Limit stop assembly for a screw and nut linear actuator

A screw and nut linear actuator has a threaded drive screw. A drive nut is threaded onto the drive screw and is axially translatable thereover by rotating the drive screw relative to the drive nut. The drive nut has a first member translatable along the drive screw upon rotation of the drive screw relative to the first member, and a second member mounted to the first member for rotation relative thereto. The drive screw and first member are allowed to rotate but not translate relative to the second member with the first member restrained from rotation relative to the drive screw. A limit stop is threaded onto the drive screw and is engageable by the first member to restrain rotation of the first member upon engagement by the first member. Engagement of the first member with the limit stop restrains further rotation of the first member thereby restraining further translation of the drive nut while allowing rotation of the drive screw and the first member relative to the second member.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a limit stop assembly for a screw and nut 
linear actuator. In particular, the present invention relates to an 
improved limit stop assembly that is adjustably screw threaded and secured 
on a drive screw of the actuator apparatus. The improved limit stop 
assembly limits the motion of a drive nut over the drive screw while 
preventing frictional binding of the drive nut against the stop assembly. 
(2) Description of the Related Art 
Screw and nut linear actuator apparatus having a limit stop of the type 
provided by the present invention commonly comprise a threaded drive 
screw, a drive nut assembly, and a limit stop assembly. The drive nut 
assembly is screw threaded on the drive screw and is axially translatable 
over the drive screw by rotating the nut assembly relative to the screw. 
The limit stop assembly is releasably secured at a predetermined position 
on the drive screw, usually at opposite ends of the drive screw. 
Prior art limit stop assemblies are commonly comprised of a split collar. A 
transverse pin projects from an internal annular wall of the collar into 
the collar center bore. The split collar is secured to a drive screw by 
inserting the drive screw through the center bore of the collar, and 
tightening the collar on the drive screw. An example of this type of limit 
stop assembly is disclosed in the U.S. patent of Thompson, Jr. et al. U.S. 
Pat. No. 4,804,809. 
Prior art drive nut assemblies commonly include a nut body having an axial 
bore through the body. A plurality of circular grooves are formed in the 
annular wall of the nut body bore. A cylindrical sleeve is received in the 
bore of the nut body and retains a plurality of balls in space relation. 
The balls engage in both the threads of the drive screw and the circular 
grooves in the nut body. Opposite ends of the sleeve are formed with pairs 
of shoulders that extend from the ends of the sleeve and are spaced 
180.degree. apart on the sleeve ends. The pairs of shoulders also project 
outwardly from the nut body. A drive nut assembly of this type is also 
disclosed in the U.S. Pat. of Thompson, Jr. et al. No. 4,804,809. 
In use, the prior art drive nut assembly is axially translated fore and aft 
over the drive screw by rotating the drive screw relative to the drive 
nut. As the drive nut assembly approaches the prior art limit stop 
assembly, the sleeve shoulder projecting from the nut body engages the 
transverse pin of the limit stop assembly at some point as the sleeve of 
the drive nut assembly is rotated relative to the drive screw. When the 
sleeve shoulder engages the transverse pin the sleeve is held stationary 
relative to the drive screw and the nut body is prevented from further 
axial movement along the drive screw and rotates freely relative to the 
sleeve and screw. 
A disadvantage encountered in prior art screw and nut linear actuator 
apparatus employing a limit stop assembly of the type described is that 
the nut body of the drive nut assembly will at times engage against the 
collar of the limit stop assembly prior to the sleeve shoulder of the nut 
assembly engaging the stop pin of the limit stop. This is caused by 
failing to accurately position the pin of the prior art limit stop 
assembly relative to the drive screw thread. The prior art limit stop 
assemblies are often secured in an adjusted position on the drive screw 
without taking care that the stop pin is properly positioned relative to 
the drive screw thread. This often results in the sleeve shoulder passing 
by the stop pin as the nut assembly is rotated and translated toward the 
collar, and the nut body frictionally engaging and binding against the 
limit stop collar. The frictional binding between the nut body and collar 
often causes damage to the machine employing the linear actuator apparatus 
due to the inability of the drive nut assembly to freewheel on the drive 
screw or to separate from the limit stop assembly. 
The screw and nut linear actuator apparatus of the present invention 
overcomes the above described disadvantages encountered with prior art 
apparatus by providing an improved limit stop assembly which is threaded 
onto the drive screw and which has a stop means, the position of which 
remains constant relative to a drive screw thread regardless of the 
position of the limit stop along the drive screw. This ensures that the 
drive nut assembly will engage with the stop means and cause a member of 
the drive nut assembly to freewheel prior to the member engaging with and 
binding against the limit stop assembly. 
