Patent Description:
A linear motion actuator is a device including a ball screw device converting rotational motion into linear motion. In the linear motion actuator, when a nut rotates, the amount of protrusion of a screw shaft protruding from the nut changes. With this mechanism, an object mounted on an end of the screw shaft undergoes displacement in an axial direction. Examples of the object mounted on the end of the screw shaft include a piston. Such a linear motion actuator is used for brake boosters as shown in Patent Literature <NUM>, for example.

As shown in Patent Literature <NUM>, the linear motion actuator includes a stroke limiting mechanism. With this stroke limiting mechanism, the movement starting point in time (operation starting point in time) of the screw shaft can be made constant.

In detail, the stroke limiting mechanism of Patent Literature <NUM> is provided with a protruding part on an end face of the nut. A rotation prevention member is mounted on the end of the screw shaft. The rotation prevention member has a locking part protruding radially outward from the end of the screw shaft. When the amount of protrusion of the screw shaft becomes smaller due to the rotation of the nut, the rotation prevention member approaches the nut, and the locking part makes contact with the protruding part. With this contact, the rotation of the nut is restricted, and the position of the screw shaft in the axial direction is positioned.

<CIT> discloses an electro-mechanically actuatable vehicle service brake in which a rolling body threaded drive which changes a rotational movement of an actuator into a translatory movement is drivingly connected to a pressure piston. The pressure piston defines a working pressure chamber which is connected via a hydraulic circuit to wheel brakes on wheels. A radial stop disk is associated with a compensation element which dampens the impact of the radial stop disk on the threaded nut when returning the threaded spindle. The compensating element also makes an oriented threaded roller unnecessary when producing the threaded spindle and threaded nut.

In the stroke limiting mechanism of Patent Literature <NUM>, the rotation prevention member is a separate body from the screw shaft. In addition, the rotation prevention member is mounted on the screw shaft, and thus weight is an obstacle when improving the operability of the linear motion actuator. In addition, the rotation prevention member is mounted on the end of the screw shaft, and thus space for placing the screw shaft and the rotation prevention member is required.

The present invention has been made in view of the above problems, and an object thereof is to provide a linear motion actuator that can achieve parts count reduction, weight reduction, and size reduction.

To achieve the above object, a linear motion actuator according to claim <NUM> is proposed.

According to the present invention, the piston is provided with the stepped face instead of a rotation prevention member. Thus, the parts count is reduced. In addition, the screw shaft achieves weight reduction compared to conventional ones, thereby improving the operability of the linear motion actuator. In addition, not having the rotation prevention member, the linear motion actuator can achieve size reduction. The load input to the stepped face due to contact with the protruding part is dispersed to the piston. Thus, stress is inhibited from being concentrated on the part in which the stepped face is provided.

As a desirable embodiment of the linear motion actuator, the piston has a bottom face surrounding the clearance groove part from the first direction out of the wall faces surrounding the clearance groove part, and at least part of the bottom face is inclined to be positioned in the first direction as the at least part of the bottom face is directed to the one of the rotational direction to be spiral.

According to the above configuration, the recessed amount of the clearance groove part can be reduced compared to a case in which the recessed amount of the clearance groove part is constant in the circumferential direction. In other words, the volume of the piston is inhibited from being reduced. With this effect, the load having been input to the stepped face is easily dispersed, and stress concentration is inhibited.

As a desirable embodiment of the linear motion actuator, the piston has a bottom face surrounding the clearance groove part from the first direction out of the wall faces surrounding the clearance groove part, and at least part of the bottom face is positioned in the first direction in stages as the at least part of the bottom face is directed to the one of the rotational direction to be staircase-shaped.

As a desirable embodiment of the linear motion actuator, the piston has a bottom face surrounding the clearance groove part from the first direction out of the wall faces surrounding the clearance groove part, and at least part of the bottom face is a flat face.

As a desirable embodiment of the linear motion actuator, the piston has a stopper having a face directed to the other of the rotational direction as the stepped face.

As a desirable embodiment of the linear motion actuator, the piston has an inner tube part positioned on a radial inside of the stopper and opening in the second direction provided with a fitting hole into which the one end of the screw shaft is fit, and a radial inside end of the stopper is connected to the inner tube part.

According to the above configuration, the load acting on the stopper is dispersed to the inner tube part. Thus, stress concentration on the stopper is inhibited.

As a desirable embodiment of the linear motion actuator, the piston has an outer tube part positioned on a radial outside of the stopper and having an outer circumferential face sliding with a housing, and a radial outside end of the stopper is connected to the outer tube part.

According to the above configuration, the load acting on the stopper is dispersed to the outer tube part. Thus, stress concentration on the stopper is inhibited.

As a desirable embodiment of the linear motion actuator, the piston has a first end face directed to the first direction, the first end face is provided with a protruding streak protruding in the first direction at a position overlapping the clearance groove part when viewed from an axial direction parallel to the screw shaft, and an amount of protrusion of the protruding streak corresponds to a recessed amount of the clearance groove part.

