BALL SCREW

Abstract: Lubricant retaining slits (5, --) are formed on a pair of seal members (2) from end surfaces (2b) thereof facing inside in the axial direction of a nut (1) of a ball screw to intermediate portions of the seal members (2), and foreign bodies removing slits (6 --) are formed on the seal members (2) from end surfaces (2c) thereof facing outside in the axial direction of the nut (1) to intermediate portions thereof. The lubricant retaining slits (5--) can positively trap the lubricant invading into the gap between the seal member (2) and the screw shaft and returns the lubricant inside the nut (1). The foreign bodies removing slits (6 --) can remove the foreign bodies stored on the outer peripheral surface of the screw shaft and discharged outside the nut (1).

Description: The present invention relates to a ball screw, according to the preamble of claim 1. As a seal member of a conventional ball screw, there is known seal members mounted at both end portions of a nut (refer to Japanese utility Model JP 6006795U). Each of such seal members has a ring shape and has an inner peripheral surface at which a projection, fitted into a ball rolling groove of a screw shaft, is formed. A slit is formed on the seal member so as to extend from an end surface of the nut facing outside in the axial direction of the nut to an intermediate portion of the seal member. By mounting the seal members at both end portions of the nut, the seal members tightly contact the outer peripheral surfaces of the screw shaft to thereby seal a lubricant inside the nut. Furthermore, by forming the slit on the seal member, an edge portion of the slit contacting the outer peripheral surface of the screw shaft scrapes off foreign bodies adhering to the screw shaft and discharges the foreign bodies outside the nut, thus preventing the foreign bodies from invading inside the nut. However, in the conventional seal structure described above, the lubricant inside the nut easily invades into a gap between the screw shaft and the seal members and there is a fear that the lubricant leaks outside the nut. Moreover, the lubricant will be also scraped off as well as the foreign bodies by the edge portion of the slit, and for this reason, the scraped lubricant is discharged outside the nut and not returned to the inside of the nut. The closest prior art document EP-A-0376164 describes a ball screw comprising a screw shaft formed, on an outer peripheral surface thereof, with a ball rolling groove, a number of balls rolling along said ball rolling groove, a nut screwed with said screw shaft through the balls and seal members disposed at both end portions of the nut, each of said seal members being formed, on an inner peripheral surface thereof, with a spiral projection to be fitted into said ball rolling groove, wherein a lubricant retaining portion is formed on the seal member from an end surface thereof facing inside in the axial direction of the nut, and said lubricant retaining portion comprises a slit, and wherein the lubricant retaining portion positively traps lubricant invading into the gap between the seal member and the screw shaft through the end surface and returns the lubricant to the inside of the nut, and in that the slit of the lubricant retaining portion extends from said end surface of the seal member to an intermediate portion thereof. and wherein a foreign bodies removing portion is formed on the seal member to remove foreign bodies adhering to an outer peripheral surface of the screw shaft and discharge the foreign bodies outside the nut. The present invention seeks to provide a ball screw having a seal member capable of effectively maintaining a lubricant inside a nut and preventing foreign bodies from invading inside the nut. According to the present invention, a ball screw as defined above is characterised in that the foreign bodies removing portion is a slit formed on the seal member so as to extend from an end face thereof facing outside in the axial direction of the nut to an intermediate portion thereof. In a ball screw of the invention the lubricant retaining portion is formed on the seal member from an end surface thereof facing inside in the axial direction of the nut to an intermediate portion thereof. Therefore the lubricant invading into the gap between the seal member and the screw shaft through the end surface can be positively trapped and returned to the inside portion of the nut. Thus, the lubricant inside the nut can be prevented from leaking out of the nut. As the lubricant retaining portion comprises a slit, the lubricant is scraped off by the edge portion of a side surface of the slit on contact with the screw shaft and returned inside the nut. Furthermore, as a foreign bodies removing portion is formed on the seal member from an end surface thereof facing outside in the axial direction of the nut to an intermediate portion thereof, foreign bodies storing on the outer peripheral surface of the screw shaft (including a surface of the ball rolling groove) are removed and discharged out of the nut. The foreign bodies removing portion is formed as a slit, and foreign bodies are scraped off by the edge portion of a side surface of the slit and discharged out of the nut. In an embodiment, pressing means for pressing the seal members against the screw shaft are provided. The pressing means ensure that the lubricant retaining portion and the foreign bodies removing portion are tightly in contact with the screw shaft. Therefore, the lubricant can be surely trapped and the foreign bodies can be removed. Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a nut and seal members of a ball screw of an embodiment of the invention; Figure 2 shows an end view of Figure 1; Figure 3 shows a sectional view of Figure 1; Figure 4 shows an end view of a seal member; Figure 5 shows a sectional view of the seal member; Figure 6 shows a side view of a ring to which a spring is mounted; and Figure 7 shows a sectional view showing a screw shaft and the seal member. Figures 1 to 3 represent a ball screw having a ball screw nut 1 and seal members 2. As is known, the ball screw comprises a screw shaft (not shown) having an outer peripheral surface on which a spiral ball rolling groove is formed, The nut 1 has an inner peripheral surface on which a spiral ball rolling groove 1a, opposed to the ball rolling groove formed on the screw shaft, is formed. A number of balls (not shown) are arranged and accommodated between the ball rolling grove 1a of the nut 1 and the ball rolling groove of the screw shaft. When the screw shaft is rotated, the balls roll in the space between the ball rolling groove 1a of the nut 1 and the ball rolling groove of the screw shaft. When a ball reaches one end portion of the ball rolling groove 1a of the nut 1 it returns to an opposite side of the ball rolling groove 1a through a return tube 3 fixed to the nut 1. The nut 1 carries out a linear motion along the screw shaft by the rotation thereof. A pair of seal member mounting bores 1b are formed at respective axial end portions of the nut 1. A ring-shaped seal member 2 is mounted in each of the seal member mounting bores 1b and is fastened therein by means of snap ring 4. Each seal member 2 is produced through an injection molding of a synthetic resin or through a cutting working, and a spiral projection 2a is formed on the inner peripheral surface of the seal member 2 which is to be fitted into the ball rolling groove of the screw shaft (see Figure 3). As shown in Figure 3, a plurality of slits 5 for retaining lubricant are formed on the seal member 2 so as to each extend from an end surface 2b thereof facing inside in the axial direction of the nut 1 towards an intermediate portion of the seal member 2. A plurality of slits for removing foreign bodies are formed on the seal member 2 so as to each extend from an end face 2c thereof facing outside in the axial direction of the nut towards an intermediate portion of the seal member 2. The lubricant retaining slits 5 can prevent lubricant (grease, for example), which fills the inner portion of the nut 1, from leaking outward. The foreign bodies removing slits 6 can prevent foreign bodies from invading the nut 1 from outside. Figure 4 shows an end view of the seal and illustrates the arrangement of the lubricant retaining slits 5. The lubricant retaining slits 5 each extend substantially in a radial direction of the screw shaft so as to penetrate the seal member 2 as holes from its inner periphery to its outer periphery. Further, in Figure 4, oblique line portions represent springs 12a and 12b described below. As shown, the lubricant retaining slits 5 are formed on eight portions of the seal member 2 with equal distance between adjacent ones in the circumferential direction. The eight lubricant retaining slits 5 are formed so as to each have an inclination e 1 in the rotation direction F of the screw shaft and to be gradually displaced in the direction of arrow F from the inner periphery of the screw shaft towards the outer periphery thereof. The inclined formation of the lubricant retaining slits 5, enables one of opposing side surfaces 8a and 8b of each slit 5 to act as a lubricant retaining scraper surface having an edge portion for scraping off the lubricant. The number of the lubricant retaining slits 5 may be selected as required. Figure 5 shows a sectional view of the seal member 2. The lubricant retaining slits 5 are formed so as to each have an inclination 2 relative to the direction of an axis X of the nut 1. That is, in a case where the centre line E of the ball rolling groove 9 is inclined at a lead angle of α, in a predetermined deflected direction D with respect to the direction (direction of auxiliary line C) normal to the direction of the axis X of the nut 1, the centre line of the lubricant retaining slit 5 is included at the predetermined angle 2 in a direction reverse to the direction D with respect to the axis X of the nut 1. The lubricant retaining scraper surface 8b is inclined at an angle of β from a surface normal to the centre line E of the ball rolling groove 9 so as to displace the scraped lubricant in the axial direction X of the nut and return the lubricant inside the nut 1. The eight lubricant retaining slits 5 are all formed at an inclination 2 with respect to the axis X of the nut 1. The foreign bodies removing slits 6 extend substantially in a radial direction of the screw shaft so as to penetrate the seal member 2 as holes from its inner periphery to its outer periphery (see Figure 2). In the illustrated