Screw nut

A screw nut on which a pulley is mounted is capable of promoting reductions in the size and weight thereof. The screw nut includes: an outer ring; and a nut rotatably assembled to the outer ring via a first rolling element. The nut includes: a nut body having a screw groove in an inner surface thereof; and a recirculation component provided at an end portion of the nut body in an axial direction, the recirculation component having a turn-around path for recirculating a second rolling element rolling along a screw groove. A pulley to be mounted on the nut is fitted on an outer surface of the nut body, and is brought into contact with an end surface of the nut in the axial direction. The pulley is mounted on the end surface of the nut with a fastening member.

TECHNICAL FIELD

The present invention relates to a screw nut or a spline nut used for, for example, an actuator for an industrial robot.

BACKGROUND ART

A ball screw spline is used for a Z−θ actuator used for, for example, a tip shaft of a SCARA robot to meet demands for increases in functionality and precision. The ball screw spline includes a shaft, a screw nut assembled to the shaft, and a spline nut assembled to the shaft. A screw groove and a spline groove are formed on an outer surface of the shaft in such a manner as to overlap each other. A rotary input is provided to each of the screw nut and the spline nut to control the amount of rotation. Accordingly, the shaft performs arbitrary liner motion (Z), rotary motion (θ), and spiral motion (Z+θ).

The screw nut is a combination of a bearing and a ball screw nut in one. In other words, the screw nut includes an outer ring, and a screw nut rotatably assembled to the outer ring via a bearing ball. An inner ring of the bearing and the screw nut are integrated. The nut includes a nut body having a screw groove in an inner surface thereof, and a recirculation component having a turn-around path for recirculating screw balls rolling along the screw groove in the nut body.

The spline nut is a combination of a bearing and a ball spline nut in one. In other words, the spline nut includes an outer ring, and a nut rotatably assembled to the outer ring via a bearing ball. The nut includes a nut body having a spline groove in an inner surface thereof, and a recirculation component having a turn-around path for recirculating spline balls rolling along the spline groove.

Pulleys for rotating the screw nut and the spline nut are mounted on the screw nut and the spline nut, respectively. As a known pulley mounting method, Patent Literature 1 discloses a mounting method that provides a flange-shaped protrusion protruding in a radial direction integrally with an outer surface of a nut body, and fits a pulley on the outer surface of the nut body and brings the pulley into contact with the flange-shaped protrusion. The pulley is fitted on the outer surface of the nut body here; accordingly, the position of the pulley can be determined in the radial direction. Moreover, the pulley is brought into contact with the flange-shaped protrusion; accordingly, the position of the pulley can be determined in an axial direction.

Moreover, as another pulley mounting method, a mounting method is known which provides a recirculation component at an end portion of a nut body in an axial direction, provides an outside-diameter portion that protrudes in the axial direction in a ring shape in such a manner as to surround the recirculation component, integrally with the nut body, and fits the pulley on an outer surface of the nut body and brings the pulley into contact with the ring-shaped outside-diameter portion. The pulley is fitted on the outer surface of the nut body; accordingly, the position of the pulley can be determined in a radial direction. Moreover, the pulley is brought into contact with the ring-shaped outside-diameter portion; accordingly, the position of the pulley can be determined in the axial direction.

CITATION LIST

Patent Literature

Patent Literature 1: JP 4-224351 A

SUMMARY OF INVENTION

Technical Problem

In recent years, further reductions in the size and weight of the ball screw spline have been requested. This is because if reductions in the size and weight of the ball screw spline used for, for example, the tip shaft of a SCARA robot can be promoted, it is possible to move the SCARA robot at high speed and reduce the size of a motor.

However, in the pulley mounting method described in Patent Literature 1, the flange-shaped protrusion protruding in the radial direction is provided integrally with the outer surface of the nut body. Accordingly, there is a problem that the outside diameter of the nut is increased by the length required to provide the flange-shaped protrusion. Moreover, a return pipe as a recirculation component is provided on the outer surface of the nut body of the screw nut. Accordingly, there is also a problem that the outside diameter of the nut is increased by the length required to provide the return pipe.

