GB2185301A

GB2185301A - Ball screw nut

Info

Publication number: GB2185301A
Application number: GB08700434A
Authority: GB
Inventors: Hiroshi Teramachi
Original assignee: Individual
Current assignee: Individual
Priority date: 1986-01-09
Filing date: 1987-01-09
Publication date: 1987-07-15
Also published as: JPH0587703B2; DE3700430A1; DE3700430C2; GB8700434D0; FR2592698B1; FR2592698A1; GB2185301B; KR870007378A; IT1201118B; JPS62159853A; IT8719034A0; KR920000544B1

Abstract

A ball screw nut 2 is provided with a cylindrical extension 35 so that a rotational drive member can be connected to the extension in such a manner that the axis of rotation of the extension, and therefore the nut, is aligned with the drive member whereby vibrations, which might otherwise result from the error in the rotation transmission and the misalignment, are eliminated to ensure stable and accurate rotations. The screw 22 and nut 2 may each be connected to a respective drive motor (M1, M2 Figure 3), the motors being rotatable at slightly different speeds to provide a fine position adjusting facility. <IMAGE>

Description

SPECIFICATION Ball screw nut The present invention relates to a ball screw nutfor useina ball screw unit and, more particularly,toa nut adapted for threaded engagement with a ball screw shaft through a number of balls.

An example of a ball screw nut of the above specified type according to the prior art is shown in Figure 5.A ball screw nut2 is formed on its inner and outer circumferences respectively with a rolling groove 31, through which the nut 2 is screwed on a ball screw shaft 1 through a number of balls 3, and ball rolling grooves 32 on which the nut 2 is borne by a pair of bearing members 4 (e.g., 4a and 4b). The outer circumference of the ball screw nut 2 is further formed atone end with jointflange7which in turn is formed with a gear8fortorquetransmission. In general, the ball screw nut 2 together with the ball screw shaft 1 ,the bearing members 4 and so on constitutes a ball screw unit.

A stationary flange 6 is formed to project from the outer race 5 of one bearing member 4a, while a retaining spacer 9 is interposed between the pair of bearing members 4 of the ball screw unitfor retaining each bearing member 4 under pressure on a predetermined portion of the outer circumference ofthe ball screw nut 2. A pair of dust seals 10 are provided for plugging the gaps between the two end portions of the ball screw shaft 1 and nut 2.

A ball screw nut 2 thus constructed is rotatably borne by the bearing members 4, and can rotate relative to the ball screw shaft 1 so that the nut and shaft can reciprocate relative to each other.

With the ball screw unit described above, it is possible to construct a travelling table mechanism which can be moved quickly for large adjustments and slowlyforfine adjustments, as shown in Figure 6. In this travelling table mechanism, a moving table 12 is movably arranged on a stationary bed 11. The ball screw shaft 1 is rotatably supported near its two ends by bearing members 13 and 14. One end portion 1 a of the ball screw shaft 1 is connected through a coupling 15 to the shaft 16a of a drive motor 16. The outer race 5 of the bearing member4a is fixed on the moving table 12 through the flange 6.

Engaged with the gear 8 is a transmission gear 17 which is fixed on the shaft 1 8a of a second drive motor 18 mounted fixedly on the moving table 12.

If the drive motors 16 and 18 of the travelling table mechanism thus constructed are rotated in opposite directions, the moving table 12 isquicklyfed in a predetermined direction. If, however the drive motors 16 and 18 are rotated in the same direction but at slightly different rotational speeds, rotations perunittime,the moving table 12 is finely fed in a predetermined direction.

When the ball screw nut 2 ofthe prior art is to be rotationally driven, the gear8 at one side may be brought into meshing engagementwiththe transmission gear 17 of the drive motor 18. Since, however, the gears 8 and 17 have their axes of rotation misaligned and offset, the rotational accuracy of the ball screw nut 2 will decrease as a result of the play orthe like of the gear 8 whilethe rotating force is being transmitted from the drive motor 18 to the ball screw nut 2. The rotations ofthe ball screw nut 2 are accordingly liable to become unstable. As a result, the travelling table mechanism shown in Figure 6 suffers from the problem that it is difficult to accurately adjust the moving table 12.

According to the present invention, there is provided a ball screw nut adapted for threaded engagement with a ball screw shaft through a plurality of balls, said nut comprising a nut body formed at one end with an elongate tubular extension, the distal end of said extension being adapted for engagementwith a rotational drive member in such a mannerthatthe rotational axis of said nut is aligned coaxiallywith the axis of said drive member.

