U.S. patent number RE32,433 [Application Number 06/828,602] was granted by the patent office on 1987-06-09 for anti-backlash nut having longitudinal flexural members with ramps thereon and means to apply an axial pre-load force to said ramps.
This patent grant is currently assigned to Kerk Motion Products, Inc.. Invention is credited to Keith W. Erikson, Kenneth W. Erikson.
United States Patent |
RE32,433 |
Erikson , et al. |
June 9, 1987 |
Anti-backlash nut having longitudinal flexural members with ramps
thereon and means to apply an axial pre-load force to said
ramps
Abstract
An anti-backlash nut having one or more longitudinal flexural
members with one end fixed and one end free-floating is disclosed
which undergoes translational movement along a shaft. This
anti-backlash nut is improved by providing ramps at the
free-floating ends of the flexural members and means to apply axial
forces on the ramps to thereby create radial force vectors which
maintain the ramps in contact with the shaft even after the nut has
become worn.
Inventors: |
Erikson; Kenneth W. (Merrimack,
NH), Erikson; Keith W. (Milford, NH) |
Assignee: |
Kerk Motion Products, Inc.
(Hollis, NH)
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Family
ID: |
27125208 |
Appl.
No.: |
06/828,602 |
Filed: |
February 11, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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700346 |
Feb 13, 1985 |
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465453 |
Feb 10, 1983 |
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Reissue of: |
925269 |
Jul 17, 1978 |
04249426 |
Feb 10, 1981 |
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Current U.S.
Class: |
74/441; 411/262;
411/278; 411/280; 74/89.42 |
Current CPC
Class: |
F16B
39/36 (20130101); F16H 25/2009 (20130101); Y10T
74/18728 (20150115); Y10T 74/19902 (20150115) |
Current International
Class: |
F16B
39/36 (20060101); F16B 39/00 (20060101); F16H
25/20 (20060101); F16H 055/18 (); F16B
039/36 () |
Field of
Search: |
;74/441,424.8R,424.8A,409,89.15 ;411/262,278,280,437,433
;403/290,334,371 ;285/383,389,421 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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912009 |
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Jul 1946 |
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FR |
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1186514 |
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Aug 1959 |
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FR |
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1210274 |
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Mar 1960 |
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FR |
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1332745 |
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Jun 1963 |
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FR |
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594031 |
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Oct 1947 |
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GB |
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Other References
C T. Bower, Product Engineering, vol. 23, Sep., 1952, pp.
154-155..
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Primary Examiner: Herrmann; Allan D.
Assistant Examiner: Belisario; Martin G.
Attorney, Agent or Firm: Hamilton, Brook, Smith &
Reynolds
Parent Case Text
.Iadd.This application is a continuation of Ser. No. 700,346 filed
Feb. 13, 1985 which is a continuation of Ser. No. 465,453 filed
Feb. 10, 1983 which is a reissue of Ser. No. 925,269 filed July 17,
1978, U.S. Pat. No. 4,249,426. .Iaddend.
Claims
What is claimed is:
1. In an anti-backlash nut designed to move along a screw in either
of two longitudinal directions, said nut .[.including.].
.Iadd.comprising a nut body with .Iaddend.one or more longitudinal
flexure members having one end fixed .Iadd.to said nut body
.Iaddend.and one end free-floating:
the improvement comprising providing .[.upwardly.]. inclined ramps
on the exterior of the free-floating ends of said longitudinal
flexure members .Iadd.said ramps extending radially outwardly in a
direction away from the fixed end .Iaddend.and means for applying
an axial force to each of said ramps, said axial force being
substantially constant in either direction of motion of said
nut.
2. An anti-backlash nut designed to undergo bidirectional
translational movement along a screw having an external thread
thereon in response to relative rotational movement between the
anti-backlash nut and screw, said nut having an internal thread
complementary to the external thread on said screw and said nut
comprising, in combination:
a. one or more longitudinal flexure members having one end fixed
.Iadd.to said nut .Iaddend.and one end free-floating, said
longitudinal flexure members having .[.an upwardly.]. inclined
.[.ramp.]. .Iadd.ramps .Iaddend.on their outer surfaces.Iadd., said
ramps extending radially outwardly in a direction away from the
fixed end.Iaddend., and,
b. means for directly applying an axial force to the .[.ramp.].
