ALFRA TML 250 Mode D'emploi page 15

Basic information related to the handling of magnetic lifting gear, in
particular TML
The magnetic surface is located on the underside of the lifting magnet and has multiple magnetic poles which
generate the magnetic holding force when activated. The maximum holding force that can be achieved depends on
different factors which are explained below:
Material thickness
The magnetic flow of the lifting magnet requires a minimum material thickness to be able to flow completely through
the load. If this material thickness is not given, the maximum holding force is reduced in accordance with the
material thickness. Conventional switchable permanent magnets have a deep penetrating magnetic field similar to
tree tap roots, and require a large material thickness to achieve the maximum holding force. The compact magnetic
field of the TML magnets is similar to a shallow root and achieves maximum holding force even with small material
thicknesses. (See table 2 in this operating manual)
Material
Every material reacts in a different way to penetration of the magnetic field lines. The load-bearing capacity of the
lifting magnets is determined using a low carbon material. Steels with a high carbon content or whose structure has
been changed by heat treatment have a low holding force. Foamed or porous cast components also have a lower
holding force, so that the given load-bearing capacity of the lifting magnet can be downgraded on the basis of the
following table 1.
Table 1
Material
Non-alloyed steel (0.1-0.3% C content)
Non-alloyed steel (0.3-0.5% C content)
Cast steel
Grey castiron
Nickel
Most stainless steels, aluminium, brass
Surface quality
The maximum holding force of a lifting magnet is the result of a closed magnetic circuit in which the magnetic field
lines can connect up freely between the poles, thus creating a high magnetic flow. In contrast to iron, for example,
air has very high resistance to magnetic flow. If a kind of "air gap" is produced between the lifting magnet and the
workpiece, this reduces the holding force. In the same way, paint, rust, scale, surface coatings, grease or similar
substances all form a gap, or an air gap, between the workpiece and the lifting magnet. The increasing surface
roughness or unevenness of the surface also has a negative effect on the holding force. Reference values can be
found in the performance table of your lifting magnet.
Load dimensions
When working with large workpieces such as girders or plates, the load can deform during the lift. A large steel
plate would bend downwards at the outer edges and create a curved surface which no longer has full contact with
the bottom of the magnet. The resulting air gap reduces the maximum load-bearing capacity of the lifting magnet.
Hollow objects or those smaller than the magnetic surface will also result in less holding power being available.
Load alignment
During load transport, care must be taken that the lifting magnet is always at the center of gravity of the work piece
and that load, or lifting magnet respectively, is always aligned horizontally. In this case, the magnetic force of the
lifter acts with its breakaway force perpendicular in relation to the surface, and the maximum rated load-bearing
capacity is achieved with the 3:1 standard safety factor.
If the position of work piece and lifting magnet changes from horizontal to vertical, the lifting magnet is operated in
shear mode and the work piece can slip away to the side. In shear mode, the load-bearing capacity decreases
dependent upon the coefficient of friction between the two materials
Temperature
The high-power permanent magnets installed in the lifting magnet will begin to lose their magnetic properties
irreversibly from a temperature of more than 80°C (180°F), so that the full load-bearing capacity is never reached
again even after the magnet has cooled down.
Please note the specifications on your product or in the operating manual.
Magneticforce in %
100
90-95
90
45
11
0
15