In the above other pulley mounting method, the outside-diameter portion that protrudes in the axial direction in the ring shape in such a manner as to surround the recirculation component is provided integrally with the nut body. Accordingly, there is a problem that the outside diameter of the nut is increased by the length required for the presence of the ring-shaped outside-diameter portion. Moreover, a gap is created between the pulley and the recirculation component. Accordingly, there is also a possibility that the gap becomes a cause of the looseness of the pulley.

Hence, an object of the present invention is to provide a screw nut or a spline nut on which a pulley is mounted, the screw nut or the spline nut capable of promoting reductions in the size and weight thereof.

Solution to Problem

In order to solve the above problem, an aspect of the present invention is a screw nut or a spline nut including: an outer ring; and a nut rotatably assembled to the outer ring via a first rolling element, in which the nut has: a nut body having a screw groove or a spline groove in an inner surface thereof; and a recirculation component provided at an end portion of the nut body in an axial direction, the recirculation component having a turn-around path for recirculating a second rolling element rolling along the screw groove or the spline groove, a pulley to be mounted on the nut is fitted on an outer surface of the nut body, and is brought into contact with an end surface of the nut in the axial direction, and the pulley is mounted on the end surface of the nut with a fastening member.

Advantageous Effects of Invention

According to the present invention, the recirculation component is provided at the end portion of the nut body in the axial direction. Accordingly, the outside diameter of the nut can be reduced. Moreover, the pulley is brought into contact with the end surface of the nut in the axial direction, and is mounted on the end surface of the nut in the axial direction with the fastening member. Accordingly, it is possible to eliminate a gap between the pulley and the end surface of the nut and eliminate a cause of the looseness of the pulley. Furthermore, there is no need to provide a ring-shaped outside-diameter portion for bringing the pulley into contact with the nut body. Accordingly, the outside diameter of the nut can be further reduced.

DESCRIPTION OF EMBODIMENTS

A screw nut and a spline nut of an embodiment of the present invention are described in detail hereinafter with reference to the accompanying drawings. However, the screw nut and the spline nut of the present invention can be embodied in various modes, and are not limited to the embodiment described in the description. The embodiment is provided with the intention of enabling those skilled in the art to fully understand the scope of the invention by fully disclosing the description.

FIGS. 1 and 2illustrate perspective views (partial cross-sectional views) of a ball screw spline where a screw nut and a spline nut of one embodiment of the present invention are integrated.FIG. 1is an example where the screw nut is placed on the front side.FIG. 2is an example where the spline nut is placed on the front side. InFIGS. 1 and 2, a reference sign1denotes a shaft, a reference sign2denotes the screw nut, and a reference sign3denotes the spline nut. They are described below in turn.

As illustrated inFIG. 1, in an outer surface of the shaft1, a helical screw groove1ahaving a predetermined lead is formed, and a straight spline groove1bextending in an axial direction is formed. The screw groove1aand the spline groove1bpartially overlap with each other. The number of starts of the screw groove1aof the embodiment is one and the number of starts of the spline groove1bis four. Their numbers of starts are appropriately set.

As illustrated inFIG. 1, the screw nut2is a combination of a bearing and a ball screw nut in one. In other words, the screw nut includes an outer ring21, and a nut23rotatably assembled to the outer ring21via a plurality of bearing balls22aand22bas first rolling elements. An inner ring of the bearing and the nut23are integrated.

As illustrated in an enlarged view ofFIG. 3, the outer ring21has a substantially cylindrical shape and includes a flange21bat one end portion in an axial direction of the outer ring21. The outer ring21is mounted on, for example, a housing of a z-O actuator via the flange21b. For example, a double-start outer ring groove21ais formed in an inner surface of the outer ring21. For example, a double-start inner ring groove24afacing the outer ring groove21ais formed in an outer surface of a nut body24of the nut23. Two rows of the bearing balls22aand22bare disposed between the outer ring groove21aand the inner ring groove24ain such a manner as to be capable of rolling motion. The bearing balls22aand22bare held by a ring-shaped retainer. A seal21cthat blocks a gap between the outer ring21and the nut body24is mounted on the outer ring21.

