Note: Descriptions are shown in the official language in which they were submitted.
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ANTI-BACKLASH NUT ASSEMBLY
Background of the Invention
Many modern day machines, from data printers to
positioning devices to robots and the like, require an
element to reciprocate at relatively high speeds and with
considerable accuracy. Today's standards require such
equipment to have good positional repeatability,
reliability and long life. A typical element for
accomplishing this is an anti-backlash nut threaded onto a
lead screw that traverses back and forth when the screw
rotates.
One such element is disclosed in our U.S. Patent
4,131,031 which includes an anti-backlash nut assembly
which undergoes translational movement back and forth
along a lead screw in response to the rotation of the
screw. The nut itself is divided into two halves, both of
which have internal threads complementary to the thread on
the screw. The two portions or halves of the nut are
retained on the screw, being secured in a radial
direction, by a spacer in the form of a tube or collar. A
spring is used to apply an axial biasing force in the
longitudinal direction of the screw, forcing the nut
halves apart, to minimize or eliminate backlash.
While anti-backlash nuts made in accordance with our
above-identified patent have proven satisfactory, they are
somewhat limited because of the spring employed. The
spring biases the nut halves apart to urge their thread
flanges against opposite thread flanks of the lead screw.
w
If a load were imparted to one of the nut halves without
, the spring in place, the opposite nut half would translate
with respect to the first nut half because there is
nothing to restrain it. The anti-backlash feature of the
invention is dependent upon the axial force of the spring
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itself. Since the load is transferred through the spring,
a spring of high compressibility or stiffness causes too
great a frictional force and, thus, loss of efficiency.
Conversely, too weak a spring would not adequately reduce
backlash. Thus, the anti-backlash aspect is only as good
as the force of the spring. It is to this problem the
present invention is directed.
Another element is to be found in our U.S. Patent
4,353,264 in which we have disclosed an anti-backlash nut
assembly which also undergoes translational movement
longitudinally along a screw in response to rotational
movement of the screw. The mechanism employs a nut which
is split into two nut halves which are internally threaded
with the same thread and hand as the screw. The two nut
halves are retained in the radial direction by a coaxial
cylinder surrounding portions of the nut halves. In
addition, a spacer is mounted on the retainer means
between the nut-surfaces and is biased against at least
one of the surfaces to close any gap which may occur
between the thread of the screw and the thread of the nut.
Thus, the biasing is accomplished by employing a number of
mechanical elements.
Whenever a number of elements are used for
establishing this biasing,' they are inherently weak in
terms of axial stiffness due to mechanical and
manufacturing fit problems. The ideal anti-backlash nut
would have unlimited axial stiffness such that when a load
is applied in either direction, no discernible movement
would occur due to the elasticity of the nut itself. The
anti-backlash aspect of the nut is only as good as the
stiffness of the nut which has been created. It is the
intent of this invention to solve such a problem.
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The closest prior art may be represented by GB-A 2 1U5 816 from which
the following features are known:
An anti-bach..lash nut is designed to utxiergo bidirectiona,I translation
along a
rotational screw haying external threads, The nut has internal threads
complimentary to the screw threads. '~'t~e nut comprises: two nut halves
each having said internal threads and being movable as a unit and relative to
each other on the screw arxi a torsion spring joins the two nut halves to
induce their relative rotation on the screw in opposite directions toward a
closed abutting position to take up backlash between ttae screw threads and
the internal threads on the nut.
SUBSTITUTE SHHEET
AhII~NDED SNP
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Summarv of the Invention
The invention resides in an anti-backlash nut which
is designed to undergo bidirectional translation, i.e.
back and forth along a rotatable lead screw. The nut has
internal threads which are complementary to the thread on
the screw. The nut itself comprises two nut halves which
are movable as a unit along the rotatable lead screw and
also rotatable relative to each other on the screw.
There are means joining the nut halves to induce
their relative rotation on the screw, in opposite
directions, toward a closed position, wherein the backlash
between the threads of the screw and the threads of the
nut is taken up. A torsion spring has been used
satisfactorily to induce rotation.
