ELECTRIC ACTUATOR AND VEHICLE DRUM BRAKE INCLUDING THE SAME

An electric actuator includes a forwardly and reversely rotatable electric motor, a motion conversion mechanism configured to convert rotation of the electric motor into reciprocating linear motion of a screw shaft, an actuator case containing the electric motor and the motion conversion mechanism, and a boot covering a portion of the screw shaft that extends out of the actuator case. A vent is formed on the actuator case. The vent is configured to ventilate a sealingly closed space in the actuator case and the boot. A breathable waterproof member is provided on the vent and a cover member is also provided on the vent. The breathable waterproof member is configured to block entry and exit of water while permitting entry and exit of air. The cover member is configured to cover the vent so as to allow ventilation with outer air.

DESCRIPTION

Technical Field

The present invention relates to an electric actuator including a motion conversion mechanism that converts a rotation of a forwardly and reversely rotatable electric motor into a reciprocating linear motion of a screw shaft, and a vehicle drum brake including this electric actuator.

Background Art

An electric actuator includes a forwardly and reversely rotatable electric motor and a motion conversion mechanism that converts a rotation of this electric motor into a reciprocating linear motion of a screw shaft. When a nut of the motion conversion mechanism is rotated forwardly and reversely by the electric motor, the screw shaft, which is threadedly inserted through this nut so as to be able to advance and retreat, linearly reciprocates. Such an electric actuator is used in, for example, an electric parking brake apparatus of a vehicle drum brake (for example, refer to PTL 1).

In the electric parking brake apparatus including this electric actuator, when a parking brake lever in the drum brake is moved from a release position to an actuation position via a pulling means according to the reciprocating linear motion of the screw shaft of the electric actuator, brake shoes are pressed against the inner peripheral surface of a brake drum that rotates together with a wheel and the rotation of the wheel is braked due to a frictional resistance force generated between them, as a result of. which the vehicle is brought into a parking brake state. Then, when the electric motor of the electric actuator is rotated reversely from the parking brake state, the parking brake lever is moved from the actuation position to the release position and the brake shoes are separated from the inner peripheral surface of the brake drum, as a result of which the parking brake state of the vehicle is released.

Then, in the electric actuator, the electric motor and the motion conversion mechanism are contained in a sealingly closed space in an actuator case, but a portion of the screw shaft of the motion conversion mechanism that extends out of the actuator case is covered with an extensible and compressible bellows-like boot.

Then, in the electric actuator, when the screw shaft of the motion conversion mechanism linearly reciprocates as described above, the volume of the sealingly closed space in the actuator case changes and a pressure difference is generated therein. More specifically, when the screw shaft advances in the direction from the actuation position to the release direction to cause the boot to extend, the volume of the sealingly closed space increases and a negative pressure is generated in this sealingly closed space, as a result of which the boot dents due to this negative pressure. Conversely, when the screw shaft retreats from the release position to the actuation position to cause the boot to contract, the volume of the sealingly closed space reduces and a positive pressure is generated in this sealingly closed space, as a result of which the boot expands due to this positive pressure. A repetition of such denting and expansion of the boot raises a problem that this boot is overloaded and the durability thereof reduces. The boot also dents and expands due to a change in the altitude (the atmospheric pressure) and the temperature under which this electric actuator is disposed, besides the above-described in.crea.selreduction in the volume of the sealingly closed space according to the reciprocating linear motion of the screw shaft.

In light thereof, PTL 2 proposes a mechanism in which a ventilation unit is attached to a gear case defining a sealingly closed space together with a motor case, and the sealingly closed space and the atmosphere are in communication with each other via a vent extending through a partition wall of the motor case, an air flow path formed between the motor case and the gear case, and the ventilation unit, by which the inner pressure in the sealingly closed space is adjusted through the communication between the sealingly closed space and the atmosphere (a breather mechanism).

CITATION LIST

PATENT LITERATURE

PTL 1: Japanese Patent Application Public Disclosure No. 2019-006223 PTL 2: Japanese Patent Application Public Disclosure No. 2020-003034

SUMMARY OF INVENTION

TECHNICAL PROBLEM

However, the configuration proposed by PTL 2 necessitates the formation of the air flow path between the motor case and the gear case along with the formation of the vent through the partition wall of the motor case, and further necessitates the attachment of the ventilation unit prepared as a different member to the gear case, thereby raising a problem of causing an increase in the number of components and the complication of the structure, and leading to a cost increase accompanying them.