.Iadd.ramps .Iaddend.on each of said flexure members which axial
force is converted by the angle of the .[.ramp.]. .Iadd.ramps
.Iaddend.to a radial force which urges the flexure members inwardly
whereby the internal thread of said flexure members is maintained
in close contact with the external thread of said screw regardless
of which direction the nut travels on the thread.
3. An anti-backlash nut of claim 2 wherein said nut has a
circumferentially continuous portion at one end thereof.
4. An anti-backlash nut of claim 3 wherein said means for applying
an axial force comprises an axial compression spring .Iadd.abutting
a hollow member coaxially disposed near the base of the inclined
ramps for movement along the ramps as the flexure members are
flexed radially inwardly upon wear of screw or nut
threads.Iaddend..
5. An anti-backlash nut of claim 4 wherein said anti-backlash nut
has a plurality of longitudinal flexure members with a ramp at
their free-floating ends.
6. An anti-backlash nut of claim 5 wherein each of said plurality
of longitudinal flexure members is fixed at the same end of said
anti-backlash nut.
7. An anti-backlash nut of claim 5 wherein said anti-backlash nut
has a circumferentially continuous portion at both ends
thereof.
8. An anti-backlash nut of claim 7 wherein each of said
longitudinal flexure members is fixed at the same end of said
anti-backlash nut.
9. An anti-backlash nut of claim 7 wherein said longitudinal
flexure members are alternately fixed at opposite ends of said
nut.
10. An anti-backlash nut for translation along a shaft, comprising,
in combination:
a. one or more longitudinal flexure members having one end fixed
.Iadd.to said nut .Iaddend.and one end free-floating, said
longitudinal flexure members having an .[.upwardly.]. inclined ramp
on their outer surfaces.Iadd., said ramp extending radially
outwardly in a direction away from the fixed end.Iaddend.;
b. spring means coaxial to said flexure members for applying an
axial force to the ramp on each of said flexure members;
c. an internal structure on said flexure members complementary to
the external structure of said shaft; and,
d. said ramp having more than one angle of inclination whereby the
axial force applied to said ramp may be varied as the internal
structure of the flexure members engages with the external
structure of said shaft. .Iadd.
11. The nut of claim 1 wherein the means for applying an axial
force comprises a spring member mounted coaxial to said
longitudinal flexure members between said nut body and a hollow
member coaxially mounted about said flexure members near the base
of said ramps. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is in the field of anti-backlash nuts designed to
provide uniform translational movement in either direction along a
threaded screw in response to relative rotational movement between
the nut and screw.
2. Description of the Prior Art
In many applications, it is important to drive an element of a
machine along a screw with accurate positional repeatability and
constant drag torque in the forward and reverse direction. Data
printers and x-y tables, used as peripheral equipment in the
computer industry, for example, have such requirements.
Positioning devices designed to meet these requirements have been
proposed, and many of these employ an anti-backlash nut to achieve
the positional accuracy along the screw which is required. Examples
of two such anti-backlash nut assemblies which have been proposed
are described in the patent literature as follows.
In U.S. Pat. No. 3,656,358, issued to Kopp, a linear positioning
device is disclosed which is stated to have an improved collar for
use with a comparatively inexpensive rod having multiple grooves.
The collar is telescoped over and adapted to be translated back and
forth relative to the elongated rod. This collar includes
cantilevered fingers which are resiliently wedged into angularly
spaced grooves formed in the rod to preload the collar onto the rod
and prevent rotational play from developing between the two. In a
specific embodiment, the collar is telescoped onto a rod in the
form of a splined shaft while in another embodiment, the collar is
a nut threaded onto a screw with multiple threads.
In U.S. Pat. No. 3,997,269, issued to Linley, an anti-backlash,
self-aligning nut construction with specially constructed tubular
nut bodies which coact with concentric spring sleeves is described.
The nut bodies, in general, each have a pair of spring-biased
elements provided with internal thread formations adapted for
engagement with the external threads of a screw. In one embodiment,
a self-aligning spring sleeve is provided having solely three pairs
of oppositely-disposed transverse slots to obtain the desired
aligning features. The nut body has a base portion which is
separated from the spring-biased elements by means of two
transverse slots which, together with an adjacent pair of slots in
the spring sleeve, form in effect a universal joint. One of the
remaining slot pairs in the sleeve is oriented circumferentially
with respect to the first pair by an angle of 90.degree., with a
third pair of slots being circumferentially aligned with the first
pair.
