Bearing apparatus and lubrication unit

A bearing apparatus includes a bearing portion having an outer ring, an inner ring, a plurality of balls interposed between the outer ring and the inner ring, and a cage that holds the balls, and a lubrication unit provided adjacently to the bearing portion in an axial direction thereof to feed lubricant to a lubrication area that needs to be lubricated. The lubrication unit has a tank that stores the lubricant, and a pump that ejects the lubricant toward the lubrication area. The pump has an ejection port that is open toward the lubrication area and through which the lubricant is ejected at a predetermined initial velocity in the form of a 0.5 to 1,000 picoliter oil droplet.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-140468 filed on Jul. 14, 2015 including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a bearing apparatus including a bearing portion and a lubrication unit that allows the bearing portion to be lubricated, and a lubrication unit provided in a rotating apparatus.

2. Description of the Related Art

In recent years, for a variety of machine tools, there has been a demand to allow a main spindle to operate at a higher speed in order to increase machining efficiency and productivity. When the main spindle rotates at high speed, particularly lubricity for a rolling bearing that supports the main spindle becomes a problem. Oil lubrication is known as means for keeping the rolling bearing lubricated and includes, for example, oil air lubrication, oil mist lubrication, and oil jet lubrication. However, these lubrication systems disadvantageously consume a very large amount of oil and thus increase running costs.

Thus, a bearing apparatus has been proposed in which a lubrication unit with a tank and a pump is assembled between an inner ring and an outer ring of a rolling bearing (see Japanese Patent Application Publication No. 2004-108388 (JP 2004-108388 A)). In the bearing apparatus, the lubrication unit is installed inside an axial extension portion of the outer ring of the rolling bearing. The rolling bearing (bearing portion) and the lubrication unit are integrated together. The lubrication unit includes the tank and the pump. The tank stores lubricant. The pump discharges the lubricant in the tank into an annular space between the inner ring and the outer ring. The pump has a needle-like nozzle protruding from a pump body. The pump discharges an approximately several microliter to several tens of nanoliter oil droplet through a tip of the nozzle. The oil droplet is fed to areas such as an inner-ring raceway surface and an outer-ring raceway surface to which the lubricant need to be supplied.

In the bearing apparatus with the lubrication unit assembled therein as described above, driving of the pump allows lubricant91to seep through an opening90aat a nozzle tip90as depicted inFIG. 7A. When the amount of the seeping lubricant91increases, the lubricant is held at the nozzle tip90in the form of an approximately several microliter to several tens of nanoliter oil droplet as depicted inFIG. 7B. In the lubrication unit described in JP 2004-108388 A, the nozzle tip90is in proximity to balls92that are rolling elements. The oil droplet91held at the nozzle tip90are fed to a bearing interior through air flows occurring in the annular space between the inner ring and the outer ring as a result of rotation of the bearing.

However, in this case, the oil droplet91having left the nozzle tip90does not always reach the raceway surfaces, to which the lubricant needs to be supplied. For example, the oil droplet91may adhere to an outer surface of a cage that holds the rolling elements and fail to contribute to lubricating the rolling bearing. Thus, more lubricant (oil droplets) than needed may be discharged using the pump in order to provide for loss of the lubricant. However, the excess lubricant, which fails to contribute to lubrication, may increase rotating resistance (stirring resistance) to the rolling bearing.

In the bearing apparatus described in JP 2004-108388 A, the lubrication unit needs to be arranged in a space with a small volume, making an increase in the volume of the tank difficult. Thus, when the bearing apparatus is set to allow the pump to discharge a large amount of lubricant, consumption of the lubricant in the tank is increased, leading to the need to frequently refill the tank with lubricant. When the refilling with lubricant is frequently performed for maintenance, machine tools are shut down for each refilling, resulting in reduced production efficiency.

SUMMARY OF THE INVENTION

An object of the invention is to efficiently feed lubricant to areas that need to be lubricated to prevent excessive supply of the lubricant.

