Use of Magnets for Lifting Loads
Use of Magnets for Lifting Loads
Lifting magnet is a kind of equipment which is used as a part of the electrical overhead travelling (EOT) cranes and is used for the transfer the magnetic materials from one place to another place. It is widely used in industry dealing with ferrous materials. An iron and steel plant uses a large number lifting magnets.
Lifting magnets can be electro-magnet or permanent magnet, and can be installed and used as single magnet or as an arrangement of multiple magnets. In all the cases, making an optimum selection of lifting magnet components to handle flat or shaped steel products needs a thorough knowledge of the application. All electro-magnets use DC (direct current) power. In applications where AC (alternating current) power is available, a rectifier is needed to convert AC power to DC power. Alternatively, it is possible to operate electro-magnet from a DC battery source.
Electrically operated magnetic lifting devices, in the form of a single magnet or a group of magnets suspended from chain or wire ropes or attached to lifting equipment in any other way, can be used for lifting and transporting steels and majority of the ferrous materials or components without the need for slings. They are widely used in several industries including metal / component manufacture and storage, shipbuilding, and metal recycling industries etc.
Magnets can be provided without power supply i.e., permanent magnets, or where power is supplied from an external source or through an in-built battery. Electro-permanent magnets are also available where permanent magnets are utilized to hold the load and power is used to release the load. Power is used to both energise and release the load. They use a combination of permanent magnets and magnets whose polarity is switched by an electrical pulse rather than mechanically. Some can be partially energised to facilitate load shredding. Different shapes and types of magnets are available, e.g., 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, and magnets with adjustable pole fingers for lifting items of irregular shape.
There are three types of lift magnets namely (i) permanent magnet, (ii) electro-magnet, and (iii) permanent / electro-magnet combination. The selection of the type of magnet is based on several factors ranging from power accessibility, cost, and how close the magnet meets the shop requirements. Magnets and magnetics are changing rapidly, getting the best magnet for the application can mean one has a magnet which is stronger, lighter, contains ‘smart’ technology and / or is less expensive.
Permanent lifting magnets are of two types. One is ‘always on’ and the other is ‘on/off’. The ‘always on’ lifting magnets utilize some kind of mechanical method to separate the magnet from the steel which is being lifted. This mechanical method normally is a roller cam, solid round cam, jack screw or breaker bar. Other than the roller cam, these mechanical release methods can damage or scratch metal surfaces. Typically, this type of magnet has multi-poles meaning strips of steel with alternating polarity (N, S, N, S). These magnets are used on flat steel and have shallower magnetic penetrations which make them better suited for thinner metal.
The ’on/off’ permanent magnets have the safety of an ‘always on’ permanent magnet and the controlled ‘on/off’ of an electro-magnet. These magnets frequently use rare earth magnetic material in two separate fields. When both fields are lined up, ‘north to north’ and ‘south to south’, the magnetic field goes down into the steel. When one field is reversed, caused by rotating the ‘on/off’ handle, the field stays within the magnet, no longer holding the steel. The ‘on/off’ magnets normally have two parallel poles which give this magnet a deep penetrating magnetic field for rougher, and flat surfaces and work well on round pipe or shaft material. When this type of permanent magnet is ‘off’ all collected fuzz iron falls away. In most sizes, the ‘on/off’ magnet is required to be on steel to rotate the handle to the ‘on’ position. This is a safety feature which prevents pre-energizing of the magnet prior to being placed on steel, reducing the chance of injury or equipment damage.
Electro-magnets use electrical power to generate a magnetic field. This power is to be in the form of DC power. The DC power comes from the conversion of AC to DC from a power supply or can be provided from a battery. Electro-magnets provide controlled ‘on’ and ‘off’ from remote locations. They do however need constant electrical current or the magnet releases the load. Battery back-up power supplies can provide constant power when a power interruption occurs.
Electro-magnets with a ‘cord’ coming from a power source provide concentrated holding power and a deep reaching magnetic field to lift thick, non-flexing ferrous materials. items. Some models have pendant controls or on-board switch for ‘on, off, and release’ functions.
Battery lifting magnets operate from a self-contained, automotive-type battery, which results in maximum convenience, portability and versatility. This type of magnet is ideal for remote locations. Built-in chargers and a visible power gauge provide ease of operation. However, this type of lifting magnet needs a consistent charging schedule to make sure the batteries are charged and ready for use.
