LIFTER WITH ELECTROPERMANENT MAGNETS

Abstract

A lifter with electro-permanent magnets is provided. It has an external bearing structure closed at the bottom by a plate, provided with a heat shield, and pole pieces secured under the respective poles and protruding from the bottom plate. Each of the electro-permanent magnets has a reversible magnet arranged on top of one of the poles, of a fixed polarization magnet formed by a plurality of blocks placed along the lateral faces of the pole and of a coil arranged around the reversible magnet to cause the reversal of the polarization of the latter by means of an electrical pulse. An airtight air gap between 1 and 4 mm high is formed between each pole piece and the respective pole through the interposition of a plate of thermal insulation material that resists high temperatures provided at each pole with a rectangular window slightly smaller in size than the pole itself, with the top sides of the pole pieces and/or the bottom sides of the poles being provided with peripheral recesses suitable to act as seats for the positioning of the plate.

Claims

1-10. (canceled)

11. A lifter with electro-permanent magnets comprising an external bearing structure closed at the bottom by a plate provided with a heat shield, and pole pieces secured at the bottom of respective circuit poles of said electro-permanent magnets and protruding from said bottom plate, said lifter further comprising an airtight air gap between each of said pole pieces and the respective pole to which it is secured.

12. The lifter according to claim 11, wherein the height of the air gap is between 1 and 4 mm.

13. The lifter according to claim 11, wherein the airtight air gaps are formed in a plate of thermal insulation material that resists high temperatures arranged between the pole pieces and the poles and provided at each pole with a window slightly smaller in size than the pole itself, the top sides of the pole pieces and/or the bottom sides of the poles being provided with peripheral recesses suitable to act as seats for the positioning of said plate of thermal insulation material.

14. The lifter according to claim 13, wherein the material of which the thermal insulation plate is made is suitable to operate between 450° C. and 800° C., has a compression resistance ≧300 MPa and a thermal conductivity between 0.18 and 0.32 W*m.sup.−1*K.sup.−1.

15. The lifter according to claim 14, wherein the material of which the thermal insulation plate is made is a laminated material made up of muscovite sheets coupled through silicone resin.

16. The lifter according to claim 13, wherein the peripheral recesses are 7 to 12 mm wide and 3 to 6 mm high or 10 mm wide and 5 mm high.

17. The lifter according to claim 14, wherein the peripheral recesses are 7 to 12 mm wide and 3 to 6 mm high or 10 mm wide and 5 mm high.

18. The lifter according to claim 15, wherein the peripheral recesses are 7 to 12 mm wide and 3 to 6 mm high or 10 mm wide and 5 mm high.

19. The lifter according to claim 11, further comprising heat sinks optionally made of copper, arranged between the pole pieces under the plate of thermal insulation material.

20. The lifter according to claim 19, wherein each heat sink is made up of a plurality of transverse elements arranged between a pair of longitudinal elements extending along the opposite side walls of two adjacent pole pieces.

21. The lifter according to claim 13, further comprising heat sinks optionally made of copper, arranged between the pole pieces under the plate of thermal insulation material.

22. The lifter according to claim 21, wherein each heat sink is made up of a plurality of transverse elements arranged between a pair of longitudinal elements extending along the opposite side walls of two adjacent pole pieces.

23. The lifter according to claim 14, further comprising heat sinks optionally made of copper, arranged between the pole pieces under the plate of thermal insulation material.

24. The lifter according to claim 23, wherein each heat sink is made up of a plurality of transverse elements arranged between a pair of longitudinal elements extending along the opposite side walls of two adjacent pole pieces.

25. The lifter according to claim 15, further comprising heat sinks optionally made of copper, arranged between the pole pieces under the plate of thermal insulation material.

26. The lifter according to claim 25, wherein each heat sink is made up of a plurality of transverse elements arranged between a pair of longitudinal elements extending along the opposite side walls of two adjacent pole pieces.

27. The lifter according to claim 16, further comprising heat sinks optionally made of copper, arranged between the pole pieces under the plate of thermal insulation material.

28. The lifter according to claim 27, wherein each heat sink is made up of a plurality of transverse elements arranged between a pair of longitudinal elements extending along the opposite side walls of two adjacent pole pieces.

29. The lifter according to claim 11, wherein it is provided with an adjustment and control circuit comprising thermal probes.

30. The lifter according to claim 13, wherein it is provided with an adjustment and control circuit comprising thermal probes.

31. The lifter according to claim 14, wherein it is provided with an adjustment and control circuit comprising thermal probes.

32. The lifter according to claim 15, wherein it is provided with an adjustment and control circuit comprising thermal probes.