A bearing apparatus in an aspect of the invention includes a bearing portion having a fixed ring, a rotating ring that is coaxial with the fixed ring, a plurality of rolling elements interposed between the fixed ring and the rotating ring, and a cage that holds the rolling elements, and a lubrication unit provided adjacently to the bearing portion in its axial direction to feed lubricant to a lubrication area that needs to be lubricated. The lubrication unit has a tank that stores the lubricant, and a pump that ejects the lubricant toward the lubrication area. The pump has an ejection port that is open toward the lubrication area and through which the lubricant is ejected at a predetermined initial velocity in the form of a 0.5 to 1,000 picoliter oil droplet.

DETAILED DESCRIPTION OF EMBODIMENTS

An embodiment of a bearing apparatus in the invention will be described below.FIG. 1is a longitudinal sectional view depicting an embodiment of a bearing apparatus10. The bearing apparatus10in the present embodiment supports a main spindle (shaft7) of a main spindle apparatus provided in a machine tool so that the main spindle is rotatable. The bearing apparatus10is housed in a bearing housing8of the main spindle apparatus. InFIG. 1, the shaft7and the bearing housing8are depicted by long dashed double-short dashed lines. The bearing apparatus10includes a bearing portion20and a lubrication unit40.

The bearing portion20has an inner ring21, an outer ring22, a plurality of balls (rolling elements)23, and a cage24that holds the balls23. In the present embodiment, the outer ring22is provided so as to be non-rotatable relative to the bearing housing8, and is thus a fixed ring. The inner ring21rotates along with the shaft7, and is thus a rotating ring.

The inner ring21is a cylindrical member externally fitted over the shaft7, and has an inner-ring body portion31located on a first side in an axial direction (inFIG. 1, a right side) and an inner-ring extension portion32located on a second side in the axial direction (inFIG. 1, a left side). A raceway groove (hereinafter referred to as an inner-ring raceway groove25) is formed on an outer periphery of the inner-ring body portion31. In the present embodiment, the inner-ring body portion31and the inner-ring extension portion32are separate components. The inner-ring extension portion32functions as a spacer shaped like a short cylinder. Although not depicted in the drawings, the inner-ring body portion31and the inner-ring extension portion32may be integrated together (and may be inseparable).

The outer ring22is a cylindrical member fixed to an inner peripheral surface of the bearing housing8. The outer ring22has an outer-ring body portion35located on the first side in the axial direction (inFIG. 1, the right side) and an outer-ring extension portion36located on the second side in the axial direction (inFIG. 1, the left side). A raceway groove (hereinafter referred to as an outer-ring raceway groove26) is formed on an inner periphery of the outer-ring body portion35. In the present embodiment, the outer-ring body portion35and the outer-ring extension portion36are separate components. The outer-ring extension portion36functions as a spacer shaped like a short cylinder. Although not depicted in the drawings, the outer-ring body portion35and the outer-ring extension portion36may be integrated together (and may be inseparable).

The balls23are interposed between the inner ring21(inner-ring body portion31) and the outer ring22(outer-ring body portion35) and roll on the inner-ring raceway groove25and the outer-ring raceway groove26. The cage24is an annular member and has a plurality of pockets27formed along a circumferential direction. Each of the balls23is housed in a corresponding one of the pockets27. Consequently, the cage24can hold the balls23in juxtaposition in a circumferential direction.

A first annular space11is formed between the inner-ring body portion31and the outer-ring body portion35. A second annular space12is formed between the inner-ring extension portion32and the outer-ring extension portion36. The first annular space11and the second annular space12are continuous with each other. The balls23and the cage24are provided in the first annular space11. The lubrication unit40is provided in the second annular space12.

In the present embodiment, the inner ring21rotates with respect to the outer ring22along with the shaft7. The lubrication unit40is attached to an inner peripheral surface of the outer-ring extension portion36so as to be closely fitted to the inner peripheral surface. In contrast, a very small clearance is formed between an outer peripheral surface of the inner-ring extension portion32and an inner peripheral surface of the lubrication unit40so that the lubrication unit40does not hinder rotation of the inner ring32.

FIG. 2is a sectional view taken along line II-II inFIG. 1. The lubrication unit40is generally shaped like a circular ring. The lubrication unit40includes a holder41, a tank42, and a pump43, and further includes a circuit portion44and a power supply portion45.

The holder41is, for example, an annular member made of resin. The holder41has an inner wall46shaped like a short cylinder, an outer wall47shaped like a short cylinder, and a plurality of partition walls48a,48b,48c, and48dprovided between the inner wall46and the outer wall47. As depicted inFIG. 2, the walls define a plurality of spaces K1, K2, and K3along the circumferential direction.

