Ballscrew lubrication

Abstract

A re-circulating ballscrew assembly comprises a plurality of balls. A ballnut has a ballnut body with a radially inner surface and a helical ballnut groove formed on the radially inner surface. A ballscrew is disposed within the ballnut, the ballscrew comprising a radially outer surface and a ballscrew groove formed on the outer surface, the ballscrew groove cooperating with the ballnut groove to define a helical raceway for the plurality of balls, the helical raceway having a start point and an end point. The ballscrew assembly comprises a return track for the plurality of balls, the return track disposed radially outward of the radially inner surface of the ballnut and connecting the start point and the end point of the helical raceway. The return track extends at least in part through an annular lubricant bath.

Claims

1. A re-circulating ballscrew assembly comprising: a plurality of balls; a ballnut having a ballnut body with a radially inner surface and a helical ballnut groove formed on the radially inner surface; a ballscrew disposed within the ballnut, the ballscrew comprising a radially outer surface and a ballscrew groove formed on the outer surface, the ballscrew groove cooperating with the ballnut groove to define a helical raceway for the plurality of balls, the helical raceway having a start point and an end point; a return track for the plurality of balls, the return track disposed radially outward of the radially inner surface of the ballnut and connecting the start point and the end point of the helical raceway; wherein the return track extends at least in part through an annular lubricant bath formed in an annular cavity defined radially outwardly of the radially inner surface between a radially outer surface of the ballnut body and an annular sleeve surrounding the ballnut body, wherein the return track is defined by respective grooves formed on a radially inner surface of the annular sleeve and a radially outer surface of the ballnut body; and a plurality of pressure-balancing ports extending through the ballnut body to form passages from the annular cavity to the radially inner surface of the ballnut body.

2. The ballscrew assembly of claim 1, wherein the plurality of ports are distributed axially along the ballnut body.

3. The ballscrew assembly of claim 2, wherein the helical ballscrew groove forms a corresponding helical ridge, wherein the plurality of ports include openings on the radially inner surface of the ballnut body, and wherein the openings are aligned with the helical ridge.

4. The ballscrew assembly of claim 1, wherein each of the plurality of pressure-balancing ports has a diameter that is less than the diameter of the plurality of balls.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the present disclosure will now be described by way of example only, and with reference to the following drawings in which:

(2) FIG. 1 shows a view of a ballscrew actuator in accordance with this disclosure;

(3) FIG. 2 shows another view of the ballscrew actuator of FIG. 1.

(4) FIG. 3 shows a cross-sectional view of the ball screw actuator of FIG. 2.

DETAILED DESCRIPTION

(5) FIG. 1 shows a ballscrew assembly 2 comprising a ballnut 4, a ballscrew 6 and a plurality of balls 8. The ballnut 4 has a ballnut body 5, the ballnut body including a helical groove 10 formed on a radially inner surface 12. The ballscrew 6 includes a corresponding helical groove 14 formed on a radially outer surface 16. The ballscrew 6 is disposed within the ballnut 4, and the balls 8 sit between the ballscrew 6 and the ballnut body 5, within the helical grooves 10, 14. The ballscrew 6 can rotate within the ballnut body 5, which causes the ballscrew 6 to move along a shared axis A of the ballnut body 5 and ballscrew 6.

(6) The helical grooves 10, 14 on the ballnut body 5 and ballscrew 6 define a helical raceway for the balls 8. As the ballscrew 6 rotates, the balls 8 progress through the helical raceway from a first end 18 to a second end 20, or vice-versa. The balls 8 are then returned to a respective end 18 or 20 of the helical raceway, in the opposite direction to the movement of the ballscrew 6, to provide a continuous circulation of balls. The balls 8 are returned via a return track 22, which extends radially exterior to the helical raceway.

(7) The return track 22 may be formed within an annular cavity 24, as shown in FIGS. 1, 2 and 3. The radially inner surface of the annular cavity 24 is defined by a radially outer surface 26 of the ballnut body 5, while the radially outer surface of the annular cavity 24 is defined by a radially inner surface 28 of an annular sleeve 30 arranged around the ballnut body 5. The radially inner surface 28 of the annular sleeve 30 and the radially outer surface 26 of the ballnut body 5 comprise respective grooves 32 and 34, which correspond to the shape of the balls 8 and define the return track 22 through which the balls 8 pass.

(8) The grooves 32 and 34, and hence the return track 22, may extend through a partial helical turn, a single full helical turn, or multiple helical turns. Alternatively, the grooves 32, 34 may extend parallel to the ballscrew axis A.

(9) The ballscrew assembly 2 may comprises a plurality of helical raceways and a plurality of return tracks 22, each return track 22 joining a first and second end of a respective helical raceway to provide a continuous circuit. Each return track 22 is defined by a respective pair of grooves 32, 34 formed on the radially inner surface 28 of the annular sleeve 30 and the radially outer surface 26 of the ballnut body 5. In the embodiment illustrated, there are two helical raceways and two return tracks 22.

(10) The ballnut 4, ballscrew 6 and balls 8 are lubricated to facilitate their relative movement. The ballscrew assembly 2 may use an oil lubricant, or a grease lubricant. Lubricant can be provided to the ballscrew assembly 2 particularly effectively by applying lubricant to the balls 8. To provide spatial efficiency, the lubricant can be applied to the balls 8 within the return track 22 of the ballscrew assembly 2.

(11) In the case of the annular return track 22, the lubricant can be applied by partially or completely filling the annular cavity 24 with lubricant. This forms an annular lubricant bath, through which the balls 8 pass as they progress through the return track 22. The annular lubricant bath evenly coats the surface of the balls 8 to provide effective lubrication.

(12) The annular cavity 24 may be sealed to prevent leakage and subsequent wastage of lubricant. The annular sleeve 30 may comprise annular static seals 36 at one, or both, axial ends of the annular cavity 24.

(13) By filling the annular cavity 24 with lubricant, an adverse pressure gradient may be formed between the annular cavity 24 and the helical raceway of the ballnut body 5 and ballscrew 6. Pressure-balancing ports 40 may therefore be provided to connect the annular cavity 24 and the helical raceway.

(14) The ports 40 extend through the body of the ballnut body 5, from an outer opening 42 in the radially outer surface 26 of the ballnut body 5, to an inner opening 44 on the radially inner surface 12 of the ballnut body 5. The ports 40 thereby fluidly connect the annular cavity 24 to the helical raceway. The ports 40 may, as shown, be distributed axially with respect to the axis A of the ballnut 4, such that each port 40 is axially separated from an adjacent port 40.

(15) The helical groove 10 of the ballnut body 5 forms a respective helical ridge 46. Each of the ports 40 may be arranged so that the inner opening 44 thereof is aligned with the helical ridge 46. Each port 40 may be associated with a single full turn of the helical ridge 46.

(16) The ports 40 are sized to allow pressure-equalisation. The ports 40 may generally have a diameter that is less than that of the balls 8. The ports 40 are located a circumferential distance separate from the return track 22 in order to prevent the ports 40 interfering with the passage of the balls 8.

(17) It will be understood that the above description is of non-limiting embodiments of the disclosure. Changes and modifications can be made thereto without departing from the scope of the disclosure which is defined by the following claims.