According to the above configuration, the thickness of the part in which the clearance groove part is formed in the axial direction can be made even. The stepped face and the stopper are covered by the outer tube part, and thus the stepped face and the stopper cannot be visually recognized when assembled to the screw shaft. Thus, it is difficult to determine the phase of the stepped face and the stopper when assembling the screw shaft. However, the phase of the stepped face and the stopper can be grasped from the shape of the protruding streak. Thus, when assembling the screw shaft, phase determination of the stepped face and the stopper is made easy.

As a desirable embodiment of the linear motion actuator, the protruding part has a contact face being in contact with the stepped face, the stepped face is parallel to a first imaginary line extending in a radial direction and is placed on the other of the rotational direction when viewed from an axial direction parallel to the screw shaft, the contact face is parallel to a second imaginary line extending in the radial direction and is placed on the other of the rotational direction when viewed from the axial direction, and a distance between the stepped face and the first imaginary line is larger than a distance between the contact face and the second imaginary line.

According to the above configuration, the stepped face and the contact face are in contact with each other through the radially inside parts. Thus, it is difficult for a load to act on the radially outside parts of the stepped face and the contact face.

As a desirable embodiment of the linear motion actuator, the nut is made of an iron-based material, and the piston is made of an aluminum alloy.

According to the above configuration, the stepped face (the piston) is more easily plastically deformed when the protruding part and the stepped face make contact with each other.

The linear motion actuator of the present invention can achieve parts count reduction, weight reduction, and size reduction.

The following describes the present invention in detail with reference to the accompanying drawings. The present invention is not limited by the following modes for performing the invention (hereinafter referred to as "embodiments"). The components in the following embodiments include ones that can be readily assumed by those skilled in the art, substantially the identical ones, and ones in what is called equivalents. Furthermore, the components disclosed in the following embodiments can be combined with each other as appropriate.

<FIG> is a sectional view of a linear motion actuator of a first embodiment cut in an axial direction. <FIG> is a perspective view of a nut of the first embodiment perspectively viewed from a first direction. <FIG> is a perspective view of a piston of the first embodiment perspectively viewed from a second direction. <FIG> is a IV-IV arrow sectional view of <FIG>.

A linear motion actuator <NUM> of the first embodiment is a brake booster mounted on a vehicle and generating hydraulic pressure corresponding to the amount of depression of a brake pedal. As illustrated in <FIG>, the linear motion actuator <NUM> includes a motor <NUM>, a transmission device <NUM>, a housing <NUM>, a ball screw device <NUM>, a piston <NUM>, and a stroke limiting mechanism <NUM>.

In the following, the direction parallel to an axis O of a screw shaft <NUM> of the ball screw device <NUM> is referred to as an axial direction. In the axial direction, the direction in which the piston <NUM> is placed when viewed from a nut <NUM> of the ball screw device <NUM> is referred to as a first direction X1, whereas the direction opposite to the first direction X1 is referred to as a second direction X2.

The motor <NUM> includes a stator (not illustrated), a rotor (not illustrated), and an output shaft 101a. In the motor <NUM>, power is supplied from a power source (not illustrated) to rotate the rotor and the output shaft 101a. The motor <NUM> is supported by the housing <NUM>, with the output shaft 101a parallel to the screw shaft <NUM>.

The transmission device <NUM> includes a first gear <NUM> fit into the output shaft 101a of the motor <NUM> and a second gear <NUM> fit onto the outer circumferential side of the nut <NUM>. The second gear <NUM> is a gear with a larger diameter than the first gear <NUM>. Thus, the transmission device <NUM> reduces the rotational motion generated by the motor <NUM> and transmits it to the nut <NUM>.

The ball screw device <NUM> includes the nut <NUM>, the screw shaft <NUM>, and a plurality of balls <NUM>. The nut <NUM> is formed in a cylindrical shape about the axis O. An inner circumferential face of the nut <NUM> is provided with an inner raceway 111a. The nut <NUM> is supported by bearings <NUM> fit into an inner circumferential face of the housing <NUM>. With this structure, the nut <NUM> is free to rotate about the axis O of the screw shaft <NUM>.

The following description regarding the rotational direction of the nut <NUM> is based on the case viewed from the first direction X1. As illustrated in <FIG>, the counterclockwise rotational direction about the axis O when viewed from the first direction X1 is referred to as a first rotational direction L1. The clockwise rotational direction about the axis O is referred to as a second rotational direction L2.

As illustrated in <FIG>, the nut <NUM> has one end face <NUM> directed to the first direction X1. The one end face <NUM> is provided with a protruding part <NUM> protruding in the first direction X1. The protruding part <NUM> is substantially trapezoidal when viewed from the axial direction. The protruding part <NUM> has a contact face <NUM> directed to the first rotational direction L1.

As illustrated in <FIG>, the screw shaft <NUM> is a solid shaft component passing through the nut <NUM>. The screw shaft <NUM> includes a screw shaft main body <NUM> provided with an outer raceway 114a on its outer circumferential face and a mounting part <NUM> extending in the first direction X1 from an end face of the screw shaft main body <NUM> in the first direction X1. Although not specifically illustrated, the screw shaft main body <NUM> is supported by the housing <NUM> in a movable manner in the axial direction and in a non-rotatable manner about the axis O.

