CN211009974U

CN211009974U - Gear rack transmission assembly and gas self-closing valve

Info

Publication number: CN211009974U
Application number: CN201922163379.9U
Authority: CN
Inventors: 何原涛; 杜仁辉; 秦浩然
Original assignee: Chengdu Tnda Gas Equipment Co ltd
Current assignee: Chengdu Tnda Gas Equipment Co ltd
Priority date: 2019-12-06
Filing date: 2019-12-06
Publication date: 2020-07-14
Anticipated expiration: 2029-12-06

Abstract

The utility model discloses a rack and pinion transmission assembly and gas self-closing valve, including the valve body, be provided with self-closing valve body and ball valve assembly on the valve body, the self-closing valve body includes rack and pinion transmission assembly and sets up the current-limiting component at the inlet end, and rack and pinion transmission assembly includes rack and pinion drive mechanism, spring unit, rack and pinion drive mechanism includes intermeshing's gear and rack, the gear includes wheel tooth portion and contact site, the contact site both sides are provided with stop part, spring unit includes first spring and second spring, the stiffness coefficient of first spring and second spring is different, stop part can with first spring, second spring looks butt. The utility model provides a gas self-closing valve make the degree of difficulty high, the control accuracy is poor, life short technical problem, guarantee the security of gas pipeline when superpressure, under-voltage, damaged gas leakage.

Description

Gear rack transmission assembly and gas self-closing valve

Technical Field

The utility model relates to the technical field of valves, especially, relate to a can prevent because of gas superpressure, under-voltage, damaged gas self-closing valve who leaks automatic shutoff.

Background

The gas self-closing valve is a device which is installed on a low-pressure gas system pipeline, can be automatically closed when the gas supply pressure of the pipeline is under-pressure or over-flow, does not need external force when closed, needs to be manually opened, and is suitable for pipeline natural gas, liquefied petroleum gas and artificial gas. The basic principle of the pipeline gas self-closing valve in the prior art is to magnetize permanent magnetic materials according to design requirements to manufacture a permanent-memory multi-pole permanent magnetic linkage mechanism, identify the change of gas pressure parameters passing through the mechanism, automatically close the valve when the gas pressure parameters exceed a safety set value, and cut off a gas source. The permanent magnet is used as an automatically controlled self-closing valve, and the permanent magnet has constant attraction force in actual use, so that the normal use range under working conditions is limited. Meanwhile, in the process of manufacturing the permanent magnet, the magnetic force is difficult to accurately control, the manufacturing difficulty is high, the magnet is brittle, the service life of the magnet is short, and the magnet can adsorb impurities in the using process, so that the air tightness is influenced, the control precision is reduced, and the use safety is influenced.

SUMMERY OF THE UTILITY MODEL

An object of the utility model is to provide a rack and pinion transmission assembly and gas self-closing valve aims at solving prior art, and the gas self-closing valve makes the technical problem that the degree of difficulty is high, control accuracy is poor, life is short, guarantees the security of gas pipeline when superpressure, under-voltage, damaged gas leakage.

The utility model adopts the technical proposal that:

the utility model provides a rack and pinion transmission assembly, includes rack and pinion drive mechanism, spring unit, rack and pinion drive mechanism includes intermeshing's gear and rack, the gear includes wheel tooth portion and contact site, the contact site both sides are provided with stop part, spring unit includes first spring and second spring, the coefficient of stiffness of first spring and second spring is different, stop part can with first spring, second spring looks butt. Under normal operating condition, the low pressure spring is the spacing part butt on with the gear all the time, and the elasticity that the low pressure spring produced is less, can provide the holding power for the gear, and at this moment, rack and pinion transmission assembly is in balanced state, and when pressure reduced, the rack has decurrent motion trend, and at this moment, gear anticlockwise rotation under low pressure spring's effect. When the pressure is too high, the rack has an upward movement trend, the limiting component presses the low-pressure spring, the gear rotates clockwise, the limiting rod on the other side of the gear starts to be in contact with the high-pressure spring, if the pressure continues to increase, the rack drives the gear to rotate so that the gear overcomes the sum of the elastic forces of the high-pressure spring and the low-pressure spring, and the gear continues to rotate clockwise.

Further, the rack has a rack portion, a tie rod connected to the rack portion, and a connecting portion connected to the tie rod.

Further, the outer surface of the contact portion is an arc-shaped surface, and therefore, smoothness of contact is ensured.

Further, a center of the gear tooth portion is not coincident with that of the gear. Therefore, a lever effect taking the center of the gear tooth part as a fulcrum is formed on the gear, and the gear tooth part can be effectively rotated around the center of the gear tooth part when the push rod acts on the contact part.

