Abstract: A threaded nut of a ball-screw for a brake booster is constructed as a hydraulic piston and is produced from the group of martensitically hardening steels which are non-corroding with respect to brake fluid and which has in percent by weight: between 0.4% and 1.3% carbon (C), up to 2% silicon (Si), up to 2% manganese (Mn), between 12% and 20% chromium (Cr), and phosphorus (P) and sulphur (S) together at less than 0.015%, the balance being iron and, where applicable, melting-related impurities. Addition of the following materials to the above-described steel may be advantageous: up to 2% molybdenum (Mo), up to 0.2% vanadium (V), and up to 3% nickel (Ni).

Abstract: Provided is a separator having excellent chemical resistance, impact resistance, and productivity. A bearing retainer (1) is a separator that is interposed between two spheres adjacent to and moving in conjunction with each other for retaining the spheres configured to be movable in conjunction with and apart from each other and is formed from a resin composition containing a fluororesin which is a polymer of a monomer containing vinylidene fluoride as a main component. The bending modulus of elasticity of the resin composition is 1600 MPa or less, and the Charpy impact strength of the resin composition is 26 kJ/m2 or more.

Abstract: A ball screw apparatus includes a ball nut through which a ball screw shaft is inserted. A recessed portion is formed in an inner periphery of the ball nut and is recessed so as to extend in a tangential direction of a ball track. A ball train including a plurality of main balls and a coil spring are housed in a raceway. The recessed portion includes a first surface that supports a second end of the coil spring via a stopper ball and a second surface that regulates movement of the stopper ball in a nut axial direction. The second surface is inclined by an angle corresponding to a lead angle of the ball track of the ball nut from an axially perpendicular plane of the ball nut so as extend in the direction of the lead angle.

Abstract: The invention relates to a mechanical member wherein a first part in contact with a second part movable relative to the first part, at least one of the parts including a coating of solid lubricant at least at contact zones between the first part and the second part, the solid lubricant being suitable on its own for lubricating the mechanical member. According to the invention, the mechanical member includes retention and capillary distribution means for distributing a fluid between the parts in such a manner as to form a film between said parts, which film can be pierced temporarily at each contact zone.

Abstract: A motion guide device includes a ball nut attached onto a shaft and having helical grooves for forming a ball guiding raceway between the ball nut and the elongated shaft and for receiving ball bearing elements, a circulating device attached to the ball nut and having a pathway communicative with the raceway for forming an endless ball guiding passage between the ball nut and the elongated shaft and the circulating device and for engaging with bearing elements, and a retaining device includes a shorter chain for engaging into the helical grooves or raceway of the screw shaft and the ball nut, and a longer chain for engaging into the pathway of the circulating device.

Abstract: A ball screw comprises a screw, a nut, two return assemblies and a rolling assembly. The two return assemblies are disposed in two ends of the nut, and each of the return assemblies is defined with a return passage provided with two guiding grooves at two sides thereof for guiding the linking portions of the rolling assembly. The guiding grooves twist an angle of about 90 degrees within the return portion of each of the return assemblies to prevent the linking portions from being stretched or extruded improperly to reduce the deformation to the lowest, thus effectively extending the service life of the rolling assembly.

Abstract: A ball screw has a chain received in a circulation path of the ball screw, and the chain comprises a plurality of partition portions and a connecting portion for connecting the partition portions, a plurality of rolling elements are retained in the chain in such a manner that between each two neighboring partition portions is arranged a rolling element, each of the rolling elements is partially in contact with the return passage of each of the return assemblies to define receiving spaces in the return passage for accommodation of the connecting portion, so that the chain can be moved more smoothly in the return passage, and the manufacturing cost of the return assembly is reduced.

Abstract: Push-pull controls utilizing linear and rotational inputs to create linear output are disclosed. One control includes an elongate push rod and a roller pin. The rod has an outer surface, a generally circular cross-section perimeter, a center axis, a proximal end with a user input, and a distal end configured to directly or indirectly interact with an apparatus to be controlled. The rod is rotatable and slidable along the rod axis. The roller pin has a center axis and an outer surface that contacts the push rod outer surface. The roller pin center axis is angularly offset from and non-intersecting with the push rod center axis. The amount of angular offset is greater than zero degrees.

