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Tiny magnets for Hall effect tach - sources, adhesives?

Discussion in 'Embedded' started by Lewin A.R.W. Edwards, Jan 2, 2004.

  1. Where can one acquire small (20-30) qty magnets suitable for use in
    Hall effect tach type applications? I'm thinking of the ultra-slim
    magnet splinters you see on floppy drive spindles and VCR head drums,
    about 2mmx1mmx0.5mm. After numerous attempts on my junkpile motors, I
    have not yet succeeded in salvaging one intact :)

    In a related question, what are these normally attached with? I'm
    experimenting with motors with a max no-load speed of just under
    5000rpm, under temperature conditions expected to range from -10 to
    +50 Celsius. I'm thinking an epoxy resin is probably best, but I'd be
    curious to hear what other people use.
    Lewin A.R.W. Edwards, Jan 2, 2004
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  2. Since you just need splinters, why not shatter a larger magnet (steal
    one from the fridge...) by brute force? Shouldn't be too hard to do,
    given a solid surface and a sledge hammer ;->
    I'm not at all certain, but IMHO epoxy and cyan-acrylate
    ("super-glue") should both work out fine. Preferrably avoid using
    very much of either, ie have the mounting place fit the shape of the
    magnet as closely as possible.

    Which glue is better would depend on what you want to glue it on:
    super-glue works best between two brittle, inflexible items, becaust
    it's quite brittle itself. Epox is somewhat more flexible.
    Hans-Bernhard Broeker, Jan 2, 2004
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  3. Remove them with metal sheet and all.
    Then drop in muratic or nitric acid.
    The ceramic can withstand the acid no problem.
    Carefully grind away residual glue.

    Greetings Albert.
    Albert van der Horst, Jan 2, 2004
  4. There are people on www.ebay.com that sel the small 1/16" and 1/8"
    cylindrical or square magnets from time to time.
    That is where I got the ones I use.
    Also some of the small flexible magnets they use for advertising andsticking
    to things like the refridgerators, may work OK.
    You can cut these with a pair of sissors into small peices and stack them up
    even. But some of the hall effect sensors may not be sinsitive enough for
    Earl Bollinger, Jan 2, 2004
  5. For this purpose, I use Samarium Cobalt magnets from:

    My application is for outdoor use so I use acrylic adhesives such
    as Loctite Depend or 3M Dp-810. These work on a prettty broad range
    of substrates.
    Gary Reichlinger, Jan 2, 2004
  6. magnet splinters you see on floppy drive spindles and VCR head drums,
    I have tried cutting a very thin piece off a flexible fridge magnet,
    and also tried to saw a piece off a neodymium magnet out of a hard
    disk, but the resulting speck is not strong enough to actuate the Hall
    effect sensor. So I think I need to buy it pre-manufactured in the
    right size.
    Yeah, this is why I wanted to use the flexible magnet :) If it falls
    off, the system reverts to an open-loop control system, and this is
    going to lead to big inaccuracies. The principal application is
    direct-drive of a boat propeller. I have no way of measuring the speed
    of the boat directly most of the time (because it is submerged, can't
    use GPS), and I found that PWM duty cycle vs. speed of vehicle isn't
    well correlated (because there are other variables, esp. water depth).

    The magnet will be mounted on the shaft of the motor itself.
    Lewin A.R.W. Edwards, Jan 2, 2004
  7. For this purpose, I use Samarium Cobalt magnets from:
    Excellent response, thanks.
    My application is also outdoor, and underwater :) In other words, I'm
    close to testing a "non-flight-qualified" version of my submarine.
    I've tested parts of it, with mostly satisfactory results, but I still
    have yet to solve the problem of preventing water ingress at the
    propeller and plane glands. The electronics and software are really
    simple compared to these horrid (and EXPENSIVE) mechanical engineering
    problems. I doubt I will have a fully demonstrable prototype by the
    time my book has to hit the publisher, but at least the subsystems
    work individually [more or less ;)] and it all works together on the

    With New York the way it is right now (you can see the Empire State
    Building from my back porch, which means I have military aircraft and
    police helicopters buzzing around all the time), it seems an
    inopportune time for field tests, anyway.
    Lewin A.R.W. Edwards, Jan 2, 2004
  8. Lewin A.R.W. Edwards

    Rich Webb Guest

    This is Usenet so, Without knowing any of the details of your
    application, I'll leap in with something that's probably obviously not
    workable if one's actually holding the application, but ...

    Why not an LED/photo-transistor detector pair? I'd imagine that painting
    alternating white and black areas around the shaft circumference would
    be a whole lot simpler than reliably mounting tiny magnetic chips.

    Or, if a Hall effect really is indicated, tear apart a muffin fan of the
    appropriate diameter (if it's a small fan and your app is a large
    motor). I sacrificed one recently (broken blade) to salvage the Hall
    sensor and found that the permanent magnet on the rotor was nothing more
    than a "refrigerator-style" magnet molded into a circle to fit inside
    the hub. You would have only two poles but would gain a much larger
    surface area for the adhesive.
    Rich Webb, Jan 2, 2004
  9. Lewin A.R.W. Edwards

    Rich Webb Guest

    Could you accept that some leakage is inevitable and install a bilge
    pump system? A small amount of leakage may even be desirable in that it
    will help to lubricate the shaft packing. Perhaps a small shield just
    inboard of the gland(s) to collect the leakage and pipe it to a bilge
    tank. Adds another variable to the buoyancy issue, of course, but
    designing to be leak-tollerant up to some defined rate (depending on
    depth and pump capacity) may be a Good Thing.
    Rich Webb, Jan 2, 2004
  10. Lewin A.R.W. Edwards

    Bob Stephens Guest

    How about pressurizing the hull?
    Bob Stephens, Jan 2, 2004
  11. You could try http://www.leevalley.com/ as they have a range of rare
    earth magnets in some quite small sizes.

