Patent Description:
Examples of stator units according to the related art are disclosed in <CIT> (Patent Literature <NUM>) and <CIT> (Patent Literature <NUM>).

The stator unit (coil for a motor-operated valve) disclosed in Patent Literature <NUM> includes a cover and a stator. The cover is in the shape of a cap. The stator has a hollow cylindrical shape. The stator includes a metal component, a coil wire, and a mold made of rigid resin. The peripheral part of the body constituted of the metal component and the coil wire is covered with the mold. The stator is accommodated in the cover. The space between the cover and the stator is filled with filling resin that is flexible.

The process for manufacturing such a stator unit may include the steps of: (<NUM>) forming a stator by insert molding; (<NUM>) placing the stator in a cover; (<NUM>) inserting a mandrel (jig) into a stator to position the cover and the stator; (<NUM>) clamping the mandrel and the cover; (<NUM>) pouring filling resin in liquid form into the cover with its lower end facing upward; and (<NUM>) removing the mandrel after the filling resin sets. The stator unit obtained by this process is in finished form.

Likewise, the stator unit (electromagnetic actuator) disclosed in Patent Literature <NUM> includes a cover and a stator. The space between the cover and the stator is filled with filling resin.

The stator described in Patent Literature <NUM> is formed by insert molding in such a manner that a gap on an internal circumferential surface of a metal component is filled with rigid resin. The stator includes an annular rib. The rib fits into an annular groove in an upper-end wall portion of the cover. When filling resin in liquid form is poured into the cover, the rib in the groove keeps the filling resin from leaking into the stator. This structure eliminates the need for a jig such as a mandrel during the manufacturing of a stator unit.

It is required that the stator included in the stator unit disclosed in Patent Literature <NUM> and the stator included in the stator unit disclosed in Patent Literature <NUM> be both formed by insert molding. More man-hours are needed to manufacture such a stator unit, thus leading to an increase in the cost of manufacturing. The need of clamping the mandrel and the cover during the manufacturing of the stator unit disclosed in Patent Literature <NUM> causes a further increase in the cost of manufacturing.

The stator unit disclosed in Patent Literature <NUM> eliminates the need for a jig. To that end, the gap on the internal circumferential surface of the metal component of the stator is filled with rigid resin. Unlike the case with filling resin, cracks are likely to form between the rigid resin and the metal component. Thus, this structure may fail to prevent moisture from entering the stator.

<CIT> discloses a stator unit and a motor-operated valve with the same, wherein a detent member comprises: an annular flat plate part; detent engage piece parts provided in three or more engage plate parts which are provided from an inner peripheral side of the annular flat plate part and bent to a direction opposite to side of a yoke; and three or more projected piece parts which are bent up from the annular flat plate part to the side of the yoke. Distal end portions of the projected piece parts are welded to the yoke.

<CIT> discloses a water proof coil for an electric motor operated valve. An electro-magnetic coil body portion is so constructed to be sealed up as against the open air, wherein a cover which encloses a top part and a side part of the coil body portion is made of synthetic resin of cuplike form as a whole, wherein further when the electric motor operated valve is assembled with a bottom side of a lead wire drawing part being sealed by the soft synthetic resin materials and with the electric motor operated valve main body being inserted into the coil main body through an opening formed in the sealed part, the outer periphery of the electric motor operated valve is arranged in close contact with the internal circumference of the opening, whereby water is prevented from being encroached into the interior of the electro-magnetic coil body portion.

<CIT> discloses a stator of a drive motor for an electrically operated valve, wherein the stator is integrally formed with a resin mold by arranging a stator assembly within metal molds and injecting a molten resin from a gate at one position. An annular plate is mounted to an upper portion of the stator assembly so as to prevent the molten resin from intruding into magnetic pole teeth and prevent a weld line from being generated in an outer peripheral surface of the resin mold.

<CIT> discloses an electromagnetic drive coil unit and a molding method thereof. In a method of molding an electromagnetic drive coil unit in which a stator assembly includes bobbins having stator coils wound thereon and yokes in which an inner yoke having magnetic pole teeth and an outer yoke having magnetic pole teeth are combined and in which the stator assembly and a power supply terminal supplying power to the bobbins are molded in a resin, an inner molding process is performed on the inside of the outer yoke in a state where ends of the stator coils of the stator assembly are connected to the power supply terminal and an outer molding process is performed to cover an outer circumference of the stator assembly having an inner molded portion and an outside of the power supply terminal after the inner molding process.