In the present embodiment, the first space K1forms the tank42, the pump43is stored in the second space K2, and the circuit portion44and the power supply portion45are stored in the third space K3. Thus, the lubrication unit40is integrally configured which includes the holder41, the tank42, the pump43, the circuit portion44, and the power supply portion45. The lubrication unit40is attached to the outer-ring extension portion36so as to be integrated with the bearing portion20. As depicted inFIG. 1, the lubrication unit40in the second annular space12is provided adjacently to the first annular space11in the axial direction.

InFIG. 2, the tank42stores lubricant3, and has a storage portion49that stores the lubricant3. The tank42is connected to the pump43through a channel in order to allow the lubricant3in the storage portion49to flow out to the pump43. A retainer that retains the lubricant3(for example, felt or sponge) may be provided in the tank42. Thus, the tank42stores the lubricant3and allows the lubricant3stored in the tank42to be fed to the pump43. InFIG. 1, the pump43feeds (ejects) the lubricant3fed from the tank42into the first annular space11. The pump43will further be described below.

InFIG. 2, the power supply portion45has a power generating portion45aand a secondary battery portion45b. The power generating portion45ais enabled to generate power as a result of rotation of the inner ring21such that the power generated is stored in the secondary battery portion45b. The circuit portion44is a circuit board including a programmed microcomputer and transmits a control signal (driving signal) to the pump43. In other words, the circuit portion44applies driving power (applies a predetermined voltage) to the pump43(to a piezoelectric element55described below).

As described above, the lubrication unit40is provided adjacently to the bearing portion20in the axial direction to feed the lubricant to a lubrication area T (seeFIG. 1) of the bearing portion20that needs to be lubricated. In the following description, the “lubrication area T” corresponds to the inner-ring raceway groove25. A part of the cage24(inFIG. 1, a circular ring portion24aon the left side of the cage24) can come into slidable contact with a part35aof an inner peripheral surface of the outer-ring body portion35. Thus, the cage24is positioned in a radial direction, and the part35aof the inner peripheral surface may correspond to the lubrication area.

The pump43of the lubrication unit40is configured to eject the lubricant3toward the lubrication area T. Thus, the pump43in the present embodiment internally has a piezoelectric element55. The piezoelectric element55operates to vary the volume of the internal space in the pump43to allow the lubricant3in the internal space to be ejected through an ejection port50.

The pump43in the present embodiment has the ejection port50that is open toward the lubrication area T, to blow the lubricant3directly onto the lubrication area T. In this regard, the present embodiment is different from a technique in which lubricant (oil droplet91) discharged through a nozzle90of a conventional pump (seeFIG. 7B) is carried to an area that needs to be lubricated, through air flows occurring in a bearing interior. Thus, the pump43in the present embodiment ejects, based on the operation of the piezoelectric element55, the lubricant3inside the pump43to the lubrication area T through the ejection port50at a predetermined initial velocity (flight velocity) in the form of an oil droplet. In the present embodiment, the ejection port50is set to have a very small opening area. Thus, the pump43ejects the lubricant3to the lubrication area T in the form of a 0.5 picoliter or more and 1000 picoliter or less oil droplet. An opening50aof the ejection port50has a circular shape and preferably has a diameter (bore) of less than 100 micrometers, for example, 25 to 60 micrometers.

An operation in which the pump43ejects the lubricant3is controlled by the circuit portion44(seeFIG. 2). The pump43is controlled to eject the lubricant3toward the lubrication area T in the form of an oil droplet. Consequently, the oil droplet ejected through the ejection port50has the predetermined initial velocity and can spurt through the ejection port50and impact the lubrication area T. As described above, a very small amount of lubricant of the order of picoliters per shot is ejected from the pump43.

As depicted inFIG. 1, the opening50aof the ejection port50is located away from the balls23and outside the first annular space11formed between the inner-ring body portion31and the outer-ring body portion35. In the present embodiment, the opening50aof the ejection port50is positioned within the second annular space12formed between the inner-ring extension portion32and the outer-ring extension portion36.

