CN217440358U

CN217440358U - Blade, impeller and ventilation equipment

Info

Publication number: CN217440358U
Application number: CN202220982013.3U
Authority: CN
Inventors: 李建建; 王烨; 汪健; 李仕铭; 莫建樷; 陈明琪
Original assignee: Gree Green Refrigeration Technology Center Co Ltd of Zhuhai
Current assignee: Gree Green Refrigeration Technology Center Co Ltd of Zhuhai
Priority date: 2022-04-25
Filing date: 2022-04-25
Publication date: 2022-09-16
Anticipated expiration: 2032-04-25

Abstract

The utility model relates to a blade, impeller and ventilation equipment, this blade include the pressure surface and the suction surface of relative setting to and connect pressure surface and suction surface and relative top surface and the bottom surface that sets up, wherein, the direction of bottom surface to top surface is first direction, and the height that the pressure surface is greater than the height that the suction surface is followed first direction along the first direction. This application is through the height that sets up the pressure surface height of blade and be greater than the suction surface, can separate the air current that strikes the top surface rapidly, and partly gets into the pressure surface of blade, and another part flows to the suction surface along the top surface to reduce the impact of air current and top surface, reduce the windage, reduce the air current degree of disorder, both increase the amount of wind, reduce the aerodynamic noise again, reduce the consumption, be favorable to improving ventilation equipment's aerodynamic performance.

Description

Blade, impeller and ventilation equipment

Technical Field

The application relates to the technical field of household appliances, in particular to a blade, an impeller and a ventilation device.

Background

The impeller is a common component in ventilation equipment, and the use performance of the impeller is mainly embodied in the aspects of air volume and noise control. When an impeller in the prior art works, air enters from an air inlet and impacts the top of a blade, so that airflow cannot smoothly enter a suction surface and a pressure surface of the blade, the wind resistance is high, the air flow is disordered, the air volume is reduced, and the noise is increased.

SUMMERY OF THE UTILITY MODEL

An object of the application is to provide a blade, an impeller and a ventilation device, which can reduce pneumatic noise while increasing air volume.

In a first aspect, an embodiment of the present application provides a blade, including a pressure surface and a suction surface that are disposed opposite to each other, and a top surface and a bottom surface that connect the pressure surface and the suction surface and are disposed opposite to each other, where a direction from the bottom surface to the top surface is a first direction, and a height of the pressure surface along the first direction is greater than a height of the suction surface along the first direction.

In one possible implementation, the height of the pressure surface in the first direction is H1, the height of the suction surface in the first direction is H2, and (H1-H2)/H1 is 0.08-0.15.

In one possible implementation, the top surface is a curved surface, and the junction of the top surface and the suction surface is arranged tangentially.

In a possible implementation manner, an included angle between a tangent of a connection between the top surface and the pressure surface is α, and the following condition is satisfied: alpha is more than or equal to 35 degrees and less than or equal to 40 degrees.

In one possible implementation, the top surface has a cross-sectional shape along a plane perpendicular to the meridian plane that is a spline curve or a bezier curve.

In one possible implementation, the projections of the suction surface and the pressure surface in a plane perpendicular to the first direction are each any one of a single arc, a tangent double arc, and an airfoil arc.

In a second aspect, embodiments of the present application further provide an impeller, including: a hub; a chassis provided on an outer peripheral side of the hub; the front plate is opposite to and concentric with the chassis; and the blade is arranged between the base plate and the front plate, and the top surface of the blade is arranged towards the front plate.

In a possible implementation manner, the number of the blades is at least two, at least two blades are arranged at intervals along the circumferential direction of the hub, and the value range of the turning angle of the airflow between the front edge close to one side of the hub and the rear edge far away from the front edge of the blade is 0-40 degrees.