<CIT> discloses a motor eliminating the use of a complicated metal mold, including more design freedom. <CIT> discloses a mold forming method and a vacuum pump equipped with a stator column manufactured by this method.

It is an object of the present invention to provide a stator unit and a motor-operated valve that are highly waterproof and enable a reduction in the cost of manufacturing. It is another object of the present invention to provide a method for manufacturing the stator unit.

To that end, an aspect of the present invention is directed to a stator unit comprising the features of claim <NUM>. The stator unit includes a cover, a stator, and a mold. The cover includes a circumferential wall portion and an end wall portion contiguous with an end of the circumferential wall portion. The stator is accommodated in the cover and has a hollow cylindrical shape. The mold is made of filling resin with which the inside of the stator and the space between the stator and the cover are filled. The end wall portion has a dome portion, the cover includes a rib which has an annular shape and protrudes from an inner surface from the dome portion, the rib including a circumferential surface that faces outward. The circumferential surface defines a region having an axis that coincides with an axis of the stator. The region defined by the circumferential surface has a diameter equal to or smaller than the inside diameter of the stator.

The circumferential surface preferably has a height, in the axial direction of <NUM> to <NUM>.

The stator preferably has an internal circumferential surface covered with the mold.

Another aspect of the present invention is directed to a motor-operated valve. The motor-operated valve includes a valve main body, a can, and a stator unit. The can is attached to the valve main body. The stator unit is mounted on the outside of the can. The stator unit is the stator unit described above.

To attain the aforementioned objective, still another aspect of the invention is directed to a method for manufacturing a stator unit. The stator unit includes a cover and a stator having a hollow cylindrical shape. The cover includes a circumferential wall portion and an end wall portion contiguous with an end of the circumferential wall portion. The end wall portion has an inner surface including a circumferential surface that faces inward. The circumferential surface defines a region having an axis that coincides with an axis of the stator. The region defined by the circumferential surface has a diameter equal to or smaller than the inside diameter of the stator. The method includes: placing the stator in the cover; inserting, into the stator, a jig having a coefficient of linear expansion higher than a coefficient of linear expansion of the cover and having a hollow cylindrical shape or a solid cylindrical shape; placing the jig in such a manner that an external circumferential surface at a tip of the jig faces the circumferential surface; heating the cover and the jig in such a manner that the external circumferential surface at the tip of the jig and the circumferential surface come into contact with each other along an entire circumference; and pouring filling resin in liquid form in a state in which another end of the circumferential wall portion of the cover faces upward in such a manner that the inside of the stator and the space between the stator and the cover are filled with the filling resin.

To attain the aforementioned objective, still another aspect of the invention is directed to a method for manufacturing a stator unit. The stator unit includes a cover and a stator having a hollow cylindrical shape. The cover includes a circumferential wall portion and an end wall portion contiguous with an end of the circumferential wall portion. The end wall portion has an inner surface including a circumferential surface that faces outward. The circumferential surface defines a region having an axis that coincides with an axis of the stator. The region defined by the circumferential surface has a diameter equal to or smaller than the inside diameter of the stator. The method includes: placing the stator in the cover; inserting, into the stator, a jig having a coefficient of linear expansion lower than a coefficient of linear expansion of the cover and having a hollow cylindrical shape; placing the jig in such a manner that an internal circumferential surface at a tip of the jig faces the circumferential surface; heating the cover and the jig in such a manner that the internal circumferential surface at the tip of the jig and the circumferential surface come into contact with each other along an entire circumference; and pouring filling resin in liquid form in a state in which another end of the circumferential wall portion of the cover faces upward in such a manner that the inside of the stator and the space between the stator and the cover are filled with the filling resin.

The stator preferably has an inside diameter greater than the outside diameter of the jig.

The jig preferably includes a stator-holding portion that is in contact with the stator in such a way as to restrict movement of the stator along an axis of the stator in a state in which the jig is in contact with the circumferential surface.

