A PIVOTABLE SCREW JACK DRIVE

Abstract: A pivotable screw jack drive for a tiltable element (11,12) such as a rear view mirror of a motor vehicle, wherein a motor driven worm wheel (31) also performs the function of a jacking nut (29) which rotates about a jacking screw (35), the projecting end (41) of the jacking screw being connected to the tiltable element (11,12) for universal pivotal movement but being restrained against relative rotation, the jacking nut (29) being mounted in a housing (22) with a ball joint so that the nut and screw pivot as an assembly.

Description: This invention relates to a pivotable screw jack drive, in combination with a tilting element, for use in tilting control of position of that element of a rear view mirror for a motor vehicle, of the type comprising a housing, a static bearing surface of concave part-spherical shape in a wall of the housing, a single-piece jacking nut, having an outer surface which is a complementary convex bearing surface, of part-spherical shape which bears against said static surface, an inner threaded surface, and a series of gear teeth spaced around its periphery to form a worm wheel, a motor assembly within the housing having an output shaft, a drive worm on the output shaft, said drive worm having threads in direct meshing contact with the gear teeth of the worm wheel to drive the worm wheel, a jacking screw having a first end with a threaded male portion which is threadably engaged in direct mating contact with said inner threaded surface of the jacking nut, and a second end which engages said tiltable element for relative pivotal movement about an axis which is perpendicular to the axis of the screw, and restraining means for restraining the screw from rotation, the arrangement being such that, upon motor rotation, the nut extends or retracts the jacking screw and effects tilting of the jacking screw and nut and movement of the gear teeth across the drive worm. A pivotable screw jack drive of this type is disclosed in FR-A-2545768. The centre of curvature of the static bearing surface for the jacking nut is so located that tilting movement of the jacking nut causes a variation in the distance between the gear teeth thereon and the drive worm. The present invention aims to overcome this disadvantage. According to the invention, in a pivotable screw jack drive of the type described above, the gear teeth spaced around the periphery of the jacking nut are positioned so as to intersect a plane containing both the centre of curvature of the convex part-spherical bearing surface and the axis of rotation of said drive worm. An embodiment of the invention is described hereunder in some detail with reference to, and is illustrated in, the accompanying drawings, in which: Fig. 1 is a rear elevation of a rear view mirror (that is, viewed in a rearward direction) with respect to a motor vehicle on which it is mounted; Fig. 2 is a cross-section taken on line 2-2 of Fig. 1, but omitting the motor and worm wheel assemblies; Fig. 3 is a front elevation of the motor housing assembly, Fig. 3 also identifying the cross-section plane 2-2; Fig. 4 is an "exploded" view of the motor housing assembly, with one screw jack and its drive motor removed; Fig. 5 is a cross-section through a screw jack assembly, which best illustrates the invention herein claimed; and Fig. 6 is a geometric representation of the movement of the gear teeth across the drive worm, which takes place upon pivotal movement of the nut and screw assembly. In this embodiment, the tiltable element of the invention is the rear vision mirror which is contained within an outer shroud 10 which is hinged in known manner to a mounting (not shown) secured to the outer surface of a motor vehicle in such a way that the shroud 10 will be displaced upon striking an obstruction. The invention however is not limited to such use and can be applied to other tiltable elements, or to rear vision mirrors which do not have this provision. The rear vision mirror 11 is carried on a mirror backing plate 12 (not shown in Fig. 3), the backing plate 12 being provided with a first ball socket 13 about which it can both universally pivot and rotate, the ball socket 13 engaging a ball 14 carried on a mounting plate 15 secured by screws 16 to the outer shroud 10. The requirement is to pivot the rear vision mirror 11 and its backing plate 12 with respect to the shroud, for rotation (in this embodiment) about both vertical and horizontal axes of the ball 14. The mounting plate 15 is provided with female lugs 20 (Fig. 3) which are engaged by protruding lugs 21 (Fig. 4) of a housing 22. The housing 22 contains two pivotal screw jack drives generally designated 23, and each being as shown in a larger scale in Fig. 5. Each screw jack drive 23 comprises an electric motor assembly 24 of the reversible type, the output shaft of which carries on it a