Preloaded ball nut and screw assembly and method of manufacture

A ball nut and screw assembly preadjusted and preloaded by injecting a liquefied pressurized thermoset resin through a mold and into the area between adjacent ball nuts and allowing the thermoset resin to cure and harden while pressurized. After hardening, the mold is removed and the external preload supplied to the resin is maintained by a hardened preload ring formed by the epoxy ring.

This invention relates to a force and motion transfer device and more 
particularly to a new and improved preloaded ball nut and screw assembly 
and to a new and improved method of preloading such assemblies. 
Various ball nut and screw assemblies utilize a preload between a pair of 
ball nuts operatively mounted on a ball nut screw to obtain maximum 
stiffness and to reduce lash or the relative longitudinal shifting of the 
ball nuts and ball nut screw. An example of a preloaded ball nut and screw 
assembly is disclosed in U.S. Pat. No. 3,234,810, issued Feb. 15, 1966, 
issued to H. Orner and entitled "Preloaded Ball Screw and Nut Mechanism". 
The preloaded ball nut and screw assembly of the present invention is of 
this general category but further advances the ball nut and screw art by 
providing consistent preloading of a plurality of ball nut and screw 
assemblies with parts having a wide tolerance range and by providing a new 
and unobvious manufacturing method of preloading ball nut and screw 
assemblies. 
The ball nuts of this invention can be manufactured with simple turned down 
diameters with loose tolerances on interfacing ends of each pair of ball 
nuts to provide cooperating shoulders that telescopically interfit. The 
squareness on the interfacing shoulders can also be held to a very loose 
tolerance and the parts do not require fine finishing as in many of the 
prior art ball nut and screw constructions. The ball nuts also are 
provided with a very loose tolerance key and key way. After the ball nuts 
are assembled on a ball nut screw and loaded into interconnecting ball 
trains, a key is inserted in a longitudinal key way formed in each ball 
nut to keep the ball nuts from back driving during the preload operation 
of this invention. An O-ring or other suitable seal is inserted on a gap 
side of the preload area formed by the shoulders at the interfacing ends 
of the ball nuts to prevent a liquefied preload material such as thermoset 
resin hereinafter described, from flowing from an annular pocket or groove 
provided radially outwardly of the telescopically interfitted shoulders of 
the ball nuts onto the ball nut screw. After this O-ring seal is inserted, 
a fully sealed molding tool is placed around the pocket or groove of the 
preload area of the two ball nuts. Subsequently, the liquid thermoset 
resin with a low shrink rate and quick set or cure time is forced through 
the molding tool and into the annular pocket or groove with a 
predetermined pressure. The pressurized liquid resin acts on the opposing 
faces of the ball nuts forming side walls of the annular groove and pushes 
the ball nuts apart thereby preloading the load bearing components of the 
ball nut and screw with a predetermined and constant load for each 
assembly produced. Subsequently, the resin cures into a hardened epoxy 
ring with the preload forces being effectively transferred from an initial 
external source to a permanent, internal ring so that the mold can be 
removed for use on a second ball nut and screw assembly requiring the same 
or different preload. Accordingly, this invention provides an economical 
and optimized preload system for quantity production ball nut assemblies 
with consistently high quality results and without involving parts with 
closely held tolerances, elaborate preloading and often inconsistent 
constructions and complex and costly procedures of the prior art 
assemblies.