In addition, there are means between the nut halves
to limit their relative rotational movement when they are
in the closed telescoping position to prevent the backlash
from recurring. A high friction elastomeric washer has
been employed between the nut halves for this purpose.
In one embodiment of the invention, one of the nut
halves includes a face plate for attaching the nut to a
load.
In another embodiment of the invention, one of the
nut halves telescopes rotatably within the other.
In yet another embodiment of the invention, each of
the nut halves has a face extending transversely of the
screw at least one of which faces is coated with a high
friction elastomeric material.
The above and other features of the invention
including various and novel details of construction and
combination of parts will now be more particularly
described with reference to the accompanying drawings and
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pointed out in the claims. It will be understood that the
particular anti-backlash nut assembly embodying the
invention is shown by way of illustration only and not as
a limitation of the invention. The principles and
features of this invention may be employed in varied and
numerous embodiments without departing from the scope of
the invention.
_Brief Description of the DrawincZs
Figure 1 is a side elevation of an anti-backlash nut
assembly on a lead screw illustrating the invention.
Figure 2 is the anti-backlash nut of Figure 1 shown
in exploded view and partly in section.
Figure 3 is a sectional detailed view on an enlarged
scale of two anti-backlash nut halves and the lead screw
of Figure 1.
Detailed Description of the Invention
Referring to Figures 1 and 2, an anti-backlash nut
embodying the invention will be described. A right hand
threaded lead screw 10 is shown mounting an anti-backlash.
nut assembly generally designated 12. The nut comprises a
first or left hand nut portion 14 and a second or right
hand nut portion 16 which are driven in bidirectional .
translation, back and forth along the screw 10 by the
rotation of the screw. The nut portions 14 and 16 are
also called nut halves. The left hand nut half 14 has a. '
face plate 18 for joining the nut to a load to be
reciprocated back and forth by the lead screw. A circular
portion 20 projects from the right hand surface of the
face plate 18. A second cylindrical member 22 (Figure 2)
projects from the circular member 20 and terminates in a
circular or ring-shaped flat face 24 which is formed at
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right angles to the axis a of the lead screw 10. A
circular bore 26 is formed in the face 24 of the
cylindrical member 22 and terminating at a wall 27 (Figure
2) .
The center of the face plate 18, the cylindrical
member 20 and a portion of the cylindrical member 22 are
formed with internal screw threads 29 along the axis a.
They are the same size and hand as the threads on the lead
screw 10. A small bore 30 is formed in the circular
portion 20.
The right nut half 16 will now be described. It
includes a cylindrical portion 40 and a second smaller
cylindrical portion 42. The interior of the nut half 16
is also provided with threads 31 of the same size and hand
as the threads of the lead screw 10. The cylindrical
portion 40 has a flat face 44 formed normal to the axis a
of the lead screw and the threads 31. Face 44 and face 24
are parallel and may be engagable when the nut halves are
assembled as will be explained hereinafter. Formed in the
cylindrical portion 48 is a second small bore 48.
Anti-friction means in the form of a high friction, ,
elastomeric washer 50 is provided. It has a circular bore
52 just slightly larger than the diameter of the
cylindrical portion 42 of the nut half 16 such that the
elastomeric washer may be slid over the cylindrical
portion 42 and abut. the face 44 when the nut halves are
~.. assembled. It will also then abut the face or circular
ring 24. A torsion spring 60 having tangs 62 and 64 is
provided to induce relative rotation (in opposite
directions) of the nut halves on the screw when they are
assembled as shown in Figure 1.
Each of the nut halves 14, 16 are threaded onto the
lead screw 10 as shown in Figure 1 with the cylindrical
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portion 42 of the right nut half telescoping into the
cylindrical opening 26 in the left nut half. The torsion
spring 60 is given a twist to tighten it slightly. The
tang 62 is inserted into the bore 60 and the tang 64 is
inserted into the bore 48. In the assembled position
shown in Figure 2, the elastomeric washer 52 is located
between the face 24 on the nut half 14 and the face 46 on
the nut half 16 which parts are in closed abutting
position against the washer 50 due to the torsion spring
rotating the nut halves in opposite directions.
