BALL SCREW UNIT

Abstract: A ball screw device provided with a screw shaft having on the outer peripheral surface thereof a spiral ball rolling groove, a nut assembly having a load region composed of a spiral load rolling groove facing the ball rolling groove of the screw shaft and a non-load region connecting for communication start and terminal ends of the load region with each other to define an endless track for balls, and a ball connecting body formed of a multiplicity of balls rolling on the endless track of the nut assembly and a connecting body for rotatably holding the balls in an aligned state, wherein a guide groove is formed in the endless track of balls for guiding the connecting body of the ball connecting body to the non-load region so that the respective balls move the connecting body in the load region and the connecting body guides and moves the respective balls in the non-load region of the endless track, and leading portions are formed at least both end portions of the non-load region near the load region for guiding the connecting body along a direction in which the balls moving in the load region advance. Transfer of the balls between the load region and the non-load region in the endless track is made smoothly to achieve smooth circulation of the balls.

Description: The present invention relates to a ball screw unit comprising a screw and a nut assembly which are screw-fitted with each other through a number of balls and adapted to covert the rotary motion of a motor or the like to a linear motion at the slide portion of a machine tool or an industrial robot, and more particularly, to a tube type ball screw unit using a nut and a ball circulating cylindrical body or a side cover type ball screw unit using a nut and a pair of covers respectively fixed to both ends of the nut for the purpose of forming an endless track for the balls. BACKGROUND TECHNOLOGY Generally, the ball screw unit basically comprises a screw provided with a helical ball rolling groove on the outer peripheral surface thereof, a nut provided on the inner peripheral surface thereof with a helical load rolling groove held opposite to the ball rolling groove of the screw and having the same lead angle as the latter and a number of balls interposed between the load rolling groove and the ball rolling groove so as to roll while bearing a load and wherein an endless track for causing the balls to circulate therethrough is formed by constructing a nut assembly by fixing a ball circulating tubular body or a pair of covers to the nut. Conventionally, there have been known several methods for forming the above-described ball endless track. For example, regarding the tube type ball screw unit, there is known a tube system (Japanese Unexamined Utility Model Publication No.S49-64,672) in which as shown in Figs. 23 and 24, ball tubes(f)(ball circulating tubular bodies) each provided on both ends thereof with tongues (g) are passed between the starting end and the terminating end of a load zone formed by a ball rolling groove(b) of a screw(a) and a load rolling groove(d) of a nut( c) such that each of balls(e) rolling within the ball rolling groove(b) of the load zone is picked up or fed into the groove (b) by the tongues (g) projecting into the groove (b) so that each of the balls(e) coming out from the terminating end of the load zone is circulated again into the starting end of the load zone through the ball tubes (f). According to this tube system, however, when each of the balls(e) moves from the terminating end of the load zone of the endless track to the no-loaded zone thereof, the ball(e) runs against the tongue(g) of each of the tubes (f) so that the rolling ball(e) is picked up into the ball tube(f) forming the no-load zone and is then circulated again into the starting end of the load zone. Further, as regards the side cover type ball nut and screw unit, there is known a side cover system disclosed in the United States Patent No. 4,148,226 (entitled "Ball nut and screw assembly") in which as shown in Figs. 25 through 27, a ball return hole(h) is drilled in a nut(c) along an axial direction of the outer peripheral solid portion thereof and a pair of side covers (k) are respectively fixed to both ends of the nut(c) through spacers(i) so that the progressing direction of each of balls(e) coming out of the terminating end of a load zone formed by a ball rolling groove (b) of a screw(a) and a load rolling groove(d) of the nut(c) is inclined by each of the side covers(k) and each of the spacers ( i) thereby guiding the ball(e) from the terminating end of the load zone up to the outer cylindrical surface of the screw (a). Further, a change direction path(j) for guiding the ball(e) up to the ball return hole(h) of the nut(c) is formed in each of the side covers thereby forming a ball endless track. However, this system is also constructed such that when the ball(e) moves from the terminating end of the load zone of the ball endless track to the no-load zone, the ball (e) runs against the side wall of the change direction path (j) inclined with respect to the ball progressing direction at the terminating end of the load zone, that is, from the tangential direction along the lead angle of the load rolling groove(d), so that the rolling ball(e) is picked up into the change direction path(j) forming the no-load zone and is circulated again to the starting end of the load zone. Therefore, in the cases of the above-described conventional ball screw units, when each of the balls rolling in the load zone is picked up into the no-load zone or the balls in the no-load zone are fed to the load zone, the presence of the tongue (g) for changing the progressing direction of the rolling balls is indispensable so that the smooth circulation of the balls is impaired and as a result, the uniform and stabilized rotation of the ball screw unit is impaired, the working life of the unit is shortened or noises generated. Further, in Unexamined Published Japanese Utility Model Application No.H5-27,408, there is disclosed a ball screw unit wherein as shown in Fig. 28, a flexible belt-like retainer (i) is slidably inserted into a ball circulation path (h) provided with a ball circulating pipe (a ball tube (f)) so as to extend over the entire length of the path (h) and a number of ball pockets (not shown) are mounted on the retainer (i) at predetermined intervals over the entire length thereof so that balls(e) are rotatably retained within the