Evaluation of Magnetic Field Leakage out of a Cup Yoke Type of Magnetic Attachment

M. Nishida, Y. Tegawa¹ and Y. Kinouchi²

Faculty of Engineering, ¹Faculty of Medicine, ²Institute of Technology and Science,
The University of Tokushima, Tokushima, Japan

Introduction

The dental magnetic attachments are a device used in the mouth to retain a denture. The tissues in the mouth are exposed to the magnetic fields leaking out of the magnetic attachments for a long time. It may therefore be important to discuss biological effects of the leakage magnetic fields.

A cup yoke type of magnetic attachment is focused here for evaluating the leakage magnetic fields because the attachment is used most generally.

Materials and Methods

Fig.1. shows the longitudinal section and the dimension of the magnetic attachment considered here. The magnetic attachment is constructed by two parts, i.e., a magnetic assembly and a keeper. The shape of magnetic assembly is designed to be a disk with 4mm in diameter and 1.5mm in thickness, and a keeper is a disk with 4mm in diameter and 1mm in thickness. Magnet is a rare-earth magnet whose magnetization J is assumed to be 1.3 Tesla. Keeper is soft magnetic stainless steel with 1.6 Tesla in magnetic saturation flux density Bs. Direction of magnetization is indicated by an arrow. Non-magnetic stainless steel is also used to protect the magnet from corrosion in the mouth. The evaluation of magnetic force and magnetic field are based on numerical analysis for the attachment model of three dimensions using a finite element method (μ-MF, μ-TEC Co.,LTD.,Tokyo).

Results and Discussions

1. Optimized magnetic circuit and magnetic field

Fig.2 shows the leakage magnetic field for the optimized cup yoke type of magnetic attachment that the air gap is 0mm between a magnetic assembly and a keeper. It shows that the largest magnetic field more than 2mm from the out side the yoke is at the center of the height of magnetic assembly.

2. Magnetic circuit and magnetic field

Fig.3a. shows the leakage magnetic field V.S. the distance from the surface of the side yoke. It shows that the leakage magnetic field decreases for the increase of the distance, and for the increase of the side yoke Wy.

Fig.3b. shows the leakage magnetic field V.S. the distance from the surface of the side yoke. It shows that the leakage magnetic field does not exceed 40mT(WHO's guideline) ¹⁾ at the distance of 0.1mm from the surface of the magnetic assembly.

Fig.4 shows the leakage magnetic field density at the distance of 0.1mm, 1.0mm and 2.0mm from the outer side of the yoke and the attractive force V.S. the length of the yoke Wy, and also shows the variation of the attractive force to the Wy. The largest attractive force is obtained at 0.425mm in Wy. The length is getting larger or smaller than that, the attractive force decreases. It means that the most efficient yoke length for the largest attractive force is 0.425mm. The leakage magnetic field density becomes larger as the distance from the outside the yoke smaller.

3. The length of the yoke and magnetic field contour

Fig.5a-5c show the leakage magnetic field when the Wy varied from 0.35 to 0.45mm. Fig.5a shows that the leakage magnetic field density at the side of the yoke of the magnetic assembly is larger than that of the top of the magnetic assembly. This means that the magnetic resistance of the side yoke is large for the small length of the yoke (Wy=0.35mm). And so the leakage magnetic field density becomes large at that place. Fig.5b shows that the leakage magnetic field density at the side of the yoke of the magnetic assembly is a little smaller than Fig.5a, and that of the top of the magnetic assembly is a little larger than that of Fig.5a. This means that the magnetic resistance of the side yoke becomes smaller for the large length of the yoke (Wy=0.45mm). And so the leakage magnetic field density becomes smaller than the case in fig.5a. Fig.5c shows that the leakage magnetic field exists only the top of magnetic assembly, and doesn't exist at the side of the yoke. The larger length of yoke (Wy=0.45mm) means the smaller magnetic resistance at that part. It causes the small leakage magnetic field.

4. The air gap and magnetic field contour

Fig.6a-6c show the leakage magnetic field when the length of the air gap between a magnetic assembly and a keeper varied from 0mm to 0.1mm. When the gap is zero (Fig.6a), the leakage magnetic field is invisible at the outer side the contacting line of a magnetic assembly and a keeper. According to the increase of the air gap between a magnetic assembly and a keeper, the leakage magnetic field at the out side the contacting line becomes large (Fig.6b). And it spreads out (Fig.6c).

Fig.7 shows the leakage magnetic field V.S. the distance from the air gap. The distance from air gap means the length from the edge of the contacting line between a magnetic assembly and a keeper. The leakage magnetic field becomes large as the gap length. The density will not exceed the WHOÕs guideline 40mT when the distance from the air gap is larger than 0.13mm (air gap =0.01mm) and 0.3mm (air gap=0.1mm).

Conclusions

We evaluated the leakage magnetic fields of cup yoke type of magnetic attachment by changing the parameter of the magnetic circuit.

1) The leakage magnetic field is smaller and the attractive force is largest when the magnetic circuit is optimized.

2) The leakage magnetic field becomes large when the gap length between a magnetic assembly and a keeper becomes long.

According to the WHO's guideline in “INTERNATIONAL GUIDELINE ON EXPOSURE TO STATIC MAGNETIC FIELD”¹⁾ , the limits of exposure to static magnetic fields for the general public in continuous is 40mT.

3) Leakage magnetic fields will not exceed the WHO's guideline 40mT when the distance between a magnetic attachment and the tissue is 0.4mm or more, even if it exists the air gap of 0.1mm between a magnetic assembly and a keeper,

References

1. Environmental Health Criteria; 232 STATIC FIELDS:WHO 2006.

Discussion Board