The nut23includes the tubular nut body24, recirculation components25aand25bprovided at end portions of the nut body24in the axial direction, and lid members26aand26bmounted on end surfaces of the nut body24in the axial direction. A helical screw groove24bfacing the screw groove1aof the shaft1is formed in an inner surface of the nut body24. A plurality of screw balls27as second rolling elements is placed between the screw groove24bof the nut body24and the screw groove1aof the shaft1in such a manner as to be capable of rolling motion. A turn-around path28for recirculating the screw balls27is formed in each of the recirculation components25aand25b. A through-hole29penetrating in the axial direction is formed in the nut body24. The turn-around path28is connected to the screw groove24band the through-hole29of the nut body24.

A loaded rolling path20between the screw groove24bof the nut body24and the screw groove1aof the shaft1, the turn-around paths28, and the through-hole29configure a recirculation path. The screw ball27that has moved to one end of the loaded rolling path20passes the turn-around path28of one recirculation component25a, the through-hole29, and the turn-around path28of the other recirculation component25b, and then returns again to the other end of the loaded rolling path20.

As illustrated inFIG. 4, the recirculation components25aand25bare housed in recesses24ein the end surfaces of the nut body24in the axial direction. The recirculation components25aand25bare fixed, covered with the ring-shaped lid members26aand26b. The recirculation components25aand25bare made of resin. The nut body24and the lid members26aand26bare made of metal. The lid members26aand26bcover substantially entire end surfaces24cof the nut body24in the axial direction. The lid members26aand26bare fastened to the nut body24with fastening members30such as screws. Note that the outside diameter of the lid members26aand26bis slightly smaller than the outside diameter of the nut body24. This is for making it easy to fit a pulley41described below on an outer surface24dof the nut body24.

FIG. 5Aillustrates a pulley mounting method of the embodiment.FIG. 5Billustrates a known pulley mounting method. The pulley41has a substantially bottomed cylindrical shape, and includes a cylindrical portion41aand a bottom portion41b. A plurality of teeth41cthat meshes with a timing belt is formed on an outer surface of the cylindrical portion41a. The pulley41is made of metal such as aluminum.

As illustrated inFIG. 5A, the pulley41is fitted on an outer surface24dof the nut body24, and is brought into contact with an end surface26a1of the lid member26ain the axial direction, that is, the end surface26a1of the nut23in the axial direction. More specifically, the cylindrical portion41aof the pulley41is fitted on the outer surface24dof the nut body24with a predetermined fit, and the bottom portion41bof the pulley41is brought into contact with the end surface26a1of the lid member26a. The pulley41is fitted on the outer surface24dof the nut body24to allow determining the position of the pulley41in a radial direction. The pulley41is brought into contact with the end surface26a1of the lid member26ato allow determining the position of the pulley41in the axial direction. After the position is determined in the radial and axial directions, the pulley41is fastened to the nut body24with a fastening member42such as a bolt. A screw hole24f(refer toFIG. 4) that threadedly engages with the fastening member42is formed in the nut body24. A through-hole26a2(refer toFIG. 4) or a notch through which the fastening member42is threaded is formed in the lid member26a.

The nut body24and the lid member26aare made of metal. Accordingly, the pulley41can be fixed firmly to the nut23. Moreover, substantially the entire end surface26a1of the nut23comes into contact with the pulley41. Accordingly, a gap between the pulley41and the end surface26a1of the nut23can be eliminated. Consequently, it becomes possible to control the fastening torque of the fastening member42and eliminate a cause of looseness.

As illustrated inFIG. 5B, in the known pulley mounting method, an outside-diameter portion24gthat protrudes in the axial direction in a ring shape in such a manner as to surround the recirculation component25ais provided integrally with the nut body24to prevent the resin recirculation component25afrom coming into contact with the pulley41. The pulley41is brought into contact with the outside-diameter portion24g. However, the outside diameter of the nut body24is increased by the length required to provide the outside-diameter portion24g. The diameter of the pulley41is also increased. Moreover, a gap44is also created between the pulley41and the recirculation component25a. Accordingly, the gap44may become a cause of the looseness of the pulley41.

