One of the most spectacular applications of the principle of electro-magnetism is the use for industrial purposes of giant electric magnets capable of lifting loads of up to 25 tons in weight
LIFTING A RAIL-CROSSING ASSEMBLY with a large circular electro-magnet suspended from a crane. The cable which supplies current to the magnet is visible between the magnet and the jib of the crane. In the cabin of the crane is a switchboard from which the operator can control the current passing through the coils of the magnet.
ITis a far cry from the ancient discovery of the attracting powers of natural magnets to the powerful electric lifting magnets so commonly and extensively used to-day in the iron and steel industries all over the world.
Thousands of years ago, probably, amber was known, not only as an ornament in the form of beads and necklaces, but also for its peculiar quality of attracting to itself tiny particles of certain materials. Amber, after having been rubbed on a soft material such as wool or cloth, will attract and lift not only small bits of paper, but also a much greater variety of materials than will an ordinary steel magnet. Amber will lift tinfoil, iron filings, aluminium filings and even wood.
There is something romantic about the silent unseen forces of Nature. Nearly all the products of man’s inventive genius are audible as well as visible in their operation, but the powerful lifting electro-magnet does its work by silent and invisible power, with the entire absence of any moving parts.
The phenomenon of metallic magnetism was observed by the ancient Greeks and Romans in the loadstone, or magnet as it came to be called, having been associated with Magnesia, in Asia Minor. It was noticed that magnetic iron ore stone had the power of attracting to itself small pieces of iron and that, if it was suspended freely, it would point to the magnetic North and South Poles of the Earth. The lifting power of such natural magnets was, however, small.
In 1820 the discovery, associated with the names of H. C. Oersted and D. F. J. Arago, was made that a wire carrying an electric current had the power of attracting and holding pieces of iron. In 1825 a Lancashire shoemaker named Sturgeon, with a flair for experimenting in electricity, produced the first electro-magnet. This consisted of a bent bar of iron with a diameter of about ½ in. and a length of 12 in., round which a coil of copper wire was wound. When Sturgeon passed a small electric current through the wire he found that the two ends of the bar would attract and lift pieces of iron. Though the whole appliance weighed only about half a pound, it was capable of lifting 9 lb. of iron, or nearly twenty times its own weight. This was considered of such scientific importance as to warrant an account of Sturgeon’s experiments being read before the Royal Society of Arts in 1825.
POWERFUL ELECTRIC LIFTING MAGNET at work in a scrap yard, moving empty shells and other pieces of scrap metal. The magnet, which is of the circular type, is attached to a 5-tons overhead travelling crane. Lifting magnets of circular shape are most suitable for such loads as pig iron, solid ingots and scrap metal of varying sizes. Direct, or continuous current is used for electric lifting magnets, and they are generally built for a 110-volts or 200-volts supply.
Six years later Joseph Henry, in the United States, found that by increasing the number of turns of wire round the soft iron bar, or core, greater lifting power was obtained. He made quite large electro-magnets which were capable of lifting up to a ton.
In general principle the electromagnet has not changed. In form it comprises a massive circular, or mush-room-headed body, or a rectangular one, made of high-permeability cast steel. Inside are placed the coils of insulated copper wire for the smaller sizes; in the larger ones strap- or ribbon-shaped copper or aluminium conductors are used. The electric supply, which must be direct, not alternating current, is provided through connecting cables from the most convenient source on the spot. The reason for using direct current is that if alternating current were used, the whole of the magnet would have to be built up of laminations. which would make the price exorbitant. It is cheaper to use a direct current magnet and a motor generator set.
The bottom face of the magnet forms, or carries the pole shoes which make the contact surface with the object to be lifted. The rough usage which these magnets suffer requires the most robust construction possible. The nuts and heads of all bolts used in the body must be protected either by being sunk into corresponding pockets in the casting or by adjacent protecting ribs.
British construction seems to favour making the centre and outer pole pieces separate from, and securely bolted to the main shell. The joints in such instances are accurately machined so as to make close metallic contact — to avoid magnetic leakage. One advantage of this method is that the coils can more easily be placed inside and removed when necessary. It also has the further advantage that the pole shoes can be easily replaced at relatively less cost when they are worn out by the severe blows they get, than if the whole were made in one casting.
FLAT PLATES LIFTED BY A GIANT ELECTRO-MAGNET. Magnets will lift hundreds of small metal parts, although they will not lift such a heavy weight in these conditions as when they are dealing with a solid load. This illustration shows how the upper pieces, in the full magnetic field, become magnets themselves and attract the pieces below them.
A German firm, however, makes the whole body in one casting, leaving the central core which contains the coils open at the underside so that these can be inserted or withdrawn. The coils are secured in place by split spring rings that can be closed in slightly when they are put into place and then allowed to spring out into the undercut groove in which they remain. It is a simple and ingenious method of avoiding the use of many bolts or screws; at the same time it secures an absolutely homogeneous mass of metal for the main shell.
On the other hand the coils, which are the most vulnerable part, may not be quite so well protected from the heavy shocks and blows to which the whole appliance is subjected.
