Electric motor comprising a lubricant support of the bearing structure

Abstract

A lubricant supported electric motor includes a stator having a stator raceway and a rotor movable relative to the stator about an axis. The rotor has a rotor raceway disposed in radially spaced and opposing relation to the stator raceway to define a gap therebetween. A lubricant is disposed in the gap to support the rotor relative to the stator. The stator raceway includes a load bearing structure including a plurality of hydrodynamic surfaces and a plurality of hydrostatic pockets. Wherein the plurality of hydrodynamic surfaces are aligned in parallel relation along the stator raceway and the plurality of hydrostatic pockets are disposed in radially recessed relation relative to the plurality of hydrodynamic surfaces.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This PCT International Patent Application claims priority to U.S. Utility Patent Application No. 17 / 065,800, filed October 8, 2020, which claims priority to U.S. Provisional Application No. 62 / 912,130, filed October 8, 2019, the entire disclosures of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to electric machines, such as electric motors and generators. More particularly, the present disclosure relates to lubricant supported electric motors. BACKGROUND

[0004] This section provides a general summary of background information and the comments and examples provided in this section are not necessarily prior art to the present disclosure.

[0005] Automotive, truck, and certain off-highway applications have various drive trains that derive power from a central prime mover and use mechanical devices, such as transmissions, transfer cases, propeller shafts, and live axles, to distribute power to the wheels. These configurations work well when the prime mover can be bulky or heavy, such as various internal combustion engines (“ICE”). However, more attention is being directed toward alternative arrangements of prime movers that provide improved environmental performance, eliminate mechanical drive train components, and result in lighter weight vehicles with more passenger and payload space.

[0006] “On-wheel,” “in-wheel,” or “near-wheel” electric motor configurations are an alternative arrangement to traditional ICE prime movers that distribute prime mover functionality to each or some of a plurality of wheels via one or more electric motors disposed on, in, or near the plurality of wheels. For example, in one instance, a traction motor, using a central shaft through a rotor and rolling element bearing to support the rotor, can be used as an “on-wheel,” “in-wheel,” or “near-wheel” electric motor configuration. In another instance, a lubricant supported electric motor, as described in U.S. Application No. 16 / 144,002, the disclosure of which is incorporated herein by reference, can be used as an “on-wheel,” “in-wheel,” or “near-wheel” electric motor configuration. Each of these configurations results in a small size and light weight as compared to ICE based prime movers, each of which has certain drawbacks or deficiencies.

[0007] For example, the use of traction motors in "on-wheel," "in-wheel," or "near- wheel" configurations still results in motors that are too heavy and not robust enough for the impact loads to be used in a wheel end application. These traction motors also have to be supported by rolling element bearings, typically at each end of the motor shaft, which also makes them too heavy and too large for use in a wheel end application. These conventional rolling element bearings have limited ability to withstand (absorb) large sudden impacts and also consume significant space within the motor, which is detrimental to torque production. Similarly, the use of lubricant supported motors in "on-wheel," "in-wheel," or "near-wheel" motor applications in automotive or land vehicle applications results in arrangements that have some performance issues when subjected to a wide range of power that occurs during operation over a wide range of speeds that occur in prime mover applications. Lubricant supported motors also typically do not include rotor and stator structures that are sufficiently designed to act as load bearings. Thus, there is a continuing need for lubricant supported motors that can improve performance during operation in high impact and vibration environments while providing the lighter and smaller footprint sought after in alternative prime mover implementations. SUMMARY

[0008] The present invention generally relates to a lubricant supported motor including a stator having a stator raceway and a rotor movable relative to the stator about an axis. The rotor has a rotor raceway disposed in radially spaced and opposing relation to the stator raceway to define a gap therebetween. A lubricant is disposed in the gap to support the rotor relative to the stator. The stator raceway includes a load bearing structure including a plurality of hydrodynamic surfaces and a plurality of hydrostatic pockets. Wherein the plurality of hydrodynamic surfaces are aligned in parallel relation along the stator raceway and the plurality of hydrostatic pockets are disposed in radially recessed relation relative to the plurality of hydrodynamic surfaces. The load bearing structure in combination with the stator provides a lubricant supported motor having improved rotor-stator system stiffness to allow the lubricant supported motor to be used in high impact and high vibration environments, such as the transportation and manufacturing industries, despite being designed to be light and small relative to existing motors. The load bearing structure also allows the lubricant supported motor to have higher efficiency compared to existing designs. Other advantages can be understood by the following more detailed description of the invention. BRIEF DESCRIPTION OF DRAWINGS

