Lifting magnets10 November 2010
Continuing the new series of buyer’s guides, last month I dealt with vacuum lifters, the first of the methods of attaching to the load by adhesion. This month we look at the options presented by magnetic lifters.
If you need to lift a long thin steel plate, a component profile cut from a piece of thick steel plate or a short length of large diameter bar stock, there are not many options when it comes to lifting accessories. The difficulty of attaching to such items is often exacerbated by their flexibility, variations in shape, the need to avoid surface damage, the material being part of a stack without separators, etc. Lifting magnets can provide a cost effective solution for all these applications and many more.
Generally, lifting magnets are designed and rated to lift loads only in the horizontal plane. Attempting to use them at an angle may result in the load slipping so such applications should not be considered without reference to the manufacturer or supplier.
There is a harmonised European Standard, EN 13155 Cranes—Safety—Non-fixed Load Lifting Attachments, which covers the essential requirements for lifting magnets. There are four main types of magnets, each having particular advantages and disadvantages.
The simplest is the permanent magnet type. Modern permanent magnets can be very powerful and the obvious problem in making practical use of them is how to detach them when required. We have all played with small magnets at some time and are well aware that opposite poles attract and same poles repel. Within the magnetic lifter are two sets of magnets, one of which can be rotated to reverse its polarity thereby controlling the magnetic field. In one position the field is directed into the load thereby effectively switching the magnetic lifter on. In the other the field is directed internally away from the load thereby effectively switching the magnetic lifter off.
The rotation is usually controlled by a manually operated lever although some manufacturers offer the option of electrical or pneumatic powered mechanisms.
Permanent magnets are most suitable for simple regular shaped loads and are frequently employed to handle rectangular or round materials such as may be required to position heavy components for machining. The need to operate the lever means that this type of lifting magnet is only suitable for applications where the operator is not exposed to danger and can easily reach and operate the lever.
The second type is the electro-permanent magnet which is a variation on the mechanically controlled permanent magnet. In this type, the moving magnet is replaced by a coil enclosing magnetic material. When a pulse of electricity is passed through the coil it creates a permanent magnet, the polarity of which can be created either way. Changing the polarity of this magnet has the same effect as the mechanical movement within a permanent magnetic lifter. It has the obvious advantage that the operator can be well away from the magnet and the danger zone near the load. There are no moving parts and the electric current is only required to create the magnet, not sustain it. Therefore, once switched on, the power supply can be disconnected from the magnet if necessary.
A further advantage is that, if the degree of magnetisation is controlled, the magnetic lifter can be partially magnetised or demagnetised. When lifting a load from a stack, it provides the facility to shed excess load before being fully magnetised to secure the remaining load. This type can be designed to use either electrical energy from batteries or the mains power supply.
Permanent magnets of both types need good contact between the magnet face and the load. They have little tolerance of an air gap. Therefore care is required if the load is not perfectly flat or if there is a surface texture or finish which prevents good contact. The magnet face may be shaped to suit particular loads. Many incorporate a V shape in the face to facilitate lifting loads with a round cross-section. The magnetic properties of the material the load is made from also affect the magnet’s lifting capacity. Therefore when lifting a particular type of load for the first time, it is good practice to check that an adequate adhesive force is achieved.
For safe operation, permanent lifting magnets should have a tear off force of at least three times the working load limit when used under the conditions specified by the manufacturer. To verify that a similar level of safety is achieved under the actual operating conditions, the user can make a simple test. Using the performance information provided by the manufacturer, the user can estimate the air gap which is expected to cause a loss of capacity of about 60%. To test, introduce an artificial air gap of the estimated thickness by interposing, for example, thin card between the magnet face and the load. If the magnet can lift the load with that artificial gap then the user can have confidence in the adhesive force when the artificial gap is removed. Clearly this test should only be made under controlled conditions which assume that the load might detach and the load need only be lifted a few millimetres.
The alternative to the permanent magnet types are the electromagnets. Electromagnets do not contain permanent magnets. The magnetic field is generated by passing direct current through a coil enclosing a magnetically soft core. Electromagnets rely upon a continuous electrical supply but offer the potential of much higher lifting capacity. They can also generate a much deeper magnetic field than permanent magnets enabling them, for example, to penetrate through several plates in a stack. The greater depth of field makes them generally more tolerant of a small air gap. Because of that, the minimum tear off force need only be two times the working load limit. They may be divided into two types, those using energy from batteries and those using the mains power supply.
Battery powered electromagnets have the advantage of portability as the battery is built into the unit. However they need to be re-charged at regular intervals. The standard requires that they have an automatic warning device which monitors the power supply and provides a warning at least ten minutes before the supply reaches the level where the load will be released. This is an important safety feature. As well as limiting the time available, their reliance on batteries also limits the power available. Generally they cannot generate as deep a field as a mains powered magnet can.
Mains powered electromagnets offer the ultimate solution if high capacity and continuous operation are required and they can be controlled by the crane operator. They also offer several other potential advantages, particularly in terms of safety features.
From a safety point of view there are three distinct situations. One is where they operate in no-go areas. A typical example is in scrap handling. The random nature of scrap metal means that some is always likely to fall off during the process. The easiest and safest solution is usually to exclude people from the area. Such magnets do not need any extra safety features.
The second situation is the more usual industrial application such as plate handling in a fabrication shop. In this case people are present but, in the event of power failure, they are able to leave the danger area within a very short time. The standard requires that the lifting magnet has a device to provide an automatic warning if the mains power supply fails. It also requires a standby battery capable of providing the current needed to hold the working load for at least ten minutes.
The third situation is where people are present but may be unable to leave the area quickly, for example when working in the hold of a ship. In this case there is a further requirement to have redundancy of critical components such as the flexible cables of the DC supply lines to the magnet. Alternatively the problem can be addressed by having a secondary means of securing the load in the event of power failure. This is usually a mechanical device which can be deployed once the load is clear of its support but before endangering the persons in the area.
Magnets used for lifting loads such as plates, sheets or bars from the top of a stack also need controls to reduce the power supply so as to facilitate the shedding of excess load following which the controls must restore full power to secure the remaining load.
Lifting long flexible loads will require the use of several magnet heads suspended from a lifting beam. Lifting beams and spreaders will be the subject of next month’s article so I will not go into any detail about them here. However, so far as the magnet heads are concerned, the number required and their position along the lifting beam needs to be matched to the load. The load must be shared so that no one magnet head takes more than its capacity. Also the load must be supported so that it doesn’t flex so far as to damage it or to start it peeling from any of the heads. Because an air gap reduces a magnet’s capacity, once the load starts to peel off, it is almost certain to continue. To allow for variations in positioning of the magnets onto the plate there may need to be some redundancy built into the arrangement.
To summarise, if the load has magnetic properties, lifting magnets complying with the standard offer a range of options to suit a variety of loads and working environments. Chosen and specified correctly they are another reliable and safe tool in the armoury of lifting accessories.