Turning now in greater detail to the drawings, there is shown in FIG. 1 a 
ball nut and screw assembly 10 having an elongated metallic screw 12 which 
has an external helical ball groove 14 formed therein. Mounted on this 
screw are a pair of metallic ball nuts 16 and 18 in a side-by-side 
relationship. The nuts are equipped with conventional ball return tubes 
22, 24 accommodated in conventional openings in the nut to endlessly 
interconnect the ball tracks provided by the helical grooves 14 of the 
screw and the complementary helical grooves 26, 28 formed internally of 
the two nuts. Endless trains of balls 30, 32 operatively interconnecting 
the respective ball nuts to the ball nut screw for smooth operation with 
reduced friction as the ball nut and screw are relatively rotated to 
produce linear motion of one of their elements. 
In accordance with the present invention, the interior faces of nuts 16, 18 
are respectively formed with projecting shoulders 34, 36 which overlap or 
telescope on one another as best shown in FIGS. 2 and 4. These shoulders 
can be readily formed on the ends of the nuts by a lathe or other machine 
tool and close tolerance machining is not required with the preloading 
provided by this invention. 
When overlapped, the shoulders 34 and 36 form an annular groove or pocket 
40 into which an annular O-ring seal 42 is placed. This seal blocks the 
clearances in the interface between the ball nuts including clearances 46, 
48 appearing at the end of the shoulders and the adjacent end faces of the 
associated ball nut which are allowable tolerance variations in the 
present invention. After installation of the O-ring seal, a liquefied 
thermoset resin 50 is injected into the annular groove 40 under 
predetermined pressure to force the ball nuts apart and load the ball 
trains and grooves and any other load bearing components to eliminate lash 
of the ball nut and screw assembly. Before the resin is injected into the 
annular groove 40, an elongated key 52 is inserted in the aligned key ways 
54, 56 formed in the two adjacent ball nuts 16, 18 prior to injection of 
the thermoset resin to prevent or limit relative rotation of the ball nuts 
when the pressurized epoxy exerts the parting forces on the ball nuts. 
This allows the ball nuts to be urged apart for preloading the balls and 
the helical grooves of the ball nut and screw assembly. 
FIGS. 4 and 5 illustrate the molding apparatus used to injection mold the 
thermoset resin into the assembly. This includes a ring-like annular mold 
60 in the form of two semi-circular mold halves 62, 64 hinged together by 
a hinge 66. Diametrically across from hinge 66, there is a conventional 
cam lock 70 shown in the locking position. In this position, the cam 
halves are used to tightly squeeze the mold around the ball nuts with 
annular seals provided by O-rings 72, 74 being outside of the key and key 
way and sealingly held against the outer surfaces of the associated ball 
nuts 16 and 18. In this position, the liquefied resin can be injected into 
the annular groove 40 for preload purposes. By turning the cam lock handle 
76 outwardly 90.degree. from the position illustrated, the attached cam 78 
is rocked to a release position with respect to a bifurcated keeper 80 
projecting from upper mold half 62. The cam can then be swung on its lower 
pivot 82 on retainer 84 extending from lower mold half 64 to a released 
position so that the halves of the mold can be opened after the epoxy has 
set and preload has been accomplished. 
Welded to the top half 62 of the mold is a neck 86 projecting from opening 
88 leading into annular groove 40. This neck is internally threaded at 90 
to receive a pressure fitting 92 which is like a conventional grease 
fitting with a one-way ball check valve, not shown, through which liquid 
epoxy resin is fed into the annular groove 40. This fitting receives the 
nozzle end 94 of a pressure hose 96 which is connected to a source 100 of 
thermoset resin by the pressure hose 96. A conventional fluid pump 98 in 
this feed line pumps the liquid resin from the source 100 into the annular 
groove 40 at a predetermined pressure which is read from a pressure gauge 
102 installed in the line provided by hose 96. 
The thermoset resin has a low shrink rate and quick set or cure time and 
when fed at a predetermined pressure by pump 98 into the mold and groove 
40 the ball nuts are pushed in opposite direction by the pressurized epoxy 
acting on the annular side faces 104, 106 of the groove 40. This preloads 
the load bearing components of this assembly formed by the trains of balls 
and the helical grooves or tracks in which these balls operate. If the 
pressure is increased, the preload is accordingly increased so that any 
desired preload can be obtained. The liquid resin cannot escape from the 
mold because of the blocking action of the O-rings 72 and 74 sealed on the 
outer periphery of the ball nut and because of the fast cure time. Also 
heat can be applied to increase cure time. The external pressure is 
maintained until the epoxy resin 50 hardens into an epoxy ring 110 which 
maintains the preload pressure on the ball nuts. The epoxy also surrounds 
a portion of key 52 and when hard locks it into position to inhibit 
relative rotation of the ball nuts. After the epoxy hardens into ring 110, 
the mold 60 is removed and resin flash is cut away to complete the 
assembly. A second assembly is made with the same procedure. 
Uniform preload from assembly to assembly can be easily obtained by 
monitoring and regulating the input pressure of the thermoset resin. By 
using this method, the ball nut preload is very precise due to the ability 
to regulate the pressure. Since ball nut assemblies are preloaded using 
the same pressure, the preload would remain constant from assembly to 
assembly and the finish or squareness of the ball nut ends and other 
tolerance looseness would not adversely effect the preload. This process, 
accordingly, results in a much higher quality product while simplifying 
the machining process of making ball nut hardware. 
While a preferred embodiment and method disclosing this invention has been 
presented by this disclosure, other embodiments and methods in kind will 
become apparent to those skilled in the art. Accordingly, the scope of 
this invention is set forth in the following claims.