By means of this arrangement the ball screw nut and the drive member have their axes of rotations aligned coaxially so as to minimize the error in the rotations transmitted from the drive memberto the ball screw nut thereby to enhance the rotating accuracy of the nut. When such a ball screw nut is incorporated in a travelling table mechanism, the accuracy of the fine and quick feeds of the table is enhanced.

The ball screw nut is preferably supported by a frame through angular contact bearings so that it may be rotationally driven. The angular contact bearings are desirably of the inner race omitted type, in which the ball rolling grooves are formed directly ontheoutercircumferencesoftwo end shoulders of the nut body by omitting the inner races. Preferably, the extension formed at one end of the nut body has an external diameter smaller than that of the ball rolling grooves so that the angular contact bearings may be fitted thereon, while the internal diameter of the extension is made largerthan the external diameterofthe ball screwshaftwhich isto be movably screwed in the ball screw nut.Furthermore while the length of the extension may be varied as desired, it is preferably of sufficient length thatthere is always a portion to which the rotational drive member can be connected thereto.

The rotational drive member may be a well known motor which is connected to the extension of the ball screw nut by connecting means such as a coupling so that it can transmit its rotationsthrough an output shaft projecting from its body. In an alternative form of rotational drive member, the extension itself ofthe ball screw nut may form the output shaft of the motor. In this embodiment, the extension is formed in its outer circumference with ball rolling grooves which are appropriately spaced to allow the balls of the paired angular contact bearings to roll. At the same time, a rotor is attached to that extension so thatthe former may rotate integrally with the latter.

Moreover, the extension acting asthe output shaft is rotatably attached to the motor housing through the outer races of the angular contact bearings by fitting these bearings in the ball rolling grooves. And, the extension carrying the rotor is induced to rotate by a statorwhich is attached to the housing. Incidentally, in such cases, the ball screw nut itself functions as the motor shaft so that more stable and accurate rotations can be attained.

The nut body is preferably supported in aframe through angular contact bearings and formed on its inner and outer circumferences respectively with a ball rolling groove for the ball screw shaft and the ball rolling grooves for the bearings so that the ball screw shaft and nut can be accurately coaxial.

In a preferred embodiment, the ball screw nut constitutes a ball screw unittogetherwith the ball screw shaft, the frame and so on such that rotational drive members are connected directlyto one end of the ball screw shaft and the extension of the ball screw nut, the vibrations based on both the error in the rotation transmission and the misalignment while the rotational force is being transmitted from the individual rotational drive members to the ball screw shaft and nut are minimised. As a result, the rotations ofthe screw shaft and nut can be made accurate relative to each other. Therefore, if a travelling mechanism is constructed by using such a ball screw unit, its travelling member can befed finely or quickly with high accuracy by adjusting the directions and speeds of rotations of the individual rotational drive members.

Some embodiments of the invention will now be described byway of example and with reference to the accompanying drawings, in which: Figure 1 is a partially broken perspective view showing a ball screw nut according to one embodimentofthe present invention screwed on a ball screw shaft and in which angular contact bearings are fitted on the outer circumference ofthe ball screw nut; Figure2 is a partially broken perspective view showing the ball screw nut shown in Figure 1 assembled in a frame to form a ball screw unit; Figure 3 is an explanatoryviewshowing one example of a travelling table mechanism incorporating the ball screw unit shown in Figure 2; Figure4is an explanatoryviewshowing a modification ofthetravelling table mechanism shown in Figure 3.

Figure 5 is an explanatory section showing one example of a ball screw unit incorporating the ball screw nut of the prior art; and Figure 6 is an explanatory view showing one example of a travelling table mechanism using the ball screw unitofthe prior art.

Referring to Figures 1 to 3, a ball screw shaft 1 has a grooved portion A except at its two end portions, and is formed at one end portion with a journal 22 which isto be inserted into and retained bya connecting coupling 21 so as to provide a portion to be connected with a drive motor M1 (as shown in Figure 3).

A ball screw nut 2,to be screwed on the ball screw shaft 1 through a number of balls 3, comprises a nut body 30 and an elongatetubularextension 35 which is formed integrally with one end ofthe nut body 30.

