GB2115901A

GB2115901A - Ball screw assemblies

Info

Publication number: GB2115901A
Application number: GB08301213A
Authority: GB
Inventors: Alfons Remensperger
Original assignee: Warner Electric Brake and Clutch Co
Current assignee: Dana Inc
Priority date: 1982-02-12
Filing date: 1983-01-17
Publication date: 1983-09-14
Also published as: FR2521674A1; CH650569A5; FR2521674B1; JPS58142061A; GB8301213D0; DE3204919C1; GB2115901B

Abstract

Ball nuts (18, 20) are preloaded onto a ball screw (12) by rotating the nuts relative to one another to jam adjacent end faces (40, 42) of the nuts together. Thereafter the nuts are held against relative rotation by rotating a retaining ring (22) which spans the joint between the nuts, the retaining ring acting in the manner of a jam nut to lock the ball nuts rigidly together. <IMAGE>

Description

SPECIFICATION Ball screw assemblies The present invention related to ball screw assemblies of the type in which a pair of axial ly aligned ball nuts are adapted to be preloaded on a ball screw. One way to effectthe preloading is to rotate the nuts relative to one another to jam adjacent end faces of the nuts together. Thereafter, the nuts are locked against relative rotation in order to maintain the preload.

Various arrangements have been devised for locking the nuts against relative rotation after the preloading has been effected. Some prior arrangements require that the nuts be turned to predetermined relative angular positions before locking can be achieved. Examples of such arrangements are disclosed in Whicker United States Patent No.

3,382,731 and Ploss et al United States Patent No.

3,851,541. The arrangement disclosed in the Whicker Patent requires that the nuts be placed in a surrounding housing before the nuts can be locked together.

In the arrangement disclosed in the Ploss et al Patent, set screws extend into a retaining ring and are threaded into the nuts. Allowable manufacturing tolerances result in some play existing between the set screws and the retaining ring and, in addition, the set screws may tend to loosen if the ball screw assembly is subjected to vibration either during shipping or operation. As a result, the two nuts may tend to rotate relative to one another in spite of the retaining ring and may not always be held together as a rigid unit.

Other arrangements allow the two nuts to be locked together regardless of their relative angular positions (i.e., stepless preloading). One such arrangement is disclosed in our copending United States application Serial No. 174,214, filed July 31, 1980 in the name of Ploss. Again, however, the nuts may have a tendency to rotate relative to one another since the nuts are held together by a frictional locking ring which is secured by set screws.

The present invention provides a ball screw assembly comprising a screw having a ball groove, first and second axially aligned nuts each having a ball groove, balls located between the groove of said screw and the grooves of said nuts and adapted to circulate within said grooves, said nuts having adjacent end faces, a retainer ring spanning the joint between the adjacent end faces of said nuts, and coacting means on said retainer ring and said nuts and operable when said retainer ring is rotated relative to said nuts to lock said nuts against relative rotation and to hold said balls in a preloaded condition in said grooves.

The invention also provides a ball screw assembly comprising a screw having a ball groove, first and second axially aligned nuts each having a ball groove, balls located between the groove of said screw and the grooves of said nuts and adapted to circulate within said grooves, said nuts having adjacent end faces adapted to be jammed against one another by relative rotation of the nuts to preload said balls in said grooves, a retainer ring telescoped over the adjacent end portions of said nuts and spanning the joint between said nuts, and coacting means on the inner side of said ring and on the outer sides of said nuts and operable when said ring is rotated relative to said nuts to draw one of said nuts toward the other of said nuts and lock said nuts against relative rotation.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a side elevational view of a ball screw assembly being broken away and shown in section; Figure 2 is a view similar to figure 1 but shows another embodiment of a ball screw assembly; Figure 3 is a side elevational view of yet another embodiment of a ball nut, the retaining ring of the nut being shown in section; and Figure 4 is a cross-sectional view taken substantially along the line 4-4 of Figure 3.

A ball screw assembly 10 comprises a screw 12 and a multi-sectional nut 16 consisting substantially of two seperate and axially aligned nuts 18 and 20.

Circulating in the helical grooves or races of the screw and the nuts are balls 14 which are made to return in a known manner from one race to, for example, the one immediately adjacent it, by ball transfer members (not shown). In this way, rotational movement of one part, for example, the screw 12 is converted into linear displacement of the other part, in this case, the multisectional nut 16. A mounting flange 24 is integral with the right end of the nut 20 and is formed with holes 26 which are adapted to receive screws (not shown) for fastening the multisectional nut 16 to a machine component such as a machine tool carriage.

