Ball screw

There is provided a ball screw that is used to apply a preload to balls and that has a configuration of achieving a balance between the cooling efficiency and ease of assembly and disassembly. To this end, the ball screw includes a screw shaft, and a plurality of nuts movable relative to the screw shaft in an axial direction. Each of the nuts includes flow channels, which are cooling mechanisms and which respectively serve as independent channels to allow a cooling medium to pass through them. These flow channels are provided symmetrically with respect to a space between the nuts, and are configured so that the cooling medium can independently circulate in each of the nuts.

TECHNICAL FIELD

The present invention relates to ball screws, more particularly to a ball screw having a function to cool a nut.

BACKGROUND ART

There has heretofore been known a ball screw which includes a screw shaft, and a nut that is screwed to the screw shaft via plural rolling elements (e.g., balls) and in which the screw shaft and the nut are relatively rotatable. In this ball screw, frictional heat resulting from a point contact or surface contact is generated between the screw shaft and the nut during rotation. Hence, a cooling portion to reduce the frictional heat is provided in some cases.

Types of installing the cooling portion in the conventional ball screw include a shaft center cooling type and a nut cooling type. The shaft center cooling type is a configured such that the screw shaft is a cooling target so that the cooling portion is provided in the screw shaft. As an example of this shaft center cooling type, the screw shaft is hollow to allow a cooling medium to flow through the screw shaft. The nut cooling type is configured such that the nut is a cooling target and the cooling portion is provided in the nut.

Here, the shaft center cooling type may have a drawback in cost for making a hollow hole in the screw shaft, when the shaft center cooling type is used in a large and long ball screw device. Therefore, the nut cooling type is often used.

The technique disclosed in Patent Literature 1 is given as the ball screw using such a nut cooling type is. Specifically, according to the technique in Patent Literature 1, a cooling medium is made to pass through a flow channel provided in a nut in an axial direction to cool the nut.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

The mainly used ones of recent large and long ball screws are ball screws manufactured by using double nut preload. In such ball screws, it is important to efficiently cool plural nuts, and prevent leakage of the cooling medium during disassembly, assembly, and maintenance.

However, even when the ball screw disclosed in Patent Literature 1 is applied to the ball screw manufactured by using the double nut preload, there is room for improvement in providing a ball screw which achieves a balance between efficient cooling of plural nuts and the prevention of the leakage of the cooling medium.

The present invention has been made to address the above drawbacks, and has an object to provide a ball screw, which achieves a high cooling efficiency, which prevents the leakage of a cooling medium even during disassembly, assembly, and maintenance, and which achieves a well balance between the cooling efficiency and the prevention of leakage.

Solution to Problem

To achieve the above object, a ball screw according to one embodiment of the present invention includes: a screw shaft in which a spiral groove is arranged on an outer circumferential surface;

two nuts, each of which includes a spiral groove facing the spiral groove of the screw shaft, and which are screwed to the screw shaft via a plurality of balls disposed between the spiral grooves of the two nuts and the spiral groove of the screw shaft; and

a preload application member configured to apply a preload to the balls,

wherein cooling portions are independently provided in the two nuts, respectively,

wherein the cooling portions are respectively provided in the nuts to be symmetric with respect to a space between the two nuts,

wherein each of the cooling portions is a flow channel to pass a cooling medium,

wherein the flow channel comprises at least one of an axial flow channel extending in an axial direction or a circumferential flow channel provided to intersect perpendicularly to the axial direction,

wherein the preload application member is a spacer which is coaxial with the two nuts and which is disposed between the two nuts,

wherein the axial flow channel is provided through each of the two nuts in the axial direction, and

wherein a seal portion is provided coaxially with the two nuts, the seal portion being provided at an end of each of the two nuts and having one surface forming the axial flow channel and the circumferential flow channel and another surface in contact with the spacer.

That is, in a ball screw according to one embodiment of the present invention including a screw shaft in which a spiral groove is arranged on an outer circumferential surface; two nuts, each of which includes a spiral groove facing the spiral groove of the screw shaft, and which are screwed to the screw shaft via a plurality of balls disposed between the spiral grooves of the two nuts and the spiral groove of the screw shaft; and a preload application member configured to apply a preload to the balls,

cooling portions (cooling mechanisms) are independently provided in the two nuts, respectively, and

the cooling portions are respectively provided in the nuts to be symmetric with respect to a space between the two nuts. The above-described two nuts are incorporated by coupling, connecting, or adjacently arranged.

