Patent Description:
When a pneumatic actuator such as a pneumatic cylinder or a pneumatic motor is actuated, it is required to use clean compressed air with no dust nor liquid, such as oil or water, being mixed therein. In order to remove foreign matter such as liquid or dust from the compressed air, a pneumatic filter is normally disposed in a pneumatic circuit for supplying compressed air to the pneumatic actuator. An example of such a pneumatic filter is disclosed in Patent Literature (PTL) <NUM>. As illustrated in <FIG>, this type of pneumatic filter normally includes a hollow filter element <NUM> for removing foreign matter. The filter element <NUM> is disposed inside a filter case <NUM> that has an inlet port <NUM> and an outlet port <NUM>.

The filter element <NUM> includes a filter body <NUM> shaped like a hollow cylinder, an upper cap <NUM> attached to the upper end of the filter body <NUM>, and a lower cap <NUM> attached to the lower end of the filter body <NUM>.

The filter body <NUM> includes an inner core <NUM>, an inner filter member <NUM>, an outer core <NUM>, and an outer filter member <NUM>. The inner core <NUM> is shaped like a hollow cylinder that defines a central space <NUM>. The inner filter member <NUM> surrounds the periphery of the inner core <NUM>. The outer core <NUM> is shaped like a hollow cylinder and surrounds the periphery of the inner filter member <NUM>. The outer filter member <NUM> surrounds the periphery of the outer core <NUM>.

The inner core <NUM> and the outer core <NUM> are made of perforated metal sheets, and vent holes are disposed uniformly over the entire core.

In the pneumatic filter, compressed air is introduced from the inlet port <NUM> of the filter case <NUM> through an air introduction port <NUM> of the upper cap <NUM> into the central space <NUM> of the filter body <NUM>. As indicated by arrows in <FIG> and <FIG>, the compressed air subsequently enters the inner filter member <NUM> through vent holes 49a of the inner core <NUM>, passes through the inner filter member <NUM>, and enters the outer filter member <NUM> through vent holes 51a of the outer core <NUM>. After passing through the outer filter member <NUM>, the compressed air flows out of the filter element <NUM> toward the outlet port <NUM> of the filter case <NUM>. In this process, liquid such as oil and water contained in the compressed air is collected in the inner filter member <NUM> and the outer filter member <NUM>. The liquid is gradually aggregated into large droplets, and the liquid droplets move down by their own weight and drip into the filter case <NUM>.

In the filter element <NUM>, the compressed air that is introduced into the central space <NUM> from the air introduction port <NUM> of the upper cap <NUM> normally takes the shortest route to flow through the filter body <NUM> from the central space <NUM> toward the outside of the filter element <NUM>. More specifically, the vent holes 49a and vent holes 51a are formed uniformly over respective entire surfaces of the inner core <NUM> and the outer core <NUM>. Accordingly, the compressed air introduced into the central space <NUM> flows out so as to horizontally transverse the vent holes 49a of the inner core <NUM>, the inner filter member <NUM>, the vent holes 51a of the outer core <NUM>, and the outer filter member <NUM>. As a result, liquid droplets once collected in the inner filter member <NUM> and the outer filter member <NUM> tend to move laterally with the flow of the compressed air and disperse again from the outer filter member <NUM> into the compressed air, which leads to a problem that the compressed air mixed with the liquid droplets flows out of the outer filter member <NUM>. Liquid droplets tend to redisperse more especially at an upper part (near the inlet) of the central space <NUM> because the air pressure is higher as the distance from the inlet is smaller.

<CIT> discloses a filter element with an inside filtration member surrounding a center space, an outside filtration member surrounding the inside filtration member, an upper end cap having an introduction opening for introducing compressed air inside the center space, and a lower end cap closing off the lower end part of the center space. The outer periphery of the lower end part of the inner filtration member is surrounded by an inner side wall and the outer periphery of the lower end part of the outer filtration member is surrounded by an outer side wall, with the upper end surface of the outer side wall being positioned higher than the upper end surface of the inner side wall.