embodiment, these foreign bodies removing slits 6 are also formed on eight portions of the seal member 2 with equal distance between adjacent ones in the circumferential direction. Each foreign bodies removing slit 6 has an inclination 1 in the rotating direction F of the screw shaft with respect to the radial direction of the screw shaft. One of opposing surfaces 10a and 10b of each of the slits 6 acts as a foreign bodies removing surface having an edge portion for scraping off the foreign bodies. As shown in Figure 5, the foreign bodies removing slits 6 are formed so as to each have an inclination 2 with respect to the direction of the axis X of the nut 1. That is, in a case where the centre line E of the ball rolling groove 9 is inclined, at a lead angle of α, in a predetermined deflected direction D with respect to the direction (direction of auxiliary line C) normal to the direction of the axis X of the nut 1, the centre line of the foreign bodies removing slit 6 is inclined at the predetermined angle 2 in the direction reverse to the direction D with respect to the axis X of the nut 1. The foreign bodies removing scraper surface 10b is inclined at an angle of β from a surface normal to the centre line E of the ball rolling groove 9 so as to displace the scraped foreign bodies in the axial direction X of the nut and discharge them outside the nut 1. As is shown in Figures 6 and 7, each seal member 2 is formed with two grooves 11a and 11b in its outer peripheral surface which each extend in the circumferential direction thereof. A respective spring 12a, 12b is wound around each one of the grooves 11a, 11b. By winding springs 12a and 12b around the outer peripheries of the seal members 2, an elastic property is imparted to each seal member 2. The flexible elastic seal member 2 can be tightly fitted to the screw shaft to thereby improve the sealing performance. In addition, the edge portions of the lubricant retaining scraping surface 8b and the foreign bodies removing scraping surface 10b can be urged into sure contact with the screw shaft to thereby reliably scrape off the lubricant and the foreign bodies. The number of grooves, as 11a and 11b extending in the circumferential direction of each seal member 2 is not limited to two and may be selected as required. In one example, only a single groove is provided in each seal member. The lubricant scraping function and the foreign bodies removing function of the seal members 2 will now be described in the case where the screw shaft is rotated in the direction F shown in Figures 2 and 4. When the ball screw is rotated in the direction F in Figure 2 or 4, the edge portion of the scraper surface 8b of each lubricant retaining slit 5 of the seal member 2 will slide in contact with the outer peripheral surface of the screw shaft. For this reason, the lubricant adhering to the outer peripheral surface of the screw shaft will be scraped off by the edge portion of the scraper surface 8b of the lubricant retaining slit 5 and scraped on the scraping surface 8b thereof. Since this scraping surface 8b of the lubricant retaining slit is inclined at an angle of β with respect to the direction normal to the centre line E of the ball rolling groove 9, the lubricant is returned to the inside of the nut 1. Accordingly, lubricant invading from the end surface 2b of the seal member 2 into the gap between the nut 1 and the screw shaft can be positively trapped and returned again to the inside of the nut 1. Further, in the case where the ball screw is rotated in the direction reverse to the direction F in Figure 2 or 4, the lubricant can be sealed at the end surface 2b of the seal member 2 as in the case of no formation of a lubricant removing slit (see Figure 3). Similarly, when the screw shaft is rotated in the direction F in Figure 2 or 4, the edge portion of the scraper surface 10b of the foreign bodies removing slit 6 slides on the outer peripheral surface of the screw shaft in contact therewith. Accordingly, foreign bodies adhering to the outer peripheral surface of the screw shaft can be scraped off and the scraped foreign bodies are then scraped on the scraper surface 10b for removing the foreign bodies. Further, since this scraper surface 10b has an inclination β to a direction normal to the centre line E of the ball rolling groove 9, the scraped foreign bodies are discharged out of the nut 1. As explained above, in embodiments of the invention, each of the lubricant retaining portions is formed on the seal member from an end surface of the seal member facing inside in the axial direction of the nut to an intermediate portion thereof. Similarly, each of the foreign bodies removing portions is formed on the seal member from an end surface of the seal member facing outside in the axial direction of the nut to the intermediate portion thereof. By these means, lubricant invading into the gap between the seal member and the screw shaft through the end surface of the seal member can be positively trapped and returned to the inside of the nut, and foreign bodies remaining on the outer peripheral surface of the screw shaft can be removed and discharged outside. Therefore, the lubricant can be retained in the inside portion of the nut and the foreign bodies can be prevented from entering inside the nut.