This nut body 30 is formed on its inner circumference with an appropriate ball rolling groove 31 receiving the balls 3 in a rolling manner, and onthetwo ends of its outer circumference with ball rolling grooves 32 forming part of an angular contact bearing 45. The extension 35 of the ball screw nut 2 is in the form of a cylinder which has an external diametersmallerthan the diameter of the ball rolling grooves 32, so thatthe angular contact bearings 45 may be fitted on the grooves 32, and an internal diameter largerthan the external diameter of the ball screw shaft 1 so thatthe shaft 1 can move therein.When the end face ofthe nut body 30 opposed to the extension 35 is positioned atthe end ofthe grooved portion Aofthe ball screw shaft 1, as is shown in the drawings, the extension 35 still projects from one end portion of the ball screw shaft 1. The extension 35 is formed at the end portion of its inner circumference with a receptacle 37 into which a coupling 36 acting as a member to be connected with another drive motor M2 (as shown in Figure 3) is inserted and retained.

The shaft 38 of the drive motor M2 isfixedly connected to the coupling 36.

Aframe 4 is formed of a cylindrical outer sleeve 40 which has at one end an opening to match the external diameter of the aforementioned extension 35. Aflange 41 is formed integrally with the other end of the outer sleeve 40. Between the inner circumference of the outer sleeve 40 and the aforementioned nut body 30, there are sandwiched the angular contact bearings 45, each of which comprises a ball rolling groove 32 formed on the outer circumference of the nut body 30; balls 46 arranged in contact along the ball rolling grooves 32; and an outer race 47 fitted in the inner circumference of the outer sleeve 40 for holding the balls 46 in a rolling mannertogetherwith the corresponding ball rolling groove 32.Reference numeral 42 appearing in Figure 2 denotes a retaining plate which isfitted removably in one end opening of the outer sleeve 40 and assembled in the outer sleeve 40 so as to retain the ball screw nut 2 and the corresponding bearing 45. Numeral 43 denotes a dust seal which is sandwiched between the ball screw shaft 1 and the end portion ofthenutbody30opposedtothe extension 35.

The ball screw nut 2 thus constructed is machined by clamping the two ends of a cylindrical pipe by a centering member, grinding and finishing the outer circumference of one end portion (which correspondstothe extension 35 of the nut2) ofthe cylindertoform a machining reference face, grinding the inner circumference of the extension 35 with reference to the reference face, and grinding the inner circumference ofthe end portion of that extension 35 to form the receptacle 37. Then the other end portion (which corresponds to the body 30 ofthe ball screw nut 2) of the cylinder is ground at its inner circumference to from the ball rolling groove 31 and simultaneously at its outer circumference to form the ball rolling grooves 32 for the angular contact bearings 45. By adopting these machining procedures, the axis of the receptacle 37 to be formed atthe end portion of the extension 35 ofthe ball screw nut 2 can be accurately aligned with the common axis of the ball rolling groove 31 formed on the inner circumference of the nut body 30 and the ball rolling grooves 32 formed on the outer circumference ofthe nut body 30.

As a result, a ball screw unit incorporating the ball screw nut 2 thus fabricated may be assembled, by screwing the ball screw nut 2 on the ball screw shaft 1 through the numerous balls 3, fitting the individual angularcontact bearings45 on the paired ball rolling grooves 32 formed on the outer circumference of the nut body 30, inserting them from the extension 35 of the ball screw nut 2 into the opening ofthe aforementioned outer sleeve 40, and fixing the ball screw nut 2 and the bearings 45 by means of the retaining plate42.

Atravelling table mechanism using the ball screw unitwill be described as an example so asto explain the operation of the unit. As seen from Figure 3, a moving table 12 is movably arranged over a stationary bed 11 through a not-shown guide mechanism. One drive motor M1 is fixed on the moving table 12, and the other drive motor M2 is fixed on the stationary bed 11. The frame 4 is fixedly disposed on the stationary bed 11. The journal 22 of the ball screw shaft 1 and the shaft 23 of the drive motor M1 are firmly connected through the coupling 21, and the other coupling 36 is inserted into and retained by the receptacle 37 ofthe nut 2. Then,the end of the shaft 38 of the other drive motor M2 is firmly connected to the coupling 36.

In thetravellingtable mechanism thus constructed, if the moving table 12 is to be finely adjusted, the drive motors M1 and M2 have a common direction of rotation but a slightly different rotational speed. The ball screw shaft 1 and nut 2 are rotated by the rotations of the drive motors M1 and M2, respectively, to establish a slight difference in rotations therebetween. The ball screw shaft 1 is finely fed in a predetermined direction in accordance with that difference.

