Counter-rotating roller screws

Abstract

The utility model discloses a reverse circulation formula roller screw, this reverse circulation formula roller screw includes the screw cap, axle stem, retainer, a plurality of rollers and two cam rings. The screw cap is equipped with helical first thread on the inner surface. The axle stem is movably threaded in the screw cap, and the outer surface of axle stem is equipped with helical second thread and the guide groove of axial penetration second thread. The retainer is threaded between the screw cap and axle stem, and the retainer is circumferentially annularly equipped with a plurality of assembly slot holes. The outer surface of each roller is equipped with third thread, and is assembled in the assembly slot hole of retainer, and the third thread is engaged in first thread and second thread simultaneously. Two cam rings are threaded and fixed in axle stem and are set in the two ends of retainer along the axial direction, and each cam ring has a protruding part protruding along the axial direction, and corresponds to one end of the guide groove.

Description

Technical Field

[0001] This utility model relates to a roller screw, and more particularly to a reverse circulation roller screw. Background Technology

[0002] Most existing ball screws are standard planetary ball screws, and even those using recirculating designs are only standard recirculating ball screws. Standard planetary ball screws typically have a complex overall structure. Furthermore, planetary designs are primarily used for heavy-duty, low-speed applications, performing poorly in the medium-speed range. Their structures often require customization for various applications. Standard recirculating ball screws use a nut that moves along with the cage, rollers, and cam rings on the screw. The cam ring is fixed to the nut, and roller return is achieved by a guide component. Typically, in a standard recirculating ball screw structure, the cage is located closer to the screw, while the protrusion on the cam ring is closer to the outside of the screw than the cage. Because the nut, cage, rollers, and cam ring all move along the screw, standard recirculating designs are prone to foreign matter intrusion over long-term operation, affecting rolling quality and service life. Utility Model Content

[0003] Therefore, a simpler, more compact structure is needed to make the overall structure smaller, suitable for space-constrained equipment, and to simplify the manufacturing process.

[0004] In one embodiment, this invention provides a reverse-circulating roller screw including a nut, a shaft, a cage, multiple rollers, and two cam rings. The nut has a helical first thread on its inner surface. The shaft is movably inserted into the nut, and its outer surface has a helical second thread and a guide groove extending axially through the second thread. The cage is a hollow annular shape, inserted between the nut and the shaft, and has multiple assembly slots circumferentially arranged around it. Each roller has a third thread on its outer surface and is assembled into the assembly slots of the cage, with the third thread simultaneously engaging with both the first and second threads. The two cam rings are inserted into and fixed to the shaft and axially positioned at both ends of the cage; each cam ring has an axially protruding portion corresponding to one end of the guide groove.

[0005] The aforementioned reverse-circulation roller screw features a straightforward and simple roller circulation structure, allowing for greater overall miniaturization and suitability for space-constrained equipment. Furthermore, its simpler structure compared to standard planetary roller screws simplifies manufacturing. Additionally, its modular design facilitates easier maintenance and replacement.

[0006] Furthermore, reverse-circulating roller screws offer greater flexibility in the mid-speed range, broadening their application scenarios. For example, they can be used in servo electric cylinder systems or replace hydraulic cylinders to provide precise control, high repeatability, and a clean, leak-free solution. They can also be used in high-frequency reciprocating motion, such as testing instruments (fatigue testing) or simulation motion platforms (six-axis dynamic platforms). Additionally, they can be used in aerospace and defense equipment, such as flight control actuators (e.g., flaperon control), door drives, and landing gear systems. They can also be used in automated and high-end production line equipment, such as humanoid robots, heavy-duty grippers, positioning platforms, pressing modules, and robotic arm joint axes.

[0007] In some embodiments, the guide groove has a first arcuate surface, a bottom surface, and a second arcuate surface. The first arcuate surface, the bottom surface, and the second arcuate surface extend axially and are sequentially connected in the circumferential direction. The protrusion of one of the two cam rings corresponds to one end of the first arcuate surface, and the protrusion of the other cam ring corresponds to one end of the second arcuate surface.

[0008] In some embodiments, a lubrication groove is provided on the first thread, the second thread, or the third thread.

[0009] In some embodiments, the third thread includes a plurality of annular teeth arranged circumferentially along the axis, and the annular teeth are parallel to each other. Each annular tooth has a first side surface and a second side surface between its crest and root. The lubrication groove is provided only on the first side surface, only on the second side surface, or on both the first and second side surfaces.

