Shield For Permanent Magnet Structure

Abstract

A shield or guide to reduce leakage flux in the vicinity of the air gap of a permanent magnet is provided by positioning magnetic sleeves or plates in engagement with surfaces of the permanent magnet adjacent the air gap. The sleeves or plates are polarized at their surfaces which engage the permanent magnet. Each sleeve or plate has its surface which abuts the permanent magnet of the same polarity as the portion of the permanent magnet which it engages. The sleeves or plates may be flexible. Ferrites and cobalt-rare earths are satisfactory magnetic materials for the sleeves and plates.

Description

BACKGROUND OF THE INVENTION

Magnetic flux from a permanent magnet is subject to straying or leaking. This is particularly the case where it is necessary for such flux to pass through a long air gap. There is no known material which can insulate magnetic flux in the sense that insulating materials can insulate an electrical conductor. Leakage flux is useless flux as far as uses of permanent magnets are concerned. Consequently, permanent magnets must be designed much larger than would be necessary if leakage flux could be eliminated or greatly reduced. It is an aim of this invention to provide a structure which includes a barrier to leakage flux.

SUMMARY OF THE INVENTION

The present invention provides a shield or guide to reduce leakage flux in the vicinity of the air gap of a permanent magnet. The permanent magnet is provided with sleeves or plates in engagement with its surfaces adjacent to the air gap. The engaging surface of the sleeve or plate possess the same polarity as the surface of the permanent magnet with which it is in contact. This establishes a barrier to leakage flux which would otherwise emanate from the magnet. The sleeves or plates may be formed of flexible material and may be in the form of ferrites or cobalt-rare earth materials. Preferably, the shield provided by the sleeves or plates should have a high coercive force and should have just sufficient magnetic potential to neutralize the magnetic potential producing the leakage flux.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 a hollow cylindrical alnico magnet has its north pole indicated by the letter "N" and its south pole indicated by the letter "S". Adjacent to the north pole is a sleeve 11 of magnetic material. Adjacent to the south pole is a sleeve 12 of magnetic material. The sleeves 11 and 12 are magnetically oriented to provide an exterior surface of one polarity and an interior surface of the opposite polarity. The polarity of the interior surface of the sleeve magnets 11 and 12 is the same as the polarity of the magnet 10 with which it is in contact. This has the effect of repelling what would otherwise be leakage flux emanating from the magnet 10. Thus, the sleeves 11 and 12 serve as barriers to leakage flux from magnet 10. The result is that the normal distribution of flux from the alnico sleeve is modified and a larger percentage of flux is returned through the inside of the alnico sleeve. This arrangement is typical of the arrangement used to focus an election tube. In an actual test case the weight of an alnico 8 sleeve to produce 1000 gauss in the center of sleeve (point H.sub.i) is 10 pounds with the conventional approach using no magnetic shielding. A design to incorporate shielding has an alnico 8 weight of 4 pounds and gives 700 gauss before application of shielding. As a flexible ferrite shield is applied the flux density increases to 1000 gauss. Three pounds of magnetic shielding is required. Consequently, there is an overall weight saving of three pounds or 30 percent and an economic advantage in that the cost of the flexible ferrite is less than one-third that of alnico.

The correct choice of thickness of magnetic sleeves 11 and 12 enables the flux density at the outer surface of the magnet 10 to be held at approximately zero. In this condition the magnetomotive force of the sleeves 11 and 12 exactly balances the magnetomotive force of the surface flux leakage of the magnet 10. This flux cancellation system will work for any permanent magnet system but confers maximum advantage in permanent magnet circuits having long limbs supplying flux to very high permeance air gaps.

FIGS. 2 and 3 illustrate the invention applied to a magnetic structure having a rectangular configuration. In this embodiment a pair of alnico magnets 13 and 14 have the polarity indicated by the letters "N" and "S", respectively. A steel member 15 provides a magnetic flux path. Immediately adjacent the north pole of the magnet 13 a plurality of magnetic plates 16, 17 and 18 engage the surface of the magnet 13. Similarly, a plurality of magnetic plates 21, 22 and 23 are in contact with the surface of the magnet 14 immediately adjacent the air gap between the magnets 13 and 14.

Each surface of the plate magnets 16-18 and 21-23 has the same polarity as the surface of the alnico magnet 13 and 14 with which it is in contact as indicated by the letters "N" and "S". Thus, the plate magnets 16-18 and 21-23 serve as guides and barriers for the magnetic flux emanating from the magnets 13 and 14.

The plates 16-18 and 21-23 are preferably composed of cobalt-rare earth magnetic materials prepared in accordance with U.S. Pat. Nos. 3,625,779, 3,639,181, and 3,652,343 which are incorporated herein by reference. The use of cobalt-rare earth magnetic plates as flux leakage barriers adds very little to the weight of the permanent magnet and greatly increases the effective flux output. Thus, a smaller and lighter magnet can produce the same effective flux output as a larger magnet which does not have the type of flux leakage barrier disclosed herein. In a high permeance gap, tests indicate that the useful gap density can be increased by 35 percent by using such a shielding approach.

While the invention has been described with reference to particular embodiments, it is obvious that there may be variations which properly fall within the scope of the invention. Accordingly, the invention should be limited in scope only as may be necessitated by the scope of the appended claims.

Claims

What I claim as new and desire to secure by letters patent of the United States is:

1. A magnetically shielded permanent magnet structure comprising:

a permanent magnet having an air gap;

and at least one flux guide of permanent magnet material outlying said magnet adjacent said air gap, said guide having magnetic poles oriented to provide counter magnetic potential directionally displaced 90.degree. to the flux of said magnet whereby leakage flux at said air gap is reduced.

2. A structure as claimed in claim 1 wherein the shielding sleeve is flexible.

3. A structure as claimed in claim 1 wherein the shielding sleeve is composed of a cobalt-rare earth alloy.

4. A structure as claimed in claim 2 wherein the shielding sleeve is composed of a ferrite magnet material.

5. A magnetically shielded permanent magnet structure comprising:

an elongated permanent magnet having an air gap;

and a magnetic shield for reducing leakage flux at said air gap, said shield comprising flat magnets positioned around, and in contact with, said elongated magnet structure adjacent said air gap, said flat magnets being magnetized in a direction whereby the interior surface of each flat magnet possesses the same polarity as the portion of the elongated magnet which it engages.