[0009] The drawings described herein are for illustrative purposes only of selected aspects and are not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0010] Figure 1is a cross-sectional side view of an exemplary lubricant supported electric motor showing a rotor extending along an axis and rotatably disposed within a stator to define a gap therebetween, and a lubricant disposed within the gap for supporting the rotor within the stator;

[0011] Figure 2 is Figure 1 is a perspective view of an exemplary stator formed of a plurality of stator laminations stacked along an axis with one another;

[0012] Figure 3 is a perspective view of an alternative arrangement of a stator for implementation in an interchanged arrangement of a lubricant supported electric motor, wherein the stator extends along an axis and the rotor is rotatably arranged about the stator;

[0013] Figure 4 is a cross-sectional side view of a lubricant supported electric motor showing a load bearing structure disposed on an inner stator raceway of a stator;

[0014] Figure 5 is Figure 4 is a cross-sectional side view of a lubricant supported electric motor showing a load bearing structure formed of a plurality of stator laminations;

[0015] Figure 6 is Figure 4 is a cross-sectional side view of a lubricant supported electric motor showing a load bearing structure formed of a continuous sleeve disposed axially about a stator lamination between a first stator end and a second stator end;

[0016] Figure 7 is a cross-sectional side view of a lubricant supported electric motor showing a load bearing structure disposed on an outer stator raceway of a stator and formed of a plurality of stator laminations;

[0017] Figure 8 is a cross-sectional side view of a lubricant supported electric motor showing a load bearing structure disposed on an outer stator raceway of a stator and formed of a continuous sleeve;

[0018] Figure 9 is Figure 8 is a perspective side view of a stator of

[0019] Figure 10 is Figure 5 is an enlarged cross-sectional view of a portion of each of a plurality of stator laminations forming a hydrodynamic surface, the plurality of stator laminations having a circumferential outer edge that is inclined at a first edge side and a second edge side to define a plurality of lubricant release channels, wherein the first edge side is disposed toward a first stator end and the second edge side is disposed toward a second stator end;

[0020] Figure 11 is Figure 7 part of a magnified cross-sectional view showing each of a plurality of stator laminations forming a hydrodynamic surface having a circumferential outer edge that is angled at the first edge side and the second edge side to define a plurality of lubricant release channels, wherein the first edge side is disposed toward a first stator end and the second edge side is disposed toward a second stator end; and

[0021] Figure 12 is a magnified cross-sectional view of a stator lamination forming a hydrodynamic surface showing a plurality of lubricant supply tubes passing between adjacent stator laminations. DETAILED DESCRIPTION

[0022] Example embodiments of a lubricant supported electric motor according to the present disclosure will now be more fully described. Each of the example embodiments is provided to make the disclosure thorough and to convey the full scope of the inventive concept, features, and advantages of the present invention to those skilled in the art. For this purpose, numerous specific details are set forth, such as examples of specific components, devices, and methods, associated with the lubricant supported electric motor, in order to provide a thorough understanding of each embodiment associated with the present disclosure. It will be apparent, however, to one skilled in the art that the example embodiments can be practiced without employing all of the specific details provided. In other instances, well-known

[0023] Figure 1 and Figures 4-11 A lubricant supported electric motor 10 according to an aspect of the present invention is shown. As best shown in Figure 1 , the lubricant supported electric motor 10 includes a stator 12 and a rotor 14 that extends along an axis A and is movably (i.e., rotatably) disposed within the stator 12 to define a gap 16 (see Figures 7-8 and Figure 11 , also shown as "G" in Figure 1 ). In alternative arrangements, the stator 12 and the rotor 14 can be interchanged, where the stator 12 extends along the axis A and the rotor 14 is rotatably disposed about the stator 12 (see, for example, Figures 3-6 and Figure 10 ) without departing from the scope of the present disclosure. In either arrangement, and as best shown in Figures 2-3 , the stator 12 is formed from a plurality of stator laminations 17 that are stacked one on top of another along the axis A.

