Magnetic Lifting Devices
Magnetic Lifting Devices
Magnetic lifting devices are used for lifting and transporting of steel and ferrous metal stock or manufactured components. They are normally installed and used as single magnets or as arrangements of multiple magnets. In all of these cases, they are suspended from chains or wires or otherwise attached to the lifting equipment such as cranes. Electrically operated magnetic lifting devices are widely used in the iron and steel plants.
Lifting magnets can be provided with no power supply i.e. permanent magnets, or where power is supplied by cable from an external source or through an in-built battery. The magnet and any associated electrical equipment are to be designed for its intended purpose and constructed to withstand the environment in which it is required to operate. When used correctly magnetic lifting devices handle magnetic materials and components safely, and without the need for slingers.
All electromagnets use DC (direct current) power. In applications where AC (alternating current) power is preferred, a rectifier is needed to convert AC power to DC power. Accordingly, it is possible to operate electromagnets from a DC battery source.
Magnetic lifting devices are not to be treated as ‘general purpose’ pieces of lifting gear as they have to be designed to suit particular types of loads and environments. The hazards associated with their use are falling material and electromagnetic field (EMF). The magnetic lifting device is to be clearly marked with its safe working load (SWL) and is not to be operated above its SWL. Some of the factors which have a bearing on the selection of lifting magnets and operating conditions, particularly the SWL, include (i) magnetic properties of the load materials, (ii) load weight, thickness, shape and area in contact with the magnet, (iii) surface profile of materials to be lifted, (iv) stiffness or flexibility of the load, (v) range of sizes to be lifted and frequency of the operations, (vi) surface conditions of the magnet and the load, (vii) temperature of the magnet and the load and (viii) capacity of the external electrical power supply.
Lifting magnets can be specially constructed for a particular purpose. A number of different types of magnet are available. For example, flat pole plate magnets for handling sheet metals and bulk goods such as scrap iron etc., specially-shaped pole plate magnets for lifting pipes, round steel bars, or sectional steel, magnets with adjustable pole fingers for lifting items of irregular shape, and magnets with safety devices e.g. grab claws, as additional equipment.
Lifting magnet devices have lifting magnets which fall into two general shape classifications namely (i) round, and (ii) rectangular. The round lifting magnet is built with a centre pole (or core) and a round outer pole concentric with the centre pole. This magnet can be either a permanent magnet or an electro-magnet. The round electro lifting magnet is the most efficient when considering its face area as related to the lift power. Since the outer pole is a consistent distance from the core or centre pole, the result is a uniform magnetic strength over the magnet face area. Black iron pipe or steel tubing is used for outer poles of round lifting magnets, and hot rolled steel round bar stock is used for the centre core of round electro magnets.
The rectangular lifting magnet is built with either of two basic magnetic circuits, the two-pole or three-pole circuit, and it also can be permanent or electro-magnet. Pole plate and core material are normally hot rolled steel or low carbon steel. For the plates of rectangular lift magnets hot rolled steel plate is welded and joined into the required box shape. Using these stock materials, there is no limitation to lift magnet geometry. Both the round and rectangular lift magnets normally are adaptable to almost any lifting application.
Large electromagnets are extensively used in iron and steel plants for the handling of various materials. Power is fed to the magnets through trailing cables from a control box. These magnets, with ratings of tens of kilowatts, possess considerable stored magnetic energy, and it is hence necessary to provide a discharge resistor to absorb this energy when the magnet is switched off. It is normal for the discharge resistor to be situated in the control box with the rest of the control gear. Lifting magnets are frequently subjected to rough treatment and it is not uncommon for the cable to be broken. If this happens while it is switched on, the magnet is deprived of its discharge resistor and the final result is a breakdown of the magnet insulation, followed by a costly re-wind.
In case of the permanent magnets, the attraction and holding of the ferromagnetic material is made by permanent magnets incorporated in the lifting device. It has not got any plunger and its magnetic circuit is opened. Apart from the permanent magnets there is a coil mounted in, when the coil is fed with the electric current, cancels part of the magnet field of the magnet and allows loosening of the piece. The magnet of the lifting device recovers its initial force when the feeding of the electric current to the coil stops. In case of the lifting devices with the electro magnets the attraction and holding of the ferromagnetic material is made when the coil is turned on. It has not got any plunger and its magnetic circuit is open. When the coil is turned off, the piece drops. Fig 1 shows the principle of working of electromagnets.