The inner raceway 111a and the outer raceway 114a form a spiral track therebetween. The ball <NUM> are placed in this spiral track. When the nut <NUM> rotates, the inner raceway 111a pushes the outer raceway 114a in the axial direction via the balls <NUM>. With this pushing, the screw shaft <NUM> moves in the axial direction. In the present embodiment, when the nut <NUM> rotates in the second rotational direction L2, the screw shaft <NUM> moves in the first direction X1. On the other hand, when the nut <NUM> rotates in the first rotational direction L1, the screw shaft <NUM> moves in the second direction X2.

The mounting part <NUM> has a smaller diameter than the screw shaft main body <NUM>. Thus, an annular stepped face 115a directed to the first direction X1 is provided at the boundary between the mounting part <NUM> and the screw shaft main body <NUM>.

The piston <NUM> is a cylindrical component placed coaxially with the axis O. Although the piston <NUM> is preferably manufactured by forging, it may be formed by known methods of machining such as cutting. The piston <NUM> is placed inside a cylinder <NUM>, closer to the end in the second direction X2. Although the cylinder <NUM> of the present embodiment is provided integrally with the housing <NUM>, the cylinder <NUM> and the housing <NUM> may be separate from each other. Inside the cylinder <NUM> is brake fluid, not illustrated. The piston <NUM> includes a first end face <NUM> directed to the first direction X1 and a second end face <NUM> directed to the second direction X2.

The first end face <NUM> is provided with a concave face 121a recessed in the second direction X2. The concave face 121a is opposite a bottom face 107b of the cylinder <NUM>. A coil spring, not illustrated, is placed between the concave face 121a and the bottom face 107b. When the piston <NUM> is pushed in the first direction X1, the piston <NUM> moves against the coil spring, not illustrated. Note that the piston is not necessarily provided with the concave face 121a.

As illustrated in <FIG>, the central part of the second end face <NUM> is provided with a fitting hole <NUM> opening in the second direction X2. The mounting part <NUM> (one end of the screw shaft <NUM>) is inserted into the fitting hole <NUM> (refer to <FIG>). The inner diameter of the fitting hole <NUM> is slightly smaller than the outer diameter of the mounting part <NUM>, providing a tightening allowance. Thus, the piston <NUM>, without separating from the screw shaft <NUM>, moves in the axial direction integrally with the screw shaft <NUM>.

In the following, the part of the piston <NUM> fit onto the mounting part <NUM> (the tubular wall part surrounding the outer circumferential side of the fitting hole <NUM>) is referred to as an inner tube part <NUM>. As illustrated in <FIG>, an end face 124a of the inner tube part <NUM> in the second direction X2 is in contact with the annular stepped face 115a of the screw shaft <NUM>.

As illustrated in <FIG>, an outer circumferential face of the piston <NUM> is in sliding contact with a seal member <NUM> on the inner circumferential side of the cylinder <NUM>. With this structure, the brake fluid, not illustrated, is sealed in so as not to flow toward the nut <NUM> and the screw shaft <NUM>.

The outer diameter of the piston <NUM> is larger than the outer diameter of the nut <NUM>. The second end face <NUM> of the piston <NUM> is provided with an annular outer tube part <NUM> protruding in the second direction X2 to surround the outer circumferential side of the nut <NUM>. In other words, the outer circumferential face of the piston <NUM> is expanded in the second direction X2 by the outer tube part <NUM>. Thus, even when the piston <NUM> moves in the first direction X1, the outer tube part <NUM> and the seal member <NUM> make sliding contact with each other to maintain sealability.

Part of the second end face <NUM> of the piston <NUM> forms an opposite face <NUM> opposite the one end face <NUM> of the nut <NUM>. The opposite face <NUM> is positioned on the radial outside of the inner tube part <NUM> and on the radial inside of the outer tube part <NUM>.

As illustrated in <FIG>, part of the opposite face <NUM> is provided with a clearance groove part <NUM> recessed in the first direction X1. The clearance groove part <NUM> extends in the rotational direction about the axis O and is arc-shaped (C-shaped) when viewed from the axial direction. This clearance groove part <NUM> is a space for avoiding contact with the protruding part <NUM> of the nut <NUM>. In the following, the wall face surrounding the clearance groove part <NUM> from the first direction X1 out of wall faces surrounding the clearance groove part <NUM> is referred to as a bottom face <NUM>.

As illustrated in <FIG> and <FIG>, the recessed amount (depth) of the clearance groove part <NUM> gradually increases from the opposite face <NUM> toward the first rotational direction L1. Thus, the bottom face <NUM> of the clearance groove part <NUM> is a spiral-shaped spiral face, which is positioned in the first direction X1 as it is directed to the first rotational direction L1. The end of the clearance groove part <NUM> in the first rotational direction L1 is provided with a stepped face <NUM> with respect to the opposite face <NUM> and the bottom face <NUM>. On the other hand, the end of the clearance groove part <NUM> in the second rotational direction L2 is provided with a ridgeline <NUM> as a boundary line between the opposite face <NUM> and the bottom face <NUM>.