    Mike Anton
    Michael Anton, Jan 2, 2004
  12. Lewin A.R.W. Edwards

    happyhobit Guest

    For small magnets try http://wondermagnet.com/ .

    I recently got some small (.0635 dia. X .03125 thick $1.25/10) for wheel
    encoders for a miniature robot I'm building.

    happyhobit, Jan 2, 2004
  13. Could you accept that some leakage is inevitable and install a bilge
    I do accept that some leakage is inevitable, and the electronics is on
    a raised platform to keep it dry as long as possible. I'm getting more
    leakage than I can deal with, though. So far the best solution I can
    come up with is to run the prop shaft through a piece of metal tube of
    just slightly larger bore than the shaft, and pack the space with
    viscous grease. It doesn't work very well.

    Due to expense and danger (I had a couple of narrow escapes from being
    shot by pieces of bursting fittings), I had to abandon my original
    plan for active depth control, which means I don't have an internal
    source of compressed air - so there's only so much pumping I can do.
    Lewin A.R.W. Edwards, Jan 2, 2004
  14. Lewin A.R.W. Edwards

    Jim Stewart Guest

    I can't help but think that this problem was
    solved about 80 years ago, and not by a bunch
    of EE's :)

    I am not familiar with propeller shafts, but
    I have seen the packing on old steam engines.
    They generally have something called a stuffing
    box or a packing gland, where grease-covered
    flax or cotton string is forced between the shaft and
    the box, sealing the joint but still allowing
    shaft motion.

    This link might be of help:

    Jim Stewart, Jan 2, 2004
  15. Lewin A.R.W. Edwards

    Rich Webb Guest

    What about a variation on a labyrinth seal? Should be manufacturable
    with a drill press/hole saw rig and some sheets of teflon.

    inboard outboard
    side_____ _ _ _____side
    | | |

    Where the # are teflon (or brass?) plates with a hole through the center
    slightly larger than the shaft and the o are o-rings that seal between
    adjacent plates.

    The | are teflon sheets that interdigitate with the above but are
    affixed to the shaft.

    The [] are the walls of the stuffing tube held together by a bolt circle
    right through, inboard to outboard.
    Rich Webb, Jan 3, 2004
  16. Lewin A.R.W. Edwards

    CBFalconer Guest

    In this part of the world attempting to acquire nitric or muriatic
    (hydrochloric) acid is likely to bring down the Ashcroft/Bushian
    forces and result in incarceration in the Gitmo archipelego with
    no further communication with the outside world, lawyers, etc.
    CBFalconer, Jan 3, 2004
  17. Last time I was in the building supply store, they still sold
    gallon jugs of muriatic acid without an ID check. It may
    not be reagant grade, but it is strong enough to etch concrete
    very rapidly (its intended purpose).

    Getting nitric acid of a purity and strength suitable for
    the manufacture of nitrate-based explosives is probably
    more difficult. None of the procurement problems are
    anything that couldn't be solved by an attentive student
    of a college freshman course in chemistry. And you can
    get the same education in a lot of places without professors.

    Mark Borgerson
    Mark Borgerson, Jan 3, 2004
  18. Your problem is that you have to seal around your driveshaft, because you put
    your motor in the dry space.

    Have you considered going to motors that were DESIGNED for unprotected
    submersion, putting them OUTSIDE the dry space, and just running non-moving
    electrical connections?

    There was a do-it-yourself dive scooter project many years ago that used a
    salvaged trolling motor just this way.
    John R. Strohm, Jan 3, 2004
  19. Well, in this part of the world you can get muriatic acid in a
    diluted form in the super market.
    If you add household chlorine (sodium hypoclorate) it starts to
    stink. This is extremely unhealthy for any metal (even gold)
    and yourself.
    Albert van der Horst, Jan 5, 2004
  20. Hi John,
    I thought about it, but they're really expensive. Also, I need stepper
    motors as well as straight DC motors - search for "waterproof stepper
    motor" and see just how incredibly expensive they are!

    Besides, this engineering problem has solutions that work - I just
    haven't found one that lies within my mechanical skills yet.

    I have here on my desk some sealed bearings that, if mated with an
    exactly fitting shaft, perform very effective water exclusion up to
    2atm, but unfortunately I need 3atm - and I would prefer to have an
    actual "rated for xxxkPa" part rather than just testing some scrounged
    parts. Oh, well. I'm just about to order the "chassis" (skeleton) for
    the preflight version, I don't have to make final decisions on all of
    this until a little later - there's plenty of room to add extra
    sealing hardware, or to bolt on outboard motors, if necessary.
    Lewin A.R.W. Edwards, Jan 7, 2004
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