pockets, respectively, whereby adjoining balls do not come into contact with each other, the ball screw unit can have an excellent operation characteristic free of torque dispersion and can allow the balls to smoothly circulate through the ball circulation path(h). However, in the case of such a ball screw unit, the belt-like retainer(i) assembled into the ball circulation path(h) is inevitably twisted and bent and it is necessary to absorb such twisting and bending of the retainer (i) within the ball circulation path(h) so that although the idea of "providing a ball screw unit having an excellent operation characteristic by preventing adjoining balls from coming into contact with each other by the belt-like retainer assembled-into the ball circulating path" may be accepted as reasonable, there is actually no suitable means as to how to absorb the twisting and bending of the belt-like retainer (i) within the ball circulating path (h) and it is the actual situation that such an idea has not been materialized as a complete product. Thus, in the cases of the conventional ball screw units, irrespective of whether or not they employ the system for forming a ball endless track, when each of the balls rolling in the load zone is picked up into the no-load zone or each of the balls in the no-load zone is fed into the load zone, it becomes necessary to provide a means for changing the progressing direction of the balls so that the smooth circulation of the balls is inevitably impaired which results in that the uniform and stabilized rotation of the ball screw unit is impaired, the working life of the unit is shortened or noises generated. Therefore, as a result of their intensive investigation of means for fundamentally solving the above-described problems, the present inventors have completed the present invention by finding out the following facts. When a ball coupler is formed by making use of a plurality of flexible coupling members having the function of rotatably retaining a number of balls in their aligned state, a guide groove for guiding the coupling members of the ball coupler is formed in a no-load zone of a ball endless track, the ball coupler is moved by each of the balls in a load zone of the endless track while the balls are guided to move in the no-load zone of the endless track by the coupler and guide sections for guiding the coupling members of the ball coupler along the progressing direction of the balls moving in the load zone are respectively provided at both ends of the no-load zone at least near the load zone, the delivery and reception of the balls between the load zone and the no-load zone of the endless track are performed smoothly to thereby achieve the smooth circulation of the balls within the endless track. Accordingly, an object of the present invention is to provide a ball screw unit which comprises a screw, a nut assembly screw-fitted with the screw through balls and having a ball endless track and a ball coupler assembled into the endless track of the nut assembly which makes smooth the delivery and reception of the balls to be performed between a load zone and a no-load zone of the endless track thereby achieving the smooth circulation of the balls. Another object of the present invention is to provide a tube type ball screw unit including a nut assembly comprising a nut having a load rolling groove forming a load zone of a ball endless track and a ball circulating tubular body having a no-load ball path forming a no-load zone of the ball endless track whereby the delivery and reception of the balls to be performed between the load zone and the no-load zone of the ball endless track can be performed smoothly thereby achieving the smooth circulation of the balls. These objects are achieved by a ball screw unit according to claim 1. JP 05 027408 discloses a ball screw unit according to the preamble of claim 1 DISCLOSURE OF THE INVENTION The present invention relates to a ball screw unit comprising a screw having a helical ball rolling groove on the outer peripheral surface thereof, a nut assembly having a load zone formed by a helical load rolling groove opposing to the ball rolling groove of the screw and a no-load zone forming a ball endless track by establishing communication between the starting end and the terminating end of the load zone, a number of balls rolling within the endless track of the nut assembly and a ball coupler formed of coupling members for retaining the balls to rotate in their aligned state, wherein the no-load zone of the ball endless track is provided with a guide groove for guiding the coupling members of the ball coupler, the coupling members are moved by the balls in the load zone while they guide and move the balls in the no-load zone of the endless track and further, guide sections for guiding the coupling members along the progressing direction of the balls moving in the load zone are respectively provided at least at both ends of the no-load zone near the load zone. The ball screw unit of the present invention described above is of a tube type wherein the nut assembly comprises a nut having on the inner peripheral surface thereof a helical load rolling groove opposing to the above-described ball rolling groove of the screw and a ball circulating tubular body fixed to the nut and having a no-load ball path forming a ball endless track by establishing communication between the starting end and the terminating end of the-load rolling groove wherein a guide groove for guiding the coupling members of the ball coupler is formed in the no-load zone of the endless track that is formed by the no-load ball path such that the coupling members are moved by the balls in the load zone of the endless track that is formed by the load rolling groove while the balls are guided to move by the coupling members in the no-load zone of the endless track, and guide sections for guiding the coupling members along the progressing direction of the balls moving along the load rolling groove are formed in the no-load ball path at least on the side of the load rolling groove. First, the tube type ball screw unit of the present invention will be described in detail. The first structure of the tube type ball screw unit of the present invention is such that the unit comprises a screw having a helical load rolling groove on the outer peripheral surface thereof, a nut having on the inner peripheral surface thereof a helicalload rolling groove opposing to the ball rolling groove of the screw, a