JP3728917B2 - Ball screw device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a ball screw device, and more particularly, to a ball screw device that is used for driving a high-speed feed of a machine tool and realizes high speed and high accuracy.
[0002]
[Prior art]
Conventionally, as a ball screw device, for example, as disclosed in Japanese Utility Model Publication No. 6-11433, a tube type in which a nut is screwed onto a screw shaft having a spiral groove via a rolling element such as a hard ball. One using a ball screw is known. A spiral groove corresponding to the spiral groove of the screw shaft is provided on the inner surface of the nut. Then, by the rotation of the screw shaft, the ball moves while rolling in both spiral grooves, and the nut moves linearly along the screw shaft.
[0003]
In the ball screw device as described above, conventionally, a steel ball made of SUJ2 of bearing steel has been used as the rolling element. In such a ball screw drive using a steel ball, for example, when a high speed feed of 60 m / min or more is performed, the amount of heat generation increases, the temperature rises, the noise increases, and the tongue portion of the ball screw tube further increases. Therefore, it is difficult to use the ball screw device at a high speed.
[0004]
In this field, in order to solve the above problem, for example, a ball made of a ceramic material having a small specific gravity and a small linear expansion coefficient has been used as a rolling element.
[0005]
[Problems to be solved by the invention]
In ball screws, for high-precision positioning, it is common to apply a preload to the ball screw in order to increase the rigidity by reducing the axial clearance of the ball screw to zero, further reducing the amount of elastic displacement with respect to the axial load. is there. For this reason, when a rolling element made of a material having a relatively large linear expansion coefficient is used, the amount of preload tends to increase due to thermal expansion or centrifugal force of the rolling element resulting from a temperature rise due to high-speed rotation of the screw shaft.
[0006]
However, when ceramic spheres are used as rolling elements, if high-speed feeding is performed, the linear expansion coefficient is small and the temperature rise is small, but the specific gravity is small, so the increase in preload is small. In addition, since the linear expansion coefficient of the rolling element is smaller than that of the ball shaft and ball nut, there is a problem that clearance tends to occur inside the ball screw and preload is lost. As a result, the positioning accuracy of the ball screw device deteriorates. There was something to do.
[0007]
As described above, when ceramic balls are used as rolling elements, internal clearance is likely to occur because the diameter of the rolling elements is small with respect to the ball shaft / ball nut and the linear expansion coefficient is small. It is because it becomes easy to be influenced by.
[0008]
An object of the present invention is to solve the above problems. That is, an object of the present invention is to suppress the generation of internal clearance caused by a small linear expansion coefficient of ceramic spheres when a ceramic material is used for a high-speed feed drive and a rolling element is used. It is an object of the present invention to provide a ball screw device that can prevent a decrease and can realize high speed and high accuracy.
[0009]
[Means for Solving the Problems]
As a result of intensive studies, the present inventors have found that reducing the amount of thermal expansion on the ball nut side is effective in achieving the above object. In particular, by cooling the ball screw device and setting the temperature on the ball nut side lower than the ball shaft side, or by setting the linear expansion coefficient of the material on the ball nut side smaller than the ball shaft side, It has been found that the generation of internal clearance caused by the low coefficient of linear expansion of ceramic spheres can be suppressed, the decrease in internal preload can be suppressed, high-speed feed can be achieved, and high accuracy can be realized. It came to achieve.
[0010]
That is, a ball screw device according to the present invention includes a screw shaft having a spiral ball screw groove on the outer peripheral surface, a ball nut having a ball screw groove on the inner peripheral surface facing the ball screw groove of the screw shaft, A ball screw comprising a ball screw groove of a ball nut and a rolling element that is movably fitted between the ball screw groove of the screw shaft, wherein the rolling element is made of a ceramic material having a specific gravity of 2 to 4. And at least the amount of thermal expansion on the ball nut side is reduced. In particular, as a means for reducing the amount of thermal expansion on the ball nut side, the ball screw device is provided with a cooling mechanism or the ball nut is made of stainless steel.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described. In addition, embodiment of this invention is not limited to the following examples.