The circumferential surface of the inner surface of the end wall portion of the cover includes faces inward or outward. The axis of the region defined by the circumferential surface coincides with the axis of the stator. The diameter of the region defined by the circumferential surface is equal to or smaller than the inside diameter of the stator. The circumferential surface is useful in manufacturing the stator unit. More specifically, the aforementioned methods for manufacturing a stator unit each include: (<NUM>-a) inserting, into a stator, a jig having an outward-facing circumferential surface (external circumferential surface) that can come into contact with an inward-facing circumferential surface of a cover and having a coefficient of linear expansion higher than a coefficient of linear expansion of the cover or (<NUM>-b)inserting, into a stator, a jig having an inward-facing circumferential surface (internal circumferential surface) that can come into contact with an outward-facing circumferential surface of a cover and having a coefficient of linear expansion lower than a coefficient of linear expansion of the cover; (<NUM>) placing the jig in such a manner that the (external or internal) circumferential surface of the jig faces the circumferential surface of the cover; and (<NUM>) heating the cover and the jig in such a manner that the circumferential surface of the jig and the circumferential surface of the cover come into contact with each other along an entire circumference. In this way, the (external or internal) circumferential surface of the jig and the circumferential surface of the cover come into close contact with each other, and the jig is fixed to the cover without the need for a clamp or the like. With the jig being fixed to the cover, filling resin in liquid form is poured into the cover. Consequently, the inside of the stator and the space between the stator and the cover are filled with the filling resin. This procedure eliminates the need for forming the stator by insert molding and the need for clamping, thus enabling a reduction in the cost of manufacturing the stator unit. The inside of the stator is filled with the filling resin, and the stator unit is thus rendered highly waterproof.

Hereinafter, a motor-operated valve manufactured by a method according to an embodiment of the present invention will be described with reference to <FIG>.

<FIG> is a front view of a motor-operated valve including a stator unit according to an embodiment of the present invention. In <FIG>, the stator unit is viewed in section. <FIG> illustrate a method for manufacturing the stator unit illustrated in <FIG>. More specifically, <FIG> respectively illustrate a state in which the stator unit is disassembled, a state in which the stator unit is assembled, and a state in which the stator unit is in finished form. In <FIG>, a cover, a stator, and a mold are viewed in section. <FIG> is an enlarged sectional view of the region enclosed with a dash-dot-dot line in <FIG>.

Referring to <FIG>, a motor-operated valve <NUM> includes a valve main body <NUM>, a can <NUM>, and a stator unit <NUM>. Unless otherwise specified, expressions "upper" and "lower" herein refer to the positional relationship in the top-bottom direction in <FIG>.

The valve main body <NUM> has a hollow cylindrical shape with a bottom. The valve main body <NUM> includes a valve chamber and a valve port communicating with the valve chamber. A first conduit <NUM> and a second conduit <NUM> are fixed to the valve main body <NUM> by brazing. The first conduit <NUM> is connected to the valve chamber. The second conduit <NUM> is connected to the valve port. The valve chamber is provided with a valve element (not illustrated) for opening and closing the valve port.

The can <NUM> has a hollow cylindrical shape with its upper end closed. The can <NUM> includes a hollow cylindrical portion <NUM> and a ceiling portion <NUM>, which are constructed as one piece. The valve main body <NUM> is attached to a lower end of the hollow cylindrical portion <NUM>. The hollow cylindrical portion <NUM> has an external circumferential surface, on which protrusions <NUM> are arranged. The protrusions <NUM> are each hemispherical. Four protrusions <NUM> are arranged at <NUM>° intervals on the external circumferential surface of the hollow cylindrical portion <NUM>. The ceiling portion <NUM> is dome-shaped. The ceiling portion <NUM> is contiguous with an upper end of the hollow cylindrical portion <NUM>. The upper end of the hollow cylindrical portion <NUM> is closed by the ceiling portion <NUM>. A rotor (not illustrated) is disposed within the can <NUM>. The rotor and the stator unit <NUM> constitute a stepping motor. The valve main body <NUM> is made of stainless steel or brass. The can <NUM> is made of stainless steel.

The stator unit <NUM> is disposed on the outside of the can <NUM>. The stator unit <NUM> includes a cover <NUM>, a stator <NUM>, and a mold <NUM>.

