B) Pull tests are the most common method. This requires a pull test scale ( spring
loaded scale graduated in ounces or pounds ) and various test fixtures. When
comparing test results, whether between different magnet manufacturers or an
in-house scheduled audit, it is imperative that the same procedure and apparatus
are used in each test. If a smaller ferrous test fixture is used than what was used in
the previous test, the results will differ greatly. The reason for this is the larger the
ferrous test fixture, more of the magnetic field will be absorbed, therefore,
producing higher test results.
Pull tests produce two end results- pulling power and holding power. Both of these
results are of equal importance. Pulling power relates to the "reachout" of the
magnetic field. Depending on the application, a magnet that has the power to draw
ferrous contaminants out of a product stream should have the ability to retain that
ferrous contaminant. Therefore in comparison the reachout would be more critical
than the holding power. Plate magnets typically fall into this category.
Holding power relates to the breakaway force required to remove the ferrous test
fixture from the working surface of the magnet. Grates and drawer type magnets
typically have 1" diameter magnetic tubes mounted on 2" centers. Quite often there
are two or more rows of staggered tubes. While reachout is still important, it is more
critical to the performance of the separator that it have the maximum achievable
holding power. The staggered configuration of the magnetic tubes causes the product
to cascade over one row of tubes and fall directly below on top of the following row
of tubes. This ensures the weakly magnetic particles will be captured and retained
and not return to the cleaned product.