Further, a push rod is further provided, and one end of the push rod is abutted to the contact part.

Further, one end of the push rod is provided with a first convex part, and the other end of the push rod is provided with a second convex part.

Further, the size of the first convex part is smaller than the outer diameter of the push rod.

Furthermore, the outer surfaces of the first convex part and the second convex part are spherical surfaces, so that the smoothness of contact is further ensured.

Furthermore, the gear and the push rod are made of high-wear-resistant materials, so that the gear and the push rod are further guaranteed to have longer service life, and the stability and the use safety of the gas self-closing valve are improved.

The gas self-closing valve comprises a valve body, wherein a self-closing valve body and a ball valve assembly are arranged on the valve body, the self-closing valve body is internally provided with the gear rack transmission assembly, and the air inlet end of the valve body is provided with a flow limiting component.

Furthermore, the device also comprises a manual control mechanism, wherein the manual control mechanism comprises a pull rod connected with the gear rack transmission mechanism, a button connected with the pull rod, and a pull ring matched with the button.

The utility model discloses technical effect for prior art is: the gas self-closing valve automatic control mechanism when having the superpressure, under-pressure and the overflow volume, solved various gas leakage problems effectively and when stopping gas, can the self-closing air supply, prevent to resume the problem that gas leakage gas causes the calamity accident because of not closing the gas valve when air feed, simultaneously, the utility model discloses a drive mechanism uses rack and pinion and first spring and second spring to realize closing under the self-closing valve high-low pressure state, the long service life of spring, it is easier to make, has improved control accuracy.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments of the present invention or the description of the prior art will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

FIG. 1 is a schematic view of the overall structure of the gas self-closing valve of the present invention

Fig. 2 is a schematic view of the gear structure of the present invention;

FIG. 3 is a schematic structural view of the spring assembly of the present invention;

fig. 4 is a schematic view of the rack structure of the present invention;

FIG. 5 is a schematic view of the push rod of the present invention;

fig. 6 is a schematic structural view of the limiting bracket of the present invention;

fig. 7 is a schematic structural view of the spring support of the present invention;

fig. 8 is a schematic structural view of the sliding member of the present invention;

fig. 9 is a schematic structural diagram of the housing of the present invention;

FIG. 10 is a schematic view of the housing and valve body assembly of the present invention;

fig. 11 is a schematic structural view of a second housing of the present invention;

FIG. 12 is a schematic view of the normally closed state of the self-closing valve of the present invention;

FIG. 13 is a schematic view of the low pressure critical closed state of the self-closing valve of 0.8kpa of the present invention;

fig. 14 is a schematic view of the 8kpa high pressure critical closed state of the self-closing valve of the present invention;

fig. 15 is a schematic view of the closing state of the self-closing valve with overpressure above 8 kpa.

Description of reference numerals:

Detailed Description

Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are exemplary and intended to be used for explaining the present invention, and should not be construed as limiting the present invention. The specific meaning of the above terms in the present invention can be understood according to specific situations by those skilled in the art.

In order to make the objects, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

The utility model discloses at the concrete implementation as follows:

as shown in FIG. 1, the embodiment of the utility model provides a gas self-closing valve, including valve body 1, be provided with self-closing valve body 4 and ball valve assembly 12 on the valve body 1, self-closing valve body 4 includes rack and pinion transmission assembly and sets up the current-limiting component 2 at the inlet end, rack and pinion transmission assembly includes rack and pinion drive mechanism and rack and pinion drive mechanism cooperation's spring unit.

Preferably, the rack and pinion mechanism is disposed in a housing assembly, the housing assembly includes a first housing 20 and a second housing 21, and the housing assembly is fixedly disposed in the cavity 33 of the valve body 1.

The ball valve assembly 12 comprises a valve core 18 arranged on the outlet side of the valve body, a channel is arranged in the valve core 18, the valve core is supported by a support seat 19, a valve rod 16 is connected to the upper part of the valve core 18, a cover body 15 is arranged on the upper part of the valve rod 16, and a knob 13 is further arranged, and the knob 13 is connected with the valve rod 16 through a screw 14. An air outlet joint 17 is also connected to the outlet side of the ball valve assembly 12; the manual opening or closing of the gas passage is achieved by rotating the knob 13 of the ball valve assembly 12.

