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Starrett magnetic base
Years ago an acquaintance took apart his Starrett magnetic base. When he
re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold |
On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. The characteristic that controls this is the product of the magnet length and the material Coercive Force - roughly speaking this controls the ability of the material to retain much of it's magnetic strength during or after exposure to an airgap in the magnetic circuit. A magnet is open circuit if it's on the bench with no pole pieces attached - the magnetic circuit is closed if a continuous path through soft iron connects North pole to South pole. All the older types of magnet steels and even the latest Alnicos behave this way and 50 to 90% loss of field strength from incautious disassembly is possible. Ceramic (ferrite) and rare earth (samarium etc) magnets behave differently. Their coercive force is so high that they can withstand being magnetically open circuited without serious loss of field strength. Jim |
On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan
Vordos" spake: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! SWAG: He didn't quite get the mag release assembled properly and the magnet wasn't seating to the bottom when down. As there are no wires, releasing of the Magic Smoke is -not- a possibility here. My SWAG is interpreted from ownership of a fine HF (Chiwanese) mag base (which has never been disassembled.) --------------------------------------------------------------- Never put off 'til tomorrow | http://www.diversify.com what you can avoid altogether. | Dynamic Website Applications --------------------------------------------------------------- |
Harold and Susan Vordos wrote in message ... Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold Because the type of magnet used in it lost its magnetism very quickly on dissassembly (low coercive force) when the pole pieces were removed. This site explains about magnet properties and coercive force. http://www.stanfordmagnets.com/magnet.html |
On Tue, 01 Feb 2005 06:17:55 -0800, Larry Jaques novalidaddress@di wrote:
On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan Vordos" spake: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! SWAG: He didn't quite get the mag release assembled properly and the magnet wasn't seating to the bottom when down. Or, he put one of the magnet assemblies in backwards? That or the whole flux-capacitor theory the other guy posted, that's good too. (???) |
Larry answered cryptically: "SWAG: He didn't quite get the mag release
assembled properly and the magnet wasn't seating to the bottom when down." And further, picquing our curiosity, he goes, "As there are no wires, releasing of the Magic Smoke is -not- a possibility here. My SWAG is interpreted from ownership of a fine HF (Chiwanese) mag base (which has never been disassembled.)" Now I, suspecting Larry was trying to interleague himself with co-responding English friends "Jim" and Dave, looked up "SWAG". Sure 'nuf, there it was in Newton's Telecom Dictionary: "Scientific Wild-Ass Guess". Bob ( how to you make the grin symbol?) Swinney "Larry Jaques" wrote in message ... On Tue, 1 Feb 2005 03:13:52 -0800, the inscrutable "Harold and Susan Vordos" spake: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! --------------------------------------------------------------- Never put off 'til tomorrow | http://www.diversify.com what you can avoid altogether. | Dynamic Website Applications --------------------------------------------------------------- |
On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable
spake: This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. Alnicos are semi-permanent magnets? Interesting. I'll have to read more about them. I'd never heard of them. --------------------------------------------------------------- Never put off 'til tomorrow | http://www.diversify.com what you can avoid altogether. | Dynamic Website Applications --------------------------------------------------------------- |
Larry Jaques wrote:
On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable spake: This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. Alnicos are semi-permanent magnets? Interesting. I'll have to read more about them. I'd never heard of them. They're permanent, just not forgiving. It was _the_ magnetic material until the hard-ferrite ceramics came about. AlNiCo can be cast, and magnetized in place (this is the exact flip side of being demagnetized in place), it requires a lot of length to develop the coercivity necessary to cross an airgap. Ferrite can be mass-produced cheaply, it's much less likely to demagnetize than AlNiCo, but it _can_ be demagnetized if you run too much current in a DC motor with ferrite magnets. Neodymium-Iron-Boron is tough stuff, "supermagnet" material. Its strong and nearly (magnetically) indestructible. It tends to temporarily loose magnetization with elevated temperatures; AlNiCo beats it out there. Physically it corrodes readily and it's very brittle, and if you let two magnets whang into each other the impact can shatter one or both magnets and let little sharp bits of shrapnel fly around. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com |
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On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable spake: This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. That's dibble and fluff. Alnico magnets do decay, albeit slowly (over years). But no magnetic base uses a "keeper" while it's in use (none I know of have a keeper, at all), or it wouldn't stick to anything. The keeper is designed to _complete_ the flux circuit, in order to lengthen the life of the magnet. If the circuit were completed with a keeper (shunting the field between the poles), the magnet couldn't stick to your work surface. He's confused. More likely, anyway (if the base is relatively new - say twenty years or less), it has a ceramic/ferrite magnet. They hardly decay in strength at all, with or without a keeper. LLoyd |
