Machine element or mechanism – Mechanical movements – Reciprocating or oscillating to or from alternating rotary
Reexamination Certificate
1998-10-01
2001-06-12
Bucci, David A. (Department: 3682)
Machine element or mechanism
Mechanical movements
Reciprocating or oscillating to or from alternating rotary
C074S424830
Reexamination Certificate
active
06244122
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a nut mechanism for creating translational motion by engagement with a rotating threaded drive shaft or feed screw.
BACKGROUND OF THE INVENTION
In a variety of mechanical devices and systems, feed screw and nut arrangements are frequently used for providing translational motion to a moving carriage or the like. Typically, the nut is rigidly affixed to the carriage and is prevented from rotating about its axis, such that rotation of the feed screw causes the nut, and hence the carriage, to translate along the feed screw axis.
In some applications, precision of movement of the carriage is not of particular concern, and hence factors such as dimensional imperfections and friction which are common or inherent in feed screw
ut arrangements are relatively insignificant problems. However, in other applications requiring precise movement and positioning of a carriage, these factors can be significant problems. For example, small errors in the manufacture of the various components of a feed screw
ut arrangement can lead to various imperfections including misalignment of the feed screw axis with the direction of carriage movement, slight eccentricity of the nut with respect to the feed screw, and other problems. These dimensional imperfections can result in forces being exerted on the carriage in directions other than the intended direction of movement, which can cause deflection of the carriage or other undesirable consequences. Furthermore, in all feed screw
ut arrangements, there is friction between the rotating feed screw and the nut. In most conventional feed screw
ut arrangements wherein the nut is rigidly connected to the carriage, frictional forces exerted on the nut are transmitted to the carriage, which again can cause deflection of the carriage and other problems.
Because of problems such as those mentioned above, efforts have been made toward developing feed screw
ut arrangements having self-aligning nuts which can tolerate a certain amount of misalignment between the feed screw and the carriage or other structure on which the nut is carried. For example, U.S. Pat. No. 3,977,269 discloses a self-aligning nut mechanism having an elongate tubular nut body formed of a base portion and an internally threaded portion connected thereto. The internally threaded portion has a plurality of longitudinal slots that divide it into multiple elements whose thread formations are adapted for contact with the screw threads. The base portion of the body has a pair of oppositely disposed transverse slots enabling the threaded elements to have limited movement in radial directions with respect to the screw axis. Yieldable means are carried by the threaded elements for biasing them toward one another so as to maintain an intimate engagement of the threaded formations with the screw threads. An elongate spring sleeve surrounds and is concentric with the nut body, and has three pairs of oppositely disposed transverse slots spaced apart along the length of the sleeve. The three pairs of slots are indexed 90° with respect to one another to enable pivotal movement of the adjacent sleeve parts. One extreme sleeve part carries the nut body and the other extreme sleeve part is adapted to be secured to a reciprocating part of a machine.
The self-aligning nut of the '269 patent thus purports to address the problem of misalignment between a feed screw and the reciprocating machine part, by allowing the nut to resiliently conform to the feed screw and maintain threaded engagement therebetween, and by allowing the sleeve parts to move relative to each other to compensate for slight misalignment between the screw axis and the machine part. However, the '269 patent does not purport to address the problem of forces being transmitted to the machine part in directions other than the intended direction of movement of the machine part. Even though the nut and sleeve arrangement purportedly compensates for misalignment and remains engaged with the feed screw, such misalignment would result in forces on the nut and/or on the machine part in directions other than the intended direction of movement, which forces are undesirable where highly precise movement of the machine part is required. Additionally, frictional forces between the feed screw and the nut would also result in such undesirable forces.
SUMMARY OF THE INVENTION
The aforementioned problems are overcome and other advantages are realized by a nut mechanism in accordance with the present invention. In accordance with one preferred embodiment of the invention, a nut mechanism for translating a carriage along an X-axis includes an externally threaded feed screw which is rotatable about a fixed screw axis parallel to the X-axis, and a nut having an internally threaded bore which threadingly receives the feed screw. The nut mechanism further includes a stationary runner adapted to be fixed relative to the feed screw axis, the stationary runner defining a stationary runner surface which extends parallel to the X-axis, and a runner bearing attached to the nut and projecting outwardly therefrom along a Y-axis which is perpendicular to the X-axis. The runner bearing engages the stationary runner surface to prevent rotation of the nut when the feed screw is rotated such that rotation of the feed screw causes the nut to translate along the X-axis and the runner bearing to travel along the stationary runner surface. The mechanism also includes a dummy runner bearing attached to the nut and projecting outwardly therefrom along an axis which is perpendicular to the X-axis and angularly displaced about the screw axis from the runner bearing, and a dummy runner defining a dummy runner surface which extends parallel to the X-axis and engages the dummy runner bearing. The dummy runner bearing and dummy runner are biased toward each other so as to rotatably bias the nut in a direction to maintain the runner bearing in contact with the stationary runner surface. Thus, the dummy runner bearing and dummy runner ensure continuous contact of the runner bearing with the stationary runner surface such that rotation of the feed screw tends to cause translation of the nut rather than rotation of the nut with the feed screw.
Preferably, the nut mechanism includes bearing elements which substantially prevent the nut from transmitting forces to the carriage in directions nonparallel to the X-axis along which the carriage moves. Specifically, the nut mechanism in a preferred embodiment includes a drive bearing attached to the nut and projecting outwardly therefrom, the drive bearing having an outer generally cylindrical drive surface defining an axis which is perpendicular to the X-axis; and a driven bearing which has an outer generally cylindrical driven surface defining an axis and which is adapted to be attached to the carriage such that the axis of the driven surface is perpendicular to both the axis of the drive surface and the X-axis, and such that the driven surface is engaged by the drive surface to form a crossed bearing coupling. The drive and driven bearings advantageously are freely rotatable about their axes. The crossed bearing coupling formed by the cylindrical surfaces of the drive and driven bearings perpendicularly oriented relative to each other approximates a frictionless contact between a sphere and a flat surface wherein only forces normal to the flat surface can be transmitted to the surface by the sphere. Thus, any erroneous motions of the nut which would otherwise result in forces on the carriage in directions non-parallel to the X-axis will instead result in rotation of one or both of the drive and driven bearings about their axes, and accordingly the force transmitted from the drive bearing to the driven bearing is substantially entirely in a direction parallel to the X-axis. Nonaxial forces on the carriage are thereby minimized.
In accordance with another preferred embodiment of the invention, the nut mechanism includes a slave carriage connected with the nut and adapted to engage the carria
Hsu Chi-Hua
Singh Gurpreet
Smith Stuart
Alston & Bird LLP
Bucci David A.
Joyce William C.
University of North Carolina at Charlotte
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