Magnetic Wood and Magnet Selection

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We find hanging utensils in the kitchen to be the best storage method; most of the vertical space in the kitchen is covered with kitchen equipment.  However, the sides of the cabinets above the sink had nothing hanging on them! This problem demanded a solution. Last time I did a project like this I made pegboard, but pegboard is inflexible both in spacing and in the type of hooks available. This time: magnets.

I started with 1/16″ thick sheets of steel, which I covered with oak veneer. Technical support at 3M advised me to use spray adhesive, so I adhered the veneer with Super 77. The trim on the cabinets conceals the side edges, but not the bottom edges, so I epoxied small oak strips along the top and bottom to hide those edges and blend into the cabinetry.

Once I had the veneered steel screwed to the cabinets I realized that I couldn’t find magnetic hooks suitable for hanging kitchen tools.  I could hang a few things with Super Fridge Magnets and some things with the Standoff Magnetic Tool Holder, but most of the things I wanted to hang would require a different solution.

These  magnetic bases have a protruding threaded rod. To transform them into hanging rods I attached standoffs, metal tubes that are threaded on the inside. At McMaster I could buy standoffs in a variety of lengths, though for some reason the stainless steel ones were twice the price of aluminum.  To get a finished look I found two options. McMaster sells elegant end caps for the stand offs, but they are expensive—more than the shorter length standoffs. You can see them below in stainless steel (which is the same price as aluminum). I also bought a box of stainless fillister head machine screws which make reasonable looking caps for the standoffs at just pennies a piece. This scheme enabled me to make hooks as long or as short as needed. I worried that the absence of a hooked end might allow tools to fall off the wall and plunge into the sink, but that hasn’t been a problem for most things.




The one exception is the red funnel which slips right off of a rod. A simple solution is to insert a screw that is longer than the threaded rod so the screw head acts as a stop.

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Hanging this funnel required a trick to keep it from sliding off the rod.

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We also have some spatulas whose hang holes do not admit a 1/4″ shaft, and the threaded rod does not come in a smaller size that is compatible with the magnetic screw bases. I attached a 1/2″ diameter magnet to a small piece of wood and inserted a 3/16″ diameter stainless rod to create a suitably sized hanger. It didn’t work: I planned to hang four spatulas from this rod, but the hook would slide down and fall off the wall if I loaded it with more than two of the utensils.

Next I tried polymagnets. I recently learned about these magnets where the magnetic field varies spatially across the magnet to produce different effects. They make several types of magnet pairs, including the spring, latch, and centering magnets. For this task I was testing the “attach” type magnets, which supposedly concentrate the magnetic field closer to the magnet so that the magnet grips more strongly. I had on hand some 1″ diameter, 1/32″ thick “attach” magnets as well as two 1″ diameter, 1/8″ thick “attach” magnets (one with a mounting hole and one without). The 1/32″ thick magnets have the same volume of magnetic material as the 1/2″ diameter, 1/8″ thick magnets I used to make my unsuccessful hooks. Another observation is that two of the 1/32″ magnets got stuck to each other at one point and they were extremely difficult to separate. It was impossible to slide them apart like one usually does to separate rare earth magnets. I had to separate them using a wedge, something I’ve never had to do before, even with larger 3/4″ diameter, 1/8″ thick magnets. These magnets are not weak.

I glued a 1″ diameter 1/32″ thick “attach” magnet to a small piece of wood and again inserted a length of steel rod.   This new hook was very weak and slide right down the wall.  I glued on a second “attach” magnet, and now it could hold the weight of 3 spatulas, but not 4.  When attached to the refrigerator, however, the hook would hold the necessary weight—the four spatulas I wanted to hang weighed about a pound in total. Note also that the rolling pin weighed more than a pound and yet was easily held to the veneered steel by one hook with a 1/2″ magnet.  The hook with two polymagnets has double the amount of magnetic material,  but it fails to do the job.

I abandoned that design.  The hanging rod in my original design was below the magnet instead of centered over it.  The wood was 1/4″ thick.  The magnets are 1/8″ thick.  I didn’t think the remaining 1/8″ of wood sufficed to support the hanging rod.  The off-center hanging rod seems to make the shear hold weaker because the load on the rod can tilt the magnet away from the surface.  I made the centered rod design (right) by using a magnet with a screw mounting hole in the center. The rod goes through the screw mounting hole, and I filled the space with epoxy to ensure that the rod would be well supported.  This new hook holds all four spatulas without any difficulty.

