GB2034049A

GB2034049A - A Rotating Head for Testing Elongated Ferromagnetic Test Pieces

Info

Publication number: GB2034049A
Application number: GB7934489A
Authority: GB
Original assignee: Institut Dr Friedrich Foerster Pruefgeraetebau GmbH and Co KG
Current assignee: Institut Dr Friedrich Foerster Pruefgeraetebau GmbH and Co KG
Priority date: 1978-11-03
Filing date: 1979-10-04
Publication date: 1980-05-29
Also published as: DE2847716B2; DE2847716A1; GB2034049B; DE2847716C3; FR2440553A1; FR2440553B1

Abstract

Diametrically opposed poles 13 and 14 of the rotating head are energised by windings 7 and 8 and carry adjustable pole shoes 21 and 22. The shoes slide on arcuate surfaces 20 so that the pole surfaces 30 and 31 are adjusted on circular arcs 33 and 34 which are tangential at the axis of the wire or rod 3 under test. (Different size pole pieces 28 and 29 at different distances from the axis are shown to illustrate alternatives for a thin wire 3 and a thick rod 32). Two sensing heads 45 containing coils for sensing stray flux at defects are mounted on levers 17 and 18. The heads move on arcs which also pass through the axis. This arrangement preserves uniform angular relationships between the energising flux and the sensing heads over a wide range of adjustment to suit test pieces of different diameters. <IMAGE>

Description

SPECIFICATION A Rotating Head for Testing Elongated Ferromagnetic Test Pieces The present invention relates to a rotating head for testing elongated ferromagnetic test pieces of essentially circular cross-section for defects, comprising an annular magnet yoke incorporated in a rotating head and having two inwardly directed, diametrically opposite poles that can be adjusted to different diameters of the test pieces and at least one exciting winding, and comprising further at least one probe lever mounted to swing in a plane perpendicular to the axis of the test piece and carrying on its forward end at least one magnetic stray flux probe for scanning the test piece surface.

A rotating head of this type has been described in German Laid-Open Patent Application No.

1 946 142. The magnet yoke of the rotating head described therein comprises a soft-iron hollow cylinder and two pole pieces of like material which are supported on adjusting means fastened at two opposite points of the cylinder walls and which can be moved by said adjusting means in radial direction for the purpose of adapting them to different test piece diameters. Two probe levers of the rotating head are mounted for swivelling action in two supports which can be likewise adjusted in the radial direction.

It is a drawback of the described rotating heads that in addition to the pole pieces the supports of the probe levers must also be readjusted each time the test piece diameter changes. Moreover, the worm gear used for adjusting the supports will easily get fouled as the probe shoe sliding along the test piece surface causes a lot of metailic dust. Accordingly, the worm gear must be protected by a folding bellows, which is expensive and undesirable. Another drawback consists in the fact that the described adjusting means for the heavy pole pieces allows only a relatively small adjusting range.

Now, it is the object of the present invention to provide a rotating head in which no adjustment of the probe lever is required even for a relatively wide diameter range of the test pieces. This object is achieved by a rotating head in accordance with claim 1.

The rotating head of the invention offers the advantage that although the adjusting means for the support of the probe lever are eliminated, the angular distance between the stray flux probe and the points of entry of the magnetic flux into the test piece remain the same for any test piece diameter. The poles can be adjusted to a wide diameter range. In addition, the angle formed between the fixed axis on the probe lever which extends through the point of contact between the probe lever and the test piece surface and which may for instance be identical with the axis of the stray flux probe, and a perpendicular line erected upon the test piece surface at the point of contact remains relatively small even for the largest diameter of the given test piece range when the said axis forms a right angle with the test piece surface in the case of very small test piece diameters.In one embodiment of the invention, the angular deflection of this axis from the perpendicular line erected upon the test piece surface may be further reduced by making the axis form a right angle with the test piece surface in the case of a medium diameter of the given test piece range. Another embodiment of the invention suggests a removable insert containing the adjustable parts required for the adaptation to different test piece diameters. Other advantageous embodiments of the invention will be apparent from the sub-claims.

Hereafter, the invention will be described in detail with reference to one embodiment of the invention and to the drawings in which Figure 1 is a view of a rotating head of the invention; Figure 2 shows a probe shoe of the latter; Figure 3 is a schematic representation of a probe lever and Figure 4 shows an alternative embodiment of pole shoes Figure 1 shows the essential components of a rotating head of the invention. These parts are mounted between two plates 1, the front plate having been eliminated to give a better view so that only the rear plate is visible in the drawing.