ball circulating tubular body mounted on the nut and having a no-load ball path forming a ball endless track by establishing communication between the starting end and the terminating end of the load rolling groove, a number of balls rolling through the endless track and a ball coupler comprised of a plurality of coupling members capable of retaining the balls in their rotatable and aligned state wherein a guide groove for guiding the coupling members of the ball coupler is formed in a no-load zone of the ball endless track formed of the no-load ball path such that the coupling members are moved by each of the balls in the load zone of the endless track formed by the load rolling groove while the balls are guided to move by the coupling members in the no-load zone of the endless track, and guide sections for guiding the coupling members along the progressing direction of the balls rolling along the load rolling groove are formed in the no-load ball path at least on the side of the load rolling groove. Further, the second structure of the tube type ball screw unit of the present invention is such that the unit comprises a screw having a helical ball rolling groove on the outer peripheral surface thereof, a nut having on the inner peripheral surface thereof a helical load rolling groove opposing to the ball rolling groove of the screw and a no-load ball path for circulating each of balls from one end to the other end of the load rolling groove and screw-fitted with the screw through the balls, a ball circulating tubular body mounted in the nut with its one end opening at the terminating end of the load rolling groove while the other end opens at the starting end thereof and forming a no-load ball path such that each of the balls coming out of the terminating end of the load rolling groove is fed to the starting end of the load rolling groove through the no-load path to thereby circulate the balls, a number of balls rolling through an endless track formed by the load rolling groove of the nut and the no-load ball path of the ball circulating tubular body and a ball coupler comprised of a plurality of flexible coupling members capable of coupling the balls in a chain-like fashion. The coupling members of the ball coupler comprise a plurality of interposers put between the balls and a plurality of coupling sections for establishing communication between the interposers so that the balls are respectively retained by the coupling members in their rotatable and aligned state. Further, the ball circulating tubular body is formed by a pair of tubular pieces and a tubular body connecting these tubular pieces. Each of the tubular pieces is provided at least on the side of its load rolling groove with a guide hole having a guide zone for guiding each of the balls in the tangential direction along the lead angle of the load rolling groove. Further, in the guide holes of the pair of tubular pieces and the communication hole of the tubular body forming a no-load ball path by establishing communication between the guide holes there are formed guide grooves with which the coupling members of the ball coupler slidably fit so as to be guided thereby so that in the load rolling groove forming the load zone of the endless track, the coupling members are moved by the balls rolling along the load rolling groove and in the no-load ball path forming the no-load zone of the endless track, the balls are guided to move by the coupling members moving through the path. In the case of this tube type ball screw unit, the ball rolling groove to be formed on the outer peripheral surface of the screw may be circular in shape having substantially a semi-circular ball rolling surface in section or in the shape of a gothic arch having two substantially quadrant-like ball rolling surfaces intersecting each other and one or two or more such ball rolling grooves may be formed. Further, as regards the nut to be screw-fitted with the screw through a number of balls, it has on the inner peripheral surface thereof a helical load rolling groove opposing to the ball rolling groove of the screw, and as regards this helical load rolling groove, like the ball rolling groove of the screw, it may be circular in shape having substantially a semi-circular ball rolling surface in section or in the shape of a gothic arch having two substantially quadrant-like ball rolling surfaces intersecting each other and further, one or two or more such ball rolling grooves may be formed in correspondence to the number of the ball rolling grooves. In the present invention, the ball circulating tubular body forming the no-load ball path of the endless track is formed of a pair of tubular pieces and a tubular body establishing communication between the tubular pieces and each of these pieces is provided at least on the side of its load rolling groove with a guide hole having a guide zone for guiding each of the balls tangentially along the lead angle of the load rolling groove and further, the guide hole of each of the pair of tubular pieces and the communication hole of the tubular body forming the no-load ball path by establishing communication between the guide holes of the tubular pieces are respectively provided with guide grooves with which the coupling sections of the ball coupler slidably fit so as to be guided thereby. As regards the guide holes of the pair of tubular pieces, it is preferable that the guide holes be twisted and/or bent over the range from their starting ends on the side of the load rolling groove of the nut to their terminating ends on the side of the communication hole of the tubular body in such a manner that the guide grooves are held in a symmetrical positional relationship with each other at their starting ends on the side of the load rolling groove while they are held in the same positional relationship with each other at their terminating ends on the side of the communication hole, whereby the tubular body which establishes communication between the pair of tubular pieces can be formed with ease. It is particularly preferable that the guide hole of each of the pair of tubular pieces be formed to be a guide zone over the entire length thereof and a part or the whole of the guide grooves of the guide hole be twisted, that the guide grooves of the tubular body for establishing communication between the pair of tubular pieces be so formed as to be held in the same positional relationship with each other over the range from the starting end to