The cover <NUM> is made of synthetic resin. The cover <NUM> is in the shape of a cap. The cover <NUM> includes a circumferential wall portion <NUM> and an upper-end wall portion <NUM>, which are constructed as one piece. The circumferential wall portion <NUM> is larger than the contour of the stator <NUM>. The stator <NUM> is disposed on the inner side with respect to the circumferential wall portion <NUM>, with a space left between the stator <NUM> and the circumferential wall portion <NUM>. The upper-end wall portion <NUM> is contiguous with an upper end 41a (an end) of the circumferential wall portion <NUM>. The upper-end wall portion <NUM> has a dome portion <NUM>. The dome portion <NUM> is at an area in the upper-end wall portion <NUM> facing the can <NUM> in the top-bottom direction. The upper-end wall portion <NUM> and the can <NUM> face each other along a circumference, from which the dome portion <NUM> protrudes upward. The dome portion <NUM> may have a top part having an opening. The dome portion <NUM> has an inner surface including a circumferential surface <NUM>, which faces inward. The diameter of the circumferential surface <NUM> is equal to the outside diameter of the can <NUM>. The diameter of the circumferential surface <NUM> is not limited to a particular value. It is only required that the diameter of the circumferential surface <NUM> be equal to or smaller than the inside diameter of the stator <NUM>. The diameter of the region defined by the circumferential surface <NUM> is equal to the outside diameter of the can <NUM>. In this embodiment, the circumferential surface <NUM> has a height of <NUM>; that is, the circumferential surface <NUM> is <NUM> millimeters long along an axis L. The circumferential surface <NUM> preferably has a height of <NUM> to <NUM>. The diameter of the region defined by the circumferential surface <NUM> is not limited to a particular value. It is only required that the diameter of the region defined by the circumferential surface <NUM> be equal to or smaller than the inside diameter of the stator <NUM>.

The cover <NUM> is made of rigid resin, such as polybutylene terephthalate (PBT) (coefficient of linear expansion: <NUM> × <NUM>-<NUM>/°C) or polyphenylene ether (PPE) (coefficient of linear expansion: <NUM> × <NUM>-<NUM>/°C).

The stator <NUM> has a hollow cylindrical shape. The stator <NUM> includes a first stator <NUM>, a second stator <NUM>, a fastening member <NUM>, and a lead wire connection section <NUM>. The first stator <NUM>, the second stator <NUM>, and the fastening member <NUM> are aligned in the top-bottom direction on the drawing plane (i.e., along the axis L).

The first stator <NUM> includes metal components such as a yoke 51a, a bobbin 51b, and a coil 51c. The coil 51c in the yoke 51a is made of a coil wire wound around the bobbin 51b. The yoke 51a includes pole teeth 51d, which are claw poles. The pole teeth 51d include downward-pointing pole teeth and upward-pointing pole teeth. Tips (tapered tip portions) of the downward-pointing pole teeth face downward, and tips (tapered tip portions) of the upward-pointing pole teeth face upward. The downward-pointing teeth and the upward-pointing teeth are alternately arranged in the circumferential direction.

The second stator <NUM> is structurally similar to the first stator <NUM>. More specifically, the second stator <NUM> includes metal components such as a yoke 52a, a bobbin 52b, a coil 52c, and pole teeth 52d. The inside diameter of the first stator <NUM> and the inside diameter of the second stator <NUM> are each equal to the outside diameter of the can <NUM>. The inside diameter of the first stator <NUM> and the second stator <NUM> are each equal to the diameter of the circumferential surface <NUM> of the cover <NUM>.

The fastening member <NUM> includes a flat plate portion <NUM>, connecting tabs <NUM>, and holding tabs <NUM>. The flat plate portion <NUM> has an annular shape. The fastening member <NUM> is formed by using the following procedure: stamping out the flat plate portion <NUM>, the connecting tabs <NUM>, and the holding tabs <NUM> from a metal sheet; folding the connecting tabs <NUM> in one direction in such a manner that each connecting tab <NUM> and the flat plate portion <NUM> form a right angle; and folding the holding tabs <NUM> in the opposite direction in such a manner that each holding tab <NUM> and the flat plate portion <NUM> form a right angle. The flat plate portion <NUM> has through-holes 54a. The connecting tabs <NUM> are welded to the yoke 52a of the second stator <NUM>. The first stator <NUM>, the second stator <NUM>, and the fastening member <NUM> have coincident axes.