33. The lifter according to claim 16, wherein it is provided with an adjustment and control circuit comprising thermal probes.

34. A method of manufacturing a lifter with electro-permanent magnets according to claim 13, wherein the circuit poles, the pole pieces and the plate of thermal insulation material are taken at a temperature of at least 150° C. so as to eliminate any trace of moisture and then the pole pieces are secured on the poles through mounting screws sufficiently tightened as to compress the plate such that it acts as a gasket for the airtightness of the air gaps.

Description

[0015] FIG. 1 is a transverse cross-sectional view along the midplane of a lifter according to the present invention, resting on a load to be lifted, with an enlarged detail; and

[0016] FIG. 2 is an enlarged detail of a bottom plan view of a corner of the lifter of FIG. 1.

[0017] With reference to these figures, there is seen that a lifter with electro-permanent magnets according to the present invention conventionally includes an external bearing structure, a plurality of electro-permanent magnets and an adjustment and control circuit.

[0018] The bearing structure consists of a top cover 2, provided with couplings for the connection to lifting means (e.g. a crane), two peripheral walls 3 and a bottom closure plate 10 provided with a heat shield 9 to protect the magnets from the heat radiated by the hot ferromagnetic materials to be lifted. Said structure is obviously made of high magnetic conductivity materials, typically carbon mild steel, in order to minimize the reluctance of the magnetic circuit, same as the circuit poles 1 and the pole pieces 5, intended to contact the load to be lifted, which protrude below the closure plate 10 and are secured to poles 1 through screws 11.

[0019] Each of the electro-permanent magnets includes a reversible magnet 6, arranged on top of a pole 1 and in contact therewith, and a fixed polarization magnet 7 formed by a plurality of blocks placed along the lateral faces of said pole 1. Around the reversible magnet 6 there is arranged a commutation coil 8 that controls the reversal of the polarization thereof, to commute between the condition of inoperative lifter illustrated in FIG. 1 with the poles in series North-South-North-South and the condition of operative lifter with the poles in parallel North-South-South-North.

[0020] The adjustment and control circuit preferably includes a device of the type described in EP 0929904 B1. In brief, said device includes for each polarity a first magnetic sensor arranged close to the base of pole 1 and a second magnetic sensor arranged between the fixed magnet 7 and the reversible magnet 6, so as to measure substantially only the magnetic flux passing through the reversible magnet 6, as well as a control unit for processing the signals transmitted by said magnetic sensors (not shown in the drawings) and obtaining the operating point of the lifter on the magnetization curve of the reversible magnet 6.

[0021] The above device guarantees absolute safety during any load lifting and transporting operation by checking that the sum of the reversible losses of the magnetic masses 6, 7 and of the decrease in magnetic permeability of the ferromagnetic circuit of the lifter, and in particular of the hot material to be lifted, still allows the lifter to attain the lifting safety coefficient according to the EN 13155 standard (or another similar standard applied in other countries).

[0022] Such a device also monitors the efficiency of coils 8 that are preferably made of an aluminium strip or a copper strip so as to minimize their volume and to optimize the thermal dissipation due to Joule effect. Coils 8 are designed such that they can operate correctly with reversal pulses that are either constant in current or constant in voltage, although given the critical operating conditions of high temperature of the material it is preferable to use a constant current apparatus.

[0023] The adjustment and control circuit employs also the signals of thermal probes (not shown in the drawings) extending inside the various critical components of the lifter to check that it is possible to perform safely the operations.

[0024] Turning now to the specific novel aspects of the present lifter, the applicant has found that for the particular application for which the present lifter is intended it is advantageous to have the pole pieces 5 not in contact with poles 1 but leaving a small air gap, even if this implies an increase in the magnetic reluctance of the circuit and an increased magnetic circuit leakage. Overcoming this rooted technical prejudice, the applicant has verified through tests that at the operating temperatures requested to the lifter to operate on ferromagnetic materials at high temperatures up to 600-650° C., the disadvantage due to the increased magnetic reluctance is substantially offset by the reduced transmission of heat through the air gap which allows it to operate with the same loads of a traditional lifter but for much longer times without having to put it to rest after a couple of hours to let it cool down.

[0025] More specifically, returning to the example mentioned above of a traditional lifter equipped with fixed polarization magnets of samarium-cobalt alloy which moves the ferromagnetic material at 600° C. with a 60% operating cycle, after two hours of operation its maximum strength lifting was reduced to 44% compared to 100% which it had at 20° C. at the start of the operation. Furthermore, the temperatures reached by the critical elements prevent to continue to safely use the lifter that must be put to rest to cool down.