The clearance groove part <NUM> is a space for avoiding contact with the protruding part <NUM> as described above. Thus, the inclination angle of the bottom face <NUM> of the clearance groove part <NUM> is set to be same as the inclination angle of the inner raceway 111a (refer to <FIG>) or set to be larger than the inclination angle of the inner raceway 111a.

The part out of the opposite face <NUM> not provided with the clearance groove part <NUM> is a stopper <NUM>. The side face of the stopper <NUM> in the second rotational direction L2 is the stepped face <NUM>. The stopper <NUM> is substantially trapezoidal when viewed from the axial direction. As illustrated in <FIG>, the radially inside end of the stopper <NUM> is connected to the inner tube part <NUM>. The radially outside end of the stopper <NUM> is connected to the outer tube part <NUM>.

The following describes the operation of the linear motion actuator <NUM> of the first embodiment. When the motor <NUM> is driven, rotational motion is transmitted to the nut <NUM> via the transmission device <NUM>. With this transmission, the nut <NUM> rotates. When the rotational direction of the nut <NUM> is the second rotational direction L2, the screw shaft <NUM> moves in the first direction X1. Along with this movement, the piston <NUM> also moves in the first direction X1, increasing the hydraulic pressure of the brake fluid. Consequently, the hydraulic pressure of the brake fluid is transmitted to an external device through a through hole 107a.

On the other hand, when the nut <NUM> rotates in the first rotational direction L1, the screw shaft <NUM> moves in the second direction X2. Along with this movement, the piston <NUM> moves in the second direction X2, decreasing the hydraulic pressure of the brake fluid. The distance between the second end face <NUM> of the piston <NUM> and the one end face <NUM> of the nut <NUM> gradually decreases. The protruding part <NUM> of the nut <NUM> enters the clearance groove part <NUM> of the piston <NUM> while rotating in the first rotational direction L1.

After entering the clearance groove part <NUM>, the protruding part <NUM> further rotates in the first rotational direction L1 to make contact with the stepped face <NUM> of the stopper <NUM>. With this contact, the rotation of the nut <NUM> in the first rotational direction L1 stops. After the stop of the rotation of the nut <NUM>, the contact face <NUM> of the nut <NUM> and the stepped face <NUM> of the piston <NUM> are in contact with each other (refer to <FIG>), and thus the nut <NUM> is restricted from rotating in the first rotational direction L1. With this restriction, the screw shaft <NUM> is also restricted from moving in the second direction X2. From the above, when the linear motion actuator <NUM> is operated next time, it starts from the state in which the protruding part <NUM> and the stepped face <NUM> are in contact with each other. In this way, the movement starting point in time (the operation starting point in time) of the screw shaft <NUM> in the axial direction is made constant. In other words, the protruding part <NUM> (the contact face) and the stopper <NUM> (the stepped face <NUM>) form the stroke limiting mechanism <NUM>.

When the protruding part <NUM> makes contact with the stepped face <NUM>, a load is input from the protruding part <NUM> to the stopper <NUM>. The stopper <NUM> is formed integrally with the piston <NUM>, and thus the load is dispersed to the piston <NUM>. The stopper <NUM> is continuous with the inner tube part <NUM> and the outer tube part <NUM>, and the load is easily dispersed to the inner tube part <NUM> and the outer tube part <NUM>. Thus, the load having been input to the stopper <NUM> is dispersed to the parts, and stress is not concentrated on the stopper <NUM>.

As described above, the linear motion actuator <NUM> of the first embodiment has the ball screw device <NUM> having the screw shaft <NUM>, the nut <NUM>, and the balls <NUM>, the piston <NUM> mounted on the one end of the screw shaft <NUM>, and the stroke limiting mechanism <NUM> setting the operation starting point in time of the screw shaft <NUM> toward the first direction X1 pointed by the one end. The nut <NUM> has the one end face <NUM> directed to the first direction X1 and the protruding part <NUM> protruding from the one end face <NUM>. The piston <NUM> has the opposite face <NUM> directed to the second direction X2, which is opposite to the first direction X1, and opposite the one end face <NUM>, the clearance groove part <NUM> recessed from the opposite face <NUM> in the first direction X1 and extending in the rotational direction about the screw shaft <NUM>, and the stepped face <NUM> placed on an end of the clearance groove part <NUM> in one of the rotational direction (the second rotational direction L2) out of the wall faces surrounding the clearance groove part <NUM> and directed to the other of the rotational direction (the first rotational direction L1). The protruding part <NUM> and the stepped face <NUM> are in contact with each other to form the stroke limiting mechanism <NUM>.

According to the linear motion actuator <NUM> of the first embodiment, the rotation prevention member is not required. Thus, the parts count is reduced, and the man-hours for assembly work are reduced. In addition, the screw shaft <NUM> is reduced in weight, improving the operability of the linear motion actuator <NUM>. Furthermore, the linear motion actuator <NUM> can also be reduced in size.

