Dynamically-lubricated bearing and method of dynamically lubricating a bearing

Dynamically-lubricated bearings and methods of dynamically lubricating bearings, including bearings used in gas turbine engines. Such a bearing includes an inner race having an inner race groove, an outer race having an outer race groove that opposes the inner race groove, rolling elements disposed between the inner and outer races and in rolling contact with the inner and outer race grooves, and a cage disposed between the inner and outer races to maintain separation between the rolling elements. A lubricant is introduced into a cavity between the inner and outer races, and rotation of the inner race relative to the outer race causes air to enter pockets of the cage that contain the rolling elements, which in turn causes the lubricant to exit the cavity of the bearing.

BACKGROUND OF THE INVENTION

The present invention generally relates to bearings and more particularly to bearings of the type that are dynamically lubricated, wherein the bearings are configured to inhibit viscous heating of the lubricant and thereby operate at relatively lower temperatures.

FIG. 1schematically represents a high-bypass turbofan engine10of a type known in the art. The engine10is schematically represented as including a nacelle12and a core engine module14. A fan assembly16located in front of the core module14includes an array of fan blades18. The core module14is represented as including a high-pressure compressor22, a combustor24, a high-pressure turbine26and a low-pressure turbine28. Air is drawn into the inlet duct20of the engine10and then compressed by the compressor22before being delivered to the combustor24, where the compressed air is mixed with fuel and ignited to produce hot combustion gases that pass through the turbines26and28before being exhausted through a primary exhaust nozzle30. To generate additional engine thrust, a large portion of the air that enters the fan assembly16is bypassed through an annular-shaped bypass duct32before exiting through a fan exit nozzle34.

FIG. 1schematically represents the high-pressure compressor22and high-pressure turbine26as mounted on the same shaft36so that the flow of hot exhaust gases that pass through the high-pressure turbine26turns the turbine26as well as the compressor22via the shaft36. The shaft36is supported with multiple rolling element bearings, of which a ball bearing38is represented inFIG. 1located near the entrance of the compressor22. The shaft36is mounted within an inner race of the bearing38, while an outer race of the bearing38is supported by a static structure of the core engine module14. FromFIG. 1, it should be apparent that the axis of the bearing38coincides with the centerline35of the engine10.

FIG. 2represents a cross-sectional view of a portion of the bearing38ofFIG. 1. As a ball bearing, the bearing38is shown as comprising an inner race40, an outer race42, rolling elements (balls)44(of which only one is shown inFIG. 2), and a cage46. The rolling elements44reside within grooves50and52defined in opposing surfaces of the races40and42, respectively, such that in combination the grooves50and52define the load-bearing contact surfaces of the bearing38. The cage46serves to maintain separation between the rolling elements44. InFIG. 2, each groove50and52is represented as having a semi-spherical cross-sectional shape that closely matches the curvature of the rolling elements44, though with a slightly larger radius than the rolling element44. Such a shape is commonly referred to as a circular arch, and provides a single contact point between each rolling element and each individual race40and42. The contact points or patches54and56are diametrically opposed as schematically represented inFIG. 2. The term “patches” refers to the fact that a true point contact does not exist when a bearing is loaded, and that the contact patches54and56have elliptical shapes caused by loading between the rolling elements44and the races40and42.

Due to the high rotational speeds required of the shaft36, the bearing38must operate at high rotational speeds. Specifically, though the outer race42does not rotate, the inner race40rotates at the same speed as the shaft36and the rolling elements44therebetween rotate around the inner race42at a lower speed than the inner race42. High-speed ball bearings of the type represented inFIG. 1are often dynamically cooled with a lubricant that flows through the bearing38. InFIG. 2, the inner race40of the bearing38is provided with under-race lubrication features in the form of multiple inlets48through which a lubricant (typically oil) is introduced into an annular-shaped cavity58defined by and between the inner and outer races40and42of the bearing38. The lubricant provides both lubrication and cooling of the rolling elements44and cage46within the cavity58. Under the influence of centrifugal forces caused by the spinning inner race40, the lubricant supplied through the inlets48flows radially outward to contact the cage46, the rolling elements44, and the outer race42. As represented inFIG. 2, because the bearing38is provided with an under-race lubrication system, the cage46is typically configured so that it bears against cage lands60on the inner race40.