More recently, an anti-backlash nut having oppositely-directed
longitudinal flexure members has been disclosed in U.S. application
Ser. No. 907,518 filed May 19, 1978.Iadd., now U.S. Pat. No.
4,210,033.Iaddend.. This anti-backlash nut has a continuous portion
extending longitudinally from one end of the screw to the other. In
addition, there are at least two, and usually more,
oppositely-directed longitudinal flexure members which have one end
fixed to the anti-backlash nut and one end free-floating. The
oppositely-directed longitudinal flexure members are biased towards
the screw by one or more radial springs or other means for
biasing.
While the anti-backlash nut described in U.S. application Ser. No.
907,518 .Iadd.now U.S. Pat. No. 4,210,033 .Iaddend.has proven to be
very successful, it sometimes suffers a problem after the threads
on the nut have become worn. This problem relates to the fact that
when an axial load is applied to this nut there is an opposite
force of reaction directed towards the inclined surface of the nut
thread. Since this surface of the nut thread is inclined, the force
of reaction is broken into an axial or longitudinal force vector
and a normal force vector. The normal force vector tends to force
the nut away from the screw, and in many cases can overcome the
normal force towards the screw which is created by the radial
spring.
SUMMARY OF THE INVENTION
This invention relates to an anti-backlash nut which is designed to
undergo translational movement along a shaft, such as an externally
threaded screw or a spline. The nut has a bore with shape which is
complementary to the external shape of the shaft. In addition,
there is at least one, and usually more, longitudinal flexure
members which have one end fixed to the nut and one end
free-floating along the shaft. In one embodiment, each of a
plurality of longitudinal flexure members is fixed at the same end
of the anti-backlash nut so that they extend in the same direction.
In another embodiment, each of a plurality of longitudinal flexure
members is alternately fixed at opposite ends of the nut so that
they extend in opposite directions.
The improvement of this invention comprises the provision of ramps
on the free-floating ends of the flexural members together with
axial biasing means for providing an axial preload force on these
ramps. This can be conveniently done by mounting an axial
compression spring around the nut with a hollow ring mounted at the
base of the ramps. The axial compression spring forces the hollow
ring as far up the ramps as is possible thereby producing radial
force vectors which insure that the longitudinal flexure members
maintain good contact with the shaft even after the internal
structure of the bore of the nut becomes partially worn.
Thus, the anti-backlash nut of this invention retains the
advantages of nut described in copending application Ser. No.
907,518 .Iadd.now U.S. Pat. No. 4,210,033 .Iaddend.and, in
addition, overcomes normal force vectors created when an axial load
is applied.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating an anti-backlash nut and
screw assembly wherein the nut has longitudinal flexural members of
the type heretofore described in the prior art;
FIG. 2 is a side elevational view of the anti-backlash nut
illustrated in FIG. 1;
FIG. 3 is a cross-sectional view of the anti-backlash nut of FIG.
1;
FIG. 4 is a partial diagrammatic view of an anti-backlash nut of
FIGS. 1-3 showing an enlarged view of the internal thread and
illustrating the force vectors created by the force of reaction to
an axial load force;
FIG. 5 is a perspective view of one embodiment of an anti-backlash
nut according to this invention and having two longitudinal flexure
members fixed at the same end of the nut and having ramps at their
free-floating ends;
FIG. 6 is a cross-sectional view taken along section line 6--6 in
FIG. 5;
FIG. 7 is a cross-sectional view taken along sectional line 7--7 in
FIG. 6;
FIG. 8 is a partial diagrammatic view of the anti-backlash nut of
FIGS. 5-7 illustrating the radial force vectors created by the
axial pre-load provided by a compression spring bearing on the
ramps of the longitudinal flexural members;
FIG. 9 is a perspective view illustrating an alternative embodiment
of an anti-backlash nut according to this invention in which the
nut has a continuous circumferential portion at one end only and
three longitudinal flexure members extending therefrom with ramped
portions at their free-floating ends;
FIG. 10 is a longitudinal cross-sectional view of the anti-backlash
nut of FIG. 9 taken along sectional line 10--10;
FIG. 11 is an end view of the anti-backlash nut of FIGS. 9 and 10
taken along site line 11--11 in FIG. 10; and,
FIGS. 12 and 13 are side elevation views of still further
embodiments of ramped flexural members according to this
invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
This invention can be further described by referring to the Figures
in more detail.