the terminating end of the tubular body and the tubular body be formed to have bent portions at the starting end and the terminating end thereof, respectively, that the pair of tubular pieces be made to have quite the same shape by using a material obtained by sintering a powdered metal and that the tubular body for establishing communication between the pair of tubular pieces be formed by a pair of tube strips of quite the same shape. Thus, by forming the ball circulating tubular body with a pair of tubular pieces of the same shape and a pair of tube strips of the same shape, the number of molded parts can be minimized. As regards the shape of each of the tubular pieces, it is preferable that the outer configuration thereof be formed non-circular such as elliptical, square, pentagonal or the like and that non-circular fitting holes each corresponding to the outer configuration of each of the tubular pieces are drilled in the nut so that the tubular pieces to be mounted in the nut are correctly positioned by fitting the tubular pieces into these fitting holes, respectively. Further, each of the tubular pieces may be formed as a single part in its entirety or as a plurality of divided parts such that when the divided parts are fitted into a fitting hole drilled in the nut, the divided parts are combined to become a complete tubular piece. As regards the method of connecting the pair of tubular pieces and the tubular body, there is no particular limitation thereto except that the no-load ball path formed by the tubular pieces and the tubular body and the guide grooves formed along the no-load ball path are formed smoothly and continuously . without any level difference or step. However, it is preferable that the nut be provided on the outer peripheral surface thereof with a mounting surface to which the tubular body is fixed and that the depth of each of the fitting holes drilled in the nut is larger than the length of each of the tubular pieces. Further, it is preferable that on the upper surface of each of the tubular pieces there be formed a stepped portion for positioning each end of the tubular body so that the tubular body may be fixed to the mounting surface of the nut by means of solid metal fixtures after positioning both ends of the tubular body at the stepped portions, respectively. The twisting and/or bending to be applied on the no-load ball path of the ball circulating tubular body comprising the pair of tubular pieces and the tubular body may be performed wholly on the sides of the tubular pieces but in that case, the twisting may well be applied only on the tubular pieces while the bending is applied on the tubular body. Thus, it is recommended that the twisting and/or bending processes be performed separately one for the tubular pieces and the other for the tubular body whereby improvement of workability of the unit and universalization of the molded parts are contrived and the manufacturing cost is reduced through the reduction of the number of parts used. The ball coupler to be assembled into the ball endless track is required to be formed such that the balls are coupled to one another by coupling members in a chain-like fashion lest the balls run against one another, and it is necessary for the ball coupler to retain the balls in their rotatable state without falling off in such a manner that the coupling members are moved by the balls rolling along the load zone in the load zone of the ball endless track while the balls are guided to move by the coupling members moving along the no-load zone in the no-load zone. Therefore, the ball coupler is constructed by a plurality of interposers put between the balls and a plurality of coupling members connecting the interposers. This ball coupler to be assembled into the ball endless track may be in the form of a single non-endless ball coupler of the above-described structure or a plurality of such ball couplers may be assembled into the ball endless track, or it may be in the form of a single endless ball coupler of the above-described structure. According to the tube type ball screw unit of the present invention, especially due to the structure in which the unit is provided with a ball circulating tubular body, a ball coupler retaining a number of balls through coupling members in their rotatable and aligned state is assembled into the ball endless track, guide grooves for guiding the ball coupler are formed in the no-load ball path forming the no-load zone of the ball endless track such that the ball coupler is moved by the balls in the load rolling groove forming the load zone of the endless track while the balls are guided to move by the ball coupler in the no-load zone and further, guide sections for guiding the ball coupler in the progressing direction of the balls moving along the load rolling groove are provided in the no-load ball path at least on the sides of the load rolling groove, the delivery and reception of the balls between the load zone and the no-load zone of the endless track becomes quite smooth thereby achieving the smooth circulation of the balls. Further, in the tube type ball screw unit having such a ball circulating tubular body, due to the structure in which the ball coupler retaining a number of balls in their rotatable and aligned state is assembled into the ball endless track of the unit, the ball circulating tubular body forming the no-load zone is formed by the pair of tubular pieces and the tubular body establishing communication between the pair of tubular pieces, guide holes each having a guide zone for guiding each of the balls tangentially along the lead angle of the load rolling groove (or the lead angle of the ball rolling groove) are formed at least on the side of the load rolling groove, the guide holes of the pair of tubular pieces and the communication hole of the tubular body forming the no-load ball path by connecting the guide holes are provided with guide grooves for guiding the coupling members of the ball coupler slidably coming into engagement therewith so that the coupling members of the ball coupler are guided to move by the balls in the load zone of the ball endless track while the balls are guided to move by the coupling members in the no-load zone of the ball endless track, the delivery and reception of the balls between the load zone and the no-load zone of the ball endless track becomes smooth thereby achieving the smooth circulation of the balls. Further, in the tube type ball screw unit, when