CN110953280B

CN110953280B - Liquid-gas buffer

Info

Publication number: CN110953280B
Application number: CN202010002859.1A
Authority: CN
Inventors: 毛从强; 李辛; 刘辉; 齐亚文; 郝博; 于正庭; 高晗; 谢想
Original assignee: CRRC Qingdao Sifang Rolling Stock Research Institute Co Ltd
Current assignee: CRRC Qingdao Sifang Rolling Stock Research Institute Co Ltd
Priority date: 2020-01-02
Filing date: 2020-01-02
Publication date: 2021-04-20
Anticipated expiration: 2040-01-02
Also published as: CN110953280A

Abstract

The invention provides a hydropneumatic buffer comprising: a cylinder body; a first piston assembly: the piston head is arranged in the cylinder body and divides the cylinder body into a first liquid cavity and a second liquid cavity, and the force transmission rod extends out of the cylinder body; a first fluid channel and a second fluid channel which are communicated with the first liquid cavity and the second liquid cavity are arranged on the piston head; the first fluid channel is provided with a first switch piece which is configured to be opened when the first liquid cavity pressure is greater than the second liquid cavity pressure; the second fluid channel is provided with a second switch piece which is configured to be opened when the second liquid cavity pressure is greater than the first liquid cavity pressure; a second piston assembly: the second liquid cavity is arranged around the force transmission rod and is arranged between the piston head and the cylinder body, and an air cavity is formed between the piston head and the cylinder body port. The piston assembly of the liquid-gas buffer adopts a structure design of partial built-in and partial built-out, namely, the piston assembly is used as a liquid cavity separation assembly and a force transmission assembly, so that the buffer structure is more compact.

Description

Liquid-gas buffer

Technical Field

The invention relates to the technical field of coupler buffering, in particular to a liquid-gas buffer.

Background

The inside of the liquid-gas buffer mainly comprises an oil chamber and a gas chamber which are separated by a liquid-gas separation piston. The oil cavity is filled with hydraulic oil and is internally provided with a throttling pore with a certain damping coefficient. The air cavity is filled with inert gas with certain pressure. When the buffer is pressed, on one hand, hydraulic oil flows through the throttling pore and rubs with the metal wall to generate heat, so that most of external compression kinetic energy is dissipated; on the other hand, the inert gas is compressed, and a small part of external compression kinetic energy is converted into compression potential energy of the gas, so that power is provided for the subsequent buffer recovery.

The structure of a traditional-structure hydraulic-pneumatic buffer is shown in figure 1, and comprises a cylinder body 1, a plunger 2 and a piston 3, wherein the plunger 2 is provided with a longer plunger inner cavity, a throttling damping fluid channel 301 is arranged at a port position of the plunger inserted into the cylinder body 1, the left side and the right side of the channel are communicated oil cavities 4, one part of the oil cavities is positioned in the cylinder body 1, and the other part of the oil cavities is positioned in the plunger 2. The piston 3 is positioned in the inner cavity of the plunger 2, and the piston 3 isolates an air cavity 5 at the right side of the oil cavity. In the radial space, the plunger cavity substantially fills the inner diameter of the damper cylinder. When the buffer is pressed, all hydraulic oil in the cylinder body flows into the inner cavity of the rod of the plunger 2, and pushes the liquid-gas separation piston 3 to move towards the direction of the air cavity 5, so that the air cavity 5 is compressed, and buffering and energy absorption are realized. The liquid-gas buffer with the structure has the defects that the radial space of the inner cavity of the plunger 2 is smaller than the inner diameter of the cylinder in structure, hydraulic oil is close to the characteristics of incompressible low compressibility and inert gas can not be compressed infinitely, so that a liquid-gas separation piston needs a larger stroke moving space, and the whole buffer is redundant in length and size, heavy in weight and not compact enough. Moreover, the larger the stroke, the smaller the radial dimension of the damper, and this drawback is more pronounced. In terms of performance, the static impedance of the traditional structure liquid-gas buffer under the low-speed compression working condition is completely dependent on the pressure in the gas cavity. In order to ensure the reliability of a sealing link, the pressure intensity in the air cavity is not easy to set too large, the static resistance value of the liquid-air buffer is directly low, the energy absorption under the low-speed working condition is limited, and the working condition applicability of the liquid-air buffer with the traditional structure is limited by the characteristics of the low resistance value and the low static capacity.