The fastening member <NUM> in this embodiment includes four holding tabs <NUM>. The holding tabs <NUM> are disposed so as to correspond to the respective protrusions <NUM> of the can <NUM>. The distance between two holding tabs <NUM> facing each other in the radial direction of the stator <NUM> is equal to the outside diameter of the can <NUM>. The can <NUM> is inserted into the stator <NUM> such that inward-facing surfaces of the holding tabs <NUM> come into contact with an external circumferential surface of the can <NUM>. The holding tabs <NUM> each have a holding hole 56a. The protrusions <NUM> are caught in the respective holding holes 56a such that the can <NUM> and the stator unit <NUM> are held in place.

The lead wire connection section <NUM> is a substrate connected to a tip of a coil terminal pin <NUM>, which extends from the stator <NUM> in a direction orthogonal to the axis L. The lead wire connection section <NUM> is connected with a lead wire <NUM>.

In this embodiment, the inside diameter of the stator <NUM> or, more specifically, the inside diameter of each of the first stator <NUM> and the second stator <NUM> is equal to the diameter of the region defined by the circumferential surface <NUM> of the cover <NUM>.

The inside of the stator <NUM> is filled with the mold <NUM>. More specifically, the inside of the first stator <NUM>, the inside of the second stator <NUM>, the space between the second stator <NUM> and the flat plate portion <NUM>, and the through-holes 54a of the flat plate portion <NUM> are filled with the mold <NUM>. The flat plate portion <NUM> is embedded in the mold <NUM>. The space between the cover <NUM> and the stator <NUM> is also filled with the mold <NUM>.

The mold <NUM> is made of highly flexible resin, such as urethane, epoxy, or silicone. The highly flexible resin is hereinafter referred to as filling resin. The filling resin used in this embodiment is thermoset urethane.

The valve main body <NUM>, the can <NUM>, the circumferential surface <NUM> of the cover <NUM>, and the stator <NUM> (i.e., the first stator <NUM>, the second stator <NUM>, and the fastening member <NUM>) included in the motor-operated valve <NUM> each have a central axis that coincides with the axis L.

The following describes a mandrel <NUM>, which is used to manufacture the stator unit <NUM> of the motor-operated valve <NUM> described above.

The mandrel <NUM> is a jig having a hollow cylindrical shape. Alternatively, the mandrel <NUM> may have a solid cylindrical shape. The mandrel <NUM> is made of synthetic resin. The mandrel <NUM> is formed by injection molding. The coefficient of linear expansion of the synthetic resin that is the material of the mandrel <NUM> is higher than the coefficient of linear expansion of the material of the cover <NUM>. The mandrel <NUM> is made of rigid resin, such as polytetrafluoroethylene (PTFE) (coefficient of linear expansion: <NUM> × <NUM>-<NUM>/°C), polystyrene (PS) (coefficient of linear expansion: <NUM> to <NUM> × <NUM>-<NUM>/°C), or polyacetal (POM) (coefficient of linear expansion: <NUM> to <NUM> × <NUM>-<NUM>/°C). Alternatively, the mandrel <NUM> may be made of metal.

The mandrel <NUM> includes a trunk portion <NUM> and a stator-holding portion <NUM>. The trunk portion <NUM> has a hollow cylindrical shape. The outside diameter of the trunk portion <NUM> at room temperature is equal to or slightly smaller than the diameter of the circumferential surface <NUM> of the cover <NUM>. Thus, the outside diameter of the trunk portion <NUM> at room temperature is equal to or slightly smaller than the diameter of the region defined by the circumferential surface <NUM> of the cover <NUM>. The outside diameter of the trunk portion <NUM> is substantially equal to the outside diameter of the can <NUM>. A tip 101a of the trunk portion <NUM> is placed in the region defined by the circumferential surface <NUM>. The stator-holding portion <NUM> is contiguous with the trunk portion <NUM>. The stator-holding portion <NUM> has a hollow cylindrical shape. The outside diameter of the stator-holding portion <NUM> is greater than the outside diameter of the trunk portion <NUM>. A slit <NUM> annularly extends between the stator-holding portion <NUM> and the trunk portion <NUM>. The mandrel <NUM> is inserted into the stator <NUM> such that the inward-facing surfaces of the holding tabs <NUM> come into contact with an external circumferential surface 101b of the trunk portion <NUM> of the mandrel <NUM>, and the holding tabs <NUM> are received in the slits <NUM> (see <FIG>).

The following describes a method for manufacturing the stator unit <NUM> of the motor-operated valve <NUM> with reference to <FIG>.