It is desirable to elastically deform or preload the balls 14 in the races in order to take up axial play or backlash between the balls and the races and increase the axial stiffness of the ball screwassem- bly. Preloading preferably is effected by rotating one of the nuts (e.g., the nut 18) along the screw 12 and toward the other nut 20 in order to jam the adjacent planar end faces 40 and 42 of the nuts tightly against one another. Such jamming creates oppositely directed forces and causes the balls of the two nut portions to contact opposite flanks of the races in a known manner as disclosed, for example, in Ploss et al United States Patent No. 3,851,541. After the nuts have been jammed together, they are locked against relative rotation in order to maintain the preload.

The degree of preloading depends upon how tightly the nuts 18 and 20 are jammed together and this depends upon the relative angular positions of the nuts.

The two nuts 18 and 20 are adapted to be locked against relative rotation by a stepless retaining ring 22 which acts in the manner of a precision jam nut.

The retaining ring is such that, when loosened, it premits one nut to be rotated to any desired angular position relative to the other nut to effect the desired degree of preloading in a stepless manner. When tightened, the retaining ring locks the nuts rigidly to one another so that the two nuts of the multisectional nut 16, in effect, become a single unit and remain rigidly locked together without play developing between the two either during shipment of the ball screw assembly 10 or during operation thereof.

In the present instance, the retaining ring 22 is telescoped over adjacent end portions of the nuts 18 and 20, such end portions being of reduced diameter. The retaining ring 22 includes a left hand internal thread 30 and a right hand internal thread 32 which preferably is of the same pitch as the left hand thread. The threads 30 and 32 are adapted to mate with left and right hand external threads 34 and 36, respectively, formed on the reduced diameter portions of the nuts 18 and 20. Preferably, the various threads are metric precision threads having a pitch such that a self-locking effect is achieved when the ring is tightened. A hole 28 is formed radially through the ring and is adapted to receive a tool for tightening the ring.

To assembly and preload the ball screw assembly 10, the nuts 18 and 20 first are installed on the screw 12 with the balls 14 in place but without the retaining ring 22 being present. The nuts then are rotated relative to each other so that their end faces 40 and 42 just make contact, this being a condition of unstressed contact with zero preload. Circumferentially aligned marks are placed on the nuts 18 and 20 outside of the area to be later occupied by the ring 22 so as to make it possible to again place the nuts in a condition of unstressed contact with zero preload after the ring has been installed.

Next, the ball screw 12 is screwed out of one of the nuts (e.g., to the right out of the nut 18) and is simultaneously replaced with a smooth arbor which holds the balls 14 in place in the nut 18. With the left end of the screw at the left end of the nut 20 and with the right end of the arbor at the right end of the nut 18, the arbor and the nut 18 are pulled axially away from the screw and the nut 20. This axial seperation enables the ring 22 to be threaded onto one of the nuts, for example, the nut 20. The arbor and the nut 18 then are brought back toward the screw and the nut 20 and, by turning either the ring 22 or the nut 18, the ring may be threaded onto the nut 18 while remaining threaded on the nut 20.Such turning is continued until the end faces 40 and 42 contact one another with the previously inscribed marks disposed in circumferential alignment so as to locate the nuts in a position of unstressed contact with zero preload. Thereafter, the arbor is removed from the nut 18 and, at the same time, the screw 12 is inserted into the nut 18 to hold the balls 14 therein.

The desired preload then is imparted to the ball screw assembly 10 by rotating the nuts 18 and 20 relative to one another on the screw 12 to jam the end faces 40 and 42 together under pressure and to cause the balls 14 in the two nuts to bear against opposite flanks of the races. Once the desired preload has been established, the retaining ring 22 is rotated relative to both nuts. As a consequence, the ring tightens in the fashion of a jam nut and tends to draw the nuts together to increase the surface pressure between the end faces 40 and 42. Thus, the ring locks the nuts together as a single unit and prevents relative rotation of the nuts so as to maintain the desired preload.Since the ring acts as a jam nut, the tight locking of the nuts 18 and 20 is retained even under conditions of severe vibration which might be encountered during shipment or operation of the ball screw assembly 10.

The assembly shown in Figure 2, differs from that shown in Figure 1 only in that the retaining ring 22' is constructed so as to also function as a mounting flange on the end of the nut 20.

It will be appreciated that it is not essential that the threads 30 and 34 be of opposite hand from the threads 32 and 36. Instead, all of the threads may be of the same hand with the threads 30 and 34 being of different pitch from the threads 32 and 36. Alternatively, angularly spaced shoulders may be formed on one of the nuts to coact with angularly spaced shoulders on one side of the retaining ring, the other side of the retaining ring being formed with a thread which coacts with a thread on the other nut.