Disclosure of Invention

The invention aims to provide a liquid-gas buffer with compact structure and high impedance to solve the problem that the buffer performance of the liquid-gas buffer group is limited due to low resistance and low static capacity in the prior art.

In order to achieve the purpose, the invention adopts the technical scheme that:

a hydropneumatic buffer comprising:

a cylinder body;

a first piston assembly: the piston head is arranged in the cylinder body and divides the cylinder body into a first liquid cavity and a second liquid cavity, and the force transmission rod extends out of the cylinder body; a first fluid channel and a second fluid channel which are communicated with the first liquid cavity and the second liquid cavity are arranged on the piston head;

the first fluid channel is provided with a first switch piece which is configured to be opened when the first liquid cavity pressure is greater than the second liquid cavity pressure;

the second fluid channel is provided with a second switch piece which is configured to be opened when the second liquid cavity pressure is greater than the first liquid cavity pressure;

a second piston assembly: the second liquid cavity is arranged around the force transmission rod and is arranged between the piston head and the cylinder body, and an air cavity is formed between the piston head and the cylinder body port.

Preferably, the method comprises the following steps: the first fluid passage and the second fluid passage are respectively positioned on two sides of the axial center of the piston head.

Preferably, the method comprises the following steps: the cylinder body axially comprises a first cylinder body end and a second cylinder body end, the first cylinder body end is provided with a tank liquid hole, and the force transmission rod penetrates out of the second cylinder body end.

Preferably, the method comprises the following steps: the liquid-gas damper further comprises:

the throttle rod extends into the cylinder body;

the first fluid channel comprises a throttling channel and a branch channel, the throttling channel is communicated with the branch channel and penetrates through the first liquid cavity and the second liquid cavity, and when the first piston assembly is compressed, the throttling rod can be inserted into the throttling channel and is in clearance fit with the throttling channel;

the first switching member is disposed on the bypass passage.

Preferably, the method comprises the following steps: the throttling channel is arranged along the axial direction of the piston head, and the branch channel is arranged in a mode of deviating from the extending direction of the throttling channel.

Preferably, the method comprises the following steps: the cylinder body is provided with a filling hole, and the throttle rod is arranged at the filling hole and is detachably connected with the cylinder body.

Preferably, the method comprises the following steps: the second cylinder body end is an open end, the cylinder body further comprises an end cover arranged at the open end, the end cover radially closes the open end, a through hole is formed in the end cover, and the force transmission rod penetrates out of the cylinder body through the through hole.

Preferably, the method comprises the following steps: the end cap comprises an end seat part and an extension part, wherein the extension part extends from the end seat part to the piston head direction along the axial direction, and the extension part comprises a first extension part arranged along the through hole.

Preferably, the method comprises the following steps: the second piston assembly is arranged between the first extension part and the side wall of the cylinder body, and closes the space among the first extension part, the side wall of the cylinder body and the end seat part to form an air cavity.

Preferably, the method comprises the following steps: the extension part further comprises a second extension part which is arranged at a radial interval with the first extension part, the second extension part is arranged by being attached to the side wall of the cylinder body, the second piston assembly is arranged between the first extension part and the second extension part, and the space among the first extension part, the second extension part and the end seat part is sealed to form an air cavity.

Preferably, the method comprises the following steps: the first switch part adopts a pressure increasing valve, and the second switch part adopts a one-way valve.

Compared with the prior art, the liquid-gas buffer provided by the invention has the beneficial effects that:

(1) the piston assembly adopts a structure design of partial built-in and partial built-out, namely, the piston assembly is used as a liquid cavity separation assembly and a force transmission assembly, so that the structure of the buffer is more compact. The exposed part of the piston rod is changed from a hollow cylindrical form into a solid rod shape. The reduction of the radial dimension directly brings obvious light weight effect to the buffer. Meanwhile, the exposed part of the piston rod has small volume, the volume change of the oil cavity is small after the piston rod is pressed into the oil cavity, the stroke activity of the required liquid-gas separation piston is small, and the axial space of the buffer is saved.