[0012]
(1) Example of shaft side hollow cooling FIG. 1 is an overall configuration diagram showing an example of a ball screw device of the present invention. In FIG. 1, the ball screw device mainly includes a screw nut 1 and ball nuts 2A and 2B having a flange 5 (hereinafter, collectively referred to as a ball nut 2), while being inserted between the ball nuts 2A and 2B. It comprises a seat 3, a large number of rolling elements (balls) that are movably fitted in the ball screw groove of the screw shaft 1 and the ball screw groove of the ball nut 2, and a cooling mechanism 4 that cools the screw shaft 1 hollowly. ing. The screw shaft is fixed to a building such as a factory where the machine tool is installed.
[0013]
A spiral ball screw groove 7 is formed on the outer peripheral surface of the screw shaft 1, and a ball screw groove facing the ball screw groove 7 is formed on the inner peripheral surface of the ball nut 2. A large number of balls are rotatably fitted or accommodated in the ball screw groove 7 of the screw shaft 1 and the ball screw groove of the ball nut 2, and the ball nut 2 is supported by the screw shaft 1 via the balls. . The ball is made of a ceramic material having a specific gravity of 2 to 4, and is preferably made of a ceramic material having a linear expansion coefficient of 4 × 10 ⁻⁶ / ° C. or less. In the present invention, a ceramic material made of silicon nitride (Si ₃ N ₄ ) or zirconia (ZrO ₂ ) can be used. The ball shaft 1 and the ball nut 2 are generally made of chrome molybdenum steel (SCM420H).
[0014]
The inside of the screw shaft 1 is hollow, and the whole ball screw device can be cooled by passing a coolant made of water, oil or the like through the screw shaft 1. That is, the coolant supplied from the cooling device R1 is introduced into the hollow of the screw shaft 1 from the inflow pipe 1i, passes through the shaft, cools the entire ball screw, and is discharged from the discharge pipe 1o to the cooling device R1. Return to be used again.
[0015]
The ball nut 2 is formed in a substantially cylindrical shape, and a flat portion 8 is formed on a part of the outer peripheral surface of each of the ball nuts 2A and 2B. Four holes 9 are formed in the flat portion 8 with the screw shaft 1 therebetween, and two sets of ball circulation tubes 10 are inserted into the holes 9 so as to straddle the screw shaft 1.
[0016]
In the ball screw device as described above, when the screw shaft 1 rotates, the ball moves while rolling in the ball screw groove of the screw shaft 1 and the ball nut 2, and when the ball turns around in the ball screw groove, This ball is picked up from one end of the ball circulation tube 10, passes through the tube 10, and returns to the ball screw groove from the other end of the tube 10. The ball nut 2 moves linearly in the process of the ball moving while rolling in the ball screw groove by the rotation of the screw shaft 1.
[0017]
In the process as described above, in this embodiment, when a ball made of a ceramic material having a specific gravity of 2 to 4 is used, a high positioning accuracy is achieved by providing the cooling mechanism, and the linear expansion coefficient of the ceramic material is small. Therefore, the generation of the internal clearance of the ball screw device can be suppressed. That is, by passing the coolant through the shaft hollow, the temperature rise of the entire ball screw device can be kept low, and the internal preload can be prevented from decreasing.
[0018]
(2) Example of adding nut side cooling to shaft side hollow cooling-Series system-
FIG. 2 is an overall configuration diagram showing another example of the ball screw device of the present invention. In FIG. 2, the schematic configuration of the ball screw device is substantially the same as that of the embodiment shown in FIG. 1, but in this embodiment, the nut side cooling is further connected in series to the shaft side hollow cooling of the above (1). An example added with is shown. That is, in the present embodiment, the coolant supplied from the cooling device R1 is first introduced into the cooling passage in the ball nut 2 from the inflow pipe 2i of the ball nut 2, and from the discharge pipe 2o while cooling the ball nut 2. Discharged. The discharged coolant is further introduced into the hollow of the screw shaft 1 from the inflow pipe 1i of the screw shaft 1, passes through the shaft, cools the entire ball screw, and then is discharged from the discharge pipe 1o to the cooling device R1. Returned for further circulation.
[0019]
In the cooling mechanism of this embodiment, first, the ball nut side is cooled by the coolant, and then the coolant whose temperature has been raised by this cooling action is circulated through the ball shaft hollow this time. Cooling. Therefore, according to this method, it is possible to set the temperature increase on the ball nut side lower than that on the ball shaft side.
