JP2017125413A - Valve gear - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the assembling workability of a valve gear by a simple construction.SOLUTION: A pivot is connected rotatably through a pin 27 to a pivot 25 to the rocker arm 21 of an engine. A shaft 1 is formed with a hole 2, into which the pivot 25 is inserted, thereby to regulate the position of the pin 27 for the rocking center of the rocker arm 21. A stopper 4 is attached to the pivot 25 and the shaft 1 thereby to energize the pivot 25 toward the inside of the hole 2. Moreover, a guide face 7 is protruded from the stopper 4 toward the side of the pivot 25.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a valve operating apparatus that opens and closes a valve body of an engine.

  2. Description of the Related Art Conventionally, a valve mechanism that reciprocates a valve body of an engine by converting a rotational movement of a camshaft into a rocking movement of a rocker arm is known. A variety of power transmission members (rotating members, swinging members, reciprocating members, etc.) for amplifying the swing generated by the cam fixed to the camshaft and transmitting it to the valve body are provided inside the valve mechanism. ) Is built-in. A variable valve mechanism that controls the reciprocation (valve lift) and phase (valve timing) of the valve body in multiple stages by switching the operating states of a plurality of power transmission members is also known. In recent years, a continuously variable valve mechanism that can arbitrarily adjust valve lift and valve timing by continuously changing the positional relationship and contact position between power transmission members has been developed (see Patent Document 1).

JP 2008-215103 A

  Generally, among various rocking members (rocker arms) provided in the valve operating mechanism, an intake rocker arm that drives an intake valve and an exhaust rocker arm that drives an exhaust valve are shafts such as an intake rocker shaft and an exhaust rocker shaft. The ring-shaped member is mounted so as to be swingable. However, other rocker arms are not necessarily attached to the shaft-like member, and may be attached via, for example, a pin-joined link member. The valve mechanism having such a structure has a problem that the position of the link member may be moved in the assembling process, or the link member may be detached from the shaft-like member, and it is difficult to obtain good workability.

  In the example shown in Patent Document 1, a control arm is rotatably attached to one end of a center rocker arm via a connecting pin, and the control arm is locked to the intake rocker arm. The locked state between the control arm and the intake rocker arm is a non-fastened state so that the position of the connecting pin can be finely adjusted. In addition, after the valve mechanism is completely assembled, the center rocker arm is sandwiched between the cam fixed to the camshaft and the other power transmission member. Therefore, the control arm does not move during engine operation.

  However, there is a possibility that the control arm moves in the extending direction when the valve mechanism is not completely assembled, such as during the process of assembling the engine or during maintenance work of the valve mechanism. Therefore, the operator must perform the assembling work with care so that the control arm does not fall out of the intake rocker arm.

  One of the objects of the present case has been invented in view of the above-described problems, and is to provide a valve operating apparatus that can improve the assembly workability with a simple configuration. It should be noted that the present invention is not limited to this purpose, and is an operational effect that is derived from each configuration shown in “Mode for Carrying Out the Invention” to be described later. Can be positioned as a purpose.

(1) A valve operating device disclosed herein includes an axial pivot pivotally connected to a rocker arm of an engine via a pin, and a hole into which the pivot is inserted. And a shaft for defining the position of the pin serving as a moving center. A stopper attached to the pivot and the shaft and biasing the pivot toward the inside of the hole; and a guide surface protruding from the stopper toward a side of the pivot.
In addition, the said shaft should just have a hole in which the said pivot is inserted at least, and is not limited to a shaft-shaped thing. Therefore, the shaft can be read as “pivot locking member”, “pivot position defining member” or the like.

(2) It is preferable that the guide surface has a retaining surface facing the end of the pin. In this case, it is preferable to provide a pair of retaining surfaces and face both one end and the other end of the pin.
(3) It is preferable that the retaining surface has a predetermined gap with respect to the end of the pin. That is, it is preferable that the retaining surface and the pin are not always in contact with each other.

(4) It is preferable that the retaining surface is formed in a vertically long shape along the axial direction of the pivot.
(5) It is preferable that the guide surface has a tip shape that warps in a direction away from the pivot.
(6) It is preferable that the stopper has a bent shape that sandwiches the pivot and the shaft inside.