<CIT> discloses a filter assembly with an upper end cap attached to the upper end part of the filter assembly. The upper end cap has an inner peripheral wall mated inside of a central space part of an inside filter member, an intermediate wall surrounding the outer periphery of an outside core member, and an outer peripheral wall surrounding the outer periphery of an outside filter member. The height of the intermediate wall is lower than the heights of the outer peripheral wall and the inner peripheral wall. The inside of the upper end cap is filled with an adhesive to a depth burying the intermediate wall, and the upper end part of the filter assembly is attached to the upper end cap in a state such that the upper end part is in the adhesive.

A technical problem is to prevent aggregated liquid droplets from redispersing into compressed air by controlling the flow of compressed air flowing through the filter element.

To solve the above-described technical problem, a filter element comprises the features of claim <NUM>. The filter element includes a filter body shaped like a hollow cylinder, an upper cap attached to an upper end of the filter body, and a lower cap attached to a lower end of the filter body.

The filter body has a central space formed so as to extend inside the filter body along a central axis of the filter body. The filter body includes an inner core that is shaped like a hollow cylinder so as to define the central space, an inner filter member that surrounds a periphery of the inner core, an outer core that is shaped like a hollow cylinder and surrounds a periphery of the inner filter member, and an outer filter member that surrounds a periphery of the outer core.

The upper cap has a nozzle that is fitted into an upper part of the central space and configured to introduce compressed air into the central space. The upper cap also has a skirt that is shaped tubularly and surrounds a periphery of the outer filter member. The lower cap has a plug portion that plugs a lower end of the central space and a discharge hole through which drainage separated from the compressed air is discharged. A central channel is formed in the central space between a lower end of the nozzle of the upper cap and an upper end of the plug portion of the lower cap.

In addition, vent holes are formed around a portion of the inner core that faces the central channel. The outer core has a perforated portion around which vent holes are formed and a non-perforated portion in which no vent hole is formed. A region over which the non-perforated portion is formed along the central channel extends from a position above a central position of the central channel in an up-down direction to an upper end of the central channel. A region over which the perforated portion is formed along the central channel extends from a lower end of the non-perforated portion to a lower end of the central channel.

In the filter element, an upper portion of the inner filter member may be disposed between the inner core having the vent holes and the non-perforated portion of the outer core at an upper part of the central channel. In addition, a downflow channel in which the compressed air flowing in between the inner core and the outer core from the central channel through the vent holes of the inner core flows downward along the inner filter member is thereby formed between the inner core and the non-perforated portion of the outer core.

In the filter element, the non-perforated portion of the outer core may extend to a position below a lower end of the skirt of the upper cap.

In the up-down direction, a ratio of a length of the non-perforated portion formed in the outer core to a length of the central channel may be <NUM> to <NUM>%, and more preferably <NUM> to <NUM>%.

In the filter element, the upper cap preferably has an air inlet being in communication with the nozzle. An inside diameter of the air inlet is preferably larger than any of inside diameters of the nozzle and the central channel. In addition, the inside diameter of the nozzle is preferably smaller than the inside diameter of the central channel.

According to the present invention, the non-perforated portion having no vent hole is formed at the upper portion of the outer core so as to surround the upper part of the central channel, which enables the non-perforated portion to control the flow direction of compressed air passing through the filter element. This prevents the flow of compressed air from redispersing aggregated liquid droplets into the compressed air at the upper part of the central channel where the compressed air flows at a high velocity.

<FIG> illustrate an embodiment of a filter element <NUM> according to the present invention. The filter element <NUM> includes a filter body <NUM>, an upper cap <NUM>, and a lower cap <NUM>. The filter body <NUM> is shaped like a hollow cylinder. The upper cap <NUM> is shaped circularly and attached to an upper end of the filter body <NUM>, which is an end along the central axis L of the filter body <NUM>. The lower cap <NUM> is shaped circularly and attached to a lower end of the filter body <NUM>, which is an end opposite to the upper end.

