This invention relates to a lifting apparatus and, in particular, to a magnetic lifting apparatus for heavy loads.

Conventional lifting apparatus often employ electromagnetic means for lifting heavy loads. In such apparatus, an electric current is passed through a large coil of a lifting head to produce a magnetic force, in accordance with known principles. This magnetic force attracts the load and the lifting head together in order that the load may be lifted and moved as required. In order to release the load, the current to the lifting head coil is cut off such that the magnetic force is removed and the load becomes detached from the lifting head.

In the event of a power failure or cable fracture occurring during a lifting operation, however, the current to the lifting head coil will be suddenly cut off and the load will be released prematurely. This may result in the load falling from a considerable height. Furthermore, electromagnetic lifting systems consume a large quantity of power due to the large current necessary to produce the magnetic force required.

We have now devised an improved lifting apparatus which overcomes the disadvantages described above.

In accordance with the present invention there is provided a lifting apparatus which comprises an upper unit comprising a plurality of spaced-apart permanent magnets having pole pieces disposed between them, a lower unit comprising a plurality of poles arranged to be positioned at the ends of the pole pieces between the permanent magnets to form a magnetic circuit, and means for displacing the lower unit away from the upper unit in order to break said magnetic circuit.

Preferably the upper unit comprises a plurality of generally C-shaped assemblies of permanent magnets nested together, such that the ends of the permanent magnet assemblies are positioned at intervals along the lower or lifting surface of the upper unit of the apparatus.

Preferably a section of non-magnetically permeable

material is positioned below the ends of the permanent magne assemblies.

Preferably an arrangement is provided at each end o the upper unit, for diverting end flux to the load bein lifted. Preferably each such arrangement comprises an en plate of magnetically permeable material and a series o permanent magnets positioned between that end plate and th corresponding end of the upper unit. Preferably the lower uni has opposite end pieces corresponding to the respective en plates of the upper unit.

Preferably the pole pieces of the upper unit are mad of a magnetically permeable material, such as 2.5% silico iron. The poles of the lower unit may be made of cobalt iro alloy, and may have sloping edges or may be adapted for liftin circular objects, such as tubes.

Preferably the means for displacing the lower unit awa from the upper unit comprises hydraulic rams which ca preferably re-attach the lower and upper units as required The hydraulic rams are preferably operable by means of oi pumped into or out of them. The oil may be pumped by a han pump or electrical pumps.

Preferably the system further includes means fo performing computer controlled measurements of certai parameters in order to give advanced warnings of unsaf loading, for example tilt in the load being lifted, temperatur limitations and exceeding the weight safety factor limit.

Preferably the permanent magnets are rare-eart permanent magnets. These may all comprise Neodymium-Boron-Iro (NdFeB) , or they may comprise Neodymium-Boron-Iron (NdFeB except for some sections of Samarium Cobalt (SmCo) .

In accordance with the invention, it is possible t provide a lifting apparatus having a 100% fail safe liftin capability for loads up to 1 tonne or more and with a maximu operational temperature of 300°C. It is further possible t provide a lighter and more compact lifting apparatus than prio art systems, with negligible power consumption due to it efficient design.

Embodiments of the invention will now be described b way of examples only and with reference to the accompanyin

drawings, in which:

FIGURE 1 is a schematic cross-sectional view of a first embodiment of lifting apparatus in accordance with the present invention; FIGURE 2 is a graphical representation to show an overview of the energy densities and coercivites of ferrite and rare-earth permanent magnet materials;

FIGURE 3 is a view corresponding to Figure 1, showing the lifting apparatus when the release mechanism is in operation;

FIGURE 4 is a section, on a plane perpendicular to Figure 3, through an end portion of the apparatus; and

FIGURE 5 is a schematic cross-sectional view of a second embodiment of the present invention. Referring to Figures 1 and 3, there is shown a lifting apparatus which comprises a 'Multi-C-Pole Unit' 1 (MCPU) and a 'Lower Assembly Unit' 2 (LAU) having a load 3 attracted thereto. ^' The MCPU 1 comprises a plurality of generally C- shaped magnet assemblies 4, each assembly 4 comprising a plurality of flat magnets disposed edge-to-edge with each other, with each such magnet elongated lengthwise of the apparatus (i.e. perpendicular to the plane of the paper on which Figure 1 is drawn) . The end magnets 5 of the assemblies 4 are positioned at equally spaced intervals along the lower or lifting surface of the MCPU 1. Positioned between each of the C-shaped magnet assemblies 4, are a plurality of generally C-shaped magnetic pole pieces 6, made for example of 2.5% Silicon Iron: it will be noted that the end magnets 5 of the assemblies 4 project downwardly beyond the lower ends of the pole pieces 6. The LAU 2 comprises a plurality of poles 7, made for example of cobalt iron alloy, and sections of non- magnetically permeable material 8, arranged such that the poles 7 are positioned between the projecting ends 5 of the C-shaped magnet assemblies 4, and the sections of non-magnetically permeable material 8 are positioned directly below the projecting ends 5 of the respective C-shaped magnet assemblies 4.

