JP2005133799A - Ball screw device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ball screw device to be built in an electric linear actuator capable of reducing the cost and improving service life. <P>SOLUTION: The ball screw device to be built in the electric linear actuator is provided with a screw shaft 3 having a spiral screw groove 2 in the peripheral surface thereof, a nut 4 having a spiral screw groove 8 corresponding to the screw groove 2 in the inner peripheral surface thereof and to be screwed to the screw shaft 3, and multiple balls 9 rotatably installed between both the screw grooves 2 and 8. The screw shaft 3 is manufactured by rolling, and the screw groove 2 of a finished product is formed into a gothic arch groove at 30-45° of contact angle and 54-58% of groove curvature without performing grinding after hardening and tempering. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is used for, for example, a drive device for automatically switching a gear ratio of a transmission for an automobile or for automatically connecting / disconnecting a clutch, a nursing bed, a lifting table, a lifter, an in-vehicle jack, and the like. The present invention relates to a ball screw device that is built in an electric linear actuator incorporated in various mechanical devices and the like to operate the electric linear actuator.

As an electric linear actuator that expands and contracts using an electric motor as a drive source and displaces various articles, for example, the one shown in FIG. 7 is known (see, for example, Patent Document 1 and Patent Document 2), and is incorporated in the electric linear actuator. An example of the ball screw device used is shown in FIG.
This ball screw device 1 corresponds to a screw shaft 3 having a spiral thread groove 2 on an outer peripheral surface and rotatably supported in a housing H of an electric linear actuator, and corresponding to the screw groove 2 on an inner peripheral surface. A nut 4 having a helical thread groove 8 (see FIG. 3) is screwed together.

The screw groove 8 of the nut 4 and the screw groove 2 of the screw shaft 3 are opposed to each other to form a spiral load track between the two, and a large number of balls 9 (see FIG. 3) is loaded so that it can roll.
A part of the outer peripheral surface of the nut 4 is a flat surface, and a set of two holes communicating with the load track is formed on the flat surface so as to straddle the screw shaft 3. By inserting both ends of a substantially U-shaped circulation tube 5 as an example of a ball circulation member into the set of holes, the ball 9 revolving along the load track is moved from the middle of the load track by the circulation tube 5. The ball 9 is scooped up and returned to the original load trajectory, whereby the ball 9 is infinitely circulated.

Then, when the screw shaft 3 is rotated in the forward and reverse directions by the electric motor 6 supported by the housing H of the electric linear actuator, the nut 4 screwed to the screw shaft 3 is rotated through the rolling of the ball 9. The output shaft member 7 coupled to the nut 4 is displaced in the axial direction.
JP-A-8-322189 JP 9-327149 A

By the way, while the performance and durability of the ball screw device built in the electric linear actuator are regarded as important, the demand for cost reduction is also important.
However, due to the importance of the cost of the ball screw device, for example, if the manufacturing process is simplified, performance and durability due to a decrease in product accuracy may be reduced, and variations in quality may result in extremely short lifespans. The risk of doing so will increase.
The present invention has been made to solve such disadvantages, and an object of the present invention is to provide a ball screw device incorporated in an electric linear actuator that can reduce cost and extend the life.

In order to achieve the above object, an invention according to claim 1 includes a screw shaft built in an electric linear actuator and having a helical thread groove on an outer peripheral surface, and a helical shape corresponding to the thread groove of the screw shaft. In a ball screw device comprising a nut having a thread groove on an inner peripheral surface and screwed to the screw shaft, and a large number of balls loaded in a rollable manner between the both thread grooves,
The threaded shaft is manufactured by rolling, and the finished thread is made into a gothic arch groove without quenching and tempering, so that the contact angle is 30 ° to 45 °, and the groove curvature is 54% to 58%. % Or less.

  According to the present invention, the screw shaft is manufactured by rolling, and the finished groove is a Gothic arch groove without grinding after quenching and tempering, the contact angle is 30 ° to 45 °, and the groove curvature By setting the ratio to 54% or more and 58% or less, it is possible to provide a ball screw device built in an electric linear actuator that can achieve cost reduction and long life.

Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. In this embodiment, the basic structure of the electric linear actuator and the ball screw device built in the electric linear actuator is the same as that shown in FIGS. Each figure will be described with the same reference numerals, or description thereof will be omitted.
The ball screw device built in the electric linear actuator which is an example of the embodiment of the present invention is manufactured by rolling the screw shaft 3 with reference to FIG. 8 and without grinding after quenching and tempering. The finished product is used as it is. In other words, this is a ball screw device that achieves extreme cost reduction by omitting the manufacturing process.

  On the other hand, omitting the finish grinding step may significantly reduce the accuracy of the ball screw device. In particular, it is known that the function and life of the shape of the thread groove 2 for transferring the ball 9 vary greatly depending on its accuracy. This is because the movement of the ball 9 fluctuates due to a defective shape of the thread groove 2, or a torque increase or contact with a high surface pressure occurs due to abnormal rotation, and the ball 9 or the thread groove 2 is worn or damaged. .