(2) The design of a pressurizing structure, in particular to a two-stage pressurizing structure, the throttling channel and the pressurizing valve have liquid flow pressurizing effects, and further the impedance of the liquid-gas buffer can be improved. A booster valve structure unique to the hydropneumatic buffer boosts the low air pressure from the air chamber in the second fluid chamber to the high pressure in the first fluid chamber. And, the piston head has a larger pressure acting area in the first chamber than in the second chamber, whereby the damper is able to output a higher static resistance value than the conventional type of hydro-pneumatic damper. The filling pressure of the buffer is reduced by the pressurization structure, and at any point in a compression stroke, the maximum pressure inside the buffer is lower than that of the traditional type, so that the use working conditions of a sealing piece of the buffer and a metal part can be improved, and the service life of the buffer is effectively prolonged.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.

FIG. 1 is a schematic diagram of a prior art hydropneumatic buffer;

FIG. 2 is a schematic structural diagram of a first embodiment of a hydropneumatic buffer of the present invention;

FIG. 3 is a schematic structural diagram of a second embodiment of the hydropneumatic buffer of the present invention;

FIG. 4 is a schematic view of a third embodiment of an end cap for a hydropneumatic buffer of the present invention;

FIG. 5 is a schematic view of a first piston assembly;

FIG. 6 is a schematic view of a first embodiment of a hydropneumatic buffer according to the present invention;

FIG. 7 is a schematic diagram of a recovery process of the first embodiment of the hydropneumatic buffer of the present invention;

FIG. 8 is a schematic view of a second embodiment of a hydropneumatic buffer according to the present invention;

FIG. 9 is a schematic diagram of a recovery process of a second embodiment of the hydropneumatic buffer of the present invention;

wherein, in the figures, the respective reference numerals:

1-cylinder, 2-plunger, 3-piston, 301-throttle damping fluid channel, 4-oil chamber, 5-air chamber, 6-cylinder, 601-first cylinder end, 602-second cylinder end, 603-tank liquid hole, 604-air filling hole, 605-end cover, 6051-end seat part, 6052-first extension, 6053-second extension, 701-piston head, 702-force transmission rod, 703-throttle channel, 801-first liquid chamber, 802-second liquid chamber, 901-first fluid channel, 9011-liquid inlet, 9012-branch channel, 902-second fluid channel, 10-booster valve, 11-check valve, 12-second piston assembly, 13-air chamber, 14-throttle rod.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

It will be understood that when an element is referred to as being "disposed on," "connected to" another element, it can be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or be indirectly connected to the other element.

It is to be understood that the terms "upper," "lower," "vertical," "top," "bottom," "inner," "outer," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the present invention and to simplify the description, and are not intended to indicate or imply that the referenced devices or elements must be in a particular orientation, constructed and operated in a particular orientation, and are not to be construed as limiting the present invention.

It should be noted that the terms "first", "second", "third" and "fourth" are used for descriptive purposes only and are not intended to imply relative importance.

The invention provides a hydraulic-pneumatic buffer which can be used for a vehicle and can buffer the collision energy of the vehicle.

Hydropneumatic shock absorber structure referring to fig. 2, including a cylinder, a first piston assembly and a second piston assembly.

The cylinder block 6 includes a first cylinder end 601 and a second cylinder end 602 in the axial direction.

A first piston assembly: referring to the structure of fig. 3, the piston head 701 and the force transmission rod 702 are included, the piston head 701 is arranged in the cylinder 6, is tightly matched with an inner cavity of the cylinder 6, and divides the cylinder 6 into a first liquid cavity 801 and a second liquid cavity 802, wherein the first liquid cavity 801 is arranged between the piston head 701 and the first cylinder end 601, and the second liquid cavity is arranged between the piston head 702 and the second cylinder end 802. The force transfer rod 702 extends out of the cylinder 6 through the second cylinder end 802; the piston head 701 is provided with a first fluid passage 901 and a second fluid passage 902 which communicate the first fluid chamber 801 and the second fluid chamber 802. Compared with the traditional structure of the liquid-gas buffer, the piston assembly adopts an external design, namely, the piston assembly is used as a liquid cavity separation assembly and a force transmission element, so that the structure of the liquid-gas buffer is more compact.