In one possible implementation, the inlet stagger angle β at the leading edge of the blade _A1 Has a value range of beta _A1 45-60 DEG, outlet mounting angle beta at the trailing edge of the blade _A2 Has a value range of beta _A2 ＝45°～80°。

In a third aspect, an embodiment of the present application provides a ventilation apparatus, including: an impeller as described previously; the motor is arranged at the rotating central shaft of the hub of the impeller; and the guide ring is arranged on one side of the front disk of the impeller, and the inner diameter of the guide ring is larger than that of the front edge of one side, close to the hub, of the blades of the impeller and is smaller than that of the front disk.

According to the blade, impeller and ventilation equipment that this application embodiment provided, the pressure surface height through setting up the blade is greater than the height of suction surface, can separate the air current of assaulting the top surface rapidly, and partly get into the pressure surface of blade, another part flows to the suction surface along the top surface to reduce the impact of air current and top surface, reduce the windage, reduce the air current degree of disorder, both increased the amount of wind, reduce the aerodynamic noise again, reduce the consumption, be favorable to improving ventilation equipment's aerodynamic performance.

Drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts. In addition, in the drawings, like parts are denoted by like reference numerals, and the drawings are not drawn to actual scale.

Fig. 1 is a schematic perspective view illustrating a ventilation apparatus according to an embodiment of the present disclosure;

FIG. 2 illustrates a schematic top view of an impeller provided by an embodiment of the present application;

FIG. 3 illustrates a perspective view of a blade provided by an embodiment of the present application;

figure 4 shows a projection view of the blade shown in figure 3 in a meridian plane;

FIG. 5 shows a schematic view of the blade shown in FIG. 3, as seen from the leading edge side;

fig. 6 shows an enlarged structural view of the area a in fig. 2.

Description of the reference numerals:

10. an impeller; 1. a blade; x, a first direction; 11. a pressure surface; 12. a suction surface; 13. a top surface; 14. a bottom surface; 15. a leading edge; 16. a trailing edge; 2. a hub; 3. a chassis; 4. a front plate;

20. and (4) a flow guide ring.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are some embodiments of the present application, but not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present application.

Fig. 1 shows a schematic perspective view of a ventilation device provided in an embodiment of the present application, and fig. 2 shows a schematic top view of an impeller provided in an embodiment of the present application.

As shown in fig. 1 and 2, the ventilation device provided by the embodiment of the present application includes an impeller 10, a motor (not shown in the drawings), and a baffle 20. The ventilation device may be an air conditioner, and the impeller 10 is used to allow an air flow to pass through a heat exchanger, so that the air flow exchanges heat with a refrigerant in the heat exchanger, thereby achieving a cooling or heating effect.

The impeller 10 comprises blades 1, a hub 2, a chassis 3 and a front disk 4. The chassis 3 is disposed on the outer peripheral side of the hub 2, the front disk 4 is disposed opposite to and concentrically with the chassis 3, the blade 1 is disposed between the chassis 3 and the front disk 4, and the top surface 13 of the blade 1 is disposed toward the front disk 4.

The motor is arranged at the rotating central shaft of the hub 2, the guide ring 20 is arranged at one side of the front disk 4 of the impeller 10, and the inner diameter of the guide ring 20 is larger than the inner diameter of the front edge 15 of the blade 1 of the impeller 10 close to one side of the hub 2 and smaller than the inner diameter of the front disk 4. When the impeller 10 operates under the action of the motor, air flow is sucked along the axial direction of the guide ring 20 and collides with the top surface 13 of the blade 1 to drive the blade 1 to do work, and then the air flow is conveyed by radially discharging air from the impeller 10.

As shown by the dashed circle in fig. 2, the inner diameter of the guide ring 20 is D, the inner diameter of the leading edge 15 of the vane 1 is D1, the inner diameter of the front disc 4 is D2, and D1< D2. Optionally, in order to increase the air volume and reduce the energy consumption of the ventilation device, the inner diameter of the guide ring 20 corresponds to 1/3-2/3 of the length of the blade 1 from the front edge 15 side.

