Anti-explosion centrifugal fan

Abstract

The anti-explosion centrifugal ventilator comprises a base, a transmission mechanism is arranged on the base and comprises a shaft seat, a transmission shaft, a fan wheel, a motor, a driving wheel, a driven wheel and a transmission belt, and a protection mechanism comprises a connecting sleeve, a connecting rod, a clamping block, a clamping groove, a sliding hole, a pressure applying ball, a pushing block and a pressure adjusting mechanism. The pressure adjusting mechanism comprises sliding grooves, an adjusting sleeve, a pressure spring, a sliding sleeve and a positioning mechanism. The multiple sets of sliding grooves are distributed in the outer wall of the connecting sleeve and communicate with the multiple sets of sliding holes. The adjusting sleeve slides in the multiple sets of sliding grooves and is connected with the multiple sets of pushing blocks, the effective protection effect is achieved in the fan running process, when the fan wheel is blocked by foreign matter, resistance generated by the transmission shaft can cause the clamping block to overcome the pushing force of the pressure applying ball, the pushing blocks are pushed to drive the adjusting sleeve to slide, and therefore the preset force of the pressure spring is overcome; finally, idling is formed between the connecting rod and the connecting sleeve, and the explosion risk caused by overload of the motor is avoided.

Description

TECHNICAL FIELD

[0001] The utility model relates to centrifugal fan technical field, more specifically, it relates to a kind of explosion-proof centrifugal fan. BACKGROUND

[0002] In some special environments, such as chemical plants, mines, oil and gas fields and other flammable and explosive places, it is very important to use explosion-proof centrifugal fan for air ventilation and gas discharge. Since there are a large number of flammable gases or dust in these environments, any equipment failure can cause serious safety accidents. Therefore, these devices not only require strong ventilation capacity, but also must ensure sufficient stability and safety during operation. One of the core requirements of explosion-proof centrifugal fan is to be able to run continuously and stably to prevent overload, explosion and other accidents caused by equipment failure.

[0003] However, in the prior art, the fan wheel of the explosion-proof centrifugal fan is easily affected by foreign matter during actual operation. The foreign matter may accidentally enter the inside of the fan wheel, causing the fan wheel to be stuck. When this happens, the fan wheel cannot rotate normally, which in turn causes the motor driving the fan to stop operating due to overload. If this problem is not solved in time, the overload of the motor will cause heat accumulation, further leading to equipment failure or even explosion, thereby posing a serious safety threat to personnel and equipment. UTILITY MODEL CONTENT

[0004] (I) Technical problem solved

[0005] To solve the problems existing in the prior art, the utility model provides an explosion-proof centrifugal fan to solve the technical problem that the overload of the motor mentioned in the background technology will cause heat accumulation, further leading to equipment failure or even explosion.

[0006] (II) Technical solution

[0007] To achieve the above object, the utility model provides the following technical scheme: A kind of explosion-proof centrifugal ventilator, including base, transmission mechanism is arranged on the base, the transmission mechanism includes shaft seat, transmission shaft, fan wheel, motor, driving wheel, driven wheel and transmission belt, shaft seat is provided with two groups of installation on base, transmission shaft is rotatably installed on shaft seat, fan wheel is installed at one end of transmission shaft, motor is installed on the outside of base, driving wheel is installed at motor output end, driven wheel is provided with protection mechanism between transmission shaft, transmission belt is arranged in the outer wall of driving wheel and driven wheel, the protection mechanism includes connecting sleeve, connecting rod, clamping block, clamping groove, sliding hole, pressure ball, push block and pressure regulating mechanism, connecting sleeve is installed on driven wheel, connecting rod is installed at one end of transmission rod, clamping block is provided with multiple groups sliding in connecting sleeve, clamping groove is provided with multiple groups distribution in the outer wall of connecting rod, sliding hole is provided with multiple groups distribution in connecting sleeve, pressure ball is slidably arranged in multiple groups sliding hole, push block is slidably arranged in multiple groups sliding hole, pressure regulating mechanism includes sliding slot, adjusting sleeve, pressure spring, sliding sleeve and positioning mechanism, sliding slot is provided with multiple groups distribution in the outer wall of connecting sleeve and is communicated with multiple groups sliding hole.Adjusting sleeve is slidably arranged in multiple groups sliding slot and is connected with multiple groups push block, pressure spring is provided with multiple groups installation on the top surface of adjusting sleeve, sliding sleeve is slidably arranged in the outer wall of connecting sleeve and is connected with multiple groups pressure spring.