[0010] In some embodiments, the second thread includes a plurality of threads, each thread having a third side surface and a fourth side surface between its thread crest and thread root. The lubrication groove may be located only on the third side surface, only on the fourth side surface, or simultaneously on both the third and fourth side surfaces.

[0011] In some embodiments, when the lubrication groove is provided on both the third and fourth sides, the lubrication groove is mirror-shaped with the radial straight line passing through the spiral crest as the center.

[0012] In the above embodiments, the cross-section of the bottom shape of the lubrication groove is triangular, arc-shaped, or rectangular.

[0013] In some embodiments, each cam ring includes a convex ring body and an elastic washer. The elastic washer is disposed at one end of the convex ring body, and the protrusion is formed on the elastic washer, and the convex ring body and the elastic washer are made of different materials.

[0014] In some embodiments, the hardness of the elastic gasket is less than that of the roller.

[0015] In some embodiments, the reverse-circulating roller screw further includes two end caps fixed to both ends of the nut. Each end cap has a connecting channel, and the nut has at least two heat dissipation passages through its annular wall. One end of each heat dissipation passage is connected to the connecting channel on the end cap. Attached Figure Description

[0016] Figure 1 This is a perspective view of a reverse-circulating roller screw according to an embodiment of this case.

[0017] Figure 2 This is an exploded view of a reverse-circulation roller screw according to an embodiment of this case.

[0018] Figure 3 This is a perspective view of a reverse-circulation roller screw with the nut omitted, according to an embodiment of this invention.

[0019] Figure 4 This is a perspective view illustrating two cam ring assemblies mounted on a shaft, representing only one embodiment of the present invention.

[0020] Figure 5 This is a partial enlarged view of the roller in one embodiment of this case.

[0021] Figure 6 for Figure 4 A magnified view of the first thread of the shaft at the circled area.

[0022] Figure 7 This is a cross-sectional view of a reverse-circulating roller screw according to an embodiment of this case.

[0023] In the attached figures, the following labels are used:

[0024] 100: Reverse Circulation Roller Screw

[0025] 10: Nuts

[0026] 11: First thread

[0027] 12: Heat dissipation path

[0028] 20: Shaft

[0029] 21: Second thread

[0030] 211: Screw thread

[0031] 2111: Luofeng

[0032] 2112: Spiral root

[0033] 2113: Third side

[0034] 2114: Fourth Side

[0035] 22: Guide groove

[0036] 221: First arc surface

[0037] 222: Bottom surface

[0038] 223: Second arc surface

[0039] 30: Cage

[0040] 31: Assembly slot

[0041] 40: Roller

[0042] 41: Third thread

[0043] 411: Circular teeth

[0044] 4111: Tooth crest

[0045] 4112: Tooth base

[0046] 4113: First side view

[0047] 4114: Second side view

[0048] 50: Cam ring

[0049] 51: Protrusion

[0050] 52: Fixing element

[0051] 53:Protruding ring body

[0052] 54: Elastic gasket

[0053] 60: Lubrication groove

[0054] 70: End Cap

[0055] 71: Connecting Channels Detailed Implementation

[0056] Please see Figures 1 to 3 , Figure 1 This is a perspective view of a reverse-circulation roller screw according to an embodiment of this case. Figure 2 This is an exploded view of a reverse-circulation roller screw according to an embodiment of this case. Figure 3 This is a perspective view of a reverse-circulation roller screw 100 according to an embodiment of this invention, omitting the nut. The reverse-circulation roller screw 100 of this embodiment includes a nut 10, a shaft 20, a cage 30, a plurality of rollers 40, and two cam rings 50. The nut 10 has a helical first thread 11 on its inner surface. In this embodiment, the first thread 11 may cover the entire inner surface of the nut 10. In other embodiments, it may only be provided on a portion of the inner surface of the nut 10, and can be adjusted according to application requirements.

[0057] Depend on Figure 2 and Figure 3It is evident that the shaft 20 is movably inserted into the nut 10, and the outer surface of the shaft 20 is provided with a helical second thread 21 and a guide groove 22 that passes through the second thread 21 axially. The retainer 30 is a hollow annular shape and is inserted between the nut 10 and the shaft 20. The retainer 30 is provided with multiple assembly slots 31 circumferentially. The outer surface of each roller 40 is provided with a third thread 41 and is assembled into the assembly slots 31 of the retainer 30, and the third thread 41 simultaneously engages with the first thread 11 and the second thread 21.