The filter body <NUM> has a central space <NUM> formed therein so as to extend in the up-down direction along the central axis L. The filter body <NUM> also has an inner core <NUM>, an inner filter member <NUM>, an outer core <NUM>, and an outer filter member <NUM>. The inner core <NUM> is a perforated hollow cylinder that defines the central space <NUM>. The inner filter member <NUM> is a hollow member that surrounds the periphery of the inner core <NUM>. The outer core <NUM> is a perforated hollow cylinder that surrounds the periphery of the inner filter member <NUM>. The outer filter member <NUM> is a hollow member that surrounds the periphery of the outer core <NUM>. Note that the terms "central axis direction" and "up-down direction" are used herein to have substantially the same meaning.

The inner filter member <NUM> is positioned upstream of the outer filter member <NUM> in the flow of compressed air and mainly collects dust and liquid contained in the compressed air. The liquid is in the form of mist or droplets and is made of oil, water, or the like. The inner filter member <NUM> is formed by cylindrically curling a pleated filter sheet. The inner filter member <NUM> is disposed between the inner core <NUM> and the outer core <NUM> such that fold lines 7a are aligned in parallel to the central axis L.

The inner filter member <NUM> having a pleated structure has a large filtration area compared with a cylindrically curled flat filter sheet.

The inner filter member <NUM> collects the liquid such as oil and water. The outer filter member <NUM> is positioned downstream of the inner filter member <NUM> in the flow of compressed air and mainly serves to take the liquid away from the compressed air flow and to guide the liquid toward the lower cap <NUM>. The outer filter member <NUM> is disposed at the periphery of the outer core <NUM> so as to surround the inner filter member <NUM> with the outer core <NUM> interposed therebetween. Accordingly, the outer filter member <NUM> may be referred to as a separation layer that separates the liquid from the compressed air.

For example, the inner filter member <NUM> and the outer filter member <NUM> may be formed of a fiber sheet made by regularly or randomly laminating thin chemical fibers each having a diameter of several to several tens of micrometers or of a nonwoven fabric made by bonding the laminated chemical fibers by using an adhesive or by fusing or interlacing. Alternatively, the inner filter member <NUM> and the outer filter member <NUM> may be formed of an aggregate of micro ceramic particles or of a porous sheet made of a synthetic resin. The inner filter member <NUM> is woven densely by using a fiber thinner than that of the outer filter member <NUM> so as to have a lower porosity (i.e., finely woven) so that the inner filter member <NUM> can reliably collect minute dust particles and fine mist of oil, water, and the like. In contrast, the outer filter member <NUM> has a higher porosity (i.e., coarsely woven) by using a fiber thicker than that of the inner filter member <NUM> so that the outer filter member <NUM> can send the liquid such as oil or water collected by the inner filter member <NUM> rapidly toward the lower cap <NUM>. In short, the inner filter member <NUM> is a fine filter, whereas the outer filter member <NUM> is a coarse filter.

The upper cap <NUM> includes an annular cover <NUM> that covers the upper end of the filter body <NUM> except for the central space <NUM>. The upper cap <NUM> also includes a ring-like flange <NUM> that surrounds the periphery of the cover <NUM>. The flange <NUM> and the cover <NUM> are connected by radially disposed multiple links <NUM>.

A cylindrically shaped air inlet <NUM> into which compressed air flows and a cylindrically shaped nozzle <NUM> communicating with the air inlet <NUM> are formed concentrically at the center of the cover <NUM> so as to extend along the central axis L. The air inlet <NUM> protrudes upward relative to the cover <NUM>. The nozzle <NUM> protrudes downward relative to the cover <NUM> and is fitted in an upper part of the central space <NUM> of the filter body <NUM>.

The inside diameter of the air inlet <NUM> is constant over the entire length thereof. The inside diameter of the nozzle <NUM> is also constant over the entire length thereof. The inside diameter of the air inlet <NUM> is set to be larger than any of the inside diameter of the nozzle <NUM> and the diameter of the central space <NUM>. In the illustrated example, the inside diameter of the air inlet <NUM> is set to be twice as large as the inside diameter of the nozzle <NUM>. An inclined step <NUM> having a conical surface is formed where the air inlet <NUM> is joined to the nozzle <NUM>. In addition, the inside diameter of the nozzle <NUM> is smaller than the diameter of the central space <NUM>.

