Development of a Simple Measuring Device of the Attractive Forcewith Magnetic Attachment
K. Shoji，Y. Nakamura, R. Kanbara, T. Iwai, Y. Takada 1, Yu. Ohno, Y. Tanaka
Department of Removable Prosthodontics School of Dentistry Aichi-Gakuin University*Division of Biomaterials, Tohoku University Graduate School of Dentistry

Introduction
The establishment of the attractive force measurement and measurement method is
considered important to quantitatively describe the characteristics of dental magnetic attachment
products. Various studies have been conducted to obtain accurate attractive force measurementssince the development of magnetic attachments. The combination of Aichi-Gakuin testing jig andhigh-performance tensile tester is considered the most effective method of attractive force
measurement of a dental magnetic attachment. The method meets ISO standards. Although
Aichi-Gakuin testing jig allows high accuracy measurement, it lacks versatility due to its
complicated biaxial structure and high cost.
An improved jig was devised by ISO task force of the Japanese Society of Magnetic
Application in Dentistry to solve problems of Aichi-Gakuin testing jig. The improved jig expandedversatility by simplifying the structure from biaxial to uniaxial, and lowering the cost. Although theversatility of a jig was improved, it still requires a high-performance tensile tester. Since the ISO
measurement system requires an expensive tensile tester, it lacks versatility due to difficulty ininstalling the device in institutions, introducing worldwide, and its difficult portability.

ObjectiveThe purpose of the present study was to develop a simple measuring system of theattractive force introducing a tensile tester with a built-in jig to expand the versatility of theconventional ISO measurement system, and to evaluate the measurements using the system. The
new system is designed to be cost effective, easily fabricated, and compatible with the round-robintest in each country.

Material and method
The following figures show a simple measuring system of the attractive force introducing

a tensile tester with a built-in jig developed in the present study. The total weight of a device is 5 kg,
and the size is 120 mm in width, 105 mm in length, and 370 mm in height ( Fig. 1 ).

Fig.1 a simple measuring system of the attractive force
The measurement value was displayed by the Imada digital force measurement gauge up to 50 Nwith measurement accuracy of ±0.2% FS ( Fig. 2 ).An uniaxial linear ball bearing is embedded in
the device so that the friction resistance can move minimally in the longitudinal direction during
measurement ( Fig. 3 ). A magnetic assembly was embedded in the mold bonded with a digitalforce gauge by a movable joint in the upper part of a magnet assembly, and a keeper was installedin the lower mold with X-Y stage. The lower part of the device with a keeper starts free-fallmovement by a measurement start screw ( Fig. 4 ).

Fig.2 Digital Force Gauge (IMADA)

Fig. 3 linear ball bearingFig. 4 measurement system
The lower part of the device was supported using a dash-pot (KINECHECK, Meiyu Airmatic Co.,LTD) to maintain a constant speed ( Fig. 5 ).
The Fig. 6 shows the inner structure of a dash-pot. The falling velocity was maintained at aconstant speed by oil pressure during indentation, and the piston was returned to the original
position by the restoring force of the spring. The velocity can be changed by altering the size of oil
aperture ( Fig. 6 ).

Fig.5

Dashpot (KINECHEK, meiyu)

Fig.6 the inner structure of a dash-pot

Oil pressure of a dash-pot was controlled, and a scale at a cross head speed of 5 mm/min or lesswas confirmed in the preliminary experiment. The amount of time before the device sank by 5 mm
was measured to calculate the measurement speed (mm / min).
The cross head speed was determined at 4.8 mm / min (13 on the scale) based on themeasurement results of oil pressure scale ( Table 1 ).

Table1

The cross head speed was determined by screw HydraulicControl

The measurement results of a tensile tester with a built-in jig, a simple measuring device ofattractive force, were compared with the results obtained by conventional two systems including
the combination of Aichi-Gakuin testing jig and tensile tester and improved ISO jig and tensile
tester ( Fig. 7 ). Measurement samples were (GC) ( Fig. 8 ).

Fig.7 Comparison of measuring methods

Fig.8 Measurement samples( GIGAUSS D600 and C600 )Results
The attractive force measurement of GIGAUSS D600 using a simple measuring device
was 4.04 N on average. The results showed significantly smaller attractive force compared withthe other two measurement methods, and large standard deviation ( Fig. 9 ).GIGAUSS C600
demonstrated the similar results with attractive force measurement of 4.57 on average ( Fig. 10 ).

Fig. 9 Results GIGAUSS D600

Fig.10 Results GIGAUSS D600

Discussions
The attractive force measurement using a tensile tester with a built-in jig confirmed thatthe crosshead speed during measurement can be controlled by a simple device. Fall velocity wascontrolled at a constant speed to avoid acceleration by a dash-pot, and vertical and horizontal
directions were regulated by linear ball bearing. The crosshead speed under 5 mm/min (ISOstandard) was accurately reproduced. The detaching movement between a magnetic assembly
and a keeper, which is considered the most important element of attractive force measurement,
was controlled. The previous studies showed that the most challenging part of attractive force
measurement was to control the unique movement of magnets, and to transmit the attractive force
to the load cell in the vertical direction. Therefore, overcoming these challenges was considered
the biggest achievement of the present study. Although the simple measuring device used in thepresent study controlled the detaching movement between a magnetic assembly and a keeper,
vertical and horizontal direction could not be controlled since the movable joint was used during
accurate transmission of the attractive force to the load cell. As a result, measurement values weresmall, resulting in poor reproducibility. This problem is considered to be solved by incorporating
ISO jig into the simple measuring device.
Another achievement of the present study was that the device is portable and mailable
due to its size and weight. Therefore, the same device can be used for the measurement
domestically and internationally to compare the results. The device can be fabricated at various
institutions due to its simple structure and low cost, and the measurement procedure is simple.

Conclusion
The simple measuring device was designed and the prototype of the device wasfabricated. The present study using the prototype device established the foundation of morecost-effective and versatile attractive force measurement system, and elucidated improvement
points. Further improvements of the device are being pursued to increase availability of thepresent system.

References
1. Tanaka, Y.: Dental Magnetic Attachment - new prothodontic treatment used the magnet -,Ishiyaku Publishers, Inc. (Tokyo), 1992.
2. Tanaka, Y.: Dental Magnetic Attachment, Q＆A, Ishiyaku Publishers, Inc. (Tokyo), 1995.
3. Nakamura, Y.: Stress analysis of overlay denture and a magnetic attachment using finite
element method. J Jpn Prosthodont Soc, 42: 234-245, 1998.