Because the outer race42does not rotate and the inner race40, rolling elements44and cage46are moving at different speeds, the lubricant within the cavity58tends to churn, which as used herein refers to nonhomogeneous flow patterns within the cavity58. Analysis has shown that churning primarily occurs at the outer race42, and more particularly within the groove52of the outer race42, where the lubricant tends to accumulate before exiting the bearing38. Analysis has also indicated that churning occurs between the cage46and inner race40, as a result of a low pressure area created by the rotational effects of the high-speed rolling elements44. In conventional dynamically-lubricated bearing designs, the lubricant exits the bearing38at the inner and outer diameters of the cage46on both axial ends62and64of the bearing38, with the majority of the lubricant exiting at the outer diameter of the cage46in view of the position of the cage46against the inner race cage lands60. Furthermore, when the bearing38is operating with an axial load (as represented inFIG. 2), a majority of the lubricant will exit at the outer diameter of the cage46and on the unloaded side of the bearing38.

Various approaches have been proposed to promote the lubrication of rolling element bearings, including efforts to reduce heat generation at high rotational speeds. One such approach disclosed in U.S. Pat. No. 5,749,660 to Dusserre-Telmon et al. is the inclusion of a drain feature in the outer race. The drain features are orifices having entrances that are located in the groove of the outer race and exits that are located on the outer circumference of the outer race, so that the lubricant drains from the bearing by flowing completely through the outer race in a radially outward direction. The grooves of the inner and outer races do not have circular cross-sectional shapes matching the curvature of the rolling elements, but instead are described as having rather conical shapes that define vertices which form part of a central circumference of each groove. As a result, the rolling elements never cover the drain orifices located in the outer race groove, but instead touch the outer race at two lateral contact patches on each side of the orifices. Such a configuration is similar to conventional bearing races that have what is commonly referred to as a gothic arch, in which case the race is defined by two radii with different axes of curvature, as opposed to the aforementioned circular arch defined by a single radius. Similarly, the rolling elements contact the inner race groove at two lateral contact patches on each side of inlet orifices that are formed in the inner race to introduce the lubricant into the bearing, with the result that each rolling element can have as few as two and as many as four contact points with the inner and outer races.

While not intending to promote any particular interpretation of U.S. Pat. No. 5,749,660, it appears that the four-point contact may not be capable of operating with a low axial load conditions that would occur when the rotor thrust load changes direction during transitions from low to high speed conditions, as would be required in most gas turbine applications of the type represented inFIG. 1. Furthermore, the drain orifices may contribute significant stress concentrations in the outer race and reduce the ability of the bearing to survive ultra-high load events, such as fan blade out conditions.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides dynamically-lubricated bearings and to methods of dynamically lubricating bearings, including bearings of the type suitable for use in gas turbine engines.

According to a first aspect of the invention, a dynamically-lubricated bearing includes an inner race having an inner race groove between a pair of inner race cage lands, an outer race circumscribing the inner race and having an outer race groove that is between a pair of outer race cage lands and opposes the inner race groove, rolling elements disposed between the inner and outer races and in rolling contact with the inner and outer race grooves, and a cage disposed between the inner and outer races to maintain separation between the rolling elements. The cage comprises side rails and spacers therebetween that, in combination, define pockets in which each of the rolling elements is individually received. The side rails define a pair of outer diametrical surfaces of the cage that face the outer race cage lands of the cage and an oppositely-disposed pair of inner diametrical surfaces of the cage that face the inner race cage lands of the cage. The bearing is configured for introducing a lubricant into the cavity between the inner and outer races, and features are provided in the cage for enabling air to ingress into the pockets by fluidically interconnecting the pockets to an external environment surrounding at least one of a pair of axial ends of the bearing.

Another aspect of the invention is a method of dynamically lubricating a bearing that comprises the elements described above. The method includes installing the bearing in a gas turbine engine so as to support a shaft that interconnects a compressor and a turbine of the gas turbine engine. A lubricant is introduced into the cavity between the inner and outer races, and the inner race is rotated relative to the outer race so that air enters into the pockets of the cage through the features and causes the lubricant to exit the cavity of the bearing.