The plates 1 consist of a non-magnetic material, such as brass, and are provided with openings 2 forming a passage for the test piece which takes the form of a steel wire 3 suitably guided in front of and behind the rotating head. The plates 1 are pivoted in a housing-not shown in the drawing-and can be rotated about the axis of the test piece 3 by means of drive means which are likewise not shown. The rotating head consists mainly of a ring-shaped magnet yoke 4 for producing a strong magnetic flux within the test piece 3 and an exchangeable central insert 5 containing the adjustable parts required for the adaptation of the magnet yoke to different test piece diameters and means for scanning the test piece surface to determine the existence of magnetic stray fluxes.The magnet yoke 4 comprises a ring core 6 of soft iron carrying electric windings 7, 8 for exciting the magnetic flux. The windings 7, 8 are connected back-toback so that the resulting magnetic flux closes essentially across the poles which will be discussed later and the test piece. The crosssection of the ring core 6 is preferably such that its dimension in the axial direction is smaller than or at least equal to its height h. The short construction of the magnet yoke thus obtained offers two advantages. On the one hand, the amount of dust in the form of metallic dust or loose scale that will deposit upon the inside of the magnet yoke as a result of the centrifugal force will be kept within tolerable limits. On the other hand, the overall length of the rotating head can be kept shorter.As a result, the guide means can be arranged at a shorter distance which will give improved guidance and reduce the tendency of the test piece to vibrate. The ring core 6 is supported by two U-shaped pole extensions 9, 10, which in turn are held between the plates 1 and serve in addition to pass on the magnetic flux via two plane surfaces 11 into or out of the insert 5.

The provision of a removable insert 5 which contains all adjustable parts required for the adaptation to different test piece diameters proves to be extraordinarily advantageous. On the one hand, this arrangement renders the adjustable parts easily accessible so that the adjustment can be carried out easily and quickly.

On the other hand, a second insert 5 can be adjusted to the diameter to be handled next while test pieces of a given diameter are still being tested. Thus, extremely short change-over times are reached because only the two inserts 5 will have to be exchanged when changing over to test pieces of a different diameter. The insert 5 is inserted into the rotating head from the front. For this purpose, the front plate 1-not shown in the drawings provided with a suitable opening.

Similarly to the magnet yoke 4, the insert 5 comprises two mounting plates 1 2 of a nonmagnetic material, of which only the rear one is shown in the drawing for the reason outlined before. The two mounting plates 12 comprise between them: two fixed pole pieces 13, 14, two adjustable pole shoe arrangements 1 5, 1 6, two probe lever arrangements 17, 18 and a plug-in connection 1 9. The rear faces of the two pole pieces 13, 14 are in contact with the faces 11 of the pole extensions 9, 10, while their curved front faces 20 are in contact with the similarly curved faces of two pole shoes 21, 22. Each of the two pole shoes 21,22 is laterally held by two anchors 23 mounted to pivot about a bolt 24.Again, only the rear anchor is shown in the drawing. The bolt 24 may be supported in the two mounting plates 12 by means of cams so that the distance between the two curved faces can be easily adjusted. Thus, they permit on the one hand a sliding relative movement of the two curved faces when the anchors 23 are pivoted about the bolt 24 for the purpose of adjusting the pole shoe, while permitting on the other hand a close contact between the curved faces during the passage of the magnetic flux. Instead of the cams supporting the bolt 24 in the plates 12, slots provided in the plates may also be used as a simple means to provide the desired clearance between the curved faces.In this case, the pole shoes 21, 22 are fixed by means of a clamping jaw 25 and a screw 26 which can be displaced within a slot 27 in the pole shoes 21,22 and which serve to rigidly clamp the pole shoes 21, 22 against the pole pieces 1 3, 14. The pole shoes 21, 22 carry at their forward ends exchangeable pole heads 28, 29. Pole shoe 21 is shown in its foremost position. This position will be occupied in use when testing a steel wire 3 of a diameter which is close to the lower limit of the range of the rotating head. In the drawing, a pole head 28 with a correspondingly narrow pole face 30 has been selected. Although in operation the pole shoes 21,22 are identically positioned and equipped, the pole shoe 22 has been shown in the drawing in its rearward position for demonstration purposes.The circumference of the corresponding test piece of large diameter is indicated by a broken line. Here, a pole head 29 with a correspondingly large pole face 31 is employed. Each adjustment of the pole shoes 21, 22 carried out for adapting them to different test piece diameters makes the pole shoes move in opposite senses along circular paths 33, 34, which touch each other at a point coinciding with the axis of the test piece 3.