The piston <NUM> of the first embodiment has the stopper <NUM> the face directed to the other of the rotational direction (the first rotational direction L1) of which is the stepped face <NUM>, the inner tube part <NUM> positioned on the radial inside of the stopper <NUM> and provided with the fitting hole <NUM> opening in the second direction X2 into which the one end of the screw shaft <NUM> is fit, and the outer tube part <NUM> positioned on the radial outside of the stopper <NUM> and having an outer circumferential face sliding with the housing <NUM>. The radially inside end of the stopper <NUM> is connected to the inner tube part <NUM>. The radially outside end of the stopper <NUM> is connected to the outer tube part <NUM>.

According to the linear motion actuator <NUM> of the first embodiment, the load having been input to the stopper <NUM> is dispersed to the inner tube part <NUM> and the outer tube part <NUM>. Thus, stress is not concentrated on the stopper <NUM>.

The linear motion actuator <NUM> of the first embodiment has been described. Although the piston <NUM> of the first embodiment has the inner tube part <NUM> and the outer tube part <NUM>, for example, it may be a piston including only an inner tube part, a piston including only an outer tube part, or a piston including neither an inner tube part nor an outer tube part. The shape of the clearance groove part of the piston is not limited to the example shown in the embodiment, either. The following describes modifications in which the clearance groove part is varied in shape. In the modifications, the piston does not have the outer tube part in order to make the shape of the clearance groove part easier to see.

<FIG> is a perspective view of a piston of a first modification perspectively viewed from the second direction. In a piston 120A of the first modification, the recessed amount of a clearance groove part 127A is constant in the circumferential direction. In other words, a bottom face 129A of the clearance groove part 127A is a flat face flat toward the rotational direction. Also in this first modification, as in the first embodiment, the rotation prevention member is not required, achieving parts count reduction and the size reduction of the linear motion actuator.

The end of the clearance groove part 127A in the second rotational direction L2 is a stepped face 131A between the bottom face 129A and the opposite face <NUM>. According to the first modification, the recessed amount of the clearance groove part 127A is larger than that of the clearance groove part <NUM> of the first embodiment. In other words, the piston <NUM> of the first embodiment is larger than the volume of the piston 120A of the first modification. Thus, from the viewpoint of inhibiting stress concentration, the shape of the clearance groove part <NUM> of the first embodiment is more desirable.

<FIG> is a perspective view of a piston of a second modification perspectively viewed from the second direction. As illustrated in <FIG>, in a piston 120B of the second modification, a bottom face 129B of a clearance groove part 127B has a spiral-shaped spiral face 129a and a flat-shaped flat face 129b. In other words, the spiral face 129a extends from the ridgeline <NUM> in the first rotational direction L1, and the flat face 129b extends from the end of the spiral face 129a in the first rotational direction L1 in the first rotational direction L1. Also with the thus configured second modification, the same effects as those of the first embodiment can be obtained. In other words, the bottom face may be a combination of two or more kinds of faces.

<FIG> is a perspective view of a piston of a third modification perspectively viewed from the second direction. As illustrated in <FIG>, a clearance groove part 127C of a piston 120C of the third modification has a recessed amount (depth) from the opposite face <NUM> increasing in stages toward the first rotational direction L1. In other words, a bottom face 129C of the clearance groove part 127C is a staircase-shaped staircase face positioned in the first direction X1 in stages as it is directed toward the first rotational direction L1. Also with the thus configured third modification, the same effects as those of the first embodiment can be obtained.

The recessed amount of the clearance groove part 127C is substantially the same as that of the clearance groove part <NUM> of the first embodiment and the volume of the piston 120B is substantially equal to that of the piston <NUM> of the first embodiment. Thus, as in the first embodiment, this shape easily inhibits stress concentration. Besides, as to the manufacture of the piston 120C of the second modification, when the outer circumferential face of the piston 120C is cut (it is cut from the radial outside) to form the clearance groove part 127C, it is formed more easily than the spiral bottom face <NUM> of the first embodiment. Thus, the manufacture of the piston 120C can be reduced in cost.

<FIG> is a perspective view of a piston of a fourth modification perspectively viewed from the second direction. As illustrated in <FIG>, in a piston 120D of the fourth modification, a bottom face 129D of a clearance groove part 127D is a combination of a spiral face 129a, a stepped face 131A, and a flat face 129b. In other words, the recessed amount of the clearance groove part 127D significantly changes in the middle, forming the stepped face 131A. With this structure, a thick-walled reinforcing part 131D partially remains in the first rotational direction L1 of the stopper <NUM>. Thus, the fourth modification achieves weight reduction compared to the piston <NUM> of the first embodiment, while its shape inhibits stress concentration compared to that of the piston 120A of the first modification.

The modifications about the clearance groove part (the bottom face) have been described. The following describes examples in which the shapes of the contact face and the stepped face are changed.

<FIG> is a plan view of a piston of a fifth modification viewed from the second direction. As illustrated in <FIG>, a stepped face 130E of a piston 120E of the fifth modification is arc-shaped when viewed from the second direction X2. In other words, a radial central part <NUM> of the stepped face 130E protrudes in the second rotational direction L2. Thus, when the protruding part <NUM> and a stopper 128E make contact with each other, the contact face <NUM> makes contact with the central part <NUM> of the stepped face 130E. When the contact is repeated, the central part <NUM> of the stepped face 130E gradually collapses, and the stepped face 130E becomes a flat face. From the above, according to the fifth modification, the stepped face 130E has a shape in which the contact part with the contact face <NUM> of the protruding part <NUM> gradually becomes larger. Also with the fifth modification, the same effects as those of the first embodiment can be obtained.