[0058] Please see again Figure 3 and Figure 4 , Figure 4 This is a perspective view illustrating two cam rings assembled on a shaft, representing one embodiment of the present invention. Two cam rings 50 are inserted into and fixed to the shaft 20 and axially positioned at both ends of the retainer 30. Each cam ring 50 has an axially projecting protrusion 51 corresponding to one end of the guide groove 22. During assembly, one cam ring 50 is inserted into the shaft 20 and positioned corresponding to one end of the second thread 21, with the protrusion 51 corresponding to one end of the guide groove 22. Then, a fixing element 52 can be used to fix the cam ring 50 to the shaft 20. The fixing element 52 can be, for example, a... Figure 2 The straight rod shown passes radially from one side of the cam ring 50 through the cam ring 50 and the shaft 20 to the other side of the cam ring 50, thereby fixing the cam ring 50 relative to the shaft 20. In this embodiment, the fixing element 52 can be fixed to the cam ring 50 and the shaft 20 by a tight fit. In other embodiments, it can also be fixed by fixing elements such as screws or bolts.

[0059] After one cam ring 50 is assembled, the retainer 30 is inserted into the shaft 20, corresponding to the second thread 21. Then, another cam ring 50 is fixed to the other end of the retainer 30, corresponding to the other end of the second thread 21, with the protrusion 51 corresponding to the other end of the guide groove 22. Finally, all rollers 40 are inserted one by one into the assembly slots 31, so that the third thread 41 on the roller 40 engages with the second thread 21 on the shaft 20. Figure 3 As can be seen, after assembly, the two protrusions 51 will be radially closer to the shaft 20 than the cage 30. In other words, the cage 30 will be positioned further outward of the shaft 20 relative to the protrusions 51, while the protrusions 51 will be positioned inside the cage 30. This is also one of the characteristics of the reverse circulation type. In this case, the protrusions 51 are not limited to being located on the inner, outer, or middle side of the cage 30, but can all serve the purpose of guiding and pushing the rollers 40.

[0060] like Figure 3After the structure shown is assembled, the nut 10 is inserted through the second thread 21 of the shaft 20, so that the third thread 41 of the roller 40 and the first thread 11 of the nut 10 mesh with each other, thereby completing the assembly of the reverse circulation roller screw 100.

[0061] During operation, since the third thread 41 of the roller 40 simultaneously engages with the first thread 11 of the nut 10 and the second thread 21 of the shaft 20, driving the nut 10 to rotate clockwise or counterclockwise will ultimately cause the shaft 20 to reciprocate linearly relative to the nut 10. Furthermore, when the nut 10 is driven, the roller 40, where the third thread 41 engages with the first thread 11 of the nut 10, rotates around the shaft 20, causing the cage 30 to rotate accordingly. During the rotation of the roller 40 around the shaft 20, it will move towards the cam ring 50 on one side according to the direction of rotation. Figure 3 and Figure 4 As shown in the example, the roller 40 gradually moves towards Figure 3 The left side (also) Figure 4 The cam ring 50 on the left side moves to abut against the left cam ring 50. At this time, the roller 40 will be closer to the left side of the assembly slot 31 (the left end of the assembly slot 31 in the length direction). In order for the roller 40 to return to the center position of the assembly slot 31 (the middle of the assembly slot 31 in the length direction), a matching... Figure 3 and Figure 4 When the roller 40 rotates to the location of the guide groove 22, it will be guided by the guide groove 22 and sink into the guide groove 22, causing the roller 40 to disengage from the first thread 11 and the second thread 21. Driven by the retainer 30, just before leaving the guide groove 22, it will be pushed to the right by the protrusion 51 of the left cam ring 50 and return to the center position of the assembly slot 31. After leaving the guide groove 22, it will re-engage with the first thread 11 and the second thread 21 at the center position of the assembly slot 31. During reverse rotation, the protrusion 51 of the right cam ring 50 will push and reset the roller 40. Through the guidance of the guide groove 22 and the combined action of the protrusion 51 of the cam ring 50, the roller 40 can repeatedly circulate between the two cam rings 50.

