Alnico Shallow Pot Magnets
For holding, fixing, gripping applications
A Pot Magnet is a typical magnetic assembly. It is usually a two part component comprising a magnet within a cup-shaped ferromagnetic housing. The ferromagnetic ‘cup’ is often a plated mild steel material but ferritic stainless steel (400 series) is also used (popular for its corrosion resistance); other design variants exist.
The pot magnet is designed to clamp with high magnetic forces when in direct contact with ferromagnetic (e.g. mild steel) surfaces. The clamping / pull force, when in direct contact, is normally higher than just the equivalent magnets alone. This is because the magnetic circuit is optimised for clamping applications (by removing air gaps from the magnetic circuit).
The design of pot magnets is such that only one surface has the magnetic performance giving the clamping force – it is the side where the magnet is visible. If you cannot see the magnet, it is probably because pot magnets, when stored for transit, have protective mild steel keeper plates covering the magnetic face (sometimes with a plastic spacer between to aid separating the keeper plate – removing the keeper plate and spacer unleashes the magnetic performance).
At Eclipse Magnetics when we design pot magnets, we design them to provide maximum pull force for the magnet volume used – to do this we design out air gaps from the magnetic circuit of the assembly. The ferromagnetic cup carries magnetism around (taking it away from the surrounding air) and directing magnetism into the surface to be clamped to make a highly efficient complete magnetic circuit with minimal air gap (ideally zero air gap) to the surface to be clamped onto. So, if a magnet has its North face visible, the ferrous ring of mild steel around the outside of the magnet is acting like a South pole – in effect giving two poles on the clamping surface making a short distance between poles for magnetism to travel through the clamped surface.
Because of how pot magnets work, it does not matter whether the visible pole of the magnet is a North pole or a South pole, the surrounding steel ring creates the other pole and clamping is the same regardless of the polarity.
Pot magnets allow a wide array of means of fixing them onto customer’s components due to variations in the ferromagnetic cup housing design. They can include threaded bars (male threads), tapped threaded holes in the housing material (female threaded hole / boss / screwed bush / internal thread), countersunk holes for screws, counterbore holes for bolts and even blind ended for simply gluing into place – some versions are even precision ground on the outer diameter to allow an interference fit. The fixing means in the cup design may simply be a continuation of the same material used in the cup (mild steel or magnetic stainless steel) but other materials may be used as well, including brass, stainless steel and other metals, alloys and materials. It is important to remember that you cannot put threads onto magnets themselves because they are too brittle so cannot take the forces that threading requires.
Pot magnets also protect the magnets inside them, which really aids in high clamping force applications. The steel cup housings can take much higher forces, higher mechanical knocks, and higher loads, preventing the magnet from breaking or shattering if it were used on its own (magnets on their own are brittle and prone to breakage making them unsuitable for load bearing applications). The cup takes the forces instead of the magnet, protecting the magnet – ideally the pot magnet is produced such that is is ever so slightly inset relative to the face of the ring of the cup so any mechanical hits should be on the steel rather than the magnet limiting the breakage risk but not reducing the pull force that much (as insetting the pot magnet would introduce an air gap which would start to impact on pull force). Press fitting of pot magnets in place is feasible when using pot magnets (not possible with magnets on their own) – the cup protects the magnet and takes the compressive forces instead; specially produced pot magnets are required (which we offer).
Pot magnets are always rated by a clamping force (which is the maximum possible pull force the magnet can achieve). They are designed for clamping so they are rated in kg pull or N pull (1kg pull force is ~9.81N pull force). Although they contain magnets rating them by a magnetic field strength is never advised – you will measure large variations across the clamping face going from a North pole in the centre to a South pole surrounding it). Pot magnets, by having a ferrous cup surrounding the magnet do not ‘throw’ magnetic field very well away from the clamping face – the cup draws magnetism back to it rather than into the air (due to the magnetic circuit design) – this is why pot magnets clamp extremely well when in direct contact with the ferromagnetic material being clamped onto but as the gap between the pot magnet and clamped material increases the pull force drops off extremely rapidly. This property of pot magnets (best for direct clamping with no air gaps) is a reason why they usually are not ideal for use with triggering applications (such as sensors) and is the reason why field strength values are not given for pot magnets (there is almost always a better alternative solution for triggering applications such as sensors where a field strength at distance is required – we can help you on this is you need assistance).