FIGS. 1-3 illustrate an anti-backlash nut assembly as described in
copending application Ser. No. 816,983 .Iadd.now
abandoned.Iaddend.. Thus, it can be seen in FIG. 1 that this
assembly has an externally threaded screw 10 having thereon an
anti-backlash nut 12. Anti-backlash nut 12 has a faceplate 14
integrally attached to it and containing three equally spaced
attachment holes so that an element to be driven can be easily
attached to nut 12 by bolting it thereto. As relative rotational
movement occurs between screw 10 and nut 12, nut 12 undergoes
translational movement along screw 10 and thus drives the element
fastened to it in a longitudinal direction. If the relative
rotational movement is then reversed, such as by rotating screw 10
in the opposite direction, nut 12 undergoes translational movement
in the reverse longitudinal direction. As stated previously, it is
quite often important to achieve highly accurate positioning of nut
12 in both longitudinal directions in response to the same degree
of rotation of screw 10.
Some of the design features of anti-backlash nut 12 can be more
clearly seen in FIGS. 2 and 3. Therein, it can be seen that
continuous portion 16 extends from one end to the other of nut 12
in the longitudinal direction. The extent to which continuous
portion 16 extends circumferentially will vary depending upon the
exact application for anti-backlash nut 12, but is chosen to
provide sufficient flexural stiffness while still allowing for
relief of screw thread inconsistencies without significant
binding.
Anti-backlash nut 12 has three oppositely-directed longitudinal
flexure members 18, 20 and 22. Flexure members 18 and 22 are each
fixed at one end of anti-backlash nut 12, that being the end to
which faceplate 14 is attached. Flexure member 20, on the other
hand, is fixed at the opposite end of nut 12. Each of flexure
members 18, 20 and 22 extend longitudinally for a substantial
length and are free-floating at their unfixed ends. They could, of
course, be shorter or longer than illustrated.
Radial springs 24 and 26 are positioned in slots 28 and 30,
respectively, extending around the periphery of anti-backlash nut
12. The purpose of radial springs 24 and 26 is to bias flexure
members 18, 20 and 22 towards screw 10 so that good contact is
always maintained between the external thread on screw 10 and the
complementary internal threads on flexure members 18, 20 and 22 and
continuous portion 16.
While anti-backlash nut 12 illustrated in FIGS. 1-3 has proven
highly successful, it does suffer some problems which can occur
under certain conditions. For example, it can be appreciated that
radial springs 24 and 26 contribute directly to the adverse
condition of frictional drag. Because of this, it is necessary to
maintain this radial preload created to a minimum value by
employing radial springs 24 and 26 which have relatively low spring
constants.
FIG. 4 is a partial diagrammatic view of longitudinal flexure
member 20 and illustrates one problem which can occur under high
axial loading of nut 12. In response to an axial load applied,
which may be due to inertial affects or system drag, there is a
force of reaction which bears on the thread form of element 20.
Since this force of reaction bears on an inclined surface of the
thread, it is split into a longitudinal force vector and a normal
force vector. Unfortunately, the normal force vector acts to oppose
the radial force vector applied by radial spring 24 or 26. In cases
where the axial load is high relative to the radial force applied
by springs 24 and 26, the normal force vector of the reaction force
can entirely overcome the radial force of spring 24 or 26. When
this condition occurs, flexure member 20 is forced away from the
screw thread and the intimate contact necessary for proper
operation of the anti-backlash nut 12 is lost.
It should also be noted that as the thread on flexure element 20
wears, the intimate contact will also be lost.
FIGS. 5-7 illustrate one embodiment of an improved anti-backlash
nut 50 which overcomes the aforementioned problems. Anti-backlash
nut 50 has a faceplate 52 which contains equally spaced attachment
holes so that an element to be driven along a screw can be attached
to nut 50, such as by bolting it thereto. Anti-backlash nut 50 has
a circumferentially continuous portion 54 at its faceplate end and
a circumferentially continuous portion 56 at its opposite end.
Additionally, it contains two longitudinal continuous portions 58
and 60 which are located directly across from each other on
opposite sides of the screw which extends through the bore 62 of
anti-backlash nut 50.