Abstract: A chain for a motion transmission apparatus comprises a plurality of spacers and two links two links at both sides of the respective spacers for connecting spacers together. The spacers are equidistantly spaced from one another by a receiving space which is longitudinally asymmetrical and provided for holding rolling elements.

Abstract: A ball screw has a chain received in a circulation path of the ball screw, and the chain comprises a plurality of partition portions and a connecting portion for connecting the partition portions, a plurality of rolling elements are retained in the chain in such a manner that between each two neighboring partition portions is arranged a rolling element, each of the rolling elements is partially in contact with the return passage of each of the return assemblies to define receiving spaces in the return passage for accommodation of the connecting portion, so that the chain can be moved more smoothly in the return passage, and the manufacturing cost of the return assembly is reduced.

Abstract: A roller screw structure comprises a nut, a screw and a rolling assembly. The rolling assembly is movably disposed between the nut and the screw and includes at least one rolling cone. The slantwise rolling surface of the rolling cone contacts the spiral groove surfaces of the nut and the screw in a line-to-line manner, and the angle between the rolling surface and the self-rotation axis of the rolling cone is from 0 degree to 6.3 degrees. Thereby, making the distance between the self-rotation axis and the rolling surface of the rolling cone of the rolling assembly different can prevent the occurrence of speed difference caused at different rolling paths of the rolling assembly.

Abstract: A linear bearing employs spherical roller bodies which are contained in a roller nut body with two drive helixes and two return helixes which form a continuous loop in the nut body. The return helixes are offset from the drive helixes and have a pressure relief cutout to allow for movement of the spherical roller bodies through the return helix region.

Abstract: A motion guide device includes a ball nut attached onto a shaft and having helical grooves for forming a ball guiding raceway between the ball nut and the elongated shaft and for receiving ball bearing elements, a circulating device attached to the ball nut and having a pathway communicative with the raceway for forming an endless ball guiding passage between the ball nut and the elongated shaft and the circulating device and for engaging with bearing elements, and a retaining device includes a shorter chain for engaging into the helical grooves or raceway of the screw shaft and the ball nut, and a longer chain for engaging into the pathway of the circulating device.

Abstract: A motion guide device includes a ball nut attached onto a shaft and having helical grooves for receiving ball bearing elements, and a ball coupler including a flexible coupling member having a number of openings formed between spacers for receiving ball bearing elements, the ball bearing element is contacted with the ball nut at a contact point which is spaced from the inner peripheral surface of the ball nut for a spacing distance, and the ball bearing element is contacted with the screw shaft at a contact point which is spaced from the outer peripheral surface of the screw shaft for a spacing distance that is preferably smaller than the spacing distance between the contact point of the ball bearing element and the ball nut.

Abstract: A rolling element retainer is provided with two through rolling element retaining spaces. Each of the rolling elements retaining spaces is provided for accommodation of one rolling element. The rolling element retainer is provided with a linking portion at each of two opposite sides thereof. The rolling element retainer is further provided with a rolling element rolling channel in each of two external end surfaces thereof. The angular difference between the respective rolling element rolling channels and the respective linking portions is 0 degree or 90 degrees. By such arrangements, the rolling channels at two ends of the rolling element retainer are provided for contact with the rolling elements therebetween, so as to guide the movement of such rolling elements.

Abstract: A linear motion transmission apparatus with a chain comprises at least one flexible link and a plurality of spacers, the spacers are equidistantly spaced apart and connected by the link, each two neighboring spacers define a holding space for holding rolling elements, each of the spacers has two separated arcuate retaining portions used to contact and hold the rolling elements. A lubricating-oil passage is formed between each of the spacers and the rolling elements to guide the lubricating oil from the non-load area of the outer surface of the rolling element to the load area, allowing the load area to be lubricated sufficiently.

Abstract: A motion transmission apparatus with a chain, wherein the chain comprises a plurality of spacers and two links two links at both sides of the respective spacers for connecting spacers together. The spacers are equidistantly spaced from one another by a receiving space which is longitudinally asymmetrical and provided for holding rolling elements.

Abstract: An intelligent transmission element includes a first element formed in a strip shape and disposed with a slot therein; a second element disposed with a containing space for placing the first element, wherein the inner side of the containing space is disposed with a tank corresponding to the plurality of slots, the tank and the plurality of slots forms a loading path for disposing a plurality of rolling elements and a silencing element. The silencing element is placed between the plurality of rolling elements and contains a dye container for a dye. When the silencing element is damaged due to an external force, the dye inside the dye container of the silencing element would color the transmission element to let the user notice that the silencing element is damaged.

