Guide device for linear motion

A guide device includes a ram and a column translatable axially relative to each other. The ram has an outer circumferential surface of a squared cross section formed of four flat portions each of which extends axially. The column disposed around the ram has a through hole of a squared cross section formed of four flat portions corresponding to the flat portions of the ram. In each flat portion of the column is provided a needle bearing that rolls on the corresponding flat portion of the ram. Inside the column are provided a plurality of supporting shafts extending in a direction perpendicular to the extending direction or to the axial direction of each flat portion of the ram. Each needle bearing is rotatably supported by the corresponding supporting shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIGS. 1 to 4 show a first embodiment of the present invention. As shown in FIGS. 1 and 2 , a guide device 1 includes an axially extending ram 2 having a central through hole 2 a and a tubular column 3 disposed around the ram 2 and slidable relative to the ram 2 . The ram 2 has an outer circumferential surface of a squared cross shape formed of four flat portions 20 . Each of the flat portions 20 extends axially. The column 3 is disposed around the outer circumferential surface of the ram 2 and has a central through hole 3 a of a squared cross shape formed of four flat portions 30 each disposed opposite to each flat portion 20 of the ram 2 . A needle bearing 5 is provided at each flat portion 30 . The needle bearing 5 , shown in FIG. 3 , includes a cylindrical outer race 50 and a plurality of needles 51 provided on an inner circumferential side of the outer race 50 . A supporting shaft 4 passes through the needle bearing 5 and the needle bearing 5 is supported rotatably around the supporting shaft 4 . The outer race 50 of the needle bearing 5 contacts the corresponding flat portion 20 of the ram 2 . Each supporting shaft 4 is inserted into a supporting hole 35 formed in the column 3 and is supported on both ends in the supporting hole 35 . Thereby, supporting rigidity is improved and thus, adequate support of the needle bearing 5 is secured. Also, each supporting shaft 4 extends toward the direction perpendicular to the extending direction of each flat portion 20 of the ram 2 . Each flat portion 30 defining the central through hole 3 a of the column 3 has an axially extending through groove 33 formed thereon. Each needle bearing 5 is received in the through groove 33 . In this way, by forming a through groove as a supporting hole, or a bearing pocket, for each needle bearing 5 , working process of the column 3 becomes easier. An oil retaining member 6 is inserted into the through groove 33 , as shown in FIG. 1 . The oil retaining member 6 is formed of oil retaining plastics, oil retaining felt, or oil soaked porous materials with continuous air cells. The oil retaining member 6 , shown in FIG. 4 , has a plurality of notches or grooves 60 for receiving the needle bearings 5 . By utilizing such an oil retaining member 6 , installation of needle bearings 5 into the through grooves 33 can be conducted with ease. As is clearly seen in FIGS. 2 and 3 , the needle bearings 5 provided at adjacent flat portions 30 of the central through hole 3 a are disposed at each corner of the central through hole 3 a. In operation, when the ram 2 and the column 3 moves relatively, e.g. the ram 2 reciprocates relative to the column 3 fixed to a base member (not shown) through a flange portion 35 , each needle bearing 5 rotates around the supporting shaft 4 and rolls on the flat portion 20 of the ram 2 . In such a manner, travel of the ram 2 relative to the column 3 is smoothly guided. In this case, since each needle bearing 5 supported around the supporting shaft 4 acts as a linear bearing, a retainerless guide device is achieved. Thus, even when a torsion occurs between the ram 2 and the column 3 , skewing due to an inadequate support of the needle bearing 5 is effectively prevented. Also, since each supporting shaft 4 for rotatably supporting each needle bearing 5 extends in the direction perpendicular to the extending direction of each flat portion 20 of the outer circumferential surface of the ram 2 , or to the direction of relative movement of the ram 2 and the column 3 , each needle bearing 5 can be securely prevented from skewing relative to the flat portion 20 or the rolling surface. Furthermore, since each needle bearing 5 at the adjacent flat portions 30 of the central through hole 3 a of the column 3 is disposed at each corner of the central through hole 3 a, each corner portion of the outer circumferential surface of the ram 2 can be held by each roller bearing 5 , thereby enabling a firm support of the ram 2 . As a result, even when an excessive torsion