[0024] A lubricant 18 is disposed in the gap 16 for supporting the rotor 14 within or about the stator 12 and providing continuous contact between these components. Thus, the lubricant 18 can act as a cushion (e.g., suspension) between the stator 12 and the rotor 14 to minimize or prevent contact therebetween. In other words, the lubricant 18 prevents direct contact between the stator 12 and the rotor 14 and provides a lubricant-supported electric motor 10 that can be robust against impact and vibration loads due to the presence of the lubricant 18. Additionally, and optionally, a substantially incompressible lubricant 18 can be used in order to minimize the gap between the stator 12 and the rotor 14.

[0025] As shown in Figure 1 The stator 12 defines a passage 20 that is disposed in fluid communication with the gap 16 for introducing the lubricant 18. However, the passage 20 can be disposed on any other component of the lubricant-supported electric motor 10 without departing from the present disclosure. According to one aspect, the lubricant 18 can be circulated or pumped through the passage 20 and into the gap 16 in various ways. For example, a high pressure source (e.g., pump) 24 of the lubricant 18 can be fluidly connected to a low pressure source (e.g., reservoir) 26 of the lubricant 18, where the lubricant 18 can be moved from the low pressure source to the high pressure source, through the passage 20 and into the gap 16. Rotation of the rotor 14 relative to the stator 12 can operate as a self-pump to drive the lubricant 18 through the passage 20 and into the gap 16.

[0026] As further shown in Figure 1 The rotor 14 is preferably interconnected with a drive assembly 22 for connecting the lubricant-supported electric motor 10 to one of a plurality of wheels of a vehicle. For example, in one instance, the drive assembly 22 can include a planetary gear system. Alternatively, the drive assembly 22 can include one or more parallel shaft gears. However, the rotor 14 can be directly connected to a wheel of a vehicle without departing from the scope of the present disclosure. The stator 12 and the rotor 14 are configured to exert electromagnetic forces therebetween to convert electrical energy to mechanical energy to move the rotor 14 and ultimately drive a wheel connected to the lubricant-supported electric motor 10 via the drive assembly 22. The drive assembly 22 can provide one or more speed reduction ratios between the lubricant-supported electric motor 10 and the wheel in response to movement of the rotor 14.

[0027] As best shown in Figure 1 and Figures 4-8 The rotor 14 has a rotor raceway 28 (in Figure 1 and Figures 7-8The stator 12 has the stator raceway 30 (arranged as an inner raceway). Figure 1 and Figures 7-8 The stator raceway 30 is arranged opposite to the rotor raceway 28. However, when the stator 12 and the rotor 14 are arranged opposite to each other relative to axis A, the corresponding raceways 28 and 30 are similarly interchanged, with the stator raceway 30 arranged as the inner raceway and the rotor raceway 28 arranged as the outer raceway (see...). Figures 4-6 In any arrangement, and as in Figures 4-8 As best shown in the diagram, the stator raceway 30 includes a support structure 32 comprising a plurality of hydraulic surfaces 34, each hydraulic surface 34 being arranged axially spaced from each other and circumferentially arranged along the stator raceway 30 parallel to axis A. In other words, each hydraulic surface 34 extends circumferentially along the stator raceway 30 and is arranged radially spaced from adjacent hydraulic surfaces among the plurality of hydraulic surfaces 34.

[0028] The support structure 32 of the stator 12 further defines a plurality of static fluid pockets 36', 36"". The static fluid pockets 36', 36" extend circumferentially along the stator raceway 30 and are radially recessed relative to the hydraulic surfaces 34. Each of the plurality of static fluid pockets 36', 36" is axially disposed between a pair of corresponding hydraulic surfaces 34. In a preferred arrangement, each of the plurality of static fluid pockets 36', 36" is rectangular; however, other shapes may be used without departing from the scope of this disclosure. The plurality of static fluid bag portions 36', 36" include a first group of static fluid bag portions 36' and a second group of static fluid bag portions 36" wherein the first group of static fluid bag portions 36' extends along the stator raceway 30 at circumferential intervals and adjacent to a first end 38 of the stator 12, and the second group of static fluid bag portions 36" extends along the stator raceway 30 at circumferential intervals and adjacent to a second end 40 of the stator 12. However, additional groups of static fluid bag portions 36', 36" may be used without departing from the scope of this disclosure. Figures 4-8 As shown, the stator includes a plurality of lubricant supply pipes 42, which are configured to be in fluid communication with each set of static fluid bags 36', 36" for delivering lubricant 18 to the static fluid bags 36', 36" and then continuously to the hydraulic surface 34. Figure 12 As shown, the lubricant supply pipe 42 preferably passes between adjacent stator laminations 17. However, the lubricant supply pipe 42 may also pass through the stator winding area if packaging space permits.