Abstract: A motion guide device includes a ball nut attached onto a shaft and having helical grooves for receiving ball bearing elements, a tubular member is engaged with the ball nut and includes a ball circulating pathway communicative with the helical groove of the ball nut, and includes an intermediate segment, two curved segments, and two end coupling segments each having a ball guiding channel which includes a wider inner section for guiding and circulating the flexible coupling member of a ball coupler to smoothly move through the ball guiding channel of each end coupling segment and for preventing the flexible coupling member from being twisted. The tubular member is preferably formed by two half members.

Abstract: A chain type ball screw has a chain received in a circulation path of the ball screw, and the chain comprises a plurality of partition portions and a connecting portion for connecting the partition portions, a plurality of rolling elements are retained in the chain in such a manner that between each two neighboring partition portions is arranged a rolling element, each of the rolling elements is partially in contact with the return passage of each of the return assemblies to define receiving spaces in the return passage for accommodation of the connecting portion, so that the chain can be moved more smoothly in the return passage, and the manufacturing cost of the return assembly is reduced.

Abstract: A ball circulating member constructed in an external face of a nut is provided with ball scooping sections for scooping up balls rolling on between ball screw grooves of a screw shaft and the nut, along a line tangent to a center orbital circle of the balls. The path length of a ball scooping path section formed in the ball scooping sections is set to be ½ or greater of the diameter of the ball.

Abstract: A motion transmission apparatus with a chain, wherein the chain comprises a plurality of spacers and two links two links at both sides of the respective spacers for connecting spacers together. The spacers are equidistantly spaced from one another by a receiving space which is longitudinally asymmetrical and provided for holding rolling elements.

Abstract: A chain type transmission assembly doesn't have the problem that the chain interferes with the transmission assembly, the thickness of the links of the chain of the transmission assembly is precisely calculated by an equation, so that the chain can be prevented from interfering with the outer surface of the screw, thus substantially improving the efficiency and service life of the transmission assembly.

Abstract: A traction drive power transmission device, where a lubrication oil can be stably and continuously applied for a long period to portions at which rollers and a shaft are in contact with each other, shortage of the lubrication oil does not occur at the contact portions, the lubrication oil is not wasted, and which does not impair work environment by contaminating floor surface etc. A traction drive power transmission device has at least one drive roller (12a), a follower roller (12b), and a circular cylinder-like shaft (11). The shaft (11) is held between the drive roller (12a) and the follower roller (12b). When the drive roller (12a) is rotated, rotational force by the rotation is transmitted to the shaft (11) to drive it in its axial direction. The device has a lubrication oil-delivering portion (20) with application bodies (22-1, 22-2) for applying a lubrication oil only to portions where the shaft (11) is in contact with the drive roller (12a) and the follower roller (12b).

Abstract: A linear bearing employs spherical roller bodies which are contained in a roller nut body with two drive helixes and two return helixes which form a continuous loop in the nut body. The return helixes are offset from the drive helixes and have a pressure relief cutout to allow for movement of the spherical roller bodies through the return helix region.

Abstract: A ball screw comprises a shaft, a nut, two return elements, plural rolling elements and a sleeve. When being rotated, the shaft will drive the nut to move and circulate the rolling elements within a circulation path defined by a loaded path and the return passages of the return elements. The nut is provided with a circular groove in an outer circumferential surface thereof, and the sleeve is disposed in the circular groove. The sleeve is provided with plural push grooves in an inner circumferential surface thereof, and there is an angular difference between a rolling passage in the nut and the respective push grooves, and the push grooves cooperate with the rolling elements which pass through the rolling passage. By such arrangements, the push grooves of the sleeve will be pushed by the rolling elements to convert the linear motion into rotary motion, thus making the sleeve rotate.

Abstract: Provided is a ball screw module including a screw shaft, a nut member, a ball connector, and a circulating device inside the nut member. The screw shaft and the nut member constitute a first load rolling groove. The circulating device, being connected with the first load rolling groove, has a first turning section and a second turning section, and includes a first guiding groove and a second guiding groove. The first guiding groove has a radius of curvature greater than that of the second guiding groove when the ball connector passes the first turning section. The first guiding groove has the radius of curvature smaller than that of the second guiding groove when the ball connector passes the second turning section.

