Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2001-01-26
2002-11-26
Joyce, William C (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
C074S424770, C074S424880
Reexamination Certificate
active
06484599
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a nut unit for use in a ball screw drive, comprising a sleeve-shaped nut body that is made of an essentially rigid material and that has an axis and an inner circumferential surface, wherein a one-piece, ball-guiding helix of strip material—which is channel-shaped as viewed in transverse cross-section—is arranged on the inner circumferential surface of the nut body, wherein the helix of strip material is attached to the inner circumferential surface of the sleeve-shaped nut body by a convexly curved external profile, the latter being embodied with external profile flank regions and an external profile crown region, and the helix defines, by means of a concavely curved internal profile facing the axis, a ball channel with an internal profile base region and two internal profile flank regions.
In order to form a ball race, it is known from DE 27 32 896 C2 to apply to the inner circumferential surface of a sleeve-shaped nut body a channel-like, profiled metal strip which is rigidly attached along its entire length to the inner circumferential surface of the nut body. The flanks of the metal strip extend out into the central space of the nut body, which is to say that they are not supported outside of the base region. Moreover, known from DE 27 32 896 C2 is an embodiment (see
FIG. 3
) wherein a profiled metal strip is attached by vulcanization to the inner circumferential surface of a bushing made of elastically deformable material. The subject there is thus not a nut body of an essentially rigid material, but of an elastically deformable material.
It is known from DE 28 05 141 to cut a helical profiled groove in the inner surface of a sleeve-shaped nut body and to allow the balls of a continuous set of balls to run directly in this profiled groove on the nut side. In this connection, high demands are placed on the profiled groove with respect to the surface characteristics of the bearing race region, in particular with respect to hardness and smoothness.
In order to create a helicoid profiled groove on the inner circumferential surface of a nut body, it is known from DE 30 28 543 to lay a round wire helix on the inner circumferential surface of a nut body and to arrange it in a helicoid manner by means of ribs on the inner circumferential surface of the nut body. Therein, the balls of an associated ball set each run between two adjacent turns of the round wire. It is further known from DE 30 28 543 to form a cylindrical tube into a screw-like helix to form a nut unit and to slit this tube on the inner side of the helix, so that the balls of a ball set carried in the tube can project radially inward and engage the threads of a spindle.
SUMMARY OF THE INVENTION
In contrast, it is proposed in accordance with the invention that the outer profile flank regions of the strip material helix be essentially rigidly supported by a support profile of a support groove formed in the inner circumferential surface of the nut body.
The following is achieved by the arrangement in accordance with the invention, in contrast to the state of the art discussed above: in comparison to the first embodiment per DE 27 32 896 C2, an increased stiffness is achieved. Owing to the support by the support groove in the external profile flank regions, deformation of the strip material is in any case largely suppressed if not completely precluded. The strip material can neither tip nor be bent. On the other hand, the surface of the strip material helix in the region of the concavely curved internal profile of the support groove can be finished with respect to hardness and smoothness prior to the installation of the strip material helix in the nut body, and if desired at the strip material itself prior to its forming or at least prior to its final shaping. This has particularly great importance when large thread pitches are required for the ball nut, a requirement that is occasionally present in the case of machine tools in order to be able to increase the feed rates without having to raise the spindle speed to levels that are critical in terms of bending. When large nut thread pitches are required, it becomes increasingly difficult to perform smoothing operations, in particular grinding, on the finished threads as the pitch increases.
Also in contrast to the second embodiment per DE 27 32 896 C2, wherein the metal strip is attached by vulcanization to the inner circumferential surface of a bushing of elastically deformable material, the embodiment in accordance with the invention provides the important advantage, owing to the essentially rigid support of the external profile flank regions by the support profile of the support groove, that the strip material can neither tip nor be bent and cannot yield in any other way, which is precisely the goal of the known embodiment with respect to the change in pitch that is striven for therein.
In contrast to the known embodiment per DE 28 05 141, the effect is achieved that surface treatment with regard to hardness and smoothness in the ball race is possible without regard to the thread pitch, because smoothing and hardening treatments are possible before installation in the nut body and even before shaping of the strip material helix, but in any case prior to its final shaping.
In contrast to the first-described known embodiment per DE 30 28 543 A1, wherein balls are guided on the nut body between two adjacent turns of the round wire helix, the advantage is achieved that balls are guided on the nut body by a one-piece channel profile, resulting in a better and easier to manufacture precision and in higher load capacity. In contrast to the further embodiment described in DE 30 28 543 A1, wherein balls are guided in a tube that is rolled into a spiral and cut on the inside of the spiral, the advantage of increased stiffness is again achieved owing to the support of the external profile flank regions by the support profile of the support groove.
The proposed invention can in particular find application when the concavely curved internal profile is shaped such that balls of an associated ball set of predetermined nominal diameter run on bearing race tracks of the internal profile flank regions, each of which bearing race tracks lie within a track region of the relevant internal profile flank region. In such an embodiment of the ball channel, the strip material helix is stably supported by the support of its external profile flank regions by the support profile of the support groove at or in the immediate vicinity of the bearing race track and/or the bearing race tracks that are possible as a result of altering the ball diameter, so that maximum stiffness of the ball screw drive can be achieved.
When reference is made to a track region, in particular that track region is meant that is determined by the possible bearing race tracks which result from a group of ball sets with nominal diameters graduated from ball set to ball set that are available to set a specific preload range for the ball screw drive. In this connection, the nominal diameter in each case determines the “pressure angle” (defined hereinafter).
A first embodiment of the invention resides in that a support—based on physical contact—of an external profile flank region on the support profile of the support groove extends over a contact zone corresponding approximately to the total extent of the track region along the curved internal profile and—if desired—extends beyond the borders of the track region. In this first embodiment, all conceivable bearing race tracks that result from changing the nominal ball diameter are supported directly and free of bending on the back side, which is to say in the external profile flank region, so that optimal stiffness is achieved. However, in this embodiment a relatively high precision is demanded in the manufacture of the external profile flank regions and the support profile.
In accordance with a second embodiment of the invention, provision is made for the support, based on physical contact, of an external profile flank regi
Baker & Botts LLP
Joyce William C
Rexroth Star GmbH
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