Methods of making ball nuts for preloaded ball nut and screw assemblies

A method of making ball screw and nut assemblies, and the products which result, wherein the nut is provided with a deformable wall portion of decreased compressive strength interjacent the ends of the nut. A compressive force is applied to the nut to axially deform the portion of decreased compressive strength while not changing the lead of adjacent lands and grooves in the non-compressed portion of the nut. The axial position of the lands and grooves in the non-compressed portion is thereby axially displaced such as to preload a recirculating train of balls traveling along the non-compressed portion of the nut relative to the lands and grooves of the screw, when the screw, nut, and balls are assembled.

BACKGROUND OF THE INVENTION 
The present invention relates to methods of making recirculating ball nut 
and screw assemblies which are internally preloaded to reduce lash. 
Preloading of the load bearing components or balls of ball nut and screw 
actuators is common in the production of ball nut and screw assemblies to 
achieve quality results without the necessity to hold undue tolerances, 
and to provide the consistency which is necessary to the use of the 
assemblies, particularly, for example, those which may be used on machine 
tools which perform close tolerance machining. 
One method of fabricating preloaded ball nut and screw assemblies is 
disclosed in the present assignee's U.S. Pat. No. 4,643,041, issued Feb. 
17, 1987, and involves utilizing a hardenable preloading plastic ring 
between a pair of nut members which exerts a preload once the ring, which 
is an epoxy ring, hardens. 
Other patents, and I refer particularly to U.S. Pat. No. 3,479,897 granted 
Nov. 25, 1969, utilize springs, such as torsion springs, between two nuts 
which are provided in tandem relation on ball screws, and particularly, 
rolled screws which provide less precision than ground screws. Such 
preloaded ball screw and nut assemblies maintain the preload control 
necessary during the useful life of the system, despite wear, to achieve 
increased operating life. 
SUMMARY OF THE INVENTION 
The present invention, in one of its aspects, is directed to a new method 
of forming recirculating ball nut and screw systems wherein a section of 
the nut interjacent its ends is structurally weakened, as by forming an 
annular cut or groove in its peripheral surfaces, and an internal thread 
is machined in the other portions of the nut. The nut is then axially 
compressed, with the result that the weakened portion compresses slightly, 
while portions adjacent to the compressed portion remain uncompressed and 
provide thread portions adjacent to the compressed portion which have the 
same lead as the screw threads, but are displaced axially due to the 
compression of the weakened portion, such that, when the nut is assembled 
on a screw, a preload will be applied to separate ball trains in axially 
opposite directions. 
One method of internally preloading ball nuts in common use in industry 
today is to form the ball thread groove in a tapping operation, and then 
to grind the thread surface produced such as to provide the preload 
configuration. This method of producing preloaded systems requires many 
grinding passes back and forth with the result that the method is 
relatively slow and somewhat labor intensive, and, accordingly, is 
relatively costly. 
The present method employs the rough and finish tapping operations before 
the grinding step, but, instead of grinding, uses a burnishing tool to 
provide a rolled nut groove in a much less time consuming manner, while 
still, in the end, obtaining substantially the quality of a ball nut and 
screw actuator in which the preload is accomplished by grinding. The 
result is achieved without necessitating the use of a spring, or the 
imposition of an epoxy preload ring or other element. 
One of the prime objects of the present invention is to design a relatively 
simple and very economical method of providing preloaded systems by, in 
effect, re-configuring the pre-threaded nut, in a manner which can be 
readily accomplished in high production manufacturing operations. 
A further object of the invention is to provide a precision preloaded ball 
nut and screw assembly which achieves the preload of ground surfaces 
without the necessity for the grinding operation. 
Still another object of the invention is to achieve the result by the 
simple step of fashioning the nut to produce the preload, without 
requiring any changes in the standard screw shaft or the recirculating 
load bearing balls. 
A still further object of the invention is to provide a preloaded ball nut 
and screw shaft assembly of the character described which is durable, 
reliable, and has the same operating life as present, more expensive 
systems wherein the preload is ground in the nut. 
Other objects and advantages of the invention will become apparent with 
reference to the accompanying drawings and the accompanying descriptive 
matter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now more particularly to FIG. 6 in the first instance, a ball nut 
and screw actuator, generally designated A, is employed to generally 
identify the assembly which, as usual, comprises a ball screw component 10 
having a helical ball groove 11 and a helical land 12. The groove 11 is 
well suited to accommodating separate trains of load bearing balls 13 and 
14 which are accommodated in a like groove 15 provided in the nut, which 
we have generally designated N. As is usual, the nut helical groove 15 and 
screw groove 11 are complemental in the sense that they are identically 
configured and have the same lead. 
