Patent Publication Number: US-2022212648-A1

Title: Brake apparatus for vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of Korean Patent Application No. 10-2021-0000386 filed on Jan. 4, 2021 and Korean Patent Application No. 10-2021-0003321 filed on Jan. 11, 2021, which are hereby incorporated by reference for all purposes as if set forth herein. 
     TECHNICAL FIELD 
     Exemplary embodiments of the present disclosure relate to a brake apparatus for a vehicle, and more particularly, to a brake apparatus for a vehicle, which generates a braking force by operating a motor. 
     BACKGROUND 
     In general, a brake apparatus for a vehicle presses a piston by converting a rotational force of a drive motor into a rectilinear motion by using a screw-nut mechanism and produces a braking force by pressing a brake pad, which is a friction member, against a disc by moving a caliper by using a pressing force of the piston and a reaction force of the piston. After the braking, the brake apparatus eliminates the frictional force by separating the caliper and the piston from the brake pad by using knock-back between the brake pad and the disc and using a restoring force (roll-back) of a piston seal. 
     In general, an actuator provided in such a brake apparatus for a vehicle includes a motor and a power transmission device that operate friction pads installed on a caliper of the disc brake apparatus at the time of parking a vehicle. A rotational force of the motor of the actuator is transmitted to an input shaft of the caliper through the power transmission device such as a gear. As the input shaft rotates, a piston and a caliper housing move toward each other, and the two friction pads mounted on the piston and the caliper housing press two opposite surfaces of a disc, thereby restricting a rotation of the disc. 
     The actuator of the brake for a vehicle generates various types of noise such as noise amplified by rotations of gears, and the noise results in consumer dissatisfaction. To solve the problem, the actuator of the brake for a vehicle in the related art uses a helical gear that less generates noise. However, the helical gear causes axial forces between the gears, and the axial forces cause the gears to collide with another component, which generates another noise. 
     In addition, a structural limitation of the caliper of the parking brake apparatus in the related art cause the deformation of the caliper as the parking brake apparatus repeatedly operates. For this reason, an axis of a screw is distorted, and an excessive load is applied between the screw mechanism and the nut mechanism, which degrades power transmission efficiency. Further, a gap between the screw and a body is expanded, which causes a leak of oil in the caliper. 
     The background technology of the present disclosure is disclosed in Korean Patent No. 10-1094333 (registered on Dec. 8, 2011 and entitled ‘Caliper-Integrated Electronic Parking Brake Actuator for Vehicle’). 
     SUMMARY 
     An object of the present disclosure is to provide a brake apparatus for a vehicle, which is capable of offsetting axial forces applied to gears. 
     Another object of the present disclosure is to provide a brake apparatus for a vehicle, which is capable of preventing a direct collision between a gear and a housing. 
     Still another object of the present disclosure is to provide a parking brake apparatus for a vehicle, which is capable of preventing deterioration in power transmission efficiency and an oil leakage due to deformation of a caliper. 
     Various embodiments are directed to a brake apparatus for a vehicle, the brake apparatus including: a housing; a drive part configured to generate power by being a drive part configured to generate power; a power transmission part disposed in the housing and configured to transmit the power from the drive part while rotating in conjunction with an operation of the drive part; and an anti-collision part disposed between the housing and the power transmission part and configured to prevent a collision between the housing and the power transmission part. 
     The power transmission part may include: a first power transmission part configured to rotate by receiving the power from the drive part; a second power transmission part configured to engage with the first power transmission part and rotate in conjunction with a rotation of the first power transmission part; and a third power transmission part configured to engage with the second power transmission part and rotate in the same direction as the first power transmission part in conjunction with a rotation of the second power transmission part. 
     The first power transmission part and the third power transmission part may apply axial forces to the second power transmission part in a direction in which the axial forces are offset. 
     A tooth of the first power transmission part and a tooth of the third power transmission part may be inclined at a torsional angle in a second direction, and a tooth of the second power transmission part may be inclined at a torsional angle in a direction opposite to the second direction. 
     The second power transmission part may include: a second rotary shaft fixed to the housing; a first gear configured to rotate about the second rotary shaft and engage with the first power transmission part; a second gear configured to extend from the first gear, have a smaller diameter than the first gear, and engage with the third power transmission part; and a first friction reducer disposed between the first gear and the second rotary shaft and configured to support the first gear so that the first gear is rotatable. 
     The first friction reducer may include: an insertion portion in sliding contact with an inner peripheral surface of the first gear and configured such that the second rotary shaft is inserted into the insertion portion; a support portion extending in a radial direction of the insertion portion and configured to adjoin the housing and support the insertion portion; and a cut-out portion configured to penetrate the insertion portion and induce deformation of the insertion portion when the second rotary shaft is inserted into the insertion portion. 
     The anti-collision part may include: an anti-collision member configured to surround the insertion portion; and a movement prevention portion protruding from the anti-collision member, inserted into the insertion portion, and configured to prevent the anti-collision member from moving relative to the insertion portion. 
     The anti-collision member may provide an elastic restoring force, and as the second power transmission part moves toward the housing, the anti-collision member may be elastically deformed to prevent contact between the second power transmission part and the housing. 
     The drive part may be configured to receive external electric power. 
     Various embodiments are directed to a brake apparatus for a vehicle, the brake apparatus including: a caliper module having a cylinder part that faces a pad plate module; a piston module installed on the cylinder part so as to be movable forward or rearward and configured to deform the cylinder part by using a reaction force generated when the pad plate module is pressed; and a transmission module installed on the cylinder part and configured to move the piston module forward or rearward, in which an installation angle of the transmission module with respect to the piston module is changed when the cylinder part is deformed, such that a movement of the transmission module relative to the cylinder part is prevented. 
     In addition, the cylinder part may be rotated in a first direction by the reaction force generated by the piston module, and the power transmission module may rotate in the first direction together with the cylinder part. 
     In addition, the transmission module may include: a spindle part penetrating the cylinder part and configured to be rotated by a rotational force transmitted from an actuator; and a nut part configured to be in rollable contact with the piston module and press or release the piston module in conjunction with a rotation of the spindle part. 
     In addition, the nut part may include: a head portion having a front surface in rollable contact with a rear surface of the piston module in the first direction; a connection portion extending from a rear surface of the head portion and having an inner peripheral surface thread-coupled to an outer peripheral surface of the spindle part; and an anti-rotation portion disposed at a lateral side of the head portion and configured to prevent an axial rotation of the head portion by interfering with an inner surface of the piston module. 
     In addition, a curvature of the front surface of the head portion may correspond to a curvature of the rear surface of the piston module. 