A cylindrically shaped skirt <NUM> is formed around the periphery of the cover <NUM> so as to extend downward and surround the periphery of an upper end portion of the outer filter member <NUM>. The lower end 18a of the skirt <NUM> is disposed at the same position as the lower end 16a of the nozzle <NUM> in the up-down direction. The lower end 18a of the skirt <NUM>, however, may come to a position lower than the lower end 16a of the nozzle <NUM>.

The lower cap <NUM>, which plugs the lower end of the filter body <NUM>, has a plug portion <NUM>, a recessed groove <NUM>, and a peripheral wall <NUM>. The plug portion <NUM> is shaped like a cylindrical stub and is fitted in a lower part of the central space <NUM> so as to plug the lower part. The recessed groove <NUM> is formed annularly so as to surround the plug portion <NUM>. The peripheral wall <NUM> is disposed so as to surround a lower end portion of the outer filter member <NUM>. Lower end portions of the inner core <NUM>, the inner filter member <NUM>, the outer core <NUM>, and the outer filter member <NUM> are fitted in the recessed groove <NUM>.

An annular support step <NUM> is formed at the inside surface of the peripheral wall <NUM> at a position closer to, but lower than, an upper end 22a of the peripheral wall <NUM>. The lower end of the outer filter member <NUM> abuts the support step <NUM>. The surface of the support step <NUM> is disposed at the same position as the upper end 20a of the plug portion <NUM> in the up-down direction. Accordingly, the upper end 22a of the peripheral wall <NUM> is positioned higher than the upper end 20a of the plug portion <NUM>. The lower end of the outer filter member <NUM> is positioned higher than the lower ends of the inner core <NUM>, the inner filter member <NUM>, and the outer core <NUM>.

Multiple discharge holes <NUM> are formed inside the peripheral wall <NUM> so as to penetrate from the surface of the support step <NUM> to the lower surface 22b of the peripheral wall <NUM>. The discharge holes <NUM> are disposed radially around the central axis L. The liquid droplets (drainage) flowing down through the outer filter member <NUM> and the drainage filtrated by the inner filter member <NUM> and accumulated in the recessed groove <NUM> are discharged to the outside of the filter element <NUM> through the discharge holes <NUM>.

The upper cap <NUM> and the lower cap <NUM> are attached to respective upper and lower ends of the filter body <NUM> as described above. Accordingly, a central channel <NUM> into which compressed air is introduced is formed in the central space <NUM> between the lower end 16a of the nozzle <NUM> of the upper cap <NUM> and the upper end 20a of the plug portion <NUM> of the lower cap <NUM>.

The inner core <NUM> and the outer core <NUM> are formed by cylindrically curling perforated metal sheets through which many vent holes <NUM> and <NUM> are punched out. The inner core <NUM> and the outer core <NUM> serve to reinforce the filter body <NUM>.

The vent holes <NUM> are disposed uniformly around a portion of the inner core <NUM> that faces the central channel <NUM>.

On the other hand, the outer core <NUM> has a perforated portion 8b around which the vent holes <NUM> are disposed uniformly and a non-perforated portion 8a that does not have any vent hole.

As illustrated in <FIG>, the region over which the non-perforated portion 8a is formed in the length direction of the central channel <NUM> (in the up-down direction) extends from a position above the central position C of the length A of the central channel <NUM> to the upper end of the central channel <NUM> (i.e., the lower end 16a of the nozzle <NUM>). The non-perforated portion 8a is also configured to extend to a position below the lower end 18a of the skirt <NUM>.

On the other hand, the region over which the perforated portion 8b is formed in the length direction of the central channel <NUM> extends from the lower end of the non-perforated portion 8a to the lower end of the central channel <NUM> (i.e., the upper end 20a of the plug portion <NUM>).

Accordingly, an upper portion of the inner filter member <NUM> is disposed between the inner core <NUM> having the vent holes <NUM> and the non-perforated portion 8a of the outer core <NUM> at an upper part of the central channel <NUM>, and the inside surface of an upper portion of the outer filter member <NUM> is covered by the non-perforated portion 8a of the outer core <NUM>.