The present invention has been made in consideration of the above-described problems, and is directed to providing an electric actuator capable of inhibiting a change in the inner pressure in the actuator to thus achieve the improvement of the durability of a boot with a simple structure without causing an increase in the number of components and the complication of the structure and leading to a cost increase accompanying them, and a vehicle drum brake including this electric actuator.

SOLUTION TO PROBLEM

To achieve the above-described object, according to a first feature of the present invention, an electric actuator includes a forwardly and reversely rotatable electric motor, a motion conversion mechanism configured to convert a rotation of the electric motor into a reciprocating linear motion of a screw shaft, an actuator case containing the electric motor and the motion conversion mechanism, and a hoot covering a portion of the screw shaft that extends out of the actuator case. A vent is formed on the actuator case. The vent is configured to ventilate a sealingly closed space in the actuator case and the boat. A breathable waterproof member is provided on the vent and a cover member is also provided on the vent. The breathable waterproof member is configured to block entry and exit of water while permitting entry and exit of air. The cover member is configured to cover the vent so as to allow ventilation with outer air.

According to a second feature of the present invention, in addition to the - first feature, the actuator case includes a first case containing the electric motor and a second case containing the motion conversion mechanism. The vent is formed near the screw shaft on the second case.

According to a third feature of the present invention, in addition to the first or second feature, the breathable waterproof member is provided on one side of the vent where the sealingly closed space is located, and a filter is provided on the other side of the vent where the cover member is located.

According to a fourth feature of the present invention, in addition to the third feature, at least one of the breathable waterproof member or the filter is fixed to the vent1w heat sealing.

According to a fifth feature of the present invention, a vehicle drum brake is configured to spread open/contract a brake shoe provided on a backing plate by the electric actuator according to any of the first to fourth features, thereby pressing/stopping pressing the brake shoe against an inner periphery of a brake drum that rotates together with a wheel to generate/release a required braking force. The cover member is a mounting tubular portion for fixing the electric actuator to the backing plate.

According to a sixth feature of the present invention, in addition to the fifth feature, the mounting tubular portion is integrally formed on the backing plate, and the portion of the screw shaft that extends out of the actuator case is inserted in the mounting tubular portion, by which the electric actuator is fixed to the backing plate.

According to a seventh feature of the present invention, in addition to the fifth feature, the mounting tubular portion is attached to the backing plate via a mounting member fixed to the backing plate, and the portion of the screw shaft that extends out of the actuator case is inserted in the mounting tubular portion, by which the electric actuator is fixed to the backing plate.

According to an eighth feature of the present invention, in addition to the seventh feature, the mounting tubular portion is a tubular member, and a discharge port is formed near a lowermost portion thereof.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the first feature of the present invention, the electric actuator establishes the communication of the sealingly closed space in the actuator case and the boot to the atmosphere with the simple structure of only forming the vent on the actuator case, thereby inhibiting a change in the inner pressure in the sealingly closed space that otherwise would he caused due to the sliding motion of the screw shaft or due to a change in the altitude or the temperature. Therefore, the electric actuator can inhibit the expansion and the contraction of the boot to thus achieve the improvement of the durability thereof with the simple structure without causing an increase in the number of components and the complication of the structure and leading to a cost increase accompanying them. Further, water can be securely prevented from entering the sealingly closed space via the vent due to the breathable waterproof member provided on the vent, and therefore high operational stability of the electric actuator can be maintained. Further, due to the provision of the cover member that covers the vent, the vent is kept from being exposed to the atmosphere, and entry of a foreign object from outside can be blocked.

According to the second feature of the present invention, the electric actuator includes the vent formed near the screw shaft on the first case of the actuator, and therefore can effectively inhibit a change in the pressure in the boot according to the sliding motion of the screw shaft and thus achieve the improvement of the durability and the sealing performance of the boot.

According to the third feature of the present invention, the electric actuator includes the breathable waterproof member provided on the sealingly closed space side of the vent and the filter provided on the cover member side of this vent, thereby removing the foreign object contained in the outer air by the filter and preventing the breathable waterproof member from being clogged due to the foreign object Therefore, the function of the breathable waterproof member is kept from being impaired, and this breathable waterproof member can stably maintain a high performance.