[0008] The utility model further sets up, the positioning mechanism includes locating block, locating groove, push spring, rotating sleeve and give way slot, locating block is provided with multiple groups sliding installation in the outer wall of sliding sleeve, locating groove is provided with multiple groups distribution in the outer wall of connecting sleeve, push spring is provided with multiple groups installation in multiple groups the inboard of locating block and is connected with sliding sleeve, rotating sleeve is rotatably installed in the outer wall of sliding sleeve, and give way slot is provided with multiple groups distribution in the inner wall of rotating sleeve.

[0009] The utility model further sets up, the outer side of fan wheel is equipped with shell, the shell is fixedly connected with base, and the shell is set to play the protection function, prevent foreign matter from entering fan wheel inside, and simultaneously pass through the fixed connection with base, improve the overall stability of equipment.

[0010] The utility model further sets up, multiple groups the bottom of clamping block is all set to arc, multiple groups the clamping groove is all set to arc and is engaged with multiple groups the bottom of clamping block, and the bottom of clamping block and clamping groove all adopt arc design, so that it can be more smooth when engaging, improve the stability of connection, and simultaneously reduce the friction and jamming phenomenon.

[0011] The utility model further sets up, multiple groups the inner wall of sliding hole is all provided with guide slot, multiple groups the outer wall of pressure ball is all equipped with guide block, multiple groups the guide slot is slidably installed in multiple groups guide slot, and the cooperation of guide slot and guide block ensures that the movement direction of pressure ball is controlled, prevents it from deviating, to ensure the stability and reliability of mechanism in the working process.

[0012] The utility model further sets up, a plurality of groups of card block top end with push block all is provided with the arc groove of adapting with pressure ball, the setting of arc groove can better accommodate pressure ball, make pressure ball stress even, avoid the abrasion caused by uneven local stress, improve the durability and stability of mechanism.

[0013] The utility model further sets up, the slide sleeve bottom surface installation is equipped with the limit rod, the limit rod is provided with a plurality of groups and is connected with the adjusting sleeve sliding, the function of limit rod is to limit the movement range of slide sleeve, ensure that it slides in the predetermined track, simultaneously through the sliding connection with adjusting sleeve, make the adjustment of pressure spring preset force more accurate and stable.

[0014] The utility model further sets up, a plurality of groups of the let -go slot both ends are provided with inclined plane, and the inclined plane design of let -go slot both ends helps the smooth let -go of parts under stress, reduces resistance and impact, improves the stability and service life of overall structure.

[0015] (Three) beneficial effects

[0016] Compared with the prior art, the utility model provides a kind of explosion-proof centrifugal ventilator, with the following beneficial effects:

[0017] 1, protection mechanism is matched with a plurality of card block and card slot, and pressure ball, push block and pressure regulating mechanism jointly act, effectively protect in the process of fan operation, when fan wheel is jammed by foreign matter, the resistance generated by transmission shaft will cause card block to overcome the thrust of pressure ball, push push block and drive adjusting sleeve to slide, to overcome the preset force of pressure spring, finally make the idle between connecting rod and connecting sleeve, avoid the explosion risk of motor due to overload, the setting of protection mechanism ensures that fan can stop working in time when encountering abnormal situation, to prevent the damage of equipment and security risk.

[0018] 2, pressure regulating mechanism is slidably connected with slide groove, adjusting sleeve, pressure spring and slide sleeve, realizes the adjustable function of pressure spring preset force. When the preset force of pressure spring needs to be adjusted, adjusting sleeve can change the tension of pressure spring by sliding, to adjust the trigger threshold of protection mechanism, the adjustability enables equipment to be adjusted in detail according to different working environment or demand, improves the flexibility and adaptability of system, and pressure regulating mechanism ensures the accurate response of protection mechanism under different conditions, to improve the overall safety and stability.

