Machine element or mechanism – Gearing – Rotary bodies
Reexamination Certificate
1999-10-04
2001-11-06
Bucci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Rotary bodies
C074S089420
Reexamination Certificate
active
06311576
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an anti-backlash nut for engaging a lead screw.
2. Description of the Relevant Art
The use of lead screw mechanisms for providing linear motion is well known. A typical lead screw mechanism has a rotating lead screw and a non-rotating traveler that is coupled to the lead screw with a nut. The traveler is typically guided or constrained so that rotation of the lead screw results in linear motion of the traveler.
Fairly high precision positioning can be accomplished with a lead screw mechanism by accurately controlling the rotation of the lead screw. An essential component for precision positioning is an anti-backlash nut or other coupling between the lead screw shaft and the traveler, which reduces or, ideally, eliminates positioning hysteresis resulting from bidirectional rotation of the lead screw.
There are tradeoffs in the design of anti-backlash nuts in that a greater precision (less backlash) may be achieved by reducing the clearance and increasing the contact area and/or forces between the lead screw and the anti-backlash nut, but at the expense of higher frictional forces and wear. It is desirable to have the nut apply a uniform pressure to the lead screw so that there is the same torque load on the lead screw regardless of the direction of travel. It is desirable to have a torsionally stiff nut that avoids or minimizes any wind-up or torsional distortion that could cause positioning inaccuracies. It is desirable to be able to adjust the pressure applied by the nut to optimize it for a particular application. It is also desirable to provide some float or compliance in the nut to avoid binding or high torque, but at the same time not compromising positioning accuracy.
SUMMARY OF THE INVENTION
In accordance with the illustrated preferred embodiment, the present invention is an anti-backlash nut that includes a body having an axially-extending bore and internal threads that engage a lead screw inserted into the bore. The internal threads are divided circumferentially into at least two, and preferably three, inner threaded segments each independently movable in a radial direction. The anti-backlash nut also includes springs that bias the inner threaded segments radially inward to contact the lead screw. The anti-backlash nut provides wear compensation as well as anti-backlash to improve positioning repeatability when used with an open loop positioner.
In a preferred embodiment, the body has an outer shell, and each of the three inner threaded segments is compliantly attached to the outer shell by two axially-extending webs that permit radial movement of the inner threaded segment by bending of the webs. An axially-extending pocket is defined between an outside surface of an inner threaded segment, its two webs, and an inside surface of the outer shell. One of the springs is mounted in each pocket. The springs are preferably fabricated from flat pieces of stainless steel that are bent so that a central portion contacts the outside surface of the inner threaded segment (pushing it radially inward) and two ends contact the inside surface of the shell.
Also preferably, the body and its outer shell, inner threaded segments, and webs are a monolithic structure, composed of a low-friction, self-lubricating material, such as DSM Delrin AF Blend or Turcite. The outer shell preferably further includes a tubular portion spaced axially from the inner threaded segments, where the tubular portion has an inner diameter slightly greater than an outer diameter of the lead screw. The tubular portion has external threads that engage an internally-threaded mounting plate for coupling the nut to a traveler.
The inner threaded segments are independently movable in a radial direction, which provides some degree of radial compliance to avoid binding between the nut and the lead screw. By varying the shape and/or thickness of the springs, the spring force can be varied to adjust the contact force between the inner threaded segments and the lead screw. The radial compliance provides wear compensation and accommodates lateral run-out of the lead screw.
The features and advantages described in the specification are not all inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter.
REFERENCES:
patent: 3004445 (1961-10-01), Monodon
patent: 3170337 (1965-02-01), Linley, Jr.
patent: 3611818 (1971-10-01), Burns et al.
patent: 4041793 (1977-08-01), Repay et al.
patent: 4210033 (1980-07-01), Erikson et al.
patent: 4274294 (1981-06-01), Siuryj et al.
patent: 5303606 (1994-04-01), Kokinda
patent: 5937702 (1999-08-01), Erikson et al.
patent: 1-153861-A (1989-06-01), None
Bucci David A.
Gray Cary Ware & Freidenrich LLP.
Joyce William C.
Techmetric Inc.
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