Magnet Systems and Safety
Permanent magnets and electromagnets are both switchable, but there is an important difference:
permanent magnets are compact and always function – they retain their magnetic force;
electromagnets switch quickly and without wear, and can also lift heavy loads, yet there is a risk of the load falling if the power is interrupted – therefore electrical lifting magnets must always be equipped with a backup system, which can be expensive.
Monostable magnet systems are only kept activated or deactivated as long as they are supplied with an electrical or pneumatic switching signal. As soon as this signal is lost, the system switches back to its ‘idle state’. An electromagnet is always monostable and always off at idle. Permanent systems can be on or off at idle.
A bistable magnet system is switched on and off by a switching signal and remains in that state until the following switching signal is received. For lifting and hoisting applications a bistable system is safer than one that is monostable, because it actually switches off, so it cannot unexpectedly attract a steel sheet. Bistable magnetic systems are always constructed with permanent magnets (or a combination of permanent magnets and electromagnets).
All MPCO lifting systems are bistable, with the exception of the electropermanent lifting magnet type.
Permanent magnets can be electrically, pneumatically or manually switchable:
This type consists of two magnets, one of which can be moved by means of a handle or twist knob. In the off position the magnets are shorted so they cancel each other out; in the on position the magnets reinforce each other.
The magnet is moved in an enclosure by means of compressed air so that no magnetism is present outside the enclosure in the off position, only in the on position (the movement is only internal).
Electrically switchable permanent lifters
Monostable: the lifting of an object is achieved with a permanent magnet, while the release is done by an electromagnet that neutralizes the permanent magnetic field.
Example: Electrically switched lifting magnet for flat & round (type EHPM)
Bistable: an electrical signal rotates two permanent magnets relative to each other, either producing a magnetic field or aligning them so they cancel each other out.
Examples: Magswitch electrical hand-held lifting magnet
Electrically switched lifting magnet for flat & round (type MS)
Electromagnets can only be switched electrically (with DC voltage).
The specified maximum breakaway force (the force required to pull the magnet off of the product) of lifting and handling magnets applies for a flat sheet of at least a specific thickness. Less force is required if the sheet is thinner or if it has a rougher or dustier surface. In practice, the maximum holding force (breakaway force) is therefore not always feasible.
For the recommended workload, in addition to these factors, you must also include a safety factor in accordance with the EN 13155 standard. Depending on the magnet system, this is:
factor 2 for electromagnets and vacuum;
factor 3 for permanent and electropermanent systems.
Always ensure that no one walks under a lifted load. If this cannot be prevented, there must always be an additional mechanical support (fall protector) around the load, e.g. chains.
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