What are holding magnets?
Holding Magnets / Holding Solenoids
Electro-holding magnets (holding magnets) remain able to be magnetised. Workpieces in position due to magnetically created forces. Pieces that are unable to be magnetised are able to be held on a magnetisable armature plate. The specific design of the magnetic circuit maximises the holding force for the smallest possible air gap. The ideal attraction of the armature plate from a greater distance is not available for these devices. The holding force is created by an electromagnetic field in case of electro-holding magnets, or by a permanent magnetic field in case of permanent electro-holding magnets.
In terms of construction, these magnets feature an open magnetic circuit, which is completed by the workpiece and the armature plate. The size of the resulting magnetic flow specifies the holding force. The degree of magnetising ability of a workpiece is specified by the relative permeability μrel of the material.
The larger the magnetic flow Φ for a continuously steady holding surface that penetrates the workpiece, or the greater the magnetic induction B on the holding surface, the higher the holding force FH.
How is holding force calculated?
The holding force is the force that is required to remove the workpiece from the holding surface of the solenoid. Using Maxwell’s pulling force formula, you may calculate:
FH=B2A / (2μ0)
B…the median flow density in the air gap, A…magnetic holding force, μ0…magnetic flow density of air
In case of a magnetic flow density of 1.6T, approx. 1N result from approx 1mm2 holding surface. The magnetic flow and therefore the flow density are specified by the overall resistance in the magnetic circuit.
Residual holding force 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 of electro holding magnets
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.
All of the statements made above apply to both electro-holding magnets and permanent electro-holding magnets, with the exception
of dependence of the holding force FH on the operating voltage, and
the expression of residual holding force
In case of permanent electro-holding magnets, the permanent magnetically produced holding force FH is neutralised by activation of the supply voltage. In any case, the quality of neutralisation depends on the applied voltage (tolerance) and the coil temperature. A holding force FH = 0 N can only be achieved for the exact nominal current.
Figure right displays the size of the holding force FH of a permanent electro-holding magnet depending on the nominal current IN at which the force equals 0 exactly. Depending on the operating conditions, the operating current differs slightly from the nominal current so that a residual holding force FR remains. Ensure that if the current is increased above the nominal current, the holding force will also be too great.