Another aspect of the invention is a method of dynamically lubricating a bearing that comprises an inner race having an inner race groove between a pair of inner race cage lands, an outer race that has an outer race groove that is between a pair of outer race cage lands and opposes the inner race groove, rolling elements disposed between the inner and outer races and in rolling contact with the inner and outer race grooves, and a cage disposed between the inner and outer races to maintain separation between the rolling elements. The cage includes side rails and spacers therebetween that, in combination, define pockets in which each of the rolling elements is individually received. The side rails define a pair of outer diametrical surfaces of the cage that face the outer race cage lands of the outer race and an oppositely-disposed pair of inner diametrical surfaces of the cage that face the inner race cage lands of the inner race. The method includes introducing a lubricant into a cavity between the inner and outer races, and then rotating the inner race relative to the outer race so that air enters the pockets of the cage through features defined in at least one of the side rails, the outer diametrical surfaces, and the inner diametrical surfaces of the cage. The air causes the lubricant to exit the cavity of the bearing.

A technical effect of the invention is the ability to reduce heat generation within bearings that are dynamically lubricated and operate at high rotational speeds. The reduction in heat generation is achieved with the use of air to purge lubricant from regions within the bearing that are prone to lubricant churning, such as the pockets of the cage, while providing a robust construction that is capable of withstanding ultra-high load events and thrust load crossovers, as would be required in many gas turbine applications.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3schematically represents a rolling element bearing100for the purpose of describing aspects of the present invention. It should be noted that the drawings are drawn for purposes of clarity when viewed in combination with the following description, and therefore are not necessarily to scale. To facilitate the description of the bearing100provided below, the terms “vertical,” “horizontal,” “lateral,” “front,” “rear,” “side,” “forward,” “rearward,” “upper,” “lower,” “above,” “below,” “right,” “left,” etc., may be used in reference to the perspective of the orientation of the bearing10inFIG. 3, and therefore are relative terms and should not be otherwise interpreted as limitations to the construction, installation and use of the bearing100.

As represented inFIG. 3, the bearing100has a similar construction to that of the bearing represented inFIG. 2. As such, the bearing100is represented as a ball bearing that comprises an inner race102, an outer race104that circumscribes the inner race102, rolling elements (balls)106(of which only one is shown inFIG. 3) between the inner and outer races102and104, and a cage108that serves to maintain separation between the rolling elements106. For this purpose, the cage108comprises side rails134and136and spacers138therebetween that, in combination, define pockets142in which each rolling element106is individually received. Each adjacent pair of rolling elements106is separated by one of the spacers138.

Each of the inner race102, outer race104and cage106has an annular shape, as is typical for rolling element bearings. The rolling elements106reside within grooves110and112defined in opposing surfaces of the races102and104, respectively. Each groove110and112is axially disposed between a pair of cage lands124and126, respectively, defined on their corresponding inner or outer race102and104. In combination, the grooves110and112define the load-bearing contact surfaces of the bearing38. As with conventional ball bearings, the grooves110and112may have semi-spherical cross-sectional shapes that closely match the curvature of the rolling elements106to provide two or more contact patches between each rolling element106and the races102and104, as was described in reference toFIG. 2. In particular, either or both of the inner and outer races102and104may have a traditional gothic arch shape or a traditional circular arch shape (according to the previously-noted definitions for these terms). In preferred embodiments of the invention, the inner race102has a gothic arch shape and the outer race104has a circular arch shape.

