Magnetic pressure, tension, torsion and like stress measuring devices

Abstract

Abstract 442,441. Magnetic measurements. SMITH, F. D., Admiralty Research Laboratory, Teddington, Middlesex, and WRIGHT, C. S., Admiralty, London. Aug. 3, 1934, No. 22678. [Class 37] [See also Group XIX] In a magnetic device for measuring pressure, tension, torsion or like stresses, whether static, transient, variable or cyclic, the stresses are applied to a member of nickel, which has a large magnetostrictive effect, to which is also applied an alternating or pulsating magnetization at a frequency which is other than that of any natural mode of mechanical vibration of the member, the modulation of the magnetization due to the magnetostrictive effect caused by the applied stress being utilized to give a direct or indirect measurement of the stress. In the arrangement shown in Fig. 1, a nickel bar 1 is fixed at one end and surrounded by a winding 3 connected to an A.C. source. If the end 2 of the bar is subjected to compression, tension or torsion, the magnetic properties of the bar are changed, with corresponding changes in the magnitude and wave form of the current flowing in the winding, which may be recorded by connecting the ends of the winding, or of a secondary winding on the same bar, to the deflection plates 4, 5 of an oscillograph ; or to the terminals of an ammeter or other deflecting instrument. For measuring the torsion in a shaft, the nickel bar may form part of the shaft, or for measuring hydrostatic or other fluid pressure, the nickel member may be in the form of a tube containing or immersed in the medium in which the pressure exists. In an arrangement for measuring the pressure in an engine cylinder, Fig. 7, the pressure is transmitted to a nickel member 15 by means of a piston 14 which sits in a hole 12 in the head wall 13 of the cylinder. The member 15 carries windings 3, 6, and a similar member 18 is provided with windings 3a, 6a, the windings being connected in a bridge as shown in Fig. 8. The member 18 is not subjected to the cylinder pressure and compensates for temperature changes. Rectifiers may be introduced into the bridge network in order to permit of the use of D.C. instruments. In order to eliminate the effects of undesirable stresses, the device shown in Figs. 13, 14 and 15 may be used. This device comprises four thinwalled nickel tubes 28 ... 31, each surrounded by a coil 32, all four tubes being braced together at one end by members secured between nickel plates 36. The other ends of the tubes 28, 29 are soldered in a brass block 37, into which is screwed a brass ring 38 securing a steel ring 39 to which is soldered a thin diaphragm 40. The other ends of the tubes 30, 31 are braced together and attached by a plug 43 and bolt 44 to the diaphragm. Pressure may be transmitted to the bolt 44 by means of an anvil 45 fitted in the cover plate 46 of a pot 47, or by means of a piston subjected to an oil pressure to be measured. Pressure exerted on the bolt 44 places the tubes 30, 31 in compression and the tubes 28, 29 in tension. Reaction to applied pressure is taken by two steel pistons 49 soldered in the brass block 37, the pistons fitting into cylinders 50 carrying compression springs 53, which are set by washers 54 to yield at a predetermined load and prevent the nickel tubes from being stressed beyond their elastic limit. The initial state of stress in the nickel tubes may be adjusted by means of plungers 55 which press against the pot 47 and react against springs 56 mounted in cylinders 57 which, with the cylinders 50, are mounted on an adjustable member 51. Specification 410,629 is referred to.

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