Jim sez: "Does this mean that turning the base to the "on" position when it
is not in contact with a surface to complete the magnetic circuit, will ruin it?" Naw! "Leakage" magnetic effect would still practically emulate a "keeper". Bob Swinney "jim rozen" wrote in message ... In article , says... This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
wrote in message ... snip---- This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. The characteristic that controls this is the product of the magnet length and the material Coercive Force - roughly speaking this controls the ability of the material to retain much of it's magnetic strength during or after exposure to an airgap in the magnetic circuit. A magnet is open circuit if it's on the bench with no pole pieces attached - the magnetic circuit is closed if a continuous path through soft iron connects North pole to South pole. All the older types of magnet steels and even the latest Alnicos behave this way and 50 to 90% loss of field strength from incautious disassembly is possible. Ceramic (ferrite) and rare earth (samarium etc) magnets behave differently. Their coercive force is so high that they can withstand being magnetically open circuited without serious loss of field strength. Jim Thanks, Jim. You likely hit the answer right on the head. This incident occured back in ealy '67----when mag bases were, indeed, Alnico. Harold |
On 1 Feb 2005 11:27:56 -0800, jim rozen
wrote: In article , says... This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. Does this mean that turning the base to the "on" position when it is not in contact with a surface to complete the magnetic circuit, will ruin it? Jim The "associated pole pieces" are the soft iron parts of the magnetic base. The geometry and physical arrangement of these pole pieces is carefully chosen so that there is just sufficient alternate path (i e leakage flux) to enable the magnet to operate close to it's maximum energy point when NOT in contact with an iron surface. This is the BH max point - it corresponds to the most efficient use of the magnet material Because of this, the magnet is not degraded when left 'ON' and not in contact with an iron surface. This deliberate leakage flux path acts as carefully chosen partial 'keeper' to protect the magnet from the demagntising effect of a wholly open magnetic circuit. When in close contact with an iron surface the overall magnetic circuit reluctance is very much lower and almost all the available flux is then diverted away from the designed in leakage path and now flows through the pole pieces into the work surface. Jim |
On Tue, 01 Feb 2005 19:44:47 GMT, "Lloyd E. Sponenburgh"
wrote: On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable spake: This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. That's dibble and fluff. Alnico magnets do decay, albeit slowly (over years). But no magnetic base uses a "keeper" while it's in use (none I know of have a keeper, at all), or it wouldn't stick to anything. The keeper is designed to _complete_ the flux circuit, in order to lengthen the life of the magnet. If the circuit were completed with a keeper (shunting the field between the poles), the magnet couldn't stick to your work surface. He's confused. More likely, anyway (if the base is relatively new - say twenty years or less), it has a ceramic/ferrite magnet. They hardly decay in strength at all, with or without a keeper. LLoyd The decay rate of Alnico magnets in a properly designed assembly is negligible. A familiar example is the use of Alnico magnets in moving coil meters which directly rely on the long term stability of the magnetic field strength. The leakage function of the pole pieces in partially closing the magnetic circuit is covered in my reply to the valid query raised by Jim Rozen. Pentagrid |
On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand
wrote: On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos" wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? Mark Rand RTFM Alnico magnets need about 8,000 ampere turns per inch length of magnet to fully re-magnetise. This assumes that you've arranged suitable lumps of soft iron or mild steel to form a closed magnetic circuit with one leg of the coil passing through the middle. The current needs only to be present for a few milliseconds. Jim |
On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos"
wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? Mark Rand RTFM |
"Harold and Susan Vordos" wrote in message ... All the older types of magnet steels and even the latest Alnicos behave this way and 50 to 90% loss of field strength from incautious disassembly is possible. Ceramic (ferrite) and rare earth (samarium etc) magnets behave differently. Their coercive force is so high that they can withstand being magnetically open circuited without serious loss of field strength. Jim Thanks, Jim. You likely hit the answer right on the head. This incident occured back in ealy '67----when mag bases were, indeed, Alnico. Harold Nope, he's completely off-base. Totally confused. LLoyd |
In article ,