[0029] As in Figures 4-8As further shown, the carrier structure 32 of the stator 12 includes a drain 44 recessed radially from the hydrodynamic surface 34 and extending circumferentially along the stator raceway 30, the drain 44 being disposed in axially spaced relation between the first and second sets of pockets 36' and 36". The drain 44 is preferably arranged at the center of the stator raceway 30. Also, the drain 44 is designed to capture lubricant 18 from the hydrodynamic surface 34 and initiate a low pressure return / drain of the lubricant 18 through the stator 12 to the sump 26. Accordingly, the stator 12 further includes a lubricant return tube 46 disposed in fluid communication with the drain 44, the lubricant return tube 46 also preferably being arranged through and between adjacent stator laminations 17 (see Figure 12 ).

[0030] As shown in Figures 4-6 , when the stator 12 is arranged along the axis A, the carrier structure 32 is arranged along the inner raceway 30 of the stator. However, as shown in Figures 7-9 , when the arrangement of the stator 12 and the rotor 14 relative to the axis A is interchanged, and the rotor 14 is arranged along the axis A, the carrier structure 32 is arranged along the outer raceway 30 of the stator. In either arrangement, the carrier structure 32 can be formed in a variety of ways, such as but not limited to the following exemplary embodiments.

[0031] According to one aspect of the present disclosure, the carrier structure 32 can be formed from the stator laminations 17 themselves. More specifically, the outer diameter of the stator laminations (as shown in Figure 5 ) or the inner diameter of the stator laminations (as shown in Figure 7 ) can be machined to form the hydrodynamic surface 34, the pockets 36' 36", and the drain 44. Certain sets of the stator laminations 17 are machined to reduce the overall diameter of the stator laminations relative to adjacent stator laminations, thereby forming recessed areas that form the pockets 36' 36" and the drain 44. The remaining stator laminations 17 can be machined thin enough to produce a smooth hydrodynamic carrying surface 34 (e.g., an Ra (average roughness) of less than 0.4 microns) without sharp edges. The lamination surfaces are machined thin enough to minimize eddy current losses therein. Alternatively, instead of finishing these surfaces, thin sheets (e.g., rings or strips) of a carrier material can be placed on the hydrodynamic surface 34 and secured to the respective stator laminations 17 by an adhesive or the like.

[0032] According to another aspect of the present disclosure, and as shown in Figure 6 and Figure 8As best shown, the load bearing structure 32 can be formed by placing and disposing a continuous sleeve 48 over the stator laminations 17 between the first end 38 and the second end 40 of the stator 12. In this arrangement, the stacked stator laminations 17 form a continuous inner or outer surface (see, e.g. Figures 2-3 ), over which the continuous sleeve 48 is placed to form the hydrodynamic surface 34, the hydrostatic pocket 36, 36", and the drain 44. Figure 9 A perspective view of the continuous sleeve 48 arranged along the stator outer raceway 30 is shown. The continuous sleeve 38 can be secured to the stator laminations 17 using a variety of methods, including but not limited to a press-in interference fit, a shrink-in interference fit, an adhesive, or mechanical features (e.g., ribs or splines).

