Solenoid actuator

Abstract: A print head has a solenoid driving means which employs a filler mixed with a lubricating oil to fix an exciting coil to a core. The filler prevents the layer winding from being shorted due to the corrosion of the winding coating of the exciting coil and prevents a mechanical abrasion of the armature due to the shortage of the lubricating oil during a long time operation.

Description: The present invention relates to a solenoid actuator for driving a movable portion of the solenoid actuator utilizing electromagnetic force. Description of the Related Art: As a solenoid actuator there is a wire dot print head which is used as a printing means for a printer. The print head of this type is generally provided with armatures each connected to a print wire and a solenoid driving means for attracting the armature and releasing the attraction of the armature. The solenoid driving means comprises a permanent magnet and an electromagnet for canceling the attraction of the permanent magnet. The electromagnet comprises a core and an exciting coil which is wound around a convex region of the core. The exciting coil is fixed to the core by a filler such as an epoxy resin, silicone resin, or the like. The filler has a function of radiating heat generated in the exciting coil outside the solenoid driving means. However, when the epoxy resin is used as the filler, the epoxy resin is chemically resolved when the internal temperature of the print head exceeds 150 °C during printing operation whereby the resolved matter corrodes a covering member which is employed in the exciting coil, which generated such a problem that the layer winding of the exciting coil is shorted. Meanwhile, the silicone resin as the filler has a high viscosity and is very difficult to be filled uniformly inside the solenoid actuator. Since the silicone resin has an absorbency to a low polymer oil, it absorbs a lubricating oil which is applied between the core and the armature for preventing an abrasion caused by the striking between the core and the armature, which resulted in the abrasion thereof when a printing operation is carried on for a long period of time. When the internal temperature of the print head rises as high as around 100°C to 120°C due to the heat generated from the exciting coil for a long time printing operation, silicon in the silicone resin reacts on oxygen whereby an extracted low polymer cyclohexane compound enters between the core and the armature, which hinders the armature from moving. To solve the problems set forth above, there is a method to regulate the viscosity by mixing the low viscosity silicone oil with the silicone resin. However, such a problem remains unsolved in this method that the absorption of the lubricating oil by the silicone resin increases. SUMMARY OF THE INVENTION It is a first object of the invention to provide a solenoid actuator capable of preventing the layer winding of the exciting coil from being shorted. It is a second object of the invention to provide a method of manufacturing a solenoid actuator capable of completely filling a filler around the periphery of the exciting coil. To achieve the above objects there is provided a solenoid actuator according to claim 1 and a method of manufacturing a solenoid actuator according to claim 7. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of a solenoid actuator according to a preferred embodiment of the present invention; Fig. 2 is a graph showing a comparison test of lives of wire dot print heads in cases that' viscosity of a filler is reduced by a silicone oil and by an oil. Fig. 3 is a molecular formula of a silicon. DESCRIPTION OF THE PREFERRED EMBODIMENT A solenoid actuator according to a preferred embodiment of the present invention will be described hereinafter with reference to Figs. 1 to 3. Fig. 1 is a cross-sectional view of a spring charge type wire dot print head. In the same figure, a wire dot print head comprises a core 1, an annular yoke or first yoke which are layered on the periphery of the core 1, a second yoke 3 provided on the first yoke 2, a permanent magnet 4, a leaf spring 5 and a third yoke 6. The leaf spring 5 supports an armature 7 elastically. The core 1 has a convex portion 1a inside the wire dot print head and an exciting coil 9 is wound around the convex portion la for cancelling a magnetic field of the permanent magnet 4. The exciting coil 9 is embedded in a filler 10 whereby the exciting coil 9 is fixed to the filler 10 for radiating the heat. A temperature detecting thermistor 11 is also embedded in the filler 10 for detecting the internal temperature of the print head. The filler 10 is covered by a filler cap 12. A part of a patch, described later, adjacent to the convex portion 1a of the core 1 is missing for guiding wires of the exciting coil 9 and the thermistor 11 out of the print head. An operation of the wire dot print head will be described briefly hereinafter. When the exciting coil 9 is not energized, a magnetic flux of the permanent magnet 4 passes the first yoke 2, the core 1, the armature 7 and the third yoke 6 and returns to the permanent magnet 4 so that magnetic attraction is generated between the convex portion 1a of the core 1 and the armature 7. The armature 7 is attracted by the core 1 while it biases the leaf spring 5 by the attraction. When the exciting coil 9 is energized at this stage, the magnetic flux generated by the exciting coil 9 cancels the magnetic flux generated by the permanent magnet 4 between the convex portion 1a of the core 1 and the armature 7. The biased leaf spring 5 is released when the magnetic flux generated by the