Claims: A ball screw comprising a screw shaft formed, on an outer peripheral surface thereof, with a ball rolling groove, a number of balls rolling along said ball rolling groove, a nut (1) screwed with said screw shaft through the balls and seal members (2) disposed at both end portions of the nut, each of said seal members being formed, on an inner peripheral surface thereof, with a spiral projection (2a) to be fitted into said ball rolling groove, wherein a lubricant retaining portion (5) is formed on the seal member (2) from an end surface thereof (2b) facing inside in the axial direction of the nut, and said lubricant retaining portion comprises a slit (5), and wherein the lubricant retaining portion positively traps lubricant invading into the gap between the seal member and the screw shaft through the end surface and returns the lubricant to the inside of the nut (1), and in that the slit (5) of the lubricant retaining portion extends from said end surface (2b) of the seal member (2) to an intermediate portion thereof. and wherein a foreign bodies removing portion (6) is formed on the seal member to remove foreign bodies adhering to an outer peripheral surface of the screw shaft, and discharge the foreign bodies outside the nut, characterised in that the foreign bodies removing portion is a slit (6) formed on the seal member (2) so as to extend from an end face (2c) thereof facing outside in the axial direction of the nut to an intermediate portion thereof. A ball screw as claimed in Claim 1, wherein pressing means (12a, 12b) for pressing said seal member against the screw shaft are provided. A ball screw as claimed in Claim 1 or Claim 2, wherein said slit (5) of the lubricant retaining portion penetrates the seal member (2) from an inner peripheral portion to an outer peripheral portion thereof. A ball screw as claimed in any preceding claim, wherein said slit (6) of the foreign bodies removing portion penetrates the seal member (2) from an inner peripheral portion to an outer peripheral portion thereof. A ball screw as claimed in any preceding claim, wherein a lubricant retaining scraper surface (8b) having an edge portion for scraping off lubricant is formed at one of opposing side surfaces of the slit (5) of the lubricant retaining portion, and the lubricant retaining scraper surface (8b) is formed to have an inclination (1) in a rotating direction of the screw shaft and to have an inclination (2) with respect to a direction of an axis of the nut so as to displace the scraped lubricant in the axial direction of the nut and return the lubricant inside the nut. A ball screw as claimed in any preceding claim, wherein the lubricant retaining portion has a plurality of slits (5), and one of opposing side surfaces (8a, 8b) of each slit (5) acts as a lubricant retaining scraper surface having an edge portion for scraping off the lubricant. A ball screw as claimed in Claim 6, wherein the lubricant retaining slits (5) are formed so as to each have an inclination (2) relative to the axial direction of the nut (1). A ball screw as claimed in any preceding claim, wherein the foreign bodies removing portion (6) has a plurality of slits (6), and one of opposing surfaces (10a, 10b) of each slit (6) acts as a foreign bodies removing surface having an edge portion for scraping off the foreign bodies. A ball screw as claimed in Claim 8, wherein the foreign bodies removing slits (6) are formed so as to each have an inclination (2) with respect to the axial direction of the nut.

Inventors: HONMA MITSUAKI, NISHIDE TETSUHIRO, NIWA HIROSHI, TAKEDA RYUJI, HONMA, MITSUAKI, NISHIDE, TETSUHIRO, NIWA, HIROSHI, TAKEDA, RYUJI, Honma, Mitsuaki THK Co., Ltd., Nishide, Tetsuhiro THK Co., Ltd., Niwa, Hiroshi THK Co., Ltd., Takeda, Ryuji THK Co., Ltd.
Applicants: THK CO LTD, THK CO., LTD.
Application year: 1999
Main classification: F16H25
IPC codes: F16H25, F16H57