Nut assembly with rotatable sleeve for taking up backlash

A nut assembly comprises two end-to-end nuts threaded onto a screw and coupled by splines which prevent relative rotation between the nuts while permitting relative axial movement. A sleeve surrounds the nuts and is threaded onto one of the nuts. The other nut is formed with ratchet teeth adapted to fit into angularly spaced notches in the sleeve. When the sleeve is rotated relative to the nuts, it forces the nuts away from one another to take up backlash between the nuts and the screw. During such rotation, the teeth ratchet into and out of successive notches and ultimately prevent relative rotation between the sleeve and the nuts during normal service use.

BACKGROUND OF THE INVENTION 
This invention relates generally to a linear actuator in the form of a 
threaded screw and a nut assembly adapted to move along the screw when the 
screw is rotated. A device of this general type is disclosed 
More specifically, the invention pertains to a linear actuator in which the 
nut assembly is defined by two end-to-end nuts. One of the nuts carries 
essentially all of the load when the screw is rotated in one direction 
while the other nut carries substantially the entire load when the screw 
is rotated in the opposite direction. In a nut assembly of this type, it 
is desirable to preload the nuts in opposite directions along the screw in 
order to take up backlash between the threads of the screw and the threads 
of the nuts and thereby enable precise positioning of the component 
carried by the nut assembly. Various arrangements for preloading the nuts 
are known but, for the most part such arrangements are complex and 
relatively expensive and do not lend themselves to quick and easy 
adjustment in the field. 
SUMMARY OF THE INVENTION 
The general aim of the present invention is to provide a new and improved 
nut assembly capable of having its preload adjusted in a rapid and simple 
manner, capable of being adjusted in comparatively fine increments and 
capable of being manufactured at relatively low cost. 
A more detailed object of the invention is to achieve the foregoing by 
providing a nut assembly in which a sleeve is telescoped over the two nuts 
in a novel manner and may be turned manually to adjust the preload on the 
nuts and to take up backlash when the threads of the screw and nuts wear. 
In even a more specific sense, the invention resides in a nut assembly in 
which the sleeve is threadably connected to one of the nuts, is supported 
to ratchet relative to the other nut, and serves to force the nuts apart 
when rotated in one direction relative to the nuts. 
These and other objects and advantages of the invention will become more 
apparent from the following detailed description when taken in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
For purposes of illustration, the invention has been shown in the drawings 
as incorporated in a nut assembly 10 for use with an elongated screw 11. 
When the screw is rotated relative to the nut assembly, the latter travels 
axially relative to the screw. The screw may be of the same general type 
as disclosed in Kopp U.S. Pat. No. 3,656,358 and has multiple threads 12 
(herein, five threads) extending along its length and all formed on the 
same helix angle. 
The nut assembly 10 comprises first and second nuts 14 and 15 disposed in 
end-to-end relation and formed with internal threads 16 and 17, 
respectively, which mate with the threads 12 of the screw 11. In this 
instance, the nuts are molded from a relatively hard plastic such as 
Delrin. A generally triangular mounting flange 18 is molded integrally 
with and projects radially from the outboard end of the nut 14 and is 
formed with three angularly spaced holes 19 which are adapted to receive 
fasteners (not shown) for connecting the nut assembly to a movable element 
such as a component of a business machine. 
Normally, the nuts 14 and 15 are held against rotation relative to one 
another and are held in a rotationally stationary position in space. The 
screw 11 normally is held in an axially fixed position. When the screw is 
rotated first in one direction and then the other, the nut assembly 10 
traverses back and forth along the screw. The threads 16 and 17 of the 
nuts are pressed against the threads 12 of the screw in such a manner that 
one of the nuts carries essentially all of the load when the screw is 
rotated in one direction while the other nut carries essentially all of 
the load when the screw is rotated in the opposite direction. 
In order for the nut assembly 10 to function effectively, it is necessary 
to maintain tight and backlash-free engagement between the threads 16 and 
17 of the nuts 14 and 15 and the threads 12 of the screw 11. As the 
threads wear, it is necessary to adjust the nut assembly on the screw in 
order to take up the clearance created by such wear. 
In accordance with the present invention, the nuts 14 and 15 are uniquely 
constructed and the assembly 10 is equipped with a novel and extremely 
simple sleeve 20 which may be quickly and easily adjusted to take up 
clearance between the threads 16 and 17 of the nuts and the threads 12 of 
the screw 11. Such adjustment may be made without disassembling any parts 
of the nut assembly, without removing the screw from the nut assembly and 
without disconnecting the nut assembly from the component adapted to be 
driven by the nut assembly. As a result, less time and effort are required 
to adjust the nut assembly than is the case with prior nut assemblies of 
the same general type. 
More specifically, the nut assembly 10 includes means for coupling the nuts 
14 and 15 against relative rotation while permitting the nuts to move 
axially relative to one another. Herein, these means comprise four 
angularly spaced fingers 21 (FIGS. 1 and 3) formed integrally with and 
projecting axially from the inboard end of the nut 14. The fingers are 
received with a snug telescopic fit in four angularly spaced grooves 22 
(FIG. 3) formed in the inboard end portion of the nut 15. The grooves 22 
open radially out of the periphery of the nut 15 and also open axially out 
of the inboard end of the nut 15. In effect, the fingers 21 and grooves 22 
define a set of splines which keep the nut 14 from rotating relative to 
the nut 15 while permitting the nut 14 to move axially toward and away 
from the nut 15. 
The adjusting sleeve 20 also is molded of Delrin and is telescoped with the 
nuts 14 and 15 in bridging relation with the gap between the nuts. 