drive worm 25. The housing 22 contains two part spherical hollow jacking nut bearing surfaces 27 (Fig. 4), and these are engaged by complementary part spherical bearing surfaces 28 of respective jacking nuts 29, each jacking nut 29 having an inner threaded surface 30, and a series of gear teeth 31 (which are either spur gear teeth or slightly helical to match the helix angle of the worm), the gear teeth 31 being spaced around the periphery of the jacking nut 29 to constitute a worm wheel which has a cylindrical envelope shape (that is, they are not concave teeth as with the ordinary worm wheel). A jacking screw 35 is bifurcate and has two limbs 36 (Fig. 4) which are splayed out by a resilient "U"-shaped spring 37, the inner ends of the limbs 36 having male threads 38 thereon which threadably engage the inner threaded surface 30 of the jacking nut 29 as best seen in Fig. 5. The projecting end of each jacking screw 35 passes through a soft elastomeric gasket 40 located between the housing 22 and the mounting plate 15, and terminates at its upper end in a ball head 41, the ball head 41 having a projecting spigot 42 which constitutes a restraining portion, rotation of the jacking screw being restrained by spigot 42 engaging a complementary surface of a slot 43 (Fig. 2), there being second and third ball sockets 44 each having a slot 43 opening into it, on the underside of the backing plate 12 of the mirror 11. The slots are directed towards the first socket 13. However the ball head 41 is otherwise movable within the part spherical socket 44 for the required pivotal movement in the two directions for vertical and horizontal adjustment of the position of the mirror 11. The arrangement therefore is such that, upon motor rotation, the worm 25 causes rotation of the jacking nut 29 to drive the jacking screw 35 outwardly or withdraw it back into the nut. In the event of there being excessive drive, the bifurcate limbs 36 merely spring together as the threads ride over one another, and this constitutes an in-built overload protection device. By driving the jacking screw 35 outwardly or withdrawing it inwardly, the backing plate 12 is tilted about the ball socket 13 in either one of the two planes, and since the ball heads 41 of the jacking screws 35 subtend an angle of 90° with each other with respect to the bolt ball socket 13, the adjustment takes place in either one of the two planes. However, tilting of the mirror 11 necessarily means that there is some tilting of the assembly of the jacking nut 29 and jacking screw 35, and this causes the gear teeth 31 to move across the thread of the worm 25 by a small degree only. Fig. 6 illustrates the geometry of this movement, and the displacement angle A from central position is very small (a small adjustment only being required for the mirror) and this corresponds to a distance D. The difference in engagement between the teeth and the threads of the worm is therefore extremely small, being trigonomically identified by (1-R cos A), wherein R is the radius of tilt, this being so small as to be negligible in the application described herein. It is recognition of this factor which enables the entire arrangement to work very satisfactorily under a wide range of conditions, with parts which are not so small as with previously proposed similar arrangements, since there is no articulation required between the jacking screw 35 and the jacking nut 29. Parts therefore are lower in cost, fewer in number and much more rugged. As seen in Fig. 1, there are two ball heads 41 which engage in complementary ball sockets 44 on the backing plate 12, and these subtend an angle of 90° with the centre of the ball and socket joint 13/14, so that pivotal movement effected by one screw jack drive 23 in, say, an X-Y plane does not affect adjustment in the Y-Z plane, and vice versa. As the pivotal movement of the assembly of jacking screw and jacking nut takes place, there must be some lateral displacement intermediate the ends and this is accommodated by means of a three limb spring 48 which engages the upper surface of the jacking nut 29, and the limbs 49 thereof bear upwardly against the underside of the soft gasket 40 and reacts, through gasket 40, against the mounting plate 15 which performs the dual function of providing a moisture barrier for the inner space within the housing 22 and also facilitating the very small amount of lateral movement required between the limbs 49 and the mounting plate 15. A consideration of the geometry as best seen in Fig. 5 will indicate that the lateral movement of the assembly of jacking nut 29 and jacking screw 35 is extremely limited in the plane which includes the central axis of the worm 25 because the centre of rotation of the bearing surface 28 is coplanar with the axis of the drive worm 25 when the longitudinal central axis of the assembly is in its medial position.