In the fifth modification, the nut <NUM> is preferably made of an iron-based material, and the piston 120E is preferably made of an aluminum alloy. With this configuration, when the protruding part <NUM> and the stepped face 130E make contact with each other, the stepped face 130E is more easily deformed. Thus, the flattening (plastic deformation) of the stepped face 130E can be accelerated. The use of the aluminum alloy also produces a damping effect (vibration absorption) when the contact face <NUM> of the protruding part <NUM> makes contact therewith. Thus, contact noise can be reduced.

The fifth modification for example shows the arc-shaped stepped face 130E as an example of the shape of the stepped face that is easily plastically deformed. For example, the stepped face may have large surface roughness, although it is substantially flat. With this example, the stepped face becomes less uneven on the surface (the surface roughness becomes smaller) through repeated contact with the protruding part <NUM>.

Microscopic unevenness may be molded onto the stepped face 130E. With this structure, the unevenness is plastically deformed only when an excessive torque is input, allowing the contact faces to acclimate to each other to disperse stress.

<FIG> is a sectional view of a piston of a sixth modification cut in the axial direction. As illustrated in <FIG>, a piston 120F of the sixth modification has an R shape at a corner part <NUM> between the stepped face <NUM> and the bottom face <NUM>. With this structure, the volume of the part of the corner part <NUM> increases, and stress concentration can be inhibited.

<FIG> is a plan view of a piston of a seventh modification viewed from the second direction. <FIG> is a plan view of a nut of the seventh modification viewed from the first direction. <FIG> is a sectional view illustrating a state in which a stopper and a protruding part are in contact with each other in a linear motion actuator of the seventh modification. <FIG> is a sectional view illustrating a state in which a stopper and a protruding part are in contact with each other in a linear motion actuator of a comparative example.

As illustrated in <FIG>, in a piston <NUM> of the seventh modification, a stepped face <NUM> is a face parallel to a face including the axis O and an imaginary line M1 extending perpendicularly from the axis O. Thus, when viewed from the axial direction, the edge of the stepped face <NUM> in the first direction X1 and the edge thereof in the second direction X2 overlap. When viewed from the axial direction, the stepped face <NUM> of the piston <NUM> of the seventh modification is placed (offset) with respect to the imaginary line M1 passing through a central part 128a of the stopper <NUM> in the circumferential direction and the axis O in the second rotational direction L2 and is parallel to the imaginary line M1. The distance between the imaginary line M1 and the stepped face <NUM> is a.

As illustrated in <FIG>, in a nut <NUM> of the seventh modification, a contact face <NUM> is a face parallel to a face including the axis O and an imaginary line M2 extending perpendicularly from the axis O. Thus, when viewed from the axial direction, the edge of the contact face <NUM> in the first direction X1 and the edge thereof in the second direction X2 overlap. When viewed from the axial direction, the contact face <NUM> of the nut <NUM> of the seventh modification is offset with respect to the imaginary line M2 passing through the axis O in the second rotational direction and is parallel to the imaginary line M2. The distance between the imaginary line M2 and the contact face <NUM> is b. The distance a is larger than the distance b (a > b).

According to this seventh modification, when a protruding part <NUM> and a stopper <NUM> make contact with each other, the contact parts are the radially inside parts of the protruding part <NUM> and the stopper <NUM>. Thus, the load acting on the radially outside parts of the protruding part <NUM> and the stopper <NUM> is reduced.

As illustrated in <FIG>, if the distance a and the distance b are equal (a = b), a contact face <NUM> of a protruding part <NUM> and a stepped face <NUM> of a stopper <NUM> will be in contact with each other in parallel (surface contact). Thus, the load acting on the radially outside parts cannot be reduced. The following describes an example in which the first end face of the piston is varied in shape.

<FIG> is a plan view of a piston of an eighth modification viewed from the first direction. As illustrated in <FIG>, the first end face <NUM> of a piston <NUM> of the eighth modification is provided with a protruding streak <NUM> protruding in the first direction X1. The protruding streak <NUM> extends in the rotational direction and is arc-shaped (C-shaped). The protruding streak <NUM> overlaps the clearance groove part <NUM> (refer to <FIG>) viewed from the axial direction. Thus, a plane <NUM> placed between both ends of the protruding streak <NUM> in the rotational direction overlaps the stopper <NUM> (refer to <FIG> and <FIG>).

The amount of protrusion of the protruding streak <NUM> gradually increases as it is directed toward the first rotational direction L1. In other words, a protruding face 134a of the protruding streak <NUM> is a spiral-shaped spiral face. Thus, the end of the protruding streak <NUM> in the first rotational direction L1 is provided with a stepped face <NUM> between the protruding face 134a and the plane <NUM>. On the other hand, the end of the protruding streak <NUM> in the second rotational direction L2 is provided with a ridgeline <NUM> formed by the protruding face 134a and the plane <NUM>.

