Helical gear

Screw mechanism having a spindle (1) which is secured in the axial direction and has a spindle axis (1'), and a spindle nut (5) which is connected to a slide (7) which can be driven and is guided in the axial direction, a joint being provided between the spindle nut (5) and the slide (7) in order to compensate for relative transverse movements and tilting between the spindle nut (5) and slide (7), said joint being formed symmetrically to the spindle axis (1') and having two joint axes which are perpendicular to each other, intersect on the spindle axis (1') and are assigned at one end to the transverse movement of a bearing plate (6), which is connected to the spindle nut (5), and at the other end to the transverse movement of a bearing plate (8), which is connected to the slide (7), and said joint containing an intermediate plate (9) via which the bearing plates (6, 8) are connected pivotably to each other in order to compensate for any tilting of their joint axes. The joint axes are each formed by two balls (19, 20; 19', 20') which are arranged aligned with one another on opposite sides of the spindle axis (1'), are mounted in mutually opposite prism grooves (17, 18; 17', 18') in the bearing plates (6, 8) and the intermediate plate (9), and bear against one another under spring pressure (13, 16; 13', 16').

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a helical gear having a spindle which is secured
 in the axial direction and has a spindle axis, and a spindle nut which is
 connected to a slide which can be driven and is guided in the axial
 direction, a joint being provided between the spindle nut and the slide in
 order to compensate for relative transverse movements and tilting between
 the spindle nut and slide, said joint being formed symmetrically to the
 spindle axis and having two joint axes which are perpendicular to each
 other, intersect on the spindle axis and are assigned at one end to the
 transverse movement of a bearing plate, which is connected to the spindle
 nut, and at the other end to the transverse movement of a bearing plate,
 which is connected to the slide, and said joint containing an intermediate
 plate via which the bearing plates are connected pivotably to each other
 in order to compensate for any tilting of their joint axes.
 2. Description of the Related Art
 Helical gears of this type are known and are described, for example, in The
 textbook "Korstruktionselemente der Feinmechanik" [Structural elements in
 precision mechanics], edited by Werner Krause, Hanser publishers (1989),
 pp. 709 ff. The spindle is rotatable, but is not displaced in the axial
 direction. The spindle nut is retained in a longitudinal guide parallel to
 the axis of the spindle and is therefore not rotatable. Rotation of the
 spindle is therefore converted into a sliding movement of the spindle nut.
 The object carrier which is to be driven is connected to the spindle nut.
 With helical gears it is possible, through the selection of sufficiently
 long spindles, to obtain drives over very long displacement paths.
 Generally, not too much has to be demanded as concerns the quality of the
 screw thread of the spindle although, for example, hardened spindles
 having a ground screw thread for great thread precision are preferable.
 Play between the screw threads of the spindle nut and the spindle can be
 compensated for by the spindle nut undergoing a division with the parts
 spring-loaded against one another. The axes of long spindles are generally
 not exactly linear. Particularly in the case of thin spindles, they can
 either be slightly bent or even corrugated. Since they are only mounted in
 their end regions, the spindle nut therefore executes slight upward and
 downward movements and also lateral movements along its displacement path.
 Such eccentricity in the concentric running of the spindle is also
 produced if the axis of a driving motor and the spindle axis are not
 aligned with each other.
 If, on the other hand, the axial guiding of the slide defines the
 displacement path vertically and laterally, there occur, in particular in
 the case of very precise slide guides, distortions between the driven nut
 and the slide. These distortions can be further reinforced if the bearings
 of the spindle are not arranged for alignment exactly parallel to the
 displacement path. Distortions of this type considerably impair the
 running characteristics and the positioning precision of the slide.
 U.S. Pat. No. 5,392,662 discloses a helical gear in which the above
 mentioned error effects are compensated for by a joint fitted between the
 spindle nut and slide. The joint consists of a contact-pressure plate
 which is connected to the spindle nut, an intermediate plate and a
 contact-pressure plate which is connected to the slide. Each of the
 bearing plates is connected to the intermediate plate via two spring
 clips. The two spring clips of a bearing plate lie parallel to each other
 on opposite sides of the spindle axis, the two pairs of spring clips being
 arranged perpendicular to each other. The spring clips are fastened to the
 bearing plates by their rounded part and to the intermediate plate by
 their foot parts. As a result, each of the bearing plates is guided, with
 regard to a transverse movement, by a spring parallelogram and, with
 regard to tilting, can be rotated about an axis. The joint is configured
 symmetrically to the spindle axis and contains a non-positive connection
 between the driving and driven system. In this case, linear compression or
 deformation of the spring clips in the case of resistance between the
 spindle drive and slide guide or in the case of relatively high driving
 frequencies cannot be ruled out. The deflection of the spring clips during
 the transverse movement produces a force reaction on the spindle, said
 reaction increasing with the deflection.
