Machine element or mechanism – Gearing – Teeth
Reexamination Certificate
1999-05-28
2001-01-23
Bucci, David A. (Department: 3682)
Machine element or mechanism
Gearing
Teeth
C074S089100, C074S424830, C074S521000
Reexamination Certificate
active
06176149
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a ball screw used for various feed mechanisms. More particularly, it relates to a ball screw which has a high load-carrying capacity and is suitable for repetitive back-and-forth operations with minute feedings, namely, reciprocating movements.
PRIOR ART
There is a ball screw known which comprises a screw shaft including a ball screw groove on the outer peripheral surface thereof., a ball nut including on the inner peripheral surface thereof another ball screw groove opposed to the ball screw groove, and a plurality of balls installed between the both ball screw grooves and adapted to circulate by a ball circulation mechanism provided in the ball nut.
For example, in U.S. Pat. No. 2,995,947, there is disclosed a ball screw in which a “return tube” is used as a ball circulation mechanism and of which number of effective turns is 6.5 and number of circuit(s) is one. However, such a ball screw having 6.5 effective turns is not yet put into a tangible form because of the reasons mentioned below.
Incidentally, the “number of effective turns”, “one circuit” and the “number of circuits” respectively mean the number of times the balls installed in one circuit turn around the periphery of a screw shaft, one closed circuit comprising the ball screw groove in which balls are installed and the ball circulation mechanism, and the number to indicate how many such circuits are incorporated (see “Screw Thread Reference Guide”, the first edition (1966. 5.20), p315; edited by Screw Thread Reference Guide Editing Committee and issued by Nikkan Kogyo Shimbun Co., Ltd.).
Though the degree depends on the machined accuracy and so on of the ball screw groove, it has been experienced that a normal feeding operation with more than 120 balls installed in one circuit of a ball nut brings about an increase in dynamic friction torque caused by the adjoining balls rotating in the same direction and pressing against each other, a ball clogging phenomenon, an early wear and a ball slipping phenomenon, which may cause an early fracture. Further, the dynamic friction torque in a normal feeding operation is hereafter called “normal friction torque”.
Accordingly, commercial ball screws have the following features:
(1) When a “return tube” or a “guide plate” is used as a ball circulation mechanism, the number of effective turns is 3.5 at the maximum (since the number of balls in one circuit exceeds 120 in the case of more than 3.5 turns).
(2) When an “end cap” is used as a ball circulation mechanism, the number of effective turns is 2.8 at the maximum. Further, since a manufacturing cost of the end cap itself is high, above ball circulation mechanism is seldom used except for a fast lead or an ultra-fast lead which are suitable for high-speed requirements.
(3) When a “barrel” is used as a ball circulation mechanism, the number of effective turn(s) is one. Since the number of balls that can be installed in one circuit is at most 60, except the case when the later-described reciprocating movement is made, an increase in a normal friction torque, the ball clogging phenomenon and the ball slipping phenomenon rarely occur. However, since the load-carrying capacity that the balls in one circuit can bear is low, the number of circuits are adjusted to be somewhere between 2 and 6 to secure the load-carrying capacity for a ball screw.
Further, the followings must be noted:
1) The number of balls when the number of effective turn(s) is one is estimated according to a diameter of a ball used, a center circular diameter of the ball and a lead of the ball.
The formula used here is as follows:
(Number of balls in one circuit)=(Number of balls in one turn×Number of effective turns)+(Number of balls in Ball circulation mechanism)
2) The number of balls in the total number of effective turns (Number of effective turns×Number of circuits) is estimated according to working conditions (Axial load and Useful life) of the ball screw. Then, when a ball circulation mechanism is a “return tube” or a “guide plate”, the total number of effective turns chosen are 2.5×2, 3.5×1 and so on, considering a positional relation and so on between an entire length of the ball nut and an insertion hole in which a pickup tube introducing balls into a parallel portion of the return tube or the guide plate is inserted.
Even though the number of balls in one circuit is limited to 120 or less, however, it was experienced during a reciprocating movement that an increase in friction torque during the reciprocation (hereafter called “reciprocating friction torque”) and a ball clogging phenomenon occurred.
Therefore, when the reciprocating movement is made, a spacer ball having a diameter 50 to 60 &mgr;m smaller than that of a ball being used is utilized at the same time. For example, according to “Oscillation Characteristics of Ball Screws” on pages 30-35 of “NTN TECHNICAL REVIEW No. 58 (1990) issued by NTN Co., Ltd., a preferable incorporation rate of the ball and the spacer ball is 1:1.
Further, it is believed that a spacer ball is used because it alleviates a biting phenomenon of balls on a surface of a ball screw groove caused by the friction among the balls.
In recent years, ball screws have come to be used under high loading conditions such as in an injection molding machine, a press and so on. Further, some of high-loading usage of the injection molding machine and so on require a ball screw to make a reciprocating movement.
In the past, in order to cope with the high-loading usage, a shaft diameter of a screw shaft was increased (naturally, the ball nut also had to be larger), the number of circuits in the ball nut was increased, and the size of the lead was increased so that a diameter of the ball used might be larger.
However, the following inconveniences were observed:
(1) It was often impossible to adopt the method of making the shaft diameter of the screw shaft larger because of the limited space in a machine. Also, a high-capacity motor was required due to an increase in a moment of inertia, which brought about a higher cost.
(2) To increase the number of circuits in a ball nut means to make the ball nut larger and longer. Therefore, there was a problem that such a measure could not be taken when a stroke of the ball nut is limited. Also, there was another problem of a sharp increase in a processing cost. Furthermore, there were some occasions when a manufacturing of the ball nut was impossible.
(3) To make the lead larger and the diameter of the ball used larger means to upsize the ball nut. Accordingly, there were many cases when such measures could not be adopted because of the limited space in machines. Also, when the ball nut was not upsized, a thick wall between an outer surface of the ball nut and the ball screw groove remained thin and caused a problem in terms of strength.
To cope with problems in the reciprocating movement, the usage of the above-described spacer ball is effective. (More specifically, it is preferable that an incorporation rate of the ball and the spacer ball is 1:1).
However, when the incorporation rate is 1:1, an axial loading that can be borne is reduced to half its original value (the basic dynamic load rating showing a load-carrying capacity is reduced to about 60% of its original value), which brings about a problem that the application to the high load-carrying usage such as in the above injection molding machine and so on is not possible.
The present invention was made in view of the above problems, and it is an object of the invention to provide a ball screw which doesn't have to be upsized, being compact and possessing the high load-carrying capacity, and further being preferable for a reciprocating movement.
SUMMARY OF THE INVENTION
In accordance with the present invention, the aforementioned problems can be solved by a ball screw, comprising: a screw shaft including a ball screw groove in the outer peripheral surface thereof; a ball nut including in the inner peripheral surface thereof another ball screw groove
Bucci David A.
Hansen Colby
Howson & Howson
Tsubaki Nakashima Co., Ltd.
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