[0019] 3、The positioning mechanism provides accurate sliding sleeve positioning function through cooperation of multiple sets of positioning blocks, positioning grooves, push springs, rotating sleeves and let-position grooves. Rotation of the rotating sleeve can adjust the cooperation relationship between the positioning blocks and the positioning grooves, and then change the pressure of the push spring, realize the adjustment of the preset force of the set pressure spring, the setting of the positioning mechanism not only ensures the stability of the sliding sleeve, but also can accurately adjust the preset force of the set pressure spring according to the need, ensures that the fan can be effectively protected in abnormal conditions, and improves the adjustment accuracy and reliability of the equipment. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 It is a whole structure schematic view of the explosion-proof centrifugal fan in the utility model.

[0021] Figure 2 It is an installation structure schematic view of the protection mechanism in the utility model.

[0022] Figure 3 It is a structure schematic view of the connecting sleeve in the utility model.

[0023] Figure 4 It is a sectional structure schematic view of the connecting sleeve in the utility model.

[0024] Figure 5 It is a sectional structure schematic view of the positioning mechanism in the utility model.

[0025] In the drawing: 1, base; 2, shaft seat; 3, transmission shaft; 4, fan wheel; 5, motor; 6, driving wheel; 7, driven wheel; 8, transmission belt; 9, connecting sleeve; 10, connecting rod; 11, clamping block; 12, clamping groove; 13, sliding hole; 14, pressure ball; 15, push block; 16, sliding groove; 17, adjusting sleeve; 18, set pressure spring; 19, sliding sleeve; 20, positioning block; 21, positioning groove; 22, push spring; 23, rotating sleeve; 24, let-position groove; 25, shell; 26, guide groove; 27, guide block; 28, limiting rod. DETAILED DESCRIPTION

[0026] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with embodiments.

[0027] It should be noted that, unless otherwise specified, all technical and scientific terms used in the present application have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0028] In the utility model, in not making opposite explanation case, the orientation such as "up, down" usually is for the direction shown in the drawing or is for vertical, perpendicular or gravity direction; similarly, for the convenience of understanding and description, "left, right" usually is for the left, right shown in the drawing; "inner, outer" refers to the inner, outer of the contour of each component itself, but the above orientation words are not used to limit the utility model.

[0029] Please refer to Figures 1-5 An explosion -proof centrifugal ventilator, including base 1, base 1 is provided with transmission mechanism, transmission mechanism includes shaft seat 2, transmission shaft 3, fan wheel 4, motor 5, driving wheel 6, driven wheel 7 and transmission belt 8, shaft seat 2 is provided with two groups of installation on base 1, transmission shaft 3 is rotatably installed on shaft seat 2, fan wheel 4 is installed on the one end of transmission shaft 3, motor 5 is installed on the outside of base 1, driving wheel 6 is installed on the output end of motor 5, driven wheel 7 is provided with protection mechanism between transmission shaft 3, transmission belt 8 is arranged in the outer wall of driving wheel 6 and driven wheel 7, protection mechanism includes connecting sleeve 9, connecting rod 10, clamping block 11, clamping groove 12, sliding hole 13, pressure ball 14, push block 15 and pressure regulating mechanism, connecting sleeve 9 is installed on driven wheel 7, connecting rod 10 is installed on the one end of transmission rod, clamping block 11 is provided with multiple groups of sliding in connecting sleeve 9, clamping groove 12 is provided with multiple groups of distribution on the outer wall of connecting rod 10, sliding hole 13 is provided with multiple groups of distribution in connecting sleeve 9, pressure ball 14 is slidably arranged in multiple groups of sliding hole 13, push block 15 is slidably arranged in multiple groups of sliding hole 13, pressure regulating mechanism includes sliding groove 16, adjusting sleeve 17, pressure spring 18, sliding sleeve 19 and positioning mechanism, sliding groove 16 is provided with multiple groups of distribution on the outer wall of connecting sleeve 9 and is communicated with multiple groups of sliding hole 13.Adjusting sleeve 17 is slidably arranged in multiple groups of sliding groove 16 and is connected with multiple groups of push block 15, pressure spring 18 is provided with multiple groups of installation on the top surface of adjusting sleeve 17, sliding sleeve 19 is slidably arranged on the outer wall of connecting sleeve 9 and is connected with multiple groups of pressure spring 18.