The bearing100can be adapted for use in high-speed rotational applications, including mounting of the shaft36ofFIG. 1. As previously described, typically in such applications the outer race104does not rotate, the inner race102rotates at the same speed as the shaft36, and the rolling elements106rotate around the inner race102at a lower speed than the inner race102. Also similar to the bearing38represented inFIG. 2, the bearing100is configured to be dynamically cooled with a lubricant that enters the bearing100through inlet orifices118located in the inner race102, providing what may be referred to as an under-race lubrication system. With these orifices118, the bearing100is provided with a lubrication capability in which a lubricant (typically oil) is introduced into an annular-shaped cavity120defined by and between the inner and outer races102and104of the bearing100to provide both lubrication and cooling of the rolling elements106and cage108. Under the influence of centrifugal forces induced by the spinning inner race102, the lubricant supplied through the orifices118flows radially outward to contact the cage108, the rolling elements106, and the outer race104.FIG. 3represents a preferred configuration in which three orifices118are present. With this approach, a relatively high percentage of the total lubricant flow can be caused to flow through the center orifice118to feed the rolling elements106, while a smaller percentage of the total lubricant flow is delivered to each of the two remaining orifices118to lubricate the cage lands124. Since the lubricant tends to centrifuge radially outward, a benefit of this configuration is that lubricant is provided directly to the cage lands124, especially during critical operating phases such as start up and shut down when cage rubs are likely to occur.

Because the outer race104does not rotate and the inner race102, rolling elements106and cage108rotate at different speeds, the lubricant within the cavity120would ordinarily tend to churn. Churning tends to occur within the groove112of the outer race104where the lubricant accumulates before exiting the bearing100, as well as that part of each cage pocket142between the cage108and inner race102due to a low pressure area created by the rotational effects of the rolling elements106. To alleviate churning within this low pressure area, the cage108is provided with features122that dynamically promote the ingress of air into the cage pockets142, which in turn is capable of promoting the flow of lubricant through the bearing cavity120, including the outflow of lubricant at the inner and outer diameters of the cage108at both axial ends114and116of the bearing100. InFIG. 3, the features122are represented as surface features122defined in each of a pair of outer diametrical surfaces130of the cage108defined by the side rails134and136of the cage108, opposite a corresponding pair of inner diametrical surfaces132(FIG. 6) that are also defined by the side rails134and136and bear against the cage lands124of the inner race102. Two embodiments of the surface features122are represented inFIGS. 4 and 5as continuous channels or passages that entirely extend across their corresponding outer diametrical surfaces130. As a result, each surface feature122defines a continuous channel or passage that is recessed below the surrounding surface of one of the outer diametrical surfaces130, and fluidically interconnects the cage pockets142to the external environment surrounding one of the axial ends114and116of the bearing100. The surface features122promote the ingress of air into the aforementioned low pressure area created within the cage pockets142by the rotational effects of the rolling elements106, which relieves the low pressure condition to promote the flow of lubricant throughout the bearing cavity120as well as egress of the lubricant from the cage pockets142to the surrounding environment. In this manner, the invention is capable of reducing the viscous heat generation that would otherwise occur due to churning of the lubricant within the bearing cavity120.

As evident fromFIGS. 4 and 5, the surface features122are angled relative to the direction of rotation or travel140of the cage108and its rolling elements106. InFIG. 4, the surface features122are also angled relative to the axis128of the bearing100(FIG. 3), which coincides with the engine centerline35(FIG. 1), whereas inFIG. 5the surface features122are parallel to the bearing axis128. As surface features122defined in the outer diametrical surfaces130of the cage108, the features122can be readily created by machining the outer diametrical surfaces130of the cage108using a variety of conventional machining equipment.

The number, depth, width, and orientation of the features122relative to the travel direction140of the rolling elements106can be readily tailored to promote the ability of air to enter the cage pockets142and reduce the degree of churning that occurs prior to the lubricant exiting the bearing100. Preferred numbers, depths and widths of the features122will depend in part on the size (diameter and axial length) of the bearing100, the properties of the lubricant, and the desired flow rate of the lubricant through the bearing100. For the application represented inFIG. 1, suitable depths (d inFIG. 3) for the features122are believed to be about 0.03 inch (about 0.75 mm) below the surrounding surfaces of the outer diametrical surfaces130of the cage108, though lesser and greater depths are foreseeable. The width (w inFIG. 4) of each feature122is preferably greater than its depth, with suitable widths believed to be about 0.12 to about 0.13 inch (about 3 to 3.3 mm), though lesser and greater widths are foreseeable. To provide an adequate airflow capacity and reduce localized churning of the lubricant within the cage pockets142, it is believed that at least one feature122should be provided for each rolling element106to promote a more efficient removal of lubricant from the outer race groove112. Certain other geometrical considerations are believed to exist, including the desirability for the features122to have flat sidewalls to promote the capture of lubricant from the bearing cavity120.