permanent magnet 4 was cancelled. As a result, a print wire 8 fixed to a free end of the armature 7 strikes a printing medium, not shown, by way of an ink ribbon where by a dot printing is carried out. When such a printing operation is carried out for a long time, the temperature of the interior of the wire dot print head rises due to heat produced by the current applied to the exciting coil 9 and an eddy-current loss generated in the core 1 so that the temperature of the exciting coil 9 rises. The thus generated heat is conducted to the thermistor 11 through the filler 10 and is detected by the thermistor 11 as temperature. The dot print head having the structure set forth above is manufactured as follows. The annular yoke 2 and the yoke 3 are layered on and fixed to the circumferential edge portion of the core 1 to thereby assemble the solenoid actuator portion of the wire dot print head. The exciting coil 9 is embedded around the convex portion 1a of the first yoke 2 while the thermistor 11 is disposed in the solenoid actuator portion of the wire dot print head at the position so as not to contact the core 1 or the exciting coil 9. At this state, the filler 10 is potted in the solenoid actuator portion of the wire dot print head. Since the filler 10 is low in the viscosity thereof, described hereafter, it penetrates into the gap defined around the convex portion 1a of the core 1 and spreads out in the lower half portion of the wire dot print head. In this case, the portion under the thermistor 11 is preferable to be clogged by the patch. The filled filler can be hardened by heating it for about three hours at 130 °C. Successively, a filler cap 12 is placed on the filler 10 while the permanent magnet 4, the leaf spring 5 and the third yoke 6 are layered on and fixed to the second yoke 3 in this turn. At this time, the permanent magnet 4, the leaf spring 5 and the third yoke 6 are preferable to be previously assembled as the upper half portion of the wire dot print head. The filler 10 will be described more in detail. A silicone resin, which has a molecular formula as illustrated in Fig. 3, is employed as the filler 10. A preferable silicone resin is, e.g. a product called as KE1204 manufactured by The Shin-Etsu Chemical Co. Ltd. and becomes red and gummy after it has been hardened. Two liquids are mixed with each other as a main agent and a hardening agent and they are hardened by heat after they are mixed with each other. The viscosity of the mixture of the main agent and the hardening agent is about 35 centipoise and does not penetrate the gap defined between the core 1 and the exciting coil 9 in this viscosity. In order to reduce the viscosity of the silicone resin, after two liquids are mixed with each other, the machine oil having a viscosity of 35 centipoise is mixed with the silicone resin wherein the ratio of the machine oil to the silicone resin is 3:10 by weight. As a result, the viscosity of the silicone resin is reduced to about 15 centipoise. It is possible to fill the filler 10 composed of the silicone oil into every nook and corner of the complicated internal structure of the wire dot print head with such the viscosity. When the print head, in which the filler 10 is filled, is more complicated, the amount or the viscosity of the machine oil to be mixed with the silicone oil are varied to thereby regulate the viscosity of the filler 10 so that the filler is sufficiently filled in the gap. The oil to be mixed with the silicone resin is preferable to be of the same component as the lubricating oil so as to prevent the abrasion caused by the striking between the metals which constitute the convex portion 1a of the core 1 and the armature 7 serving as the movable portion or the oil which does not react on the silicone resin. As the oil having the same component of the lubricating oil, there is the oil comprising mainly a synthetic hydrocarbon or a mineral oil, e.g. SHINLUB compressor oil SB (name of product produced by Nippon Steel Chemical Co., Ltd.). The filler cap 12 comprises a paper or a felt. Since the paper or the felt is superior in absorbing or keeping the oil, it is possible to supply the lubricating oil to the driving portion for a long time when the paper or the felt absorbs the lubricating oil sufficiently. Fig. 2 is a graph showing a comparative test of lives of wire dot print heads A to D employing the filler 10 comprising the mixture of the silicone oil and the silicone resin and having low viscosity and lives of wire dot print heads E to I employing the filler 10 according to the present invention. The silicone oil to be mixed with the silicone resin is a dimethyl silicone oil called as KF 96 (name of product produced by The Shin-Etsu Chemical Co., Ltd.). The lives of the wire dot print heads A to D usually expire when they print 20 million to 60 million characters while the lives of the wire dot print head E to I do not expire even if they print more than 200 million characters. The reason why the lives of the wire dot print head E to I are extended is that the lubricating oil, which is applied between the core 1 and the armature 7, is not absorbed by the filled silicone resin. On the contrary, the oil in the oversaturated state is stored in the silicone resin and this oil or the oil absorbed by the filler cap 12 is supplied to the movable portion as a lubricating oil. Accordingly, the abrasion in the movable portion is hardly generated. Even if the internal temperature of the wire dot print head exceeds 130 °C which is the hardening temperature when it is practically used, a volatile low polymer silicon compound is reduced in the absolute amount thereof