     In addition, upper and lower surfaces of the head portion may be spaced apart from the inner surface of the piston module at predetermined intervals. 
     In addition, the brake apparatus may further include a sealing member provided between the cylinder part and the spindle part and configured to prevent a leak of oil, and a center of curvature of the head portion may be positioned rearward from the sealing member. 
     According to the brake apparatus for a vehicle according to the present disclosure, the anti-collision part may prevent the second power transmission part from colliding with the housing. Therefore, it is possible to reduce the occurrence of noise due to the instantaneous axial force. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the first power transmission part and the third power transmission part may apply the axial forces to the second power transmission part in the direction in which the axial forces are offset. Therefore, it is possible to prevent an excessive movement of the second power transmission part. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the first power transmission part, the second power transmission part, and the third power transmission part may each are provided in the form of a helical gear. Therefore, it is possible to transmit higher power and reduce noise due to the engagement. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the first friction reducer may reduce the frictional force generated by the rotation of the second power transmission part. Therefore, it is possible to prevent a loss of power transmission efficiency. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the anti-collision member may have the elastic restoring force and absorb impact due to the rapid movement of the second power transmission part. Therefore, it is possible to prevent damage to the second power transmission part. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the anti-collision member is disposed to surround the insertion portion and in close contact with the outer peripheral surface of the insertion portion. Therefore, it is possible to prevent the insertion portion from arbitrarily spreading out and separating from the second rotary shaft. 
     In addition, according to the brake apparatus a vehicle according to the present disclosure, the movement prevention portion may restrict the movement of the anti-collision part relative to the insertion portion. Therefore, it is possible to stably fix the position of the anti-collision member. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the installation angle of the power transmission part with respect to the piston module may be changed when the cylinder part is deformed by the reaction force of the piston module, such that the power transmission part may rotate together with the cylinder part in the first direction, which makes it possible to prevent deterioration in performance of the brake. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the front surface of the head portion may be in rollable contact with the inner rear surface of the piston module in the first direction, which makes it possible to prevent deterioration in power transmission efficiency due to the fitting between the nut part and the spindle part. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the curvature of the front surface of the head portion may correspond in value to the curvature of the rear surface of the piston module, which makes it possible to allow the head portion to smoothly roll without interfering with the rear surface of the piston module. 
     In addition, according to the brake apparatus for a vehicle according to the present disclosure, the center of curvature of the head portion may be positioned rearward from the sealing member, which makes it possible to prevent a leak of oil caused by the expansion of a gap between the cylinder part and the coupling portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically illustrating a configuration of a parking brake apparatus for a vehicle according to an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view schematically illustrating the configuration of the parking brake apparatus for a vehicle according to the embodiment of the present disclosure. 
         FIG. 3  is a cross-sectional view schematically illustrating a state in which the nut part according to the embodiment of the present disclosure is installed. 
         FIG. 4  is a perspective view schematically illustrating a configuration of a nut part according to the embodiment of the present disclosure. 
         FIGS. 5 and 6  are operational views schematically illustrating an operating process of the parking brake apparatus for a vehicle according to the embodiment of the present disclosure. 
         FIG. 7  is a perspective view schematically illustrating a configuration of an actuator according to the embodiment of the present disclosure. 
         FIG. 8  is a cross-sectional view schematically illustrating the configuration of the actuator according to the embodiment of the present disclosure. 
         FIGS. 9 and 10  are exploded perspective views schematically illustrating the configuration of the actuator according to the embodiment of the present disclosure. 
         FIG. 11  is an enlarged perspective view schematically illustrating configurations of a first friction reducer and an anti-collision part according to the embodiment of the present disclosure. 
         FIGS. 12 and 13  are views schematically illustrating an installed state of the anti-collision part and a coupling relationship between the anti-collision part and the first friction reducer according to the embodiment of the present disclosure. 
         FIG. 14  is an operational view schematically illustrating operations of a power transmission part offsetting axial forces according to the embodiment of the present disclosure. 
         FIG. 15  is an operational view schematically illustrating an operation of the anti-collision part preventing a collision according to the embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     Hereinafter, an embodiment of a brake apparatus for a vehicle according to the present disclosure will be described with reference to the accompanying drawings. 
     Here, thicknesses of lines, sizes of constituent elements, or the like illustrated in the drawings, may be exaggerated for clarity and convenience of description. In addition, the terms used below are defined in consideration of the functions thereof in the present disclosure and may vary depending on the intention of a user or an operator or a usual practice. Therefore, such terms should be defined based on the entire contents of the present specification. 
     In addition, in the present specification, when one constituent element is referred to as being “connected to (or coupled to)” another constituent element, the constituent elements can be “directly connected to (coupled to)” each other, and can also be “indirectly connected to (coupled to)” each other with other elements interposed therebetween. Unless explicitly described to the contrary, the word “comprise (or include)” and variations such as “comprises (or includes)” or “comprising (or including)” will be understood to imply the further inclusion of stated elements, not the exclusion of the stated elements. 
     In addition, throughout the specification, the same reference numerals denote the same constituent elements. Even though the same or similar reference numerals are not mentioned or described with reference to specific drawings, the same or similar reference numerals may be described with reference to the other drawings. In addition, even though there are parts denoted by no reference numeral in specific drawings, the parts may be described with reference to the other drawings. In addition, the numbers, shapes, sizes, relative differences in sizes, and the like of the detailed constituent elements illustrated in the drawings of the present application are set for convenience of understanding, do not limit the embodiments, and may be variously implemented. 
       FIG. 1  is a perspective view schematically illustrating a configuration of a parking brake apparatus for a vehicle according to an embodiment of the present disclosure, and  FIG. 2  is a cross-sectional view schematically illustrating the configuration of the parking brake apparatus for a vehicle according to the embodiment of the present disclosure. 
     Referring to  FIGS. 1 and 2 , a parking brake apparatus  1  for a vehicle according to an embodiment of the present disclosure includes a carrier unit  1000 , a caliper module  2000 , pad plate modules  3000 , an actuator  4000 , a piston module  5000 , a transmission module  6000 , and a sealing member  7000 . 
     The carrier unit  1000  is fixed to a vehicle body and supports the caliper module  2000  to be described below. The carrier unit  1000  according to the embodiment of the present disclosure may be fixed to a knuckle of the vehicle body by means of a mounting bolt or the like. The carrier unit  1000  may be made of a material having high rigidity, such as steel, to sufficiently endure a load transmitted from the caliper module  2000 . The shape of the carrier unit  1000  is not limited to the shapes illustrated in  FIGS. 1 and 2  and may be variously changed in design within the technical spirit of the shape capable of being fixed to the vehicle body. 