[0033] Similar to the method of forming the stator 12, the structure of the rotor 14 can also be formed in a variety of similar ways. For example, the inner or outer rotor raceway 28 can be finish formed to produce a smooth load bearing surface (e.g., less than 0.4 microns Ra). Alternatively, the inner or outer rotor raceway 28 can be formed from a thin sheet (e.g., a ring or strip) of load bearing material held by an adhesive. Additionally, the inner or outer rotor raceway 28 can be formed from a continuous rotor sleeve secured to the rotor 14 using a variety of methods, including a press-in interference fit, a shrink-in interference fit, an adhesive, or mechanical features (e.g., ribs or splines). The continuous rotor sleeve can also provide mechanical support for the integrity of the rotor 14, such as when the lubricant supported motor 10 is operating at high speeds where the structure of the rotor 14 is subjected to large outward forces. This structure is particularly useful for magnet retention, typically used for laminated support of the rotor 14, and also for squirrel cage robustness in induction machines, when the lubricant supported motor 10 is a permanent magnet brushless type. According to an aspect of the present disclosure, some lubricant supported motors 10 have a solid rotor 14 (e.g., special induction machines) whose surface can be used directly as a load bearing surface.

[0034] Figures 10-11 An enlarged cross-sectional view of a hydrodynamic surface 34 formed from the inner or outer diameter of a stator lamination 17 is shown. As shown in Figure 10 When the rotor 14 is rotatably disposed about the stator 12, each stator lamination 17 forming the hydrodynamic surface 34 can include a circumferential outer edge 50 that is beveled at a first edge side 52 disposed toward the first end 38 of the stator 12 and a second edge side 54 disposed toward the second end 40 of the stator. Alternatively, as shown in Figure 11As shown, each stator lamination 17 forming the hydrodynamic surface 34 can include a circumferential inner edge 56 similarly beveled at a first edge side 52 disposed toward the first end of the stator 12 and a second edge side 54 disposed toward the second end of the stator, when the rotor 14 is arranged along the axis A. The beveled circumferential outer edge 50 or circumferential inner edge 56 advantageously incorporates lubricant flow features into the hydrodynamic surface 34 of the bearing structure 32, thereby forming a slight recessed lubricant release channel 58 disposed between each adjacent stator lamination 17, which can allow proper lubricant flow to improve bearing performance and stability. The release channel 58 formed between stator laminations 17 can be connected to lubricant conducting channels and lubricant flow control devices.

[0035] Additionally and alternatively, the stack of stator laminations 17 can include stator laminations of slightly different diameters, such that every other or every third lamination (generally, one or more stator laminations in "n") is slightly recessed (larger inner diameter) than the other laminations. The stator laminations with the smaller inner diameter are ground to form the hydrodynamic surface 34, and the stator laminations with the larger inner diameter are not ground, thus forming a recess or "release area" every "n" laminations.

[0036] The bearing structure 32 according to the present application advantageously allows for a number of bearing control configurations and methods not provided by prior art lubricant supported electric motors. For example, the bearing structure 32 provides increased bearing stiffness by pumping more lubricant 18 to the hydrodynamic surface 34 via the radial lubricant supply tube 46 and release channel 58. Such configurations and methods can be used, for example, to minimize the effects of critical speed resonance in the rotor 14 and stator 12 of the lubricant supported electric motor 10. Additionally, by allowing lubricant to flow out of the hydrodynamic surface 34 via the release channel 58, the lubricant supply tube 42, and the lubricant return tube 46, pressure in the hydrodynamic surface 34 can be reduced, thereby providing reduced bearing shear losses. Both of these effects can be further controlled by increasing or decreasing bearing pressure radially around the bearing structure 32 or axially along the bearing structure 32. Additionally, thermal control of the hydrodynamic surface 32 can be further controlled on a segment by segment basis.

[0037] According to an aspect of the present disclosure, the stator laminations 17 preferably include the following properties, features, and support. The stator laminations 17 include an effective flux conduction similar to the current in the motor windings. The stator laminations 17 also include radial mechanical stiffness to accommodate bearing loads without buckling the stator laminations 17. The stator laminations 17 also include stator slot clearances for winding and lubricant flow. The stator laminations 17 also include stator tooth widths for lubricant passage and lamination jointing to provide electrical insulation between stator laminations and mechanical adhesion of the stator laminations. The stator laminations 17 also preferably include mechanical structures that create compression of the lamination stack (e.g., stator housing, rotor hub, weldments, and through-pins), and internal diameter variations that create shoulders (e.g., for retaining bearing rings, bearing sleeves, hydrostatic pocket sleeves, and drain gutter sleeves).

[0038] The improvements to the lubricant supported motor detailed herein have advantages including higher levels of efficiency, rigid and stable load bearing structures to support the rotor of the lubricant supported motor, and bearing capability that can withstand high impact (vibration). Additional advantages include space savings (compact), and adequate cooling (through the lubricant) of the motor windings to increase the torque density (i.e., torque per volume) of the lubricant supported motor 10.