InFIG. 4, the features122are represented as straight channels that are oriented so as to be inclined at an acute angle, α, or at an obtuse angle, β, to the direction140that the rolling elements106travel with the cage108. The features122represented inFIG. 4are inclined at an acute angle (α) of about thirty degrees or an obtuse angle (β) of about 150 degrees to the travel direction140of the rolling elements106, though it is believed that other acute and obtuse angles can be used.FIG. 5represents another embodiment of the invention in which both sets of features122are disposed approximately perpendicular to the travel direction140of the rolling elements106. Though within the scope of the invention, models have indicated that the configuration represented inFIG. 5would not be as effective as that ofFIG. 4. The features122are represented inFIGS. 4 and 5as straight, though it is foreseeable that the features122could be formed to have an arcuate shape. A potential benefit of curved features122would be that the capture angle for the lubricant could be more shallow (less than 30 degrees), and a curved shape could allow more features122to be accommodated within an available space. Furthermore, it is foreseeable that the features122on either or both outer diametrical surfaces130could differ from each other, for example, the features122could differ from each other in terms of their shape, width, depth and orientation.

The features122are capable of promoting the flow of lubricant from the bearing cavity120, and therefore reduce heat generation within the bearing100by reducing churning of the lubricant within the regions of the cage pockets142adjacent the inner race102. By reducing the heat generation within the bearing100, the invention further has the capability of reducing the capacity of the lubrication system coolers that would otherwise be required to cool the bearings of a gas turbine engine. In turn, reducing the size of the coolers reduces the weight and performance losses of the engine and consequently improves the fuel consumption for the engine and aircraft.

The surface features122depicted inFIGS. 4 and 5are also believed to provide advantages over prior attempts to reduce heat generation within dynamically-lubricated bearings of the type represented inFIG. 2. One advantage is that the features122are limited to the outer diametrical surfaces130of the cage108, which are nonfunctional surfaces of bearings equipped with an under-race lubrication system as a result of the cage108bearing against the cage lands124of the inner race102(as represented for the bearings38and100ofFIGS. 2 and 3). By avoiding the use of drain orifices that pass entirely through the outer race104(as done in U.S. Pat. No. 5,749,660), the invention avoids any loss in structural integrity that would occur as a result of stress concentrations associated with through-holes in the outer race104. As such, it is believe that the bearing100would be more capable of surviving ultra-high load events, such as fan blade out conditions.

Because the features122are not located within the outer race groove112, the invention also avoids any concern for damage occurring to the rolling elements106as a result of contact with the features122. This advantage is in contrast to U.S. Pat. No. 5,749,660, whose race grooves must each be machined to have a conical shape so that the inlet and drain orifices formed in the race grooves never come into contact with the rolling elements. Consequently, the invention can make use of an inner groove110having a gothic arch shape, while the outer groove112may have a circular arch shape that more closely matches the curvature of the rolling elements106. As a result, depending on the loading conditions, contact between each rolling element106and the races102and104may occur at two locations that are diametrically opposed (similar to what is schematically represented inFIG. 2), or at two locations to one side of the element106, or at more than two locations. It is believed that, in contrast to the bearing of U.S. Pat. No. 5,749,660, the bearing100described above is likely to be more capable of operating with a thrust load crossover, as is typically required in gas turbine applications of the type represented inFIG. 1.

The invention can also be adapted to bearings that do not utilize under-race lubrication. For example, for bearings that are supplied a lubricant through a side jet directed at the cavity120, the cage108could be configured to bear against the outer race cage lands126, as represented inFIG. 6. In this case, the features122can be formed in the inner diametrical surfaces132of the cage108in the same manner as described above for the features122formed in the outer diametrical surfaces130of the cage108, in which case air ingress into the cage pockets142is promoted along the inner diametrical surfaces132.