Permanent / electro-magnet combination type lifting magnets provide the best of both worlds. They provide safety of a permanent magnet for the entire lift cycle and only uses a momentary pulse of electricity to redirect the magnetic field inward to release the load. The design of these magnets use virtually no energy, do not need battery back-up, and can be controlled remotely. These magnets are Ideal for lifting applications which need fast cycle times. However, surface conditions are to be flat and clean.
Ceramic magnets are made of strontium ferrite (SrFe) in a sintering process. Ceramic magnets are staples in the electronics, automotive, medical, mining, industrial, oil industries and more. Ceramic magnets are medium strength magnet material with a high resistance to demagnetization, has long time stability (loses 0.5 % of its magnetic strength in 100 years), and are brittle material which has to be cut with diamond tipped blades.
Rare earth neodymium-iron-boron (NdFeB) magnets are made in sintered as well as bonded forms. These magnets are normally referred to as Neo. This magnet material provides the highest magnetic strength of any magnet material, very high resistance to demagnetization and is ideal for applications needing maximum strength in a limited area. Because of its high iron content, Neo is normally coated or plated to prevent oxidization, hence grinding is to be avoided.
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 such as fine metal build-up on or between the poles of the magnet, nicks or gouges in the poles of the magnet, and rust build-up etc., (ii) exposure to extreme temperatures (ceramic lifts lower than -60 deg C and higher than 148 deg C, neodymium-iron-boron rare earth lifts lower than -22 deg C and higher than 82 deg C, and electro-magnet and battery lifts higher than 60 deg C), (iii) severe blow or shock to the magnet i.e., a blunt instrument to position the magnet on the load is not to be used, and (iv) exposure to electrical currents i.e., placing of the magnet next to a large motor or generator and use of the magnet as part of a welding ground circuit are not to be done.
Factors which affect lifting capacity of a magnet are (i) load thickness, (ii) load material, (iii) load surface condition, (iv) load length or width, (v) attitude of the load, (v) temperature of the load, and (vi) portion of lifting magnet in contact with load.
Load thickness – The magnetic flux flowing from a lifting magnet into a load increase as the thickness of the load increases. Hence, as a lifting capacity of the lifting magnet is a function of this flux, the lift capacity increases with load thickness. For every lifting magnet, there is a critical load thickness where all of the available flux of the lifting magnet flows into the load and the lift capacity reaches maximum.
Load material – Several alloys of iron do not accept magnetic flux as easily as do low carbon steels. Hence, loads of such alloys do not accept all of the flux available in the lifting magnet, which reduces the lifting lift capacity of the lifting magnet.
Load surface conditions – Anything which creates an air gap or non-magnetic separation between a lifting magnet and the load, reduces the flux flowing from the lifting magnet into the load, which reduces the lifting capacity of the lifting magnet. A rough surface finish, paper, dirt, rust, paint, and scale produces such gaps.
Load length or width – As the length or width of the load increases, the load begins to deflect and to peel at the lifting magnet face. This can create an air gap between the load and the lifting magnet, which reduces the lifting capacity.
Attitude of the load – As the attitude of the surface of the load to which a lifting magnet is attached (lifting surface) changes from horizontal to vertical, the lifting capacity of the lifting magnet reaches a minimum and becomes dependent upon the coefficient of friction of the lifting surface.
Load temperature – The temperature of the load can cause damage to the lifting magnet and, if high enough, even change the magnetic characteristics of the load. The standard magnet is designed to operate in an environment with a temperature range of -25 deg C to 50 deg C. The load temperature can be between -40 deg C and 80 deg C.
Portion of lifting magnet face in contact with load – The full face of the lifting magnet is required to contact the load if the lifting magnet is to achieve maximum capabilities.
The factors which dominate the lifting magnet selection for any specific application are (i) weight, shape, and contact area of the objects to be lifted, (ii) surface conditions of load and magnet, (iii) stiffness of 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 load material, and (viii) ambient temperature. Consideration of each of these factors lead to identification of the magnet type, shape, number, and face contour best suited for economy, efficiency, and safety. Lifting magnets fall into two normal shape classifications namely (i) round magnet, and (ii) rectangular magnet.
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 lifting electro-magnet is most efficient when considering its face area as related to lift power. Since the outer pole is at 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 a permanent magnet or an electro-manet. Pole plate and core material are hot rolled steel or low carbon steel. For the plates of rectangular lift magnets hot rolled steel plate is welded and joined into the needed box shape. Using these stock materials, there is no limitation to lift magnet geometry, but round and rectangular lift magnets normally prove adaptable to almost any lifting application.
