A: Dennis O’Leary, Industrial Magnetics, says:
Magnetic separation equipment has been drawn into the many titles used to structure a plant’s program in addressing foreign contamination concerns. Whether it’s the Food Safety Modernization Act (FSMA), Global Food Safety Initiative (GFSI), British Retail Consortium Global Standard (BRCGS), Safe Quality Food (SQF), or some variation thereof, there’s an ever-increasing trend in which processing facilities require manufacturers to meet a minimum and measurable threshold of repeatable performance to meet federal and industry processing standards.
There’s a framework forming around magnetic specifications, which generally results from a plant’s interaction and knowledge of food schema documentation, as well as cooperation with magnet manufacturers in developing and defining requirements and verification documents. The main components include a callout for gauss and pull strength figures. These figures aren’t magical nor are they difficult to obtain from the manufacturer but understanding each figure and how they can have the most positive influence on a plant’s quality program is critical.
So, here’s a quick education on magnet terminology:
- Gauss measures the amount of magnetic lines of flux in one square centimeter of a magnet’s surface.
- Pull strength measures how many ounces or pounds of force it takes to remove a piece of ferrous metal from a magnet.
- Magnetic circuit is a combination of magnet material and mild steel pole pieces.
- Air gap refers to any nonferrous material space between the magnet and ferrous material.
Let’s start with the easy one, which is pull strength. This measurement is accomplished with a manual or digital pull scale and a ¼-inch or ½-inch ferrous sphere with test results recorded in ounces or pounds. The measurement results that you record from this test should be entirely repeatable and easy to perform. Gauss is defined as the stored energy in a magnet — called magnet performance or maximum energy product (often abbreviated BHmax) — and is typically measured in units of megagauss-oersteds (MGOe). One MGOe is approximately equal to 7957.74715 joule per cubic meter (J/m3).
Measuring a magnet’s gauss is a bit trickier than finding the pull strength but is accomplished using an electric gauss meter equipped with either a transverse or axial probe. The meter readings generated with this device tend to have a broader delta between the high and the low results than those readings resulting from pull strength testing. That said, many plants are opting to write gauss and pull strength numbers into their programs.
This sounds easy, right? Just simply write the specification calling for the high range of gauss and pull strength found in commercially available magnetic separators, but therein lies the rub. A garden variety magnetic circuit is housed in a ~1-inch OD tube and the “guts,” which compose that circuit as defined earlier present many options from various manufacturers. Magnet material and pole-piece dimensions vary, as does the tube’s wall thickness, which serves as a housing to the circuit. So, when it comes to dialing in a specification, it’s akin to buying a new automobile. Do you want speed? Yes. But you may want the vehicle to have storage for better family vacations, good fuel economy, comfortable legroom, and a sunroof. Add it all up and you’re looking at a manual transmission touring bus that goes from 0 to 60 mph in 5 seconds and is equipped with leather recliners and gets 30 mpg.
While that’s a tall order, magnet manufacturers have been challenged with a similar task and many have responded with numerous offerings to meet specification requirements within reason. If gauss is the desired testing parameter, there are circuit designs optimized to reflect high gauss readings. Generally, such a circuit would yield lower pull strength because the pole pieces and tube wall thickness are designed in such a manner to maximize the gauss results. Conversely, a circuit optimized with an entirely different set of pole and magnet material dimensions will yield a higher pull-strength result. A third option, which would generally yield marginally lower gauss and pull strength numbers, would result from using a thicker-walled tube. This could strike a balance for certain plants and applications, considering it provides less tube wear for potentially abrasive applications, while still providing industrial-grade mechanical strength. The best news is that options exist and manufacturers can assist processing facilities with determining the proper parameters for magnetic separation equipment that will ensure the best path to mitigating foreign contamination incidents.
Industrial Magnetics, Boyne City, MI, manufactures various magnets for tramp metal removal as well as supplies other related products and services.