FIG. 3is a diagram illustrating the pump43and the inner-ring raceway groove25. The ejection port50in the present embodiment is configured to eject an oil droplet (lubricant3) along a center line direction J of the ejection port50(an opening direction of the opening50a). The opening50aof the ejection port50directed toward the lubrication area T allows the oil droplet ejected through the opening50ato be fed directly to the lubrication area T. Thus, the center line direction J of the ejection port50is inclined to a virtual line L parallel to the center line of the bearing portion20. InFIG. 3, the virtual line L is a line passing through a center C of the ball23. The center line direction J of the ejection port50is inclined at an inclination angle α to the virtual line L.

In the bearing apparatus10configured as described above (seeFIG. 1), the pump43of the lubrication unit40ejects the lubricant3, in the form of an oil droplet, to the lubrication area T of the bearing portion20that needs to be lubricated. Thus, the lubricant3can be efficiently fed to the lubrication area T. This enables prevention of excessive supply of the lubricant3and an increase in stirring resistance offered by the lubricant3. Consumption of the lubricant3is suppressed so that wasteful use of the lubricant3can be prevented. This enables extension of intervals between maintenance operations for refilling the tank42with the lubricant3. As a result, a machine tool provided with the bearing apparatus10needs to be less frequently shut down for maintenance, suppressing a decrease in production efficiency. When an excess amount of the lubricant3is fed to the bearing portion20, the lubricant3offers increased stirring resistance to rotation of the bearing. Then, temperature is elevated so that the lubricant3is more likely to be degraded. However, the present embodiment prevents the excessive supply of the lubricant3and thus an increase in stirring resistance, thereby suppressing degradation of the lubricant3resulting from an elevated temperature.

In the bearing apparatus10in the present embodiment, the lubricant3ejected toward the lubrication area T is in the form of a 0.5 to 1000 picoliter oil droplet. Consequently, the pump43needs to provide reduced power. If an attempt is made to eject the lubricant3directly to the lubrication area T at a predetermined initial velocity in the form of an approximately several microliter to several tens of nanoliter oil droplet, the pump43needs to provide high power. It is difficult to downsize the pump43providing high power to such a size that the pump43can be housed in the second annular space12. In contrast, in the present embodiment, the lubricant3fed to the lubrication area T is in the form of a 1000 picoliter or less oil droplet. Therefore, the size of the pump43can be reduced, facilitating assembly of the pump43in the second annular space12.

When the inner ring21rotates along with the shaft7, air flows occur in the first annular space11in the same direction as a rotating direction of the balls23and the cage24. Thus, in the present embodiment, the opening50aof the ejection port50is positioned outside the first annular space11. This makes the oil droplet ejected through the opening50aless likely to be affected by the air flows occurring in the first annular space11. Thus, the oil droplet can be fed directly to the target lubrication area T.

To allow the lubricant3to be ejected directly to the lubrication area T in the form of an oil droplet, the oil droplet (0.5 to 1000 picoliter oil droplet) ejected through the pump43need to have an increased flight velocity. Specifically, as depicted inFIG. 3, when an axial distance from the opening50aof the ejection port50to a point Ta included in the lubrication area T is denoted as Y (meters), the oil droplet as ejected through the ejection port50has such an initial velocity V0(meters/sec) has a value that is greater than or equal to 30 times the value of the axial distance Y. In the present embodiment, the initial velocity is set such that the value of the initial velocity V0is equal to 400 times the value of the axial distance Y, which allows the oil droplet to more easily reach the lubrication area T. An upper limit on the initial velocity V0(meters/sec) can be set such that, for example, the value of the initial velocity V0is less than or equal to 500 times the value of the axial distance Y. Each of the balls comes into point contact with the inner-ring raceway groove25, and thus, the point Ta included in the lubrication area T serves as a contact point on the inner-ring raceway groove25with which the ball23comes into contact. The bearing portion20in the present embodiment forms an angular ball bearing. The contact point (Ta) between the ball23and the inner-ring raceway groove25is positioned closer to a shoulder portion33of the inner-ring body portion31than a bottom point25aof the inner-ring raceway groove25.