the guide hole of each of the pair of tubular pieces establishing communication between the load rolling groove and the no-load ball path of the nut is twisted and/or bent over the range from the load rolling groove side starting end to the no-load ball path side terminating end thereof in such a manner that the guide grooves are held in a symmetrical positional relationship with each other at their starting ends on the side of the load rolling groove while they are held in the same positional relationship with each other at their terminating ends on the side of the no-load ball path, the ball coupler rolling through the ball endless track is adjusted in its posture by the change direction path whereby the balls forming part of the ball coupler do not interfere with one another and can circulate in the ball endless track smoothly in their aligned state. In the tube type ball screw unit, when the entire length of the guide hole of each of the pair of tubular pieces forms itself a guide zone and the guide grooves of the guide hole are partly or wholly twisted and also when the guide grooves are formed at the same position over the range from the starting end to the terminating end of the tubular body connecting the pair of tubular pieces and a bent portion is formed at each of the starting and terminating ends of the tubular body, the no-load ball path can be twisted and bent as required with ease whereby the ball coupler can circulate through the no-load zone of the ball endless track in a stabilized state and at the same time, the tubular pieces and the tubular body can be constructed by using the minimum possible number of parts. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front view of a tube type ball screw unit according to a first embodiment of the present invention; Fig. 2 is a schematic perspective view (partly cut away) of the tube type ball screw unit shown in Fig. 1 which view shows a portion of a ball endless track; Fig. 3 is an illustrative front view of a non-endless ball coupler assembled into the tube type ball screw unit shown in Fig. 1; Fig. 4 is a enlarged partial view of an A-portion of the ball coupler shown in Fig. 3; Fig. 5 is a bottom view of the A-portion of the ball coupler shown in Fig. 4; Fig. 6 is a plan view of a nut as one of components of the tube type ball screw unit shown in Fig. 1; Fig. 7 is a schematic sectional view of an endless track of the tube type ball screw unit shown in Fig. 1; Fig. 8 is a schematic sectional view of a no-load ball path formed by the nut and a ball circulating tubular body of the tube type ball screw unit shown in Fig. 1; Fig. 9 is a perspective view of a tubular piece forming part of the endless track shown in Fig. 7; Fig. 10 illustrates the tube piece shown in Fig. 9, wherein Fig. 10(a) is a front view of the tubular piece, Fig. 10(b) is a plan view of the same and Fig. 10(c) is a bottom view of the same; Fig. 11 is a plan view of one of tubular pieces forming a tubular body shown in Fig. 8; Fig. 12 is a front view of the tubular piece shown in Fig. 11; Fig. 13 is a sectional view of the tubular piece shown in Fig. 12 when taken along the A-B line thereof; Fig. 14 is a sectional view of the tubular piece shown in Fig. 12 when taken along the C-D line thereof; Fig. 15 is a sectional view of the tubular piece shown in Fig. 12 when taken along the B-C line thereof; Fig. 16 is a sectional view of the ball circulating tubular body shown in Fig. 8; Fig. 17 is a schematic perspective view (partly in section) of the ball endless track of the tube type ball screw unit shown in Fig. 1; Fig. 18 is a perspective view (partly in section) of the non-endless ball coupler traveling within a load rolling groove of the nut of the tube type ball screw unit shown in Fig. 1; Fig. 19 is a perspective view (partly in section) of the non-endless ball coupler traveling along the ball rolling groove of the screw forming part of the tube type ball screw unit shown in Fig. 1; Fig. 20 is an illustrative sectional view of a tube type ball screw unit which does not form part of the present invention the view being similar to Fig. 7; Fig. 21 is an illustrative sectional view of a ball circulating tubular body forming part of the ball screw unit shown in Fig. 20; Fig. 22 is a perspective view of the ball circulating tubular body of the ball screw unit of Fig. 20; Fig. 23 is a plan view of a conventional tube type ball screw unit; Fig. 24 is a sectional view of the conventional tube type ball screw unit shown in Fig. 23; Fig. 25 is a partial front sectional view of a conventional side cover type ball screw unit; Fig. 26 is a partial side sectional view of a nut of the ball screw unit shown in Fig. 25; Fig. 27 is a view illustrating how the side covers of the ball screw unit shown in Fig. 25 are mounted; and Fig. 28 is a front sectional view of another conventional tube type ball screw unit. [Description of Reference Symbols] 1...Screw 2...Nut 3...Ball circulating tubular body 4...Non-endless Ball coupling 5...Ball rolling groove 6...load rolling groove 7...Mounting surface 8...Fitting hole 9...Fixture 10...No-load ball path 10a... Guide hole 10b...Communication hole 11...Ball 12...Connecting section 13a and 13b (13)... Interposers 14...Belt portion (connecting section) 15...Tubular piece 15a...Upper surface 15b...Bottom surface 15c...Stepped portion for positioning 16...Tubular body 16a...Bent portion 17...Guide groove 18...Tube strip 18a...Bent portion 119...Guide groove. BEST MODE FOR CARRYING OUT THE INVENTION The modes for carrying out the present invention will be described on the bases of embodiments shown in the accompanying drawings. [First embodiment: Tube type ball screw unit] A tube type ball screw unit according to the present invention is shown in Figs. 1 and 2. This ball screw unit basically comprises a screw 1 provided on the outer peripheral surface thereof with a helical ball rolling groove 5, a cylindrical nut 2 provided with a central through hole for receiving the screw 1, a helical load rolling groove 6 formed on the inner peripheral surface thereof so as to lie opposite to the ball rolling groove 5 of the screw 1, a flat mounting surface 7 on the outer peripheral surface thereof and a pair of fitting holes 8 drilled at positions on a diagonal line of the mounting surface 7 so as to extend from the mounting surface 7 toward a starting end and a terminating end of the load rolling groove 6, respectively, a ball circulating tubular body 3 mounted