According to the fourth feature of the present invention, the electric actuator allows at least one of the breathable waterproof member or the filter to be easily fixed by heat sealing.

According to the fifth feature of the present invention, the vehicle drum brake uses the mounting tubular portion for fixing the electric actuator to the backing plate provided to the vehicle drum brake as the cover member, and therefore can reduce the number of components and achieve a weight reduction and a cost reduction.

According to the sixth feature of the present invention, the electric actuator can be easily attached to the vehicle drum brake using the mounting tubular portion formed integrally with the backing plate, Further, because the portion of the screw shaft that extends out of the actuator case is inserted in the mounting tubular portion, the boot covering the portion of the screw shaft that extends out of the actuator case is protected by the mounting tubular portion, and this boot is prevented from incurring damage due to an external force, thereby enhancing the durability thereof.

According to the seventh feature of the present invention, the electric actuator is configured to be mounted on the backing plate using the mounting tubular portion configured as a different member from the backing plate, and therefore the position at which this electric actuator is mounted can be easily changed. Further, because the portion of the screw shaft that extends out of the actuator case is inserted in the mounting tubular portion, the boot covering the portion of the screw shaft that extends out of the actuator case is protected by the mounting tubular portion, and this boot is prevented from incurring damage due to an external force, thereby enhancing the durability thereof.

According to the eighth feature of the present invention, the discharge port is formed near the lowermost portion of the mounting tubular portion that is the tubular member, and therefore ventilation in the mounting portion is excellently conducted via this discharge port, and water and the like introduced into the mounting portion can be discharged to outside via the discharge port.

REFERENCE SIGNS LIST

DESCRIPTION OF EMBODIMENTS

First, the schematic configuration of a vehicle drum brake according to the present invention will be described now with reference toFIGS.1to3.

FIG.2is a front view of main portions of the vehicle drum brake according to the present invention.FIG.2is a perspective view of the main portions of this vehicle drum brake.FIG.3is a cross-sectional view taken along a line 3-3 illustrated inFIG.1.

A vehicle drum brake30according to the present invention is intended to be mounted on each wheel of a vehicle, and is equipped with an electric parking brake apparatus for electrically locking a rotation of the wheel when the vehicle is parked.

The vehicle drum brake30includes a disk-shaped backing plate31fixed to the vehicle body side, and a circular hole31ais formed at the center of this hacking plate31. A not-illustrated axle is inserted through the circular hole31a. Then, a pair of circular arc-shaped (arched) brake shoes, a rotatable parking brake lever, a return spring, a wheel cylinder, an automatic adjuster, and the like are mounted on the back surface side (the back side in the direction vertical to the paper ofFIG.1) of this backing plate31, although they are not illustrated. The pair of brake shoes is contactable to and separable from the inner peripheral surface of a brake drum that rotates together with the wheel. The parking brake lever spreads open these brake shoes with the aid of the function of an electric actuator1according to the present invention, thereby pressing the brake shoes against the inner peripheral surface of the brake drum. The return spring biases the pair of brake shoes in a closing direction (a direction away from the inner peripheral surface of the brake drum). The wheel cylinder operates according to a hydraulic pressure generated in a master cylinder in reaction to an operation on a brake pedal, thereby pressing the pair of brake shoes against the inner peripheral surface of the brake drum. The automatic adjuster functions to automatically adjust a radial space between the brake shoes and the brake drum.

Then, the above-described electric actuator1. for driving the electric parking brake apparatus is mounted in an obliquely tilted state via a mounting tubular portion22engaged with a mounting bracket32at an upper portion of the front side of the backing plate31. More specifically, the electric actuator1is mounted in an obliquely tilted state in such a manner that the extension end (the front end) of a screw shaft3(refer toFIG.3) is located on the lower side. The mounting tubular portion22also has a function as a cover member that covers a vent29of the electric actuator1, which will be described below.

Now, the mounting bracket32is a metal plate member configured as a different member from the backing plate31, and is attached to the surface of the backing plate31at three portions thereof using studs33and nuts34threadedly engaged with them as illustrated inFIGS.1and2. The electric actuator1is mounted and supported in a state that a snap ring21wound around the outer periphery of an actuator case10of the electric actuator I is engaged with the mounting tubular portion22, and the mounting tubular portion22is correctly positioned on the mounting bracket32.