Anti-backlash nut 50 has two longitudinal flexure members 64 and
66, each of which is fixed at the faceplate end of nut 50 and
free-floating at the opposite end. Longitudinal flexure members 64
and 66 have ramps 68 and 70, respectively, positioned at the outer
surface of their free-cloating ends. Ramps 68 and 70 are shown to
have an incline of about 20.degree. degrees, but can have a wide
range of slopes depending upon the length of the flexure members,
the particular application involved, etc.
Axial spring 72 is mounted around anti-backlash nut 50 and held in
compression between faceplate 52 and ring member 74 which is
located at the base of ramps 68 and 70. As can be appreciated, ring
74 will slide up ramps 68 and 70 as far as possible under the axial
force supplied by spring 72.
FIG. 8 illustrates the force vectors created by the combination of
ramps 68 and 70, axial spring 72, and ring 74. Because axial spring
72 is held in compression, it creates an axial pre-load which bears
longitudinally on ring 74. This force is transmitted by ring 74 to
the inclined surfaces of ramps 68 and 70, which convert part of the
axial pre-load force into a radial force vector which opposes the
normal reaction-force vector tending to force the longitudinal
flexure members 64 and 66 away from the thread of the screw. It
should also be noted that, as the internal thread of anti-backlash
nut 50 wears, longitudinal flexure members 64 and 66 are maintained
in intimate contact with the thread of the screw because of the
radial force vectors created by the combination of names with the
axial compression spring.
FIGS. 9-11 illustrate an alternative embodiment of an anti-backlash
nut 80 according to this invention. In these figures, elements
which are the same as elements in FIGS. 5-7 have been given same
numerals.
Anti-backlash nut 80 has only one continuous circumferential
portion 54, which is located at its face-plate end. At the opposite
end, there is no circumferentially continuous portion.
Three longitudinal flexure members 82, 84 and 86 are fixed to nut
80 at the faceplate end, but are free-floating at their opposite
ends. Longitudinal flexure members 82, 84 and 86 have ramps 88, 90
and 92, respectively, at the outside surface of their free-floating
ends. Additionally, each of ramps 88, 90 and 92 terminate in
circumferentially planar surfaces 94, 96 and 98, respectively, each
of which have a larger circumferential diameter than the rest of
anti-backlash nut 80.
Anti-backlash nut 80 operates in a similar manner to anti-backlash
nut 50 previously described, except for the difference in the
longitudinal flexure members. The axial compression spring 72 still
applies an axial load to ring 74 which slides up ramps 88, 90 and
92 as far as it can. The axial pre-load force is then broken down
by ramps 88, 90 and 92 into its various vectors, one of which is a
radial force vector which maintains each of longitudinal flexure
members 82, 84 and 86 in intimate contact with the screw extending
through the bore of anti-backlash nut 80.
It will be recognized, of course, that the anti-backlash nut of
this invention can have any number of longitudinal flexural
elements. In addition, these elements can be fixed at the same end
of the anti-backlash nut or fixed in an alternate fashion, or
otherwise, at opposite ends thereof.
It should also be recognized that the term "ramps" is used herein
in its most generic sense. It is not limited to a surface having a
linear slope or curve. In fact, it is believed that ramps with any
linear or higher order curve, such as a paralolic curve, or any
combination of these, may be suitable. Examples of other
embodiments or ramps having different surfaces thereon are shown in
FIGS. 12 and 13.
In FIG. 12, flexural member 100 is illustrated with external thread
102 on the inner wall of its bore. Ramp 104 has a curved surface
106 thereon. This can be an advantage in some cases since it tends
to apply less force in the radial direction when the anti-backlash
nut is new, and more force in the radial direction as the
anti-backlash nut wears during its life.
In FIG. 13, flexural element 110 has a spline-matching surface 112
thereon. Ramp 114 has an operative surface formed from three linear
surfaces, 116, 118, and 120, which have decreasing slopes. Thus,
more radial force is applied by surface 118 than surface 116, and
still further radial force is applied by surface 120.
The anti-backlash nuts described herein can be fabricated to form a
wide variety of materials including many metals and polymeric
compositions. Preferred materials are polymeric compositions which
have low friction and are self-lubricating. An example of such a
polymeric composition is self-lubricating polyacetal.
Those skilled in the art will recognize, or be able to ascertain
using no more than routine experimentation, many equivalents to the
specific components, elements, steps, materials, etc., described
herein. Such equivalents are intended to be covered by the
following appended claims.
* * * * *