Note that the diameter of each vent hole <NUM> formed in the inner core <NUM> is equal to that of each vent hole <NUM> formed in the outer core <NUM>. The density of the vent holes <NUM> formed in the inner core <NUM> is also equal to the density of the vent holes <NUM> formed in the outer core <NUM>.

The filter element <NUM> with the above-described configuration is accommodated inside a filter case <NUM> as indicated by the chain lines in <FIG>. The filter case <NUM> has an inlet port <NUM> into which uncleaned compressed air flows and an outlet port <NUM> from which cleaned compressed air flows out. The compressed air from the inlet port <NUM> is introduced into the central channel <NUM> through the air inlet <NUM> and the nozzle <NUM> of the upper cap <NUM>. The compressed air subsequently passes successively through the vent holes <NUM> of the inner core <NUM>, the inner filter member <NUM>, the vent holes <NUM> of the outer core <NUM>, and the outer filter member <NUM>. In this process, the inner filter member <NUM> and the outer filter member <NUM> remove dust and liquid such as oil, water, and the like, from the compressed air. Consequently, the compressed air flows out through the outlet port <NUM> of the filter case <NUM> and is supplied to a fluid-pressure apparatus (not illustrated).

The liquid collected by the inner filter member <NUM> is initially in the form of minute particles, which are gradually aggregated into large particles. Due to gravity, the large particles move down through the inner filter member <NUM> and also through the outer filter member <NUM>. Meanwhile, the particles are further combined into even larger liquid particles, which finally reaches the lower cap <NUM>. After the liquid flowing down the inner filter member <NUM> reaches the recessed groove <NUM> of the lower cap <NUM>, the liquid flows over the support step <NUM> into the discharge holes <NUM> and drips into the filter case <NUM>. Meanwhile, the liquid flowing down the outer filter member <NUM> reaches the support step <NUM>, and the liquid further flows through the discharge holes <NUM> and drips into the filter case <NUM>. The drainage accumulated in the filter case <NUM> flows out appropriately through a discharge hole <NUM>.

Here, the flow of the compressed air will be described in detail with reference to <FIG>. In <FIG>, the inner filter member <NUM> is omitted to show the arrangement of the vent holes <NUM> of the outer core <NUM> clearly. The inner filter member <NUM> is indicated by the chain line.

In <FIG>, compressed air enters the small-diameter nozzle <NUM> from the large-diameter air inlet <NUM>, which squeezes the flow and thereby increases the velocity of flow. The compressed air is introduced, in a diffusion state (in a turbulent state), from the nozzle <NUM> into the central channel <NUM> having a diameter larger than that of the nozzle <NUM>.

The compressed air introduced into the central channel <NUM> tends to take the shortest route to flow through the filter body <NUM> from the central channel <NUM> toward the outside of the outer filter member <NUM>. Here, the pressure of air is higher in an upper part of the central channel <NUM> (near the inlet) than in a lower part thereof. Accordingly, the velocity of the compressed air normally becomes higher in the upper part.

The upper portion of the outer core <NUM> is the non-perforated portion 8a. Accordingly, the compressed air that has flown into the inner filter member <NUM> through the vent holes <NUM> of the inner core <NUM> is blocked by the non-perforated portion 8a at the upper part of the central channel <NUM> and prevented from flowing laterally (horizontally) through the outer core <NUM>. The compressed air is thereby caused to flow downward along the non-perforated portion 8a. A portion of the compressed air flows straight down along the inner filter member <NUM>, and a remaining portion of the compressed air flows obliquely downward. Accordingly, the liquid collected by the inner filter member <NUM> is moved rapidly downward due to gravity and due to the downward flow of the compressed air.