 BRIEF SUMMARY OF THE INVENTION
 The invention was therefore based on the object of realizing a helical gear
 of a simple type of construction, in which tolerances caused by the
 manufacturing and type of construction in the straightness of the spindle
 and the alignment of its bearings with respect to the slide guide do not
 have any negative effects on the slide guide. The intention is for the
 rotatory movement of the spindle to be converted with the highest possible
 quality into a lateral movement which is as strictly proportional as
 possible. At the same time, the intention is for the mutual reactions
 between the driving and driven system to be as small as possible. In
 addition, during the transmission of the actuating force the intention is
 that the active vector, if possible, does not produce any secondary
 torques which stress the system, particularly at relatively high driving
 frequencies.
 According to the invention, this object is achieved in the case of a
 helical gear of the type mentioned at the beginning by the joint axes each
 being formed by two balls which are arranged aligned with one another on
 opposite sides of the spindle axis, are mounted in mutually opposite prism
 grooves in the bearing plates and the intermediate plate, and bear against
 one another under spring pressure.
 Advantageous refinements emerge from the features of subclaims.
 The interconnection of the joint configured in accordance with the
 invention enables the spindle nut to execute, in the case of deflection
 and tilting taking place exclusively by rolling, all necessary
 compensating movements with respect to the slide with the smallest
 possible frictional forces. The joint, which is free of play and is
 secured rotationally by the form-fitting bearing of the joint elements in
 the driving direction, ensures that the positioning precision of the drive
 is not affected during a reversal of movement.
 The joint axes lie staggered at a short distance one behind another in the
 axial direction. The actuating vector for uncoupling actuating forces from
 the spindle or coupling them back into the spindle therefore always passes
 back through the spindle axis. In this manner, axial forces from mass
 accelerations act on the spindle nut in a manner free of secondary
 torques. The effects of different flank inclinations of the spindle and
 also of a twisted spindle axis are not reinforced during rotation about
 the spindle axis, but are halved on account of the symmetrical coupling
 arrangement.
 Movements taking place radially between the nut and driven slide are
 absorbed via the balls rolling is prisms. The frictional torque which can
 be transmitted to the nut from the rotation of the spindle is therefore
 very small, with the result that only very small mutual rotatory reactions
 are produced.
 As the bearings for the balls use is made of mutually opposite prism
 grooves which can be restricted in their longitudinal extent. The balls
 lying in prism grooves execute translatory compensating movements and
 tiltings at the lowest possible frictional forces.
 The prism grooves are expediently open on both sides, it being possible for
 them to be milled in the workpiece in one working step. The balls can then
 be secured in a special cage against lateral migration. The cage or the
 balls in the cage are to be able to follow the compensating movements.
 The cage can be designed as a thin disk which has elongate holes lying
 transversely to the direction of the prism grooves for holding the balls
 and contains centering edges on both sides with a lug to secure against
 rotation, said centering edges engaging in the holes in the mutually
 opposite bearing and intermediate plates, at least the centering edge
 being enclosed in the hole in the intermediate plate with radial play.
 The production of very precisely aligned prism grooves as ball paths of
 high surface quality is possible cost-effectively only in the case of
 continuous, laterally open paths. Run-on slopes, which are unfavorable in
 particular also for the positioning precision of the balls, at the end of
 separately milled ball paths are therefore avoided.
 The advantage of continuous prism grooves without the necessity of fitting
 a cage can be realized by the fact that one of the mutually opposite
 plates has a linear prism groove intersecting the spindle axis, and the
 other plate has two linear prism grooves which are at a distance from the
 spindle axis and run perpendicular to the prism groove intersecting the
 spindle axis. The ball lies in each case in the intersecting point of the
 prism grooves.

DETAILED DESCRIPTION OF THE EMBODIMENTS
 In FIG. 1, a spindle 1 is driven by a stepping motor 2 which is connected
 to the spindle 1 via a shaft coupling 3. At its other end, the spindle 1
 runs in a ball bearing 4. The shaft coupling compensates for any alignment
 deviations between the spindle axis 1' and the motor axis.
 A spindle nut 5 which has a bearing plate 6 is coupled to the spindle 1. A
 further bearing plate 8 is fastened to a slide 7. Between the bearing
 plates 6 and 8 is an intermediate plate 9 which can be tilted about two
 axes, which are to be described later, and can be displaced along these
 axes. The slide 7 is connected to a pretensioned, highly precise ball
 guide 10 which runs along a cylindrical rod 11. The bearing brackets of
 the rod 11, the ball bearing 4 and the motor 2 are not illustrated. They
 are aligned in such a manner that the spindle 1 and the rod 11 are
 parallel to each other and lie in one plane. The aim is for the spindle 1
 to pass through the mass center of gravity of the slide 7 in order to
 avoid tilting moments during the drive acceleration. However, this problem
 is virtually negligible when highly precise ball guides 10 are used for
 the slide 7.
