26.4.2 Permanent Magnets

Some substances are naturally magnetic and others are capable of being “magnetized” and retaining that magnetism. The most common form of permanent magnet is a substance called Magnetite and this is used for domestic applications. Industrial permanent magnets use “rare earth” substances such as Samarium or Neodymium which produce very much stronger magnetic fields. An industrial permanent magnet is switched “on and off” by mechanically rotating a moveable magnet or magnets within the device thereby arranging it to add to or cancel out the field of the static magnets.

Most permanent magnetic lifters are switched manually by means of a lever, but some manufacturers offer the option of electrical or pneumatic powered mechanisms.

Figure 26.4.2-1 

Key

  1. Ferromagnetic crown

  2. Static permanent magnet

  3. Reversible permanent magnet

  4. Magnetic flux collectors

  5. Ferromagnetic work piece

This type of magnetic lifter shall provide a tear off force at least three times the working load limit under the conditions specified by the manufacturer. The controls shall clearly indicate whether the magnet is on or off.