How to Conduct a Magnet Pull Test

HOW TO CONDUCT A PROPER PULL TEST
Most magnet manufactures have adopted standard test equipment that measures the holding force of a magnet. Eriez offers a Pull Test Kit that includes a variety of pull test pieces to measure holding force. (You may not need all these pieces, depending upon which magnet is being tested.):

• Scale with snap ring
• Aluminum spacers, including one-inch thick, 1/2 in thick (2), 1/4 in thick
• Pull test pieces including 1/4 in sphere, 1 in sphere, 1 x 3 x 1/8 in plate, 1 x 3 x 1/8 in plate with radius mount.

Two test pieces are recommended when measuring holding force for tube magnets. Use the 1 x 3 x 1/8 in plate with radius mount for ceramic tube magnets and the 1/4 in steel sphere for all RE tube magnets. Ceramic tube magnets remove tramp metal from free flowing products.

Tramp metal consists of large iron such as hex nuts, bolts, nails and other metallics in that size range, so using the 1 x 3 x 1/8 in test piece is ideal. RE magnets capture iron particles associated with abrasion, scale, or small metallic slivers such as threads stripped from a bolt. Since it is difficult to measure the force associated with removing a metal sliver from a tube magnet, use the 1/4 in steel sphere.

PERFORMING THE TEST

The pull test procedure begins with cleaning the magnet surface. Even a small gap between the pull test piece and magnet will adversely affect test results. Then use the following steps:

• Select the appropriate pull test piece and spacer for the separator, based on guidelines found in your pull test kit, and attach to the scale. For example, it’s best to use the ¼ in steel sphere attached to a stainless steel bar for testing a grate or tube magnet and the 1 x 3 1/8 in plate for plate magnets.
• Zero the scale with the appropriate pull test piece by rotating the adjustment knob on top until the scale line measures zero.
• Allow the piece to attract to the magnet and position the scale perpendicular to the magnet surface.
• Pull the scale directly away from the magnet surface slowly, smoothly and evenly. Be careful, as the piece may snap unexpectedly from the magnet surface. Wear protective eyewear during the test. Note the measurement by reading the metal band as it slides down the barrel of the scale.
• Repeat the test to verify results.

After three to four pulls, the results should be within a reasonable range, i.e., a ¼ in steel sphere might measure 88 to 92 oz in pull strength during the test sequence on an RE tube magnet. However, if the test is repeated in six months and pull strength drops to 75-80 oz, definite magnetic erosion can be detected.

It is not necessary to pull test every tube of every grate assembly. Spot check the grate assembly and record an average of approximately three pull tests. Do not pull test near the ends or edges of any magnetic separator.

The magnet strength will measure less at the edges because of magnet design constraints. Measures around the center of a tube or plate should represent the magnet’s true working strength.

Pull test results can help determine if (or when) a magnet needs replacing. Magnets with visible cracks or wear holes should be removed from operation immediately, regardless of test results. Magnets showing 15-20 percent erosion in holding force over a six-month to one-year period should also be replaced, particularly if the magnet is part of a Critical Control Point.

Not sure when to replace your magnets? Most magnet suppliers will offer advice on test results and replacement recommendations.  Some magnet manufacturers, including Eriez, will lend assistance with pull test inspections and help with evaluations.

Eriez Manufacturing Co., 2200 Asbury Road Erie, PA 16506, 814-835-6000, [email protected], www.eriez.com