     The caliper module  2000  defines a schematic external appearance of the parking brake apparatus  1  for a vehicle according to the embodiment of the present disclosure and supports the piston module  5000  and the transmission module  6000  to be described below. The caliper module  2000  according to the embodiment of the present disclosure includes a bridge part  2100 , a finger part  2200 , and a cylinder part  2300 . 
     The bridge part  2100  defines an upper external appearance of the caliper module  2000 . The bridge part  2100  supports the finger part  2200  and the cylinder part  2300  to be described below. The bridge part  2100  is slidably connected to the carrier unit  1000  by means of a guide rod or the like. The bridge part  2100  is slid by a reaction force generated between the pad plate modules  3000  and the piston module  5000 . The specific shape of the bridge part  2100  is not limited to the shapes illustrated in  FIGS. 1 and 2  and the shape of the bridge part  2100  may be variously changed in design. 
     The finger part  2200  extends from one side of the bridge part  2100  and faces any one of the pair of pad plate modules  3000 . The finger part  2200  may be integrated with the bridge part  2100  by welding, pressing, bending, or the like. Alternatively, the finger part  2200  may be detachably coupled to the bridge part  2100  by bolting or the like. The finger part  2200  presses or releases the pad plate module  3000  in conjunction with the sliding of the bridge part  2100 . The finger part  2200  according to the embodiment of the present disclosure perpendicularly extends downward from a front end of the bridge part  2100 . The finger part  2200  has an inner surface disposed to face one of the pair of pad plate modules  3000  disposed at an outer side (left side based on  FIG. 2 ) based on the disc. 
     The cylinder part  2300  extends from the other side of the bridge part  2100  and faces the other of the pair of pad plate modules  3000 . The cylinder part  2300  has therein a vacant space in which the piston module  5000  and the transmission module  6000  may be installed. The cylinder part  2300  may have an oil port through which brake oil is introduced so that a hydraulic pressure may be applied to the inside of the cylinder part  2300 . The cylinder part  2300  according to the embodiment of the present disclosure extends downward from a rear end of the bridge part  2100 . The cylinder part  2300  has a hollow cylindrical shape opened at one side thereof, and the opened side faces the other of the pair of pad plate modules  3000  disposed at an inner side (based on FIG.  2 ) based on the disc  2 . 
     The pad plate modules  3000  are installed to be movable rearward or forward toward the disc  2  that rotates together with a vehicle wheel, and thus apply a braking force to the vehicle. According to the embodiment of the present disclosure, the pair of pad plate modules  3000  is provided. The pad plate modules  3000  respectively face outer and inner surfaces of the disc  2  with the disc  2  interposed therebetween. The pair of pad plate modules  3000  may be connected to the caliper module  2000  or the carrier unit  1000  so as to be movable forward or rearward. A friction pad made of a material, such as rubber, with a high frictional coefficient may be attached to one surface of the pad plate module  3000  facing the disc  2 . 
     The actuator  4000  is installed at a rear side of the caliper module  2000  and generates driving power. The actuator  4000  is connected to the transmission module  6000  to be described below and transmits driving power to the transmission module  6000 . A specific configuration of the actuator  4000  according to the embodiment of the present disclosure will be described below. 
     The piston module  5000  is installed on the cylinder part  2300  so as to be movable forward or rearward and presses or releases the pad plate module  3000  while being moved forward or rearward by the transmission module  6000  to be described below. The piston module  5000  deforms the cylinder part  2300  by using a reaction force generated when the pad plate module  3000  is pressed. More specifically, the piston module  5000  rotates the cylinder part  2300  in a first direction A by using the reaction force generated when the pad plate module  3000  is pressed. In this case, for example, the first direction D may be a counterclockwise direction based on  FIG. 2  in a direction in which the cylinder part  2300  is moved outward from the pad plate module  3000 . 
     The piston module  5000  according to the embodiment of the present disclosure has a cup shape opened at one side thereof. The closed side of the piston module  5000  faces the pad plate module  3000 . An outer surface of the piston module  5000  is spaced apart from an inner surface of the cylinder part  2300  at a predetermined distance, thereby forming a clearance. In this case, the piston module  5000  is elastically supported in the cylinder part  2300  by a corrugated tube installed at a front side of the cylinder part  2300  or an O-ring installed in the cylinder part  2300 . Therefore, the piston module  5000  may prevent deterioration in movement performance due to friction with the inner surface of the cylinder part  2300 . Inner left and right surfaces of the piston module  5000  extend perpendicularly to an axial direction of the cylinder part  2300  and are disposed in parallel with each other, thereby preventing an axial rotation of a nut part  6200 . An inner rear surface of the piston module  5000  may be curved at a predetermined curvature to guide a rolling motion of the transmission module  6000  to be described below. In this case, a center of curvature of the inner rear surface of the piston module  5000  may be positioned rearward from the sealing member  7000  to be described below. Alternatively, the inner rear surface of the piston module  5000  may have a straight shape. 
     The transmission module  6000  is installed in the cylinder part  2300  and moves the piston module  5000  forward or rearward by receiving driving power from the actuator  4000 . When the cylinder part  2300  is deformed, an installation angle of the transmission module  6000  with respect to the piston module is changed, such that the movement of the transmission module  6000  relative to the cylinder part is prevented. More specifically, when the cylinder part  2300  is rotated in the first direction D by a reaction force generated by the piston module  5000 , the transmission module  6000  integrally rotates in the first direction D together with the cylinder part  2300 . Therefore, the transmission module  6000  may prevent a gap of a connected portion with the cylinder part  2300  from being expanded when the cylinder part  2300  is deformed or prevent deterioration in power transmission performance due to a load. 
     The transmission module  6000  according to the embodiment of the present disclosure includes a spindle part  6100  and a nut part  6200 . 
     The spindle part  6100  penetrates the cylinder part  2300  and is rotated by the rotational force transmitted from the actuator  4000 . The spindle part  6100  according to the embodiment of the present disclosure includes a bolt portion  6110 , a flange portion  6120 , and a coupling portion  6130 . 
     The bolt portion  6110  is disposed in the piston module  5000  and extends in a direction parallel to the axial direction of the piston module  5000 . The bolt portion  6110  has a screw thread formed on an outer peripheral surface thereof and is thread-coupled to the nut part  6200  to be described below. The bolt portion  6110  has a front end thereof spaced apart from the inner rear surface of the piston module  5000  at a predetermined interval and facing the inner rear surface of the piston module  5000 . 