SUMMARY OF THE INVENTION 
The screw and nut linear actuator apparatus of the present invention 
comprises a threaded drive screw, a drive nut assembly, and an improved 
limit stop assembly. The limit stop assembly is adjustably positioned and 
secured on the drive screw. As in prior art screw and nut linear actuator 
apparatus, the drive nut assembly of the present invention is screw 
threaded on the drive screw and is axially translatable fore and aft over 
the drive screw by rotating the drive screw relative to the nut. 
In one preferred embodiment of the invention, the drive screw is provided 
with a double screw thread of predetermined pitch. A single screw thread 
or multiple threads greater than two may also be employed. 
Also, in a preferred embodiment of the invention, the drive nut assembly 
may include a nut body with a center axial bore. Circular grooves are 
formed in the inner annular wall of the bore. A cylindrical sleeve is 
received in the bore of the nut body. The sleeve retains a plurality of 
balls in spaced relation, and the balls engage in the circular grooves of 
the nut body and the screw thread of the drive screw. Shoulders extend 
from opposite ends of the sleeve and project from opposite ends of the nut 
body. Other types of drive nut assemblies may be used with the present 
invention. 
The improved limit stop assembly of the present invention is formed from an 
annular collar with an axial center bore. Unlike prior art limit stop 
assemblies, the limit stop assembly of the present invention includes an 
internal screw thread that mates with the threads of the drive screw. In a 
preferred embodiment of the invention, a circular cavity is also formed in 
the annular wall of the collar center bore at an end of the collar that 
faces the drive nut assembly on the drive screw. Stop means which may be 
in the form of a pin project into the collar center bore from the annular 
wall of the circular cavity. The position of stop means relative to a 
drive screw thread remains constant regardless of the position of the 
limit stop along the drive screw. In a preferred embodiment of the 
invention, the center axis of a projecting pin always generally intersects 
the center line of a thread groove of the drive screw when the limit stop 
assembly is adjustably screw threaded on the drive screw. With the 
projecting pin so located, as the drive nut assembly is axially translated 
toward the limit stop assembly, the sleeve shoulder of the drive nut 
assembly will always engage a projecting pin prior to the nut body of the 
drive nut assembly engaging the collar of the limit stop assembly, thereby 
preventing frictional binding between the drive nut assembly and the limit 
stop assembly of the invention. 
The improved limit stop assembly of the present invention may also be used 
at both ends of the drive screw to limit the fore and aft axial 
translation of the drive nut assembly over the drive screw, while 
preventing binding between the drive nut assembly and the two limit stop 
assemblies.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a screw and nut linear actuator apparatus 10 employing the 
limit stop assembly 12 of the present invention. The screw and nut linear 
actuator apparatus also comprises a threaded drive screw 14 and a drive 
nut assembly 16. The drive nut assembly is screw threaded on the drive 
screw and is axially translatable fore and aft over the drive screw by 
rotating the drive screw relative to the drive nut. The axial translation 
of the drive nut assembly over the drive screw to the right as viewed in 
FIG. 1 is limited by the limit stop assembly 12 secured on the drive 
screw. The axial translation of the drive nut assembly to the left as 
viewed in FIG. 1 is limited by a pin 18 press fit in a hole extending 
transversely through the drive screw. While the pin 18 is shown, it should 
be understood that a limit stop assembly 12 may also be adjustably 
positioned on the left end of the drive screw 14 in lieu of the pin 18 if 
so desired. In many applications of the limit stop assembly 12 of the 
present invention, a pair of limit stop assemblies will be adjustably 
positioned and secured on opposite ends of the drive screw. 
The drive screw 14 is provided with a double screw thread of predetermined 
pitch that includes first and second threads 22, 24 that spiral, side by 
side, over the exterior surface of the drive screw. In alternate 
embodiments of the invention a single screw thread or multiple threads 
greater than two may be employed. As is conventional, the threads 22, 24 
of the drive screw 14 have thread minor diameters 26, 28 and thread major 
diameters 32, 34. The thread grooves have center lines at their deepest 
indentation into the drive screw. The groove center lines spiral along the 
drive screw and the center lines of the respective thread grooves are 
separated at axially adjacent points by the pitch of the drive screw. 
The drive nut assembly 16 includes a member in the form of a nut body 36 
having a center axial bore 38 and a counter bore 40 of somewhat greater 
diameter than the bore 38. A plurality of circular grooves 42 are formed 
in the inner annular wall of the bore 38. Another member in the form of a 
cylindrical sleeve 46 is rotatably received in the center bore 38 of the 
nut body. A plurality of holes 48 are provided in the sleeve 46. The holes 
48 retain a plurality of balls 52 in spaced relation in the sleeve. The 
balls 52 engage in both the circular grooves 42 of the nut body 36 and the 
screw threads 22, 24 of the drive screw. Pairs of shoulders 54, 56 extend 
from opposite right and left ends of the cylindrical sleeve, respectively. 
