Patent GB-1518994-A (GB)
Title: Apparatus for non-destructive testing of metal blanks for surface defects
IPC: G01N27/87
Filed: 1975-06-25
Abstract: 1518994 Flaw detectors GRANGES OXELOSUNDS JARNVERK AB 25 June 1975 [19 Aug 1974] 27004/75 Heading G1N In a flaw detecting apparatus pressure and tension members are provided to ensure that the or each inspection head remains in contact with the surface of the body being tested e.g. steel slabs, flat steel, billets or blooms. A supporting arm 3 which is mounted adjacent to and transversely of the steel plate 8 to be inspected moves in the direction of arrow 7 and pulls a plurality of spaced inspecting heads 1 slidingly over the surface 10. Each head 1 is coupled by a universal joint 12 to a pressure member 4 so as to be tiltable both about a first axis extending transversely of direction 7 and parallel to surface 10 and about a second axis extending parallel to direction 7. Member 4 is of variable length e.g. a pneumatic or hydraulic cylinder influencable on both sides and is pivotably mounted at its upper end by a swivel joint 14, Fig. 3 (not shown) fastened to support 16 an arm 3, the pivot axis extending perpendicular to direction 7 and parallel to surface 10. Each head 1 is also coupled by crossed tension members 2, 5 to the arm 3, the ends each comprising ball head joints 18, 20 which permit a pivoting of the members about an axis parallel to surface 10 and an axis transversely of direction 7 with simultaneous mutual twisting which is produced by a tilting of the inspecting head about an axis parallel to direction 7. To avoid mutual obstruction of the members 2, 5 the member 5 includes a U-shaped portion 22 extending around member 2. In an alternative arrangement Figs. 4, 5 (not shown) a connecting pin is passed through the members 2, 5 at their intersection in the direction perpendicular to the plane formed by the members in the untwisted position the pin being non-displaceably mounted along its axis in the member 5 but displaceably  mounted along its axis in the member 2. Instead of using two crossed members 2, 5 a drawbar-like tie member 44 may be used (Fig. 6) which consists of rod 46 and support 48 which are rotatable with respect to each other about the longitudinal axis of member 44, the effective length of the tie member remaining substantially constant on relative rotation of the rod 46 and support 48. Rod 46 is fixed to sleeve 50 shrunk on rod 54 which is mounted parallel to surface 10 transversely of direction 7 so that the tie member 44 may pivot about the axis of rod 54. Similarly support 48 is pivotably mounted about rod 56 mounted on head 1 by bearings 70, 72. In order to reduce sliding friction of surface 26 over the slab 8 the pressure members 4 may be influenced in the stroke direction to compensate for part of the weight of the head. Inspecting heads. Each head incorporates an inspection means 6 which may be of eddy current type or magnetic types. In an eddy current head an energizing coil having a scanning width equal to the distance between two receiving coils is provided with the receiving coils being differentially connected. In the magnetic heads a device is provided for magnetizing the slabs together with probes measuring a magnetic leakage field e.g. magnetic field sensitive resistors or coils. Two probes are mounted on each head along a line perpendicular to direction 7 and connected in a differential circuit. In order to detect elongated cracks independently of the direction 7 a first magnetization device magnetizes the slab 8 at 45 degrees to direction 7 and the field measured by the two probes and a second magnetizing device is disposed behind the first which magnetizes the slabs at right angles to the first device and the stray field therefrom is also measured by the probe pair. Alternatively three probes may be arranged in a line at an angle between 45 degrees to 135 degrees to direction 7.