Weight, shape, and contact area are to be considered at the very beginning in lifting magnet selection. The contact area of the load controls the needed magnet number and size 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 that ‘penetrates’ the load thickness, so that holding power is sufficient for the load involved. But if the same load offers a much larger contact area, then 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. Fig 1 shows load lifting magnet construction.
Fig 1 Load lifting magnet construction
The number of poles a magnet 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 3 or more poles. The design of the magnet circuit determines the depth of the magnet field produced.
Normally, 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 higher gaps than those with three poles and, and hence, they are normally better suited for thicker loads. Conversely, three-pole magnets are normally the logical choice for thinner loads.
Fig 2 shows magnets for 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 is used 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. A magnet for handling the billet is to penetrate the billet material to get adequate holding or lift power. But this same magnet is inefficient for lifting the plate since the shape of its penetrating field results in considerable fringing outside the relatively thin plate. For lifting the plate an arrangement of multiple smaller lifting magnets is more efficient.
Fig 2 Magnets for a billet and a flat plate
Anything which prevents the face of the magnet from making full contact with the part being lifted is considered an ‘air gap’. Rust, dirt, dust, machine grooves, and holes are just a few examples of an ‘air gap’. It is desired that the magnet makes full contact with a ‘clean’ part before lifting.
Maximum lift force achieved by a magnet is when the direction of force is perpendicular (90-degree) to the surface of the load material. If a load is tipped at an angle, shear forces, slide forces, friction, peeling forces associated with movement or impact forces from bumping the load as it is conveyed, can cause the lift to fail.
The full lifting ability of the magnet is put to work only when there is no space between the mating surfaces of the magnet and load. The famous ‘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 1/4 unit of the pulling power at 2 units of distance, and 1/9 unit of the pulling power at 3 units of distance and so on.
Fig 3 Effect of distance on the pulling power of a magnet
For holding or lifting magnet applications, the concern is with distances between magnet and the load measured is to be at the distance of only a few millimetres. These distances are very small in comparison to the dimensions of the magnetic poles and loads themselves. Also, another frequent concern is with the magnetic field shapes which are more complex than those created by a single magnetic pole. Although the inverse square law continues to apply in these cases, the ‘real world’ effects can reduce its apparent significance. In case of real-world situation, the pulling power of a given 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 lifting power when surface separations as small 2.5 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.
Because of the above, it is necessary 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 to be derated as per the degree of any unevenness or separation from full contact with the load to be lifted.
If the load is balanced, and is stiff enough so that there is insignificant sag (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 peeling effects. Also, even if sag is not a factor, and if the magnets cannot be placed on centre, the strength of the magnets is 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 sag of the load overhang causes a force which is not perpendicular to the magnet face. Fig 4a 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 because 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 certain limits. Outside these limits the magnets are to be de-rated appropriately.
Fig 4 Handling of loads with magnets
In cases where there can be large quantities of load sag, two other factors are be considered by the system designer and operators. First case is the possibility which the load can assume an overall ‘arc’ shape which does not match the linear arrangement of magnets on the lifting beam, i.e., the lifted load is effectively lifting any central magnet(s) at the expense of increased forces on the end magnets. In the second case, there is the possibility of load ‘bounce’ during transition portions of the lift where the effective peeling load can be multiplied by large factors because of the transient acceleration of the unsupported portions.
Both of the above effects are to be minimized by careful lifting beam design, where the lifting loads at each magnet position are calculated by taking into account 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 sag.
For practical reasons, majority of the lift-beam 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-wagons or trucks where the load can be plates of varying sizes at one time and bars or beams or other structural shapes at another. The assembly is to be designed with the proper selection and arrangement of magnets so that it is efficient on the full-range of items handled.
For example, if a magnet and lift-beam assembly is used to unload plates which vary in size from 3,600 mm x 2,400 mm down to 1,200 mm x 600 mm, the magnet layout to handle the smaller plates does not handle 3,600 mm x 2,400 mm plates efficiently. Even if there is adequate lift power in the layout for the larger plates, overhang is excessive. Hence, it is necessary to arrive at a magnet layout dictated by the larger plates. This layout is then results in magnets overhanging the smaller loads as shown in the Fig 4b.
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 it is needed to lift only one beam or bar from a storage rack where the beams or bars are of varying lengths and are lying end to end. The Fig 4c shows the way a short piece can be lifted from a storage rack in this situation.
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 desirable to select and space the magnets for an efficient lift on the larger load. But when this difference, between the largest as seldom-encountered load, and the smaller as normally encountered load, is vastly different, then it is desired to examine the costs involved to see if the system is truly economical. Such an analysis can lead for a compromise for the number of magnets to be used.