Then, as illustrated inFIG.3. a portion of the screw shaft3of the electric actuator1that extends out of the actuator case10is covered with an extensible and compressible bellows-like boot15made from a flexible and elastic rubber material or the like, and this boot15is covered with the mounting tubular portion22joined to the mounting bracket32by welding. Therefore, a part of the screw shaft3and the boot15are inserted inside the mounting tubular portion22. Now, one longitudinal end of the mounting tubular portion22. is attached to the end portion of the actuator case10and extends along the screw shaft3, and a cylindrical plug23is attached to an opening portion on the other longitudinal end of the mounting tubular portion22. Then, a discharge port22a shaped like a circular hole is formed. near the lowermost end of the mounting tubular portion22as illustrated inFIG.2with the electric actuator1mounted on the hacking plate31.

In the present embodiment, the mounting tubular portion22and the mounting bracket32are configured as different members from the backing plate31, and the mounting bracket32is configured to be attached to the front side of the backing plate31using the pluralities of studs33and nuts34(three for each of them in the present embodiment), so that the position at which the electric actuator1is mounted on the backing plate31can be easily changed. However, the mounting tubular portion22may be configured integrally with the backing plate31as illustrated inFIG.4. Employing such a configuration can contribute to reducing the nwriber of components and the number of assembling man-hours, thereby achieving a cost reduction of the electric parking brake apparatus.

Then, as illustrated inFIG.3, one end of a deflectable tube24is fixed to the plug23by being inserted therein, and this tube24is bent in a U-like shape after extending from the plug23obliquely downward and the other end thereof is fixed to a connector25attached at a. lower portion of the front side of the hacking plate31by being inserted therein as illustrated inFIGS.1and2. Now, the connector25is attached to the lower portion of the front side of the hacking plate31using two studs37and two nuts38threadedly engaged with them as illustrated inFIGS.1and2.

Further, as illustrated inFIG.3, one end of a brake cable26is attached to the distal end portion of the screw shaft3of the electric actuator1via a connector27. More specifically, the connector27is fixed using a pin28inserted in a circular hole3a provided at the distal end portion of the screw shaft3so as to pierce therethrough, and the one end of the brake cable26is fixed to this connector27by being inserted therein. Now, the brake cable26is inserted through inside the tube24, and the other end thereof is coupled with one end of the not-illustrated parking brake lever mounted on the back surface side of the backing plate31.

Next, the detailed configuration and the operations of the above-described electric actuator1according to the present invention will be described now with reference toFIGS.5to8.

FIG.5is a perspective view of the electric actuator according to the present invention.FIG.6is a plan cross-sectional view illustrating this electric actuator in a state thereof when the screw shaft advances.FIG.7is an enlarged detailed view of a portion labeled7inFIG.6.FIG.8is a plan cross-sectional view illustrating the electric actuator according to the present invention in a state thereof when the screw shaft retreats. in the following description, a direction indicated by arrows illustrated inFIGS.5,6, and8will be defined to be a “front-rear direction”,

The electric actuator1according to the present embodiment is intended to be used as the actuator of the electric parking brake apparatus provided to the vehicle drum brake30illustrated inFIGS.1and2. Then, as illustrated inFIG.6, the electric actuator1includes an electric motor2, a motion conversion mechanism4that converts a rotation of this electric motor2into a reciprocating linear motion of the screw shaft3, and a transmission mechanism5that functions to transmit the rotation of the electric motor2to the motion conversion mechanism4, and these electric motor2, motion conversion mechanism4, and transmission mechanism5are contained in the actuator case10.

The above-described actuator case10includes a case main body11, a motor cover12, and a gear cover13. The case main body11integrally includes a cylindrical first case11A and a second case11B. The motor cover12is attached to the opening end of the first case11.A of this case main body11so as to cover it. The gear cover13is attached to the openinu end of the case main body11so as to cover it from the opposite side from this motor cover12(the rear side inFIG.6).