occurs between the ram 2 and the column 3 , skewing of each needle bearing 5 can be securely prevented. Moreover, in operation of the device, when each needle bearing 5 rotates, oil gradually bleeds from the oil retaining member 6 contacting the needle bearing 5 , thereby preventing seizure or wear to the rolling surface resulting from breakage of oil film. Also, in this case, since oil bleeds for a long time, a long-term lubrication of the rolling surface becomes possible, thus allowing for maintenance-free device. In addition, assembly error between the ram 2 and the column 3 can be adjusted by utilizing a needle bearing 5 with an outer race 50 of different outer diameters, which facilitating adjustment of the whole device. Next, FIG. 5 shows a guide device incorporating a ball screw. In this guide device 1 , a spiral groove 2 b is formed on the inner circumferential surface of the central hole 2 a of the ram 2 . A screw shaft 7 is inserted into the central hole 2 a of the ram 2 . A spiral groove 7 a is formed on the outer circumferential surface of the screw shaft 7 . Between the inner circumferential surface of the ram 2 and the outer circumferential surface of the screw shaft 7 is interposed a thin-walled, cylindrical retainer 9 for rotatably supporting a plurality of balls 8 engaging rollably with both the spiral groove 2 b of the ram 2 and the spiral groove 7 a of the screw shaft 7 . In operation, when the screw shaft 7 rotates, each ball 8 rolls and travels along the spiral grooves 7 a and 2 b of the screw shaft 7 and the ram 2 , and thus, the ram 2 moves axially along the screw shaft 7 . During this movement, each needle bearing 5 guides the movement of the ram 2 and the ram 2 functions as a nut of a ball screw. In the aforementioned embodiments, the ram 2 has an outer circumferential surface of a rectangular cross shape and the column 3 has a central through hole 3 a of a rectangular cross shape, but the present invention is also applicable to a ram and a column of other polygonal cross shapes. Additionally, in the aforementioned embodiments, the ram 2 generally has a shorter length, but the present invention also has application to a linear guide having an infinite or longer rail. FIGS. 6 and 7 illustrate an alternative embodiment of the present invention. As shown in FIGS. 6 and 7 , a guide device 100 includes a solid cylindrical ram 102 and a cylindrical column 103 disposed around the ram 102 and translatable axially relative to the ram 102 . The ram 102 has an outer circumferential surface 102 a of a round cross shape. The column 103 is disposed outside the outer circumferential surface 102 a of the ram 102 and has a through hole 103 a of a round cross shape. A plurality of pockets 104 are formed on an inner circumferential surface of the through hole 103 a. Each pocket 104 receives a needle bearing 105 for slidably supporting the column 103 relative to the ram 102 in the axial direction. Each pocket 104 is formed at least at both end openings of the through hole 103 a and preferably spaced equally in a circumferential direction. Here, four pockets 104 are formed at 90-degree intervals circumferentially, but three pockets may be provided at 120-degree intervals, or six pockets at 60-degree intervals. Different number of pockets may be used. The number of pockets is suitably determined according to a diameter of the ram, an allowable load to the guide device and so on. As shown in FIG. 8 , the needle bearing 105 includes a cylindrical outer race or rolling element 150 that rolls on the outer circumferential surface 102 a of the ram 102 axially or in the direction perpendicular to the page, and a plurality of needle rollers 151 supported rotatably on an inner circumferential surface of the outer race 150 . The needle bearing 105 is supported by a supporting shaft 106 inserted thereinto through the needle rollers 151 and thus, the outer race 150 is rotatable around the supporting shaft 106 . As shown in FIG. 9 , the outer race 150 preferably has a concavely curved cylindrical surface 150 a i.e. a generating line of the cylindrical surface 150 a is concavely curved. The cylindrical surface 150 a of the outer race 150 has a radius of curvature slightly greater than that of the outer circumferential surface 102 a of the ram 102 . There exists an inequality, 0.52D&lE;r&lE;0.58D, wherein r: radius of curvature of the cylindrical surface 150 a ; D: diameter of the outer circumferential surface 102 a of the ram 102 contacting the cylindrical surface 150 a, which equals to 2R (R: radius of the outer circumferential surface 102 a ). Thereby, contact area with the outer circumferential surface 102 a of the ram 102 increases and surface pressure of the rolling surface decreases, thus improving wear resistance and allowing for a greater allowable load. Furthermore, a smooth