(7) It is preferable that the stopper includes a locking portion that is locked to the shaft and a spring portion that applies an elastic force that biases the pivot toward the locking portion.
(8) It is preferable that the hole extends in a direction orthogonal to the central axis of the shaft, and the shaft is rotatably provided around the central axis.

  By providing a stopper that urges the pivot toward the inside of the hole of the shaft, the pivot can be prevented from moving or falling off, and the assembly workability of the valve gear can be improved. In addition, since the guide surface is protruded from the stopper, the stopper can be easily attached at the correct position, so that the workability of assembling the stopper can be improved.

It is sectional drawing of the engine to which the valve gear of this embodiment was applied. It is a disassembled perspective view of a valve mechanism. It is a perspective view of a stopper. It is a sectional side view of the valve mechanism which expands and shows a stopper. FIG. 5 is a cross-sectional view (cross-sectional view taken along line AA in FIG. 4) of the stopper and the pin.

  A valve gear as an embodiment will be described with reference to the drawings. The embodiment described below is merely an example, and there is no intention of excluding various modifications and technical applications that are not explicitly described in the following embodiment. Each configuration of the present embodiment can be implemented with various modifications without departing from the spirit thereof. Further, they can be selected as necessary, or can be appropriately combined.

[1. Device configuration]
The valve gear 10 of this embodiment is applied to the reciprocating engine 50 shown in FIG. The engine 50 is equipped with a SOHC-type continuously variable valve lift valve operating mechanism, and the valve lift amounts of the intake valve 51 and the exhaust valve 52 can be freely changed. As shown in FIG. 1, the upper end portion of the intake valve 51 is supported by an intake rocker arm 41 so as to be reciprocally movable in the vertical direction. Similarly, the upper end portion of the exhaust valve 52 is supported by an exhaust rocker arm (not shown) so as to reciprocate in the vertical direction. The intake valve 51 and the exhaust valve 52 are always urged upward by a valve spring, and the intake port and the exhaust port are opened by pushing down the upper ends of the respective valves.

  The power transmission members for driving the intake valve 51 are the intake cam 12, the intake center rocker arm 21 (rocker arm), the intake swing cam 31, and the intake rocker arm 41 in the order in which the power is transmitted. Further, although a part of the description is omitted in FIG. 1, the power transmission members for driving the exhaust valve 52 are the exhaust cam 13, the exhaust center rocker arm, the exhaust swing cam, and the exhaust rocker arm. The valve lift amounts of the intake valve 51 and the exhaust valve 52 can be changed by changing the swing amounts of the intake rocker arm 41 and the exhaust rocker arm, and these swing amounts change the positional relationship and contact position between the power transmission members. It can be changed by changing continuously.

  The intake cam 12 and the exhaust cam 13 are cam members having an oval cross section fixed to the cam shaft 11. The camshaft 11 is a shaft member that rotates at a predetermined rotation speed ratio in conjunction with the crankshaft of the engine 50, and is pivotally supported by the cylinder head. The intake cam 12 is disposed so as to contact the center roller 23 of the intake center rocker arm 21 and presses the intake center rocker arm 21 with an amplitude corresponding to the cam profile. Similarly, the exhaust cam 13 swings and drives the exhaust center rocker arm.

  These cams 12, 13 are attached to the camshaft 11 so that the portion protruding in the radial direction of the intake cam 12 and the portion protruding in the radial direction of the exhaust cam 13 have different phases. Thereby, a phase difference is generated between the swing of the intake rocker arm 41 and the swing of the exhaust rocker arm, and the opening timing (valve timing) of the intake valve 51 and the exhaust valve 52 is determined according to this. Hereinafter, the intake center rocker arm 21, the intake swing cam 31, and the intake rocker arm 41, which are power transmission members for driving the intake rocker arm 41, will be described in detail.

  As shown in FIG. 1, the intake center rocker arm 21 is a member that is locked to the intake rocker shaft 1 in a posture sandwiched between the intake cam 12 and the intake swing cam 31. As shown in FIG. 2, the intake center rocker arm 21 is provided with a main body portion 22, a center roller 23, a center pressing portion 24, a pivot 25, and a pin 27. The main body 22 is a member having a shape in which boomerang-shaped side plates are arranged and connected in the extending direction of the camshaft 11. The center roller 23 is a part that receives a pressing force from the intake cam 12, and is pivotally supported between a pair of side plates that form the main body 22. Further, a center pressing portion 24 for transmitting the swing of the main body portion 22 to the intake swing cam 31 is formed in a planar shape (or a curved surface shape) at a portion where the pair of side plates are connected. On the other hand, on the opposite side of the center pressing portion 24 across the center of rotation of the center roller 23, a shaft-like pivot 25 that is rotatably connected via a pin 27 is provided.