The amount of protrusion of the protruding streak <NUM> in the first direction X1 is equal to the recessed amount of the clearance groove part <NUM> in the first direction X1. In other words, the thickness of the protruding streak <NUM> in the axial direction from the protruding face 134a to the bottom face <NUM> of the clearance groove part <NUM> is constant in the circumferential direction.

According to the thus configured eighth modification, the axial thickness is equal in the piston <NUM>. In the piston <NUM> including the outer tube part <NUM> (refer to <FIG>), when the piston <NUM> and the screw shaft <NUM> are assembled to each other, the stopper <NUM> and the stepped face <NUM> are covered by the outer tube part <NUM> and cannot be visually recognized (refer to <FIG>). Thus, it is difficult to determine the phase of the stopper <NUM> and the stepped face <NUM> during assembly. On the other hand, according to the eighth modification, the stopper <NUM> and the stepped face <NUM> can be grasped through the plane <NUM> (the protruding streak <NUM>). Thus, phase determination of the stopper <NUM> and the stepped face <NUM> is made easier.

Modifications facilitating phase determination of the stopper <NUM> and the stepped face <NUM> are not limited to the one described above. For example, the piston may be provided with a keyway for rotation prevention on its outer circumferential face. This piston may then enable the phase of the stopper and the stepped face to be grasped with reference to the keyway. In addition, the first end face <NUM> or the outer circumferential face of the piston <NUM> may be marked with a mark.

<FIG> is a sectional view of a linear motion actuator according to a second embodiment. <FIG> is a perspective view of a nut of the second embodiment. <FIG> is a perspective view of a piston of the second embodiment. <FIG> is a sectional view of a linear motion actuator <NUM> according to an embodiment. As illustrated in <FIG>, the linear motion actuator <NUM> has a ball screw device <NUM>, a stroke limiting mechanism <NUM>, a piston <NUM>, a motor <NUM>, and a housing <NUM>.

The ball screw device <NUM> includes a screw shaft <NUM>, a nut <NUM>, and a plurality of balls <NUM>. The screw shaft <NUM> is provided with an outer raceway (first threaded groove) <NUM> on its outer circumferential face. The screw shaft <NUM> passes through the nut <NUM>. The nut <NUM> is provided with an inner raceway (second threaded groove) <NUM> corresponding to the outer raceway (first threaded groove) <NUM> on its inner circumferential face. A spiral track (rolling path) is formed by the outer raceway (first threaded groove) <NUM> and the inner raceway (second threaded groove) <NUM>. The balls <NUM> roll along the track (rolling path). The ball screw device <NUM> is supported by the housing <NUM> via ball bearings <NUM>. As to the ball bearings <NUM>, inner rings <NUM> are fit to both ends of the nut <NUM>, whereas outer rings <NUM> are fit to the housing <NUM>. With this structure, the screw shaft <NUM> and the nut <NUM> can move smoothly relative to each other. The inner rings <NUM> may be molded integrally with the nut <NUM>.

The stroke limiting mechanism <NUM> includes a protruding part <NUM> (refer to <FIG>) provided on one end face of the nut <NUM> and on the radial outside thereof and a locking part <NUM> provided in the piston <NUM>, which is described below. With this structure, the relative displacement between the screw shaft <NUM> and the nut <NUM> is restricted at the stroke end of the screw shaft <NUM> in a contraction direction.

As illustrated in <FIG>, the piston <NUM> is provided with the locking part <NUM>. The locking part <NUM> is provided in a concave shape on an end face of the piston <NUM>. The locking part <NUM> is formed from a stepped face (contact part) <NUM> and a clearance groove part <NUM>. The protruding part <NUM> is in contact with the stepped face (contact part) <NUM>. The clearance groove part <NUM> is deepened in accordance with the lead of the outer raceway (first threaded groove) <NUM>. The piston <NUM> is coupled to a mounting part (shaft part) <NUM> provided on one end of the screw shaft <NUM>, coaxially with the screw shaft <NUM>. The piston <NUM> has a bottomed tubular shape, and the mounting part (shaft part) <NUM> is inserted into its inner diameter side. The piston <NUM> and the mounting part (shaft part) <NUM> are coupled to each other by serration fitting and press fitting to be coupled to each other in such a manner that the piston <NUM> and the mounting part (shaft part) <NUM> cannot rotate and the mounting part (shaft part) <NUM> does not slip out of the piston <NUM> in the axial direction. The material of the piston <NUM> is suitably an aluminum alloy or the like. The clearance groove part <NUM> may have the same depth over the entire face, or it may have a constant depth to the extent that the protruding part <NUM> is not in contact therewith.

The motor <NUM> is placed in the housing <NUM>. The motor <NUM> has an output shaft (drive shaft) <NUM>. A first gear (drive gear) <NUM> is provided at an end of the output shaft (drive shaft) <NUM>. The first gear (drive gear) <NUM> meshes with a second gear (driven gear) <NUM> provided on an outer circumferential face of the nut <NUM>. The first gear (drive gear) <NUM> transmits the rotation of the motor <NUM> to the nut <NUM> via the second gear (driven gear) <NUM>. When the nut <NUM> rotates, the screw shaft <NUM> moves in the axial direction. In this way, the ball screw device <NUM> converts rotational motion into linear motion.