on the mounting surface 7 of the nut 2 by means of a fixture 9 and having a no-load ball path 10 which establishes communication between the starting end and the terminating end of the load rolling groove 6 of the nut 2 to thereby form an endless track and a single non-endless ball coupler 4 capable of traveling in the endless track formed by the load rolling groove 6 (a load zone) of the nut 2 and the no-load ball path 10 (a no-load zone) of the ball circulating tubular body 3 fixed to the nut 2. The ball coupler 4 comprises a number of balls 11 and a plurality of flexible coupling members 12 made of a synthetic resin material and adapted to couple the balls 11 in a chain-like fashion. It should be noted that in Fig. 2, the ball coupler 4 is depicted schematically so that the twisting of the ball coupling members 12 can be understood easily. In the first embodiment, the ball coupler 4 is, as shown in Figs. 3 through 5, formed of the coupling members 12 each comprised of two kinds of claw-like interposing strips 13a and 13b (forming the interposes 13) of different lengths extending vertically along the spherical surface of each of the balls 11 and interposed between the balls 11 and a couple of belts (connecting sections) 14 for connecting the interposers 13 horizontally along the spherical surfaces of the balls 11. when the non-endless ball coupler 4 is curved, each of the interposers 13 is so formed that the claw-like interposing strip 13a located outside is longer than the claw-like interposing strip 13b located inside so that when the longitudinal coupler 4 is curved, each of the balls 11 is retained by each of these interposers 13 and the couple of belts 14 thereby preventing the balls from falling off and allowing the interposers 13 to be bent easily in a direction XY designated by the arrow with the shorter claw-like interposing strip 13b being held located inside. The non-endless ball coupler 4 is formed in such a manner that the balls 11 are arranged within a mold as cores, a molten synthetic resin is injected into the mold to thereby insert-molding the cores, the insert-molded material is released from the mold to produce a molded non-endless ball coupler 4 comprising a plurality of coupling members 12, then the coupler 4 is immersed into a mineral lubricating oil to cause the coupling members 12 to swell to provide a gap between each of the balls 11 and each of the coupling members 12 so that each of the balls 11 can rotate freely while it is retained by each of the coupling members 12. As shown in Figs. 6 through 8, the ball circulating tubular body 3 comprises a pair of tubular pieces 15 to be fitted into a pair of fitting holes 8 formed in the nut 2 and a tubular body 16 connecting the tubular pieces 15 so as to establish communication between the tubular pieces 15. Further, the no-load ball path 10 forming the endless track for the balls 11 comprises guide holes 10a respectively formed in the pair of tubular pieces 15 and a communication hole 10b formed in the tubular body 16. In this first embodiment, the pair of tubular pieces 15 are molded to the same shape such that as shown in Figs. 8 through 10, each of the tubular pieces 15 is substantially in the shape of an elliptical cylinder made of a sintered metal with an upper surface 15a inclined in the longitudinal direction and a bottom surface 15b and each of the guide holes 10a formed in the tubular pieces 15 forms in its entirety a guide zone extending tangentially along the lead angle of the load rolling groove 6 of the nut 2 from the bottom surface 15b toward the upper surface 15a of the tubular piece 15. Further, the guide hole 10a is provided with a pair of guide grooves 17 with which the connecting sections 14 of the coupling members 12 of the ball coupler 4 slidably fit to be guided thereby for guiding the coupling members 12 of the ball coupler 4. The guide grooves 17 are held opposite to each other anywhere in the guide hole 10a and are twisted by a predetermined angle over the range from their starting ends on the side of the load rolling groove 6 of the nut 2 (on the side of the bottom surface 15b of the tubular piece 15) toward their terminating ends on the side of the communication hole 10b of the tubular body 16 (on the side of the upper surface 15a of the tubular piece 15) whereby when the tubular pieces 15 are assembled into the nut 2, the guide grooves 17 which are held in symmetrical positional relationship with each other in the pair of tubular pieces 15 are held in the same positional relationship with each other at their terminating ends on the side of the communication hole 10b. Each of the fitting holes 8 in the nut 2 is formed substantially elliptical in correspondence to the outer configuration of each of the tubular pieces 15 and the depth thereof is larger than the length of each of the tubular pieces 15 so that by fitting the tubular pieces 15 into the fitting holes 8, respectively, the tubular pieces 15 are positioned with respect to the nut 2. Further, on the upper surface 15a of each of the tubular pieces 15 there is formed a stepped portion 15c for positioning each of the ends of the tubular body 16 such that when bent portions 16a formed respectively at the starting end and the terminating end of the tubular body 16 are inserted into the fitting holes 8, the top ends of the bent portions 16a engage the stepped portions 15c, respectively, and further, when the tubular body 16 is fixed to the mounting surface 7 of the nut 2 by means of a fixture 9, the pair of tubular pieces 15 and the tubular body 16 are fixed to the nut 2. In this case, as shown in Figs. 11 through 16, the tubular body 16 is formed of a pair of synthetic resin tube strips 18 having quite the same shape and at both ends of each of the strips 18, there are formed bent portions 18a, respectively, which form the bent portions 16a of the tubular body 16. Further, there is formed, along the central portion of the inner surface of each of the tube strips 18, a guide groove 19 into which : each of the connecting sections 14 connecting the coupling members 12 of the ball coupler 4 slidably fits. In this first instant embodiment, in order for the pair of tube strips 18 to have quite the same shape, each of the strips 18 is made to have such cross-sectional configurations that in the region A→B, it has a cross-section as shown in Fig. 13, in the region C→D, it has a cross-section as shown in Fig. 14 and in the region B→C, it has a cross-section as