Fig 1 Principle of working of electromagnets
Under normal use conditions, a permanent magnet can experience a decrease in its original holding value. The most common factors which can cause a loss of strength include (i) every day wear and tear on the magnet face, (ii) exposure to extreme temperatures outside the magnet’s temperature range, (iii) severe blow or shock to the magnet, (iv) exposure to electrical currents, and (v) exposure to vibrations.
Material as thin as 5 mm can be lifted from a stack with an electromagnet by using a variable voltage rectifier. Fixed voltage rectifiers are used on thicker materials and in applications where materials are not being lifted from a stack. Similar single sheet lifts from a stack can be achieved with a device known as a ‘drop controller’.
Ferromagnetism – Ferromagnetism is a physical phenomenon (long-range ordering), in which certain materials like iron strongly attract each other. It is one of the common phenomena which are responsible for magnetism in the magnets. One of the vital requirements of ferromagnetic material is that ions and atoms are to possess permanent magnetic moments. Some ions and atoms consist of the permanent magnetic moment which can be considered as a dipole that comprises a north pole separated from a south pole.
Ferromagnetism is caused in ferromagnets and the ferromagnets need to have net angular momentum which is obtained either through the orbital component of the spin component.
In electromagnetism, permeability is the measure of magnetization which a material obtains in response to an applied magnetic field. Permeability is typically represented by the Greek letter ‘mu’.
Attraction faces are the faces where the ferromagnetic materials are held, and the points where the magnetic flux (F) goes in and out.
Holding force (Fm) – It is the force perpendicular to the attraction faces needed to hold the attracted piece. It is normally shown in the specification sheets of the magnets and it refers to the whole contact face. The holding force for a magnet is affected by the composition of the material being lifted. Alloys with higher iron content are typically more susceptible to magnetic fields than those with lower iron content.
Shift force (FL) – It is the parallel force needed for the loosening of the attracted piece. Depending on the surface finish of the attracted piece, the force (FL) can vary between 20 % to 35 % of the holding force (Fm).
Breakaway force – it is the force needed to separate the load from the magnet when pulled in a direction perpendicular to the magnet’s face. The breakaway force of a lifting magnet is proportional to the thickness of the material being lifted. A magnet’s breakaway force increases until the material being lifted exceeds the saturation thickness. Accordingly, thinner materials do not yield as high a breakaway force while thicker materials do not yield a greater breakaway force.
Air gap (dL) – It is the medium distance between the attraction face of the holding electromagnets and the ferromagnetic piece surface. The shape and the roughness of these two surfaces and the non-magnetic materials between them, such as galvanic protection, and dust, etc. determine its value. The air gap between the magnet and the load also affects the performance of the magnetic lifting devices.
Standard voltage (Vn) – It is the value of the voltage of the electric power for which the holding electromagnet coil has been made.
Duty-cycle (ED %) – It is the value obtained dividing the connection time and the total cycle duration expressed in % and given as ED % = [time on / (time on + time off)] x 100 = (time on / a cycle duration) x 100. Standard holding electromagnets are prepared for an ED value of 100 % duty-cycle. The magnets of the magnetic lifting devices are to meet the requirement of the voltage limits set by the standards and codes.
Remanence (Br) – It is the force the electromagnet uses to hold the ferromagnetic piece after cancelling the magnetic field. Its approximate value is 5 % of Fm depending on the piece (size, roughness, and material etc.).
Polarity inversion – It is the reversal of polarity with limited duration and intensity which is needed to cancel the remnant magnetism of the attraction face in electromagnetic holding electromagnets after cutting of the voltage feeding.
Standard power demand (Pn) – It is the power demand which each of the holding electromagnets has.
Hot rate – It is the temperature rise of the holding electromagnet over the determined ambient temperature due to the power absorption under voltage. The temperature for reference is normally 35 deg C, if nothing against is indicated.
Material insulation class – It is the correspondence between coil insulation and a temperature limitation of the material used for coils manufactured. Normally, B thermal class insulation (130 deg C) is used.
Maximum performance room temperature – It normally considered as 55 deg C.