since the machine oil is used as an additive so that the adverse effect caused by the low polymer silicon is reduced. Furthermore, since the sufficient amount of the lubricating oil is applied on the surface of the core 1, the internal temperature of the wire dot print is not liable to rise even if the striking between the core 1 and the armature 7 is repeated during the printing operation and production of silicon dioxide is reduced. In order to improve the reliability of the wire dot print head, the inner diameter of the annular second yoke 3 is increased and the filler cap 12 is composed of a rubber sheet which does not contain such a component as sulfur, nitrogen oxide, organic metallic salt and phosphoric compound which prevent the silicone resin from being hardened, wherein the filler cap made from a rubber sheet is placed on the filler 10 so as to completely cover the filler 10 and the second yoke 3 is clamped by the first yoke 2 and the permanent magnet 4. In such a manner, it is possible to shut the filler 10 from the atmosphere of the armature 7 serving as the movable portion. As a result, it is possible to restrain the low polymer silicon compound from volatilizing to thereby stabilize the operation of the armature. Since the filler 10 is shut off from the atmosphere, the moisture in the air is shut off so that the filler 10 does not absorb the moisture. Accordingly, the production of the silicone oxide is reduced so that more stable mechanical property can be obtained in the wire dot print head. If the filler cap 12 employes a magnetic body such as a metal plate, etc. instead of the rubber sheet, such an additional peculiar effect can be obtained that the filler cap 12 can control the magnetic flux generated by the permanent magnet 4. According to the embodiments set forth above, the filler employs two liquid type hardening silicone resin which hardens reacting on the moisture in the air but can employ one liquid type hardening resin, wherein the latter case has the same effect as the former case. Furthermore, the present invention is not only adapted in the wire dot print head but to an apparatus incorporating the solenoid actuator therein wherein the latter case has the same effect as the former case. Described hereinafter is an example of the case where a filler is employed to fix the exciting coil of a DC brushless motor. In this case, being caused in a wire dot print head, the exciting coil corrodes at high temperature and the low polymer filler sticked between the rotor and the stator impedes the rotary motion of the rotor. At this time, a silicone resin is employed as a filler while a lubricating oil such as machine oil, synthetic oil, etc. is mixed with a silicone resin, whereby a low polymer is less generated and the stable operation can be continued for a long time. As explained above, according to the present invention, since a lubricating oil such as synthetic oil, machine oil, etc. is mixed with a silicone resin, the following effects can be achieved. It is possible to fill the filler in a complicated internal structure and also possible to restrain the mechanical abrasion. It is further possible to eliminate an operation failure of the movable portion caused by the influence of a lower polymer compound which is produced from the filler. Still furthermore, the effects set forth above can be achieved by shutting off the filler from the interior space by forming a shut off layer therebetween. It is possible to restrain the filler from absorbing moisture whereby the adverse effect given to the coil in the actuator by the filler involved in moisture absorption can be eliminated.

Claims: A solenoid actuator comprising an exciting coil (9) wound around a core member (1a) composed of ferromagnetic substance, and a silicone resin as an internal filler for fixing the exciting coil to the core member and for embedding the exciting coil, characterised in that the silicone resin is oversaturated with low polymer lubricating oil. A solenoid actuator according to Claim 1, wherein the lubricating oil has the same component as a lubricating oil in the solenoid actuator. A solenoid actuator according to Claim 1 further comprising a filler cap (12) covering a surface of the filler for isolating the filler from the atmosphere inside the solenoid actuator. A solenoid actuator according to Claim 3, wherein the filler cap (12) comprises a fiber member saturated with the oil. A solenoid actuator according to Claim 3, wherein the filler cap (12) is made of a rubber sheet. A solenoid actuator according to Claim 3, wherein the filler cap (12) is formed of a magnetic substance. A method of manufacturing a solenoid actuator which is incorporated in a print head comprising the steps of : providing an exciting coil (9), which is wound around a core member (1a) composed of ferromagnetic substance, in a base body of the solenoid actuator; mixing a silicon resin with a low polymer lubricating oil in such proportion, that the silicon resin is oversaturated with the lubricating oil; dropping the mixture into the base body of the solenoid actuator, thereby fixing the exciting coil to the core member and embedding the exciting coil; and hardening the silicon resin.

Inventors: ANDOU HIROKAZU C O OKI ELEC IN, ISHIKAWA MASAYUKI C O OKI ELEC, ISHIMIZU HIDEAKI C O OKI ELEC, KISHIMOTO MITSURU C O OKI ELEC, ANDOU, HIROKAZU, C/O OKI ELEC. IND. CO., LTD., ISHIKAWA, MASAYUKI, C/O OKI ELEC. IND. CO., LTD., ISHIMIZU, HIDEAKI, C/O OKI ELEC. IND. CO., LTD., KISHIMOTO, MITSURU, C/O OKI ELEC. IND. CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1992
Main classification: H01F7
IPC codes: B41J2, H01F7