[0020]
In the present invention, as long as the object of the present invention can be achieved, the combination of the hollow shaft cooling and the ball nut cooling and the circulation system such as the order are not particularly limited, and the system is based on various combinations and orders. Can be adopted.
[0021]
In the present embodiment, by providing the cooling mechanism as described above, high positioning accuracy can be achieved, and generation of internal clearance of the ball screw device can be more effectively suppressed.
[0022]
(3) Example of adding nut side cooling to shaft side hollow cooling-Parallel system-
FIG. 3 is an overall configuration diagram showing still another example of the ball screw device of the present invention. In FIG. 3, the configuration of the ball screw device is substantially the same as that of the embodiment shown in FIG. 1, but in this embodiment, the nut side cooling is further provided in parallel with the shaft side hollow cooling of (1) above. Therefore, two cooling devices (R1, R2) are provided. That is, in the present embodiment, the coolant supplied from the cooling device R1 is introduced into the hollow cylinder of the screw shaft 1 from the inflow pipe 1i of the screw shaft 1, and passes through the shaft to cool the entire ball screw. The exhaust pipe 1o is discharged and returned to the cooling device R1 for circulation. On the other hand, the coolant supplied from the cooling device R2 is introduced into the cooling passage in the ball nut 2 from the inflow pipe 2i of the ball nut 2, and is discharged from the discharge pipe 2o while cooling the ball nut 2, to the cooling device R2. Used for return circulation.
[0023]
The cooling mechanism of this embodiment is a system in which the ball shaft side and the ball nut side can be cooled by separate cooling devices, and these temperatures can be freely set. Can achieve the object of the present invention more effectively by setting the temperature setting of the cooling device R2 on the ball nut side slightly lower than the setting of the cooling device R1 on the shaft side.
[0024]
That is, in this embodiment, by providing the cooling mechanism as described above, high positioning accuracy is achieved, and generation of internal clearance of the ball screw device can be more effectively suppressed.
[0025]
(4) Example of imparting cooling effect by oil air and oil mist lubrication from the ball nut side FIG. 4 is an overall configuration diagram showing another example of the ball screw device of the present invention. In FIG. 4, the configuration of the ball screw device is substantially the same as that of the embodiment shown in FIG. 1, but this embodiment does not perform hollow cooling on the shaft side, and simple methods such as oil air, oil mist, etc. This is a system that has a cooling effect on lubrication. That is, in the present embodiment, when oil ball or oil mist is supplied from the ball nut side inflow pipe 2i to lubricate the ball screw, the air nut has an effect of cooling the ball screw. It can be set so that the ascent can be made lower than the ball shaft side.
[0026]
(5) Example using stainless steel as the material on the ball nut side FIG. 5 is a block diagram showing another example of the ball screw device of the present invention. In FIG. 5, the ball screw includes a screw shaft 1, ball nuts 2A and 2B having a flange 5, and a spacer 3 inserted between the ball nuts 2A and 2B.
[0027]
A spiral ball screw groove 7 is formed on the outer peripheral surface of the screw shaft 1, and a ball screw groove facing the ball screw groove 7 is formed on the inner peripheral surfaces of the ball nuts 2A and 2B. The ball screw groove 7 of the screw shaft 1 and the ball screw grooves of the ball nuts 2A and 2B accommodate a large number of balls 11 made of a ceramic material having a specific gravity of 2 to 4 so that they can freely roll. 2A and 2B are supported by the screw shaft 1 through the balls 11.
[0028]
In the present embodiment, the ball nuts 2A and 2B are made of a material having a linear expansion coefficient smaller than that of the material of the ball shaft 1 because the ceramic material is used as the ball (rolling element). In order to suppress the occurrence of internal clearance of the ball screw, in the present invention, as such a material, a material having a linear expansion coefficient of 10.5 × 10 ⁻⁶ / ° C. or less, particularly stainless steel such as SUS440C, is used. Preferably used.
[0029]
That is, in this embodiment, by having the material configuration as described above, high positioning accuracy is achieved, and generation of internal clearance of the ball screw device that occurs due to the low coefficient of linear expansion of the ceramic material is effectively suppressed. can do.