In the related art, when the airflow collides with the top surface 13 of the blade 1, the top surface 13 is vertically intersected with the suction surface 12 and the pressure surface 11, so that the airflow cannot smoothly enter the suction surface 12 and the pressure surface 11 through the top surface 13, the wind resistance is large, the energy loss is large, the air flow is turbulent, the air volume is reduced, and the aerodynamic noise is increased.

In order to solve the above problem, the present embodiment provides a vane 1, which can reduce aerodynamic noise while increasing air volume.

Fig. 3 shows a perspective view of a blade provided in an embodiment of the present application.

As shown in fig. 3, the blade 1 includes a pressure surface 11 and a suction surface 12 which are oppositely arranged, and a top surface 13 and a bottom surface 14 which are oppositely arranged and connect the pressure surface 11 and the suction surface 12, wherein the direction from the bottom surface 14 to the top surface 13 is a first direction X, and the height of the pressure surface 11 along the first direction X is greater than the height of the suction surface 12 along the first direction X. The first direction X is an axial direction of the impeller 10. The bottom surface 14 may be a flat surface or a curved surface, and is not limited herein.

Since the height of the pressure side 11 of the blade 1 is greater than the height of the suction side 12, the top side 13 connecting the pressure side 11 and the suction side 12 is a non-planar surface, such as an inclined surface or a curved surface. When airflow sucked along the axial direction of the guide ring 20 impacts the top surface 13 of the blade 1, the airflow can be quickly separated, one part of the airflow enters the pressure surface 11 of the blade 1, and the other part of the airflow flows to the suction surface 12 along the top surface 13, so that the impact of the airflow and the top surface 13 is reduced, the wind resistance is reduced, the airflow disorder degree is reduced, the air volume is increased, the aerodynamic noise is reduced, the power consumption is reduced, and the aerodynamic performance of the ventilation equipment is improved.

The specific structure of the blade provided by the embodiments of the present application is described in further detail below with reference to the accompanying drawings.

Fig. 4 shows a projection view of the blade shown in fig. 3 on a meridian plane, and fig. 5 shows a schematic view of the blade shown in fig. 3 as seen from the leading edge side.

Here, the "meridian plane" refers to a plane passing through the central axis of the impeller 10, and a projection view of the blade on the meridian plane refers to a projection view of the blade on the meridian plane when the blade rotates around the central axis of the impeller 10 to intersect with the meridian plane at any point thereon with reference to any meridian plane.

Optionally, the shape of the front edge 15 of the blade 1 may be an elliptical arc surface, or may be an arc surface, so as to play a role in guiding flow. When the air flow sucked in from the axial direction of the guide ring 20 reaches the leading edge 15, a part of the air flow enters the pressure surface 11 side and a part of the air flow enters the suction surface 12 side while flowing toward the blade root of the bottom surface 14, so that the axial air flow is converted into the radial air flow.

In some embodiments, the height of the pressure surface 11 of the blade 1 along the first direction X is H1, and the height of the suction surface 12 along the first direction X is H2, so that (H1-H2)/H1 is 0.08-0.15. So set up, can make the blade satisfy at the air separation speed, can also improve the area of contact size of air current and blade 1, increase the amount of wind.

As shown in fig. 4 and 5, the pressure surface 11 of the blade 1 has a height H1 in the first direction X, and the suction surface 12 has a height H2 in the first direction X. When the ratio of the height difference (H1-H2) between the pressure surface 11 and the suction surface 12 to the height H1 of the pressure surface 11 is close to 0.08, the inclination angle of the top surface 13 is small, so that the contact area between the airflow and the blade 1 can be increased, the air volume is increased, and the airflow separation speed is relatively slow. When the ratio of the height difference (H1-H2) between the pressure surface 11 and the suction surface 12 to the height H1 of the pressure surface 11 is close to 0.15, the inclination angle of the top surface 13 is large, the airflow separation speed is high, and meanwhile, the contact area between the airflow and the blade 1 is relatively small and the airflow volume is small. In actual use, a balance value can be obtained between the airflow separation speed and the air volume. For example, in one example, (H1-H2)/H1 is 0.1.