As illustrated inFIG. 2, the spline nut3is a combination of a bearing and a ball spline nut in one. In other words, the spline nut3includes an outer ring31, and a nut33rotatably assembled to the outer ring31via a plurality of bearing balls32aand32bas the first rolling elements. An inner ring of the bearing and the nut33are integrated.

As illustrated in an enlarged view ofFIG. 6, the outer ring31has a substantially cylindrical shape and includes a flange31bat one end portion in an axial direction of the outer ring31. The outer ring31is mounted on, for example, a housing of a z-O actuator via the flange31b. For example, a double-start outer ring groove31ais formed in an inner surface of the outer ring31. For example, a double-start inner ring groove34afacing the outer ring groove31ais formed in an outer surface of a nut body34. Two rows of the bearing balls32aand32bare disposed between the outer ring groove31aand the inner ring groove34ain such a manner as to be capable of rolling motion. The bearing balls32aand32bare held by a ring-shaped retainer. A seal31cthat blocks a gap between the outer ring31and the nut body34is mounted on the outer ring31.

The nut33includes the tubular nut body34, and recirculation components35aand35bprovided at end portions of the nut body34in the axial direction. A straight spline groove34bthat faces the spline groove1bof the shaft1and extends in the axial direction is formed in an inner surface of the nut body34(FIG. 6illustrates a row of balls rolling along the spline groove34b, but the spline groove34bis formed in the inner surface of the nut body34). A plurality of spline balls37as the second rolling elements is placed between the spline groove34bof the nut body34and the spline groove1bof the shaft1in such a manner as to be capable of rolling motion. A U-shaped turn-around path38for recirculating the spline balls37is formed in each of the recirculation components35aand35b.FIG. 6illustrates the balls moving through the U-shaped turn-around paths38, but the turn-around paths38are formed in the recirculation components35aand35b. Moreover, a straight return path39parallel to the spline groove34bis provided in the nut body34.FIG. 6illustrates the balls moving along the return path39, but the return path39is provided in the nut body34. The return path39is formed in the nut body34or a cage mounted on the nut body34. The turn-around path38is connected to the spline groove34band the return path39of the nut body34.

A loaded rolling path36between the spline groove34bof the nut body34and the spline groove1bof the shaft1, the turn-around paths38, and the return path39configure a recirculation path. The spline ball37that has moved to one end of the loaded rolling path36passes the turn-around path38of one recirculation component35a, the return path39, and the turn-around path38of the other recirculation component35b, and then returns again to the other end of the loaded rolling path36.

The recirculation components35aand35bare mounted on end surfaces34cof the nut body34in the axial direction. The recirculation components35aand35bare made of metal, and are manufactured by, for example, MIM (Metal Injection Molding). The recirculation components35aand35bhave a ring shape, and cover substantially the entire end surfaces34cof the nut body34. The outside diameter of the recirculation components35aand35bis slightly smaller than the outside diameter of the nut body34. This is for making it easy to fit the pulley41described below on an outer surface of the nut body34. The recirculation components35aand35bare fastened to the nut body34with fastening members40such as screws.

FIG. 7Aillustrates a pulley mounting method of the embodiment.FIG. 7Billustrates a known pulley mounting method. The pulley41is the same as the pulley41mounted on the screw nut2. Accordingly, the same reference signs are assigned thereto, and a description thereof is omitted.

As illustrated inFIG. 7A, the pulley41is fitted on an outer surface34dof the nut body34, and is brought into contact with an end surface35a1of the recirculation component35ain the axial direction, that is, the end surface35a1of the nut33in the axial direction. More specifically, the cylindrical portion41aof the pulley41is fitted on the outer surface34dof the nut body34with a predetermined fit, and the bottom portion41bof the pulley41is brought into contact with the end surface35a1of the nut33. The pulley41is fitted on the outer surface34dof the nut body34to allow determining the position of the pulley41in the radial direction. The pulley41is brought into contact with the end surface35a1of the nut33to allow determining the position of the pulley41in the axial direction. After the position is determined in the radial and axial directions, the pulley41is fastened to the nut body34with the fastening member42such as a bolt. A screw hole that threadedly engages with the fastening member42is formed in the nut body34. A through-hole or a notch25a2(refer toFIG. 6) through which the fastening member42is threaded is formed in the recirculation component35a.