[0030] In this embodiment, the motor 5 is installed outside the base 1 and connected to the driving wheel 6 through the output end. After the motor 5 is powered on, it starts to work, and the output shaft rotates, thereby driving the driving wheel 6 installed at the output end of the motor 5 to rotate, and the outer wall of the driving wheel 6 is connected with the transmission belt 8. The transmission belt 8 runs around the driving wheel 6 and the driven wheel 7 to transmit power to the driven wheel 7, which is connected with the driving wheel 6 through the transmission belt 8 and rotates accordingly. The other end of the transmission shaft 3 is connected with the fan wheel 4, so when the driven wheel 7 rotates, the transmission shaft 3 also rotates, and the rotation of the transmission shaft 3 drives the fan wheel 4 to rotate. The rotation of the fan wheel 4 generates airflow, so that the fan can effectively perform air exhaust. When the fan wheel 4 is blocked by foreign matter, the transmission shaft 3 will generate resistance, and the motor 5 will continue to drive the driven wheel 7 to rotate through the driving wheel 6. When the resistance is greater than the preset force of the plurality of pressure springs, the plurality of clamping blocks 11 cooperate with the arc-shaped clamping grooves 12, the clamping blocks 11 overcome the pushing force of the pressure balls 14, the pressure balls 14 overcome the pushing force of the pushing blocks 15 to drive the adjusting sleeve 17 to slide and overcome the preset force of the pressure springs 18, so that the connecting rod 10 and the connecting sleeve 9 form an idle state, and then the transmission rod stops rotating, the motor 5 is turned off, and explosion caused by overloading of the motor 5 can be avoided.

[0031] Please refer to Figures 2-5 , as an embodiment of the pressure regulating mechanism: the positioning mechanism includes a positioning block 20, a positioning groove 21, a push spring 22, a rotating sleeve 23, and a give-way groove 24. The positioning block 20 is provided with a plurality of slidingly installed on the outer wall of the sliding sleeve 19, the positioning groove 21 is provided with a plurality of distributed on the outer wall of the connecting sleeve 9, the push spring 22 is provided with a plurality of installed inside the plurality of positioning blocks 20 and connected with the sliding sleeve 19, the rotating sleeve 23 is rotatably installed on the outer wall of the sliding sleeve 19, and the give-way groove 24 is provided with a plurality of distributed on the inner wall of the rotating sleeve 23.

[0032] An outer shell 25 is installed outside the fan wheel 4, and the outer shell 25 is fixedly connected with the base 1. The outer shell 25 protects the fan wheel 4 from external substances, and through the fixed connection with the base 1, the stability of the whole device is enhanced, and the normal operation of the fan is ensured.

[0033] The bottom ends of the plurality of clamping blocks 11 are arc-shaped, and the plurality of clamping grooves 12 are arc-shaped and engaged with the bottom ends of the plurality of clamping blocks 11. The arc-shaped design of the bottom end of the clamping block 11 and the clamping groove 12 ensures smooth engagement during operation, reduces friction and jamming, thereby improving the stability and smoothness of the connection.

[0034] The inner walls of the plurality of sliding holes 13 are provided with guide grooves 26, the outer walls of the plurality of pressure balls 14 are provided with guide blocks 27, and the plurality of guide grooves 26 are slidingly installed in the plurality of guide grooves 26. The cooperation of the guide grooves 26 and the guide blocks 27 ensures the correct guidance of the pressure balls 14, prevents them from deviating from the track, and thus maintains the stable stress state of the pressure balls during operation.

[0035] The top end of each group of card blocks 11 and the upper end of the push block 15 are provided with arc-shaped grooves adapted to the pressure ball 14. The arc-shaped grooves can better adapt to the pressure ball 14, so that the force is evenly distributed, avoiding local concentrated pressure, reducing wear and tear and improving the service life of the assembly.

[0036] The bottom surface of the sliding sleeve 19 is provided with a limiting rod 28, which is provided with multiple groups and is in sliding connection with the rich adjustment sleeve 17. The limiting rod 28 limits the movement range of the sliding sleeve 19, preventing unnecessary displacement, and the sliding connection with the adjustment sleeve 17 ensures the accuracy of the preset force adjustment.

[0037] The two ends of each group of displacement grooves 24 are provided with inclined surfaces. The design of the inclined surfaces allows the parts to smoothly displace when under stress, reducing resistance and impact, thereby improving the stability of the system and the reliability of the overall structure.

