ALFRA 41100.A Mode D'emploi page 15

Basic information concerning the handling of magnetic lifting gear – in
particular TML / TMH / TMC
The magnetic surface is located on the underside of the magnet incorporating 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 flux of the permanent magnet requires a minimum material thickness to flow completely into the load.
Below this minimum thickness of material, the maximum holding force is reduced depending on material thickness.
Conventional switchable permanent magnets have a deep penetrating magnetic field similar to the tap root of a
tree,and require a large material thickness to achieve maximum holding force. The compact magnetic field of TML,
TMH and TMC magnets is similar to a shallow root and achieves maximum holding force even when used on thin
materials (see performance data in table 2, page 16).
Material
Every material reacts in a different way to penetration of the magnetic field lines. The breakaway force of the
magnet is determined using a low carbon material. Steels with high carbon content or whose structure has been
changed by heat treatment have a lower holding force. Foamed or porous cast components also have a lower
holding force, so that the given load-bearing capacity of the 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 permanent magnet can be achieved in case of a closed magnetic circuit in which
the magnetic field lines can connect up freely between the poles, thus creating a high magnetic flux. In contrast to
iron, for example, air has very high resistance to magnetic flux. If an air gap is formed between the work piece and
the magnet, the holding force will be reduced. In the same way, paint, rust, scale, surface coatings, grease or
similar substances all constitute a space (i.e. an air gap), between work piece and magnet. Furthermore, an
increase in surface roughness or unevenness has an adverse effect on the magnetic holding force. Reference
values for your TMC 300 can be found in table 2 (page 16).
Load dimensions
When working with large workpieces such as girders or plates, the load can deform during the application. 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 magnetic
clamp. Hollow objects or those smaller than the magnetic surface will also result in less holding power being
available.
Load alignment
During lateral load ('shear' mode), the holding force of the magnetic clamp decreases dependent upon the
coefficient of friction between the two materials.
Temperature
The high-power permanent magnets installed in the magnetic clampwill 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 and in the operating
manual.
Magneticforce in %
100
90-95
90
45
11
0
15