[0030]
In any of the above embodiments (1) to (5), in any of them, two nuts, ball nuts 2A and 2B, are used, and a spacer 3 is inserted between them to give a preload to the ball screw. Nut preload (D preload) method is adopted. In this method, a spacer that is thicker than the clearance between nuts by a preload amount is used. From this, for example, in FIG. 5, the ball nuts 2A and 2B are assembled by being elastically displaced in advance by the preload Fa0. When an external load Fa is applied to the ball nut 2A in this state, Fa is buffered and absorbed by the reduced displacement of the ball nut 2B, and as a result, the elastic displacement of the ball nut 2A is reduced. Thus, in the ball screw, by applying a preload, the amount of elastic displacement with respect to the axial load can be reduced and the rigidity can be increased.
[0031]
FIG. 6 is a cross-sectional view showing a normal state of contact between the ball nut 2 and the thread groove 7 of the screw shaft 1 and the ball 11 in the double nut preloading method. When a steel ball having a high linear expansion coefficient as in the prior art is used as the ball, when the screw shaft 1 rotates at a high speed, the linear expansion coefficients of the ball 11 and the ball shaft 1 and the ball nut 2 are the same. Even if the ball diameter is small to some extent with respect to the ball nut 2, as shown in FIG. 6, the ball 11 moves in the direction indicated by the arrow due to the centrifugal force, and the value of L becomes narrow and the internal preload increases. As a result, the increase in the internal preload becomes larger than the fluctuation in the preload due to the increase in internal clearance (difference in expansion) due to the difference in diameter between the ball 11 and the corresponding ball shaft 1 and ball nut 2. Even at times, the internal preload did not decrease.
[0032]
However, when a ceramic ball having a specific gravity of 2 to 4 is used as the ball, the increase in the internal preload due to the centrifugal force of the ball as described above is small, and as a result, the ball 11 and the corresponding ball in high speed rotation. The effect of increasing the internal clearance (difference in expansion) due to the diameter difference between the shaft 1 and the ball nut 2 and the small coefficient of linear expansion of the ceramic sphere is greater, reducing the internal preload during rotation. Become.
[0033]
The above point will be explained by specific calculation. For example, in a high-speed screw having a shaft diameter of 40 mm and a lead of 30 mm, if the ball diameter is 6.35 mm and the average temperature rise of the ball shaft is 20 ° C., generally the ball shaft Since the heat dissipation is larger, the temperature rise of the ball nut becomes larger. Assuming that the temperature rise of the ball nut is 25 ° C., the temperature rise of the ball is considerably larger than that of the ball shaft / ball nut, so that it can be assumed to be about 40 ° C.
[0034]
Assuming that the linear expansion coefficient of the steel material is 12.5 × 10 ⁻⁶ / ° C., the expansion amount of the ball nut is 0.0072 mm, the expansion amount of the ball shaft is 0.0042 mm, and the expansion amount of the steel ball is 0.0032 mm. If the linear expansion coefficient of the ceramic sphere is 3.2 × 10 ⁻⁶ / ° C., the expansion amount is 0.0008 mm. The difference between the expansion amount of the ball nut and the ball shaft, excluding the expansion amount of the ball, is the increase amount of the internal clearance. If this is calculated from the following formula, the increase amount of the internal clearance in the case of the steel ball is almost In contrast to 0, an increase of 0.0028 mm is caused in the ceramic sphere. That is, it is understood that the increase in the internal clearance is larger when the ceramic sphere is used.
[0035]
R = [(Dm + Da) / 2] × t1 × α1-[(Dm−Da) / 2] × t2 × α2-Da × t3 × α3
(Where Dm is the ball screw pitch circle diameter, Da is the ball diameter, α1 is the linear expansion coefficient of the ball nut, α2 is the linear expansion coefficient of the ball shaft, α3 is the linear expansion coefficient of the ball, and t1 is the temperature rise of the ball nut. Value, t2 is the temperature rise value of the ball shaft, and t3 is the temperature rise value of the ball.)
In the above example, in order not to generate internal clearance using ceramic balls, the temperature rise on the ball nut side is set to 3 ° C. lower than that on the ball shaft side, or the material line on the ball nut side is set. It is necessary to set the expansion coefficient to 10.8 × 10 ⁻⁶ / ° C. or less. It is unclear how much the temperature rise of the ball is actually, but if the temperature rise of the ball screw is further increased by high-speed feeding, it can be easily predicted that the effect will increase. In order to achieve the object of the present invention, the present inventors have intensively studied from the above knowledge, and that the linear expansion coefficient of the material on the ball nut side is set to 10.5 × 10 ⁻⁶ / ° C. or less. I found it preferable.
