Magnetic gripper

A permanent magnet holds without energy. It is switched mechanically, usually by a hand lever that diverts the magnetic flux inside. An electromagnet holds only while current flows; it can be switched remotely and its force can be controlled, but it needs protection against power failure, such as a backup battery.

The electro-permanent magnet combines both: a current pulse switches it on or off, and it holds without power. It is the type for grippers that are switched automatically or frequently and where a power failure must not release the load.

The capacity on the nameplate applies to ideal conditions: thick, flat workpiece of unalloyed steel, bright surface, full contact. Every deviation reduces the force – and the influences multiply.

The working load limit of a lifting magnet is only a fraction of its tear-off force. EN 13155 requires lifting magnets to have a tear-off force that is a multiple of the working load limit; for hand-operated permanent lifting magnets three times is common. That reserve is not leeway for the operator; it covers the scatter in material and surface.

The operating instructions therefore contain curves or tables giving the permissible load as a function of thickness, air gap and material. They must be consulted before every new application.

A magnet holds perpendicular to its pole face. If the load hangs on a vertical pole face, only friction carries – a fraction of the tear-off force. Loads are therefore lifted with the pole face horizontal, and long parts with several magnets on a lifting beam so that they do not sag and peel off.

With thin sheets in a stack, the field passes through the top sheet and picks up more. The remedy is magnets with a shallow field depth and fanning magnets that separate the sheets.

Magnetic grippers rarely fail suddenly; they fail because the conditions of use do not match the sizing: painted or scaled parts instead of bright ones, swarf between pole face and workpiece, damaged pole faces. Parts may also retain residual magnetism that attracts swarf – to be considered where machining or assembly follows.