Here, in the ball screw, each of the cooling portions is a flow channel to pass a cooling medium, and the flow channel may include at least one of an axial flow channel extending along the axial direction and a circumferential flow channel provided so as to intersect at right angles with the axial direction. That is, each of the cooling portions has a flow channel to pass the cooling medium as an independent channel. Each of these flow channels includes at least one of an axial flow channel and a circumferential flow channel. The axial flow channel is a flow channel axially provided between the inner circumference of the nut and the outer circumference of the nut. The circumferential flow channel is a flow channel provided between the inner circumference of the nut and the outer circumference of the nut in a direction that intersects at right angles with the axis of the nut. The cooling portions are configured so that the cooling medium can independently circulate in each of the nuts.

The ball screw may be configured so that each of the nuts is provided with an inflow opening which is coupled to the flow channels and into which the cooling medium flows, and a discharge opening which is coupled to the flow channels and which discharges the cooling medium. That is, each of the nuts may be configured to have at least one pair of an inflow opening which is an inflow portion for the cooling medium and a discharge opening which is an outflow portion.

The ball screw may be configured so that the preload application member is a spacer which is coaxial with the two nuts and which is disposed between the two nuts.

As the preload application member, it is possible to use a configuration which uses, for example, fixed position preload produced by a positional adjustment between the nuts, constant pressure preload produced by an elastic body such as a spring, or variable control preload to set a predetermined preload load by using fluid pressure or an actuator such as a piezoelectric element. More specifically, it is possible to use a configuration in which the above configuration of the preload is disposed between the nuts so that a plurality of nuts press or pull one another.

Advantageous Effects of Invention

According to one aspect of the present invention, it is possible to provide a ball screw, which achieves a high cooling efficiency, which prevents leakage of a cooling medium even during disassembly, assembly, and maintenance, and which achieves a well balance between the cooling efficiency and the prevention of leakage.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a side view of a configuration of a ball screw in one embodiment of the present invention.FIG. 2is a partial sectional view of a configuration of the ball screw in one embodiment of the present invention.FIG. 3is a side view of a configuration of the ball screw in one embodiment of the present invention, and flow channels are highlighted by hatching.

As illustrated inFIG. 1, a ball screw1in one embodiment of the present embodiment includes two nuts (which are a first nut100and a second nut200), a screw shaft300, and a preload application member which is provided between the two nuts and which is configured to apply preload to the ball.

The first nut100includes a body portion100A cylindrically formed to have an inside diameter larger than the outside diameter of the screw shaft300, a cap110, and tubes120and121. The cap110is attached to one end of the body portion100A by, for example, unillustrated screws via a seal material111. The tubes120and121are members for rolling element circulation provided in a flat portion115of the outer circumferential surface of the body portion100A, and are fixed by a holding plate122fastened to the first nut100by screws123to125. The first nut100is known as a flangeless type nut.

The second nut200includes a body portion200A cylindrically formed to have an inside diameter larger than the outside diameter of the screw shaft300, a flange portion201provided at one end of the body portion200A, a cap210, and tubes220and221. The cap210is attached to an end of the body portion200A by, for example, unillustrated screws via a seal material211. The tubes220and221are members for rolling element circulation provided in a flat portion215of the outer circumferential surface of the body portion200A, and are fixed by a holding plate222fastened to the second nut200by screws223to225.

Here, as illustrated inFIG. 2, a spiral groove101is formed on an inner circumferential surface100aof the first nut100so as to face a spiral groove301spirally formed on an outer circumferential surface300aof the screw shaft300. The spiral groove201is also formed on an inner circumferential surface200aof the second nut200so as to face the spiral groove301of the screw shaft300. The first nut100and the second nut200are coaxially arranged in the axial direction of the screw shaft300, and are screwed with the screw shaft300by plural rolling elements B provided between the spiral groove101and the spiral groove201. Thus, the rolling elements B are capable of rolling through a rolling channel formed by the spiral groove301and the spiral grooves101and201, so that the screw shaft300, and the first nut100, and the second nut200can relatively move in the axial direction.