For example, if a plate of size 1,200 mm x 2,400 mm x 25 mm) is to be handled almost all the time with a lift magnet assembly, and occasionally, much heavier, larger, and thicker plates are to be encountered. Rather than sizing the magnetic system for the heavier plates, then it is to be considered sizing the magnet lift assembly to the common light weight plates and to plan to use other approaches on the rare heavier and larger plates. The alternatives can be (i) to use a sling and hook arrangement to lift the heavier plates by welding temporary lift eyes on the heavier plates, or (ii) to perform any burning or cutting which is normally be done on the larger plates before they are unloaded.
Frequently, a very large load variation as described above has 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 can make the system completely automatic. The point is that a study of all factors of 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.
‘Break-away force’ is the force needed to separate the load from the magnet when pulled in a direction perpendicular to the face of the magnet. The break-away force of a lifting magnet is proportional to the thickness of the material being lifted. The break-away force of a magnet increases until the material being lifted exceeds the saturation thickness. Hence, thinner materials do not yield a high break-away force while thicker materials do not yield a higher break-away force because of saturation.
Whenever the load surface is not uniform over the magnet face, flat lifting magnets can be used but are to 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 of these 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 a person to estimate magnet holding power. For example, corrugated sheet is to be handled as shown in Fig 5a. 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. Fig 5a shows lifting of corrugated sheet and protection against failure.
Fig 5 Lifting of corrugated sheet and protection against failure
The situation is more complex when considering loads which offer little flat contact surface to the magnet. There are several 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, and / or actual comparative pull tests, using physical samples, can be needed. The important point is that a partial contact load needs appropriate de-rating of the magnet for reasons of safety and efficient operation as well as proper design of the magnets.
If a load is lifted with one or more electro-magnets, 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 between pile and railway wagon, (ii) damage to the load, if finished items or loads subject to bending or breaking are being handled, (iii) damage to structures below, and (iv) disastrous, if the lift and transfer is made in an area where personnel might inadvertently be located.
When protection against failure of the lifting capability of the magnet is needed, then the use of magnets with special safety features is to be considered. The protection against failures in different types of magnets are (i) permanent magnets with electro-magnetic load cancelling, (ii) permanent turn-off magnets with manually activated turn-off cycle, and (iii) electro-magnets with auxiliary, automatic cut-in power supply. These are shown in Fig 5b, Fig 5c, and Fig 5d.
Permanent magnets are now available which use electricity only to negate the field and which needs deliberate action by the operator to release a load. The lifting power is generated by the permanent magnet component. When the electro-magnetic 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 negate the lifting field. The turn-off cycle is manually activated, thereby making the holding and releasing function completely independent of an electrical power source. The magnetic holding power is never lost with this type of switchable permanent magnet.
Battery back-up systems are available 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 up to 20 minutes allowing the operator to manually lower the load or clear the area.
Electro-magnets 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 time (normally 15 minutes to 20 minutes) so that temporary slings can be rigged to secure the load and other precautionary measures are taken.
In short, special safety can be achieved by three different ways, and the one selected depends on the load involved.
Design factor – It is the quantity of lifting value a magnet is labelled against the lifting value under ideal conditions. Ideal conditions are when a magnet is new and pulled off a newly machined, thick, low carbon steel plate. The kilograms (kg) of pull it takes to break the magnet away from the steel surface is the ‘maximum’ lifting value. Design factor (de-rating) values are then determined by taking this maximum lifting value and dividing it by the manufacturers design factor. Design factors are minimum 2:1 and in the majority of cases 3:1.
This means a magnet with a 3:1 design factor and labelled to lift 1,000 kgs has a break-a-way force of 3,000+ kgs. The labelled lifting value is stated for the benefit and safety of the user, because of the fact that ideal conditions rarely exist in the field. The steel which is being lifted can have scale, rust, dirt, or coatings on its surface, or the surface of the magnet itself can be worn. Any of these conditions causes lower lifting values.
Safety factor – It is to be noted that the published lifting magnet capacities represent ultimate lift strength i.e., the lifting capacity of the magnet on different loads under ideal conditions unless clearly specified otherwise. It is almost impossible to foresee all 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 is normally specified by applying a safety factor to the ultimate lift strength of the magnet so as to ensure that the safety factor applied represents actual conditions as far as practical.
