GB2292838A

GB2292838A - Magnetic lifting apparatus

Info

Publication number: GB2292838A
Application number: GB9517963A
Authority: GB
Inventors: Amitava Basak; Nabeel Shirazee
Original assignee: University College Cardiff Consultants Ltd; Cardiff University
Current assignee: University College Cardiff Consultants Ltd; Cardiff University
Priority date: 1994-09-02
Filing date: 1995-09-04
Publication date: 1996-03-06
Anticipated expiration: 2015-09-04
Also published as: GB9517963D0; GB2292838B

Abstract

The lifting apparatus comprises an upper unit (1) comprising a plurality of spaced-apart permanent magnets (5) having pole pieces (6) disposed between them, a lower unit (2) comprising a plurality of poles (7) arranged to be positioned at the ends of the pole pieces (6) between the permanent magnets (5) of the upper unit, so as to form a magnetic circuit for lifting a load (3), and hydraulic rams (9) for displacing the lower unit (2) away from the upper unit (1) in order to break the magnetic circuit in order to release the load (3). <IMAGE>

Description

LIFTING APPARATUS 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 '; intervals along the lower or lifting surface of the upper uni of the apparatus.

Preferably a section of non-magnetically permeable material is positioned below the ends of the permanent magnet assemblies.

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

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

Preferably the means for displacing the lower unit away from the upper unit comprises hydraulic rams which can preferably re-attach the lower and upper units as required.

The hydraulic rams are preferably operable by means of oil pumped into or out of them. The oil may be pumped by a hand pump or electrical pumps.

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

Preferably the permanent magnets are rare-earth permanent magnets. These may all comprise Neodymium-Boron-Iron (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 to provide a lifting apparatus having a 100% fail safe lifting capability for loads up to 1 tonne or more and with a maximum operational temperature of 300he. It is further possible to provide a lighter and more compact lifting apparatus than prior art systems, with negligible power consumption due to its efficient design.

Embodiments of the invention will now be described by way of examples only and with reference to the accompanying 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 Cshaped 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 nonmagnetically 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 of non-magnetically permeable brackets 9a. The hydraulic rams 9 are operated by means of oil which is pumped into them from a small oil reservoir 10 at the top of the MCPU 1. The apparatus comprising the MCPU 1 and the LAU 2 is covered by a layer of non-magnetically permeable material. The MCPU 1 is suspended by steel ropes (not shown) via load cells 11. The load cells 11 may have means which are computer controlled for measuring certain parameters in order to give advanced warnings of unsafe loading, for example, tilt in the load 3 being lifted, temperature limitations and exceeding the weight safety factor limit.

Figure 4 shows an arrangement which is provided at each end of the apparatus, for diverting the end flux to the load 3 being lifted. This arrangement comprises permanent magnets 15 positioned across the ends of the vertical limbs of respective adjacent pairs of the magnets of the C-shaped assemblies 4 of the MCPU 1 (the magnets 15 being elongated in the direction perpendicular to the plane of the paper on which Figure 4 is drawn). Also, magnets 15a are positioned against the ends of alternate pole pieces 6, at the upper regions thereof: it will be noted that like poles of the magnets 15,15a face outwardly. An end plate 16 of magnetically permeable material is positioned across the outer faces of the 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 high strength bronze or aluminium. In the LAU 2, the end-piece 17 is mounted to the remainder of the unit by means of a member 19, also typically of high strength bronze or aluminium.

In order to determine what materials should be used as permanent magnets for lifting purposes, two main material properties must be considered. These are coercivity (Hcj) and energy density tBdttd)max. The higher the coercivity of a material, the less likely it is to become demagnetised.

Therefore, a permanent magnet with a high coercivity should be chosen for lifting purposes. Energy density is an indication of the quantity of heat that can dissipate per square metre.

If a high maximum operating temperature is required, than a 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 Ba0.6Fe20321 has a very low magnetic strength (i.e. a (Bdtta max of 34.4 kJ/m3 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 300C. However, they may alternatively be NdFeB permanent magnets if the operating temperature will not exceed 120he. 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 first brought into contact with the surface of the LAU 2 in the deenergised state, i.e. as shown in Figure 3. Oil is then pumped from the oil sumps 9b of the hydraulic rams 9 back to the oil reservoir 10 until the LAU 2 is in complete contact with the MCPU 1.

The pumping of oil to and from the rams 9 may be carried out by a simple hand pump or by electrical pumps.

The shape of the pole surface can be varied in order to take into account the roughness of the surface of the load to be lifted (and, therefore, the resulting air-gap between the load and the pole face). For example, a similar apparatus to 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 a much larger air-gap 54 between the load 43 and the pole face 47. In order to lift such a load 43, it is preferably to 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.

Claims

1) 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.

2) A lifting apparatus as claimed in claim 1, in which 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.

3) A lifting apparatus as claimed in claim 1 or 2, in which a section of non-magnetically permeable material is positioned below the ends of the permanent magnet assemblies.

4) A lifting apparatus as claimed in any preceding claim, in which the poles pieces of the upper unit comprise silicon iron.

5) A lifting apparatus as claimed in any preceding claim, in which the poles of the lower unit comprise cobalt iron alloy.

6) A lifting apparatus as claimed in any preceding claim, in which the permanent magnets comprise rare-earth permanent magnets.

7) A lifting apparatus as claimed in any preceding claim, in which the means for displacing the lower unit relative to the upper unit comprise hydraulic actuators.

8) A lifting apparatus as claimed in any preceding claim, in which an arrangement is provided at each end of the upper unit, for diverting end flux to the load being lifted.

9) A lifting apparatus as claimed in claim 8, in which each said flux diverting arrangement comprises an end plate of magnetically permeable material and a series of permanent magnets positioned between that end plate and the corresponding end of the upper unit.