The first fluid channel 901 is provided with a first switch member configured to be opened when the pressure of the first liquid chamber 801 is greater than the pressure of the second liquid chamber 802; in this embodiment, the first switching member employs a pressure increasing valve 10;

the second fluid channel 902 is provided with a second switch member configured to be opened when the pressure of the second liquid chamber 802 is greater than the pressure of the first liquid chamber 801; in this embodiment, the second switch member is a check valve 11.

The first fluid passage 901 and the second fluid passage 902 may be configured as shown in fig. 3, on either side of the axial center of the piston head 701. Besides, the first fluid channel 901 and the second fluid channel may also adopt an integrated branch structure, which is similar to a "Y" shaped structure, and the two fluid channels share a main channel, one branch of the main channel is used as the first fluid channel 901 on which the pressure increasing valve 10 is disposed, and the other branch is used as the second fluid channel 902 on which the check valve 11 is disposed. However, the method shown in fig. 3 can reduce the influence between the two fluid channels, and ensure the stable flow of the fluid. The booster valve 4 is distinguished by a boosting effect, and can have various structural forms, can be a fixed-ratio booster structure which ensures that the ratio of the pressure intensity in the first liquid cavity 801 to the pressure intensity in the second liquid cavity 802 is certain when the buffer is pressurized, can be a quantitative booster structure which ensures that the pressure intensity in the first liquid cavity 801 is always higher than the pressure intensity in the second liquid cavity 802 by a certain value when the buffer is pressurized, and can also be a combination of the two structural forms.

Second piston assembly 12: disposed around the force transfer rod 702, and between it and the piston head 701 is a second fluid chamber 802, which forms an air chamber 13 with the cylinder second port 602.

The impedance of the buffer is a main performance index of the buffer, and is directly related to the quality of the buffer performance. As a variation of the embodiment shown in fig. 2, another implementation structure of the fluid channel is further provided, which has higher resistance than the above embodiment, and specifically refer to fig. 4.

The hydro-pneumatic buffer further comprises a throttle rod 14 extending into the cylinder body 6; the throttle lever 14 is specifically arranged on the first cylinder end 601, can be arranged at the position of the tank liquid hole 603, and is detachably connected with the cylinder 6. When the liquid is filled, the throttle lever 14 is detached, and after the liquid is filled, the throttle lever 14 is attached. In this configuration, the throttle passage 703 serves as a primary booster mechanism, and the booster valve 10 serves as a secondary booster mechanism, thereby improving the resistance of the shock absorber.

Referring to fig. 5, the first fluid passage 901 includes a throttle passage 9011 and a branch passage 9012 in cooperation with the throttle lever 14, the throttle passage 9011 communicates with the branch passage 9012 and passes through the first fluid chamber 801 and the second fluid chamber 802, and the throttle lever 14 is inserted into the throttle passage 9011 and is in clearance fit with the throttle passage 9011 when the first piston assembly is compressed. When the first piston assembly is compressed, the throttle rod 14 can be inserted into the throttle passage 9011 and is in clearance fit with the throttle passage 703, and hydraulic oil can flow in the clearance between the throttle rod 14 and the throttle passage 703.

In a more preferred embodiment, the throttle passage 9011 is provided axially along the piston head 701, and the branch passage 9012 is provided offset from the direction in which the throttle passage 9011 extends. Specifically referring to the structural schematic diagram of the piston head, a branch passage 9012 is arranged on the side wall of the throttling passage 9011 and extends to the side face of the piston head 701, and the first switch piece is arranged on the branch passage 9012.

Further, for ease of servicing, the second cylinder end 602 of the cylinder is designed as an open end, the cylinder further comprises an end cap 605 disposed at the open end, the force transfer rod 702 extends from through the end cap 605 out of the cylinder 6, and the end cap 605 radially closes the open end. Specifically, the end cover 605 is detachably mounted on the opening end, and is engaged with the step of the opening end, a through hole is provided on the end cover 605, and the force transmission rod 702 passes through the through hole to the outside of the cylinder 6.