The assembly shown in Figure 3 and 4 is somewhat similar to that described immediately above. In the assembly shown in Figure 3 and 4, the nuts are indicated by the reference numerals 18" and 20" while the retaining ring is indicated by the reference numeral 22".

As shown in Figures 3 and 4, the thread 36 is replaced with a shoulder 36" formed around the nut 20" and having a flank 70" with a helical lead. The thread 34 is replaced with a shoulder 34" formed around the nut 18" and having a flank 71" with zero lead. The ring 22" is formed with an internal annular groove having a set of four angularly spaced shoulders 76" and a set of four angularly spaced shoulders 78" whose inboard sides define flanks 72" and 74", respectively. The flanks 72" have a zero lead while the flanks 74" have a helical lead corresponding to that of the flank 70". Four axially extending notches 90" are formed through the shoulders 34" and 36" to accommodate the shoulders 76" and 78" during installation of the retaining ring.

After the nut 18" has been separated axially from the nut 20" in the manner described previously, the retaining ring 22" is installed on the shoulder 34" by slipping the shoulders 76" through the notches 90" in the shoulder 34". Thereafter, the nut 18" is moved axially towards the nut 20" and, as an incident thereto, the shoulders 78" pass through the notches 90" in the shoulder 36". After the mandrel has been removed and the nuts have been rotated relative to one another to effect the preload, the ring 22" is turned relative to the nuts (in a clockwise direction when viewing the components shown in Figure 3 from the left). Such turning causes the flanks 72" to bear against the flank 71" and causes the flanks 74" to bear against the helical flank 70". In a manner similarto a quarter-turn fastener, the ring locks the nuts against relative rotation, the pitch of the helical flanks 70" being so small that a self-locking effect is obtained.

Claims

1. A ball screw assembly comprising a screw having a ball groove, first and second axially aligned nuts each having a ball groove, balls located between the groove of said screw and the grooves of said nuts and adapted to circulate within said grooves, said nuts having adjacent end faces a retainer ring spanning the joint between the adjacent end faces of said nuts, and coacting means on said retainer ring and said nuts and operable when said retainer ring is rotated relative to said nuts to lock said nuts against relative rotation and to hold said balls in a preloaded condition in said grooves.

2. A ball screw assembly as claimed in claim 1, in which said retaining ring is telescoped over the adjacent end portions of said nuts, said coacting means comprising an internal thread on said ring and an external thread on one of said nuts.

3. A ball screw assembly as claimed in claim 1, in which said retaining ring is telescoped over the adjacent end portions of said nuts, said coacting means comprising first and second different internal threads on said ring and respectively mating with first and second external threads on said first and second nuts, respectively.

4. A ball screw assembly as claimed in claim 3, in which said first threads are of a different hand than said second threads.

5. A ball screw assembly as claimed in claim 3, in which said first and second threads are of the same hand but are of different pitch.

6. A ball screw assembly as claimed in claim 1, in which said retaining ring is telescoped overthe adjacent end portions of said nuts, said coacting means comprising a groove formed around the inner side of said retaining ring and defining a first pair of first and second oppositely facing flanks, said coacting means further comprising first and second shoulders formed around the outer side of said first and second nuts, respectively, and defining a second pair of first and second oppositely facing flanks, the first and second flanks of said groove engaging the first and second flanks, respectively, of said shoulders, one pair of the engaging flanks being formed with a helical lead.

7. A ball screw assembly as claimed in claim 6, in which the other pair of the engaging flanks is formed with zero lead.

8. A ball screw assembly as claimed in claim 6 or 7, in which axially extending notches are formed through said shoulders.

9. A ball screw assembly comprising a screw having a ball groove, first and second axially aligned nuts each having a ball groove, balls located between the groove of said screw and the grooves of said nuts and adapted to circulate within said grooves, said nuts having adjacent end faces adapted to be jammed against one another by relative rotation of the nuts to preload said balls in said grooves, a retainer ring telescoped over the adjacent end portions of said nuts and spanning the joint between said nuts, and coacting means on the inner side of said ring and on the other sides of said nuts and operable when said ring is rotated relative to said nuts to draw one of said nuts toward the other of said nuts and lock said nuts against relative rotation.

10. A ball screw assembly as claimed in claim 9, in which said coacting means comprise first and second internal threads on said ring and respectively mating with first and second external threads on said first and second nuts, respectively, said first and second threads being of different hand.

11. A ball screw assembly substantially as herein described with reference to Figure 1, Figure 2, or Figures 3 and 4 of the accompanying drawings.