Protection types – The magnets of the lifting devices have protection against corrosion using galvanic treatments and protection against intrusion of solid object dust, accidental contact and water. The magnets of the magnetic lifting devices are to meet the requirement of the protection set by the standards and codes.
Magnetic flux F – It is the flux which the electromagnets generate on the surface of the piece to hold a magnetic field between the north and south poles. When the piece to hold is near the magnet, then the magnetic circuit is closed so the magnetic flux increases. The number of force lines per square centimetre which crosses perpendicular to the surface is the flux density which is also called magnetic induction B.
Piece to hold and contact surface – It is the contact surface between the electromagnet and the piece to hold is the attraction face. The surface of the piece is the one which is in contact with holding electromagnet attraction face. The holding force on the attraction surface is constant. The piece to hold determines the maximum holding force value (Fm). It depends on the size and thickness of its contact surface.
For field intensity H determined by a magnet or a coil, the induction which can be reached depends on the material type to handle. Magnetic induction is a function of field intensity [B=f(H)]. In the same electromagnet the holding forces can vary due to the magnetic properties of the material to hold. Among other things, saturation induction of the material determines the maximum holding force.
The material of the electromagnet to hold – It is that part of the material of the electromagnets where the magnetic field takes place. It is made of soft iron, with high magnetic permeability.
Internal structure and composition vary depending on the different materials. Carbon, chrome, nickel, manganese, molybdenum, and copper reduce the magnetic conductivity. The tempered pieces present a further reduction of the holding force. The harder is the temper, the inferior is the conductivity.
Battery back-up systems – These systems are provided to ensure that the load being lifted remains held by the magnets in the event of a power failure. Battery back-up systems typically hold the load for around 20 minutes thus allowing the crane operator to manually lower the load or clear the area.
Selection of lifting magnetic device
The optimum selection of a suitable lifting magnetic device and its components for the handling of steel and ferrous metal stock or manufactured components need a thorough knowledge of the application. The factors which dominate the lifting magnet selection for a specific application are (i) weight, shape, and contact area of the objects to be lifted, (ii) surface conditions of the load and the magnet, (iii) stiffness of the load, (iv) range of sizes and shapes to be lifted, (v) frequency of occurrence of the different sizes and shapes, (vi) adjustment of lifting power for less than full magnet face utilization, (vii) temperature of the load material, and (viii) the ambient temperature. Consideration of each of these factors lead to the identification of the magnet type, shape, number, and face contour best suited for the economy, efficiency, and safety.
Weight, shape, and contact area of load – Weight, shape, and contact area are to be considered at the very beginning for the selection of the lifting magnet. The contact area of the load controls the needed magnet number and size which is to be almost as much as the weight of the load. If a given load offers a relatively small surface area to the magnet face, then a magnet has to be selected which has a magnetic field which ‘penetrates’ the load thickness, so that the holding power is sufficient for the load involved. But if the same load offers a much larger contact area magnets of a different type, or smaller magnets of the same type, but more of them, can be used. These smaller magnets need not produce a field with such deep penetration into the load since the power of each magnet can be multiplied by the number used.
The number of poles a magnet has is determined by its intended use. As a general rule of thumb, the more ‘poles’ are on the face of the magnet, the shallower is the magnetic field. A two-pole magnet typically has a deeper field (extending farther from the face) than a magnet with three or more poles. The design of the magnet circuit determines the depth of the magnet field produced.
Generally speaking, as gaps between poles increase, the depth of field, or penetrating ability of the magnet, increases. For a given physical size, magnets with two poles have greater gaps than those with three poles and, accordingly are normally better suited for thicker loads. Conversely, three-pole magnets are normally the logical choice for thinner loads.
Fig 2 shows a sketch of a billet and a flat plate, each with the same weight. The type of magnet arrangement used for efficient lifting in each case is shown. As indicated above, a two-pole magnet is used for the billet and three pole magnets for the plate. The billet offers comparatively small contact area in relation to its weight. The plate of the same weight offers a considerable contact area. The magnet for handling the billet is required to penetrate the billet material to get adequate holding or lift power. But this same magnet is not efficient for lifting the plate since the shape of its penetrating field results in considerable fringing outside the relatively thin plate. Hence, for lifting the plate an arrangement of multiple smaller lifting magnets is more efficient.
