HOLDING APPLICATIONS
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There are three ways to directly clamp a non-magnetic part in a sandwich.
A. A non-magnetic part is sandwiched in between an
electromagnet and a steel plate. A non-magnetic part is considered as an air-gap.
Unless a non-magnetic part is very thin, it is not an efficient
way.
B. A non-magnetic part is sandwiched in between an
electromagnet and a permanent magnet. Usually, it consists of a parallel
electromagnet and a U shape permanent magnet. Adjust polarity of the
electromagnet to attract the permanent magnet.
C. A non-magnetic part is sandwiched in between two
electromagnets. Usually, it consists of two identical electromagnets.
Adjust polarity of the electromagnets to attract each other.
There is another way to indirectly clamp a non-magnetic part with
maximum clamping force.
An electromagnet directly contact with a steel plate or another
electromagnet. Attach a bracket to the electromagnet. Use that bracket to
clamp an non-magnetic part. In this way, you can clamp any thickness
material without loosing the holding power of the electromagnet.
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Electromagnets have less holding value in presence of air-gap(s) between an
electromagnet and a plunger (workpiece). Holding value diminishes exponentially with
air-gap(s).
Electromagnets have maximum holding value at direct contact with plungers (workpieces).
There are four common situations causing less holding value.
A. An uneven surface of a plunger (workpiece) creating
distance between en electromagnet and the plunger (workpiece).
B. Air-gap(s) between an electromagnet and a plunger (workpiece).
C. Thin plungers (workpieces), like sheet metal. Because a thin plunger (workpiece) reaches
magnetic saturation and cannot carry all magnetic flux through, all the stray
flux is wasted. So holding value is reduced and limited on the flux retained in
the thin plunger (workpiece).
D. Unconsciously apply AC voltage to DC electromagnets, even
the magnitude of the voltage is same. Usually, there is a vibration and a
buzz.
In application of holding a sheet metal part, there is a magnetic flux
saturation problem due to thin materials. Holding value is limited on the
thickness of the sheet metal. The thinner the sheet metal, the less holding
value. In large piece of sheet metal part, use more electromagnet
contact surface to hold. Multiple small electromagnets work better than a
single large electromagnet, even the total contact surface is the same.
Only if the duty cycle is small. In order to avoid over heating
electromagnets, the rule of thumb is that for every 50% more voltage cuts 50%
duty cycle. e.g. for a 12 Volts DC electromagnet, you can apply 18 Volts DC on
it if the duty cycle is 50% or less. 24 Volts DC on it if 25% or less duty
cycle.
For safety in holding applications, do not use electromagnets at more than
1/2 of listed holding value. In lifting applications, do not use at more than 1/4
of listed holding value. Electromagnets will lose their magnetic holding
power, when losing electrical power. So do not use electromagnets over people.
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