When the downward flow of the compressed air reaches the perforated portion 8b of the outer core <NUM>, the compressed air passes through the vent holes <NUM> of the outer core <NUM> and flows obliquely downward through the outer filter member <NUM>. Consequently, the compressed air flows out of the filter element <NUM> and finally flows out through the outlet port <NUM> of the filter case <NUM>. Here, the aggregated liquid particles are subjected to an obliquely downward force by the compressed air in the outer filter member <NUM>. The liquid particles are moved rapidly downward through the outer filter member <NUM> due to the downward force and gravity and separated from the compressed air flow, thereby preventing the liquid from redispersing into the compressed air.

Put another way, a downflow channel <NUM> is formed between the inner core <NUM> and the non-perforated portion 8a of the outer core <NUM>. In the downflow channel <NUM>, the compressed air coming from the central channel <NUM> through the vent holes <NUM> of the inner core <NUM> flows downward along the inner filter member <NUM>.

Drainage particles, such as dust, that have large inertia in the compressed air are transported to a deeper or lower position in the central space <NUM> due to a high velocity flow of the compressed air at the nozzle <NUM> and collected mainly by lower portions of the inner filter member <NUM> and the outer filter member <NUM>. Accordingly, the drainage particles are prevented from redispersing into the compressed air flow compared with a case in which the drainage particles are collected by upper portions of the inner filter member <NUM> and the outer filter member <NUM> and moved down along these filter members.

According to experiments performed by the inventors, it was observed to be quite effective in preventing aggregated liquid droplets from dispersing again when the ratio of the non-perforated portion 8a, in other words, the ratio of the length B of the non-perforated portion 8a to the length A of the central channel <NUM> along the central axis L as in <FIG>, was in the range of <NUM> to <NUM>%, and more preferably in the range of <NUM> to <NUM>%.

When the ratio of the non-perforated portion 8a was <NUM>% or smaller, it was observed not to be ineffective in prevention of redispersing but the degree of effectiveness was small. The smaller the ratio, the more the liquid droplets collected by the inner filter member <NUM> and the outer filter member <NUM> were dispersed again from the outer filter member <NUM> into the compressed air.

Claim 1:
A filter element (<NUM>) comprising:
a filter body (<NUM>) shaped like a hollow cylinder;
an upper cap (<NUM>) attached to an upper end of the filter body (<NUM>); and
a lower cap (<NUM>) attached to a lower end of the filter body (<NUM>), wherein
the filter body (<NUM>) has
a central space (<NUM>) formed so as to extend inside the filter body (<NUM>) along a central axis (L) of the filter body (<NUM>),
an inner core (<NUM>) that is shaped like a hollow cylinder so as to define the central space (<NUM>),
an inner filter member (<NUM>) that surrounds a periphery of the inner core (<NUM>),
an outer core (<NUM>) that is shaped like a hollow cylinder and surrounds a periphery of the inner filter member (<NUM>), and
an outer filter member (<NUM>) that surrounds a periphery of the outer core (<NUM>),
the upper cap (<NUM>) has
a nozzle (<NUM>) that is fitted into an upper part of the central space (<NUM>) and configured to introduce compressed air into the central space (<NUM>), and
a skirt (<NUM>) that is shaped tubularly and surrounds a periphery of the outer filter member (<NUM>),
the lower cap (<NUM>) has
a plug portion (<NUM>) that plugs a lower end of the central space (<NUM>), and
a discharge hole (<NUM>) through which drainage separated from the compressed air is discharged,
a central channel (<NUM>) is formed in the central space (<NUM>) between a lower end (16a) of the nozzle (<NUM>) of the upper cap (<NUM>) and an upper end (20a) of the plug portion (<NUM>) of the lower cap (<NUM>),
vent holes (<NUM>) are formed around a portion of the inner core (<NUM>) that faces the central channel (<NUM>),
characterized in that
the outer core (<NUM>) has a perforated portion (8b) around which vent holes (<NUM>) are formed and a non-perforated portion (8a) in which no vent hole is formed,
a region over which the non-perforated portion (8a) is formed along the central channel (<NUM>) extends from a position above a central position (C) of the central channel (<NUM>) in an up-down direction to an upper end of the central channel (<NUM>), and
a region over which the perforated portion (8b) is formed along the central channel (<NUM>) extends from a lower end of the non-perforated portion (8a) to a lower end of the central channel (<NUM>).