[0062] The aforementioned reverse-circulation roller screw 100 features a straightforward and simple roller circulation structure, allowing for greater overall miniaturization and making it suitable for space-constrained equipment. Furthermore, its simpler structure compared to standard planetary roller screws simplifies manufacturing. The integrated modular design of the reverse-circulation roller screw 100 also facilitates easier maintenance and replacement. Additionally, when used in short-distance operations such as those in humanoid robots, the reverse-circulation roller screw 100 can achieve a significantly smaller outer diameter compared to standard circulating roller screws.

[0063] Next, please refer to the following: Figure 2 and Figure 4 In this embodiment, the guide groove 22 has a first arc surface 221, a bottom surface 222, and a second arc surface 223. The first arc surface 221, the bottom surface 222, and the second arc surface 223 extend axially and are sequentially connected in the circumferential direction. One of the two cam rings 50 (e.g. Figure 4 The protrusion 51 of the left cam ring 50 corresponds to one end (the axial left end) of the first arc surface 221. The other (e.g., the other cam ring 50) Figure 4 The protrusion 51 of the right cam ring 50 corresponds to one end of the second arc surface 223 (the right end of the axial direction).

[0064] Continuing from the previous example, by Figure 3 and Figure 4 It can be seen that when the roller 40 rolls to the second arc surface 223 of the guide groove 22, it is already relatively close to the left side of the assembly slot 31 and will not be pushed by the protrusion 51 on the right side. As it continues to roll towards the bottom surface 222, it will roll down along the second arc surface 223 to the slightly concave bottom surface 222. When the roller 40 enters the second arc surface 223, it has disengaged from the second thread 21. At the same time, since it begins to slightly concave towards the axis of the shaft 20, it also disengages from the first thread 11 and can be pushed to move axially.

[0065] As the roller 40 moves towards the bottom surface 222 under the influence of the retainer 30, it will be guided by the inclined surface of the protrusion 51 to move to the right side of the guide groove 22 until it reaches the top of the protrusion 51. At this time, the roller 40 will be located at the center of the assembly slot 31 and will also be at the first arc surface 221, and will be pushed upward under the guidance of the arc surface 221. As the roller 40 leaves the guide groove 22, the third thread 41 will resume engagement with the first thread 11 and the second thread 21. In this way, by setting the first arc surface 221 and the second arc surface 223, the roller 40 can move more smoothly along the arc surface to or away from the bottom surface 222.

[0066] The reverse-circulation roller screw 100 may also include a lubrication groove 60 disposed on the first thread 11, the second thread 21, or the third thread 41. Next, please refer to... Figure 5 , Figure 5This is a partially enlarged view of a roller according to an embodiment of this invention. In this embodiment, the third thread 41 of the roller 40 includes a plurality of annular teeth 411 arranged circumferentially along the axis, and each annular tooth 411 is parallel to each other, forming a thread with a zero helix angle. Each annular tooth 411 has a first side surface 4113 and a second side surface 4114 between its crest 4111 and root 4112. The lubrication groove 60 may be provided only on the first side surface 4113, only on the second side surface 4114, or simultaneously on both the first side surface 4113 and the second side surface 4114. In this embodiment, the lubrication groove 60 is provided simultaneously on both the first side surface 4113 and the second side surface 4114 as an example. When the lubrication groove 60 is provided simultaneously on both the first side surface 4113 and the second side surface 4114, the lubrication groove 60 is mirror-shaped with a radial straight line passing through the crest 4111 as the center.

[0067] In the reverse-circulation roller screw 100, the first thread 11, the second thread 21, and the third thread 41 contact each other via line-to-line or / and face-to-face contact, rather than the point-to-point contact of a traditional ball screw. While increasing load-bearing capacity, this may also increase the friction between the two components, requiring greater continuity and stability in lubrication distribution. By providing lubrication grooves 60 on the first side 4113 and the second side 4114 of the third thread 41, which simultaneously meshes with the first thread 11 and the second thread 21, adding lubricating oil to the lubrication grooves 60 helps retain the lubricating oil on the contact surfaces of the first thread 11 and the second thread 21 and the third thread 41, thereby reducing the friction between the surfaces of the two threads. This further solves problems such as wear, inefficiency, localized abnormal wear, surface pitting, noise, and vibration that may arise from the large contact area and uneven lubrication distribution during transmission.

