuited by a connection 6, which preferably is not strained by torsional deflections of the shaft and reacts on the primary 7 to give a reading on the meter 11 proportional to torque. To compensate for changes in ambient temperature and for errors due, for example, to bending or thrust in the shaft, two units similar to that shown in Fig. 1, but in which the helical filaments are wound in opposite directions, are provided adjacent one another on the shaft, Fig. 2 (not shown). The two transformer primaries are connected in adjacent arms of the bridge and so act differentially to give double sensitivity. In Fig. 3, the strain sensitive filament 4 is mounted on the shaft outside of the magnetic field of the transformer and connected across the secondary 12. This permits of a more efficient disposition of the filament. Such an arrangement may be duplicated corresponding to the Fig. 2 arrangement to compense for ambient temperature and other errors, Fig. 4 (not shown). In such an arrangement, Fig. 5, the effect of changing electrical properties of the shaft may be eliminated by mounting the transformer secondaries 12a, 12b on highly conducting sleeves 15a, 15b secured to the shaft. These sleeves act as short-circuited secondaries and substantially prevent the flow of magnetic flux through the shaft 1. The sensitivity of this arrangement may be increased by disposing unstrained magnetic sleeves 16a, 16b over the conducting sleeves. They may be of powdered iron bonded directly to the conducting sleeves. The sensitive filament 4 may be mounted between the rings 2 underneath the secondary winding 12 of Fig. 3, Fig. 6 (not shown). In another arrangement the filament 4, Fig. 7, is looped back and forth between and secured to fingers F, each secured at one end only to a shaft ring 20 and extending axially of the shaft. In this case the filament responds not only to torque transmitted by the shaft but also to its speed owing to the filament strain caused by centrifugal force, so that the total wire strain may serve to indicate the horsepower transmitted. Two such arrangements may be combined, Fig. 8 (not shown), to eliminate the speed response and so indicate the torque transmitted. In a further ernbodiment, one filament 4a, Fig. 10 (not shown), is secured axially to the shaft and the filament 4b is secured circumferentially so that a thrust along the -shaft as well as torque may be measured.