Suitable lifting lugs are embodied on the casting itself, and the hole through which the cable passes to the terminal boxes is protected by side ribs and covers. Projecting parts are avoided and the corners are rounded off with sloping sides to prevent the magnet from catching among the mass of disordered pieces of metal which arc often handled. The circular magnets vary in diameter from about 18 in. to over 6 feet, and weigh up to as much as 5 tons.
The coils are the most vulnerable part of the structure. They have to carry heavy electric currents without excessive heating, they must be efficiently insulated against short circuiting and they must be able to endure a great deal of rough usage. The coils may be of either copper or aluminium, wire being used for the smaller sizes and strap, ribbon or square section construction for the larger. When in strap form the coil is wound round similarly to a tightly coiled clock spring. Mica is largely used for insulation because of its high insulating and good heat-conducting qualities. Asbestos ribbon is used as well when magnets are subjected to great heat, as sometimes they are when they have to lift hot steel ingots, or billets.
AN ELECTRIC LIFTING-MAGNET HOIST, which will perform the duties of a small travelling crane. Loads may be conveyed from one part of a factory to another. The operator, who travels with the crane, is in full control of the raising and distribution of the magnet’s load.
The passage and maintenance of a heavy electric current through the coils heats them, and this rise in their temperature is the limiting feature in magnet design. If the rise is excessive the efficiency will be reduced because of increased resistance to the flow of current through the coil, which, so to speak, throttles the amount of current which that particular size of conductor will carry. Reduced current means a weaker magnet and therefore less lifting capacity. This fundamental electrical feature is important The coils being buried inside the great mass of magnet frame, whatever heat is generated must be carried away as quickly as possible or some method must be devised for limiting it.
When the whole magnet has been assembled it is subjected to an impregnation process. A pipe from a melting tank containing an insulating compound is connected up to the interior of the magnet body containing the coils, and another pipe connects it to a vacuum pump. For three hours the magnet is subjected to a vacuum of 20 in. of mercury, during which time the coil is maintained at a temperature of about 200° centigrade by the passage through it of a strong electric current. This heat drives out all moisture and volatiles, which are immediately sucked out by the vacuum pump.
When the magnet has been thoroughly dried out, a valve to the melting tank is opened and a hot insulating compound in the form of thick varnish is drawn into the magnet interior as well as to an external heated receiver, vacuum pump valves are then versed and the magnet is subjected to a pressure of 100 lb. per sq. in. for another three hours. This forces the compound into every interstice surrounding the coils and complete impregnation is effected with the elastic insulating compound, making it impossible for moisture (a deadly enemy to electrical insulation) to enter — even the minute particles of moisture contained in air.
Magnets can in this way be made sufficiently waterproof to be used under water, and they have been operated at depths of about 70 feet, which corresponds to a hydrostatic pressure of some 30 lb. per sq. in. Use has been made of this characteristic to salve all kinds of iron and steel objects from the water. Even when such objects are contained in wooden packing cases it has been possible to lift them, as the magnetic field round the coils is powerful enough to exert its pull on the metal in spite of the intervening non-magnetic wood.
CONTACTOR PANEL for a lifting magnet, with the three-positions master switch on the right. This switch has settings for “lift”, “off” and “drop”; the “off” position cuts off the current, and the “drop” position reverses it.
Direct current electro-magnets are generally built for 110 volts or 220 volts supply. If higher voltages are used in the source of supply a fixed resistance is inserted, but this is not efficient enough to be used continuously. When only alternating current is available this has to be converted into direct current by a motor generator or by a rotary converter. When magnets are used in conjunction with steam-driven cranes it is necessary, if no other electric supply is near at hand, to provide a small steam-driven dynamo on the crane or close by.
The operation of the lifting magnet is simple, but certain precautions are necessary. Suspended by short chains from the hook of the crane, its own weight having to be included in the lifting capacity of the crane, the magnet has the necessary long cables leading to the switchboard controls in the crane cabin. To obviate tangling of the cable, sometimes a spring-loaded cable drum is provided which automatically winds in the cable as it slackens during the lifting operation and pays it out when lowering. For fairly heavy currents electrically operated “contactor” switches are used, these being under the control of a small master switch which has to carry only a relatively small current.
The master switch has three positions — “lift”, “off” and “drop”. The magnet is energized by moving the master switch lever to the “lift” position. The load may be dropped instantaneously or with a slight delay of a second or two, because of the residual magnetism in the magnet still exerting a small but diminishing pull.
To effect the slightly delayed drop, the switch lever is pushed into the middle of the “off” position, when the circuit is opened, or cut off. If the load is to be dropped instantaneously, the lever is moved to the “drop” position, when not only is the current cut off, but it is also immediately reversed in the magnet coil, thereby overcoming the residual magnetism, and the load drops at once. The lever will remain in the quick release position only while held there by the operator, as a strong spring tends to force it off. When released it does not swing over the central position and energize the magnet, but is checked in the mid, or “off” position by a mechanical stop, from which it may again be moved to the “lift” position, energizing the magnet. A skilled operator can, by controlling the amount of current, drop separate pieces, such as plates, one at a time, from a bundle and distribute them in different places.
