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HOLDING APPLICATIONS

  • Clamping Non-magnetic Parts

    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 an 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.   

    direct clamp diagramdirect clamp diagram

    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.

    indirect clamp diagram

  • Direct Contact with an Air-gap

An electromagnet will have less holding value in presence of an air-gap between the electromagnet and a plunger (workpiece). Holding value diminishes exponentially with an air-gap.

An electromagnet will have maximum holding value at direct contact with a plunger (workpiece). 

There are some common situations that cause less holding value.
A.    Contact area of a plunger (workpiece) is smaller than the contact area of an electromagnet.
B.    An uneven surface of a plunger (workpiece) creating distance between an electromagnet and the plunger (workpiece).
C.    An air-gap or any non-magnetic material (like paint, tape, etc.) between an electromagnet and a plunger (workpiece).
D.    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 magnetic flux retained in the thin plunger (workpiece).
E.    Unconsciously apply AC voltage to DC electromagnets, even the magnitude of the voltage is same. Usually, there is a vibration and a buzzing sound. 

There are some misapplications that feel like less holding value.
F.    Pulling force is not holding force, usually much less than holding value. Holding value is the magnetic force directly holding a plunger (workpiece) at zero gap. All Magnetech electromagnets are posted with holding value. Pulling force or pull force is widely misunderstood by users to think that an electromagnet pulls or draws a plunger (workpiece) at a gap or distance.
G.    Shearing (sliding) force is not holding force, usually much less than holding value. Holding value is the magnetic force directly holding a plunger (workpiece) at zero gap. All Magnetech electromagnets are posted with holding value. Shearing or sliding force is like a frictional force that is multiplication of a holding value and a frication factor. You might have experienced that it takes more force to take away a refrigerator magnet than to slide the magnet up or down.
H.    Prying or Peeling force is not holding force. When trying to separate an electromagnet from a plunger by opening a corner of contact surface a little bit, it creates a gap that dramatically reduces the holding value. Holding values are tested by separating the plunger from the contact surface of the electromagnet simultaneously. The magnetic holding force and separating force must be on the same axis and in opposite direction.
I.    Leverage force is not holding force. If an application has a leverage with a physical or unseen pivot, the holding force must be calculated by leverage equation. Posted holding values are based on that magnetic force and separating force are on the same axis and in opposite direction.

When using multiple electromagnets to hold a large metal part, it is recommended to use a floating mount method to minimize gap between the electromagnets and the large metal part. Insert leaf springs between electromagnets and their rigid mounting base, so that the electromagnets could have some wiggling space to automatically make a good surface contact with the large metal part. Sometime, in order to minimize the total height increasing, machine a pocket or groove around a mounting hole to hide a leaf spring.

Using a flat surface electromagnet to hold a non-flat metal part will experience less holding value, due to an air-gap between contact surfaces. The best way to avoid the air-gap is to machine the surface of the electromagnet to match the surface of the metal part. For a cylindrical surface, using a V-Block electromagnet is better than a flat surface electromagnet.

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 usually work better than a single large electromagnet, even the total contact surface is the same.

Only if the duty cycle is intermittent or not continuous. 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.

Heat rise:
An electromagnet uses electricity to generates magnetism. It also generates heat like any other electrical components. An electromagnet needs a mean of convection for heat dissipation and should be mounted in open space in the area where there is enough free air circulation to dissipate heat from the electromagnet. User should test an electromagnet in actual assembly for the desired period of time to ensure the temperature rise from the electromagnet is acceptable. Special custom electromagnets can be made for lower temperature rise.

Holding strength:
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 and do not use an electromagnet for a secure holding application.