The probe lever arrangements 1 7, 1 8 are of identical design, comprising each a bearing block 35, a stop arrangement 36 and a probe lever 37.

The bearing block 35 is fixed at the pole pieces 13 and 14 respectively, and pivotally attached to the probe lever 37 by means of a bolt 38. The stop arrangement 36 consists essentially of a plate 39 which is supported at the mounting plate 1 2 to pivot about the same axis as the probe lever 37, being guided in this movement by a lockable screw 40 coacting with a slot 41. An angularly projecting piece of the said plate 39 forms a stop 42 against which the probe lever 37 will come to bear when no test piece is present in the rotating head.The probe lever 37 comprises a rigid body piece 43 with a probe arm 44 forming a forward extension thereof, a probe shoe 45 fastened to the said probe arm 44 and suited for sliding contact with the test piece surface, and a counterweight 47 fastened to the rearward end of the body piece 43 which equalizes approximately the centrifugal forces encountered on both sides of the probe lever 37.

Figure 2 shows a separate lateral elevation of the probe arm 44 and the probe shoe 45. For the sliding contact, the probe shoe 45 is provided with a sliding sole 46 of carbide metal. Further, six stray flux probes 48 are incorporated in the probe shoe 45 in spaced arrangement in the longitudinal direction of the test piece. These probes enable the test piece surface to be scanned over the full width of the probe shoe 45.

To ensure that the probe shoe 45 is always in contact with the test piece surface over its full width, the probe arm 44 is preferably made of a U-section of an elastic material. In this case, the probe arm 44 will on the one hand exhibit great rigidity against deformation under the influence of the centrifugal force, while on the other hand it will distort in the presence of curvatures or irregularities of the test piece surface so as to ensure a perfect contact between the probe shoe 45 and the test piece surface. This property may even be improved by nesting two or more thin Usections in each other. The contact pressure between the probe shoe 45 and the test piece surface or the stop 42 may be derived from the centrifugal force by giving the counterweight 47 a size which is slightly above that required to produce the state of equilibrium. However, a spring force may also be used instead of or in addition to the counterweight. This latter arrangement offers the advantage that the probe lever will always occupy its foremost position when the rotating head is at a standstill or rotates slowly. Chamfered iead in surfaces 49 guarantee that the pole shoes 45 are pressed outwardly by the incoming test piece.

A point of particular importance is the alignment of the stray flux probes in relation to the test piece surface. Any deviation from the optimal contact angle will frequently mean an increase of the interfering voltage encountered in the probes and, thus, a deterioration of the signalto-noise ratio. On the other hand, any change of the contact angle will entail a change in the sensitivity of the probe. Therefore, a fixed axis on the probe shoe should always occupy the same angle in relation to the normal erected at the point of contact upon the test piece surface. Figure 3 is a diagrammatic representation showing a probe lever 51 in contact with test pieces of different diameters near the upper limit, the middle and the lower limit of the range of a rotating head.All points of contact between the probe shoe 52 and the circumference 55 of the test piece lie along a circular path 50 extending through the centre of the test piece. In each case, an axis 53 is indicated at the pole shoe 52. This axis may be identical with the axis of the probes. According to figure 3a, the axis 53 is arranged in a manner to coincide with the normal erected at the point of contact upon the test piece circumference 55 when the test piece diameter is very small. In the case of larger test piece diameters, angular deflections , and a, will be obtained between the axis 53 and the normal erected at the point of contact upon the test piece circumference 55.

These angular deflections will decrease as the length of the probe lever 51 rises. If as shown in the drawing the length 1 of the probe lever 51 is approximately equal to the largest test piece diameter of the given range, a, will be approximately 1 60.

Even smaller angular deflections can be obtained if-in the arrangement shown in figure 3b-the fixed axis 56 on the pole shoe 57 is arranged to coincide with the normal erected at the point of contact upon the test piece circumference 55 when a medium diameter of the given test piece range is being handled. In this case, the angular deflections Ct3 and a, obtained when handling the smallest and the largest diameters of the test piece range will always be considerably smaller than a2 in figure 3a.