[0038] More specifically, when the preset force of the pressure spring 18 needs to be adjusted, the rotating sleeve 23 is rotated to drive the multiple displacement grooves 24 to rotate to the top end of the multiple positioning blocks 20, thereby removing the abutment of the inner wall of the rotating sleeve 23 against the positioning blocks 20. The multiple push springs 22 push the positioning blocks 20 outward to slide and disengage from the positioning grooves 21, thereby removing the restriction on the sliding sleeve 19. The sliding sleeve 19 is stretched to increase the preset force of the pressure spring 18, or the pressure spring 18 is contracted to reduce the preset force. Then, the rotating sleeve 23 is rotated again, and the inclined grooves at both ends of the displacement grooves 24 push the positioning blocks 20 to slide to the inner wall of the rotating sleeve 23 and apply pressure. The positioning blocks 20 slide into the positioning grooves 21 and press the multiple push springs 22, thereby positioning the sliding sleeve 19 and adjusting the preset force of the pressure spring 18.

[0039] In summary, the overall device in use or operation: motor 5 is installed outside the base 1 and through the output end connection driving wheel 6. Motor 5 power on and start working, output shaft rotation, thus driving the driving wheel 6 installed in the output end of the motor 5 rotation, and the outer wall of the driving wheel 6 with the transmission belt 8 connection. Transmission belt 8 around the driving wheel 6 and driven wheel 7 operation, power transmission to the driven wheel 7, driven wheel 7 through the transmission belt 8 and driving wheel 6 connected, with the rotation. The other end of the transmission shaft 3 and fan wheel 4 connection, thus when the driven wheel 7 rotation, transmission shaft 3 also with the rotation, the rotation of the transmission shaft 3 will drive the fan wheel 4 rotation. The rotation of the fan wheel 4 produces airflow, so that the fan can effectively carry out air exhaust, when the fan wheel 4 is jammed by foreign matter, transmission shaft 3 will produce resistance, motor 5 through the driving wheel 6 continuously driven wheel 7 rotation, when the resistance is greater than the pre-set force of the plurality of pressure spring, a plurality of blocks 11 and the setting of the arc shaped slot 12 cooperation, block 11 overcome the thrust of the pressure ball 14, pressure ball 14 overcome the thrust of the push block 15 make the push block 15 drive the adjustment sleeve 17 slide and overcome the pre-set force of the pressure spring 18, so that the connecting rod 10 and connecting sleeve 9 form idling, then the transmission rod stop rotation, close the motor 5, can avoid because the motor 5 overload and lead to explosion.

[0040] But need to adjust the pre-set force of the pressure spring 18, rotate the rotating sleeve 23 drive a plurality of let go of the slot 24 rotation to the top of the plurality of positioning block 20, remove the rotating sleeve 23 inner wall of the abutment of the positioning block 20, through a plurality of push spring 22 push the positioning block 20 slide outward and away from the positioning slot 21, can remove the limit of the sliding sleeve 19, sliding sleeve 19 stretching the pressure spring 18 can increase the pre-set force of the pressure spring 18, otherwise the pressure spring 18 contraction can reduce the pre-set force, then rotate the rotating sleeve 23 again, through the let go of the slot 24 both ends of the inclined slot to push the positioning block 20 slide to the rotating sleeve 23 inner wall and the pressure under the pressure, positioning block 20 slide to the positioning slot 21 and a plurality of push spring 22 extrusion, complete the positioning of the sliding sleeve 19, then complete the pre-set force of the pressure spring 18 adjustment.

[0041] In the above-mentioned all the schemes, the connection between the two components can be selected according to the actual situation, welding, bolt and nut cooperation connection, bolt or screw connection or other known connection mode, not here one by one, in the above, whenever it involves a fixed connection, preferably consider is welding, although the embodiments of the utility model have been shown and described, for those skilled in the art, it can be understood that without departing from the principles and spirit of the utility model, these embodiments can be varied, modified, replaced and modified, the scope of the utility model is defined by the appended claims and their equivalents.

