Which other forces play a role?
Residual holding force FR of electro-holding magnets
The holding force remaining due to the magnetic remanence after deactivation of the previously indicated nominal voltage of electro-holding magnets. Depending on the workpiece, this is between 20 and 40% of the holding force while the device is activated. In case of door holding solenoids (GTR types), the residual holding force on the armature is overcome by a springoperated pin.
Shifting force Fv
The force required to shift a workpiece parallel to the holding surface while the device is switched on. Depending on the texture of the workpiece surface, this amounts to 20... 33% of the holding force FH while under current.
What factors influence the holding force?
Air gap δL
This indicates the average clearance between the holding surface of the magnet and the held workpiece surface. The shape of the surfaces facing each other and non-magnetic substances (e.g. galvanic overcoats, paint, soot) form its size.
The roughness and unevenness of the surface acts as an additional air gap. Due to the low permeability of air (μ0), the air gap is the relevant size for the magnetic flow. Influences that affect the holding force:
Configuration of the holding surface
The configuration of the holding surface is the contact surface (in %) that the workpiece contacts the holding solenoids. The holding force per surface unit of a holding solenoid is nearly the same across the complete holding surface. The configuration of the holding surface is at maximum (100%) when the complete magnetic holding surface is occupied by the workpiece.
The holding solenoid housing, which guides the magnetic flow, consists of highly permeable steel. For this reason, the high holding force indicated in the data sheets is able to be reached with armature plates or workpieces consisting of steel S235JR (formerly referred to as St37) or comparable materials. In any case, the actual holding force able to be reached is reduced by different application parameters,
including the low permeability of the workpiece. This therefore depends on the material type. Hardened materials also possess a lower permeability. The basic rule is: The higher the hardening degree, the lower the magnetic conductance and therefore also the achievable holding force.
Thickness of the workpiece
For every size of device, there is an optimal workpiece thickness, which is indicated in the device sheets of the catalogue as “armature plate thickness”. The associated diagrams specify the influence of low workpiece thickness. An armature plate thickness larger than the indicated thickness, will not result in increased holding force.
Reduction of input power
The input power may be reduced via upstream activation of a voltage regulator. The voltage reduction reduces the heating and the holding force FH of the individual devices.