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7.4
LOAD THICKNESS
The magnetic flux flows from one pole of the magnet to the other through the load being lifted. If the
thickness of the load is smaller than the width of the poles, the magnetic flux is restricted. Thus, the flux
density at the contact area between the poles and the load is reduced which results in reduced holding
force. Steel thickness of at least 20mm (SB200), 30mm (SB500) or 40mm (SB950) is required to absorb
the entire flux and achieve the maximum holding force.
7.5
LOAD TEMPERATURE
The magnetic holding force also varies with temperature of the load to be lifted. Generally, the holding
force decreases as the material temperature rises. With loads, whose temperature exceeds 80°C, the
magnetic force steadily diminishes. Therefore, any load whose temperature has exceeded 80°C should
not be hoisted with the lifting magnet.
(Notice: Contact with hot loads may cause failure of the lifting magnet. With longer contact, the magnets
are damaged by the penetrating heat.)
7.6
AIR GAP
The air gap is the average distance between the poles of the lifting magnet and load surface. Air gaps are
caused by foreign bodies or improper contact between the magnet poles and the load.
The magnetic field cannot pass so easily through non-magnetic materials (air, dust, non-ferrous materials
such as stainless steel, brass, aluminium, wood, foreign matter, concavities/convexities etc.) so the
holding force is reduced. Thus, the magnets output the full power only when their poles are directly in
contact with the surface of the load. The force - air gap curve (see graph below) shows how the holding
force (F
h
) of the lifting magnet diminishes as the air gap (mm) increases.
In order to avoid an air gap, remove the foreign matter from the load surface before positioning the lifting
magnet.