  The pivot 25 is a shaft-like member that is inserted and locked in a pivot hole 2 formed in the intake rocker shaft 1 to be described later, and functions to define the position of the pin 27 that serves as the swing center of the intake center rocker arm 21. have. The upper end side of the pivot 25 is formed in a cylindrical shape having a diameter corresponding to the size of the pivot hole 2, and the lower end side is formed in a slightly enlarged diameter cylindrical shape or a spherical shape. Further, a hollow cylindrical pin hole 26 that is perforated in a direction perpendicular to the axial direction of the pivot 25 is provided on the other end side.

  The pin 27 is a shaft-like member that is inserted into the pin hole 26 and fitted into a pin hole 28 that is drilled at one end of the intake center rocker arm 21. A predetermined gap is provided between the pin 27 and the pin hole 26 so that the pivot 25 can take an arbitrary angle with respect to the intake center rocker arm 21. On the other hand, the pin 27 is inserted without a gap so that the pin 27 does not fall off. The pin 27 may be press-fitted and fixed to the pin hole 28 or may be fixed by caulking.

  The intake rocker shaft 1 is a shaft member (shaft) on which the intake rocker arm 41 is swingably supported, and is supported so as to be rotatable about an axis with respect to the cylinder head of the engine 50. The intake rocker shaft 1 is formed with a pivot hole 2 into which a pivot 25 is inserted and locked. Of the inner peripheral surface of the pivot hole 2, the lower end side into which the pivot 25 is inserted is formed into a smooth cylindrical surface on which the pivot 25 can slide. On the other hand, a thread groove is formed on the inner peripheral surface on the upper end side of the pivot hole 2, and the adjusting screw 3 for defining the position of the pivot 25 is fastened and fixed. Thereby, the position of the pivot 25 is a position where the tip of the pivot 25 is in contact with the lower end surface of the adjustment screw 3. Further, by rotating the intake rocker shaft 1 around the axis, the pivot 25 inserted into the intake rocker shaft 1 rotates around the axis of the intake rocker shaft 1 together with the pin 27, so that the intake center rocker arm 21 swings. The center also moves around the axis of the intake rocker shaft 1.

  The intake swing cam 31 is a power transmission member for transmitting the swing of the intake center rocker arm 21 to the intake rocker arm 41. The intake swing cam 31 is provided with a main body portion 32, a cam roller 33, and an arm pressing portion 34. The main body portion 32 is a portion that is swingably attached to the swing cam shaft 35 and is mounted on the outer peripheral surface of the swing cam shaft 35 in a state of being biased in the clockwise direction in FIG. The swing cam shaft 35 is a shaft member (shaft) on which the intake swing cam 31 is swingably supported, and is supported or fixed to the cylinder head of the engine 50. If the overall shape of the main body 32 is similar to the shape of a boot, the upper end of the boot is supported by the swing cam shaft 35. Moreover, the cam roller 33 is attached to the heel portion of the boots, and the arm pressing portion 34 is a portion corresponding to the shoe sole.

  The cam roller 33 is a part that is pressed in contact with the center pressing portion 24 of the intake center rocker arm 21, and is pivotally supported with respect to the main body portion 32. The arm pressing part 34 is a part that transmits the swing of the main body part 32 to the intake rocker arm 41. The surface shape of the arm pressing portion 34 is a complex curved surface shape in which a plurality of curved surfaces having different curvatures are connected. As a result, the swing amount transmitted to the intake rocker arm 41 changes according to the contact position of the intake rocker arm 41 with respect to the arm pressing portion 34.