Harold and Susan Vordos wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! The magnet needs a "keeper" to keep the magnetic flux high. The body works as one normally. When you pull the magnet out, you have to slide it into a mild steel cylinder as it exits the one in the base. It should be possible to re "pole" the magnet with a serious magnetic pulse applied to the pole pieces of the base once it is reassembled and switched to the "on" position. But it is probably cheaper to just buy a new one, if you don't have the materials on hand to build a device to "pole" the magnet. Now to see what others have said. :-) Enjoy, DoN. -- Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand wrote:
On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos" wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? The copper involved would probably cost more than a new mag base. |
On Wed, 02 Feb 2005 00:21:26 GMT, "Lloyd E. Sponenburgh"
wrote: Nope, he's completely off-base. Totally confused. LLoyd He's right on. Trolling? |
On 2 Feb 2005 18:23:27 GMT, Dave Hinz wrote:
On Tue, 01 Feb 2005 23:15:04 +0000, Mark Rand wrote: On Tue, 1 Feb 2005 03:13:52 -0800, "Harold and Susan Vordos" wrote: Years ago an acquaintance took apart his Starrett magnetic base. When he re-assembled it, it no longer worked, although the magnet still was one. No parts were lost in the disassembly, and everything was replaced where it belonged. I witnessed the problem, so I know it's true. Can anyone explain why it no longer functioned? If so, thanks! Harold As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? The copper involved would probably cost more than a new mag base. Depends mostly on the internal airgaps. Figure about 30 amp-turns per .001" of total airgap to saturate the steel parts and the alnico. (That's 15,000 oersted) Non-magnetic material (brass or aluminum) in the flux path counts as airgap. You can get several hundred amps out of an auto battery for the short time it would take to blow the wire. #16 or #14 wire might be about right; it'd fuse in a second or two but the job would be long done by then. 50 turns of #16 should get its attention. More turns of finer wire might work too, but take more winding. I won't strongly assert that this will work, but it's easy to try. |
Mark Rand wrote:
As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? I tried re-magnetizing a base a while ago. I screwed around with it a lot and never did get very good results. I dumped charged capacitors to get the large currents. It is definitely not worth the effort, but I knew that and was just doing it for kicks, er ... science, yeah "science". I used the wisdom of this group and posted about it. Google-group the RCM archives on "magnetic base", author "engelhardt". Bob |
On Tue, 01 Feb 2005 11:13:13 -0800, the inscrutable Tim Wescott
spake: Larry Jaques wrote: On Tue, 01 Feb 2005 11:53:54 +0000, the inscrutable spake: This is because it uses Alnico type metal magnets which must NEVER be removed from their associated pole pieces (not even for milliseconds!) unless you have the facilities to re-magnetise in situ after final assembly. Alnicos are semi-permanent magnets? Interesting. I'll have to read more about them. I'd never heard of them. They're permanent, just not forgiving. It was _the_ magnetic material until the hard-ferrite ceramics came about. To clarify, I was aware of alnicos (most speakers had them--for most of my pre-teen and teen years, anyway) but wasn't aware of the possibility of demagnetizing any "permanent" magnets. AlNiCo can be cast, and magnetized in place (this is the exact flip side of being demagnetized in place), it requires a lot of length to develop the coercivity necessary to cross an airgap. Ferrite can be mass-produced cheaply, it's much less likely to demagnetize than AlNiCo, but it _can_ be demagnetized if you run too much current in a DC motor with ferrite magnets. Neodymium-Iron-Boron is tough stuff, "supermagnet" material. Its strong and nearly (magnetically) indestructible. It tends to temporarily loose magnetization with elevated temperatures; AlNiCo beats it out there. Physically it corrodes readily and it's very brittle, and if you let two magnets whang into each other the impact can shatter one or both magnets and let little sharp bits of shrapnel fly around. I have some tiny NIB mags which are strong enough to give my fingers blood blisters when I'm careless with 'em. Amazing. I wish I'd had them during experiments when I was a small child. -- The clear and present danger of top-posting explored at: http://www.netmeister.org/news/learn2quote2.html ------------------------------------------------------ http://diversify.com Premium Website Development |
On Wed, 02 Feb 2005 16:20:30 -0500, Bob Engelhardt
wrote: Mark Rand wrote: As a corollary to all the comments, Anyone have an idea how many thousand ampere turns I might need to re-magnetise an old mag base after building a good core around it? I tried re-magnetizing a base a while ago. I screwed around with it a lot and never did get very good results. I dumped charged capacitors to get the large currents. It is definitely not worth the effort, but I knew that and was just doing it for kicks, er ... science, yeah "science". I used the wisdom of this group and posted about it. Google-group the RCM archives on "magnetic base", author "engelhardt". Bob If you want an economical method of re-magnetising a useful variant of the old little gem fuse blower trick works like a charm. For the first quarter cycle of the 50/60 Hz supply frequency the ordinary domestic supply will deliver a peak current many times the rated current of the primary fuse or circuit breaker without discommoding the fuse or tripping the breaker. A quarter cycle is ample time for a magnetising pulse. In the UK, domestic 240v ring mains are typically 30A fuse or breaker protected and feed a number of sockets each rated to deliver 13A. Plugs for these sockets can be fitted with a 3, 5, or 13A fuse. A 13A fused plug delivers a peak current of over 500A into a dead short in the instant