Fig 2 Lifting of billet and plate by magnet
Surface condition of load magnet – Anything which prevents the face of the magnet from making full contact with the part being lifted is considered an ‘air gap’. Rust, dirt, ice, snow, machine grooves, and holes are just a few examples of the ‘air gap’. It is normally desired that the magnet make full contact with a ‘clean’ part before its lifting.
Only when there is no space between the mating surfaces of the magnet and the load then only the full lifting ability of the magnet can be put to work. The familiar ‘inverse square law’ indicates that the pulling power of a magnetic pole decreases rapidly as the distance between magnet face and load is increased. The graph in Fig 3 shows this effect. It can be seen that a magnetic pole with a given pulling power at 1 unit of distance has only one fourth of the power which pulls at 2 units of distance and one ninth of the power which pulls at 3 units of distance and so on.
Fig 3 Relationship between distance and pulling power of the magnet
For holding or lifting magnet applications, the distances between magnet and load which are important are of the order of a few millimetres. These distances are very small in comparison to the dimensions of the magnetic poles and the loads themselves. The other issue which matters is the magnetic field shapes which are more complex than those created by a single magnetic pole. Although the inverse square law is applicable in these cases, these ‘real world’ effects can reduce its apparent significance, which means that the pulling power of a given ‘real world’ magnet does not necessarily drop off as the square of the gap between the magnet face and the load as long as that gap is small. However, there is a considerable loss of the lifting power when surface separations as small as 2 mm occur. These separation distances are caused by normal machining grooves on the magnet face or by normal irregularities such as scale, pitting, or paint on the load.
It follows then that for maximum efficiency from a lift magnet, both the magnet face and the load surface are to be as smooth and as clean as practical. The magnet is required to be derated according to the degree of any unevenness or separation from full contact with the load to be lifted.
Stiffness or flexibility of load – If the load is balanced, and is stiff enough so that there is insignificant sag or droop of the portions of the load which overhang the lift magnet(s), then no consideration of load stiffness is needed. But, if the overhanging portions of the load can sag, the magnets are to be positioned on the load to minimize the peeling effects. Also, even if sag is not a factor and if the magnets cannot be placed on the centre then the strength of the magnets is needed to accommodate the resulting unbalanced load condition.
The holding or lift power of a magnet is rated with the pull of the load perpendicular to the face of the magnet, but droop of the load overhang causes a force which is not perpendicular to the magnet face. Fig 4 shows how this force causes a peeling action opposing the holding force. This peeling action tries to take the load off one pole of the magnet at a time. This is not resisted by the full rated magnet strength since the magnet rating depends on contact and pull of load against all magnet poles at the same time. Normally, assuming the magnets are of sufficient strength to cover all other factors, the peeling effect is not detrimental if the overhang of a flexible load falls within the limits. Outside these limits the magnets are to be derated appropriately.
Fig 4 Peeling action due to load overhang
In cases where there can be large amounts of load droop, two other factors are to be considered by the system designer and operators. These are (i) the possibility that the load can assume an overall ‘arc’ shape which does not match the linear arrangement of magnets on the lifting beam i.e. the load effectively lifting any central magnet(s) at the expense of increased forces on the end magnets, and (ii) the possibility of load ‘bounce’ during transition portions of the lift i.e. where the effective peeling load can be multiplied by large factors due to the transient acceleration of the unsupported portions. Both of these effects are to be minimized by careful lifting beam design, where the lifting loads at each magnet position are calculated by taking account of the flexibility and possible transient behaviour of the load, and by appropriate operating restrictions. In some cases, the magnets can be attached to the lifting beam using springs to help equalize the forces where there is extreme load droop.
Sizes and shapes to be lifted – For practical reasons majority of the lift-beams and magnet assemblies are used on a wide range of load sizes. This is particularly true when the assembly is used to unload or load flat rail cars or trucks where the load can be plates of varying sizes at one time and bars or beams or other structural shapes at another time. The assembly is to be designed with the proper selection and arrangement of the magnets so that it is efficient on the full-range of the items being handled. If, for example, a magnet and lift-beam assembly is used to unload plates which can vary in size from 4 m x 2.5 m down to 1 m to 0.5 m, the magnet layout to handle the smaller plates is not going to handle 4 m to 2.5 m plates efficiently. Even if there is adequate lift power in the layout for the larger plates, overhang is going to be excessive. Obviously, a magnet layout is needed which is dictated by the larger plates. This layout then results in magnets overhanging the smaller loads as shown in the Fig 5(a).