The housing <NUM> includes a first housing <NUM> and a second housing <NUM>. The first housing <NUM> has a first large-diameter recessed part <NUM> with a larger diameter and a first small-diameter recessed part <NUM> with a smaller diameter. To the first large-diameter recessed part <NUM>, the ball bearing <NUM> fit to one side of the nut <NUM> out of both ends of the nut <NUM> is fit. In the first small-diameter recessed part <NUM>, the motor <NUM> is placed. The first large-diameter recessed part <NUM> has a second small-diameter recessed part <NUM> with a smaller diameter than the first large-diameter recessed part <NUM>. The piston <NUM> is slidably fit into the second small-diameter recessed part <NUM>. The second small-diameter recessed part <NUM> serves as a cylinder. The second housing <NUM> has a second large-diameter recessed part <NUM> of the same diameter as the first large-diameter recessed part <NUM> of the first housing <NUM>. Into the second large-diameter recessed part <NUM>, the ball bearing <NUM> fit to the other side of the nut <NUM> is fit.

Although in the present embodiment the stroke limiting mechanism <NUM> includes the protruding part <NUM> provided on the one end face of the nut <NUM> and the locking part <NUM> provided in the piston <NUM>, the protruding part <NUM> of the nut <NUM> and the locking part <NUM> of the piston <NUM> may be provided in an opposite manner. Instead of providing the protruding part <NUM> directly on the end face of the nut <NUM>, a hole may be drilled in the end face of the nut <NUM>, and a pin may be inserted into the hole to make the protruding part <NUM>. Furthermore, in accordance with the pin-shaped protruding part <NUM>, the shape of the locking part <NUM> with which the pin-shaped protruding part <NUM> makes contact may be arc-shaped.

As described above, the linear motion actuator <NUM> of the present embodiment has the ball screw device <NUM>, the stroke limiting mechanism <NUM>, the piston <NUM>, the motor <NUM>, and the housing <NUM>. The ball screw device <NUM> includes the screw shaft <NUM>, the nut <NUM>, and the balls <NUM>. The stroke limiting mechanism <NUM> includes the protruding part <NUM> provided on the one end face of the nut <NUM> and the locking part <NUM> provided in the piston <NUM>. The locking part <NUM> is provided in a concave shape on the end face of the piston <NUM>. The locking part <NUM> is formed from the stepped face (contact part) <NUM> and the clearance groove part <NUM>.

With this structure, by providing the conventional locking part <NUM>, which has been provided in a separate component from the piston <NUM>, in the piston <NUM>, the strength of the locking part <NUM> can be improved without increasing the size of the component. Consequently, excessive stress concentration can be prevented from occurring in the stroke limiting mechanism <NUM> by a simple configuration.

In addition, by providing the locking part <NUM> in the piston <NUM>, the separate component in which the conventional locking part <NUM> has been provided can be eliminated, or in other words, the parts count can be reduced. Furthermore, by the elimination of the separate component in which the conventional locking part <NUM> has been provided, the linear motion actuator <NUM> can be reduced in size.

Claim 1:
A linear motion actuator (<NUM>,<NUM>) comprising:
a ball screw device (<NUM>,<NUM>) having a screw shaft (<NUM>,<NUM>), a nut (<NUM>,<NUM>,<NUM>), and a plurality of balls (<NUM>,<NUM>);
a piston (<NUM>,<NUM>,120A,120B,120C,120D,120E,120F,<NUM>,<NUM>) mounted on one end of the screw shaft (<NUM>,<NUM>); and
a stroke limiting mechanism (<NUM>,<NUM>) setting an operation starting point in time of the screw shaft (<NUM>,<NUM>) toward a first direction pointed by the one end,
the nut (<NUM>,<NUM>,<NUM>) having:
one end face (<NUM>) directed to the first direction; and
a protruding part (<NUM>,<NUM>,<NUM>) protruding from the one end face (<NUM>),
the piston (<NUM>,<NUM>,120A,120B,120C,120D,120E,120F,<NUM>,<NUM>) having:
an opposite face (<NUM>) directed to a second direction, which is opposite to the first direction, and opposite the one end face (<NUM>),
characterized by
a clearance groove part (<NUM>,<NUM>,127A,127B,127C,127D) recessed from the opposite face (<NUM>) in the first direction and extending in a rotational direction about the screw shaft (<NUM>,<NUM>); and
a stepped face placed on an end of the clearance groove part (<NUM>,<NUM>,127A,127B,127C,127D) in one of the rotational direction out of wall faces surrounding the clearance groove part (<NUM>,<NUM>,127A,127B,127C,127D) and directed to another of the rotational direction, and
the protruding part (<NUM>,<NUM>,<NUM>) and the stepped face are in contact with each other to form the stroke limiting mechanism (<NUM>,<NUM>).