  The intake rocker arm 41 is a power transmission member for transmitting the swing transmitted from the intake swing cam 31 to the intake valve 51. The intake rocker arm 41 is provided with a main body portion 42, an arm roller 43, and a valve pressing portion 44. The main body 42 is a portion that is swingably attached to the intake rocker shaft 1 and is mounted on the outer peripheral surface of the intake rocker shaft 1. The main body portion 42 of the present embodiment is formed in a shape in which triangular side plates are connected side by side in the extending direction of the intake rocker shaft 1, and the intake rocker shaft 1, the arm roller 43, and the valve are located at the apex of the triangle. Each of the pressing portions 44 is disposed. Further, the pivot hole 2 of the intake rocker arm 41 is drilled at a position between the pair of side plates.

  The arm roller 43 is a portion that receives a pressing force from the arm pressing portion 34, and is pivotally supported between a pair of side plates that form the main body portion 42. The valve pressing portion 44 is a portion that presses the upper end portion of the intake valve 51 and is provided on each of the pair of side plates. Thus, the intake rocker arm 41 of this embodiment includes two valve pressing portions 44 in order to correspond to the engine 50 in which two intake valves 51 are provided in one cylinder.

  A stopper 4 is mounted between the intake rocker shaft 1 and the pivot 25 to fix them while sandwiching them. The stopper 4 is a member that is attached to the intake rocker shaft 1 and the pivot 25 and has both a function of urging the pivot 25 toward the inside of the pivot hole 2 and a function of preventing the pin 27 from dropping or moving. The overall shape of the stopper 4 is shown in FIG. 3, and the cross-sectional shape is shown in FIGS.

  As shown in FIG. 3, the stopper 4 is provided with a locking portion 5, a spring portion 6, a guide surface 7, and a retaining surface 8. The locking part 5 is a part on one end side that is locked to the intake rocker shaft 1, and the spring part 6 is a part on the other end side. These locking portions 5 and spring portions 6 are formed in a shape that sandwiches the pivot 25 and the intake rocker shaft 1 inside, for example, by bending a vertically long steel plate into a C shape. As a result, the spring portion 6 becomes a portion for applying an elastic force that urges the pivot 25 in the direction of the locking portion 5.

  The locking portion 5 is formed in a planar shape so as to come into surface contact with the seating surface of the adjusting screw 3 attached to the intake rocker shaft 1 and has a shape cut out in an arc shape so as not to interfere with the adjusting screw 3. Have. Further, a wall surface contact portion 46 and a protrusion 48 are provided in the vicinity of the distal end portion of the locking portion 5. The wall surface contact portion 46 is a portion that is in surface contact with the wall surface 45 of the intake rocker shaft 1 erected from the seating surface of the adjustment screw 3. Further, the protrusion 48 is a portion that fits with a recess 47 formed in the axial direction of the intake rocker shaft 1 on the surface facing the wall surface 45 with which the wall surface contact portion 46 comes into surface contact. Thereby, the arrangement | positioning position of the stopper 4 with respect to the intake rocker shaft 1 will be pinpointed uniquely, and favorable positioning accuracy will be obtained.

  The spring portion 6 is formed in a concave shape corresponding to the shape on the lower end side of the pivot 25. Further, the dimension of the spring part 6 in the width direction (the dimension of the pin 27 in the axial direction) is such that the spring part 6 contacts only the pivot 25 and does not contact the main body part 22 of the intake center rocker arm 21. The width of the pivot 25 is smaller than the width. Thereby, the swingability of the intake center rocker arm 21 is ensured, and the occurrence of malfunction due to the contact between the stopper 4 and the intake center rocker arm 21 is prevented.

  A pair of guide surfaces 7 projecting in an arm shape toward the side surface of the pivot 25 is provided slightly above the spring portion 6. As shown in FIG. 3, each guide surface 7 is formed in a crank shape while avoiding contact with the intake rocker shaft 1 and the intake center rocker arm 21. In addition, a tip portion 9 having a shape that warps in a direction away from the pivot 25 is provided at the tip of the guide surface 7. As shown in FIG. 5, the distance between the left and right tip portions 9 is formed to be larger than the thickness dimension of the portion where the pin hole 28 is formed at least in the main body portion 22 of the intake center rocker arm 21. Thereby, interference with the intake center rocker arm 21 when the stopper 4 is assembled is prevented.