The lifting apparatus further comprises two hydraulic rams 9 positioned on either side, with their opposite ends

mounted to the MCPU 1 and the LAU 2 respectively by means o non-magnetically permeable brackets 9a. The hydraulic rams are operated by means of oil which is pumped into them from small oil reservoir 10 at the top of the MCPU 1. The apparatu comprising the MCPU 1 and the LAU 2 is covered by a layer o non-magnetically permeable material. The MCPU 1 is suspende by steel ropes (not shown) via load cells 11. The load cell 11 may have means which are computer controlled for measurin certain parameters in order to give advanced warnings of unsaf loading, for example, tilt in the load 3 being lifted temperature limitations and exceeding the weight safety facto limit.

Figure 4 shows an arrangement which is provided at eac end of the apparatus, for diverting the end flux to the loa 3 being lifted. This arrangement comprises permanent magnet 15 positioned across the ends of the vertical limbs o respective adjacent pairs of the magnets of the C-shape assemblies 4 of the MCPU 1 (the magnets 15 being elongated i the direction perpendicular to the plane of the paper on whic Figure 4 is drawn) . Also, magnets 15a are positioned agains the ends of alternate pole pieces 6, at the upper region thereof: it will be noted that like poles of the magnet 15,15a face outwardly. An end plate 16 of magneticall permeable material is positioned across the outer faces of th magnets 15,15a, and the LAU 2 has a corresponding end-piece 17 The MCPU 1 has an outer casing 18 which extends over its top sides and ends as shown, and typically may comprise hig strength bronze or aluminium. In the LAU 2, the end-piece 1 is mounted to the remainder of the unit by means of a membe 19, also typically of high strength bronze or aluminium.

In order to determine what materials should be used a permanent magnets for lifting purposes, two main materia properties must be considered. These are coercivity (Hcj) an energy density (B _d tt _d ) max. The higher the coercivity of material, the less likely it is to become demagnetised Therefore, a permanent magnet with a high coercivity should b chosen for lifting purposes. Energy density is an indicatio of the quantity of heat that can dissipate per square metre If a high maximum operating temperature is required, than

permanent magnet with a high energy density should be chosen. Together these two properties give an indication of the magnetic strength of a magnet.

As shown in Figure 2, a ferrite permanent magnet such as Ba0.6Fe ₂ 0 ₃ 21 has a very low magnetic strength (i.e. a (B _d tt _d ) max of 34.4 kJ/m ³ and a Hcj of 190 kA/m) compared with the magnetic strength of rare-earth permanent magnets such as samarium cobalt (SmCo) 22,24 and neodymium iron boron (NdFeB) 23. Although ferrite permanent magnets are substantially cheaper than rare-earth permanent magnets, it is preferable to use rare-earth permanent magnets for lifting purposes because a smaller, more compact and lighter lifting apparatus can be constructed due to the high magnetic strength of such magnets. A ferrite system having the same lifting capability would be heavy, bulky and inefficient due to flux leakage.

Referring back to Figure 1, the individual magnets 12 of the C-shaped permanent magnet assemblies 4 are preferably Neodymium-Iron-Boron (NdFeB) permanent magnets. The end magnets 5 are preferably Samarium Cobalt (SmCo) permanent magnets for a maximum operating temperature of 300°C. However, they may alternatively be NdFeB permanent magnets if the operating temperature will not exceed 120°C. This would make the apparatus less expensive to manufacture because NdFeB permanent magnets are less expensive than those made of SmCo. In use, when a load 3 is to be lifted, the LAU 2 and the MCPU 1 are held together by the double-acting hydraulic rams 9. In this mode, the load 3 is attracted to the lifting apparatus via the LAU 2. The apparatus is designed such that the flux level at the lifting pole face 13 is maximised such that the poles 7 can attract a rough surface (up to 4mm ridges) with ease and the system is able to lift a load 3 of 2 tonnes at an airgap 14 of 3-4mm.

As shown in Figure 3 , when the load is required to be released, oil is pumped into the oil sumps 9b of the hydraulic rams 9 from the small oil reservoir 10 at the top of the MCPU 1. The LAU 2 moves down to produce an air gap 35, at which point there is no longer any attracting force at the LAU surface 2 and the load 3 can be easily detached by lifting up the whole unit 1,2 via the steel ropes (not shown) attached to

the load cells 11.

In order to lift another load, the load is firs brought into contact with the surface of the LAU 2 in the de energised state, i.e. as shown in Figure 3. Oil is then pumpe from the oil sumps 9b of the hydraulic rams 9 back to the oi reservoir 10 until the LAU 2 is in complete contact with th MCPU 1.

The pumping of oil to and from the rams 9 may b carried out by a simple hand pump or by electrical pumps. The shape of the pole surface can be varied in order t take into account the roughness of the surface of the load t be lifted (and, therefore, the resulting air-gap between th load and the pole face) . For example, a similar apparatus t that of Figure 1 is shown in Figure 5. However, in this case the roughness of the surface of the load 43 has resulted in much larger air-gap 54 between the load 43 and the pole fac 47. In order to lift such a load 43, it is preferably t further maximise the flux level at the lifting pole face 47 This is achieved by increasing the slopes 55 of the poles 47 thereby increasing the concentration of flux at each pole face The poles may also be shaped to lift circular objects such as pipes.