     The flange portion  6120  extends from the outer peripheral surface of the bolt portion  6110  in a radial direction of the bolt portion  6110 . Therefore, the flange portion  6120  may have an approximately circular plate shape. The flange portion  6120  is in contact with a thrust bearing installed on a rear surface of the cylinder part  2300  and supports an axial load, i.e., a reaction force transmitted through the bolt portion  6110  during the braking operation. 
     The coupling portion  6130  extends from a rear end of the bolt portion  6110  and penetrates a rear surface of the cylinder part  2300 . The coupling portion  6130  is disposed in parallel with an extension direction of the bolt portion  6110 . The coupling portion  6130  has a spline screw thread formed on an inner peripheral surface thereof and is connected to an output shaft of the actuator  4000 . The coupling portion  6130  is integrally connected to the bolt portion  6110  and axially rotates together with the bolt portion  6110  when the actuator  4000  operates. 
     The nut part  6200  is connected to the spindle part  6100  and moves forward or rearward in a longitudinal direction of the spindle part  6100  in conjunction with the rotation of the spindle part  6100 . The nut part  6200  presses or releases the piston module  5000  while coming into contact with the piston module  5000  or separating from the piston module  5000  in the rotation direction of the spindle part  6100 . The nut part  6200  is in rollable contact with the piston module  5000 . 
       FIG. 3  is a cross-sectional view schematically illustrating a state in which the nut part according to the embodiment of the present disclosure is installed, and  FIG. 4  is a perspective view schematically illustrating a configuration of the nut part according to the embodiment of the present disclosure. 
     Referring to  FIGS. 3 and 4 , the nut part  6200  according to the embodiment of the present disclosure includes a head portion  6210 , a connection portion  6220 , and an anti-rotation portion  6230 . 
     The head portion  6210  defines a front external appearance of the nut part  6200  and faces the inner rear surface of the piston module  5000 . A front surface of the head portion  6210  has a convex shape and is in rollable contact with the rear surface of the piston module  5000  in the first direction D. When the inner surface of the piston module  5000  is curved, a curvature of the front surface of the head portion  6210  may correspond in value to a curvature of the inner rear surface of the piston module  5000 . Upper and lower surfaces of the head portion  6210  are spaced apart from the inner surface of the piston module  5000  at predetermined intervals. Therefore, the head portion  6210  may smoothly rotate in the first direction D without interfering with the rear surface of the piston module  5000 . A center B of curvature of the head portion  6210  is disposed rearward from the sealing member  7000  to be described below on the basis of a case in which the head portion  6210  is in contact with the rear surface of the piston module  5000 . Therefore, the head portion  6210  may allow the coupling portion  6130  to rotate integrally with the cylinder part  2300 , thereby preventing the portion between the coupling portion  6130  and the cylinder part  2300  from being expanded. In consideration of abrasion of the pad plate module  3000 , a radius R of curvature of the head portion  6210 , i.e., a distance from the center B of curvature to a front surface of the head portion  6210  may be longer, by a predetermined length, than a distance from the sealing member  7000  to the front surface of the head portion  6210 . In this case, for example, the predetermined length may be about 20 mm. 
     The connection portion  6220  extends from the rear surface of the head portion  6210  and is connected to the spindle part  6100 . The connection portion  6220  has a screw thread formed on an inner peripheral surface thereof and is thread-coupled to the spindle part  6100 , more specifically, the outer peripheral surface of the bolt portion  6110 . The connection portion  6220  moves the head portion  6210  forward or rearward in conjunction with the rotation of the spindle part  6100 . 
     The anti-rotation portion  6230  is disposed at a lateral side of the head portion  6210  and prevents an axial rotation of the head portion  6210  by interfering with the inner surface of the piston module  5000 . The anti-rotation portion  6230  according to the embodiment of the present disclosure may have a planar shape disposed along a periphery of a lateral side of the head portion  6210 . The anti-rotation portion  6230  is disposed to be perpendicular to the radial direction of the head portion  6210 . The anti-rotation portion  6230  is in surface contact with the in the left and right surfaces of the piston module  5000 , thereby preventing the head portion  6210  from being axially rotated by the spindle part  6100 . Therefore, the anti-rotation portion  6230  may allow the nut part  6200  to be rectilinearly moved by the rotation of the spindle part  6100 . 
     The sealing member  7000  is provided between the cylinder part  2300  and the spindle part  6100  and prevents a leak of oil stored in the cylinder part  2300 . The sealing member  7000  according to the embodiment of the present disclosure may be provided in the form of an O-ring that seals a portion between the coupling portion  6130  and a rear inner surface of the cylinder part  2300 . 
     Hereinafter, an operation of a parking brake apparatus  1  for a vehicle according to the embodiment of the present disclosure will be described in detail. 
       FIGS. 5 and 6  are operational views schematically illustrating an operating process of the parking brake apparatus for a vehicle according to the embodiment of the present disclosure. 
     Referring to  FIGS. 1 to 6 , to apply the braking force to the vehicle, the spindle part  6100  is axially rotated toward one side by the rotational force transmitted from the actuator  4000 . 
     The nut part  6200  thread-coupled to the outer peripheral surface of the spindle part  6100  moves forward in conjunction with the axial rotation of the spindle part  6100 . 
     As the nut part  6200  moves forward by a predetermined distance or more, the front surface of the head portion  6210  comes into contact with the inner rear surface of the piston module  5000  and presses the piston module  5000  forward. 
     The piston module  5000  is moved to the front side of the cylinder part  2300  by the pressing force and brings the pad plate module  3000  disposed therein into contact with the disc  2 . 
     As the piston module  5000  presses the pad plate module  3000  disposed therein, the reaction force is applied to the piston module  5000  in the direction opposite to the pressing force. 
     The caliper module  2000  is slid rearward by the reaction force, and the finger part  2200  brings the pad plate module  3000  disposed at the outside into contact with the disc  2 . Therefore, the pair of pad plate modules  3000  generates the braking force by using the frictional force with the two opposite sides of the disc  2 . 
     Meanwhile, the reaction force applied to the piston module  5000  is transmitted to the cylinder part  2300  sequentially through the piston module  5000 , the nut part  6200 , and the spindle part  6100 . 
     The reaction force transmitted to the cylinder part  2300  presses the rear surface of the cylinder part  2300  in a perpendicular direction, such that torque for rotating the cylinder part  2300  in the first direction D is applied to the cylinder part  2300 . 
     The cylinder part  2300  is deformed while being rotated in the first direction D by the torque. 
     The piston module  5000 , which defines a clearance having a predetermined interval with the inner surface of the cylinder part  2300 , is kept in a horizontal state. 
