A: Dennis O’Leary, Industrial Magnetics, says:
Magnetic separation equipment is used to structure a plant’s program for 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 magnet manufacturers to meet a minimum and measurable threshold of repeatable performance to meet federal and industry processing standards.
Magnetic specifications generally result from a plant’s interaction with and knowledge of food schema documentation and with the cooperation of magnet manufacturers to develop and define requirements and verification documents. Two of the main specification components are callouts for gauss and pull-strength figures. These figures aren’t magical or difficult to get from the magnet manufacturer, but understanding each figure and how it can influence a plant’s quality program is critical.
First, let’s define some critical magnet terms:
- Gauss measures the number of magnetic lines of flux in one square centimeter of a magnet’s surface.
- Pull strength measures how many ounces or pounds of force are required to remove a piece of ferrous metal from a magnet.
- A 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 pull strength, which is the easy one. Pull strength is measured using a manual or digital pull scale and a ¼-inch or ½-inch ferrous sphere, with test results recorded in ounces or pounds. The test results should be easily repeatable.
Gauss is the stored energy in a magnet — also called magnet performance or maximum energy product (often abbreviated as BHmax) — and is measured in units of megagauss-oersteds (MGOe). One MGOe is approximately equal to 7,957.74715 joules per cubic meter (J/m3).
Measuring a magnet’s gauss is trickier than finding the pull strength but can be accomplished using an electric gauss meter equipped with either a transverse or axial probe. Gauss meter readings have a broader delta range between the high and the low results than pull-strength test results. That said, many plants are opting to write gauss and pull-strength numbers into their programs.
Sounds easy, right? 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 typical magnetic circuit is housed in a tube with an outside diameter (OD) of approximately 1 inch, but manufacturers have many options for how to design the “guts” that compose that circuit, as defined earlier. Magnet material and pole-piece dimensions vary, as does the tube wall thickness. Dialing in a specification is akin to buying a new car. Do you want speed? Yes. But you may also want cargo space, 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, is equipped with leather recliners, and gets 30 mpg. That’s a tall order for a car.
Magnet manufacturers face a similarly difficult task, and many have responded with numerous offerings to meet specification requirements within reason. If gauss is the desired testing parameter, some circuit designs are optimized to reflect high gauss readings. Generally, such a circuit yields lower pull strength because the pole pieces and tube wall thickness are designed to maximize the gauss results. Similarly, some circuits are optimized to yield higher pull-strength results and have entirely different pole and magnet material dimensions. A third option, using a thicker-walled tube, yields marginally lower numbers for both gauss and pull strength. This option can strike a balance for certain applications, as it reduces tube wear from abrasive materials while still providing industrial-grade mechanical strength.
The good news is that options exist, and magnet manufacturers can assist processing facilities with determining the parameters that will best mitigate foreign contamination incidents.
Industrial Magnetics, Boyne City, MI, manufactures various magnets for tramp metal removal and supplies other related products and services.