Abstract: A ball screw assembly has an axially extending screw with an external helical groove and a nut with a through bore for receipt of the screw. The through bore has an internal helical groove adapted for cooperation with the external helical groove to define a ball raceway. A plurality of load bearing balls is disposed in the raceway for rolling contact with the internal and external grooves. A spacer is disposed between adjacent ones of the load bearing balls to prevent the balls from contacting one another. Each of the spacers has at least three pairs of diametrically opposite concave dimples adapted for contact with the load bearing balls at separate times from one another.

Abstract: A roller retainer mounted on a roller comprises a first guiding frame and a second guiding frame that are integrally connected with each other in a vertically staggered manner. The first guiding frame and the second guiding frame are located along the two diagonals of the roller respectively. The thickness of the first guiding frame and the second guiding frame is utilized to keep the space between the two non-loading surfaces of roller and a track. The roller retainer can further keep the space between each roller. Thereby, the roller retainer can prevent the rollers from contacting each other; the loading surfaces can still contact the track and the non-loading surfaces are prevented from colliding with and rubbing against the track, thus reducing the noise and ensuring the moving direction.

Abstract: In a ball screw mechanism, a retainer is provided between an outer circumference of a ball screw shaft and an inner circumference of a ball nut and has a plurality of retainer grooves arranged in the circumferential direction for retaining a plurality of rolling balls. Each retainer groove of the retainer takes a form that allows the rolling balls therein to move radially outward but restrains the rolling balls therein from moving radially inward. Therefore, in the ball screw mechanism and an electric power steering device incorporating the same, the movement of the retainer in radial directions can be restricted by the plurality of rolling balls which are rolled along ball rolling grooves spirally formed respectively on the outer circumference of the ball screw shaft and the inner circumference of the ball nut.

Abstract: Between two mutually adjoining load balls 5, there is interposed a retaining piece 10 having two concave surfaces 11 which are respectively opposed to the two adjoining load balls 5. The retaining piece 10 has an outside diameter dimension equal to or more than 0.5 times the outside diameter dimension of the load ball 5. Further, the retaining piece 10 has an outside diameter dimension which, when the retaining piece 10 passes through a screw groove circulation passage and a tube circulation passage, prevents the retaining piece 10 from touching the inner walls of these screw groove circulation passage and tube circulation passage, or has an outside diameter dimension which is equal to or less than 0.9 times the outside diameter dimension of the load ball.

Abstract: A ball screw unit has a plurality of partition-rooms each of them accommodating a spacer ball interposed between two load carrying balls. The partition room is formed into two oblique frustum shaped recesses each of them extends a flared mouth tilted downwards to the longitudinal axis of the ball screw unit. The partition room is made of elastic polymer material to facilitate pushing the spacer ball into an oblate ellipsoidal hole opened on the partition room, and the hole is also communicated with the two recesses. With this structure, the spacer balls can be held stably in the partition rooms without the fear of escaping or occurrence of disorganized ball squeezing during operation of the ball screw unit. Besides, the dimension of spacer ball may be made as small as possible so as to increase the number of load carrying balls thereby resulting in improvement of load carrying capability of the structure.

Abstract: A ball screw in which surface hardness of balls is higher than both of surface hardness of a ball screw groove of a screw shaft and surface hardness of a ball screw groove of a nut, and a solid lubricant film is formed on a surface of the balls with a thickness of 1 ?m or more and 5 ?m or less. Further, cross-sectional shape of grooves 11 and 21 of a screw shaft 1 and a nut 2 is a Gothic arc. A ratio (R/D) of a radius (R) of the arcuate cross-section of the groove 11 with respect to diameter (D) of balls 2 is set to 51.0% or more and 52.0% or less. The solid lubricant film made of MoS2 is formed on the surface of the balls 3 with a thickness of 0.5 ?m and 3.0 ?m or less.

Abstract: A ball screw has a screw shaft, a nut, and balls running in a circulating manner along a ball rolling groove of the screw shaft and a ball rolling groove of the nut. A circulating portion (16) for balls (22) is provided on a screw shaft (12) side, and permanent magnets (24) are arranged at an inlet and an outlet of the circulating portion (16).