Another embodiment of the invention is to coat the
faces 24, 44 of the nut halves with high-friction
elastomeric material, as for example, by spraying. Thus,
the high-friction elastomeric washer 50 could be replaced.
Wherein the torsion inducing means is shown as a
conventional torsion spring 60 with tangs 62 and 64, any
equivalent means could be employed to cause opposite
rotation of the nut halves, as for example, a pre-stressed
elastomeric sleeve or elastomeric binders secured to the
nut halves on a bias.
Referring to Figure 3, when the torsion spring is in
the assembled position shown in Figure 1, the left hand
nut half 14 is induced to rotate in the direction of the
downward pointing arrow on the nut half. The nut half 16
is induced to rotate in the direction of the upward
pointing arrow on the nut half in Figure 3. The nut
halves rotate in opposite directions. Gaps between the
flanks of the lead screw l0 and the threads of the nut
half 14 is thus taken up with engagement taking place at
a-a and b-b. Conversely, nut half 16 being induced to
rotate in the opposite direction results in engagement of
the thread flanks at c-c and d-d, the nut halves moving in
the direction of the arrows a and f, in other words,
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toward each other. This continues while the face 24 of
nut half 14 moves towards the face 44 of nut half 16, The
nut halves then come to rest against the opposite surfaces
of the high-friction elastomeric washer 50. If the washer
is not used and the sprayed anti-friction material is, the
faces 24 and 44 will abut each other. This is called the
locked position whereupon no further relative rotation of
the nut halves 14, 16 takes place. They are jammed
against the thread flanks at a-a, b-b, c-c and d-d. No
further rotation takes place and, more importantly, no
reverse rotation takes place because of the high
frictional contact between the washer 50 and the faces 24,
44 or the coated faces themselves. Rotation of the lead
screw 10 in the direction of the upward facing arrow (i.e.
clockwise when viewed from the right in Figure 3) will
cause the nut halves to move toward the right as a unit
carrying whatever load is attached to the face plate 18 on
the nut half 14.
Upon the end of the traverse of the lead screw 10 to
the~right, with the nut half 14 pushing the nut half 16 to
the right as viewed in Figure 3, the length of the lead
screw, the direction of the lead screw is reversed. At
that time, the thread flanks are still in engagement at
c-c, d-d and there will be no lost movement because of
backlash since these surfaces are already in intimate
contact. The nut halves being biased in such a manner
creates a wedging effect and locking of the two nut
s stem
. ' If wedging did not occur, the nuts would act
independently of each other and backlash, due to gaps
between the thread flanks, would take place. The load
applied to the left nut half 14 or mounting nut would
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merely cause the right nut half 16 to rotate with respect
to the left.
However, the utilization of the high-friction
elastomeric washer 50 or the coated faces 24, 44 in
conjunction with the torsion spring's ability to rotate
one nut half with respect to the other to maintain
intimate contact between the two nut halves with the
elastomeric friction washer and with the engaged thread
flanks, the backlash is eliminated.
Consequently, any axial force applied to the left nut
half 14 passes through the friction washer 50 before
reaching the right nut half 16. The friction restricts
the right nut half's ability to rotate relative to the
left nut.
As wear occurs at the thread nut halves and screw
flank's contact areas a-a, b-b, c-c, d-d, because the
torsion spring 60 is acting on the nut halves constantly
to urge them against the flanks of the screw thread, gaps
do not appear at the contact areas. However, the force
applied to the elastomeric washer 50 or, in the
alternative, to the coated surfaces 24, 44 tends to lessen
permitting the torsion spring 60 to continue to rotate the
nut halves together rather than to allow a gap to occur at'
the washer 50 or at the coated surfaces 24,.44.
Consequently, the system is not dependent upon force of a
compression spring, as in.our earlier patent, U.S.
4,131,031. Wear is compensated for and a relatively low
lock force is required to maintain the system's stiffness.
Through its-simplicity, lack of numerous parts, and
, manner of functioning, the problems of our U.S. Patent
4,353,264 are obviated.