Then, as illustrated inFIG.6, the electric motor2is contained in the first case11A of the case main body11, and a small-diameter driving gear6is attached to the end portion (the rear end portion) of an output shaft (a motor shaft)2aextending from this electric motor2rearward (to the rear side inFIG.6) that faces inside the gear cover13. Now, one axial end (the front end) of the electric motor2is fittedly held in a recessed portion12aformed on the motor cover12, and the other axial end (the rear end) of this electric motor2is fittedly held in a circular hole11bformed on an end wall11aof the first case11A. The output shaft (the motor shaft)2aof the electric motor2is rotatably supported on the first case11A via a bearing7.

Further, the second case11B of the case main body11is integrally disposed on the side of the first case11A, and the motion conversion mechanism4is contained in this second case1113. This motion conversion mechanism4includes a rotatable nut8, and the above-described screw shaft3threadedly inserted through this nut8reciprocally and linearly movably.

The above-described nut8integrally includes a sleeve portion8A and a drum portion8B large in diameter than this sleeve portion8A, and is rotatably supported on the second case11B via a radial bearing (a ball bearing)9set on the outer periphery of the sleeve portion8A. Now, a female screw8a is helically provided by being engraved on the inner periphery of the sleeve portion RA,

The screw shaft3is a round rod-shaped slide member elongated in the front-rear direction, and the one end of the brake cable26is coupled with the front end portion thereof via the connector27as illustrated inFIG.3. Further, a flange portion3A is integrally formed at the rear end of the screw shaft3, and the outer periphery of this flange portion3A is engaged with a pair of guide grooves14a formed along the axial direction on the inner periphery of a tubular guide member14contained in the gear cover13. Therefore, the rotation of the screw shaft3is prohibited by the guide member14, and the screw shaft3linearly reciprocates in the front-rear direction without rotating.

Then, a male screw3bis helically provided by being engraved in a predetermined range of the outer periphery of the screw shaft3. The screw shaft3is threadedly inserted through the sleeve portion8A of the nut8so as to be able to advance and retreat. The male screw3bprovided by being engraved on the outer periphery of this screw shaft3and the female screw8a. provided by being engraved on the inner periphery of the sleeve portion8A of the nut8are threadedly engaged with each other at the portion of the screw shaft3inserted through the nut8.

Further, the portion of the screw shaft3that extends out of the actuator case10is covered with the bellows-like boot15as described above. One end (the front end) of this boot15is fittedly attached to the outer periphery of the front end portion of the screw shaft3, and the other end (the rear end) of the boot15is fittedly attached to the outer periphery of a tubular portion11cprovided in a protruding manner integrally on the second case11B of the actuator case10.

Then, in the present embodiment, the vent29is formed near the screw shaft3of the second case11B of the actuator case10as illustrated inFIGS.5and6. The vent29serves as a breather mechanism that ventilates a sealingly closed space in this actuator case10and the boot15. More specifically, an end wvall11d, through which the screw shaft3extends, is provided to the second case11B, and the vent29shaped like a circular hole is formed near the screw shaft3on this end wall11d. Then, as illustrated inFIG.7in detail, a breathable waterproof member40is attached on the inner space side (the right side inFIG.7) of the vent29, and a filter41is attached to the outer space side (the left side inFIG.7) of this vent29. Now, the breathable waterproof member40is a member that has a function of blocking entry and exit of water while permitting entry and exit of air into and out of the sealingly closed space in the actuator case10arid the boot15, and this breathable waterproof member40and the filter41are fixed to around the vent29on the end wall11dof the second case11B by heat sealing in the present embodiment. Both the breathable waterproof member40and the filter41are fixed by heat sealing in the present embodiment, but any one of them may be fixed by heat sealing and the attachment of these members is not limited to the present embodiment. Then, the present embodiment employs the configuration in which the vent29formed on the actuator case11B is covered with the mounting tubular portion22.

Further, as illustrated inFIG.6, a plurality of disc springs16is contained in an axially stacked state in an axial space formed between the drum portion8B and the flange portion3A of the screw shaft3inside the drum portion8B of the nut8, and the axial positions of these disc springs16are regulated by a retainer17in abutment with a driven gear20. These disc springs16form a biasing unit that biases the screw shaft3in a direction away from the nut8(rearward).

Now, in the following description, the configuration of the transmission mechanism5will be described with reference toFIG.6.

The transmission mechanism5functions to transmit the rotation of the output shaft2aof the electric motor2to the nut8of the motion conversion mechanism4, and includes the above-described driving gear6attached to the output shaft2aof the electric motor2, an intermediate gear19rotatably supported by a support shaft18, and the driven gear20attached on the outer periphery of the drum portion8B of the nut8.