Referring to <FIG>, the cover <NUM> is placed in a roomtemperature atmosphere in such a manner that a lower end 41b (another end) of the circumferential wall portion <NUM> faces upward. The stator <NUM> is then placed in the cover <NUM> in such a manner that the first stator <NUM> is in contact with the upper-end wall portion <NUM>. Subsequently, the trunk portion <NUM> of the mandrel <NUM> is inserted into the stator <NUM>. The mandrel <NUM> is placed as illustrated in <FIG>. More specifically, the tip 101a of the trunk portion <NUM> is slid into the dome portion <NUM> of the cover <NUM> such that the external circumferential surface 101b of the trunk portion <NUM> and the circumferential surface <NUM> face each other in the direction orthogonal to the axis L. The tip 101a is placed in the region defined by the circumferential surface <NUM>. In this state, the holding tabs <NUM> of the stator <NUM> are in contact with the trunk portion <NUM>, and each holding tab <NUM> is located on the inner side of the slit <NUM> provided in the mandrel <NUM>. The stator <NUM> and the mandrel <NUM> held in place have coincident axes. The stator-holding portion <NUM> of the mandrel <NUM> is in contact with the flat plate portion <NUM>.

Then, the cover <NUM>, the stator <NUM>, and the mandrel <NUM> assembled to each other are heated. The coefficient of linear expansion of the mandrel <NUM> is higher than the coefficient of linear expansion of the cover <NUM>. Accordingly, the amount of change in the outside diameter of the mandrel <NUM> is greater than the amount of change in the diameter of the region defined by the circumferential surface <NUM> of the cover <NUM>. Consequently, the external circumferential surface 101b at the tip 101a of the mandrel <NUM> and the circumferential surface <NUM> come into contact with each other along the entire circumference. The height of a contact portion where the external circumferential surface 101b and the circumferential surface <NUM> are in contact with each other (the length of fit) is <NUM>. The length of fit is preferably equal to or more than <NUM>. With the contact portion being provided, the mandrel <NUM> is fixed to the cover <NUM> in such a manner that the region defined by the circumferential surface <NUM> and the mandrel <NUM> have coincident axes. Likewise, the mandrel <NUM> and the stator <NUM> have coincident axes. The stator <NUM> is held between the cover <NUM> and the stator-holding portion <NUM> of the mandrel <NUM>.

As the filling resin, urethane in liquid form is then poured into the cover <NUM> so as to flow into the stator <NUM> and the space between the cover <NUM> and the stator <NUM>. The mold <NUM> is obtained by setting of urethane. The cover <NUM>, the stator <NUM>, and the mandrel <NUM> then cool to room temperature. Subsequently, the mandrel <NUM> is pulled out of the stator <NUM>. The stator unit <NUM> obtained by this process is in finished form.

The stator unit <NUM> included in the motor-operated valve <NUM> in this embodiment has been described so far. The inner surface of the dome portion <NUM> on the upper-end wall portion <NUM> of the cover <NUM> includes the circumferential surface <NUM>, which faces inward. The axis of the region defined by the circumferential surface <NUM> coincides with the axis of stator <NUM>. The diameter of the region defined by the circumferential surface <NUM> is equal to the inside diameter of the stator <NUM>. The circumferential surface <NUM> is useful in manufacturing the stator unit <NUM>. More specifically, the stator unit <NUM> is manufactured by using the procedure including: (<NUM>) inserting, into the stator <NUM>, the mandrel <NUM> having a coefficient of linear expansion higher than a coefficient of linear expansion of the cover <NUM>; (<NUM>) placing the mandrel <NUM> in such a manner that the external circumferential surface 101b at the tip 101a of the mandrel <NUM> faces the circumferential surface <NUM>; and (<NUM>) heating the cover <NUM>, the stator <NUM>, and the mandrel <NUM> in such a manner that the external circumferential surface 101b at the tip 101a of the mandrel <NUM> and the circumferential surface <NUM> come into contact with each other along the entire circumference. In this way, the external circumferential surface 101b of the mandrel <NUM> and the circumferential surface <NUM> of the cover <NUM> come into close contact with each other, and the mandrel <NUM> is fixed to the cover <NUM> without the need for a clamp or the like. With the mandrel <NUM> being fixed to the cover <NUM>, urethane in liquid form is poured into the cover <NUM> and is consequently formed into the mold <NUM>, with which the inside of the stator <NUM> and the space between the stator <NUM> and the cover <NUM> are filled. This procedure eliminates the need for forming the stator <NUM> by insert molding and the need for clamping, thus enabling a reduction in the cost of manufacturing the stator unit <NUM>. The inside of the stator <NUM> is filled with urethane, and the stator unit <NUM> is thus rendered highly waterproof.