Therefore, the screw shaft 3 in this embodiment is subjected to grinding processing after heat treatment by devising the shape and processing method of the rolling die in consideration of deformation during rolling processing and deformation after heat treatment. Is manufactured by a precision rolling method that achieves the target accuracy without performing the above. By applying this precision rolling method and controlling the shape of the thread groove 2 within the following range, a ball screw device built in an electric linear actuator having a long life and a low cost can be provided.
That is, in the embodiment of the present invention, the screw shaft 3 is manufactured by rolling, and after grinding and tempering, the grinding is not performed and the finished product is used as it is, and the screw groove 2 is used as a Gothic arch groove to obtain a contact angle. Is 30 ° to 45 °, and the groove curvature is 54% to 58%.

Details will be described below.
(Thread groove contact angle is 30 ° or more and 45 ° or less)
The contact angle of the screw groove 2 of the screw shaft 3 is an important set value for determining the track position of the ball 9 and transferring the ball 9 stably. In the ball screw device built in the electric linear actuator of this embodiment, precision rolling is performed, but there is also a final accuracy relationship. It is a specification that secures.

  As shown in FIG. 3A, the clearance specification is a state in which a ball 9 incorporated between the thread groove 2 of the screw shaft 3 and the thread groove 8 of the nut 4 has a clearance and is assembled. Say. Therefore, when actually used, the ball 9 incorporated between the screw groove 2 of the screw shaft 3 and the screw groove 8 of the nut 4 is not provided with a gap by the axial load Fa as shown in FIG. It will be in the built-in state. At this time, due to the axial load Fa, the contact angle θ1 shown in FIG. 3A becomes the contact angle θ2 when the gap shown in FIG. 3B disappears, and the axial load Fa is applied, so that the gap is reduced. It can be seen that the contact angle increases.

  As described above, in this embodiment, since the ball screw device built in the electric linear actuator is assembled with the clearance specification, the contact angle becomes large during actual use. In the ball screw device, when the contact angle becomes large, as shown in FIG. 4, the component force becomes larger in the case where θ2 has a larger contact angle with respect to the axial load from the ball 9 than in the case where θ1 has a smaller contact angle. It can be seen that the load that the screw shaft 3 can receive increases. That is, when the contact angle is large, the load capacity increases.

  However, if the contact angle is increased too much, the so-called “riding” phenomenon, in which the elastic contact area between the ball 9 and the screw groove 2 of the screw shaft 3 rides on the screw groove 2 as shown in FIG. Will occur. Since the surface pressure at the end of the thread groove 2 suddenly rises due to the ride, it is often seen that breakage occurs early. Therefore, it is necessary to set the contact angle strictly in consideration of processing accuracy in rolling, deformation in heat treatment, and the gap amount.

On the other hand, if the contact angle is lowered too much, the efficiency of the force transmitted from the ball 9 to the screw groove 2 of the screw shaft 3 is lowered, and the surface pressure is increased even with a small load, so that the life tends to be shortened.
In view of the above, the present inventors conducted a durability evaluation test using a ball screw device built in an electric linear actuator. The results are shown in FIG.
As can be seen from FIG. 1, when the contact angle is less than 30 °, the service life is decreased, and when it exceeds 45 °, the service life is rapidly decreased. Therefore, the contact angle is set to 30 ° to 45 °. However, since the variation in accuracy is large in the rolling process, it is more preferable that the contact angle is 30 ° or more and 40 ° or less in order to reliably prevent the ball 9 from climbing. In the present invention, the contact angle refers to θ1 in the state of FIG.

(The groove curvature is 54% or more and 58% or less)
Next, regarding the groove curvature of the thread groove 2, generally, when the groove curvature is reduced, the contact area between the ball 9 and the thread groove 2 increases, so that the surface pressure decreases and the life tends to be extended. However, in the case of the ball screw device according to the present invention, in consideration of accuracy, when the groove curvature is extremely reduced, as shown in FIG. There is a possibility of becoming more than the place. If the number of contact points is two or more, the trajectory of the ball 9 is not fixed, and an unnecessary movement causes an increase in the torque of the ball screw device or a malfunction, and in some cases, wear or breakage occurs.

Therefore, it is necessary to set the groove curvature carefully in consideration of processing accuracy in rolling and deformation in heat treatment. -On the other hand, if the groove curvature is too large, the contact pressure will increase and the service life will decrease.
In view of the above, the present inventors conducted a durability evaluation test using a ball screw device built in an electric linear actuator. The results are shown in FIG.
As can be seen from FIG. 2, when the groove curvature is less than 54%, the life is reduced, and when it exceeds 58%, the life is abruptly reduced. Therefore, the groove curvature is set to 54% or more and 58% or less.
In the ball screw device according to the embodiment of the present invention, the effect is not exhibited unless both the contact angle and the groove curvature are within the above ranges, and the life is shortened if either of them is removed.