Some of the operating conditions which decide the applicable safety factor are (i) surface condition of the load, (ii) surface condition of the magnet, (iii) smoothness of lift, (iv) flatness and stiffness of load, (v) centering of load on magnet, (vi) environment, (vii) voltage fluctuations (for electro-magnets), and (viii) unknown factors. When any of these conditions are not ideal, a safety factor which accounts for corresponding adverse effects on the ultimate lift strength of the magnets is to be applied.
The holding force for a magnet is affected by the composition of the material being lifted. Ferrous material with higher iron content are typically more susceptible to magnetic fields than those with lower iron content. It is necessary for the operator to know the material being lifted.
Safety factor can be arrived at by assigning a value to each adverse condition and adding the results. The mathematical equation is ‘Safety factor = A + B + C + D + E’, where A is the load itself (say 2 to 4), B is the surface condition of the load which is not perfect (say 1), C is the possible non-centered load (say 2), D is the undesirable conditions of environment (say 1), and E is all other unknown factors (say 3). Then in the case which is considered in brackets, the safety factor is (2 to 4) + 1 + 2 + 1 + 3 or equal to 9 to 11. In this case the magnets which theoretically lift 9 to 11 times the maximum load involved are to be selected. These magnets have the capacity to account for actual conditions expected.
The above 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 to be the responsibility of the engineer who s specifying for the lifting magnet application.
If a magnet is applied to a vertical load surface, the load tends to slide or shear from the pole face. In this case, the ultimate lift strength is typically reduced to 1/4 of its normal value. Safety factors as outlined above are then be applied to the reduced shear rating.
Number of lift magnets for maximum efficiency – The most economical number of lift magnets for any particular application is one which consists of a single magnet with a capacity rated for the load. The cost 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 going to have prohibitive cost, and, because of its size, produces a field too deep for anything but very thick plates.
For the lift of large plates and sheets, then, more than one magnet is needed. From the wide range of magnet sizes, shapes, and capacities available, it is necessary for selection to normally consider multiple variations so as to select the best size and least 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 size of the crane needed. Hence, when the size and hence weight of magnets and attachments are minimized. It results in the savings in the crane and the 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 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 cannot be perfectly aligned and a stiff load, which do not deflect to match the magnet faces, is handled. Fig 6a 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 Fig 6a, 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 load.
Fig 6 Lifting of different loads with magnets
Lifting ferrous items using a magnet needs a good look at the length, width, and thickness of the item. Thin materials do not absorb as many of the magnetic flux lines (magnetic energy) as thicker materials. Thin materials also flex, causing the material to peel-off the magnet. Equally important is the physical size, flatness, surface conditions, and type of steel.
Material as thin as 5 mm can be lifted from a stack with an electro-magnet 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’.
On the contrary, 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 counter-act this effect.
In short, it is necessary (i) to use the least magnets possible for any particular lift application, (ii) to engineer the magnet support arrangement carefully to maintain alignment of magnets and load, (ii) to avoid arrangements which need the load to ‘push’ the magnets into position, and to consult with the magnet manufacturer when setting up a beam assembly to lift flexible loads.
Lfting loads with undefined shapes – Normally, lift magnets are designed to handle loads which can be classified into plate, bars, structural shapes, and round stock. These configurations all 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 be handled as long as it is realized that since these shapes do not match the magnet lifting surface the lift magnets need derating. Some of these shapes are shown in Fig 6b, Fig 6c, and Fig 6d, 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.
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 inter-changed on a magnet at different times for matching different loads. Normally such poles do not reduce the holding power of a magnet from that produced if its integral poles has 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 or holding power gets diminished. For avoiding this, the contact surfaces of both 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 the integral poles.
Since the holding and lifting efficiency of a magnet are based on maximum contact between magnet and load, the shape of the auxiliary pole faces is to match the load as closely as possible. However, the pole shape can be ‘compromised’ or averaged over the range of 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 it is if each round shape has exactly matching poles. Fig 7 shows these ‘average’ auxiliary poles. The auxiliary poles most closely match the configuration of the 200 mm bar since this bar is having the highest specific weight. Less contact is needed on the lighter smaller bars.
Fig 7 Magnets with shaped auxiliary poles
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 rather than on the main magnet face. After the machining is completed, these poles can be permanently attached to the magnet. Use of 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 magnetic coil further from the source of heat and increase space for passage of circulating air.
Drop controllers – Electro-magnets 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.
There are some typical application examples of auxiliary pole as described here. 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 to be used. Since the majority of the 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 needed spacing and the needed magnet face contact on the angles and bars to make the lift.