  The guide surface 7 is provided with a retaining surface 8 that faces both ends of the pin 27. As shown in FIG. 4, the retaining surface 8 is disposed at a position overlapping the pin 27 when viewed from the extending direction of the pin 27. As described above, the center position of the pin 27 with respect to the intake rocker shaft 1 can be changed by adjusting the adjusting screw 3. In order to cope with such a change in position, the retaining surface 8 is formed in a vertically long shape along the axial direction of the pivot 25 as shown in FIG. Thereby, even if the position of the pivot 25 changes in the axial direction, both ends of the pin 27 are always covered by the retaining surface 8 (opposite the front). A predetermined gap is formed between the retaining surface 8 and both ends of the pin 27. Thereby, the swingability of the intake center rocker arm 21 is ensured.

[2. Action, effect]
(1) In the valve operating apparatus 10 described above, the pivot 25 is rotatably connected to the intake center rocker arm 21 via the pin 27. A pivot hole 2 for inserting the pivot 25 is formed in the intake rocker shaft 1, and the pivot 25 is locked to the intake rocker shaft 1. In such a structure, by providing the stopper 4 that urges the pivot 25 toward the inside of the pivot hole 2, the pivot 25 can be prevented from moving or falling off, and the assembly workability of the valve gear 10 can be improved. be able to. Further, since the guide surface 7 is projected from the stopper 4, it is easy to attach the stopper 4 to the correct position, so that the workability of assembling the stopper 4 can be further improved.

  (2) The guide surface 7 is formed with a retaining surface 8 that faces the end of the pin 27. As a result, a function of preventing the pin 27 from falling off can be added to the guide surface 7, and the occurrence of noise and rattling due to the loosening of the pin 27 can be prevented. Further, the press-fitting work and caulking work of the pin 27 can be omitted, or the press-fitting torque and the caulking strength can be reduced, so that the work load can be reduced. Further, by assembling the stopper 4, the pin 27 does not fall off in the assembling process, so that the assembling workability can be further improved.

  (3) Since the retaining surface 8 has a predetermined gap with respect to the end portion of the pin 27 as shown in FIG. 5, abnormal noise or rattling due to contact between the pin 27 and the retaining surface 8 occurs. Can be prevented. In the assembling process, if the retaining surface 8 is in contact with the end of the pin 27, it can be seen that some assembling error has occurred, so that the assembling quality can be improved.

  (4) As shown in FIG. 4, the retaining surface 8 is formed in a vertically long shape along the axial direction of the pivot 25. Thereby, even if the position of the pivot 25 changes in the axial direction, both ends of the pin 27 can always be covered so as to face the front surface of the retaining surface 8. Therefore, it is possible to further enhance the function of preventing the pin 27 from coming off.

  (5) As shown in FIG. 3, the distal end portion 9 of the guide surface 7 is formed in a shape that warps in a direction away from the pivot 25. Thereby, interference with the intake center rocker arm 21 when the stopper 4 is assembled can be prevented. Since the intake center rocker arm 21 is a member disposed on the inner side of the main body 42 of the intake rocker arm 41, it is difficult to visually check and the stopper 4 is provided so as not to interfere with the intake center rocker arm 21. Installation work is complicated. However, since the distal end portion 9 of the guide surface 7 is widened so as to receive the intake center rocker arm 21 on the inner side, interference with the intake center rocker arm 21 can be more reliably prevented and assembled. Workability can be further improved.

  (6) As shown in FIG. 3, the overall shape of the stopper 4 is a bent shape that sandwiches the pivot 25 and the intake rocker shaft 1 inside. As a result, the attaching operation of the stopper 4 is completed simply by sandwiching the pivot 25 and the intake center rocker arm 21 inside the stopper 4, so that an error in attachment is unlikely to occur and the assembling quality and assembling workability are improved. be able to.

  (7) The stopper 4 is provided with a locking portion 5 that is locked to the intake rocker shaft 1 on one end side, and an elastic force that biases the pivot 25 in the direction of the locking portion 5 on the other end side. The spring part 6 to provide is provided. Thus, by structurally separating the locking portion 5 and the spring portion 6, the pivot 25 can be stably fixed to the intake rocker shaft 1 regardless of the amount of insertion of the pivot 25. Assembling workability of the stopper 4 can be improved. In addition, since the spring portion 6 is provided at the end opposite to the locking portion 5, elastic deformation can be distributed over almost the entire stopper 4, and a stable elastic force can be exerted on the pivot 25. Can continue to grant.