 In FIG. 2a, the spindle nut 5 is illustrated in a view of the lower part 12
 of the nut together with a section through the joint. The bearing plate 6
 is situated on the lower part 12. The spindle 1, which is not illustrated
 here, lies in the screw thread 12' of the lower part 12 and extends
 through holes in the bearing plates 6 and 8 and also the intermediate
 plate 9, which plates are held together resiliently by means of a screw 13
 and a screw 13' arranged mirror-symmetrically with respect to the spindle
 axis 1'.
 In the sectional illustration through the spindle nut in FIG. 2b, which
 illustration is rotated by 90.degree., the upper part 14 of the nut is
 also illustrated. It is connected to the lower part 12 by means of a screw
 15 which is under spring pressure. The screw 13 is under the pressure of a
 spring 16. The same applies to the screw 13', which is also indicated
 here, with spring 16'.
 FIG. 3a shows details of the joint in an illustration which is partially
 cut open. A continuous prism groove 17 is milled into the bearing plate 6.
 An identical prism groove 18 is milled into the intermediate plate 9.
 Lying in these prism grooves 17, 18 are two balls 19, 20 which are
 retained by a cage 21. The cage 21 has a centering edge 22 and a lug 23 on
 both sides, by means of which it is retained in a manner secured against
 rotation in the central hole in the bearing plate 6 and the intermediate
 plate 9 and also by engagement of the lug 23 in the grooves 17, 18. The
 fit of the centering edges is designed with play. In this case, it is of
 advantage if the play is present both in the bearing plate 6 and in the
 intermediate plate 9. During translatory compensating movements, the balls
 can then roll along the direction of the groove and carry the cage along
 with them. The frictional forces produced by the point bearing of the
 balls in the cage are minimal, particularly if a plastic material having
 good sliding properties is selected for the cage. In principle, however,
 it is also sufficient for the translatory relative movement if, for
 example, the play is only present in the intermediate plate. The balls can
 then be positioned with the cage in a more precise manner With respect to
 their stroke central point. However, during translatory compensating
 movements the balls then slide because of the fixing of the cage on one
 side.
 As can be gathered from the illustration in FIG. 3b which is rotated by
 90.degree., there is an identical construction of prism grooves 17', 18',
 balls 19', 20'.degree. and cage 21' between the bearing plate 8 and the
 other side of the intermediate plate 9. The two prism grooves which are at
 90.degree. with respect to each other on both surfaces of the intermediate
 plate ensure a translatory yielding in all directions. The centering edges
 22 also serve to cause a restriction of the transverse movement so as to
 prevent the intermediate plate 9 and the bearing plate 6, 8 from coming
 into contact with the spindle 1, the screws 13 and springs 16.
 FIGS. 3c and 3d show a view of the cages 21, 21'. The holes for holding the
 balls 19, 20; 19', 20' are slightly stretched perpendicular to the
 direction of the prism grooves, with the result that only a pointlike
 bearing of the balls against the cage takes place in the piercing point of
 the tilting axis and this point bearing is retained even in the case of
 possible alignment tolerances of the holes with respect to the prism
 grooves during the positioning of the balls. The respective joint axis is
 provided by the connecting line between the ball central points, which
 line cuts across the spindle axis 1'.
 FIG. 3e shows a view of another refinement of the holding of the balls in
 the cage 21. The holes for holding the balls 19, 20 are of circular design
 here and have a relatively large diameter. Oppositely directed spring
 elements 29, 30 which support the balls 19, 20 are integrally formed on
 the hole edges in the radial direction. The free ends of these spring
 elements 29, 30 lie outside the plane of projections. The cage 21 is
 expediently centered without play, so that a defined stroke central point
 of the balls 19, 20 with respect to the part surrounding them is produced.
 In the case of translatory compensating movements, only the very small,
 supporting spring forces in the direction of the spindle 1 act on the
 balls, with the result that the latter can roll virtually unhindered along
 the grooves.
 Two further holes 24, 25; 24', 25' in the cage serve as leadthroughs for
 the two screws 13, 13', which are arranged mirror symmetrically with
 respect to each other, with springs 16, 16' which hold the joint parts
 together in the axial direction in a manner free of play. The construction
 of the joint from prism grooves which are perpendicular to one another and
 have balls running in them allows exclusively translatory relative
 movements. Rotations are excluded The forward feed of the spindle nut,
 which feed corresponds to the rotation of the spindle, is therefore
 transmitted to the slide in a manner free of play.
 In the case of the exemplary embodiment illustrated in FIG. 4, a linear
 prism groove 17, which cuts across the spindle axis, and two linear prism
 grooves 27, 28 which run perpendicularly to the prism groove 17 and are at
 a distance from the spindle axis interact in each case. The balls are
 retained here in the intersecting point of the prism grooves without a
 cage. In the exemplary embodiment illustrated, the prism grooves, two in
 each case parallel to each other, are situated in the upper surface of the
 intermediate plate 9 and the bearing plate 6, as can be gathered from the
 comparison of the sectional illustrations from FIG. 4a and FIG. 4b which
 are rotated through 90.degree.. FIGS. 4c and 4d show the balls in the
 intersecting point of the prism grooves.