With respect to the second piston assembly 12, its main function is to close off the air chamber 13 and compress the air chamber 13 when subjected to an impact force. Second piston assembly 12 is directly engaged with force transfer rod 702, but during compression, relative motion occurs between force transfer rod 702 and second piston assembly 12. Further, to address this issue, the end cap 605 is further configured. The end cap 605 includes an end seat 6051 and an extension extending axially from the end seat 6051 toward the piston head 701, the extension including a first extension 6052 disposed along the bore.

With regard to the fitting structure of the second piston assembly 12 and the short message 605, the following two embodiments are further provided.

The first implementation structure: the second piston assembly 12 is disposed between the first extension 6052 and the cylinder sidewall, closing the space between the first extension 6052, the cylinder sidewall, and the end seat 6051, forming the air chamber 13. The end cap 605 is provided with an aeration hole 604 for aerating the air chamber 13.

The second implementation structure: the extension further includes a second extension 6053 radially spaced from the first extension 6052 portion, the second extension 6053 disposed against the cylinder sidewall, and the second piston assembly 12 disposed between the first extension 6052 and the second extension 6053 to close the space between the first extension 6052, the second extension 6053, and the end seat 6051, forming the air cavity 13. This arrangement is more convenient to install than the first embodiment and facilitates the determination of the initial position of the second piston member. Specifically, the second piston assembly may be installed between the first extension 6052 and the second extension 6053 of the end cap 605 prior to the end cap 605 being installed on the cylinder. The mounting position of the second piston assembly on the end cap 605 is determined by the initial volume of the desired air chamber 13.

The structure of the end cap 605 may be omitted as above. A hole through which the force transmission rod 702 can pass is provided directly in the second cylinder end 602, which also performs the function of a hydraulic-pneumatic shock absorber.

The principle mechanism of the liquid-air damper will be explained below:

in the initial state, hydraulic oil is filled into the first liquid chamber 801, and inert gas is filled into the air chamber 5. Specifically, a tank liquid hole 603 may be formed in the first cylinder end 601, hydraulic oil may be filled through the tank liquid hole 603, and an air filling hole 604 may be formed in the second cylinder end 602, and an inert gas may be filled through the air filling hole 604. Hydraulic oil will flow into the second fluid chamber 802 through the first fluid passage 901 until the pressures of the two fluid chambers are equal.

a compression process

The compression process of the first hydropneumatic buffer implementation is illustrated in figure 6.

An external force is applied to the force-transmitting rod 702, compressing the piston head 701 in the direction of the first fluid chamber 801. The pressure in the first liquid cavity 801 is increased, the pressure increasing valve 11 is opened, hydraulic oil enters the second liquid cavity 802 through the first fluid channel 901, the pressure of the second liquid cavity 802 is increased, the second piston assembly 12 is compressed, the air cavity 13 is compressed, and buffering and energy absorption are achieved.

The second hydropneumatic buffer implementation is structurally restored as shown in figure 8.

When the buffer is compressed by external force, the force transmission rod 702 retracts into the cylinder body 6, and the hydraulic oil in the compressed first liquid cavity 801 sequentially flows into the second liquid cavity 802 through the throttling channel 703 and the booster valve 10 to push the second piston assembly 12 to move right and compress the inert gas in the gas cavity 13. During compression, the pressure in the second oil chamber 802 is transferred from the gas pressure in the gas chamber 13. In the pressing-down process, the pressure increasing valve 4 and the throttle lever damping hole successively increase the oil pressure of the oil way step by step, so that the pressure in the first liquid cavity 801 is higher than that in the second liquid cavity 802. For the piston head 701, the acting area of the pressure in the first liquid cavity 801 is the inner hole section of the cylinder body 1, and the acting area is larger than the sectional area of the exposed end of the piston head 701 relative to the second liquid cavity 802. The pressure in the first chamber 801 is greater than that in the second chamber 802, and the pressure acting area of the piston head 701 in the two chambers is greater than that in the latter, so that the resistance value output by the buffer is higher than that of the traditional type. The liquid-gas buffer has a two-stage pressurization structure design of the pressurization valve 4 and the throttling channel 703, when the buffer is compressed at a low speed, the pressurization function of the pressurization valve 4 (a one-stage pressurization structure) is greater than that of the throttling channel 703 (a two-stage pressurization structure), and the buffer outputs a high static resistance value and has excellent characteristics of high static capacity and high energy absorption rate. Under the high-speed compression working condition, the two-stage pressurization structure exerts the speed-related characteristic, the pressurization capacity of the two-stage pressurization structure is obviously higher than that of the one-stage pressurization structure, so that the pressure intensity of the first liquid cavity 801 is greatly improved, the buffer outputs a higher resistance value, and the buffer is endowed with excellent performances of buffering impact and dissipating impact kinetic energy.