The nut body34and the recirculation component35aare made of metal. Accordingly, the pulley41can be fixed firmly to the nut33. Moreover, substantially the entire end surface35a1of the nut33comes into contact with the pulley41. Accordingly, a gap between the pulley41and the end surface35a1of the nut33can be eliminated. Consequently, it becomes possible to control the fastening torque of the fastening member42and eliminate the cause of looseness.

As illustrated inFIG. 7B, in the known pulley mounting method, an outside-diameter portion34g, which protrudes in the axial direction in a ring shape in such a manner as to surround the recirculation component35a, is provided integrally with the nut body34to prevent the resin recirculation component35afrom coming into contact with the pulley41. The pulley41is brought into contact with the outside-diameter portion34g. However, the outside diameter of the nut body34is increased by the length required to provide the outside-diameter portion34g. The diameter of the pulley41is also increased. Moreover, the gap44is created between the pulley41and the recirculation component35a. Accordingly, there is also a possibility that the gap44becomes a cause of the looseness of the pulley41.

Effects of the Screw Nut and Spline Nut of Embodiment

The recirculation components25aand35aare provided at the end portions of the nut bodies24and34in the axial direction. Accordingly, the outside diameters of the nuts23and33can be reduced. Moreover, the pulleys41are brought into contact with the end surfaces26a1and35a1of the nuts23and33in the axial direction, and are mounted with the fastening members42on the end surfaces26a1and35a1of the nuts23and33in the axial direction. Accordingly, it is possible to eliminate the gaps between the pulleys41and the end surfaces26a1and35a1of the nuts23and33and eliminate the cause of the looseness of the pulleys41. Furthermore, there is no need to provide the nut bodies24and34with the ring-shaped outside-diameter portions24gand34gsurrounding the recirculation components25aand35a. Accordingly, the outside diameters of the nuts23and33can be further reduced.

The pulleys41are brought into contact with the end surfaces26a1and35a1of metal components (for example, the lid member26aand the recirculation component35a) mounted on the nut bodies24and34. Accordingly, the pulleys41can be fixed firmly to the nuts23and33.

The resin recirculation component25aof the screw nut2is housed in the recess24ein the end surface24cof the nut body24. The recirculation component25ais covered with the metal lid member26a. Accordingly, there is no need to dispose the resin recirculation component25abetween the pulley41and the nut body24, and the pulley41can be fixed firmly to the nut23.

The recirculation component35aof the nut33of the spline nut3is made of metal. Accordingly, the pulley41can be fixed firmly to the nut33.

The present invention is not limited to the realization of the above embodiment, and can be modified to other embodiments within the scope that does not change the spirit of the present invention.

In the screw nut of the above embodiment, the pulley is brought into contact with the lid member. However, it is also possible to omit the lid member and bring the pulley into contact with the end surface of the nut body.

In the screw nut of the above embodiment, the pulley is brought into contact with the lid member. However, it is also possible to dispose an optional component such as a seal or lubricant supply apparatus including a metal body portion between the pulley and the lid member and bring the pulley into contact with the metal optional component.

In the screw nut of the above embodiment, the lid member is mounted on the end surface of the nut body. However, it is also possible to dispose an optional component such as a seal including a metal body portion between the lid member and the end surface. Moreover, it is also possible to cause the lid member to have the function of an optional component such as a seal.

In the spline nut of the above embodiment, the pulley is brought into contact with the recirculation component. However, it is also possible to dispose an optional component such as a seal or lubricant supply apparatus including a metal body portion between the pulley and the recirculation component and bring the pulley into contact with the metal optional component.

The description is based on Japanese Patent Application No. 2017-210039 filed on Oct. 31, 2017, the entire contents of which are incorporated herein.

REFERENCE SIGNS LIST