  (8) A continuously variable valve lift valve operating mechanism is applied to the valve operating apparatus 10 described above. That is, the intake rocker shaft 1 to which the stopper 4 is attached rotates around the axis according to the operating state of the engine 50, thereby changing the valve lift amount. On the other hand, the stopper 4 can continue to support the pivot 25 even if the valve lift amount is changed, and can prevent the pin 27 from dropping or moving. Thus, the assembly workability of the valve gear 10 can be improved without affecting the existing continuously variable valve lift valve mechanism.

[3. Modified example]
In the above-described embodiment, the structure in which the stopper 4 is applied to the power transmission member for driving the intake valve 51 is illustrated, but the same stopper 4 is applied to the power transmission member for driving the exhaust valve 52. Is also possible. For example, a pivot connected to the exhaust center rocker arm via a pin and an exhaust rocker shaft into which the pivot is inserted may be sandwiched by the stopper 4. Thereby, there exists an effect similar to the above-mentioned embodiment.

  In the above-described embodiment, the stopper 4 is configured to sandwich the pivot 25 of the intake center rocker arm 21 and the intake rocker shaft 1, but the object to be sandwiched by the stopper 4 is not limited thereto. In a valve operating apparatus having a rocker arm (swinging member) in a power transmission path that drives at least the valve body of the engine 50, a pivot (shaft-like member) is connected to one end of the rocker arm via a pin. As long as the pivot is inserted into the hole and locked, the stopper 4 similar to that of the above-described embodiment can be applied, and the same effect as that of the above-described embodiment can be obtained.

  In the above-described embodiment, as shown in FIG. 3, the stopper 4 having the crank-shaped guide surface 7 is shown, but the specific shape of the stopper 4 is not limited to this. It is preferable that the guide surface 7 has no interference with the intake rocker shaft 1 or the intake center rocker arm 21, but some interference is allowed unless the interference state is maintained for a long time. The The same applies to the gap between the retaining surface 8 and the pin 27, and it is not necessary to provide an excessively large clearance, and the retaining surface 8 and the pin 27 may be in contact with each other for a short time.

1 Intake rocker shaft (shaft)
2 Pivot hole 3 Adjustment screw 4 Stopper 5 Locking portion 6 Spring portion 7 Guide surface 8 Retaining surface 9 Tip 10 Valve operating device 11 Camshaft 12 Intake cam 13 Exhaust cam 21 Intake center rocker arm (rocker arm)
22 Main body portion 23 Center roller 24 Center pressing portion 25 Pivot 26 Pin hole 27 Pin 28 Pin hole 31 Intake swing cam 32 Main body portion 33 Cam roller 34 Arm press portion 35 Swing cam shaft 41 Intake rocker arm 42 Main body portion 43 Arm roller 44 Valve Pressing portion 45 Wall surface 46 Wall surface contact portion 47 Recess 48 Protruding portion 50 Engine 51 Intake valve 52 Exhaust valve

Claims (8)

An axial pivot pivotally connected to the rocker arm of the engine via a pin;
A shaft having a hole into which the pivot is inserted and defining a position of the pin serving as a rocking center of the rocker arm;
A stopper attached to the pivot and the shaft and biasing the pivot toward the inside of the hole;
A guide surface projecting from the stopper toward the side of the pivot;
A valve operating device comprising: 2. The valve operating device according to claim 1, wherein the guide surface has a retaining surface facing an end of the pin. The valve operating device according to claim 2, wherein the retaining surface has a predetermined gap with respect to an end of the pin. The valve operating device according to claim 2 or 3, wherein the retaining surface is formed in a vertically long shape along the axial direction of the pivot. The valve operating device according to any one of claims 1 to 4, wherein the guide surface has a tip shape that is bent back in a direction away from the pivot. The valve operating device according to claim 1, wherein the stopper has a bent shape that sandwiches the pivot and the shaft inside. The said stopper has a latching | locking part latched by the said shaft, and a spring part which provides the elastic force which urges | biases the said pivot in the direction of the said latching | locking part, It is characterized by the above-mentioned. The valve gear according to any one of the above. The hole extends in a direction orthogonal to the central axis of the shaft;
The valve operating apparatus according to any one of claims 1 to 7, wherein the shaft is provided to be rotatable around the central axis.

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