     The head portion  6210 , which is in rollable contact with the inner rear surface of the piston module  5000 , rotates in the first direction D together with the cylinder part  2300 . In this case, a curvature of the front surface of the head portion  6210  may correspond in value to a curvature of the inner rear surface of the piston module  5000 , and the upper and lower surfaces of the head portion  6210  may be spaced apart from the inner upper and lower surfaces of the piston module  5000  at predetermined intervals. Therefore, the head portion  6210  may roll in the first direction D without separate interference in the state in which the head portion  6210  is in contact with the inner rear surface of the piston module  5000 . 
     As the head portion  6210  rolls in the first direction D, the connection portion  6220 , which is integrally connected to the head portion  6210 , and the spindle part  6100 , which is thread-coupled to the connection portion  6220 , also rotate in the first direction D together with the head portion  6210 . Meanwhile, since the connection portion  6220  rotates integrally with the spindle part  6100 , it is possible to prevent a high load from being applied between the connection portion  6220  and the spindle part  6100  and prevent deterioration in efficiency due to the fitting between the connection portion  6220  and the spindle part  6100 . 
     Since the center B of curvature of the head portion  6210  is positioned rearward from the sealing member  7000 , the transmission module  6000  rotates together with the cylinder part  2300  in the first direction D based on the center B of curvature. Therefore, it is possible to prevent the expansion of the gap between the cylinder part  2300  and the coupling portion  6130  disposed at the rear side based on the center B of curvature. 
     Hereinafter, a configuration of the actuator  4000  according to the embodiment of the present disclosure will be described in detail. 
       FIG. 7  is a perspective view schematically illustrating a configuration of an actuator according to the embodiment of the present disclosure,  FIG. 8  is a cross-sectional view schematically illustrating the configuration of the actuator according to the embodiment of the present disclosure, and  FIGS. 9 and 10  are exploded perspective views schematically illustrating the configuration of the actuator according to the embodiment of the present disclosure. 
     Referring to  FIGS. 7 to 10 , the actuator  4000  according to the embodiment of the present disclosure includes a housing  100 , a drive part  200 , a power transmission part  300 , a fixing part  400 , and an anti-collision part  500 . 
     The housing  100  defines a schematic external appearance of the actuator  4000  according to the embodiment of the present disclosure. The housing  100  has therein a space in which the drive part  200 , the power transmission part  300 , the fixing part  400 , and the anti-collision part  500  may be installed. The housing  100  may be openable and closable to easily install components in the housing  100  and easily manage the components installed in the housing  100 . The housing  100  according to the embodiment of the present disclosure may have a vacant interior and include a housing main body  110  opened at one side thereof, and a cover part  120  detachably coupled to the open side of the housing main body  110  and configured to open or close the interior of the housing main body  110 . 
     The drive part  200  is installed at one side of the housing  100  and generates power by being supplied with electric power from the outside. The drive part  200  may be supplied with the electric power by being electrically connected to a terminal part  401  installed in the fixing part  400  to be described below. For example, the drive part  200  according to the embodiment of the present disclosure may be an electric motor for generating a rotational driving power by being supplied with electric power. A first rotary shaft  201  may be provided at one side of the drive part  200 , extend into the housing  100 , and rotate. 
     The power transmission part  300  are installed in the housing  100 . The power transmission part  300  rotate in conjunction with the operation of the drive part  200  and transmit power to the spindle part  6100  and the piston module  5000 . 
     The power transmission part  300  according to the embodiment of the present disclosure include a first power transmission part  310 , a second power transmission part  320 , a third power transmission part  330 , and a fourth power transmission part  340 . 
     The first power transmission part  310  is rotated by the power transmitted from the drive part  200 . The first rotary shaft  201  of the drive part  200  is inserted into the first power transmission part  310  according to the embodiment of the present disclosure. The first power transmission part  310  is disposed coaxially with the first rotary shaft  201  and rotated integrally with the first rotary shaft  201 . The first power transmission part  310  may be provided in the form of a helical gear having teeth formed on an outer peripheral surface thereof. Therefore, a contact area between the first power transmission part  310  and the second power transmission part  320  to be described below increases, which makes it possible to transmit a higher force and reduce noise occurring when the first and second power transmission part  310  and  320  rotate. The tooth of the first power transmission part  310  may be inclined at a torsional angle in a second direction D. In this case, for example, the second direction A may be a direction inclined leftward (based on  FIG. 8 ) at a predetermined angle with respect to a centerline of the first power transmission part  310 . Therefore, the first power transmission part  310  may apply an axial force downward (based on  FIG. 8 ) to the second power transmission part  320 . 
     The second power transmission part  320  engages with the first power transmission part  310  and rotates in conjunction with the rotation of the first power transmission part  310 . 
     The second power transmission part  320  according to the embodiment of the present disclosure includes a second rotary shaft  321 , a large-diameter gear  322 , a small-diameter gear  323 , a first friction reducer  324 , and a second friction reducer  325 . 
     The second rotary shaft  321  is fixed to the housing  100  and penetrates the large-diameter gear  322  and the small-diameter gear  323  which will be described below. The second rotary shaft  321  according to the embodiment of the present disclosure is provided in the form of a rod and has two opposite ends respectively fixed to the housing main body  110  and the cover part  120 . The second rotary shaft  321  is fixed so as not to rotate relative to the housing  100  when the large-diameter gear  322  and the small-diameter gear  323  rotate. A longitudinal direction of the second rotary shaft  321  is disposed in parallel with a longitudinal direction of the first rotary shaft  201 . 
     The large-diameter gear  322  rotates about the second rotary shaft  321  and engages with the first power transmission part  310 . The second rotary shaft  321  is inserted into the large-diameter gear  322 , and the large-diameter gear  322  is disposed coaxially with the second rotary shaft  321 . The large-diameter gear  322  may be provided in the form of a helical gear having teeth formed on an outer peripheral surface thereof. The outer peripheral surface of the large-diameter gear  322  engages with the outer peripheral surface of the first power transmission part  310 , such that the large-diameter gear  322  rotates in conjunction with the rotation of the first power transmission part  310 . The tooth of the large-diameter gear  322  may be inclined at a torsional angle in a direction opposite to the second direction A. That is, the tooth of the large-diameter gear  322  is inclined rightward (based on  FIG. 8 ) at a predetermined torsional angle with respect to a centerline of the second power transmission part  320 . 