Pursuant to the invention, one end portion of the sleeve is formed with an 
internal thread 25 (FIG. 1) which is screwed onto an external thread 26 
formed on the outer periphery of the inboard end portion of the nut 14. In 
this particular instance, each thread is 5/8 inches in diameter and 
includes eighteen threads per inch. If the nut 14 is held stationary and 
the sleeve 20 is rotated, the sleeve moves axially relative to the nuts 14 
and 15 by virtue of the coaction between the threads 25 and 26. 
Further in keeping with the invention, the nut 15 and the sleeve 20 are 
formed with detent means which coact with one another to normally prevent 
relative rotation between the nut 15 and the sleeve 20 but which release 
and permit such relative rotation when the nut 14 is held stationary and 
the sleeve is rotated to take up backlash. In the present instance, the 
detent means comprise several (e.g., twenty-four) angularly spaced keyways 
or notches 30 (FIGS. 2 and 4) formed in the inner side of an enlarged 
diameter collar 31 molded integrally with and projecting axially from the 
right end 32 of the sleeve. The notches 30 are spaced equally around the 
inner peripheral wall of the collar 31 and open both in a radially inward 
direction and in an axial direction. Each notch is generally arcuate in 
shape. 
The present detent means further include three teeth 35 (FIGS. 2 and 4) 
spaced angularly around the outer periphery of a flange 36 molded 
integrally with and projecting radially outwardly from the outboard end of 
the nut 15. The flange 36 is received in the collar 31 and its inboard 
side defines an axially facing shoulder 37 (FIG. 1) which abuts the 
extreme right end 32 of the sleeve 20. 
As shown in FIG. 4, each tooth 35 is arcuate and corresponds generally in 
shape to the shape of the notches 30. The teeth are received in the 
notches with a snug but slidable fit and permit the sleeve 20 to move 
axially relative to the nut 15 until the sleeve end 32 abuts the shoulder 
37. The coacting notches and teeth, however, normally lock the nut 15 and 
the sleeve 20 against relative rotation. 
With the foregoing arrangement, the nut assembly 10 is installed on the 
screw 11 by threading the screw into the nut 14 from the left end thereof, 
the right end of the screw initially being left at a position just short 
of the start of the fingers 21. The sleeve 20 then is threaded part way 
(e.g., about half-way) onto the inboard end portion of the nut 14. 
Thereafter, the nut 15 is telescoped into the sleeve 20. As an incident 
thereto, the grooves 22 slip into telescoping relation with the fingers 21 
and, in addition, the teeth 35 slip into telescoping relation with the 
notches 30. The screw 10 then is threaded through the nut 15. 
As initially installed, the nut assembly 10 usually is only in loose 
engagement with the screw 11. To take up the clearance between the threads 
12 of the screw 11 and the threads 16 and 17 of the nuts 14 and 15, the 
sleeve 20 is manually rotated in a counterclockwise direction (FIGS. 2 to 
4) while the nut 14 is held rotationally stationary. During such rotation, 
the teeth 35 ratchet first into and then out of successive notches 30 to 
permit turning of the sleeve 20. As the sleeve turns, it moves axially 
from left to right along the nuts 14 and 15. Such axial movement is 
permitted by virtue of the axially slidable fit between the notches 30 and 
the teeth 35. 
The nut assembly 10 remains loose on the screw 11 until the sleeve 20 has 
been turned counterclockwise relative to the nuts 14 and 15 sufficiently 
far to bring the right end 35 of the sleeve into engagement with the 
axially facing shoulder 37 of the nut 15. With further turning of the 
sleeve following such engagement, the right end 35 of the sleeve acts 
through the shoulder 37 to force the nut 15 to the right relative to the 
screw 11. At the same time, the sleeve acts through the threads 25 and 26 
to force the nut 14 to the left relative to the screw. As a result, the 
right flanks of the threads 17 in the nut 15 are loaded against the left 
flanks of the threads 12 of the screw 11 while the left flanks of the 
threads 16 in the nut 14 are loaded against the right flanks of the screw 
threads 12. This causes the left nut 14 to carry essentially all of the 
load when the nut assembly 10 is moved from left to right while the right 
nut carries substantially the entire load when the nut assembly is moved 
in the opposite direction. 
As the sleeve 20 is turned still further in a counterclockwise direction, 
it continues to separate the nuts 14 and 15 axially from one another and 
cause the nuts to engage the screw thread 12 even more tightly. 
Counterclockwise turning of the sleeve is stopped when the nuts are 
sufficiently tight to cause translation of the assembly 10 only when 
torque exceeding a predetermined magnitude is applied to the screw 11. 
After the screw 11 and the nut assembly 10 have been placed in service use, 
wear of the threads 12, 16 and 17 causes backlash to develop. With the 
present invention, the backlash can be eliminated simply by ratcheting the 
sleeve 20 counterclockwise while holding the nut 14 rotationally 
stationary. As a result of such rotation, the nuts 14 and 15 are spread 
even further apart so as to take up the backlash. With the present 
arrangement where there are eighteen threads 25 and 26 per inch and 
twenty-four notches 30, counterclockwise rotation of the sleeve 20 through 
1/24 of a revolution or through a distance causing the teeth 35 to enter 
the immediately adjacent notches 30 forces the nuts apart by a distance of 
0.002". By making the threads 25, 26 and the notches 30 of a finer pitch, 
an even finer degree of adjustment can be achieved. 
From the foregoing, it will be apparent that the present invention brings 
to the art a new and improved nut assembly 10 having only three 
components, namely, the two molded nuts 14 and 15 and the molded sleeve 
20. As a result, the assembly can be manufactured at relatively low cost. 
Importantly, backlash can be taken up in small increments simply by 
manually turning the sleeve 20. There is no need to assemble or 
disassemble any components in order to effect the adjustment and thus the 
adjustment may be achieved very quickly.