Provided in the usual manner, to receive the balls 13 and 14 at one end and 
return them at the other are external ball return tubes 16 and 17, 
respectively. The ball return tubes, which are U-shaped in configuration, 
have ends which lead into passage portions 18 provided in the peripheral 
wall of the nut N as shown, and a pair of passage portions 18 are 
necessary to accommodate each ball return tube 16 or 17, as usual. The 
tubes 16 and 17 extend through the annular wall of the nut to communicate 
with the nut groove 15 and deflect balls into the tubes 16 and 17 in the 
usual manner when the nut and screw are relatively moved. The nut is 
configured to preload the balls 13 and 14, when the nut is assembled in 
position, in a novel and improved manner which will now be described. 
With reference now particularly to FIG. 1, it will be noted that a metallic 
sleeve blank, generally designated 20, is provided with an annular groove, 
slit or notch 21 in its outer wall, opposite an annular interior groove 
22. The nut blank 20 may be constructed of steel of the same character 
presently employed in ball nuts, or of another suitable material. A 
typical material is SAE 8620 steel. The next step in forming the nut is to 
internally groove the interior bore of the blank 20 in a manner to provide 
the usual helical groove 15 having groove portions and defining land 
portions 15a (FIG. 2). The land portions 15a the groove portions 15 are 
formed with the same lead as the screw groove 11 and screw land 12. The 
internal threading of the blank 20 is accomplished via the usual tapping 
operation followed by a burnishing operation. The grooves 15 are first 
rough cut with a rough tapping tool and, then, finish cut with a finish 
tapping tool. After this, a burnishing tool is used to roll the groove 
surface and displace metal to provide the rolled groove 15. It is to be 
understood that a groove formed by tapping and burnishing is not a novel 
step, except in combination with the weakening of the wall of the blank 20 
which has been described, and the subsequent compression operation. 
In FIG. 3, it will be observed that the nut blank 20 has been inverted and 
placed on a fixed lower die member 23, with its girth annularly restrained 
by the annular die ring 24. The upper end of the inverted blank 20 is then 
engaged by an upper die member 25, as shown in FIG. 4. The lowering of the 
die 25 relative to the die 23, in a conventional press, then provides a 
deformed compression section generally designated "x" between end sections 
which are not compressed. 
FIG. 5 well demonstrates the compressed condition of the nut blank 20 and 
illustrates the narrowing of groove 21 and the annular bead of compressed 
material 26 which is formed during the compressing operation. With some 
expected inconsequential spring-back of the material, the nut blank 20 is 
axially compressed, under much, much higher axial loads than would ever be 
encountered in the use of the device, sufficiently to provide the desired 
compression. It is not to be understood that the deformed section "x" will 
have any axial resiliency. Rather, it is an axially fixed, non-resilient 
mass which maintains its deformed shape to provide the operation desired 
regardless of the maximum operating loads applied when the nut N and ball 
screw 10 are assembled and used. The grooves 21 and 22 are configured and 
sized to achieve this axial rigidity and maintain the pre-loads achieved. 
The axially deformed portion "x" of the nut, after compression, will have 
groove and land portions with a true lead amount minus the amount of 
compression, while the groove and land portions on each side of the 
compressed portion "x" will have the same original or true lead. For 
instance, portions "y" in FIG. 5 on opposite sides of the deformed section 
"x" have the same lead as originally, but they are displaced slightly 
axially closer together, and it is this displacement which provides the 
preload when the nut formed is in assembled position in a ball nut and 
screw actuator. The preload is exerted in opposed directions on opposite 
sides of the compressed section "x" of the nut and balls 13 traveling in 
tube 16 will be preloaded in an opposite direction from the balls 14 
traveling in tube 17. The manner of imparting the preload is well 
demonstrated in FIG. 7 in an exaggerated illustration, with the chain 
lines "a" and "b" indicating the contact points of the balls and the 
direction of preload. 
THE OPERATION 
The preloaded ball nut and screw assembly illustrated operates in the usual 
manner for translating linear motion into rotary motion, or rotary motion 
into linear motion. With the nut prevented from rotating, the rotation of 
the screw 10 will cause translation of the nut N or, if the nut N is 
prevented from moving axially, but is permitted to revolve, it will be the 
screw 10 which is moved axially. Because the preloading eliminates lash in 
either direction, the precise positioning of tools, work pieces and the 
like, can be reliably and efficiently accomplished with precision. 
ANOTHER EMBODIMENT 
In FIG. 8, another embodiment is disclosed in which like numerals have been 
used to designate like parts. In this case, six circuits are provided on 
the left side of the compressed portion "x" of the nut, and three circuits 
on the other. Here, internal passage portions 18 lead through the body of 
the nut internally to different groove portions in the screw 10. The 
groove portions 18 in FIG. 8 are connected by intermediate groove portions 
18a and the groove portions 18 and 18a are provided in nut wall inserts, 
generally designated I, in the usual manner. The six recirculating ball 
circuits at the left side of the compressed portion of the nut are 
preloaded oppositely to the three recirculating ball circuits located at 
the portion of the nut at the right side of the compressed portion in FIG. 
8. 
It is to be understood that the embodiments described are exemplary of 
various forms of the invention only and that the invention is defined in 
the appended claims which contemplate various modifications within the 
spirit and scope of the invention.