The above-described intermediate gear19is rotatably supported by the support shaft18provided so as to hang across between the case main body11of the actuator case10and the guide member14, and integrally includes a large-diameter intermediate gear19a and a small-diameter intermediate gear19b having large and small different diameters, respectively. Now, the large-diameter intermediate gear19a is meshed with the driving gear6, and the diameter thereof is set to be larger than the diameter of the driving gear6. Further, the small-diameter intermediate gear19b is meshed with the driven gear20larger in diameter than that, and the driven gear20is fittedly attached to the outer periphery of the drum portion8B of the nut8.

Then, the electric actuator I configured in the above-described manner is used as the actuator of the electric parking brake apparatus as described above, and the operations of this electric actuator1will be described now.

The state illustrated inFIG.6is a state in which the screw shaft3advances and the brake cable26(refer toFIG.3) attached at the distal end of this screw shaft3is loosened, and the brake shoes of the drum brake30illustrated inFIGS.1and2are separated from the inner peripheral surface of the brake drum in this state, Therefore, no frictional resistance force is generated between the brake shoes and the brake drum, and the electric parking brake apparatus is in a state that the parking brake is released.

When a not-illustrated parking brake switch is operated to ON from the above-described state, power is supplied to the electric motor2of the electric actuator I, and this electric motor2is started up. Then, the rotation of the output shaft2aof the electric motor2is transmitted to the motion conversion mechanism4while being slowed down by the transmission mechanism5, and the rotation of the output shaft2aof the electric motor2is converted into the linear motion (the rearward motion) of the screw shaft3,

More specifically, the rotation of the output shaft2aof the electric motor2is transmitted while being slowed down from the driving gear6to the intermediate gear19, and this intermediate gear19rotates at a predetermined speed. Then, the rotation of this intermediate gear19is transmitted to the nut8of the motion conversion mechanism4while being slowed down via the small-diameter intermediate gear19band the driven gear20meshed with each other, and this nut8rotates at a predetermined speed.

As described above, when the nut8rotates, the screw shaft3threadedly inserted through this nut8linearly moves and retreats as illustrated inFIG.8, and the brake cable26(refer toFIG.3) attached to the distal end of this screw shaft3is pulled thereby. Accordingly, the parking brake lever of the electric parking brake apparatus is pulled by the brake cable26to thus spread open the brake shoes, and. these brake shoes are pressed against the inner peripheral surface of the brake drum, as a result of which a frictional resistance force is generated between them. Then, the rotations of the brake drum and the wheel are braked due to this frictional resistance force, and theretbre the vehicle is brought into the parking brake state.

In this manner, when the screw shaft3retreats as illustrated inFIG.8, this screw shaft3enters the sealingly closed space in the actuator case10and the boot15. At this time, if the vent29is not formed on the end wall11dof the second case11B, the air in the sealingly closed space compresses to thus create a positive inner pressure in this sealingly closed space because the volume of the sealingly closed space reduces by an amount corresponding to the volume of the entry of the screw shaft3, and the boot15expands due to this positive pressure, leading to occurrence of various troubles as described above.

In light thereof, in the present embodiment, the vent29is formed on the end wall11dof the second case11B serving as the breather mechanism, and therefore the air (the inner air) in the sealingly closed space is discharged to outside via the vent29as indicated by a solid arrow inFIG.7. As a result, the inner pressure in the sealingly closed space is kept at approximately the atmospheric pressure, and a trouble such as the expansion of the boot15is kept from occurring, The present configuration allows the electric actuator1to also inhibit a change in the inner pressure due to a change in the altitude or the temperature without being limited to the reciprocating linear motion of the screw shaft3.

Next, when the not-illustrated parking brake switch is operated to OFF with the aim of releasing the above-described parking lock state, the electric motor2rotates reversely, and the rotation of the output shaft2a thereof is transmitted to the nut8of the motion conversion mechanism4via a transmission route similar to the above-described route, and therefore this nut8rotates reversely. When the nut8rotates reversely in this manner, the screw shaft3threadedly inserted through this nut8linearly moves and advances, and the electric actuator1returns to the state illustrated inFIG.6, as a. result of which the parking brake of the electric parking brake apparatus is released.