Fig 5 Typical layouts of beam and magnets assembly
Whenever the magnets overhanging the load cannot be energized because of some limiting factor of the operation, the control can be arranged so the overhanging magnets are ‘off’ when they are not in contact with a load. An example of this can be when there is requirement to lift only one beam or bar from a storage rack where the beams or bars (of varying lengths) are lying end to end. Fig 5(b) shows the way a short piece can be lifted from a storage rack in this situation.
Varying load sizes and shapes – It is obvious that if an assembly of lift magnets is to handle loads with relatively consistent size, weight, and shape 95 % of the time, and some much larger and heavier loads around 5 % of the time, then it is necessary to select and space the magnets for an efficient lift on the larger load. But when this difference between the largest, seldom-encountered load and the smaller, commonly encountered load is vastly different, then it is necessary to examine the costs involved to see if the system is truly economical. Such an analysis can lead to a compromise as to the numbers of the magnet to be used.
For explaining the above, an example of handling of the plate of dimensions 2.5 m x 1 m x 20 mm is taken. The plate is to be handled almost all the time with a lift magnet assembly. Sometimes, much heavier, larger, and thicker plates are to be handled. Rather than sizing the magnetic system for the heavier plates, sizing the magnet lift assembly is needed to be considered for the more frequent light-weight plates and to plan to use other approaches on the less frequent heavier and larger plates. These other approaches can be (i) using a sling and hook arrangement to lift the heavier plates by welding temporary lift eyes on the heavier plates, or (ii) performing any burning or cutting which is normally being done on the larger plates before unloading them.
A very large load variation as described frequently dictates a high initial magnet cost for total magnetic handling. With the approach described the user gets the manpower saving and speed advantages of magnetic handling on 95 % of the material handled. Further, the lift beam can be designed for future possible magnet additions which make the system completely automatic. The point is that a study of all factors such as economy, cycle time, manpower available, and a clear definition of the actual number of times an outsized load is encountered can result in a considerably reduced initial magnet system cost.
Partial magnet face use for lift – Whenever the load surface is not uniform over the magnet face, flat lifting magnets can be used but must be derated in proportion to the load area actually contacting the magnet face. When lifting a sheet of expanded metal, there is poor contact on the magnet face because of the high points on the expanded sheet. Further, the openings in the sheet do not offer a solid path for the magnetic circuit. Almost the same consideration is needed when handling flat perforated sheet. Although these sheets are flat, with no high points which prevent flush load and magnet contact, the holes in the sheet also prevent a solid path for the magnetic circuit. With both applications the effective lifting power of the magnet is a function of (i) percentage of sheet area actually contacting flush with the magnet face, and (ii) quality of the path the sheet offers to the magnetic circuit from pole to pole.
When a load is flat for a definable portion of its overall area, a simple calculation of the proportion of the magnet face which is covered allows estimating the magnet holding power. As an example, corrugated sheet is to be handled as shown in Fig 6a. In this case half of the magnet face is covered, so the lifting power on this corrugated sheet is estimated to be around one half what it is on a solid sheet of the same thickness.
The situation is more complex when considering loads which offer little flat contact surface to the magnet. There are so many varied-shaped loads possible which cannot be classified easily. In several cases, comparing the contact area with the contact area of a flat solid load allows an approximation of the lifting power of a magnet on the irregular load. In other cases, particularly when more accurate estimates are needed, a 3D finite element magnetic field analysis can be carried out by a magnet supplier, and / or actual comparative pull tests by using physical samples are needed. The important point is that a partial contact load needs appropriate derating of the magnet, for the reasons of safety, efficient operation, and proper design.
Fig 6 Lifting of corrugated sheet and special safety features of the lifting magnets
Lifting magnets with special safety features – If a load is lifted with one or more electromagnets, a failure of electric power to the magnet(s) during lift or transfer causes the load to drop. Depending on the load and location of lift and transfer, the results of dropping the load can be (i) of no consequence, as in the case of scrap yard magnet handling scrap, (ii) damage to the load, if finished items or loads subject to bending or breaking are being handled, (iii) damage to structures below, or (iv) disastrous, if the lift and transfer is made in an area where personnel can be inadvertantly located.