before the fuse ruptures so that many tens of kilowatts of peak power is available. This is conveniently switched on by inserting the plug and safely switched off by rupture of the fuse. I've no experience of North American supplies but it's reasonable to expect that a similar technique is workable. The magnetising coil can be fairly elementary. There is a sizeable heat pulse into the coil but, provided the wire thickness is about twice as thick as the protecting fuse wire thickness, it doesn't get hot enough to matter. For small magnets ten or twenty turns of ordinary hookup wire is all that is needed. Two diodes are needed to control the polarity of the pulse and to damp out transient current reversals. The first diode is series connected anode to supply live, cathode to coil. The second diode is shunted across the coil, cathode to hot end (i.e.cathode to cathode). The series diode controls the polarity of the current pulse. The shunt diode acts as a reverse diode and provides a safe path for the inductive overswing which occurs at the abrupt cessation of the input current pulse. Surprisingly small diodes are adequate because of the very high non-repetitive single pulse current ratings of most power rectifiers. Motorola MR754 is typical. Although nominally a 6amp rectifier its single pulse peak current rating is 400A for 8mS! Theoretically you should carefully choose the resitance and inductance of the magnetising coil to achieve maximum magnetising capability within the peak current capabilities of the rectifiers. In practice, the available peak power is so high that it is sufficient to just cram turns into the available space and to ensure that, including connecting wire, a total of at least 20 ft of wire is used. Jim |
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"Larry Jaques" wrote in message ... To clarify, I was aware of alnicos (most speakers had them--for most of my pre-teen and teen years, anyway) but wasn't aware of the possibility of demagnetizing any "permanent" magnets. I'm in my late fifties. I have a fairly large collection of odd-n-ends alnico magnets from speaker pull-outs I've kept since my teens. They're all weaker than they were, but they're all still magnets, and still useful for holding drawings, etc. To those insisting that alnico magnets lose their strength almost instantly in the absence of a keepe, consider: Millions of kitchen pot holders were made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4" alnico bar magnet within the hanging loop (for hanging the thing on your fridge or oven door). They were made, shipped, stored, and usually used in the home without any keeper of any sort -- usually when thrown in a drawer, they didn't even have the benefit of being near a ferrous substance. But they kept their strength - enough to hang the things up - for _years_. LLoyd |
On Thu, 03 Feb 2005 13:01:39 GMT, "Lloyd E. Sponenburgh"
wrote: "Larry Jaques" wrote in message .. . To clarify, I was aware of alnicos (most speakers had them--for most of my pre-teen and teen years, anyway) but wasn't aware of the possibility of demagnetizing any "permanent" magnets. I'm in my late fifties. I have a fairly large collection of odd-n-ends alnico magnets from speaker pull-outs I've kept since my teens. They're all weaker than they were, but they're all still magnets, and still useful for holding drawings, etc. To those insisting that alnico magnets lose their strength almost instantly in the absence of a keepe, consider: Millions of kitchen pot holders were made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4" alnico bar magnet within the hanging loop (for hanging the thing on your fridge or oven door). They were made, shipped, stored, and usually used in the home without any keeper of any sort -- usually when thrown in a drawer, they didn't even have the benefit of being near a ferrous substance. But they kept their strength - enough to hang the things up - for _years_. LLoyd Just because they shipped magnets without keepers doesn't mean that they are at their fullest possible strength. And if alnicos they weren't. I've seen the mag base effect where taking one apart weakens the magnet. Curious, I asked an engineer I know about it and he told me what had happened. A similar effect can be seen in stepper motors. They are assembled and then magnetised. If the motor is taken apart it will lose much of it's torque. I've been told that some of the newer motors are not as sensitive to this. Even if this is true, the motors I modify on a regular still come with instructions saying not to remove the rotor. The rotors are not alnico though. ERS |
On Thu, 03 Feb 2005 13:01:39 GMT, "Lloyd E. Sponenburgh"
wrote: I'm in my late fifties. I have a fairly large collection of odd-n-ends alnico magnets from speaker pull-outs I've kept since my teens. They're all weaker than they were, but they're all still magnets, and still useful for holding drawings, etc. To those insisting that alnico magnets lose their strength almost instantly in the absence of a keepe, consider: Millions of kitchen pot holders were made in the fifties, sixties, and seventies with a small 3/16^2 x 3/4" alnico bar magnet within the hanging loop (for hanging the thing on your fridge or oven door). They were made, shipped, stored, and usually used in the home without any keeper of any sort -- usually when thrown in a drawer, they didn't even have the benefit of being near a ferrous substance. But they kept their strength - enough to hang the things up - for _years_. The flux density in those alnico household magnets is considerably less than that of saturated alnico. It became so almost instantly when removed from the magnetizer at the factory, before it ever arrived at the store. |
In article , Don Foreman says...