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Spatula hooks. The bloodwood hook on the left with a conventional half inch magnet has the hang bar off center and can hold only two spatulas. The chechen wood hook on the right, with a one inch polymagnet and centered bar easily holds four spatulas.

 

Measuring magnets

So what was going on with my magnets? I tried to measure the capacity by filling a bag with weights (dried beans) and determining the point at which the hook began to slip. This was kind of messy and I didn’t feel very confident about my accuracy, but I did find that the hook with the two polymagnets held 4.25 lbs on the refrigerator and only 1.1 pounds on the wood covered steel. One important factor in this sort of holding power is friction. Is it possible that my finish on the wood was too slippery? Or did the magnets simply hold with less strength on the wood.

forcesheer_smTo get to the bottom of this I purchased a Chatillon force gauge cheaply on eBay. This device measures force in pounds and enabled me to measure the pull force to detach magnets from surfaces as well as the shear force (the hanging load that would cause the magnet to begin sliding). Measuring shear force is relatively easy: I attached a loop of string to the hook and pull downward with the gauge.

Measuring pull force is a bit more difficult. I used a strip of Tyvek and placed the magnet over it. This means that the magnetic force is decreased by the extra distance created by the layer of Tyvek. It also is difficult to be sure that the pull is applied directly perpendicular to the plane of the magnet, and I suspect that an angled pull may lift the magnet more easily.

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Measurement of the magnets resulted in a complicated set of data with some inconsistencies. I tested magnets on two surfaces, the refrigerator and the wood veneered steel. If the pull force of the magnet is F then the shear force is supposed to be equal to σF where the coefficient of friction, σ, depends just on the surfaces. Lee Valley sells two self-adhesive high friction materials. One is their High Friction Disks, specifically marketed for use with rare earth magnets. The other is the Super High Friction Tape. When I ordered the polymagnets I also included some Traction Tape to use with them. My hook that wouldn’t support a pound of spatulas gave the following test results

Two thin Polymagnets glued to wood
Pull Force (lb) Shear Force (lb) σ
Fridge Wood Fridge Wood Fridge Wood
Bare magnets 5 1 1.5 0.25 0.3 0.25
Friction disks 3 0.5 3.75 1 1.25 2
Friction tape 1.75 0.5 5.5 2 3.1 2
Traction tape 4.25 1.1

When placed on the refrigerator the pair of 1/32″ thick 1″ diameter “attach” polymagnets gave a respectable pull force, five pounds with nothing but a sheet of Tyvek separating them from the refrigerator. The friction disks are only 0.022″ thick and the friction tape is 0.016″ thick. But this small extra separation from the metal created by the friction disk or friction tape reduced the pull force considerably to 1/3–1/2 of the original amount. Despite this decrease in pull force, the shear force increased. Without a friction tape of some kind the coefficient of friction was around 0.25, but with friction tape it rose as high as 3.1. Note that for this hook the high friction tape gave the best result: with two pounds of holding power even on the wood it would have sufficed to hold the 1 pound load of spatulas.

Why is the magnetic force of attraction to the refrigerator so different from the force of attraction to the veneered steel?  And why can’t two magnets hold on as strongly as a single magnet hook? Is the high friction tape really so much better than the other tapes?  I made the same measurements of the screw bases:

Small (16mm) screw base magnet
Pull Force (lb) Shear Force (lb) σ
Fridge Wood Fridge Wood Fridge Wood
Bare magnet 4.5 3.5 2.75 1 0.61 0.28
Friction disk 2.5 2.75 2 2 0.8 0.73
Friction tape 2.5 1.5 2 2.5 0.8 1.7

The strength of attraction to the wood and the fridge are closer together in this case, and the shear forces are very similar. In some cases the attraction to the wood is higher than the attraction to the fridge. This result is quite different from the previous one. One anomaly stands out: the shear force for the bare magnet on the fridge is very high. I noticed that when I dragged this magnet on the fridge it was scraping little white bits of the vinyl off, so I think that rough edges of the magnet were digging in and providing a huge boost to friction. I tried sanding the base but it didn’t significantly change the results, though. (Maybe I didn’t go far enough.)  Another anomaly is the high σ value with friction tape on wood; I have no explanation for this one.  These screw bases feature a 1/2″ diameter magnet that I assumed was 1/8″ thick, which would mean they have half the magnetic material of the two polymagnets I used above, and yet the holding power on the wood is higher. Moving on to the larger screw base:

Large (20mm) screw base magnet
Pull Force (lb) Shear Force (lb) σ
Fridge Wood Fridge Wood Fridge Wood
Bare magnet 4.75 6.25 1.5 1.25 0.3 0.2
Friction disk 3 4 3.25 4 1.1 1
Friction tape 3 4 3.5 4.5 1.2 1.1

This larger base clings to the fridge with greater strength than the smaller base, and on the wood it is even more powerful. The shear force strength is also somewhat stronger on the wood. So a bigger magnet is gripping more tightly and giving more holding power, right?