When protection against failure of the lifting capability of the magnet is needed, the use of magnets with special safety features is to be considered. Permanent magnets are now available which use electricity only to negate the field and which need deliberate action by an operator to release a load. The lifting power is generated by the permanent magnet component. When the electromagnetic coils are activated, load holding flux lines from the permanent magnet are temporarily diverted so that the holding power at the pole face is zero. Since there are no moving parts, there is no need for concern about a mechanical malfunction.
Similar protection can be achieved from permanent turn-off type magnets. These permanent lifting magnets use additional permanent elements which can be rotated into position to cancel out the lifting field. Electric power is used only to activate the turn-off cycle. The turn-off cycle also can be manually activated, thereby making the holding and releasing function completely independent of the electrical power source. The magnetic holding power is never lost with either type of switchable permanent magnet. A failure of the electric power supply does not cause loss of load though it makes the crane inoperable.
Electromagnets can be used in conjunction with an auxiliary power source (battery) which cuts in instantaneously if the main power source fails. The battery capacity can be sized to provide power for a reasonable period (normally around 20 minutes) so that temporary slings can be rigged to secure the load and other precautionary measures taken.
In short, special safety can be achieved by three different ways, and the one which is to be selected depends upon the load involved. These three approaches are (i) permanent magnets with electromagnetic load cancelling (Fig 6b), (ii) permanent turn-off magnets with automatic or manually activated turn-off cycle (Fig 6c shows manually activated turn off cycle), and (iii) electromagnets with auxiliary, automatic cut-in power supply (Fig 6d).
Safety factor – Published capacity of the lifting magnet represents ultimate lift strength i.e. the lifting capacity of the magnet on different loads ‘under ideal conditions’ unless clearly specified otherwise. It is normally impossible to foresee all the varying conditions of operation from one installation to another and to try to rate the magnet for each and every possible condition of operation. Instead, the magnet capacity is specified by applying a ‘safety factor’ to the ultimate lift strength of the magnet, making sure that the safety factor applied represents actual conditions as far as practical.
Some of the operating conditions which dictate the applicable safety factor are (i) surface condition of the load, (ii) surface condition of the magnet, (iii) smoothness of the lift, (iv) flatness and stiffness of the load, (v) centering of the load on magnet, (vi) environment, (vii) voltage fluctuations (for electromagnets), and (viii) unknown conditions. When any of these conditions are something which is not ideal then a safety factor is applied for the ultimate lift strength. This safety factor is to take into account the corresponding adverse effects on the ultimate lift strength of the magnets.
For example, a safety factor can be arrived at by assigning a value to each adverse condition and adding the results such as ‘safety factor = A + B + C + D + E, where A is the load itself with assigned value 2 to 4, B is the surface condition of the load which is not perfect and hence has an assigned value of 1, C is possible non-centered load with the assigned value of 1, D is the undesirable conditions of the environment with the assigned value of 1, and E consists of all other unknown with assigned value of 3. Thus the safety factor in this example is (2 to 4) + 1 + 1 + 1 + 3 which is 8 to 10. In this case the magnets are selected which theoretically can lift 8 to 10 times the maximum load involved and has the capacity to account for actual conditions expected. This is an example only.
The particular way of defining and applying a safety factor, as well as assigning values to the effects covered by the safety factor, is the responsibility of the specifying engineer for the lifting magnet application. If a magnet is applied to a vertical load surface then the load tends to slide or shear from the pole face. In this case, the ultimate lift strength is typically reduced to one-fourth of its normal value. Safety factors as previously outlined are then to be applied to the reduced shear rating.
Number of lift magnets for maximum efficiency – The most economical number of lift magnets for a particular application is one i.e. a single magnet with a capacity rated for the load. The expense of attachment to a crane and wiring is minimized when a single magnet can be used. However, using a single large magnet to lift large plates and sheets is not efficient. It is necessary to distribute the load-carrying capacity of the magnet over the entire plate area.
