Machine element or mechanism – Gearing – Rotary bodies
Reexamination Certificate
2000-09-27
2001-11-20
Fenstermacher, David (Department: 3682)
Machine element or mechanism
Gearing
Rotary bodies
C074S089420
Reexamination Certificate
active
06318204
ABSTRACT:
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
Crema Rolf
Kranz Gerhard
Schmidt Peter
Fenstermacher David
Foley & Lardner
Leica Camera AG
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