Claims

1. An explosion-proof centrifugal fan comprising a base (1), characterised in that: The base (1) is provided with a transmission mechanism, the transmission mechanism includes an axle seat (2), a transmission shaft (3), a fan wheel (4), a motor (5), a driving wheel (6), a driven wheel (7) and a transmission belt (8), the axle seat (2) is provided with two groups of installation on the base (1), the transmission shaft (3) is rotatably installed on the axle seat (2), the fan wheel (4) is installed on one end of the transmission shaft (3), the motor (5) is installed on the outside of the base (1), the driving wheel (6) is installed on the output end of the motor (5), the driven wheel (7) is provided with a protection mechanism between the transmission shaft (3), the transmission belt (8) is arranged on the outer wall of the driving wheel (6) and the driven wheel (7), the protection mechanism includes a connecting sleeve (9), a connecting rod (10), a clamping block (11), a clamping groove (12), a sliding hole (13), a pressing ball (14), a push block (15) and a pressure regulating mechanism, the connecting sleeve (9) is installed on the driven wheel (7), the connecting rod (10) is installed on one end of the transmission rod, the clamping block (11) is provided with a plurality of groups of sliding in the connecting sleeve (9), the clamping groove (12) is provided with a plurality of groups of distribution on the outer wall of the connecting rod (10), the sliding hole (13) is provided with a plurality of groups of distribution in the connecting sleeve (9), the pressing ball (14) is slidably arranged in a plurality of sliding holes (13), the push block (15) is slidably arranged in a plurality of sliding holes (13), the pressure regulating mechanism includes a sliding groove (16), an adjusting sleeve (17), a pressure spring (18), a sliding sleeve (19) and a positioning mechanism, the sliding groove (16) is provided with a plurality of groups of distribution on the outer wall of the connecting sleeve (9) and is communicated with a plurality of sliding holes (13), the adjusting sleeve (17) is slidably arranged in a plurality of sliding grooves (16) and is connected with a plurality of push blocks (15), the pressure spring (18) is provided with a plurality of groups of installation on the top surface of the adjusting sleeve (17), the sliding sleeve (19) is slidably arranged on the outer wall of the connecting sleeve (9) and is connected with a plurality of pressure springs (18).

2. A flameproof centrifugal fan according to claim 1, characterised in that: The positioning mechanism includes a positioning block (20), a positioning groove (21), a push spring (22), a rotating sleeve (23) and a let go groove (24), the positioning block (20) is provided with a plurality of groups of slidingly installed on the outer wall of the sliding sleeve (19), the positioning groove (21) is provided with a plurality of groups of distribution on the outer wall of the connecting sleeve (9), the push spring (22) is provided with a plurality of groups of installation on the inner side of a plurality of positioning blocks (20) and is connected with the sliding sleeve (19), the rotating sleeve (23) is rotatably installed on the outer wall of the sliding sleeve (19), and the let go groove (24) is provided with a plurality of groups of distribution on the inner wall of the rotating sleeve (23).

3. A flameproof centrifugal fan according to claim 2, characterised in that: The outer side of the fan wheel (4) is provided with an outer shell (25), and the outer shell (25) is fixedly connected with the base (1).

4. A flameproof centrifugal fan as claimed in claim 3, characterized in that the plurality of groups of blades are arranged in a plurality of rows. The bottom end of the clamping block (11) is provided in an arc shape, and a plurality of clamping grooves (12) are provided in an arc shape and are clamped with the bottom end of the clamping block (11).

5. A flameproof centrifugal fan as claimed in claim 4, characterized in that the plurality of groups of blades are arranged in a plurality of rows. The inner wall of the sliding hole (13) is provided with a guide groove (26), a plurality of pressing balls (14) are provided with a guide block (27) on the outer wall, and a plurality of guide grooves (26) are slidably arranged in a plurality of guide grooves (26).

6. A flameproof centrifugal fan according to claim 5, characterized in that the plurality of groups The top end of the clamping block (11) and the push block (15) are provided with an arc-shaped groove matched with the pressing ball (14).

7. A flameproof centrifugal fan according to claim 6, characterised in that: The slide (19) bottom surface is provided with a limiting rod (28), which is provided with multiple groups and is connected with the rich adjustment sleeve (17) in sliding mode.

8. A flameproof centrifugal fan according to claim 7, characterised in that: Multiple groups of the letting slot (24) are provided with inclined surfaces at both ends.

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