A single magnet properly sized to distribute the lift capacity over a large area is prohibitive in cost, and, because of its size, produces a field too deep for anything but very thick plates. Hence, to lift large plates and sheets more than one magnet is needed. From the wide range of magnet sizes, shapes, and capacities available, the possibility normally exists to consider multiple variations for the selection of the best size and the smallest number of magnets to do an efficient lift job on the load involved. Magnet weight, linkage weight, lift beam weight, and the load itself all add up to decide the crane size needed. Hence, if this weight of magnets and attachments can be minimized results into savings in crane and crane support costs.
When more than two lift magnets are used on a beam assembly, attachment to the beam is to be arranged so that all the magnets can reach the load and none can be stripped off the load when the lift is made. This is very important when the magnet faces are not perfectly aligned and a stiff load is handled i.e. the one which does not deflect to match the magnet faces. Fig 7(a) shows how one of the magnets is prevented from maintaining contact on the load during lift. The unevenness of the magnet face elevations is exaggerated in the Fig 7 (a), but in effect this is the result when multiple magnet assemblies are not mounted correctly. The same end result occurs if the lift beam has excessive deflection under the load.
Fig 7 Incorrect magnet assembly on the beam and magnet construction
Conversely, when several lift magnets are mounted loosely on a single beam assembly, while the magnet faces can be able to align easily with the load, it is possible for the load flexibility and overhang to combine in such a way that one or more of the magnets is actually lifted by the load. This not only eliminates those magnets as effective lifting components, but it adds their weight to the load which is to be carried by the remaining magnets. This is a particularly deceptive variation of the ‘peeling’ phenomenon. Springs in the magnet linkage can be used to counteract this effect.
For the use the fewest magnets possible for any particular lift application, the magnet support arrangement is engineered carefully for maintaining the alignment of the magnets and the load. The arrangements which need the load to ‘push’ the magnets are to be avoided.
Construction of lifting magnets – Lifting magnets fall into two general shape classifications namely (i) round (Fig 7b), and (ii) rectangular (Fig 7c and 7d). The round lifting magnet is built with a centre pole (or core) and a round outer pole concentric with the centre pole. This magnet can be either permanent or electro round lifting magnets. Hot rolled steel round bar stock is used for the centre core of round electro magnets. The rectangular lifting magnet is built with either of two basic magnetic circuits, the two-pole or three-pole circuit, and it also can be permanent or electro magnet.
The round electro lifting magnet is very efficient when considering its face area as related to the lift power. Since the outer pole is a consistent distance from the core or centre pole, the result is a uniform magnetic strength over the magnet face area. Black iron pipe or steel tubing is used for outer poles of pole plate and core material is hot rolled steel or low carbon steel. For the plates of rectangular lift magnets hot rolled steel plate is welded and joined into the required box shape. There is no limitation to lift magnet geometry for using the stock materials, but round and rectangular lift magnets normally prove adaptable to almost any lifting application.
Lifting loads with undefined shapes – Normally, lift magnets are designed to handle loads which can be classified into plate, bars, structural shapes, and round materials. These configurations offer a consistent and predictable surface to the magnet face. Hence, the magnet face can be matched to the shape of the load. But there are unusual shapes and loads which can also to be handled as long as it is realized that since these shapes do not match the magnet lifting surface, the lifting magnets are to be derated. Some of these shapes are shown in Fig 8, each being handled by flat-face lift magnets. It can be seen that it is impossible to match the face of a practical magnet to all such varying loads, but it is possible to design the internal circuit of the magnet to achieve maximum efficiency on any of these loads.
Fig 8 Lifting of loads of consistent defined surface by flat magnets
Auxiliary pole plates – Auxiliary pole plates are added to the ‘integral’ poles of a magnet on its working face when the integral magnet poles do not match a load shape closely enough for efficient handling. Auxiliary poles can be interchanged on a magnet at different times to match different loads. Normally such poles do not reduce the holding power of a magnet from that produced if its integral poles had the same configuration as the auxiliary poles. However, the mating surfaces where the integral poles join the auxiliary poles are not to introduce an air gap into the magnetic circuit which diminishes the holding power. To avoid this, the contact surfaces of both the poles are to be machined smooth.