Preload Application Member

As illustrated inFIG. 1andFIG. 2, a spacer128is provided coaxially with the first nut100and the second nut200between the other end of the first nut100and the other end of the second nut200via seal materials129and130. The first nut100and the second nut200are coupled to each other by a coupling member127via the spacer128and the seal materials129and130. Only one coupling member127is illustrated inFIG. 1andFIG. 2, but the first nut100and the second nut200may be coupled to each other at plural places by plural coupling members127as needed.

Thus, the first nut100and the second nut200are coupled to each other by the coupling member127via the spacer128, so that the spacer128and the coupling member127function as preload application members, leading to a state in which what is known as fixed position preload is applied. In the ball screw1in one embodiment of the present invention, two-point contact preload in a pulling direction is applied to the first nut100and the second nut200to cancel an increase in preload torque caused by cooling the first nut100and the second nut200. This configuration enables efficient cooling of the first nut100and the second nut200.

As the preload application member, it is possible to select a configuration which uses, for example, not only the fixed position preload produced by a positional adjustment between the above nuts100and200but also constant pressure preload produced by an elastic body such as a spring, fluid pressure preload, or variable control preload to set a predetermined preload by using an actuator such as a piezoelectric element.

Cooling Portions

The first nut100and the second nut200respectively include cooling portions150and250which independently cool the nuts100and200. These cooling portions150and250are respectively arranged in the nuts100and200symmetrically with respect to a virtual area between the two nuts100and200. The area is a virtual plane A (seeFIG. 2) extending through the midpoint in the axial direction of the spacer128and passing perpendicularly to the axial direction. The provision of the cooling portions150and250keeps the weight balance of a ball screw device, and does not prevent smooth activation. Here, the configurations of the cooling portions150and250are not limited in particular, as long as the cooling portions150and250are provided symmetrically with respect to the virtual area between the two nuts100and200, and as long as the cooling portions150and250independently cool the first nut100and the second nut200, respectively. A suitable configuration can be selected depending on the purpose.

For example, as illustrated inFIG. 1andFIG. 2, the cooling portions150and250include plural flow channels152to154and252to256which are respectively pierced in the first nut100and the second nut200so that a cooling medium can pass through them. Each of these flow channels can include at least one of an axial flow channel extending in the axial direction and a circumferential flow channel provided to intersect perpendicularly to the axial direction. The aforementioned “axial flow channel” includes the axial flow channels152and154extending in the axial direction in the first nut100, and axial flow channels253and255extending in the axial direction in the second nut200. Among these axial flow channels152,154,253, and255, the axial flow channel152and the axial flow channel253are flow channels symmetrically provided between the two nuts100and200. The axial flow channel154and the axial flow channel255are flow channels symmetrically provided in the nuts100and200, respectively.

The aforementioned “circumferential flow channel” includes the circumferential flow channel153provided in the circumferential direction to intersect perpendicularly to the axial direction in the first nut100, and the circumferential flow channels252,254, and256provided in the circumferential direction to intersect perpendicularly to the axial direction in the second nut200. Among these circumferential flow channels153,252,254, and256, the circumferential flow channel153and the circumferential flow channel254are flow channels symmetrically provided in the nuts100and200, respectively. In this way, the cooling portions150and250are configured to allow the cooling medium to independently circulate in each of the nuts100and200.

In the ball screw1in one embodiment of the present invention, an inflow opening151which is coupled to the flow channels152to154and into which the cooling medium flows, and a discharge opening155configured to discharge the cooling medium may be provided in the first nut100. Moreover, in the ball screw1in one embodiment of the present invention, an inflow opening251which is coupled to the flow channels252to256and into which the cooling medium flows, and a discharge opening257configured to discharge the cooling medium may be provided in the second nut200. That is, each of the nuts100and200may include at least one pair of the inflow opening151or251, which is an inflow portion for the cooling medium, and a discharge opening155or257, which is a discharge portion.

Therefore, as illustrated inFIG. 1toFIG. 3, the cooling portion150of the first nut100includes the inflow portion151provided at the end of the cap110in the axial direction, the axial flow channel152, the circumferential flow channel153, the axial flow channel154, and the discharge portion155provided at the end of the cap110in the axial direction. That is, a single independent system of flow channel is formed in the first nut100. Two or more systems of flow channels may be provided as long as the flow channels are independently provided in each of the nuts100and200.