In this manner, when the screw shaft3advances as illustrated inFIG.6, this screw shaft3exits the sealingly closed space in the actuator case10and the boot15, and therefore the inner pressure in the seal ingly closed space reduces by an amount corresponding to the volume of this exit portion. At this time, if the vent29is not formed on the end wall11dof the second case11B, the air in the sealingly closed space expands to thus create a negative inner pressure in this sealingly closed space because the volume of the sealingly dosed space increases by an amount corresponding to the volume of the exit of the screw shaft3, and the boot15dents due to this negative pressure, leading to occurrence of various troubles as described above.

In light thereof; in the present embodiment, the vent29is formed on the end wall11dof the second case11B, and therefore the outer air flows into the sealingly closed space via the vent29as indicated by a dashed arrow inFIG.7. As a result, the inner pressure in the sealingly closed space is kept at approximately the atmospheric pressure, and a trouble such as the denting of the boot15is kept from occurring. In this case, because the outer air flows into the sealingly closed space after passing through the filter41and the breathable waterproof member40, a foreign object contained in the outer air is removed by the filter41, and the breathable waterproof member40is kept from being clogged due to the foreign object and the function of this breathable waterproof member40is kept from being impaired. Further, because entry of moisture contained in the outer air into the sealingly closed space is blocked by the breathable waterproof member40, the functions of the electric motor2, the motion conversion mechanism4, the transmission mechanism5, and the like contained in the sealingly closed space are kept from being impaired, and the operational stability of them can be enhanced. Further, due to the provision of the cylindrical mounting tubular portion22serving as the cover member that covers the vent29, the vent29is kept from being exposed to the atmosphere, and entry of a foreign object from outside can be blocked.

In this manner, the present embodiment establishes the communication of the sealingly closed space in the actuator case10and the boot15to the atmosphere with the simple structure of only forming the vent29on the second case11B of the actuator case10, thereby inhibiting a change in the inner pressure in the sealingly closed space due to the sliding motion of the screw shaft3. Therefore, the present embodiment can inhibit the expansion and the contraction of the boat15to thus achieve the improvement of the durability thereof with the simple structure without causing an increase in the number of components and the complication of the structure and leading to a cost increase accompanying them. Especially, the present embodiment includes the vent29formed near the screw shaft3of the second case11B, and therefore can effectively inhibit a change in the pressure in the boot15according to the sliding motion of the screw shaft3and thus achieve the improvement of the durability and the sealing performance of the boot15. The present embodiment can achieve the simplification of the structure and the cost reduction of the electric actuator1and also enhance the durability of the boot15in this manner, thereby achieving the simplification of the structure, a cost reduction, and the improvement of the durability and the operational stability of the vehicle drum brake30including this electric actuator1as a result thereof.

Further, water can be securely prevented from entering the sealingly closed space via the vent29due to the breathable waterproof member40provided on the vent29, and therefore high operational stability of the electric actuator1can be maintained. Then, the present embodiment includes the breathable waterproof member40provided on the inner space side of the vent29and the filter41provided on the outer space side of this vent, thereby removing the foreign object contained in the outer air by the filter41and preventing the breathable waterproof member40from being clogged due to the foreign object. Further, the present embodiment allows the breathable waterproof member40and the filter41to be easily fixed by heat sealing.

Then, the present embodiment includes the discharge port22aformed near the lowermost portion of the mounting tubular portion22as illustrated inFIG.2, thereby realizing excellent ventilation in the mounting tubular portion22via this discharge port22aand also allowing water and the like introduced into the mounting tubular portion22to be discharged to outside via the discharge port22a.

Further, because the portion of the screw shaft3that extends out of the actuator case is inserted in the mounting tubular portion22, the boot15covering the portion of the screw shaft3that extends out of the actuator case10is protected by the mounting tubular portion22and this hoot15is prevented from incurring damage due to an external force, thereby enhancing the durability thereof.

The above-described embodiments have been described citing the configuration in which the electric actuator accordinc to the present invention is applied to the electric parking brake apparatus of the vehicle drum brake, but the electric actuator according to the present invention is also applicable to an actuator used in another arbitrary apparatus.

Besides that, it is apparent that the applicability of the present invention shall not be limited to the above-described embodiments, and can he modified in various manners within the scope of the technical idea recited in the claims, the specification, and the drawings.