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# Re: [femm] Re: Incorrect force direction

- To: <femm@egroups.com>
- Subject: Re: [femm] Re: Incorrect force direction
- From: "Greg Watson" <gowatson@optusnet.com.au>
- Date: Thu, 4 Jan 2001 08:18:45 +1030
- References: <5b.fefc9e5.27823eac@aol.com> <000901c07451$cd9c00e0$52e78ec6@ibmbn145hv> <3A534275.E66BF094@foster-miller.com>

    ----- Original Message -----
    From: "David Meeker" <dmeeker@foster-miller.com>
    To: <femm@egroups.com>
    Sent: Thursday, January 04, 2001 1:47 AM
    Subject: Re: [femm] Re: Incorrect force direction

    > Anyhow, I modelled your problem on Amperes by IES, which is a rather expensive
    > commercial 3-D magnetostatic solver based on boundary elements, rather than
    finite
    > elements. This ought to give a good idea of how things would run in a real
    3-D test
    > rig, assuming everything is carefully aligned. I arbitrarily made the machine
    1"
    > deep in the "into the page" direction, for the purposes of making a 3-D
    geometry. I
    > chose a relatively fine density for the surface elements, and then set it to
    > evaluate the forces on the magnet at a bunch of different axial positions.
    What it
    > predicts is that the centring force peaks when the front edge of the magnet is
    just
    > past the iron, with the centring force dropping to near zero when the magnet
    is
    > completely between the iron bars. femm predicts the same sort of behaviour,
    except
    > that femm predicts the peak force to be more like 15 Newton/inch, rather than
    9 N.
    > The force from the 3-D geometry is smaller than in the 2-D model because some
    of the
    > magnet's flux leaks into the sides of the bars--this is an intrinsically 3-D
    effect
    > that can't be modelled by a 2-D solver.

    Hi David,

    Thanks for the effort.

    My concern was not that FEMM should report a large centring force but that a
    program which I normally have found to be very realistic would report a
    repulsive & not a centring force, no matter how fine the mesh or how much care I
    took in making sure the geometry of the mesh and / or of the model didn't effect
    the result.

    My duplication involved a round Neo, 4 on point right angle plastic guides hot
    glued to the vertical faces of 2 right angle steel pieces (two guides per face,
    1 top, 1 bottom). Alignment isn't too critical. Although the plastic guides
    aren't ideal, you can get a feel for the forces by the amount of effort needed
    to roll the magnet back & forth between the guides.

    Greg

- Follow-Ups:
  - Re: [femm] Re: Incorrect force direction
    - From: "Steven Stretz" <stretzsj@execpc.com>

- References:
  - Re: [femm] Re: Incorrect force direction
    - From: dcm3c@aol.com
  - Re: [femm] Re: Incorrect force direction
    - From: "Greg Watson" <gowatson@optusnet.com.au>
  - Re: [femm] Re: Incorrect force direction
    - From: David Meeker <dmeeker@foster-miller.com>

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