Abstract: A ball connecting body for a ball screw or a linear guide-way, comprises spacers, guide linking members, and connecting members. The spacers are interposed between two adjacent balls, thereby preventing the two balls from hitting. The guide linking members are used for linking the two spacers, so that the ball may be maintained in a space formed by the guide linking members and the spacers. The connecting member connects two adjacent spacers. The total width of the connecting member in a direction of a connecting line of the two guide linking members is smaller than the total height of the connecting member in a direction vertical to the connecting line of the two guide linking members, so that the connecting member has an excellent bendable capability in the direction of the connecting line of the two guide linking members.

Abstract: A linear motion device has: a linear motion body externally fitted onto a shaft and making a relative linear motion along the shaft; a plurality of balls retained in a rolling element groove formed in an inner surface of the linear motion body, and rolling between the rolling element groove and the shaft; separators interposed among the balls; and a circulating path formed in the linear motion body and for circulating the balls from one end side of the rolling element groove to the other end side thereof; wherein the linear motion device is charged with grease containing base oil which is 220-395 in worked penetration defined by JIS K2220 and 50-500 mm2/s in kinematic viscosity at 40° C.

Abstract: Ball screw, between the spindle nut (1) and threaded spindle (2) of which at least one ball chain (6, 8) formed from balls (9) rolls in each case along a ball channel (5) which is delimited by ball grooves (3, 4) formed on the spindle nut (1) and on the threaded spindle (2), the ball chain (6, 8) being deflected from one end of the ball channel (5) to a start of this ball channel (5) in deflection portions of the ball channel (5), one or more spring elements (11, 17, 27, 35) arranged in the ball chain (6, 8) between balls (9) of the ball chain (6, 8) exerting a pressure force on the balls (9) in the axis of the ball chain (6, 8).

Abstract: The invention provides a ball screw apparatus which suppresses abrasion and damage by competition of balls, controls spaces between retaining pieces and balls, and prevents retaining pieces from falling, thereby enabling to more improve operating ability. The retaining pieces are positioned between the balls running in a ball rolling path, such that a sum total of the spaces between the retaining pieces and the balls is 50% or less of an outer diameter size of the retaining piece, and at the same time a contact angle between the retaining piece and the ball is 25° or above to 40° or below.

Abstract: An electric power steering apparatus comprises a rack-and-pinion mechanism and a ball screw mechanism. The ball screw mechanism comprises a shaft formed a screw groove and a nut formed a screw groove. A ball-roll path is created spirally by the screw grooves of the shaft and the nut being opposite each other. A plural number of balls are contained into the ball-roll path. A ball-return path is connecting both ends of the ball-roll path so that the balls go through. A circulation path consists of the ball-roll path and the ball-return path, in which the balls move endlessly. A plural number of separators is arranged between the balls respectively in order to prevent balls from contacting each other. A number of the balls is fewer from one ball to three balls than a maximum number of the balls which can be contained into the circulation path.

Abstract: In a ball screw, a nut is fitted with the outer periphery of a screw shaft, ball rolling grooves opposed to each other are formed in the screw shaft and the nut The ball rolling grooves cooperate in forming a ball rolling path therebetween. A tube as a return path disposed on the nut for connecting one end portion of the ball rolling path to the other end portion thereof, the ball rolling path and tube cooperate in forming an endless circulation path. A plurality of load balls are disposed within the endless circulation path. The position of the tube cannot be limited to a single specific position. In the ball screw, retaining pieces are respectively inserted between the mutually adjacent ones of the load balls disposed within the endless circulation path.

Abstract: A linear motion device includes an inner member and an outer member cooperatively forming a ball circulating path therebetween, a plurality of balls rollably disposed in the ball circulating path, and a plurality of retaining pieces interposed between the balls, wherein the ratio of a ball-to-ball direction force applied to the retaining piece by the balls disposed at the both sides thereof to the elastic displacement of the retaining piece by the force is adjusted to a range of from 0.1 to 10 [N/?m] from 0.1 to 10 [N/?m] by predetermining a bore diameter of a through-hole formed at the central portion of the retaining piece to a range of from 35% to 65% of a diameter of the ball.