In some embodiments, the top surface 13 is curved, and the junction of the top surface 13 and the suction surface 12 is disposed tangentially. The top surface 13 of the blade 1 is tangentially arranged at the joint of the suction surface 12, so that the airflow can smoothly enter the suction surface 12 along the top surface 13, and the airflow separation speed is improved.

In some embodiments, the tangent to the junction of the top surface 13 and the pressure surface 11 forms an angle α with the pressure surface 11, and the following condition is satisfied: alpha is more than or equal to 35 degrees and less than or equal to 40 degrees. In one example, α ═ 38 °. The included angle alpha between the tangent line of the joint of the top surface 13 and the pressure surface 11 is set so as to ensure that the top surface 13 has a good drainage effect.

As shown in fig. 4, the tangent to the connection of the top surface 13 to the pressure surface 11 forms an angle α with the pressure surface 12. When the airflow hits the top surface 13, separation can be performed quickly, with a portion of the airflow entering the pressure surface 11 side and another portion of the airflow flowing along the top surface 13 to the suction surface 12 side. Because the top surface 13 has a good drainage effect, the pressure surface 11 can generate positive pressure, and the suction surface 12 can generate negative pressure, so as to drive the blade 1 to rotate.

In some embodiments, the top surface 13 has a polynomial curve in cross-sectional shape along a plane perpendicular to the meridian plane. Polynomial curves include, but are not limited to, spline curves, bezier curves, and the like. The cross-sectional shape of the top surface 13 along a plane perpendicular to the meridian plane is also referred to herein as the "profile" of the top surface 13. In one example, the profile of the top surface 13 is a bezier curve to ensure that the top surface 13 has a good smoothing effect, increasing the speed of the gas flow separation.

In some embodiments, the projections of the suction surface 12 and the pressure surface 11 in a plane perpendicular to the first direction X are each any one of a single arc, a tangent double arc, and an airfoil arc. The term "tangent double arc" means that one arc on the side of the front edge 15 is opposite to the other arc and is tangent to the other arc at the intersection. That is, the suction surface 12 and the pressure surface 11 may each be any one of a single arc surface, a tangential double arc surface, and an airfoil cambered surface in a radial direction from the leading edge 15 to the trailing edge 16. Because the suction surface 12 and the pressure surface 11 both have smooth curved surfaces, the axial sucked air flow is converted into radial air flow which is thrown out from the air outlet. The curvatures of the suction surface 12 and the pressure surface 11 may be the same or different.

Therefore, when the airflow sucked from the axial direction of the guide ring 20 collides with the top surface 13, the airflow entering the suction surface 12 side smoothly flows toward the rear edge 16 side of the blade 1 along the shape of the suction surface 12, and the airflow entering the pressure surface 11 side smoothly flows toward the rear edge 16 side of the blade 1 along the shape of the pressure surface 11, so that the resistance of the airflow flowing through the suction surface 12 or the pressure surface 11 can be reduced, the air volume can be increased, and the power consumption of the ventilator can be reduced.

Further, as mentioned above, when the impeller 10 operates under the action of the motor, the air flow is sucked in along the axial direction of the flow guiding ring 20 and collides with the top surface 13 of the blade 1, so as to drive the blade 1 to apply work, and then the air flow is discharged from the impeller 10 in the radial direction to realize the air flow delivery. The guide ring 20 is disposed on the front disk 4 side of the impeller 10, and the inner diameter of the guide ring 20 corresponds to 1/3-2/3 of the length of the blade 1 from the front edge 15 side. In order to enable the air flow introduced by the guide ring 20 from the axial direction to rapidly wind out from the radial flow channel of the blade 1 through the top surface 13 of the blade 1, the suction surface 12 and the pressure surface 11 may have different curvatures so that the radial direction of the blade 1 from the leading edge 15 to the trailing edge 16 has different thicknesses.