One typical application example is the lifting a circular coil. Coils of steel strip are to be lifted and moved using electro lift magnets. The coils vary in inner diameter (ID) and outer diameter (OD) and all have three banding straps as shown in the Fig 8. Rectangular lift magnets with a length dictated by the smallest ID and largest OD of the coil range are 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 straps, its lift capacity is greatly reduced since the magnet face do not have full contact on the load. Fig 8 shows typical application examples of auxiliary poles.
Fig 8 Typical application examples of auxiliary poles
Another typical application example is the lifting plate of different sizes. Plates from 300 mm to 1,200 mm wide and from 1,200 mm to 2,400 mm long are to be handled. The plates arrive at the unstacking location in any of the arrangements shown in Fig 9 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 a large number of small magnets, rather than a few large magnets, is the best selection. The magnets are electro-magnets or permanent turn-off magnets arranged in banks so only the magnets directly in contact with the load to be lifted are energized. The layout of the magnets is as shown in the Fig 9, and the electrical controls are designed such that all or any combination of magnets can be energized in order to selectively lift only one small plate at a time.
Fig 9 Magnets for lifting of plated of different sizes
General safety guidelines – The lifting magnet face and the lifting magnet contact area on the load are to be clean. The operator is required to avoid carrying the load over people. No person is to be allowed to stand on top of lifted work-piece. In no case, load or magnet is to come into contact with any obstruction. No hooking of two lifting magnets is allowed without the use of a properly designed spreader beam. Care is to be taken to make certain that the load is correctly distributed for the lifting magnet being used. Placing of the magnet in shear and uneven lifts are to be avoided.
Under no circumstances the ferrous materials which weigh more than the stated lift magnet value are to be lifted. Maximum lift force achieved by a magnet is when the direction of force is perpendicular (90-degree) to the metal surface. If a load is tipped at an angle, shear forces, slide forces, friction, peeling forces associated with movement or impact forces from bumping the load as it is conveyed can cause the lift to fail. Fig 10 gives safety guidelines for lifting magnets.
Fig 10 Safety guidelines for lifting magnets
Contact with work-piece – Lifting magnets, regardless of type, have two things in common namely (i) pole plates are to be in full contact with the work-piece to develop all the magnetic lifting potential for the piece being handled, and (ii) there is a given thickness of work-piece upon which any magnet develops its maximum lifting capability. When it is used on thinner material, the lifting capacity is reduced.
Multiple-magnet beams are to be used in a manner which permits all magnets to contact the work. If the end magnet on a beam for long narrow loads is not in contact, the leverage created by the overhanging weight can overload the next magnet and cause the load to progressively strip away from every magnet on the beam. The same action can take place when handling plates. For thinner plates and sheets, the material is more likely to sag from its own weight and cause a peeling action. When deciding a magnet distribution plan for a multiple system, the above factors are to be considered.
Multiple thickness sheet handling – Several lifting magnets have a magnetic field which reach through one or more thin sheets or plates. If more than one sheet is to be handled in a single lift, condition of the material is extremely important. As an example, a given magnet can lift five perfectly flat, clean sheets, whereas it can lift only two of the same sheets if they are warped, bent, covered in scale, etc.
It is good practice to lift a load, stop just a short distance above the pile and use the magnet controller to release the bottom sheet. Safe-hold magnets cannot be controlled in this manner and hence, are not desired for multiple sheet handling.
Suspension or mounting – For normal lifting applications, some flexibility in the mounting is desirable. This is particularly beneficial when more than one magnet is on a beam or fixture since, if the work-piece is not level, the magnets can align themselves to the work-piece as they are moved into position. The degree of flexibility is to be controlled so that no damage occurs to the magnet, beam, or wiring if the load suddenly breaks away from the magnet. The suspension design is to consider and prevent situations in which one or more magnets can actually be lifted by the remaining magnets because of the flexing of the beam or the load.
There are several ways of achieving flexibility and the magnet manufacturer is to be consulted on a method best suited for the specific application. If the installation is of the multiple-magnet type, it is important for safety that the pole faces of all magnets be in firm contact with the load. Normally this needs that the pole faces lie in exactly the same plane when unloaded.
Always wire of adequate size to conduct electric power to the magnet are to be used. Undersized wiring creates a potential fire hazard because of excess heating and ultimate failure of the insulation. It is to be ensured that the power connected to the magnet matches all the electrical characteristics on the name-plate. Otherwise, components can be damaged and failures occurs. Magnets are required to have provision for grounding through the power cord or in the junction box.