rotation of the outer race 150 is secured and skewing of the outer race 150 is prevented. Also, in this case, since a contact surface C between the cylindrical surface 150 a of the outer race 150 and the outer circumferential surface 102 a of the ram 102 is formed at a central portion of the cylindrical surface 150 a, a contact radius or distance between a center line of the outer race 150 and a contact surface C in every portion of the contact surface C is substantially equal to each other. Thereby, a differential slippage due to the rotation of the outer race 150 is prevented from occurring at the contact surface C, thus preventing wear to the contact surface C. In addition, in the case that a radius r of curvature of the cylindrical surface 150 a of the outer race 150 is smaller than 0.52 D, a smooth rotation of the outer race 150 is restrained and differential slippage will occur. On the other hand, in the case that a radius r of curvature of the outer race 150 is greater than 0.58 D, a contact area becomes smaller and an allowable load will decrease. The cylindrical surface of the outer race 150 may have a linear generating line. In this case, working of the outer race 150 will become easier. Alternatively, the outer race 150 may include a first outer race having a cylindrical surface that satisfies the above-mentioned inequality and a second outer race having a linear cylindrical surface. In this case, the first outer race may be disposed at regions where a relatively greater load is applied, and the second outer race may be disposed at the other regions. Each supporting shaft 106 , shown in FIG. 7 , is inserted into a supporting hole 130 formed at each pocket 104 inside the column 103 and both end portions of the supporting shaft 106 is supported in the supporting hole 130 . Thereby, supporting rigidity of the supporting shaft 106 is improved and adequate support of the needle bearing 105 is secured. Also, each supporting hole 130 penetrates the outer circumferential surface of the column 103 , which facilitates boring process of the column 103 . Furthermore, since each supporting hole 130 is a through hole, a pitch or a distance from a center of the ram 102 to a centerline of each supporting hole 130 can be made accurate using a working method such as a wire cut electrical discharge machining. Between the ram 102 and the column 103 is interposed a thin-walled, cylindrical member 107 . The thin-walled cylindrical member 107 , shown in FIG. 10 , has a plurality of apertures 170 each corresponding to the pocket 104 . The cylindrical member 107 is provided for sustaining a radial load applied between the ram 102 and the column 103 , and may be formed of bearing materials such as an oil retaining metal or plastics in view of lubricating properties and wear resistance. Especially, a dry-type, Teflon bearing is preferable because no lubricants are required. In addition, the column 103 has a flange 131 at its lower portion to bolt a base member (not shown) through a bolt hole 131 a. Also, there is provided a dust seal 108 at both openings of the central through hole 103 a to prevent dust from entering the through hole 103 a. In operation, when the ram 102 and the column 103 moves relatively, e.g. the ram 102 reciprocates relative to the column 103 fitted to the base member (not shown) through the flange 131 , each outer race 150 of the needle bearings 105 rotates around each supporting shaft 106 and rolls axially on the outer circumferential surface 102 a of the ram 102 . Thereby, movement of the ram 2 relative to the column 103 is smoothly guided. In this case, since each needle bearing 105 functions as a linear bearing, a retainerless guide device is achieved. Furthermore, according to this embodiment, it is not a needle roller of a small diameter but a roller-shaped outer race 150 of a larger diameter rotatably supported around the supporting shaft 106 through a plurality of needle rollers 151 that rolls in the axial direction on the outer circumferential surface 102 a of the ram 102 . Thus, rotation of the outer race 150 is hard to be hindered by dust or small particles on the rolling surface, thereby allowing for use of the guiding device under the dusty atmosphere. Those skilled in the art to which the invention pertains may make modifications and other embodiments employing the principles of this invention without departing from its spirit or essential characteristics particularly upon considering the forgoing teachings. The described embodiments and examples are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. Consequently, while the invention has been described with reference to particular embodiments and examples, modifications of structure, sequence, materials and the like would be apparent to those skilled in the art, yet fall within the scope of the invention.