shown in Fig. 15 so that when the pair of such tube strips 18 are combined with each other to form the tubular body 16, the pair of guide grooves 19 are present on the inner surface of the communication hole 10b of the tubular body 16 in opposite relationship with each other as shown in Fig. 16. In this first embodiment, the relationship between the endless track formed by the load rolling groove 6 (load zone) of the nut 2 and the no-load ball path 10 (no-loaded zone) of the ball circulating tubular body 3 formed by the guide holes 10a of the pair of tubular pieces 15 and the communication hole 10b of the tubular body 16, and the non-endless ball coupler 4 traveling through the endless track is simulatively shown in Figs. 17 through 19. That is, Fig. 17 shows the movement and the posture of the non-endless ball coupler 4 within the endless track, especially the manner of how the coupler 4 is twisted within the guide holes 10a and bent at the bent portions 16a of the tubular body 16, Fig. 18 shows the condition of the non-endless ball coupler 4 traveling along the load rolling groove 6 of the nut 2 and Fig. 19 shows the condition of the non-endless ball coupler 4 traveling along the ball rolling groove 5 of the screw 1. In these figures 17 through 19, the non-endless ball coupler 4 is depicted schematically so that the twisting of the coupling members 12 may be understood easily. The connecting sections 14 connecting the coupling members 12 of the ball coupler 4 traveling through the endless track are held symmetrically with each other in position immediately before the ball coupler 4 enters each of the guide holes 10a of the tubular pieces 15 or immediately after it comes out of each guide hole 10a (the starting and terminating ends of the load rolling groove 6) but they are twisted by a predetermined angle within the guide holes 10a of the tubular pieces 15 and come to be held in the same positional relationship with each other immediately before the outlets of the guide holes 10a of the tubular pieces 15 on the sides of the tubular body 16 and in the communication hole 10b of the tubular body 16 having the bent portions 16a at both ends thereof, they are held in the same positional relationship with each other from the starting end to the terminating end of the communication hole 10b. Accordingly, in this first embodiment, the guide hole 10a of each of the pair of tubular pieces 15 forms itself a guide zone extending tangentially along the lead angle of the load rolling groove 6 of the nut 2 whereby the delivery and reception of the balls 11 between the load zone and the no-load zone of the endless track become smooth, a predetermined twisting process is performed at each of the pair of tubular pieces 15 and a predetermined bending process is performed at both ends of the tubular body 16 so that the ball couplers 4 roll smoothly within the endless track thereby achieving the smooth circulation of the balls. Figs. 20 through 22 concern a tube type ball screw unit as in the case of the first embodiment but unlike the first embodiment, the whole of the ball circulating tubular body 3 is in the shape of a substantially U-shaped tube having both ends 3a thereof fixedly fitted into a pair of fitting holes 8 formed in the nut 2. This example does not form past of the invention and is only includes as addition information. Further, the ball circulating tubular example body 3 is provided with guide holes 10a at both ends 3a thereof as in the case of the tubular piece 15 of the first embodiment and at the central portion 3b thereof there is provided a communication hole 10b as in the case of the tubular body 16 of the first embodiment so that these guide holes 10a and the communication hole 10b form themselves a no-load ball path 10 for forming a no-load zone of the endless track for the balls 11. The guide holes 10a which are respectively formed at both ends 3a of the tubular body 3 form themselves guide zones each extending tangentially along the lead angle of the load rolling groove 6 of the nut 2 and further, each of the guide holes 10a is provided with a pair of guide grooves 19 with which the connecting sections 14 connecting the coupling members 12 of the ball coupler 4 slidably fit so as to allow the coupling members 12 of the ball coupler 4 to be guided thereby. These guide grooves 19 lie opposite to each other within each of the guide holes 10a and are twisted by a predetermined angle over the range from the load rolling groove 6 of the nut 2 to the communication hole 10b whereby when both ends 3a of the ball circulating tubular body 3 are assembled into the nut 2, the guide grooves 19 which are in symmetrical positional relationship with each other at both ends 3a thereof are held in the same positional relationship with each other on the sides of the communication hole 10b. Accordingly the guide holes 10a of both ends 3a of the ball circulating tubular body 3 form themselves the guide zones each extending tangentially along the lead angle of the load rolling groove 6 of the nut 2 whereby the delivery and reception of the balls 11 between the load zone and the no-load zone of the endless track become smooth, a predetermined twisting process is performed at both ends 3a of the ball circulating tubular body 3 and a predetermined bending process is performed over the range from both ends 3a toward the central portion 3b of the body 3 so that the ball coupler 4 smoothly travels within the endless track thereby achieving the smooth circulation of the balls 11. INDUSTRIAL APPLICABILITY According to the ball screw unit of the present invention, the ball coupler which retains a number of balls in their rotatably aligned state by means of coupling members forming the ball coupler is assembled into the endless track of the unit, the guide grooves for guiding the ball coupler are formed in the no-load zone of the endless track, the ball coupler is moved by each of the balls in the load zone of the endless track while the balls are guided to move by the ball coupler in the no-load zone and the guide sections for guiding the ball coupler along the progressing direction of the balls moving along the load rolling groove are provided in the no-load ball path at positions at least on the sides of the load rolling groove so that the delivery and reception of the balls between the load zone and the no-load zone of the ball endless track can be performed quite smoothly thereby achieving the smooth circulation of the balls.