As the TP of the ball screw device incorporated in the electric linear actuator, a nominal number of 14 × 05 × 100-Ct10 corresponding to a hole screw whose lead diameter and lead length are shown in JIS 1192 was used.
For the screw shaft, SAE4150 material was used as a material, and after rolling the material, a hardened layer was formed on the surface by induction hardening to obtain a finished product.
The test machine was a ball screw endurance life tester built in an electric linear actuator manufactured by NSK Ltd. (NSK), and a mode test was conducted assuming gear shift of an automobile transmission.

The mode conditions are shown in FIG. As will be described in detail below, this is a mode in which bidirectional loads are alternately applied. The life until the operating torque was doubled or more than the initial due to wear or damage was recorded, and the test time was recorded. Then, from the Weibull function distribution, out of the 10 test TPs, the time required for 10% of the ball screw device to reach the lifetime from the short lifetime side was determined. Furthermore, the calculated life was calculated from the test conditions, and the ratio was expressed as the ratio of the test time to the calculated life time.
In this embodiment, the evaluation is performed with various changes in the contact angle and groove curvature. However, the processing accuracy in rolling and the deformation in heat treatment are all measured, and 10TP values close to each other are taken as one group, and the average values are taken as the contact angle and groove curvature. Table 1 shows a list of contact angles and groove curvatures of TP used in this example.

○ in Table 1 “Preventing Riding” indicates that there is no possibility of getting on this time, △ indicates that there was no possibility of getting on this time, △ indicates that there was no possibility of getting on this time, but there is a possibility of getting on depending on variation in dimensions, and × indicates that this was taken , And.
Next, life evaluation conditions are shown.
Test condition identification number: NSK ball screw 14 × 05 × 100-Ct10 equivalent (ball diameter 1/8 inch) Test machine name: NSK, ball screw endurance life tester for electric linear actuator Test load: Axial load = 2000N ( P / C = 0.38)
Maximum load time: 1 second Maximum load time: 2 seconds No load time: 1 second No load hold time: 0.5 second Stroke: 20 mm
Lubricating grease: Albania No. 2 (Showa Shell Sekiyu)

  The test results are shown in Table 1 and FIGS. In each of Examples 1 to 13 of the present invention, the lifetime was 1.5 times longer than the calculated lifetime. On the other hand, although the groove curvature is within the range of the present invention, in Comparative Examples 14 and 15 in which the contact angle is too small, the contact surface pressure is increased, the short life is sometimes generated, and the life is shortened. Further, although the groove curvature is within the range of the present invention, in Comparative Examples 16 and 17 in which the contact angle is too large, there is a case where a high contact pressure portion is generated due to the ball climbing, and a short life is generated. Declined.

Furthermore, the contact angle is within the range of the present invention, but the groove curvature is too small. In Comparative Examples 18 and 19, there are two or more contact points between the ball and the screw groove, and abnormal movement of the ball occurs. Short life due to torque increase occurred. Although the contact angle is within the range of the present invention, the groove curvature is too large. In Comparative Examples 20 and 21, the contact surface pressure is increased, the short life is sometimes generated, and the life is shortened.
In addition, this invention is not limited to the said embodiment and Example, In the range which does not deviate from the summary of this invention, it can change suitably.

It is a graph which shows the relationship between a contact angle and a lifetime ratio. It is a graph which shows the relationship between a groove curvature and a life ratio. (A) is explanatory drawing for demonstrating the state of the ball | bowl integrated with the clearance gap between the screw groove of a screw shaft, and the screw groove of a nut, (b) is the screw groove of a screw shaft by axial load. FIG. 6 is an explanatory diagram for explaining a state of a ball incorporated with no gap between the nut and the thread groove, and FIG. It is explanatory drawing for demonstrating the difference of the component force by the magnitude of the contact angle with respect to the load from a ball | bowl. It is explanatory drawing for demonstrating the shape collapse of the thread groove of a screw shaft. It is a graph for demonstrating the mode conditions of a durable life test. It is sectional drawing which shows an example of an electric linear actuator. It is a perspective view which shows an example of the ball screw apparatus incorporated in the electric linear actuator of FIG.

Explanation of symbols

1 Ball screw device 2 Thread groove (screw shaft side)
3 Screw shaft 4 Nut 8 Thread groove (Nut side)
9 balls

Claims (1)

A screw shaft built in an electric linear actuator and having a spiral thread groove on the outer peripheral surface, and a spiral thread groove corresponding to the screw groove of the screw shaft on the inner peripheral surface, and screwed onto the screw shaft A ball screw device including a nut and a plurality of balls loaded so as to roll between the screw grooves,
The threaded shaft is manufactured by rolling, and the finished thread is made into a gothic arch groove without quenching and tempering, so that the contact angle is 30 ° to 45 °, and the groove curvature is 54% to 58%. % Or less ball screw device.

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