     The small-diameter gear  323  extends from the large-diameter gear  322  and has a smaller diameter than the large-diameter gear  322 . The small-diameter gear  323  is integrally connected to the large-diameter gear  322  and rotated together with the large-diameter gear  322 . The small-diameter gear  323  engages with a third power transmission member  331  of the third power transmission part  330  and transmits rotational power to the third power transmission part  330 . The small-diameter gear  323  according to the embodiment of the present disclosure is provided in the form of a helical gear extending downward (based on  FIG. 8 ) from a central portion of the large-diameter gear  322 . The small-diameter gear  323  is disposed coaxially with the large-diameter gear  322  and rotated about the second rotary shaft  321 . The small-diameter gear  323  may be integrally connected to an inner peripheral surface of the large-diameter gear  322 . Alternatively, the small-diameter gear  323  may be integrally connected to a rear surface of the large-diameter gear  322 . Like the large-diameter gear  322 , the tooth of the small-diameter gear  323  is inclined at a torsional angle in the direction opposite to the second direction A. The small-diameter gear  323  has a smaller diameter than the large-diameter gear  322  so that the small-diameter gear  323  and the large-diameter gear  322  collectively define an approximately T-shaped longitudinal section. Therefore, the small-diameter gear  323  may greatly increase a gear ratio despite a narrow installation space. 
     The first friction reducer  324  is disposed between the large-diameter gear  322  and the second rotary shaft  321  and supports the large-diameter gear  322  so that the large-diameter gear  322  is rotatable relative to the second rotary shaft  321 . More specifically, the first friction reducer  324  fills a clearance between the large-diameter gear  322  and the second rotary shaft  321 , thereby inhibiting a radial movement of the large-diameter gear  322 . An outer peripheral surface of the first friction reducer  324  is in sliding contact with the inner peripheral surface of the large-diameter gear  322 , thereby reducing a frictional force caused by the rotational motion of the large-diameter gear  322 . 
       FIG. 11  is an enlarged perspective view schematically illustrating configurations of a first friction reducer and an anti-collision part according to the embodiment of the present disclosure. 
     Referring to  FIG. 11 , the first friction reducer  324  according to the embodiment of the present disclosure includes an insertion portion  324   a , a support portion  324   b , and a cut-out portion  324   c.    
     The second rotary shaft  321  is inserted into the insertion portion  324   a . An outer peripheral surface of the insertion portion  324   a  is in sliding contact with the inner peripheral surface of the large-diameter gear  322 . The insertion portion  324   a  according to the embodiment of the present disclosure has an approximately cylindrical shape. The second rotary shaft  321  is inserted into the insertion portion  324   a , such that the insertion portion  324   a  surrounds the second rotary shaft  321 . The insertion portion  324   a  is fixed and press-fitted with an outer peripheral surface of the second rotary shaft  321 , such that a movement of the insertion portion  324   a  relative to the second rotary shaft  321  is prevented. An outer surface of the insertion portion  324   a  is made of a material having a low frictional coefficient or a lubricating substance such as oil is applied onto the outer surface of the insertion portion  324   a , such that the outer surface of the insertion portion  324   a  is in sliding contact with the inner peripheral surface of the large-diameter gear  322 . An insertion groove may be formed in an outer surface of the insertion portion  324   a . The insertion groove is recessed into the insertion portion  324   a , and a movement prevention portion  520  to be described below may be inserted into the insertion groove. 
     The support portion  324   b  extends in a radial direction of the insertion portion  324   a , adjoins the housing  100 , and supports the insertion portion  324   a . The support portion  324   b  according to the embodiment of the present disclosure is disposed at an upper end of the insertion portion  324   a  and provided in the form of a circular plate extending in the radial direction of the insertion portion  324   a . An upper surface of the support portion  324   b  is in contact with an inner surface of the cover part  120  of the housing  100  and supports the insertion portion  324   a.    
     The cut-out portion  324   c  penetrates the insertion portion  324   a  and induces deformation of the insertion portion  324   a  when the second rotary shaft  321  is inserted into the insertion portion  324   a . More specifically, the cut-out portion  324   c  is made by forming a cut-out space in the insertion portion  324   a . When the second rotary shaft  321  is inserted into the insertion portion  324   a , the cut-out portion  324   c  induces a diameter of the insertion portion  324   a  to increase to correspond to a diameter of the second rotary shaft  321 . Therefore, the second rotary shaft  321  may be more smoothly inserted into the insertion portion  324   a , and the insertion portion  324   a  may be installed on the second rotary shaft  321  that may have various diameters. The cut-out portion  324   c  according to the embodiment of the present disclosure is provided in the form of a hole made by cutting the insertion portion  324   a  in the longitudinal direction. The cut-out portion  324   c  may be inclined at a predetermined angle with respect to the longitudinal direction of the insertion portion  324   a.    
     The second friction reducer  325  is disposed between the small-diameter gear  323  and the second rotary shaft  321  and supports the small-diameter gear  323  so that the small-diameter gear  323  is rotatable relative to the second rotary shaft  321 . A specific shape of the second friction reducer  325  may be identical to the shape of the first friction reducer  324 . However, the second friction reducer  325  is not limited to the shape, and the shape of the second friction reducer  325  may be variously changed in design within the technical spirit of the shape of the second friction reducer  325  for supporting the small-diameter gear  323  so that the small-diameter gear  323  is rotatable. 
     The third power transmission part  330  engages with the second power transmission part  320  and rotates in the same direction as the first power transmission part  310  in conjunction with the rotation of the second power transmission part  320 . The third power transmission part  330  transmits the rotational force to the fourth power transmission part  340 , to be described below, while rotating. The third power transmission part  330  engages with the second power transmission part  320  so that a direction of an axial force applied to the second power transmission part  320  by the third power transmission part  330  is opposite to a direction of an axial force applied to the second power transmission part  320  by the first power transmission part  310 . Therefore, the axial force applied to the second power transmission part  320  by the first power transmission part  310  and the axial force applied to the second power transmission part  320  by the third power transmission part  330  may be offset. Therefore, it is possible to prevent the second power transmission part  320  from excessively moving in the axial direction. 
     The third power transmission part  330  according to the embodiment of the present disclosure includes the third power transmission member  331  and a third friction reducer  332 . 
     A third rotary shaft  344  of the fourth power transmission part  340 , which will be described below, is inserted into the third power transmission member  331 . The third power transmission member  331  may be provided in the form of a helical gear having teeth formed on an outer peripheral surface thereof. The tooth of the third power transmission part  331  may be inclined at a torsional angle in the second direction A. For example, like the first power transmission part  310 , the second direction A may be a direction inclined leftward (based on  FIG. 8 ) at a predetermined angle with respect to a centerline of the third power transmission member  331 . Therefore, the third power transmission member  331  may apply the axial force upward (based on  FIG. 8 ) to the second power transmission part  320 . 
