GB547991A - Improvements in or relating to apparatus for the detection of cracks in ferrous metals by magnetic means - Google Patents

Abstract

547,991. Magnetic tests direct connections between conductors. JOHNSON, L. Aug. 12, 1941, Nos. 10228 and 14985. [Classes 37 and 38 (i)] In apparatus for detecting cracks in ferrous metals in which a magnetic flux and an electric current are simultaneously passed through a test piece, a flexible conductor is elastically pressed against each end of the test piece to provide the electrical contact. On a frame 20, Fig. 1, are supported two arms 21 of good permeability material with a gap 22 between them. These form part of a magnetic circuit which also includes brackets 23 of similar material extending from the outer ends of the arms 21 to a yoke 24 which is surrounded by a coil 25 to form the core of an electromagnet for energizing the magnetic system. An adjustable headstock 26 is mounted to slide on each of the arms 21 and can be clamped in any position by a screw 27. Each headstock 26 carries a clamping jaw 33 which can be adjusted axially by a handwheel nut 30 which is prevented from axial movement by a screw pin 31, Fig. 4. Held in a recess in the clamping jaw 33 is a flexible conductor 35 consisting of a disc of wire gauze backed by a cushion 36 of rubber &c. This can be pressed outwards by a plunger 40 moved by a cam 37 operated by a handle 42. The test piece is clamped between the jaws 33, with which it makes magnetic contact. Current is passed through the test piece from terminals 43 on the brackets 23, the yoke 24 being insulated from these brackets by a thin layer of insulating material such as mica. In a modification, Fig. 10, the magnet yoke is in two portions 50, 51 of rectangular cross section, each integral with one of the brackets 23. The portions 50, 51 are insulated from one another by a thin layer of mica 52. In another construction, Fig. 12, the magnet yoke comprises a number of laminations 54 separated by layers of mica, alternate laminations being connected to the same bracket 23. The test piece is energized by the flux of the magnetic circuit and electric contact is made with its ends by operating the handles 42. Magnetic testing liquid is then applied, surplus liquid draining into a trough 57, and current is applied through the terminals 43. The yoke 24 may be divided at right angles to the lines of force instead of longitudinally.