[0026] A lifter according to the present invention, which faces the same operating conditions and is provided with airtight gaps 2 mm high, initially starts with a maximum force at 20° C. reduced to 82% due to the air gap but after two hours of operation still retains 43% of the force. This means that despite the air gap of 2 mm, thanks to the presence of the airtight air gaps, the reduction of losses due to the heating of the magnetic masses after two hours of operation limits the difference between the forces of the two lifters to 1% against the initial 18% (and taking into account the safety factor of 3 of EN 13155 this difference in practice is 0.33% and 6% respectively).

[0027] However, the present lifter is still able to operate safely, as opposed to a traditional lifter, as shown by the following table which shows the temperature increase of the critical elements that even after 16-18 hours of operation have not yet reached the limit values, so that the lifter still retains about 38% of the force.

TABLE-US-00001 Time Temperature Temperature Temperature (h) SmCo (° C.) Alnico (° C.) coils (° C.) 2 106 90 65 6 200 180 140 8 229 207 160 16 281 255 180 18 286 260 182

[0028] The enclosed drawings show the preferred embodiment or best mode of the invention in which the airtight air gaps are obtained by interposing between the pole pieces 5 and poles 1 a plate 4 of thermal insulation material resistant to high temperatures, for example a laminated material commercially known as Pamitherm and consisting of sheets of muscovite coupled by means of silicone resin. In correspondence of each pole 1 there has been formed in plate 4 a rectangular window of a size slightly smaller than the pole itself, while in the top sides of the pole pieces 5 and/or in the bottom sides of poles 1 peripheral recesses were obtained, for example 7-12 mm wide and 3-6 mm high, which form seats for the positioning of plate 4.

[0029] Subsequently, the magnetic poles 1, the pole pieces 5 and plate 4 have been brought to a temperature of at least 150° C. to eliminate any presence of moisture and the mounting screws 11 of pieces 5 have been tightened so as to compress adequately plate 4 such that it acts as a gasket for the airtight sealing of the air gaps 12 thus formed.

[0030] These air gaps 12, which are preferably between 1 and 3 mm high depending on the temperatures and on the operating times, greatly reduce the transmission of heat received from the lifted material at temperatures up to 650° C. thanks to the extremely low thermal conductivity of dry air that is around λ=0.026 W*m.sup.−1*K.sup.−1.

[0031] The material of which plate 4 is composed can operate between 450° C. and 800° C., has a resistance to compression ≧300 MPa and a very low thermal conductivity λ=0.18 W*m.sup.−1*K.sup.−1 (although materials with λ up to 0.32 W*m.sup.−1*K.sup.−1 are suitable). The temperature values in the above table were obtained using a lifter according to the preferred embodiment comprising a plate 4 having these parameters.

[0032] The typical size of a magnetic pole 1 is preferably included between 200 and 350 mm in width and between 800 and 1400 mm in length, and in the illustrated embodiment the portion of plate 4 compressed between pieces 5 and poles 1 forms a frame 10 mm wide and 5 mm high that although being of reduced area transmits heat more readily since it has a thermal conductivity about 7 times higher than the dry air of the airtight air gap 12. To minimize this amount of heat it is preferable to install heat sinks 13 between the pole pieces 5, below plate 4 of heat-insulating material, each heat sink 13 being formed by a plurality of transverse elements arranged between a pair of longitudinal elements which extend along the opposite side walls of two adjacent pole pieces 5. Note that half-sized heat sinks 13′ are arranged also on the outer side walls of the outermost pieces 5 to achieve maximum efficiency in heat dissipation, and that heat sinks 13, 13′ are preferably made of copper whose thermal conductivity coefficient is λ=390 W*m.sup.−1*K.sup.−1.

[0033] A lifter with electro-permanent magnets thus manufactured and operated is therefore capable of safely moving for long periods materials such as billets, blooms, slabs, etc. at a temperature of 600-650° C. and is therefore suitable for the discharge operating cycle of the cooling plates located at the outlet of the hot rolling line of said products in a steel mill.

[0034] It is obvious that the above-described and illustrated embodiment of the lifter according to the invention is just an example susceptible of various modifications. In particular, the exact number, shape and arrangement of the magnetic polarities may vary depending on the specific application, for example by providing a lifter with a single magnetic dipole or three or more magnetic dipoles rather than the two magnetic dipoles illustrated in the present embodiment.

[0035] Moreover the air gaps 12 can be obtained in other ways, for example by arranging between pieces 5 and poles 1 gaskets of suitable material and adequate height housed in specific seats.