The flux density in those alnico household magnets is considerably less than that of saturated alnico. It became so almost instantly when removed from the magnetizer at the factory, before it ever arrived at the store. Anyone who works on magnetos knows that the rotors should always be stored with 'keepers' on the pole pieces. I always wondered what would happen if I took one of those old mags, and set it up with some of the new supermagnets. Jim -- ================================================== please reply to: JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com ================================================== |
On 3 Feb 2005 09:53:47 -0800, jim rozen
wrote: In article , Don Foreman says... The flux density in those alnico household magnets is considerably less than that of saturated alnico. It became so almost instantly when removed from the magnetizer at the factory, before it ever arrived at the store. Anyone who works on magnetos knows that the rotors should always be stored with 'keepers' on the pole pieces. I always wondered what would happen if I took one of those old mags, and set it up with some of the new supermagnets. Jim The keeper discussion still seems to be simmering gently so a few extra comments might be helpful. For the moment consider only the iron alloy based magnets. This includes Alnico, Alni and the earlier alloys using cobalt,tungsten or carbon additives. These alloys, when magnetised to saturation in a fully closed magnetic circuit magnetising jig, all retain about the same flux density after the magnetising field is removed. This is quoted in the manufacturers literature as the B remanence figure. It is typically about 12 Kilogauss. If, now, a small air gap is introduced there is an immediate drop in flux density which is only partially recovered when the air gap is returned to zero. Provided that any later experimentally introduced airgap is smaller than this first gap there is no further degradation. If however a subsequent air gap is greater than any preceding gap it ratchets the closed magnetic circuit flux density down further and establishes a new range of airgaps that the magnet can accept without further degradation. This is the behaviour that we observe when we disassemble a magnetic base or remove the armature from a servomotor that uses this type of magnet. It doesn't explain the role of keepers commonly used to protect magnets when not in use. Once a magnet has been exposed to the demagnetising influence of a large airgap, putting the keeper back on will NOT restore it to it's previous state. The keeper is there for a different reason. Permanent magnets exhibit their magnetism as a result of the saturating magnetising field forcing their randomly oriented internal magnetic domains into a nice orderly additive arrangement. As noted above this is fairly easily disturbed by the demagnetising effect of an airgap. It is also disturbed by mechanical shock and it's sensitivity to shock or stray external magnetic fields is increased if it's also fighting with the demagnetising influence of an airgap. The older permanent magnet materials which have low intrinsic coercive force are particularly sensitive to this problem and this is why you see the old schoolboy bar magnets and ancient magneto horshoe magnets religiously stored with keepers on when not in use. A keeper is also used occasionally to provide a temporary alternate flux path when it is necessary to remove part of the normal main flux path. This avoids exposing the magnet to the demagnetising effect of a large disasembled airgap. The keeper must, of course, be installed BEFORE the main flux path is removed. The mechanical shock effect is easily demonstrated in the workshop. Magnetise a screwdriver or a piece of hardened carbon steel by stroking it against a decent permanent magnet. Left undisturbed on the bench it will retain it's newfound magnetism indefinitely. Bash it hard against any sustantial lump of metal and much of its magnetism is immediately destroyed. Similar effects, but to a much smaller degree occur with ferrite and the rare earth supermagnets. With most designs, additional airgap induced degradation is not enough to matter. Designs using the old magnet materials can be uprated by changing to rare earth magnets but the vastly different optimum length to diameter ratio of the new material means that a pretty major mechanical redesign is necessary. Jim |
Very nice explanation, thank you. I've learned so much in this NG! Bob
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