     The third friction reducer  332  is disposed between the third power transmission member  331  and the third rotary shaft  344  and supports the third power transmission member  331  so that the third power transmission member  331  is rotatable relative to the third rotary shaft  344 . A specific shape of the third friction reducer  332  may be identical to the shape of the first friction reducer  324 . However, the third friction reducer  332  is not limited to the shape, and the shape of the third friction reducer  332  may be variously changed in design within the technical spirit of the shape of the third friction reducer  332  for supporting the third power transmission member  331  so that the third power transmission member  331  is rotatable. 
     The fourth power transmission part  340  is connected to the third power transmission part  330  and rotated in conjunction with the rotation of the third power transmission part  330 . The fourth power transmission part  340  serves to finally transmit the power, which is generated by the drive part  200 , to the spindle part  6100  that presses the piston module  5000 . The fourth power transmission part  340  according to the embodiment of the present disclosure includes a sun gear  341 , planet gears  342 , a carrier  343 , the third rotary shaft  344 , a ring gear  345 , and a fourth friction reducer  346 . 
     The sun gear  341  extends from the third power transmission part  330  and rotates together with the third power transmission part  330 . The sun gear  341  according to the embodiment of the present disclosure is disposed on a central portion of the third power transmission member  331  and perpendicularly extends downward (based on  FIGS. 8 and 10 ) from the third power transmission member  331 . The sun gear  341  may be provided in the form of a spur gear having teeth formed on an outer peripheral surface thereof so as to engage with the planet gears  342  to be described below. The sun gear  341  is concentric with the rotation center of the third power transmission member  331 , such that the sun gear  341  rotates about the same rotation axis as the third power transmission member  331 . The sun gear  341  may be formed integrally with the third power transmission member  331 . Alternatively, the sun gear  341  may be provided separately from the third power transmission member  331  and coupled to and integrated with the third power transmission member  331 . 
     The planet gears  342  engage with the sun gear  341 , rotate about the rotation axes thereof, and revolve around the sun gear  341 . The planet gear  342  may be provided in the form of a spur gear having teeth formed on an outer peripheral surface thereof and engages with an outer peripheral surface of the sun gear  341 . The planet gear  342  is provided in plural, and the plurality of planet gears  342  is disposed around the sun gear  341 . The number of planet gears  342  according to the embodiment of the present disclosure is four, for example. However, the present disclosure is not limited thereto, and the number of planet gears  342  may be three or less or five or more. The plurality of planet gears  342  is disposed at an equal angle about the rotation center of the sun gear  341 . The plurality of planet gears  342  may rotate about the respective rotation axes thereof or revolve around the third rotary shaft  344  in conjunction with the rotation of the sun gear  341 . 
     The carrier  343  supports the planet gears  342  and rotates in conjunction with the revolution of the planet gears  342 . When the planet gears  342  revolve, the carrier  343  rotates the spindle part  6100  while rotating together with the planet gears  342 . 
     Planet gear rotary shafts extend from one side (an upper side based on  FIGS. 9 and 10 ) of the carrier  343  and are respectively inserted into the planet gears  342 . The planet gear rotary shaft is provided in plural, and the plurality of planet gear rotary shafts is equal in number to the planet gears  342 . The planet gear rotary shaft penetrates a central portion of the planet gear  342 . Therefore, the planet gear  342  may rotate about the planet gear rotary shaft. The planet gear rotary shafts are disposed at positions spaced apart from the center of the carrier  343  at predetermined distances. The plurality of planet gear rotary shafts may be disposed at an equal interval in a circumferential direction of the carrier  343 . Therefore, the carrier  343  may axially rotate in conjunction with the revolution of the planet gears  342 . 
     An output shaft coupled to the spindle extends from the other side (a lower side based on  FIGS. 9 and 10 ) of the carrier  343 . The output shaft has spline teeth formed on an outer peripheral surface thereof and is coupled to an inner peripheral surface of the spindle by means of a spline structure. 
     The third rotary shaft  344  extends from the carrier  343  and supports the third power transmission part  330 . The third rotary shaft  344  according to the embodiment of the present disclosure is provided in the form of a rod extending to one side (the upper side based on  FIGS. 9 and 10 ) of the carrier  343 . An end of the third rotary shaft  344  is inserted into the cover part  120  of the housing  100 . The third rotary shaft  344  is integrally connected to the carrier  343  and rotates relative to the housing  100  when the carrier  343  rotates. The third rotary shaft  344  penetrates a central axis of the third power transmission member  311  and supports the third power transmission member  311 . 
     The ring gear  345  has a hollow ring shape. The ring gear  345  has teeth formed on an inner peripheral surface thereof, and the inner peripheral surface of the ring gear  345  engages with the outer peripheral surfaces of the planet gears  342 . The ring gear  345  may rotate together with the planet gears  342 . Alternatively, when the planet gears  342  rotate, the ring gear  345  may be fixed to adjust a gear ratio at which the power is transmitted through the output shaft of the carrier  343 . 
     The fourth friction reducer  346  is disposed between the housing  100  and the third rotary shaft  344  and supports the third rotary shaft  344  so that the third rotary shaft  344  is rotatable relative to the housing  100 . The fourth friction reducer  346  according to the embodiment of the present disclosure has a hollow ring shape, and an outer peripheral surface of the fourth friction reducer  346  is fixed to and press-fitted with the cover part  120  of the housing  100 . An inner peripheral surface of the fourth friction reducer  346  is in sliding contact with an outer peripheral surface of the third rotary shaft  344  and supports the third rotary shaft  344  so that the third rotary shaft  344  is rotatable, thereby reducing a frictional force caused by the rotational motion of the third rotary shaft  344 . 
     The fixing part  400  is installed in the housing  100  and supports the first rotary shaft  201 , the second rotary shaft  321 , and the third rotary shaft  344 . The fixing part  400  according to the embodiment of the present disclosure has an approximately plate shape and is installed between the third power transmission member  331  and the planet gears  342 . The fixing part  400  is disposed to be perpendicular to the longitudinal direction of the first rotary shaft  201 , the second rotary shaft  321 , and the third rotary shaft  344 . The fixing part  400  has a through-hole having a diameter corresponding to a diameter of the first rotary shaft  201 , a diameter of the second rotary shaft  321 , and a diameter of the third rotary shaft  344 . The first rotary shaft  201 , the second rotary shaft  321 , and the third rotary shaft  344  may be supported by being inserted into the through-hole. 
