Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2000-10-24
2002-07-09
Bucci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
C384S520000
Reexamination Certificate
active
06415676
ABSTRACT:
SUMMARY OF THE INVENTION
This invention relates to a ball screw used in various feeding devices, and more particularly to a ball screw having spacers, and to a method for inserting balls and spacers into the ball screw.
A typical known ball screw device comprises a screw shaft having a spiral ball rolling groove on its outer peripheral surface; a ball nut having a spiral ball rolling groove on its inner peripheral surface, the ball rolling groove of the ball nut facing and being opposed to the ball rolling groove of the screw shaft, whereby the ball rolling grooves together form a spiral ball rolling channel having first and second ends; a ball circulating passage connecting said ends of the ball rolling channel; and a plurality of balls in the ball rolling channel and in the ball circulating passage. For example, the ball screw device of
FIG. 10
, known end-cap system ball screw device, comprises a screw shaft
40
having an outer peripheral surface on which a spiral ball rolling groove
42
is formed, a ball nut
50
having an inner peripheral surface on which a spiral ball rolling groove
52
is formed. The groove
52
is opposed to, and faces, groove
42
, forming the spiral ball rolling channel. The ball nut
50
has a thick wall in which an axially extending ball return passage
54
is formed. End caps
60
are mounted on the axial end surfaces of the ball nut, and ball guide grooves
62
in the end caps connect the spiral channel to the ends of the axial ball return passage, the grooves
62
and axial passage
54
thereby forming a ball circulating passage
58
. Balls
70
are situated in the channel formed by the opposed grooves, and in the ball circulating passage.
Besides the above-mentioned end-cap ball recirculating scheme, other recirculating schemes have been used, including ball return tubes, barrels and guide plate systems. These are described in Izawa, Minoru,
Ball Screw Application Technique
First Ed., Kogyo Chosakai Co. Ltd., May 20, 1993, pages 19-21.
Most such ball screw devices adopt the “all ball” specification, meaning that the balls are disposed close to one another. The balls which support the load are large in number and hence the ball screw device has a large load-bearing capability and good rigidity. However, because of nonuniformities in the shape of the ball rolling grooves and the like, there are slight differences in the revolving speeds of the balls. In the load region, when the revolving speed of a rear ball is larger than that of a front ball as seen in the advancing direction of the balls, the rear ball bumps into the front ball, these balls jostle each other, and a compressive force is liable to act at the point at which the jostling balls contact each other.
When the compressive force acts at the contact point between two balls, sliding contact acts in a direction to prevent rolling of the balls. Consequently a large resistance is generated, which prevents rotation of the balls, giving rise to fluctuation or a remarkable increase in the dynamic torque of the ball screw device. Furthermore, a ball clogging phenomenon may occur, and sliding contact of the balls also causes problems with the generation of noise (usually expressed by “sound pressure level”) and rapid wear of the balls.
The increase in dynamic torque during low speed movement or oscillatory movement is considered to be due primarily to a phenomenon, occurring as a result of sliding contact due to jostling between load-supporting balls, in which the balls cut into the ball rolling groove surfaces.
To alleviate the above-mentioned problems in devices made in accordance with the “all ball” specification, the number of inserted balls is typically made two to four less than the full number of balls that could be inserted. The reduction in the number of balls provides a clearance between balls in the load region, in order to reduce jostling between balls.
In order to reduce fluctuations in dynamic torque, and particularly to reduce the remarkable fluctuations and increase in dynamic torque generated during low speed movement or during repeated reciprocating movement, and to reduce tilting movements occurring during minute feeding, spacer balls are used. The spacer balls have a diameter slightly less (by several tens of &mgr;m) than the diameter of the load bearing balls. When a spacer ball comes into rolling contact with a load-bearing ball, they roll in opposite directions. Accordingly, the sliding contact generated in the case of a ball screw device in accordance with the “all ball” specification is avoided, and the resulting resistance to rolling becomes extremely small, and minimum fluctuation of dynamic torque can be achieved.
Another proposed solution to the aforementioned problems it's the use of a resilient, strip-like retainer which respectively holds balls rotatably in a large number of ball pockets, as depicted in Japanese laid-open utility model publication 27408/1993. The retainer strip is capable of circulating movement.
Similarly, another ball screw device has been proposed in which a resilient ball chain and connector belt are used, as depicted in Japanese laid-open patent publication 169746/1998. The resilient ball chain consists of a large number of balls arranged in line at a given interval, and the connector belt rotatably holds these balls and connects neighboring balls with one another. The ball chain and connector belt are capable of circulating movement.
In both of the last two approaches mentioned above, contact between the balls can be prevented, and consequently fluctuations in dynamic torque can be reduced and ball clogging can be prevented. Moreover, noises due to sliding contact between the balls, and rapid wear of the balls can be prevented.
Still another proposal has been to utilize spacers with spherical concave surfaces on both axial end surfaces thereof. The spacers, which are uniform in thickness are disposed between adjacent balls and portions of the spherical concave surfaces of the balls fit slidably into the concave faces of the spacers, as shown in Japanese laid-open utility model publication 178659/1988. The spacers are all of the same thickness, the term “thickness” referring to the dimension equal to the distance between vertices of the adjacent balls when the adjacent balls are in close contact with the concave surfaces of the spacer between them. In other words, thickness of a spacer is the distance of closest approach of the surfaces of the balls when separated by a spacer.
The ball screw device utilizing spacers avoids point contact between balls, utilizing instead face contact between the balls and the concave surfaces of the spacers. Thus, the high pressure inherent in point contact in the ball screw devices according to the “all ball” or “spacer ball” specifications is avoided. Furthermore, the spacers allow the distance between load-supporting balls to be small compared with devices made according to the “spacer ball” specification, and consequently ball screw devices utilizing spacers generally have a greater load-bearing capability and greater rigidity than those utilizing spacer balls.
The above-described ball screw devices have various drawbacks, which will be described below.
The ball screw device in which the number of balls is from two to four fewer than the number which would fully load the ball rolling channel and ball circulation passage is subject to ball clogging. The ball clogging phenomenon results from the remarkable increase in dynamic torque which is caused by the jostling of the load-supporting balls when the device is subjected to low speed movement or oscillatory movement.
In the ball screw device in accordance with the spacer ball specification, the load capacity and rigidity of the device are reduced as a result of the decrease in the number of load-supporting balls. For example, when the ratio of load supporting balls to spacer balls is 1:1, the basic dynamic rated load indicative of the load capacity is reduced to approximately 60%. Further, since the balls which support the load and the spacer ball
Horimoto Hidemi
Ohga Soichiro
Takagi Hirouyki
Bucci David A.
Howson & Howson
McAnulty Timothy
Tsubaki Nakashima Co., Ltd.
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