Pot magnet pull forces are tested on high quality thick mild steel test parts. If the material being clamped onto is thin, the maximum pull force you will achieve may become less than the rated pull force – as the material gets thinner, the measured pull force reduces further. If this happens and you need more pull force, you either need to increase the thickness of the material you are clamping onto, use more pot magnets to multiply the force, or you could use more smaller pot magnets (which may work better with thinner materials).
If the material being clamped onto is of poor ferromagnetic quality (rusty, low magnetic permeability, curved, coated/painted, thin, meshed, etc) the ability of the magnetism to interact with the material to produce the high pull force becomes limited. So a pot magnet may never reach towards its rated maximum pull force value if the application isn’t ideal. For example, a 130kg rated pot magnet could pull up to that 130kg against very thick girders, but could be pulled off by hand if against thin sheet steel such as that on a fridge door (don’t try it though – you risk damaging the surface of the fridge and will dent the fridge metalwork as well).
Pot magnets could clamp to each other if you had versions with a North magnet pole visible attracting to a version with a South magnet pole visible. However, the surrounding ferromagnetic steelwork of the cup is usually close to magnetic saturation (to keep unit weight down) so you often don’t get twice the pull of a single magnet, but some amount between 1 and 2 times the force from a single pot magnet. You also have to get hold of pot magnets with poles reversed – normal pot magnets clamp directly to ferrous materials so the polarity does not matter; we can supply pot magnets with poles reversed to give North and South versions of the same design but it isn’t usually done (as such applications tend to be niche).
Pot magnets resist sliding better than many other magnets. As a general guideline rule, where a standard magnet takes one fifth (20%) of its direct pull force to start to slide over a surface (shear force), a pot magnet typically takes one third (33%) of its direct pull force to start to slide over a surface (shear force). And rubber / polyurethane coated pot magnets have even higher friction so have even higher shear forces (often having the ability to have a shear force exceeding the direct pull force). The rubber coated pot magnets not only offer superior slide resistance, they also help protect painted/coated surfaces from getting scratches. Pot magnets without such coatings can slide over surfaces risking scratch marks.
Pot magnets are therefore perfect for clamping and holding applications and for applications where the magnet may be exposed to high compressive forces (the pot helps to protect the magnet inside).
They do not work well when trying to clamp though larger air gaps. For the best pot magnet performance the pot magnets should be in direct contact with the mild steel they are to clamp onto (simply due to how pot magnets are designed). Likewise pot magnets are not really designed for pot magnet to pot magnet clamping – it is possible but we would have to produce North and South versions to get the required attraction (this would be a custom production service).
Ferrite, Alnico, NdFeB and SmCo materials are all used within pot magnets. Each type has its own properties and characteristics. Size for size, the NdFeB versions will be the most powerful and the ferrite versions will be cheaper but least powerful (size for size). SmCo versions will be not far behind NdFeB for performance but will have better temperature characteristics. Alnico versions will have a range of performances depending on their design but will be better for higher temperature applications.
Pot magnets can supply pull forces varying from less than 1kg to up to 130kg (higher is possible). Most pot magnets can work up to +80 degrees C (rubber coated version up to 60 degrees C). At higher temperatures either the magnet material becomes a limitation (standard NdFeB material limited to +80 deg C, but higher temperature grades can be used) or the production method may become the limitation (e.g. adhesive temperature limitation, or expansion coefficients causing problems). So some alnico pot magnets may be rated at 220 deg C (even though alnico may be fine up to 450-550 deg C) because the materials of the pot magnet design expand at different rates causing the assembly to risk becoming loose above that temperature. But to solve this we produce special pot magnets that can cope with up to 450-550 degrees C (for use in high temperature applications such as welding for example). We provide magnetic solutions for various applications and can take into account the environmental conditions.
The pot magnets then come with varying styles to suit nearly every application:-
With hooks or eyebolts, countersunk (for screws), through hole (for bolts and cap head screws), male and female threaded (for attachment to nuts or into threads), blind ended (for press-fitting), etc.
They may have variants to their names which usually describe how they work or their shape/style e.g. limpet pots which use a threaded hook to assist prising the magnet off from the clamped surface, shallow pots which are hot very thick, deep pots which are tall, bi-pole pots (a variant with a North and South half-moon mild steel poles and no magnet visible).
We stock a vast array of pot magnet designs – chances are that we already have an off-the-shelf solution to your requirements. But, because we produce and supply custom magnetic assemblies, we can supply pot magnets to a variety of sizes and performance criteria with the ability to add fixing means to meet the customer requirements. We make pot magnets to order.
See our range of Pot Magnets