Electrical short circuits are always a possibility wherever electricity is used, but the damaging consequences of a short are going to be much less if proper grounding is provided. Wire size, grounding techniques, and circuit protection can be determined by a competent electrical person. No chances are to be taken with improperly wired systems. The two DC leads from the DC power source are to be connected to the terminals in the magnet outlet box.
Either wire from the DC power source can be connected to either terminal post in the outlet box, unless indicated on the magnet. DC leads from the DC power source are not to be broken with a switch or fuses. The energy from the magnet is required to have a decay path either through power source, or a free-wheeling diode connected across the coil in a blocking direction.
Magnet maintenance and care – A lifting magnet, like any other tool, needs to be kept in good working order. Maintenance of the lifting magnet can only assist in a safe lift condition. Magnet maintenance and care include the following.
Occasionally checking of the mechanical operation of the magnet release handle, spring, grip, as well as the lift lug for damage or fatigue.
Keeping the surface (magnetic face) of the lift free of chips, slag, weld beads, dirt, rust, etc. This can be done by wiping the surface of the magnet off frequently with a wire brush, shop rag, or gloved hand.
Applying of oil or grease to magnetic face pole surfaces if magnet is to be stored for long periods of time.
After a period of time the magnet face poles can become somewhat rounded, nicked, or gouged, reducing the effectiveness of the magnet. Magnetic pole faces can be machined or ground to bring the magnet back to a consistent flat surface. Calibration tests can determine the magnetic strength of the lifting magnet.
Magnets are not to be subjected to weld on, hammer, throw, or drop of the magnet.
Operators are not to strike, slam, ram or forcefully impact the magnet against other objects.
Lifting magnets are normally designed to be used in dry applications. They are not to be used under water without consulting the manufacturer.
Magnets are to be always stored in a non-conductive, dry environment.
An annual calibration test is desired for ensuring that the lifting magnet is performing to its optimal level. Calibration of a lifting magnet is a test, performed by an approved testing facility which determines the lift capacity of the magnet, at the time of the test. Under an ‘ideal condition’ environment, a series of break-away tests determines the ‘de-rated’ holding value of the magnet. This holding value is to meet or exceed the value stated on the lifting magnet. If the stated holding value is met, the magnet can be returned to use and scheduled for another calibration test in one year. The outcome of the test allows the operator of the lifting magnet to know that the magnet meets the lift standards as designed by the manufacturer.
If the stated holding value is not met, the lifting magnet can possibly be machined to bring all magnet face poles back to a smooth, level condition. If that does not bring the magnet back to the manufactured lifting value, the magnet is to be removed from operation and replaced with another magnet. A certification of calibration, given at the conclusion of the testing, gives the operator documentation of the performance of the magnet.
The magnet is typically attached to an overhead lifting device properly rated to handle the maximum magnet capacity. A properly designed or selected clevis (a U-shape connector), shackle, chain hook or strap is attached through the lifting eye.
The rated load (capacity) of the lifting magnet is clearly marked on the lifting magnet or on a tagattached to it where it is visible. This rating is referred for information relating to decreases in rating because of the load thickness, load alloy, load surface conditions, load length or width, attitude of load, portion of lifting magnet face in contact with load and / or load temperature. The marked capacity is based on a specific load for which the rating applies.
Inspection of magnets – The new and reinstalled lifting magnets is to be inspected by a designated person prior to initial use to verify compliance with applicable provisions. Altered, repaired, or modified lifting magnets are to be inspected by a designated person. The inspection can be limited to the provisions affected by the alteration, repair or modification, as determined by the qualified person.
Inspection procedures for lifting magnets in regular service are divided into three general classifications, based upon the intervals at which the inspections are to be performed. The intervals, in turn, are dependent upon the nature of the critical components of the lifting magnet and the degree of their exposure to wear, deterioration, or malfunction. The three normal classifications are designated as every lift, frequent, and periodic, with respective intervals between the inspections as defined below.
In case of every lift inspection, visual examination is done by the operator before and during each lift is made by the lifting magnet. In case of frequent Inspection, visual examination by the operator or other designated persons is done. In these two cases records are not needed. The interval for frequent interval in case of normal service is monthly, for heavy service it ranges from weekly to monthly, and for severe service it is daily to weekly. In case of periodic inspection, visual inspection is done by a qualified person and records of apparent external conditions are made to provide the basis for a continuing evaluation. The interval for periodic inspection in case of normal service is yearly, for heavy service it is quarterly, unless external conditions indicate that disassembly to be done to permit detailed inspection, and for severe service it is monthly, unless external conditions indicate that disassembly is to be done to permit detailed inspection.