The pairs of shoulders project axially outward from opposite ends of the 
nut body bore 38. The shoulders of each pair are spaced 180.degree. apart 
from each other. The counter bore 40 allows the end of the drive nut to 
move past the pin 18 so that the shoulder 56 can engage the pin. 
The structure of the threaded drive screw 14 and the drive nut assembly 16 
described above is conventional. A drive screw and drive nut assembly of 
the type described are disclosed in the U.S. Pat. of Thompson, Jr. et al., 
No. 4,804,809. While the drive screw and nut assembly shown has balls 52, 
it is to be understood that rollers or the like could also be used. 
Moreover, it is to be understood that the present invention is not limited 
to any particular construction of the drive nut assembly. The invention 
can be used with any drive nut assembly that has a first member 
translatable along the drive screw upon rotation of the drive screw 
relative to the first member, and where a second member is mounted to the 
first member for rotation relative thereto, such that the drive screw and 
first member are allowed to rotate but not translate relative to the 
second member with the first member restrained from rotation relative to 
the drive screw. 
The improved limit stop assembly 12 of the present invention is generally 
an annular collar 58 with a cylindrical exterior surface 62 and a 
cylindrical axial bore 64 extending through its center. A narrow slit 66 
is cut completely through one side of the collar. A partially threaded 
transverse hole 68 extends through the collar 58 perpendicular to the 
split 66. A bolt 72 is screw threaded into the threaded hole 68. 
A circular cavity 74 is formed in the left end face 76 of the collar 58 as 
viewed in FIGS. 1-5. The circular cavity 74 is concentric with the collar 
center axis and has a diameter slightly larger than the diameter of the 
cylindrical sleeve 46 of the drive nut assembly 16 to permit the entry of 
the sleeve shoulders 54, 56 into the cavity. 
A pair of mutually opposed pin holes 78, 82 extend through the collar 
between the exterior surface 62 and an internal annular wall 84 of the 
circular cavity 74. The pin holes 78, 82 are perpendicular to the center 
axis of the collar 58. A pair of stop means in the form of pins 86, 88 are 
press fit in the pin holes 78, 82 and project a short distance into the 
interior bore 64 of the collar. The extent to which the pins 86, 88 extend 
into the collar bore is such that the sleeve shoulders 54, 56 will contact 
the pins when the shoulders are rotated into the circular cavity 74, but 
the pins will not contact the drive screw threads 22, 24 as the limit stop 
assembly 12 is adjustably positioned on the drive screw 14 in a manner to 
be described. 
In this described embodiment, the remainder of the annular wall of the 
collar interior bore 64, between the circular cavity 74 and the right end 
face 92 of the collar as viewed in FIGS. 1-5, is formed as a double start 
internal screw thread. The double start internal thread includes first and 
second internal threads 94, 96 that mate with the threads of the drive 
screw 14. 
Preferably, the positioning of the pins 86, 88 is such that the center axes 
of the pins approximately intersect the center lines of the thread grooves 
of the drive screw 14 when the limit stop assembly 12 is screw threaded on 
the drive screw. 
In operation, the limit stop assembly 12 is adjustably screw threaded on 
the drive screw 14 with the circular cavity 74 of the stop assembly 
directed toward the drive nut assembly 16 on the screw. The internal screw 
thread of the limit stop assembly 12 positively locates the center axes of 
the projecting pins 86, 88 directly above the centers of the drive screw 
thread grooves 26, 28 for all axially adjusted positions of the stop 
assembly on the drive screw. When the stop assembly is adjusted to its 
desired position on the drive screw, the stop assembly 12 is secured to 
the drive screw 14 by tightening the bolt 72 of the stop assembly. If 
desired, a second limit stop assembly 12 may be adjustably screw threaded 
on the opposite end of the drive screw 14 with the circular cavity of the 
second stop assembly directed toward the drive nut assembly 16. In this 
described embodiment of the invention, with the pair of projecting pins 
86, 88 of the stop assembly located above the centers of adjacent thread 
grooves 26, 28 of the drive screw 14, as the drive nut assembly 16 is 
axially translated toward the limit stop assembly the sleeve shoulders 54, 
56 will always engage at least one of the projecting pins prior to the nut 
body 36 of the drive nut assembly engaging the end face 76 of the stop 
assembly collar, thereby preventing frictional binding engagement between 
the drive nut assembly and the limit stop assembly. 
The improved limit stop assembly of the present invention may also be 
employed in a screw and nut actuator apparatus having a single threaded 
drive screw and drive screws having multiple threads greater than two. For 
example, where a single threaded drive screw is used, the collar of the 
limit stop assembly will be provided with a single threaded interior bore 
that, like the first embodiment, positively locates a projecting pin of 
the stop assembly relative to the thread of the drive screw. 
While the present invention has been described by reference to specific 
embodiments, it should be understood that modifications and variations of 
the invention may be constructed without departing from the scope of the 
invention defined in the following claims.