[0036]
FIG. 7 shows a case where a steel ball (linear expansion coefficient: 12.5 × 10 ⁻⁶ / ° C.) is used as a rolling element (ball) in the ball screw having no cooling mechanism in the method of the embodiment (1). When ceramic spheres (linear expansion coefficient 3.2 × 10 ⁻⁶ / ° C.) are used, when the above-mentioned ceramic spheres are used for the axial side hollow cooling of the embodiment (1), and further using ceramic spheres For each of the cases where the ball nut of the embodiment (5) is made of stainless steel (linear expansion coefficient: 10.1 × ⁻⁶ / ° C.), the result of measuring the increase value of the nut surface temperature with respect to the average rotational speed of the ball shaft is Show.
[0037]
According to this, when a ceramic sphere is used as a rolling element, the temperature increase value increases less as compared with the case where a steel ball is used as the rotational speed increases.
[0038]
From this, when the ceramic sphere is used, it is expected that not only the effect of the centrifugal force of the ball but also the decrease of the preload due to the small amount of expansion of the ball occurs. When the shaft side hollow cooling is performed, the temperature rise is extremely small even at high speed rotation. When stainless steel is used for the ball nut, the temperature rise due to high-speed rotation is slightly larger, but smaller than when steel balls are used. In this case, cooling on the ball shaft side is required to improve performance.
[0039]
The ball screw device of the present invention is preferably used, for example, for driving a machine tool that performs high-speed feed by high-speed rotation, and can achieve high speed and high accuracy of the ball screw device. .
[0040]
【The invention's effect】
As described above in detail, by reducing the amount of thermal expansion on the ball nut side as in the present invention, in particular, a cooling mechanism is provided on the ball screw so that the temperature on the ball nut side is lower than the temperature on the ball shaft side. Generation of internal clearance of ball screw due to low linear expansion coefficient of rolling elements made of ceramics by controlling the setting or by setting the coefficient of linear expansion of the material on the ball nut side lower than that of the ball shaft side It is expected to suppress the effect. Therefore, this enables high-speed feeding and suppresses a decrease in internal preload. Furthermore, thermal displacement is suppressed by the cooling effect, and high accuracy can be achieved.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram showing an embodiment (an example of shaft side hollow cooling) of a ball screw device of the present invention.
FIG. 2 is an overall configuration diagram showing an embodiment of the ball screw device according to the present invention (an example in which nut-side cooling is added to shaft-side hollow cooling—a series system—).
FIG. 3 is an overall configuration diagram showing an embodiment of the ball screw device according to the present invention (an example in which nut-side cooling is added to shaft-side hollow cooling—a parallel system—).
FIG. 4 is an overall configuration diagram showing an embodiment of the ball screw device according to the present invention (an example in which a cooling effect is provided by oil air and oil mist lubrication from the ball nut side).
FIG. 5 is a configuration diagram showing an embodiment of the ball screw device of the present invention (an example in which stainless steel is used as the material on the ball nut side).
FIG. 6 is a cross-sectional view showing a normal state of contact between a ball nut and a screw groove of a screw shaft and a ball in a double nut preload system.
FIG. 7 is a graph showing the relationship between the average rotational speed and the temperature rise in the embodiment of the present invention and the prior art.

Claims (3)

A screw shaft having a spiral ball screw groove on the outer peripheral surface; a ball nut having a ball screw groove facing the ball screw groove of the screw shaft on the inner peripheral surface; and the ball screw groove of the ball nut and the screw shaft. A ball screw device comprising a rolling element that is movably fitted between a ball screw groove and the rolling element is made of a ceramic material having a specific gravity of 2 to 4, and at least a thermal expansion amount on the ball nut side Is smaller than the amount of thermal expansion on the screw shaft side . The ball screw device according to claim 1, wherein a cooling mechanism for cooling at least the ball nut side is provided as means for reducing the thermal expansion amount on the ball nut side. 2. The ball screw device according to claim 1, wherein the means for reducing the amount of thermal expansion on the ball nut side is such that the linear expansion coefficient of the ball screw shaft member is larger than the linear expansion coefficient of the ball nut material.

Images