In case of special or infrequent service, inspection is to be done as recommended by a qualified person before the first occurrence and as directed by the qualified person for any subsequent occurrences.
In case of every lift inspection, items such as (i) lifting magnet face and surface of the load for foreign materials and smoothness, (ii) condition and operation of the control handle of a manually controlled permanent magnet, and (iii) condition and operation of indicators and meters when installed, are to be inspected by the operator before and / or during every lift for any indication of damage as specifically indicated, including observations during operation for any damage which can occur during the lift.
In case of frequent Inspection Items, items to be inspected include (i) structural and suspension members for deformation, cracks or excessive wear on any part of the lifting magnet, (ii) lifting magnet face for foreign materials and smoothness, (iii) condition of lifting tackle or sling suspension, (iv) condition and operation of control handle, (v) condition and operation of indicators and meters, where applicable, (vi) cracked housings, welds, and loose bolts, (vii) labels and markings, (viii) all electrical conductors which are visible for loose connections, continuity, corrosion and damage to insulation, and (ix) for battery operated electro-magnets, inspection of proper level of battery electrolytes and of corrosion of posts and connectors. In this inspection, items are to be inspected for damage at intervals as defined above, including observations during operation for any indications of damage which can appear between inspections. A qualified person is to determine whether any indications of damage constitute a hazard or needs more frequent inspection.
In case of periodic inspection, complete inspection of lifting magnets is to be done and recorded at intervals as defined above. Any deficiencies, such as (i) all members, fasteners, locks, switches, warning labels, and lifting parts are to be inspected for deformation, wear, and corrosion, (ii) all electrical components including meters, indicators, or alarms are to be tested for proper operation and condition, and (iii) the lifting magnet coil is to be tested for ohmic and ground readings and compared to the manufacturer’s standards. The inspection is to be done by a qualified person and determination made as to whether they constitute a hazard.
In case of lifting magnets not in regular use (the lifting magnet which has been idle for a period of one month or more) the inspection is to be done in accordance with the above before being placed into service.
Inspection reports with date are to be made on critical items, such as those listed in periodic inspections. Records are to be available to appointed personnel for each periodic inspection and when the lifting magnet is either modified or repaired.
Any indications of damage disclosed by the inspection requirements are to be corrected according to the following procedures before operation of the lifting magnet is resumed.
Operational testing – New and reinstalled lifting magnets are to be tested by a qualified person, or a designated person under the direction of a qualified person, prior to initial use to verify compliance with applicable provisions, including, but not limited to (i) moving parts, (ii) latches, (iii) stops, (iv) switches, (v) control devices, and (vi) alarms.
Altered, repaired, or modified lifting magnets are to be tested by, or under the direction of, a qualified person. The test can be limited to the provisions affected by the alteration, repair or modification as determined by a qualified person with guidance from the manufacturer. All indicator lights, gauges, horns, bells, alarms, pointers and other warning devices are to be tested. Reports with date of all operational tests are to be made available.
Load test – Prior to initial use, all new, modified or repaired lifting magnets are to be tested by a qualified person and a record to be furnished confirming the load rating of the lifting magnet. Lifting magnets with normal application are required to satisfy the rated break-away force test. The rated load of the lifting magnet is to be less than 50 % of the rated break-away force measured in this test.
Special application lifting magnets are required to comply with the application break-away force test. The specified application load is to be less than 50 % of the application break-away force measured in this test.
Safe-hold magnets have a safety latch which locks when the magnet is turned on. This is for the benefit of the operator and is not to be removed. If damaged, it is to be replaced. The break-away force test is to establish the force needed to vertically remove the lifting magnet from a low carbon, rolled steel plate of the minimum thickness stated by the lifting magnet manufacturer. The portion of this plate which is in contact with the lifting magnet is not to exceed 0.0032 mm and be flat within 0.05 mm per metre, without exceeding 0.125 mm total. The full operating face of the lifting magnet is to be in contact with the steel plate, which is to be between 15 deg C and 50 deg C.
The application break-away force test is to establish the application break-away forces of the lifting magnet under the variety of loading conditions for which the lifting magnet is specified. The details of this test are to be supplied by the manufacturer.
Altered, repaired, or modified lifting magnets are to be tested by, or under the direction of, a qualified person. This test is to be limited to the provisions affected by the alteration, repair, or modification, as determined by a qualified person with guidance from the manufacturer.