Claims: In combination with a tiltable element (11,12), a pivotable screw jack drive (23), for tilting control of the tiltable element, comprising a housing (22), a static bearing surface (27) of concave part-spherical shape in a wall of the housing (22), a single-piece jacking nut (29), having an outer surface which is a complementary convex bearing surface (28), of part-spherical shape which bears against said static surface (27), an inner threaded surface (30), and a series of gear teeth (31) spaced around its periphery to form a worm wheel, a motor assembly (24) within the housing having an output shaft, a drive worm (25) on the output shaft having threads in direct meshing contact with the gear teeth (31) of the worm wheel to drive the worm wheel, a jacking screw (35) having a first end (36) with a threaded male portion (38) which is threadably engaged in direct mating contact with said inner threaded surface (30) of the jacking nut (29), and a second end (41) which engages said tiltable element (11,12) for relative pivotal movement about an axis which is perpendicular to the axis of the screw (35), and restraining means (42) for restraining the screw (35) from rotation, the arrangement being such that, upon motor rotation, the nut extends or retracts the jacking screw (35) and effects tilting of the jacking screw (35) and nut (29) and movement of the gear teeth (31) across the drive worm (25), characterised in that the gear teeth (31) spaced around the periphery of the jacking nut (29) are positioned so as to intersect a plane containing both the centre of curvature of the convex part-spherical bearing surface (28) and the axis of rotation of said drive worm (25). A pivotable screw jack according to claim 1, wherein the worm wheel (31) has a cylindrical envelope shape. A pivotable screw jack according to claim 1 or 2, wherein said rotation-restraining means comprises a restraining spigot (42) which so engages a complementary surface of a slot (43) in the tiltable element (11,12) as to restrain the screw (35) from rotation. A pivotable screw jack according to any preceding claim, wherein said second end (41) of the jacking screw (35) also comprises a part-spherical surface, and said tiltable element (11,120) contains a part-spherical socket (44) complementary to and engaged by said part-spherical surface (41) of said jacking screw (35). A pivotable screw jack according to any preceding claim, further comprising a mounting plate (15), means (20,21) joining the housing (22) to the mounting plate (15), a spring (48) engaging said part-spherical surface (28) of said jacking nut (29), the spring (48) having projecting limbs (49) which react against a surface of the mounting plate (19), thereby retaining said jacking nut outer surface (28) in contact with said static bearing surface (27). A pivotable screw jack according to claim 5, further comprising a soft gasket (40) contiguous with a surface of said mounting plate (15) and between said mounting plate (15) and spring limbs (49). A pivotable screw jack according to any preceding claim, wherein said jacking screw first end is bifurcate, having two limbs (36) each with outer curved thread surfaces (38) which threadably engage said inner thread surface (30) of the jacking nut (29), and a "U"-shaped spring (37) between said limbs urging their threaded surfaces into that engagement. A pivotable screw jack according to any preceding claim, wherein said tiltable element (12) comprises a backing plate of a rear vision mirror for a motor vehicle. A rear vision mirror assembly for a motor vehicle comprising a backing plate (12), a surface (13) of the backing plate which defines a first element of a first ball and socket joint, a mounting plate (15,), a second element (14) on the mounting plate, said first ball and socket joint engaging the first element (13) thereof for relative pivotal and rotational movement of the backing plate with respect to the mounting plate, two further concave part-spherical surfaces (44) of the backing plate (12) defining respectively first elements of second and third ball and socket joints in the backing plate (12), which subtend an angle of 90° with the first element of the first ball and socket surface (13), two pivotable screw jack drives (23), each according to claim 1, and mounting means (22) mounting said screw jack drives (23) to the mounting plate (15), said second ends (41) of said jacking screws (35) comprising the second elements of said second and third ball and socket joints, and pivotally engaging respective said further backing plate surfaces (44). A rear vision mirror assembly according to claim 9, further comprising slot surfaces in said backing plate (12) defining two slots (43) opening into said two further concave part-spherical surfaces (44) and directed therefrom towards the first ball and socket joint, and respective spigots (42) projecting from the balls (41) of said second and third ball and socket joints engaging walls of said slots and restraining those balls (41) against rotation.

Inventors: FIMERI, GARRY GORDON LESLIE
Applicants: BRITAX RAINSFORDS PTY. LTD.
Application year: 1987
Main classification: B60R1
IPC codes: B60R1