Now, the position of the ejection port50, the inclination angle α of the ejection port50, and an ejection velocity (the initial velocity V0) of the oil droplet will be described. These values are set so as to allow the lubrication area T to be appropriately lubricated according to the size (bearing number) of the bearing portion20. Thus, the bearing portion20will be described below which corresponds to an angular ball bearing in which the outer ring22(outer-ring body portion35) has an outside diameter of 110 millimeters, the inner ring21(inner-ring body portion31) has a bore diameter of 70 millimeters, the bearing portion20has an axial dimension (width dimension) of 20 millimeters, and each of the balls23has a contact angle of 20°.

The position of the opening50aof the ejection port50in the axial direction lies in the second annular space12, located away from the balls23as described above. When the bottom point25aof the inner-ring raceway groove25is set to be a reference, the position of (the center of) the opening50ain the radial direction lies between the reference and the center C of each of the balls23. The position of the opening50ain the radial direction will further be described. With the bottom point25aset to be the reference, a radial dimension from the reference to the (center of) opening50a, positioned outside the reference in the radial direction, is denoted as H. The diameter of each ball23is denoted as d. For the bearing portion20, the radial dimension H can be set such that 0<H≤0.5×d, and in the present embodiment, the radial dimension is set such that H=0.22×d. The position of the opening50ais set such that 0<H as described above. However, when a shoulder portion of the inner-ring body portion31that is closer to the pump43is higher than the bottom point25aas depicted inFIG. 3, the opening50ainevitably needs to be positioned so as to avoid overlapping the shoulder portion.

As described above, the center line direction J of the ejection port50is inclined at the inclination angle α to the virtual line L, which is parallel to the center line of the bearing portion20and which passes through the center of each ball23. Specifically, the inclination angle α is set such that the center line direction J of the ejection port50coincides with a straight line G0connecting the center of the opening50awith the point Ta included in the lubrication area T (in other words, in the present embodiment, the contact point of the inner-ring raceway groove25with which each ball23comes into contact). That is, the inclination angle α is set equal to an angle formed between the virtual line L and the straight line G0and denoted as w.

Deviations δa and δb for the inclination angle α are set because the inclination angle may be set to any value as long as the oil droplet can reach any position of the inner-ring raceway groove25. The deviation δa is an angle formed between the straight line G0serving as a reference and a straight line connecting the center of the opening50awith an end25bof the inner-ring raceway groove25that is closer to the pump43. The deviation δb is an angle formed between the straight line G0serving as the reference and a straight line connecting the center of the opening50awith the end25cof the inner-ring raceway groove25that is farther from the pump43. Therefore, to allow the oil droplet ejected through the opening50ato impact the inner-ring raceway groove25, the inclination angle α of the center line direction J of the ejection port50to the virtual line L may be set as follows.
(Angle ψ−deviation δb)≤(inclination angle α)≤(angle ψ+δa)

The thus set range of the inclination angle may correspond to the lubrication area T. In other words, the entire area of the inner-ring raceway groove25may correspond to the lubrication area T.

The initial velocity V0at which the oil droplet is ejected through the ejection port50is set as follows. The bearing apparatus10is used in four typical forms depicted inFIGS. 4A, 4B, 4C, and 4D.FIG. 4Aillustrates that the direction of the center line L0(axial direction) of the bearing portion20coincides with the horizontal direction and that the lubrication area T is positioned below the ejection port50.FIG. 4Billustrates that the direction of the center line L0(axial direction) of the bearing portion20coincides with the horizontal direction and that the lubrication area T is positioned above the ejection port50.FIG. 4Cillustrates that the direction of the center line L0(axial direction) of the bearing portion20coincides with the vertical direction and that the lubrication area T is positioned below the ejection port50.FIG. 4Dillustrates that the direction of the center line L0(axial direction) of the bearing portion20coincides with the vertical direction and that the lubrication area T is positioned above the ejection port50.

The initial velocity V0is set such that, regardless of in whichever of the four forms the bearing apparatus10is arranged, the lubricant3ejected through the ejection port50in the form of an oil droplet can reach the lubrication area T, that is, the inner-ring raceway groove25. This will be specifically described. For the bearing portion20that is an angular ball bearing (with an axial dimension of 20 millimeters) according to the present embodiment, the deviation δa on one side and the deviation δb on the other side with reference to the straight line G0(seeFIG. 3) have been described. The smaller of the two deviations (in the present embodiment, δb) is defined as a deviation δ for the inclination angle α in the bearing portion20(δ=δb). The deviation δ is calculated to be 2.3° based on geometric shapes.