Abstract: A ball screw in a continuously variable speed transmission has an axially immovable pulley half (2) and axially movable pulley half (3) both mounted on a rotary shaft (4). The axially movable pulley half (3) is moved by a ball screw (20) so as to infinitely vary the belt wrapping radius and thus the transmission speed. A nut (22), forming one part of the ball screw (20), is immovably secured in both axial and rotational directions. A screw shaft (21), forming the other part of the ball screw (20), is movable in both axial and rotational directions. The screw shaft 21 is connected to the axially movable pulley half (3). The axially movable pulley half (3) is axially moved by rotating the screw shaft (21).

Abstract: A higher efficiency roller screw system that includes a nut body designed to engage a threaded shaft. The nut body includes a center bore with closed, partial helical grooves formed therein. Disposed inside the center bore is a shaft with open helical threads formed thereon. A plurality of rollers with non-helical grooves formed thereon are also disposed inside the nut body's center bore and longitudinally aligned with the shaft. The grooves on each roller are designed to engage the grooves on the nut body and the threads on the shaft. The nut body includes a cross-over region where the trajectory of the internal grooves extend radially outward and axially. When the rollers' grooves contact the grooves in the cross-over region, the rollers are forced outward to maintain contact with the grooves on the nut body and are deflected axially. When they move outward, they disengage from the helical threads on the shaft.

Abstract: A ball screw apparatus which can suppress retaining pieces from contacting a wall of a ball rolling path or ball circulating path, and suppress the retaining pieces between the balls from falling down, so that such a ball screw apparatus can be improved in noise reduction, operability and endurance. The retaining piece is rounded at an edge portion opposite to the ball, such that a radius of curvature of the edge portion is defined to be 0.05 mm to 1 mm, and the outer diameter of the retaining piece is in a range of 0.60 times or more to 0.85 times or less of the diameter of the ball.

Abstract: A ball screw which is little subject to deterioration of operating characteristics and occurrence of wear, torque change, noise, vibration, etc. during operation due to stagnation or competition of balls. The ball screw includes a screw shaft having a helical thread groove formed on the outer surface thereof and a nut having a thread groove formed on the inner surface thereof corresponding to the thread groove on the screw shaft which engages with the screw shaft with the interposition of a number of balls rollably provided in a helical ball rolling path formed between the two thread grooves, and spacers between adjacent ones of the number of balls, wherein the variation of the total of the gap between the balls and spacers during the rolling of the balls is smaller than 1/20 of the diameter of the ball.

Abstract: A linear-motion device has: a linear-motion element fitted on a shaft which moves straight along the shaft; a plurality of balls which are retained in a ball groove formed on the inner side of the linear-motion element and roll over between the ball groove and the shaft; a separator interposed between the balls; and, a circulating path formed in the linear-motion element through which the balls are circulated from one end of the ball groove to the other, which is filled with a grease, wherein the separator is a formed product comprising as a resin component a polybutylene naphthalate-based elastomer formed by a hard segment made of a polybutylene naphthalate and a soft segment made of a polycondensate of 2,6-naphthalanedicarboxylic acid with a polytetramethylene ether glycol.

Abstract: The present invention relates to a ball spacer and the rolling device using it. The ball spacer of the present invention makes the ball doesn't touch to his adjoining ball, so as to reduce the noise. Besides, the ball spacer comprises a cylinder surface touching to the ball, so that the spacer can rotate according to the ball and almost hasn't relative motion to his adjoining ball.

Abstract: A rolling element interference preventer for a guide device prevents a number of rolling elements rolling at regular intervals in a continuous circulation path of the guide device from interfering with each other. The rolling element interference preventer is formed of a thermoplastic resin elastomer having a physical property in accordance with an expression, (A×B)÷C?18, where A represents a 10% elongation stress, B represents a tensile strength, and C represents a bending modulus of elasticity.

Abstract: In the ball screw, a ball circulation tube 20 is formed to have such a shape that the movable amount of balls 15 in a first direction (X) intersecting at right angles to the axis of the tube 20 is larger than the movable amount of the balls 15 in a second direction (Y) intersecting at right angles to the first direction (X), and the ball circulation tube 20 is fixed to a nut 13 in such a manner that the outer surface 20a of the tube 20 extending in parallel with the first direction (X) is opposed to the outer surface of the nut 13.

Abstract: A super finishing is carried out after grinding, so that surface roughness of a ball rolling groove of a screw shaft is made 0.12 &mgr;m or less in an average roughness (Ra). Separators 7 having ball holding surfaces shaped in concave surface are disposed between balls respectively.