The circumferential surface <NUM> of the stator unit <NUM> has a height of <NUM>. Thus, the mandrel <NUM> is more reliably fixed to the cover <NUM>. The circumferential surface <NUM> preferably has a height of <NUM> to <NUM>. If the height of the circumferential surface <NUM> is greater than <NUM>, the stator unit <NUM> would be large in size.

In a state in which the external circumferential surface 101b of the mandrel <NUM> is in contact with the circumferential surface <NUM>, the stator-holding portion <NUM> is in contact with the stator <NUM> in such a way as to restrict the movement of the stator <NUM> along the axis L. This structure enables the stator <NUM> to be held between the cover <NUM> and the stator-holding portion <NUM>.

The following describes modifications of the stator unit <NUM> with reference to <FIG>. Some of the constituent elements of the stator unit according to any one of the following modifications are denoted by the reference signs of the respective constituent elements of the above-described motor-operated valve <NUM>. Unless otherwise specified, each of these constituent elements is identical or substantially identical to the corresponding constituent element of the motor-operated valve <NUM> in structural respects.

<FIG> illustrate a stator unit 30A, which is a first modification of the stator unit <NUM> described above. The stator unit 30A includes the cover <NUM>, a stator 50A, and the mold <NUM>.

Referring to <FIG>, D1 is greater than D2, where D1 denotes the inside diameter of the stator 50A, and D2 denotes the diameter of the region defined by the circumferential surface <NUM> of the cover <NUM>. D2 also denotes the diameter of the circumferential surface <NUM> of the cover <NUM>. The outside diameter of the trunk portion <NUM> of the mandrel <NUM> at room temperature is equal to or slightly smaller than the diameter of the region defined by the circumferential surface <NUM> of the cover <NUM>. The mandrel <NUM> is inserted into the stator 50A such that a clearance K1 is left between the stator 50A and the mandrel <NUM>, as illustrated in <FIG>. At the time of manufacturing of the stator unit 30A, urethane in liquid form flows into the clearance K1 such that an internal circumferential surface of the stator 50A is covered with urethane. Consequently, the internal circumferential surface of the stator 50A included in the stator unit 30A is covered with the mold <NUM>. Thus, the stator 50A (the pole teeth 51d and the pole teeth 52d) is not exposed, as illustrated in <FIG>. The stator unit 30A is rendered more highly waterproof accordingly. When the can <NUM> and the stator unit 30A vibrate, the mold <NUM> comes into contact with the can <NUM>; that is, the stator 50A is kept from contact with the can <NUM>. The mold <NUM> absorbs shock caused by vibrations, thus eliminating or reducing abnormal sounds, out-of-step in the stepping motor, and the possibility that the can <NUM> will become scratched. This structure prevents or retards electrolytic corrosion of the can <NUM> and the stator 50A.

<FIG> illustrate a stator unit 30B, which is a second modification of the stator unit <NUM> described above. The stator unit 30B includes a cover 40B, a stator 50B, and the mold <NUM>.

Referring to <FIG>, the cover 40B includes a rib 44B, which has an annular shape and protrudes from an inner surface of a dome portion 43B along the axis L. The rib 44B has a circumferential surface 46B, which faces outward. The circumferential surface 46B is an external circumferential surface of the rib 44B. The circumferential surface 46B has a height of <NUM>. The circumferential surface 46B preferably has a height of <NUM> to <NUM>. The diameter of the circumferential surface 46B is smaller than the inside diameter of the stator 50B. Thus, the diameter of the region defined by the circumferential surface 46B is smaller than the inside diameter of the stator 50B. The dome portion 43B has a circumferential surface 45B, which faces inward. The diameter of the circumferential surface 45B is equal to the inside diameter of the stator 50B. Thus, the diameter of the region defined by the circumferential surface 45B is equal to the inside diameter of the stator 50B. The inside diameter of the trunk portion <NUM> of the mandrel <NUM> at room temperature is equal to or slightly greater than the diameter of the circumferential surface 46B. Thus, the inside diameter of the trunk portion <NUM> of the mandrel <NUM> at room temperature is equal to or slightly greater than the diameter of the region defined by the circumferential surface 46B. The diameter of the circumferential surface 45B and the inside diameter of the stator 50B are each greater than the outside diameter of the trunk portion <NUM>. Thus, the diameter of the region defined by the circumferential surface 45B and the inside diameter of the stator 50B are each greater than the outside diameter of the trunk portion <NUM>. The tip 101a of the trunk portion <NUM> is placed in the region defined by the circumferential surface 45B. The axes of the regions defined respectively by the circumferential surfaces 45B and 46B coincide with the axis of the stator 50B.