[0039] Obviously, many modifications and changes can be made to the present disclosure in light of the above teachings, and therefore, the present disclosure is to be understood to not be limited by the particular description described above but is to be understood broadly within the scope of the appended claims. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but are interchangeable with other embodiments, as appropriate, and are not exclusive. Even if not specifically described or shown, an embodiment can use alternative methods and materials to practice the application.

Claims

1. A lubricant supported electric motor characterized by, The electric motor comprises: a stator having a stator raceway; a rotor movable relative to the stator about an axis, the rotor having a rotor raceway disposed in radially spaced and opposing relation to the stator raceway to define a gap therebetween; and a lubricant disposed in the gap for supporting the rotor relative to the stator; and the stator raceway includes a load bearing structure comprising a plurality of hydrodynamic surfaces and a plurality of hydrostatic pockets; the plurality of hydrodynamic surfaces are arranged in parallel relation along the stator raceway, the plurality of hydrostatic pockets are disposed in radially recessed relation relative to the plurality of hydrodynamic surfaces; a plurality of lubricant supply tubes are disposed in fluid communication with the plurality of hydrostatic pockets for delivering the lubricant to the plurality of hydrostatic pockets and then successively to the plurality of hydrodynamic surfaces and the gap for supporting the rotor relative to the stator; the stator further includes a drain disposed centrally of the stator raceway, the drain being radially recessed from the plurality of hydrodynamic surfaces, extending circumferentially along the stator raceway, and disposed in axially spaced relation to the plurality of hydrostatic pockets.

2. The lubricant supported electric motor of claim 1, wherein, the plurality of hydrodynamic surfaces are each disposed in axially spaced relation to one another, and each of the plurality of hydrostatic pockets is disposed axially between a pair of adjacent hydrodynamic surfaces of the plurality of hydrodynamic surfaces.

3. The lubricant supported electric motor of claim 2, wherein, the stator extends axially from a first stator end to a second stator end, and the plurality of hydrostatic pockets includes a first set of hydrostatic pockets extending in circumferentially spaced and aligned relation proximate the first stator end and a second set of hydrostatic pockets extending in circumferentially spaced and aligned relation proximate the second stator end.

4. The lubricant supported electric motor of claim 3, wherein, the drain is disposed between the first set of hydrostatic pockets and the second set of hydrostatic pockets.

5. The lubricant supported electric motor of claim 1 wherein, the stator includes at least one lubricant return tube disposed in fluid communication with the drain.

6. The lubricant supported electric motor of claim 5 wherein, the stator includes a plurality of stator laminations stacked one upon another along the axis, and each of the plurality of lubricant supply tubes and the at least one lubricant return tube passes between adjacent ones of the stacked stator laminations.

7. The lubricant supported electric motor of claim 6 wherein, an inner diameter or an outer diameter of the stacked stator laminations is machined to form the plurality of hydrodynamic surfaces, the plurality of hydrostatic pockets, and the drain.

8. The lubricant supported electric motor of claim 7, wherein, each of the stator laminations that collectively define the plurality of hydrodynamic surfaces includes a circumferential edge that is beveled at a first edge side disposed toward a first end of the stator and a second edge side disposed toward a second end of the stator to form a lubricant release channel; the lubricant release channel is disposed between adjacent first and second beveled side edges of each stator lamination that forms the plurality of hydrodynamic surfaces.

9. The lubricant supported electric motor of claim 5 wherein, the stator includes a continuous sleeve disposed circumferentially around the plurality of stator laminations between the first stator end and the second stator end to form the load bearing structure.

10. The lubricant supported electric motor of claim 1 wherein, the rotor extends along the axis, and the rotor is rotatably disposed within the stator.

11. The lubricant supported electric motor of claim 1 wherein, the stator extends along the axis, and the rotor is rotatably disposed about the stator.

12. The lubricant supported electric motor of claim 1 wherein, The rotor is operatively interconnected with a wheel of the vehicle.

13. The lubricant supported electric motor of claim 1 wherein, Each of the plurality of hydrostatic pockets has a rectangular cross-sectional shape.

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