Machine element or mechanism – Mechanical movements – Reciprocating or oscillating to or from alternating rotary
Reexamination Certificate
1999-03-17
2001-02-13
Bonck, Rodney H. (Department: 3681)
Machine element or mechanism
Mechanical movements
Reciprocating or oscillating to or from alternating rotary
C074S424830
Reexamination Certificate
active
06186015
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to power transmission devices and more particularly concerns a ball-nut transmission in which a pair of concentric members are connected together by a plurality of balls for transferring directional forces from one of the members to the other by rotation of one of the members.
BACKGROUND OF THE INVENTION
One form of power transmission that has found general acceptance for transferring drive from one member to an adjoining concentric member is the so called recirculating ball-nut screw device. Such devices provide low-friction, low wear and efficient operation under varying temperature and humidity conditions, and were first used on aircraft for moving the wing flaps from a stowed position to an extended position and vice versa. In general, devices of this type consist of recirculating balls guided within a helical ball duct defined between a screw and nut with the balls being continuously recirculated throught the helical duct and through ball return means provided on the nut. Apart from the aviation field, the recirculating ball-nut screw device has many other uses as a linear motion device replacing the threaded nut-and-screw jackshaft-type mechanism. In these uses, the non-rotating nut is typically attached to the linearly movable part of the mechanism while the screw rotates to drive the nut. There are some cases in the machine design field where the use of the recirculating ball-nut screw device could be the ideal choice but cannot be used because of the bulk and disposition of the recirculating ball-return duct interfering with another part of the mechanism.
One form of a ball-nut screw device that eliminates the need for recirculating the balls can be seen in U.S. Pat. No. 4,199,999 issued on Apr. 29, 1980 in the name of Joseph R. Metz. This patent discloses a ball-type mechanical transmission which includes a driver shaft having a rolled-in helical thread engaged by several balls carried in through-openings formed in a sleeve surrounding the shaft. A cylindrical jacket encircles the sleeve so as to provide a back-up bearing surface for the balls and also constitutes a slide having intermediary bearing means within an outer housing. The housing further provides bearings for the shaft and for a connector part constituting the output of the transmission.
Although the above mentioned Metz patent provides a ball-nut transmission without requiring the balls to be recirculated, the use of a separate sleeve for holding the balls and a separate cylindrical jacket sleeve for maintaining the balls in the sleeve openings increases the complexity and cost of the transmission. In addition, by positioning the balls within a cylindrical opening in the sleeve as taught by the Metz patent, a point contact exists between each ball and the surface of the jacket and a circular line contact exists between the ball and the inner edge of the cylindrical opening in the sleeve. The problem with the point contact and line contact between the parts is that high stress concentrations are created in those areas resulting in increased wear in the assembly.
SUMMARY OF THE INVENTION
The ball-nut transmission according to the present invention is functionality similar to the ball-nut transmission seen in the Metz patent but differs therefrom in that it achieves maximum compactness with minimum stress concentrations, saves weight, and provides maximum shear strength for the balls. Moreover, rather than utilizing separate members for maintaining the balls confined in cylindrical openings of a sleeve by providing a separate jacket, the ball-nut transmission according to the present invention encapsulates the individual balls within a hemispherical cavity formed in the inner cylindrical surface of the nut member. Each of the hemispherical cavities is formed by a tool such as a ballend mill which passes through an access hole located in the body of the nut member diametrically opposite to the cavity. The access holes serve to not only provide a means for forming the hemispherical cavities in the inner cylindrical surface of the nut member but also serve to lessen the weight of the nut member by removal of some of the metal from the nut member. Prior to assembly of the screw shaft member and the nut member, a viscous lubricant such as grease is placed in each of the cavities. The balls are then placed in each of the cavities and retained therein by the lubricant while the screw shaft member is inserted within the nut member and rotated into the assembled position. Once assembled, each ball is encapsulated between the accommodating cavity and the groove in the shaft member where there is limited contact (line contact) that spreads out wear over the whole length of the groove.
In a modified version of the ball-nut transmission according to the present invention, the outer cylindrical surface of the shaft member is provided with a plurality of hemispherical cavities located along a helical path. Each of the cavities in the cylindrical surface of the shaft member is provided with a ball retained therein in the same manner as explained above. In this instance, the inner surface of the cylindrical opening within the nut member is provided with a helical groove having the same pitch as the pitch of the helical path along which the cavities are formed in the outer cylindrical surface of the shaft member. Thus, when the shaft member is threaded into the nut member, the balls of the shaft member interact with the grooves for retaining the balls and providing a ball-nut transmission device that operates in the manner of the arrangement as explained above albeit providing limited axial movement of the shaft member relative to the nut member.
Accordingly, one object of the present invention is to provide a new and improved ball-nut transmission in which each of the balls used for transferring drive between a shaft member and a nut member are encapsulated between a hemispherical cavity formed in one of the members and a helical groove provided in the other of the members.
Another object of the present invention is to provide a new and improved ball-nut transmission consisting of a shaft member and a nut member and in which a cylindrical opening within the nut member has its inner surface formed with a plurality of hemispherical cavities each of which is adapted to support a ball that interacts with a groove formed in the outer surface of the shaft member for coupling one of the members to the other.
A further object of the present invention is to provide a new and improved ball-nut transmission consisting of a shaft member and a nut member and in which the outer cylindrical surface of the nut member is formed with circumferentially spaced access holes each of which permits a tool to extend through the access hole to form a hemispherical cavity in the inner surface of the cylindrical opening in the nut member.
A still further object of the present invention is to provide a new and improved ball-nut transmission in which the shaft member is formed with a helical groove and the nut member has a cylindrical outer surface and a cylindrical inner surface defining a cylindrical opening that is formed with a plurality of hemispherical cavities located along a helical path the pitch of which is equal to the pitch of the helical groove formed on the shaft member.
A still further object of the present invention is to provide a new and improved ball-nut transmission consisting of two major parts and a plurality of balls and in which one of the major parts is a shaft member having a helical groove cooperating with the other of the major parts which is a nut member for converting rotary movement into linear movement and vice versa and further in which a plurality of balls interconnect the two members and are encapsulated therebetween by having each of the balls located in a hemispherical cavity formed in the nut member and located along a helical path having the same pitch as the pitch of the helical groove.
The above and other objects are realized in accordance with the present invention
Bonck Rodney H.
DaimlerChrysler Corporation
MacLean Kenneth H.
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