In case he moving table 12 isto be quicklyfed, on the other hand, it is sufficient to rotate the individual drive motors M1 and M2 in opposite directions to each other and to set their rotational speeds at considerably high values. At this time, the ball screw shaft 1 and nut 2 are rotated in the opposite directions by the drive motors M1 and M2, respectively. As a resu It the ball screw shaft 1 is rotated at a high speed corresponding to the sum of the rotational speeds of the individual drive motors so that the moving table 12 is fed correspondingly quickly.

The performance ofthe ball screw unit incorporating the ball screw nut according to the present embodiment will now be evaluated.

Firstofall,thejournal 22 ofthe ball screw shaft 1 and the receptacle 37 ofthe ball screw nut 2 are machinedaccuratelysothattheshaftl and the nut 2 are connected directly to the shafts 23 and 38 ofthe drive motors M1 and M2 without any misalignment.

As a result, the rotations of the drive motors M1 and M2 are transmitted directly to the ball screw nut 2 while maintaining the rotational accuracies ofthe shaft 1 and the nut 2 at satisfactory levels.

Secondly, the nut body 30 is accurately formed with the ball rolling grooves 31 and 32, by which the ball screw shaft 1 and nut2 are screwed through the balls 3, and the ball screw nut 2 is supported by the frame 4through the angular contact bearings 45. As a result, the ball screw shaft land nut2 are aligned withotfail. Thus, the axes of rotations of the shaft 1 and the nut 2 are aligned with those of the drive motorsM1 and Mlto obviate any dangerthatthe ball screw shaft 1 and nut 2 may be caused to vibrate as a result of misalignment while the rotations ofthe drive motors M1 and M2 are being transmitted to the shaft 1 and the nut 2.

As a result, the relative rotations of the ball screw shaft 1 and nut 2 may be accurately varied so thatthe feed of the aforementioned moving table 12 can also be highly accurately adjusted.

Of course, the construction of the travelling table mechanism need not be limited to that shown in Figure 3 but may be exemplified by a modification shown in Figure 4, in which the drive motor M1 to be connected to the ball screw shaft 1 is fixed on the stationary bed 11 whereas the other drive motor M2 to be connected to the bail screw nut 2 is fixed on the moving table 12 and in which the aforementioned frame 4 is fixed on the moving table 12.

Claims

1. A ball screw nut adapted for threaded engagementwith a ball screwshaftthrough a plurality of balls, said nut comprising a nut body formed at one end with an elongate tubular extension,the distal end of said extension being adapted for engagementwith a rotational drive member in such a mannerthatthe rotational axis of said nut is aligned coaxially with the axis of said drive member.

2. A nut according to claim 1 wherein said nut body is formed on its outer circumference with ball grooves for engagement with the balls of a pair of angular contact bearings, and wherein said cylindrical extension member has an external diameter less than the diameter of said ball grooves and greater than the diameter of a screw shaftto which the nut is to befitted.

3. A nut according to claim 2 wherein a said ball groove is provided at each end of said nut body.

4. A nut according to claim 3 wherein said nut body and said angular contact bearings are housed within a frame, said frame having an opening at one end adapted to receive said extension and a mounting flange integrally formed at the other end.

5. A ball screw nut substantially as hereinbefore described with reference to Figures 1 to 4 of the accompanying drawings.

6. Travelling table apparatus comprising a base, a table movable relative to said base along guide means, a ball screw shaft, a ball screw nut according to any preceding claim mounted on said screw shaft through a plurality of balls, said screw shaft being rotatably mounted on either said base or said table, and said screw nut being rotatably mounted on the other of said base or table, and respective rotating drive means adapted to rotate said screw nut and said screw shaft to cause said table to move relative to said base.

7. Travelling apparatus according to claim 6 wherein said screw nut is mounted to said table and said screw shaft is mounted to said base.

8. Travelling table apparatus according to claim 6 wherein said screw nut is mounted to said base and said screw shaft is mounted to said table.

9. Travelling table apparatus according to any of claims 6,7 or 8 wherein said rotating drive means are coaxial.

10. Travelling table apparatus substantially as herein before described with reference to Figures 3 & BR< 4 of the accompanying drawings.

11. Acylindrical ball screw nut adapted to be screwed on a ball screwshaftthrough a number of balls, wherein said nut comprises a nut body having its two end shoulders formed on its outer circumferences with ball rolling grooves on which the balls of angular contact bearings are allowed to roll, and wherein said nut body has one end formed with a tubular nut extension which has an external diameter smallerthan at least said ball rolling grooves and an internal diameter largerthan the external diameter of said ball screw shaft and to which isconnecteda rotational drive member four rotating said nut body.