Specifically, taking the suction surface 12 and the pressure surface 11 of the blade 1 as a single arc surface as an example, the thickness of the blade 1 gradually becomes thicker from the front edge 15 side to 1/3 of the length of the blade 1, so as to improve the compressive strength of the blade 1, and the thickness from 1/3 of the length of the blade 1 to the rear edge 16 gradually becomes thinner, so as to increase the width of the flow channel between two adjacent blades 1, so that the axially entering airflow is decelerated when changing to the radial flow direction, so that the kinetic energy of the airflow is converted into pressure energy to drive the blade 1 to rotate, thereby further increasing the air volume and reducing the power consumption of the ventilation equipment.

Referring again to fig. 2, the blade 1 as described above is arranged between the base plate 3 and the front plate 4, the leading edge 15 of the blade 1 being arranged towards the hub 2 and the top surface 13 of the blade 1 being arranged towards the front plate 4. In some embodiments, the number of the blades 1 is at least two, at least two blades 1 are arranged at intervals along the circumferential direction of the hub 2, and the turning angle of the airflow between the front edge 15 and the rear edge 16 far away from the front edge 15 of the blades 1 ranges from 0 to 40 °.

As shown in fig. 2, the plurality of blades 1 are disposed between the base plate 3 and the front plate 4, a radial flow channel is formed between two adjacent blades 1, the inner diameter of the flow guiding ring 20 is an air inlet, and an air outlet is formed between the rear edges 16 of two adjacent blades 1. The airflow is sucked from the axial direction of the air inlet, mainly collides with the top surface 13 of the blade 1, then a part of the airflow enters the pressure surface 11 of the blade 1 to generate positive pressure, and another part of the airflow enters the suction surface 12 of the blade 1 to generate negative pressure, and then the airflow respectively flows towards the rear edge 16 of the blade 1 and the blade root facing the bottom surface 14 to drive the blades 1 to rotate along the counterclockwise direction in fig. 2. The turning angle of the airflow between the front edge 15 and the rear edge 16 far away from the front edge 15 of the blade 1 ranges from 0 to 40 degrees, smooth flow guiding of the airflow along a radial flow channel can be guaranteed, the distance between the air outlet and the air inlet of the impeller 10 is increased, and accordingly the entrainment backflow phenomenon of the airflow is weakened.

In addition, in at least two vanes 1, the profile of the top surface 13 of each vane 1 may be the same or different, depending on the specific requirements. When the top surface 13 of each blade 1 adopts the same profile, the manufacturing process can be simplified, and the manufacturing cost of the impeller 10 can be reduced.

Fig. 6 shows an enlarged structural view of the area a in fig. 2.

In some embodiments, the inlet stagger angle β at the leading edge 15 of the blade 1 _A1 Has a value range of beta _A1 45-60 DEG, outlet setting angle beta at the trailing edge 16 of the blade 1 _A2 Has a value range of beta _A2 ＝45°～80°。

As shown in fig. 2 and 6, the blade 1 has a mean camber line S, which is constituted by points at equal distances from the pressure surface 11 and the suction surface 12 and is delimited by a leading edge 15 and a trailing edge 16. Assuming that the circle in which the leading edges 15 of the plurality of blades 1 are located is a first circle C1, the first tangent line at the intersection of the first circle C1 and the mean camber line S is T1, the circle in which the trailing edges 16 of the plurality of blades 1 are located is a second circle C2, and the second tangent line at the intersection of the second circle C2 and the mean camber line S is T2, "the inlet setting angle β _A1 "refers to the angle between the mean camber line S of the blade 1 and the first tangent T1, the" outlet setting angle β _A2 "refers to the angle between the mean camber line S of the blade 1 and the second tangent T2. The blade 1 is arranged on the impeller 10 in such a way, so that the airflow can be ensured to flow according to the preset airflow turning angle of the blade 1, and the working efficiency is improved.