When there is necessary relative movement (sliding) between a magnet and load, sacrificial auxiliary poles can be used to take the resulting wear. These auxiliary poles can be hardened or plated much more economically than can the integral poles. Since holding and lifting efficiency of a magnet are based on maximum contact between magnet and load, the shape of auxiliary pole faces are to match the load as closely as possible. However, the pole shape can be ‘compromised’ or averaged over the range of the shapes handled so that it is not necessary to change poles every time load size changes.
As an example for lifting 100 mm, 150 mm, and 200 mm diameter steel bars, as well as flat loads, auxiliary poles with a single shape can be designed for handling the full range of round bar sizes. When flat loads are handled the auxiliary poles are removed. The operator is to recognize that the lifting ability of the magnet is not the same on each of these round shapes as if each round shape has exactly matching poles. Fig 9 shows these ‘average’ auxiliary poles. The auxiliary poles more closely match the configuration of the 200 mm bar since this bar is the heaviest. Less contact is needed on the lighter, smaller bars.
Even if load shape is constant and wear is not experienced, it can be more economical to do any machining of pole plates needed to match load shapes on small auxiliary poles than on the main magnet face. After the machining is completed these poles can be permanently attached to the magnet. Using auxiliary poles can readily convert standard or stock magnets to specials with maximum economy and minimum fabrication time. Auxiliary poles are beneficial for handling hot material. They space the coil further from the source of heat and increase space for passage of circulating air. Some auxiliary pole plate applications are shown in Fig 9. Chrome – faced auxiliary poles handle polished sheets without damage. Auxiliary poles allow magnet to reach flat surface of special shaped load.
Fig 9 Auxiliary pole plate for magnets
Drop controllers – Electromagnets hold a load by means of magnetic lines of force which are generated by the magneto-motive force of the energized magnet. When the magnet is de-energized, residual lines of force can remain if there is close contact between the magnet and load. Reverse current drop controllers are used to apply reverse current to the magnet for cancellation of these residual lines. This cancellation results in faster release of the load.
Examples of typical application
Auxiliary poles – The requirement is to lift and transfer plates, flat and round bars, and angles to a shot blast table. The bars and angles are to be deposited on the shot blast table at the same spacing and in the same orientation at which they are used. Because the majority of lifts are on flat plate and flat bars, the occasional handling of angles and round bars is done by adding auxiliary poles to the magnets. These are engaged only when the angles or round bars are handled, and provide the required spacing and the required magnet face contact on the angles and bars to make the lift. Fig 10 gives some examples of typical applications.
Fig 10 Examples of typical applications
Lifting of a circular coil – Coils of steel strip are to be lifted and moved using electro lifting magnets. The coils vary in inner diameter (ID) and outer diameter (OD) and all have three banding straps as shown in Fig 11. Rectangular lift magnets with a length dictated by the smallest ID and largest OD of the coil range is selected. The magnets overhang the load on the ID or OD when any but a load with a minimum ID or maximum OD is handled. The number of magnets selected is determined by the clear spaces between the three straps. If a lift magnet is allowed to rest on one of the bands, its lift capacity gets greatly reduced because the magnet face is not having full contact on the load.
Fig 11 Lifting of coil and plates by magnets
Lifting plate of various sizes – Plates of various lengths and widths are handled. The plates arrive at the unstacking location in any of the arrangements shown in Fig 11 depending on their width and length. When more than one stack is on a common skid, only one stack is to be unpiled at a time. Here is a case where numerous small magnets, rather than a few large magnets, are the ideal selection. The magnets are electro type or permanent turn-off type arranged in banks so only the magnets directly in contact with the load to be lifted are energized.
The layout of magnets is as shown in Fig 11, and the electrical controls designed so that all or any combination of magnets can be energized in order to selectively lift only one small plate at a time.
General safety guidelines
The general safety guidelines are (i) the lifting magnet face and the lifting magnet contact area on the load is to be clean, (ii) the operator is to avoid carrying the load over people, (iii) no person is to be allowed to stand on top of lifted work-piece, (iv) the load or magnet to come into contact with any obstruction is not allowed, (v) no hooking of two lifting magnets is allowed without the use of a properly designed spreader beam, (vi) care is to be taken to make certain that the load is correctly distributed for the lifting magnet being used, (vii) placing of the magnet in shear is to be avoided, and (viii) uneven lifts are to be avoided.