[0068] Simultaneously, by providing lubrication grooves 60 on both sides to form a large number of lubrication grooves 60, the lubricating oil in the lubrication grooves 60 can effectively reduce the heat generated by the mutual contact, squeezing, and friction between the surfaces of the two threads, reducing heat accumulation in high-pressure areas. This avoids the problem of localized heat accumulation affecting the lifespan and efficiency of the mechanism due to poor heat dissipation during high-frequency operation. On the other hand, since the lubrication grooves 60 are provided on the first side 4113 or the second side 4114 that will contact other threads, the lubrication grooves 60 are sandwiched between the two surfaces. This helps to retain the lubricating oil in the lubrication grooves 60 even when the roller 40 rotates and generates centrifugal force, thus avoiding the problem of the lubricating oil being difficult to maintain on the rolling surface for a long time due to the centrifugal force of rotation.

[0069] Next, please refer to Figure 6 , Figure 6 for Figure 4A partially enlarged view of the first thread of the shaft at the circled area. In addition to the aforementioned provision of a lubrication groove 60 on the third thread 41, a lubrication groove 60 can also be provided on the second thread 21 simultaneously. Figure 6 As can be seen, the second thread 21 includes multiple threads 211, and each thread 211 has a third side surface 2113 and a fourth side surface 2114 between its thread peak 2111 and thread root 2112. The lubrication groove 60 may be provided only on the third side surface 2113, only on the fourth side surface 2114, or simultaneously on both the third side surface 2113 and the fourth side surface 2114. In this embodiment, the lubrication groove 60 is provided only on the third side surface 2113. This allows for the retention or addition of more lubricating oil between the second thread 21 and the third thread 41 via the lubrication groove 60, thereby achieving the effects of improved lubrication, heat dissipation, and reduced friction noise between the threads, as mentioned above.

[0070] Furthermore, the cross-section of the bottom of the lubrication groove 60 is triangular, arc-shaped, or rectangular. The shape of the bottom of the lubrication groove 60 can be matched with lubricating oils of different viscosities to achieve the desired lubrication effect.

[0071] Please see again Figure 2 and Figure 4 Each cam ring 50 also includes a convex ring body 53 and an elastic washer 54. The elastic washer 54 is disposed at one end of the convex ring body 53, and the protrusion 51 is formed on the elastic washer 54. The convex ring body 53 and the elastic washer 54 are made of different materials. The convex ring body 53 is mostly made of metal or alloy, while the elastic washer 54 can be made of plastic, rubber, or other materials. The elastic washer 54 and the convex ring body 53 can be combined by metal embedding injection molding, embedding the elastic washer 54 into the convex ring body 53, or forming the elastic washer 54 by metal plating on the convex ring body 53. Using an elastic washer 54 with a higher elastic deformation capacity on the side of the cam ring 50 that will directly contact the roller 40 can reduce the friction between the roller 40 and harder materials (such as high-hardness metals or alloys) during operation and reduce the heat generated by friction. Furthermore, the slight deformation of the elastic pad 54 can absorb some of the impact and vibration. In addition to reducing local stress concentration and component wear, it can also effectively suppress high-frequency vibration and noise generated by the movement and contact of the rollers 40, achieving the effect of vibration reduction and noise reduction, and further improving the stability and service life of the device under high-speed operation. In order to further reduce the impact and vibration generated by the friction or pushing force between the contacting parts, the hardness of the elastic pad 54 can be made less than the hardness of the rollers 40.

[0072] Next, please refer to Figure 2 and Figure 7 , Figure 7This is a cross-sectional view of a reverse-circulation roller screw according to an embodiment of this invention. In this embodiment, the reverse-circulation roller screw 100 further includes two end caps 70 fixed to the two ends of the nut 10. Each end cap 70 has a connecting channel 71. The nut 10 has at least two heat dissipation passages 12 through its annular wall, and one end of each heat dissipation passage 12 is connected to the connecting channel 71 on the end cap 70. Figure 7 As can be seen, the right ends of the heat dissipation passages 12 located on the upper and lower sides of the nut 10 are connected to the connecting channel 71 on the right end cap 70. The left ends of the heat dissipation passages 12 located on the upper and lower sides of the nut 10 are connected to the connecting channel 71 on the left end cap 70. This forms a closed-loop cooling oil circuit. Different cooling liquids such as deionized water and cooling oil can be placed in the heat dissipation passages 12 and the connecting channel 71. The coolant placed in the closed-loop cooling oil circuit can circulate and dissipate heat using natural convection or external heat sinks. For example, such as... Figure 7 As indicated by the arrow, the coolant in the lower heat dissipation passage 12 may rise from the right connecting channel 71 to the upper heat dissipation passage 12 after being heated. The coolant in the upper passage, due to its lower temperature, sinks from the left connecting channel 71 to the lower heat dissipation passage 12, thus forming a natural backflow in the nut 10, thereby reducing the high-pressure heat accumulation in the nut 10 area.