Paradoxical as it may sound, it is possible to increase the load handled by reducing the current consumption. The hold-on pull of a magnet varies roughly as the square of the current. Twice the current gives about four times the magnetic power; treble it and about nine times the power is available. Again, the current is greatest when the magnet is coolest. When a heavy current is maintained the coil heats up, “throttling” the current, and diminishes the magnet’s lifting capacity, making it less powerful for the next lift. But this further remarkable feature was discovered — that once the magnet has gripped the load it requires much less current to hold it. Thus there is no need to maintain full current strength when once the load is held. Current can thus be reduced and money saved.
Use of Relays
Conversely, reduced current means lower temperature, and lower temperature means more current available when the maximum demand comes on again for the next lift. So this cycle of facts makes it possible to use what are called “relays”, which, after a predetermined interval of, say, 10 to 15 seconds — to let the magnetism soak in, or become “saturated” — automatically insert a resistance in series with the magnet coils, thereby reducing the current.
HANDLING TINPLATE with an electric lifting magnet suspended from the hook of an overhead travelling crane. Electro-magnets are extremely useful for work of this kind, as no chains or rope slings have to be fixed round the objects to be lifted. The magnet can be controlled by one man, with much saving of time and expense.
Thus the magnet uses less current; a certain amount may be dissipated in the resistance, but that has no effect on heating the magnet coil, which is what is aimed at. The coil has time to cool down between lifts and is fresher for the next effort when it comes along. Tests of this system in one instance showed an average saving of 16 per cent in lifting power required over a given period, and an increase of about 30 per cent in the current available for lifting.
The practical uses of lifting magnets when dealing with iron and steel products are almost innumerable. They will pick up either a large solid casting weighing 25 tons or a mass of hundreds, if not thousands, of small parts such as nails, bolts or scrap bits of metal, all of which they will by invisible force suck up in a higgledy-piggledy mass of all shapes and sizes. Those pieces next the magnets, or in the full magnetic field, pass on the magnetism to the parts below them. Electro-magnets will handle flat plates of great size, round bars or square bars, big steel billets and blocks, pig iron or scrap iron, rails or rolls, a small pipe or a boiler flue. So long as the objects are made of iron, steel or other magnetic metal, a magnet will find them out and grip them with its irresistible power.
From a Needle to an Anchor
Electro-magnets are often used to lift a steel “skull-cracker” ball, weighing perhaps 15 tons or 20 tons, to a height and drop it repeatedly on to a pile of old castings to be broken up for scrap to feed the melting furnaces. To prevent injury to adjacent buildings or people, heavy timber shelters are built round the scrap heap, or the operation may be carried out in a shallow pit. Electro-magnets can be made to work in water or in an acid bath. They can be let down deep drill holes to pull out broken drills, or they can extract all kinds and sizes of “tramp iron” from a stream of grain passing along a band conveyer. They will lift anything of suitable metal from a needle to one of the Queen Mary's anchors. All these operations the electro-magnet will do at a cost of only about a quarter of that required by the older methods. The saving in operating costs is chiefly due to the fact that the one man on the crane can do almost everything. No one is required to fix chains or rope slings round the objects to be lifted.
Circular magnets are most suitable for handling loads such as pig iron, solid ingots and various sized scrap metal. Rectangular magnets, either singly or with two or more slung from a spreader bar, are most suitable for lifting sheet material, long bars, sections, tubes and the like. The more solid the load dealt with the greater is the weight that can be lifted; the more the load is divided up the less is the weight that can be handled. For example, a 64-in. magnet will lift approximately the following weights: a solid ingot, or a piece of heavy armour plate, of up to 25 tons; a “skull-cracker” ball of 10 tons or, if fitted with a special pole shoe, of 20 tons; loose pig iron up to 2 tons; or up to 25 cwt. of such loads as thin sheet iron, scrap stampings, metal turnings and “swarf”, bolts, nails and so forth. Electro-magnets are applied to many other engineering appliances and machines. Magnetic separators in various forms extract fine particles of iron and magnetic oxides from raw materials used in the production of pottery, china and ground materials. Not only are such particles detrimental to the finished product, but also, where these materials are worked up and have to pass through costly machines, any larger pieces might easily cause damage.
Magnetic chucks are extensively used in machine tools for holding down otherwise awkwardly shaped parts to be machined, and magnetic clutches are used for transmitting power through shafting. In many electrical control systems large switches are operated by the pull of electro-magnets or solenoids. Even the ordinary telephone is dependent upon the same principle. There are scores of other applications, all of which are based on the principle which the shoemaker Sturgeon and others discovered over a hundred years ago.
SCRAP-BREAKING WITH A 10-TONS BALL, another application of the principle of the electro-magnet. This work, for the sake of safety, is generally carried out in a pit to protect workmen from flying fragments. A magnet with specially shaped pole-pieces holds the ball until the current is cut off. The magnet is afterwards lowered from the crane to retrieve the ball from the pit.