As illustrated inFIG. 1toFIG. 3, the cooling portion250of the second nut200includes the inflow portion251provided in the outer circumference of the flange portion201in a direction that intersects perpendicularly to the axis, the circumferential flow channel252, the axial flow channel253, the circumferential flow channel254, the axial flow channel255, the circumferential flow channel256, and the outflow portion257provided in the outer circumference of the flange portion201in the direction that intersects perpendicularly to the axis. That is, a single independent system of flow channel is formed in the second nut200. Two or more systems of flow channels may be provided as long as the flow channels are independently provided in each of the nuts100and200.

Here, tube taper screws for pipe fastening are provided in the inflow portions151and251and the discharge portions155and257, so that pipes are connected to these tube taper screws. This configuration enables supply and discharge of the cooling medium.

Cooling Medium

Various gases and liquids can be used as a fluid to serve as the cooling medium. As a gas, not only air or compressed air but also nitrogen, an inert gas (e.g., argon), hydrocarbon (e.g., butane or isobutane), helium, ammonia, carbon dioxide, or a mixture of the above gases can be used. As a liquid, not only water but also a coolant in which antirust is added to water, a coolant in which various additives are added to water, or various oils as cooling medium oils can be used. Specifically, mineral oils, animal and vegetable oils, or synthetic oils can be used. A suitable one of the above may be selected depending on, for example, the use environment. Further, the temperature of the cooling medium may be managed and the flow volume may also be managed. The cooling medium may be used in a turbulent state.

In one embodiment of the present invention, the temperature can be managed in each nut, and, for example, the preload can be controlled.

Furthermore, the positions and sizes of the inflow portions and the discharge portions, and the sectional shape and sectional area of each flow channel can be suitably adjusted depending on the use condition.

According to the ball screw in one embodiment of the present invention, the preload load can be higher, and the ball screw can therefore be suitably applied to what is known as a large-sized ball screw (i.e., the outside diametrical dimension of the screw shaft300is substantially 80 nm or more).

According to the ball screw in one embodiment of the present invention, the cooling medium can independently circulate in each of the nuts. Thus, the cooling efficiency is high. Such a high cooling efficiency allows the nuts to be cooled firstly. Then, and the effects of the cooling are transmitted to the balls that are the rolling elements from the spiral grooves on the inner circumferences of the nuts and further transmitted to the spiral groove of the screw shaft, and also cools the screw shaft. The preload change and the deterioration of lubrication caused by heat generation are prevented, accordingly. The cooling effects are remarkably exhibited, when the preload load is relatively high and when the contact states between the spiral grooves and the rolling elements, and the spiral grooves and the rolling elements are maintained. Therefore, the ball screw according to the present invention is suitable to a relatively large-sized ball screw.

According to the ball screw in one embodiment of the present invention, even a long (i.e., about 4 m or more) ball screw in which the use of what is known as the shaft center cooling type is difficult can be applied without the shaft center cooling, but the shaft center cooling can be used together when necessary.

Furthermore, according to the ball screw in one embodiment of the present invention, the flow channel of the cooling medium for cooling is independently provided in each of the nuts. Therefore, for example, even at the time of maintenance in which some of the nuts need to be replaced, the leakage of the cooling medium during maintenance can be effectively prevented when piping to each of the cooling medium flow channels is blocked.

Thus, the ball screw in one embodiment of the present invention can be suitably used as a ball screw which demands processing accuracy in particular and which is used in what is known as a linearly moving part of a large-sized machine tool that may be subject to the maintenance.

The ball screw in one embodiment of the present invention achieves a high cooling efficiency. Hence, there are no significant changes in the preload and the length of the screw shaft, and excessive heat generation is prevented. Therefore, no significant deterioration of a lubricant can be found. Accordingly, there is no significant deterioration in positioning accuracy of the linearly moving part caused by heat generation in the ball screw, a stable operation in which what is known as torque variation is small can be maintained for a long period of time, and such advantages can be found at even a long screw shaft. Consequently, the ball screws in some embodiments of the present invention are particularly applicable as ball screws used in linearly moving parts of large-sized machine tools for high-precision processing. The ball screw in one embodiment of the present invention can be used together with the shaft center cooling.

The ball screws in some embodiments of the present invention are applicable as ball screws used in linearly moving parts of large-sized machine tools for high-precision processing.

While the embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and improvements can be made.

REFERENCE SIGNS LIST