FIG. 7represents another alternative for the surface features122, in which the surface features122are not continuous channels or passages that entirely extend across their corresponding inner diametrical surfaces132, but instead only partially extend across the inner diametrical surfaces132. The surface features122ofFIG. 7have a different effect than the surface features122ofFIGS. 4 and 5, in that the features122ofFIG. 7are intended to reduce the amount of lubricant that flows into the cage pockets142from the inlet orifices118located in the inner race102. While not wishing to be limited to any particular theory, it is believed surface features122defined in the inner diametrical surfaces132of the cage108are capable of behaving as impellers that help to pump the lubricant out of the cage pockets142. The features122also reduce the contact area between the inner diametrical surfaces132and the inner race102.

FIGS. 8 and 9represent still another alternative for the features122that are capable of dynamically promoting the ingress of air into the cage pockets142. The features122ofFIGS. 8 and 9differ from those ofFIGS. 3 through 7, in that the former are through-holes defined in the cage108, and specifically in the side rails134and136of the cage108that separate the rolling elements106from the axial ends of the cage108. The through-hole features122ofFIGS. 8 and 9allow air to ingress to the inner diameter of the cage108, which promotes the flow of lubricant from the inner diameter of the cage108to its outer diametrical surfaces130and then beyond to the external environment surrounding the axial ends114and116of the bearing100. As such, the through-hole features122promote the ingress of air into the aforementioned low pressure area created within the cage pockets142by the rotational effects of the rolling elements106.FIGS. 10 and 11represent additional features for promoting the flow of lubricant radially outward through the cage108. The features are in the form of recesses144located in each corner of the cage pocket142, creating passages146through which the lubricant can flow around each rolling element106(shown in phantom inFIG. 11).

By relieving the low pressure condition, the flow of lubricant throughout the bearing cavity120is promoted, as well as egress of the lubricant from the cage pockets142to the surrounding environment. As evident fromFIGS. 8 and 9, the through-hole features122can be formed as straight channels that are oriented so as to be inclined at an acute angle to the direction140that the rolling elements106travel with the cage108. Due to limitations of space and stress, the through-hole features122will typically be limited to diameters of less than 0.1 inch (about 2.5 mm), for example, about 0.075 to about 0.085 inch (about 1.9 to about 2.1 mm), though it is foreseeable that larger features122could be formed to further enhance lubricant flow through the cage108and reduce heat generation. Similar to the surface features ofFIGS. 3 through 7, it is believed that the through-hole features122ofFIGS. 8 and 9can be provided in numbers roughly equal to the number of rolling elements106held by the cage108to provide an adequate airflow capacity and reduce localized churning of the lubricant within the cage pockets142.

Finally,FIG. 10represents an additional aspect of the invention by which bearing performance can be promoted by increasing the lubricant turbulence adjacent the cage spacers138in order to reduce drag forces attributable to rotation of the rolling elements106within the cage pockets142. In particular, the surfaces of the spacers138facing the rolling elements106are dimpled with numerous recesses148, similar to the surface of a golf ball. Semispherical recesses148are believed to be satisfactory, though it is foreseeable that recesses148of a variety of shapes and sizes could be used. Notably, this aspect of the invention can be used independently or in combination with any of the embodiments described in reference toFIGS. 3 through 9.

From the above, it should be appreciated that the bearings100described above are well suited for installation in a variety of applications, in addition to gas turbine engines. Generally, any of the bearings100can be installed so that the rotation of its rolling elements106results in the elements106circumferentially traveling between the inner and outer races102and104while contacting their respective grooves110and112. A lubricant injected or otherwise delivered into the cavity120is then drawn through the cavity120, more particularly from the cage pockets142, as a result of the features122promoting ingress of air into the cage pockets142, which in turn promotes the expulsion of lubricant from the cage pockets142and the bearing cavity120. As such, the lubricant is not drained from the bearing cavity120through the outer race104, but instead is drawn from the cavity120between the cage lands124and126and the cage108therebetween.

While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the bearing100could differ from that shown, and various materials and processes could be used to construct and fabricate the bearing100. Therefore, the scope of the invention is to be limited only by the following claims.