b recovery procedure

The recovery process of the first hydropneumatic buffer implementation is illustrated in figure 7. The second hydropneumatic buffer implementation is structurally restored as shown in figure 9. The principles of both embodiments are the same and will be described below.

When the buffer is pressed to the stroke and the buffer is moved, the compression potential energy stored by the inert gas in the air cavity 13 needs to be released. The piston head 701 is forced to move back in the axial direction due to the difference of left and right force areas in the oil chamber pressure environment, the one-way valve 11 is opened, the first fluid chamber 801 and the second fluid chamber 802 are in undamped communication, and the first piston assembly is pushed back to the original position. The existence of the booster valve 10 reduces the gas filling pressure of the novel liquid-gas buffer, namely, the output of the liquid-gas buffer is slightly lower than the resistance value of the liquid-gas buffer with the traditional structure in the whole return process.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims

1. A hydropneumatic buffer, comprising:

a cylinder body;

a second piston assembly: the second liquid cavity is arranged around the force transmission rod and is arranged between the piston head and the cylinder body, and an air cavity is formed between the piston head and the cylinder body port;

the throttle rod extends into the cylinder body;

the first fluid passage comprises a throttling passage and a branch passage, the throttling passage is communicated with the branch passage and communicated with the first liquid cavity and the second liquid cavity, and when the first piston assembly is compressed, the throttling rod can be inserted into the throttling passage and is in clearance fit with the throttling passage.

2. The hydropneumatic buffer of claim 1 wherein: the first fluid passage and the second fluid passage are respectively positioned on two sides of the axial center of the piston head.

3. The hydropneumatic buffer of claim 1 wherein: the cylinder body axially comprises a first cylinder body end and a second cylinder body end, the first cylinder body end is provided with a tank liquid hole, and the force transmission rod penetrates out of the second cylinder body end.

4. A hydropneumatic buffer as claimed in claim 1, 2 or 3 wherein: the first switching member is disposed on the bypass passage.

5. The hydropneumatic buffer of claim 4 wherein: the throttling channel is arranged along the axial direction of the piston head, and the branch channel is arranged in a mode of deviating from the extending direction of the throttling channel.

6. The hydropneumatic buffer of claim 4 wherein: the throttle rod is arranged at the liquid hole of the tank and is detachably connected with the cylinder body.

7. The hydropneumatic buffer of claim 3 wherein: the second cylinder body end is an open end, the cylinder body further comprises an end cover arranged at the open end, the end cover radially closes the open end, a through hole is formed in the end cover, and the force transmission rod penetrates out of the cylinder body through the through hole.

8. The hydropneumatic buffer of claim 7 wherein: the end cap comprises an end seat part and an extension part, wherein the extension part extends from the end seat part to the piston head direction along the axial direction, and the extension part comprises a first extension part arranged along the through hole.

9. The hydropneumatic buffer of claim 8 wherein: the second piston assembly is arranged between the first extension part and the side wall of the cylinder body, and closes the space among the first extension part, the side wall of the cylinder body and the end seat part to form an air cavity.

10. The hydropneumatic buffer of claim 8 wherein: the extension further comprises a second extension arranged at a radial interval with the first extension, and the second piston assembly is arranged between the first extension and the second extension to close the space among the first extension, the second extension and the end seat part to form an air cavity.