[0073] Alternatively, the coolant may not fill the entire channel of the heat dissipation passage 12 and the connecting channel 71. Instead, the centrifugal force generated when the reverse circulation roller screw 100 is actuated can drive the coolant to circulate in the heat dissipation passage 12 and the connecting channel 71, promoting heat conduction and heat convection, and carrying away the heat generated by the friction between the nut 10 and the roller 40, thereby improving the heat dissipation effect of the nut 10.

[0074] Although the present invention has been disclosed above with reference to the aforementioned embodiments, it is not intended to limit the present invention. Any person skilled in the art may make some modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of patent protection of the present invention shall be determined by the scope defined in the claims appended to this specification.

Claims

1. A reverse-circulating roller screw, comprising: A nut with a helical first thread on its inner surface; A shaft is movably inserted into the nut. The outer surface of the shaft is provided with a helical second thread and a guide groove that passes through the second thread axially. A retainer, which is hollow and circular, is inserted between the nut and the shaft. The retainer is provided with multiple assembly slots around its circumference. Multiple rollers, each having a third thread on its outer surface, are assembled into the assembly slot of the cage, and the third thread simultaneously engages with both the first thread and the second thread; and Two cam rings are inserted and fixed to the shaft and are axially disposed at both ends of the retainer. Each cam ring has a protruding portion that protrudes axially and corresponds to one end of the guide groove.

2. The reverse circulating roller screw according to claim 1, wherein the guide groove has a first arc surface, a bottom surface and a second arc surface, the first arc surface, the bottom surface and the second arc surface extend axially and are sequentially connected in the circumferential direction, the protrusion of one of the two cam rings corresponds to one end of the first arc surface, and the protrusion of the other of the two cam rings corresponds to one end of the second arc surface.

3. The reverse circulation roller screw according to claim 1, further comprising a lubrication groove disposed on the first thread, the second thread or the third thread.

4. The reverse circulation roller screw according to claim 3, wherein the third thread includes a plurality of annular teeth arranged around the axis, and each of the annular teeth is parallel to each other, and each of the annular teeth has a first side surface and a second side surface between a tooth crest and a tooth root, and the lubrication groove is provided only on the first side surface, only on the second side surface, or simultaneously on the first side surface and the second side surface.

5. The reverse circulation roller screw according to claim 4, wherein when the lubrication groove is provided on both the first side and the second side, the lubrication groove is mirror-shaped with a radial straight line passing through the tooth crest as the center.

6. The reverse circulation roller screw according to claim 3, wherein the second thread includes a plurality of threads, and each of the threads has a third side surface and a fourth side surface between a thread peak and a thread root, and the lubrication groove is provided only on the third side surface, only on the fourth side surface, or simultaneously on the third side surface and the fourth side surface.

7. The reverse circulation roller screw according to any one of claims 3 to 6, wherein the cross-section of the bottom shape of the lubrication groove is triangular, arc-shaped or rectangular.

8. The reverse circulating roller screw according to claim 1, wherein each cam ring further includes a convex ring body and an elastic washer, the elastic washer being disposed at one end of the convex ring body, and the protrusion being formed on the elastic washer, and the convex ring body and the elastic washer being made of different materials.

9. The reverse circulation roller screw according to claim 8, wherein the hardness of the elastic washer is less than the hardness of the roller.

10. The reverse circulation roller screw according to claim 1, further comprising two end caps fixed to the two ends of the nut, each end cap having a connecting channel, the nut having at least two heat dissipation passages through an annular wall, one end of each heat dissipation passage being connected to the connecting channel on the end cap.

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