From this, blade 1, impeller 10 and ventilation equipment that this application embodiment provided, the height that is greater than suction surface 12 through the height that sets up pressure surface 11 of blade 1, can separate the air current of impacting top surface 13 fast, partly pressure surface 11 that gets into blade 1, another part flows to suction surface 12 along top surface 13, thereby reduce the impact of air current and top surface 13, reduce the windage, reduce the air current degree of disorder, both increase the amount of wind, reduce aerodynamic noise again, reduce the power consumption, be favorable to improving ventilation equipment's aerodynamic performance.

It can be understood that the ventilation device provided by the embodiment of the present application is not limited to an air conditioner, and may also be a household ventilation device such as a range hood, a dust remover, and the like.

It should be noted that references in the specification to "one embodiment," "an example embodiment," "some embodiments," etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

It should be readily understood that "on … …", "above … …" and "above … …" in this disclosure should be interpreted in its broadest sense such that "on … …" means not only "directly on something", but also includes the meaning of "on something" with intervening features or layers therebetween, and "above … …" or "above … …" includes not only the meaning of "above something" or "above" but also includes the meaning of "above something" or "above" with no intervening features or layers therebetween (i.e., directly on something).

Furthermore, spatially relative terms, such as "below," "lower," "above," "upper," and the like, may be used herein for ease of description to describe one element or feature's illustrated relationship to another element or feature. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The device may have other orientations (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly as well.

It is noted that, in this document, relational terms such as "first" and "second," and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

Finally, it should be noted that: the above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some or all of the technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A blade is characterized by comprising a pressure surface, a suction surface, a top surface and a bottom surface, wherein the pressure surface and the suction surface are arranged oppositely, the top surface and the bottom surface are connected with each other and are arranged oppositely, the direction from the bottom surface to the top surface is a first direction, and the height of the pressure surface along the first direction is larger than that of the suction surface along the first direction.

2. The blade of claim 1, wherein the pressure surface has a height in the first direction of H1, the suction surface has a height in the first direction of H2, and (H1-H2)/H1 is 0.08-0.15.

3. The blade of claim 1 wherein the top surface is curved and the junction of the top surface and the suction surface is disposed tangentially.

4. A blade according to claim 3, wherein the angle between the tangent to the connection of the tip surface and the pressure surface is α, and the following condition is fulfilled: alpha is more than or equal to 35 degrees and less than or equal to 40 degrees.

5. The blade of claim 3, wherein a cross-sectional shape of the top surface along a plane perpendicular to the meridian plane is a spline curve or a Bezier curve.

6. The blade of claim 1, wherein projections of the suction surface and the pressure surface in a plane perpendicular to the first direction are each any of a single arc, a tangent double arc, and an airfoil arc.

7. An impeller, comprising:

a hub;

a chassis provided on an outer peripheral side of the hub;

the front disc is opposite to and concentric with the chassis; and

a blade according to any of claims 1 to 6, arranged between the base disc and the front disc with the top surface of the blade arranged towards the front disc.

8. The impeller according to claim 7, wherein the number of the blades is at least two, at least two of the blades are arranged at intervals along the circumferential direction of the hub, and the turning angle of the airflow between the front edge close to one side of the hub and the rear edge far away from the front edge of the blades ranges from 0 ° to 40 °.

9. The impeller of claim 8, wherein an inlet mounting angle β at the leading edge of the blade _A1 Has a value range of beta _A1 45-60 DEG, an outlet setting angle beta at the trailing edge of the blade _A2 Has a value range of beta _A2 ＝45°～80°。

10. A ventilation device, comprising:

an impeller according to any one of claims 7 to 9;

the motor is arranged at the rotating central shaft of the hub of the impeller; and

the guide ring is arranged on one side of the front disc of the impeller, and the inner diameter of the guide ring is larger than the inner diameter of the blade of the impeller, which is close to the front edge on one side of the hub, and is smaller than the inner diameter of the front disc.