Wrong. Both screw bases have the same sized magnet.  The larger base has a plastic ring encircling the magnet.

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I disassembled one of each hook and found that the magnets were slightly bigger than 1/2″ in diameter (0.54″) and at 0.15″ thick, they were thicker than expected.  And they were indeed the same.  These neodymium magnets come in different grades with different strengths, but I tested the pull force and found no difference: both held to the fridge with 2 lbs and the wood with 2.5 lbs.

So apparently the difference in magnetic forces depends on the way the magnet is enclosed. I measured the pull force for a bare 1/2″ magnet and got 1.5 pounds for both the fridge and the wood. When I put the magnet into a steel cup the force rose to 3 pounds.  The observations above suggest that I might get even more force if I had an oversized cup surrounding my 1/2″ magnet.  So I tried inserting my 1/2″ magnet into a base designed for a 5/8″ magnet.  The force rose slightly to 3.25 lb on either the wood or the fridge.  I didn’t see the striking increase I got with the magnet base on the wood.   I tried a larger 3/4″ magnet cup and the force decreased slightly.  The only curiosity was that when I inverted the magnet cup, so the magnet was backed with steel but not covered on the sides, I saw an increase in the attraction to the wood to 3.5 lb, while the attraction to the fridge declined to 2.5 lb.

What conclusions can we draw from these measurements? Clearly backing the magnet with steel makes a big difference.  It seems like it’s hard to predict the attractive force of different configurations, so measurement may be the only way to know what will happen. The thin polymagnets did not adhere well to the wood. Perhaps their magnetic force is shorter range than the normal magnets, and the veneer is thicker than the vinyl that covers the refrigerator. This behavior doesn’t offer any obvious advantages, so I don’t plan to use those magnets again.

The coefficient of friction for the bare magnets came out consistently around 0.2–0.3, and inserting friction tape of some kind improves this to about 1. The small gap created by the friction tape significantly reduces magnetic pull, but you still get more shear force. Don’t skip the friction tape if you care about shear loading! But I didn’t get consistent friction coefficients as claimed by the physics books. My measurements were all over the place, ranging from 0.73 up to 3.1. Why the big spread? The friction coefficient is supposed to be independent of contact area, but I’m observing higher values when I have more contact area and the lowest values when contact area is the least. The high friction tape seemed to perform better with the polymagnets, but not at any other time, so I can’t identify a winner among the friction tapes.

Tilting can play a role in limiting the load a magnet can hold on a vertical surface. These magnetic hooks fail  not by sliding down the fridge but by tipping forward: the failure isn’t in shear.  I tested my wooden hooks (pictured above) on the veneered steel surface.  The bloodwood hook could support 1.75 lbs of force at the base of its 1.5″ rod but only 1 lb of force at the tip.  The chechen hook has a four times bigger magnet and could support 5 lbs at its base, but again only 1 lb at the tip of its longer 2.125″ rod.  When the load was at the tip it  usually failed by tilting, or it vibrated as it moved.  Perhaps even when magnets are sliding down the fridge they are tipped forward, compressing the friction tape at the bottom and moving the magnet away from the fridge at the top.

I’ll finish with some measurements of a few other polymagnets. I have a 1″ diameter, 1/8″ thick “attach” magnet. It has a marked side that I thought was supposed to be the active side. Pull force on the fridge was 7 lbs on the active side and 13.5 lbs on the reverse side. On the wood I got 3 lbs on the active side and 5.5 lbs on the reverse side. I also have a polymagnet with ring shaped magnetic regions designed so that a pair of them will center on each other. This magnet is 3/4″ in diameter and 1/8″ thick. With the marked side to the fridge it had a 9 lb pull force and with the reverse side only 4 lbs. On the wood, this magnet gave a pathetic 2 lb pull on the active side. A normal magnet of the same dimensions clung to the fridge with 2 lbs of force, and the wood with 2.5 lbs. The 1/2″ magnets I tested above are only 44% as big and yet 75% as powerful in this situation, so for hanging stuff on the fridge, at least, save money and use the smaller magnets—or spring for that centering polymagnet.

 

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