Lifting magnet

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Lifting magnet - 2000 kg, with table
Moving a ramp plate

Lifting magnets are load-receiving means for lifting loads / articles with magnets . The loads to be lifted must be ferromagnetic and magnetize as little as possible. In most cases, lifting magnets are used for loads made of iron or steel.

commitment

Magnetic load lifters are used in steel works , in the steel and steel pipe trade, in metal processing, e.g. B. used for charging machine tools , in steel construction or shipbuilding .

Advantages and disadvantages

The advantage of a magnetic coupling is that unprocessed parts can be captured without hooks, handles or eyes. No other load handling devices such as chains or tongs have to be attached and removed again. Recording from above saves space, because there is no need to provide space for other load handling devices on the side. A magnet is a durable and low-maintenance component.

The disadvantage is the high dead weight of the load handling device, the magnetization of the load (permanent magnetic field remains) and the lack of positive coupling (safety aspect in the event of a power failure of the electromagnet).

Due to the remanence of the load material, the load does not always drop when it is switched off. One tries to counteract this with air gaps, which in turn reduce the maximum adhesive force.

Load capacity

The actual load capacity depends on the magnet, how well the magnetic field penetrates the load. The load-bearing capacity is reduced on the one hand by a larger air gap , which can be caused by curved shapes, rough surfaces or rust, on the other hand by a less magnetic load material or a low material thickness. Tables or diagrams are often given on the magnet for the dependence of the material thickness and air gap.

The load-bearing capacity is given with threefold certainty, which seems appropriate because it is reduced by the effects mentioned.

Operating instructions can be used to assess whether the load-bearing capacity is reduced by a load that is too light. There are holding magnets with an extra large or extra small penetration depth. This is achieved by arranging the pole pairs with different densities. A low penetration depth can be advantageous in order to remove only the topmost sheet from a stack of sheets. Another possibility to select is to remove the sheet metal with a low magnetic force and then switch to full force.

A load change in the direction of the gap between magnet and load (tilting, lack of balance) must be avoided - the full holding force is only achieved with a force direction perpendicular to the gap (or with a horizontal gap). Otherwise it is reduced by the coefficient of friction between the partners.

Types

In principle, electromagnets or permanent magnets are suitable for lifting loads. In the combination, a permanent magnet is partially electrically reversed. The latter solutions have the advantage that the load does not drop in the event of a power failure.

With electromagnet

Electromagnets can be conveniently switched on and off remotely via the supply line in order to couple and release the load. The disadvantage is that an electrical supply with a feed line is required and the load on mains-powered devices drops in the event of a power failure. The operation of such magnetic cranes is therefore only permitted in closed-off areas. Magnetic cranes often have emergency batteries for safety.

The stray magnetic field can damage people with active prostheses or aids.

With permanent magnet

Switching by relative movement

Since the development of neodymium-iron-boron magnets in the 1980s, very strong permanent magnets can be produced. The required switchability can be achieved by a movable arrangement of several magnets, so that a switchable magnet is created on the outside . Lifting devices with permanent magnets can lift around 20 times their own weight. The drive for changing the magnet arrangement can be done purely mechanically with a lever, but also hydraulically or with compressed air.

Switching by magnetizing

Alternatively, an arrangement of Alnico magnets can be magnetized or demagnetized by a pulse from an electromagnet in order to achieve the desired switching effect of the external magnetic field. In addition to higher security, the method offers energy savings and a safe release of the load (no remnant field in the load).

Switching through compensation

According to another design principle, the field of a permanent magnet is only compensated with an electromagnet in order to release the load. The currentless state is therefore holding on. Nevertheless, dangers arise in the event of a power failure when working in the field area.

Others

In addition to their use in electric motors and sorting / separating devices , lifting magnets are an area in which considerable quantities of rare earth magnets are used industrially.

Individual evidence

  1. Lifting magnets and their advantages. In: Website of the company Spanntech GmbH. Retrieved April 14, 2019 .
  2. ^ A b Eberhard Kallenbach, Rüdiger Eick, Peer Quendt, Tom Ströhla, Karsten Feindt, Matthias Kallenbach: Electromagnets: Fundamentals, calculation, design and application , Springer-Verlag 2009, 402 pages, page 316f
  3. a b Hand lifting magnet - operating instructions. (PDF) In: Website of Alfra GmbH. August 15, 2016, p. 5 , accessed April 14, 2019 .
  4. https://www.materialfluss.de/krane-und-verbindungen- need-und-hebetechnik/ neuer-magnetkran-neue- moegitäten.htm message from Weka Business Medien GmbH, October 25, 2016, accessed on 29. March 2020
  5. Martin Scheffler, Klaus Feyrer, Karl Matthias: Conveyors: hoists, elevators, industrial trucks, conveyor technology and construction machinery . Springer-Verlag, Berlin / Heidelberg / New York 2013, ISBN 978-3-663-16318-3 , pp. 41 .
  6. Electric lifting magnets, emergency power safety circuit and cross beams. In: Company website Beloh Magnetsysteme GmbH & Co. KG. Retrieved April 13, 2019 .
  7. ^ MH Walmer, JF Liu, PC Dent: Current Status of Permanent Magnet Industry in the United States . In: Proceedings of 20th International Workshop on RARE EARTH PERMANENT MAGNETS AND THEIR APPLICATIONS . Crete September 8, 2008 (English, groenerekenkamer.nl [PDF; accessed April 13, 2019]).
  8. https://www.pfeifer.info/de/produkte-dienstleistungen/produkte/hebezeuge-und-wind/lasthebemagnete/permanent-lasthebemagnete/hit-green-s-permanent-lasthebemagnet-standard.html Technical data of a permanent lifting magnet of the company Pfeifer / Memmingen, accessed on March 29, 2020
  9. https://stenzel-werkzeugtechnik.de/elektro-permanent-lasthebemagnete/ Information from Stenzel Werkzeugtechnik GmbH & CO. KG, accessed on March 29, 2020
  10. Matthias Buchert, Andreas Manhart, Jürgen Sutter: Investigation of rare earths: Permanent magnets in industrial use in Baden-Württemberg . Ed .: Öko-Institut e. V. Freiburg January 2014 ( oeko.de [PDF; accessed April 14, 2019]).