Figure 4 shows an alternative embodiment of the pole shoe, which renders the exchange of pole heads completely superfluous. A curved centre piece 61 of soft iron is fastened between two anchors 62, of which again only the rear one is shown in the drawing. In front of and behind the centre piece 61, several thin shells 63 of a magnetic soft material are provided which are held together and clamped against pole pieces 1 3 by means of a clamping jaw 65 and a screw 64.

When the centre piece 61 is moved back, the laterally adjacent shells 63 are simultaneously withdrawn when the front faces of the shells 63 are flush with the pole face 66 of the centre piece 61. Thus, the pole faces can be increased in steps simultaneously with the adaptation to larger test piece diameters. The configuration corresponding to the largest test piece diameter of the given range is indicated by broken lines, the line 67 representing the forward delimitation of the pole shoe, i.e. the pole face, and the lines 68 indicating the rearward delimitation of the pole shoe.

The plug-in connection 1 9 connects the stray flux probes 48 in the exchangeable insert 5 electrically with connections in the rotating head.

The signals of the stray flux probes 48 are transferred from the rotating to the fixed part of the equipment by means of slip rings (not shown in the drawing). Likewise, the exciting windings 7, 8 are supplied with the required exciting current through slip rings. The electric evaluation of the stray flux signals received from the probes 48 is carried out in conventional manner.

Claims

1. A rotating head for testing elongated ferromagnetic test pieces of essentially circular cross-section for defects, comprising an annular magnet yoke incorporated in a rotating head and having two inwardly directed, diametrically opposite poles that can be adjusted to different diameters of the test pieces and at least one exciting winding, and comprising further a probe lever mounted to swing in a plane perpendicular to the axis of the test piece and carrying on its forward end a magnetic stray flux probe for scanning the test piece surface, the mounting of the lever being such that swinging of the lever will cause the stray flux probe to move along a circular path extending through the axis of the test piece and wherein the poles are adjusted along two oppositely directed circular paths or approximately circular paths that touch each other at a point coinciding substantially with the axis of the test piece.

2. A rotating head in accordance with claim 1, wherein a probe shoe carrying the probe is in sliding contact with the surface of the test piece with the points of contact all situated on the first said circular path.

3. A rotating head in accordance with claim 2, wherein the probe is dressed along a fixed axis of the probe shoe which in the case of test pieces of small diameters coincides with the normal erected at the point of contact between the probe shoe and the test piece surface on the circumference of the test piece.

4. A rotating head in accordance with claim 2, wherein the probe is dressed in accordance with a fixed axis of the probe shoe which in the case of test pieces of medium diameters within the operative test piece range coincides with the normal erected at the point of contact between the probe shoe and the test piece surface on the circumference of the test piece.

5. A rotating head in accordance with any of the preceding claims, wherein the circular path of the stray flux probe and the circular path of the stray flux probe and the circular paths along which the poles are adjusted, intersect at right angles, the point of intersection coinciding with the axis of the test piece.

6. A rotating head in accordance with any of the preceding claims, comprising pole pieces with curved faces which are fixed in relation to the annular magnet yoke and connected to the latter, and which faces are in contact with similarly curved pole shoes that can be moved along the curved faces for the adjustment of the poles.

7. A rotating head in accordance with claim 6, wherein the pole shoes can be clamped to the faces by clamping means.

8. A rotating head in accordance with claims 6 or 7, wherein the pole shoes comprise exchangeable pole heads.

9. A rotating head in accordance with claim 6 or 7, wherein each pole shoe comprise a curved centre piece with curved shells bearing against the two sides of the centre piece and arranged for being moved in relation to the latter parallel to the curvature of the curved face of the pole piece.

10. A rotating head in accordance with claim 9, wherein when the centre piece is retracted, the adjacent shells are also retracted successively and in pairs.

11. A rotating head in accordance with any of claims 6 to 10, wherein the pole shoes are mounted in a pivoting slide.

12. A rotating head in accordance with any of claims 6 to 11, wherein the or each probe lever is supported on one of the pole pieces.

13. A rotating head in accordance with any of the preceding claims, wherein the probe lever comprises a probe arm consisting of one or several U-shaped sections nested within each other.

14. A rotating head in accordance with any of the preceding claims, and comprising an exchangeable insert containing both the adjustable poles and the probe lever or levers.

15. A rotating head in accordance with claim 14, wherein the insert is open towards the outside, thus allowing dust particles to drop out.

16. A rotating head in accordance with any of the preceding claims, wherein the annular magnet yoke comprises a ring core whose cross-section in the axial direction of the test piece (3) is smaller or at least equal to its height h.

1 7. A rotating head substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.