FIG. 5is a graph illustrating the initial velocity V0of the oil droplet (the axis of abscissas) and the deviation δ for the inclination angle α with reference to the straight line G0(the axis of ordinate) in each of the four forms depicted inFIGS. 4A, 4B, 4C, and 4D. The graph indicates results of simulation. In the graph illustrated inFIG. 5, a “thick continuous line” represents the form inFIG. 4A, a “thin continuous line” represents the form inFIG. 4B, a “thick dashed line” represents the form inFIG. 4C, and a “thin dashed line” represents the form inFIG. 4D.

As illustrated inFIG. 5, the initial velocity V0at which the deviation δ is 2.3° or less in all of the four forms is 1.2 meters/sec or more. That is, when the lubricant3is ejected through the ejection port50at an initial velocity V0of 1.2 meters/sec or more in the form of an oil droplet, the oil droplet can be attached to the lubrication area T, i.e., the inner-ring raceway groove25, regardless of the orientation in which the bearing apparatus1including the bearing portion20is used.

FIG. 6is a graph illustrating a relationship between the minimum value of the initial velocity V0of the oil droplet and the axial dimension (width) of the bearing portion20. The minimum value in the graph is the minimum value of the initial velocity V0of the oil droplet that allows the oil droplet to reach the inner-ring raceway groove25regardless of to whichever of the forms depicted inFIGS. 4A to 4Dthe bearing apparatus1is applied. For example, when the bearing portion20included in the bearing apparatus1has an axial dimension (width) of 5 mm, the initial velocity V0has a minimum value of 0.6 meters/sec. When the bearing portion20has an axial dimension (width) of 150 mm, the initial velocity V0has a minimum value of 3.3 meters/sec. As depicted inFIG. 6, an increased axial dimension (width) of the bearing portion20results in increase in the needed initial velocity V0of the oil droplet. As described above, the initial velocity V0can be set based on the size (particularly the axial dimension) of the bearing portion20and the size of the lubrication area T.

As described above, in the bearing apparatus1in the present embodiment, the lubricant3is ejected from the pump43of the lubrication unit40toward the lubrication area T in the form of an oil droplet of a picoliter size and also at an increased flight velocity. Thus, the lubricant3can be efficiently fed to the lubrication area T, preventing excessive supply of the lubricant3to suppress consumption of the lubricant. Thus, the lubricant3in the tank can be prevented from being wastefully used.

In the above-described embodiment, the bearing apparatus10is provided with the bearing portion20and the lubrication unit40. However, the lubrication unit40may be applied to rotating apparatuses other than machine tools. For example, the lubrication unit40may be an apparatus configured to lubricate a gear tooth surface of a gear mechanism. In other words, although not depicted in the drawings, the lubrication unit may be an apparatus provided in a rotating apparatus such as a gear mechanism to feed lubricant to a lubrication area of the rotating apparatus that needs to be lubricated. Also in this case, the lubrication unit has a tank that stores lubricant and a pump that ejects the lubricant to the lubrication area. The pump has an ejection port that is open toward the lubrication area. The pump is configured to eject the lubricant to the lubrication area through the ejection port at a predetermined initial velocity (flight velocity) in the form of a 0.5 to 1000 picoliter oil droplet.

The embodiment described above is illustrative in all respects and is not restrictive. That is, the bearing apparatus (lubrication unit40) in the invention is not limited to the illustrated forms but may be in any other form within the scope of the invention. In the above-described embodiment, the bearing portion20is an angular ball bearing. However, the form of the bearing is not limited to this. A deep groove ball bearing may be used or a tapered roller bearing or a cylindrical roller bearing may be used.

In the above-described embodiment, the inner ring21is a rotating ring. However, the outer ring22may be rotating ring, and the inner ring21may be a fixed ring. In this case, the lubrication unit40may be attached to the inner-ring extension portion of the fixed ring.

In the invention, the pump ejects the lubricant to the lubrication area that needs to be lubricated, in the form of an oil droplet. Thus, the lubricant can be efficiently fed to the lubrication area, enabling prevention of excessive supply of the lubricant.