     The terminal parts  401  may be installed on the fixing part  400  and electrically connected to the drive part  200 . A pair of terminal parts  401  may be provided, and the terminal parts  401  are respectively connected to a positive (+) electrode and a negative (−) electrode of the drive part  200 . The terminal parts  401  extend from the fixing part  400  and are connected to a connector installed at one side of the housing  100 . 
     The anti-collision part  500  is disposed between the housing  100  and the power transmission part  300  and prevents a collision between the housing  100  and the power transmission part  300 . More specifically, the anti-collision part  500  provides a buffer means between the housing  100  and the power transmission part  300  and prevents the second power transmission part  320  from colliding directly with the housing  100  by being moved toward the housing  100  by an instantaneous axial force during a process in which the power transmission parts  300  transmit power. Therefore, the anti-collision part  500  may prevent the occurrence of noise caused by the collision between the second power transmission part  320  and the housing  100  and prevent damage to the second power transmission part  320  due to the repeated collision. 
       FIGS. 12 and 13  are views schematically illustrating an installed state of the anti-collision part and a coupling relationship between the anti-collision part and the first friction reducer according to the embodiment of the present disclosure. 
     Referring to  FIGS. 12 and 13 , the anti-collision part  500  according to the embodiment of the present disclosure includes an anti-collision member  510  and the movement prevention portion  520 . 
     The anti-collision member  510  is disposed between the second power transmission part  320  and the first friction reducer  324  and surrounds the insertion portion  324   a . The anti-collision member  510  has an elastic restoring force and is elastically deformed as the second power transmission part  320  moved toward the housing  100 . Therefore, the anti-collision member  510  may absorb impact applied from the second power transmission part  320  because of the instantaneous axial force, thereby preventing direct contact between the second power transmission part  320  and the housing  100 . When the axial force applied to the second power transmission part  320  is eliminated, the elastic restoring force of the anti-collision member  510  may return the second power transmission part  320  to an original position. The anti-collision member  510  according to the embodiment of the present disclosure is provided in the form of a ring made of an elastic material such as rubber or silicone. The anti-collision member  510  is disposed so that two opposite surfaces thereof respectively face the support portion  324   b  and the large-diameter gear  322 . An inner peripheral surface of the anti-collision member  510  is in close contact with the outer peripheral surface of the insertion portion  324   a  in a state in which the anti-collision member  510  surrounds the insertion portion  324   a . Therefore, the anti-collision member  510  may prevent the insertion portion  324   a  from arbitrarily spreading out after the second rotary shaft  321  is inserted into the insertion portion  324   a.    
     The movement prevention portion  520  protrudes from the anti-collision member  510  and is inserted into the insertion portion  324   a , thereby preventing the anti-collision member  510  from moving relative to the insertion portion  324   a.    
     The movement prevention portion  520  according to the embodiment of the present disclosure is provided in the form of a protrusion horizontally protruding from the inner peripheral surface of the anti-collision member  510 . The movement prevention portion  520  is inserted into the insertion groove formed in the outer surface of the insertion portion  324   a  and prevents the anti-collision member  510  from moving in the longitudinal direction of the insertion portion  324   a . Therefore, the movement prevention portion  520  may stably fix a position of the anti-collision member  510 . 
     Hereinafter, an operating process of the brake apparatus  1  for a vehicle according to the embodiment of the present disclosure will be described in detail. 
       FIG. 14  is an operational view schematically illustrating operations of a power transmission part offsetting axial forces according to the embodiment of the present disclosure. 
     First, the power generated by the drive part  200  is transmitted to the first rotary shaft  201  and rotates the first power transmission part  310 . 
     As the first power transmission part  310  rotates, the large-diameter gear  322  of the second power transmission part  320  engaging with the first power transmission part  310  rotates in the direction opposite to the rotation direction of the first power transmission part  310 . 
     Since the tooth of the first power transmission part  310  is inclined at a torsional angle in the second direction A and the tooth of the large-diameter gear  322  is inclined at a torsional angle in the direction opposite to the second direction A, the axial force is applied downward (based on  FIG. 14 ) to the second power transmission part  320 . 
     The small-diameter gear  323  integrally connected to the large-diameter gear  322  rotates together with the large-diameter gear  322 . 
     As the small-diameter gear  323  rotates, the third power transmission member  331  of the third power transmission part  330  engaging with the small-diameter gear  323  rotates in the same direction as the first power transmission part  310 . 
     Since the tooth of the third power transmission member  331  is inclined at a torsional angle in the second direction A and the tooth of the small-diameter gear  323  is inclined at a torsional angle in the direction opposite to the second direction A, the axial force is applied upward (based on  FIG. 14 ) to the second power transmission part  320 . 
     The axial force applied to the second power transmission part  320  by the first power transmission part  310  and the axial force applied to the second power transmission part  320  by the third power transmission part  330  are applied in the directions in which the two axial forces are offset. Therefore, the upward and downward movements of the second power transmission part  320  are inhibited. 
       FIG. 15  is an operational view schematically illustrating an operation of the anti-collision part preventing a collision according to the embodiment of the present disclosure. 
     When the instantaneous axial force is applied upward (based on  FIG. 15 ) to the second power transmission part  320 , the large-diameter gear  322  is moved upward by the axial force. 
     As the large-diameter gear  322  is moved upward by a predetermined distance or more, an upper surface of the large-diameter gear  322  comes into contact with the anti-collision member  510  and compresses the anti-collision member  510  in the upward/downward direction. 
     The anti-collision member  510  is elastically deformed by the large-diameter gear  322 , thereby applying the elastic restoring force in the direction in which the axial forces applied to the second power transmission part  320  are offset. 
     As the anti-collision member  510  is continuously and elastically deformed by the movement of the large-diameter gear  322 , a magnitude of the elastic restoring force applied to the large-diameter gear  322  gradually increases. 
     When a magnitude of the elastic restoring force applied to the large-diameter gear  322  by the anti-collision member  510  is equal to a magnitude of the instantaneous axial force applied to the second power transmission part  320 , the large-diameter gear  322  cannot move any further and is prevented from colliding with the inner surface of the cover part  120 . 
     Thereafter, the anti-collision member  510  is restored to an original shape by the elastic restoring force and returns the large-diameter gear  322  to the original position. 
     While the present disclosure has been described with reference to the exemplary embodiment depicted in the drawings, the exemplary embodiment is described just for illustration, and those skilled in the art to the present technology pertains will understand that various modifications of the exemplary embodiment and any other exemplary embodiment equivalent thereto are available. 
     Thus, the true technical scope of the present disclosure should be defined by the following claims.