The stator unit 30B is manufactured by using the procedure including: (<NUM>) inserting, into the stator 50B, the mandrel <NUM> having a coefficient of linear expansion lower than a coefficient of linear expansion of the cover 40B; (<NUM>) placing the mandrel <NUM> in such a manner that an internal circumferential surface 101c at the tip 101a of the mandrel <NUM> faces the circumferential surface 46B; and (<NUM>) heating the cover 40B, the stator 50B, and the mandrel <NUM> in such a manner that the internal circumferential surface 101c at the tip 101a of the mandrel <NUM> and the circumferential surface 46B come into contact with each other along the entire circumference.

The coefficient of linear expansion of the mandrel <NUM> is lower than the coefficient of linear expansion of the cover 40B. Accordingly, the amount of change in the diameter of the region defined by the circumferential surface 46B of the cover 40B is greater than the amount of change in the inside diameter of the mandrel <NUM>. Consequently, the internal circumferential surface 101c at the tip 101a of the mandrel <NUM> and the circumferential surface 46B come into contact with each other along the entire circumference. The mandrel <NUM> is fixed to the cover 40B in such a manner that the region defined by the circumferential surface 46B and the mandrel <NUM> have coincident axes.

The inside diameter of the stator 50B included in the stator unit 30B and the diameter of the region defined by the circumferential surface 45B are each greater than the outside diameter of the trunk portion <NUM> of the mandrel <NUM>. The mandrel <NUM> is inserted into the stator 50B such that a clearance K2 is left between the stator 50B and the mandrel <NUM> and between the circumferential surface 45B and the mandrel <NUM>, as illustrated in <FIG>. At the time of manufacturing of the stator unit 30B, urethane in liquid form flows into the clearance K2 such that an internal circumferential surface of the stator 50B is covered with urethane. Consequently, the internal circumferential surface of the stator 50B included in the stator unit 30B is covered with the mold <NUM>. Thus, the stator 50B (the pole teeth 51d and the pole teeth 52d) is not exposed, as illustrated in <FIG>. The stator unit 30B is rendered more highly waterproof accordingly. When the can <NUM> and the stator unit 30B vibrate, the mold <NUM> comes into contact with the can <NUM>; that is, the stator 50B is kept from contact with the can <NUM>. The mold <NUM> absorbs shock caused by vibrations, thus eliminating or reducing abnormal sounds, out-of-step in the stepping motor, and the possibility that the can <NUM> will become scratched. This structure prevents or retards electrolytic corrosion of the can <NUM> and the stator 50B.

Claim 1:
A stator unit, comprising:
a cover (40B) including a circumferential wall portion (<NUM>) and an end wall portion (<NUM>) contiguous with an end (41a) of the circumferential wall portion (<NUM>);
a stator (50B) accommodated in the cover (40B) and having a hollow cylindrical shape; and
a mold (<NUM>) made of filling resin with which an internal circumferential surface of the stator (50B) and a space between the stator (50B) and the cover (40B) are filled, wherein
the end wall portion (<NUM>) has a dome portion (43B),
the cover (40B) includes a rib (44B), which has an annular shape and protrudes from an inner surface of the dome portion (43B),
the rib (44B) includes a circumferential surface (46B) that faces outward, and
the circumferential surface (46B) defines a region having an axis that coincides with an axis of the stator (50B),
wherein the circumferential surface (46B) has a diameter smaller than an inside diameter of the stator (50B), and
wherein the circumferential surface (46B) is adapted to come into contact, along an entire circumference, with the internal circumferential surface (101c) at a tip (101a) of a jig (<NUM>) during a manufacturing process of the stator unit.