Claims: A ball screw unit provided with a screw (1) having a helical ball rolling groove (5) on the outer peripheral surface thereof, a nut assembly having a load zone (6) comprising a helical load rolling groove (6) opposing to the helical ball rolling groove (5) of the screw (1) and a no-load zone (10) forming a ball endless track by establishing communication between a starting end and a terminating end of the load zone (6), a number of balls (11) rolling within the ball endless track of the nut assembly and a ball coupler (4) comprising a plurality of coupling members (12) capable of retaining the balls (11) in their rotatably aligned state, wherein a guide groove (17,19) for guiding the coupling members (12) of the ball coupler (4) is formed in the no-load zone (10) of the ball endless track such that the coupling members (12) are moved by the balls (11) in the load zone (6) of the ball endless track while the balls are guided to move by the coupling members (12) in the no-load zone (10) of the endless track, and guide sections for guiding the coupling members (12) along the progressing direction of the balls (11) moving in the load zone (6) are respectively provided at least at both ends of the no-load zone (10) lying near the load zone (6) wherein said nut assembly comprises a nut (2) having on the inner peripheral surface a helical load rolling groove (6) opposing to the ball rolling groove (5) of said screw (1) and a ball circulating tubular body (3) attached to the nut (2) and having a no-load ball path (10) for forming a ball endless track by establishing communication between a starting end and a terminating end of said load rolling groove (6) and wherein a guide groove (17,19) for guiding said ball coupler (4) is formed in the no-load zone (10) of said endless track formed by said no-load ball path (10) such that said ball coupler (4) is moved by the balls (11) in the load zone (6) of the endless track formed by said load rolling groove (6) while the balls (11) are guided to move by the ball coupler (4) in the no-load zone (10) of the endless track and guide sections for guiding said ball coupler (4) along the progressing direction of the balls (11) moving along said load rolling groove (6) are provided in the no-load ball path (10) at least on the side of said load rolling groove (6); characterized by said ball circulating tubular body (3) comprising a pair of tubular pieces (15) and a tubular body (16) for establishing communication between said pair of tubular pieces (15) and each of said tubular pieces (15) providing at least on the side of said load rolling groove (6) with a guide hole (10a) which includes a guide zone for guiding each of the balls (11) tangentially along the lead angle of said load rolling groove (6). The ball screw unit as described in Claim 1, wherein the guide holes (10a) of said pair of tubular pieces (15) are twisted and/or bent over the range from the starting ends thereof on the side of the load rolling groove (6) of the nut (2) to the terminating ends thereof on the side of the communication hole (10b) of said tubular body (16) so that the guide grooves (17,19) are held in symmetrical positional relationship with each other at the starting ends of the guide holes (10a) on the side of said load rolling groove (6) while they are held in the same positional relationship with each other at the terminating ends thereof on the side of said communication hole (10b). The ball screw unit as described in Claim 1, wherein the entire length of the guide hole (10a) of each of said pair of the tubular pieces (15) forms a guide zone and a part or the whole of the guide groove (17) of said each guide hole (10a) is twisted, the tubular body (16) for establishing communication between said pair of tubular pieces (15) is provided with guide grooves (19) at the same position over a range from the starting end to the terminating end thereof and has bent portions (16a) on the sides of the starting and terminating ends thereof, respectively. The ball screw unit as described in Claim 3, wherein said pair of tubular pieces (15) are formed to the same shape and the tubular members for establishing communication between said pair of tubular pieces (15) are formed of a pair of tubular strips (18) of the same shape. The ball screw unit as described in any one of Claims 1 through 4, wherein each of said tubular pieces (15) is formed non-circular in section and is positioned by being fitted into a non-circular fitting hole (8) formed in the nut (2). The ball screw unit as described in any one of Claims 1 through 5, wherein a single non-endless ball coupler (4) or a plurality of non-endless ball couplers (4) comprising a plurality of coupling members (12) having both ends is or are assembled into said ball endless track. The ball screw unit as described in any one of Claims 1 through 4, wherein a single endless ball coupler comprising a plurality of coupling members is assembled into said ball endless track.

Inventors: EBINA SHIGERU, TAKEDA RYUJI, EBINA, SHIGERU, TAKEDA, RYUJI, EBINA, Shigeru, THK Co., Ltd, TAKEDA, Ryuji, THK Co., Ltd
Applicants: THK CO LTD, THK CO., LTD.
Application year: 1998
Main classification: F16H25
IPC codes: F16H25, F16C33