As shown in fig. 1, 2 and 3, the gear rack transmission mechanism comprises a push rod 24, a gear 23 and a rack 22; the rack 22 includes a rack portion 22-1 engaged with the pinion 23, a tie rod 22-2 connected to the rack portion 22-1, and a connecting portion 22-3 connected to the tie rod 22-2; one end of the push rod 24 is provided with a first convex part 24-1, the other end is provided with a second convex part, preferably, the size of the first convex part 24-1 is smaller than the outer diameter of the push rod 24, and for smooth contact, the outer surfaces of the first convex part 24-1 and the second convex part are preferably spherical surfaces;

as shown in fig. 2, the gear 23 includes a gear tooth portion 23-1 and a contact portion 23-3, the gear tooth portion 23-1 has partial meshing teeth thereon, the contact portion 23-3 is preferably a circular portion or a sector portion having a circular arc edge, and an arc center of the contact portion 23-3 is spaced from a center of the gear tooth portion 23-1, that is, the center of the contact portion 23-3 is not coincident with the center of the gear, so that a lever effect with the center of the gear tooth portion 23-1 as a fulcrum is formed on the gear 23, and it is ensured that the pushing rod 24 can effectively rotate the gear tooth portion 23-3 around the center thereof when acting on the contact portion 23-3. A through hole for mounting the gear 23 is formed in the center of the gear tooth part 23-1, and preferably, the gear is mounted on a pin shaft in the shell assembly through the through hole; the first convex portion 24-1 of the push lever 24 abuts against the contact portion 23-3 of the gear 23, and in order to ensure smooth contact, the outer surface of the contact portion 23-3 is preferably an arc-shaped surface, so that the first convex portion 24-1 and the contact portion 23-3 of the gear 23 form a point contact, and the gear portion 23-1 of the gear 23 meshes with the rack 22. In a working state, the push rod 24 is always in frictional contact with the moving gear 23, and preferably, the gear 23 and the push rod 24 are made of high-wear-resistant materials, so that the gear 23 and the push rod 24 have longer service life, and the stability and the use safety of the gas self-closing valve are improved.

As shown in fig. 2 and 3, the spring assembly includes a first spring 30 and a second spring 31, the stiffness coefficients of the first spring (30) and the second spring (31) are different, two sides of the contact portion 23-3 of the gear 23 are respectively provided with a limiting component 23-2, the limiting component 23-2 is used for abutting against the first spring 30 and the second spring 31 in the working state, one end of the first spring 30 and one end of the second spring 31 are installed in the housing assembly, and the other end of the first spring 30 and the other end of the second spring 31 abut against the limiting components 23-2 in the two sides in the working state. Specifically, the position limiting component 23-2 is preferably a position limiting rod.

Preferably, the stiffness coefficient of the second spring 31 is larger than that of the first spring 30, or the stiffness coefficient of the first spring 31 is larger than that of the second spring 30, wherein a spring with a large stiffness coefficient is used as a high pressure spring acting in a high pressure state, and a spring with a low stiffness coefficient is used as a low pressure spring acting in a low pressure state. Specifically, in a normal working state, the low-pressure spring is always abutted to the limiting rod on the gear 23, the elastic force generated by the low-pressure spring is small, and can provide supporting force for the gear 23, at the moment, the rack-and-pinion transmission assembly is in a balanced state, when the pressure is reduced, the rack 22 has a downward movement trend, at the moment, the gear 23 rotates anticlockwise under the action of the low-pressure spring, if the pressure is reduced continuously, the gear 23 rotates anticlockwise continuously, and the push rod 24 moves leftwards beyond a balanced point of the contact part 23-3 until the valve is closed; when the pressure is too high, the rack 22 has an upward movement tendency, the limiting rod presses the low-pressure spring, the gear 23 rotates clockwise, the limiting rod on the other side of the gear 23 starts to be in contact with the high-pressure spring, if the pressure continues to increase, the rack 22 drives the gear 23 to rotate so that the gear 23 overcomes the sum of the elastic forces of the high-pressure spring and the low-pressure spring, the gear 23 continues to rotate clockwise, and at the moment, the push rod 24 moves leftwards beyond the balance point of the contact part 23-3 until the valve is closed. The effective closing of the self-closing valve under a high-pressure or low-pressure state can be realized through the matching action of the gear rack transmission mechanism, the first spring and the second spring, the service life of the springs is long, the gear rack structure is reliable in transmission and easier to manufacture, and the control precision of the self-closing valve can be further improved.

As shown in fig. 1, the rack and pinion transmission mechanism is arranged in a cavity 33 of the self-closing valve body 4, a valve cover 11 is arranged on an opening of the cavity 33 of the self-closing valve body 4, the manual control mechanism is arranged on the valve cover 11, the manual control mechanism comprises a pull rod 10 connected with the rack and pinion transmission mechanism, a button 9 connected with the pull rod 10, and a pull ring 8 matched with the button 9, and the manual control mechanism is used for manually opening or closing the valve.

An annular groove 32 is formed between the valve cover 11 and an opening of a cavity 33 of the self-closing valve body 4, the self-closing valve body 4 further comprises a diaphragm 5, the whole diaphragm 5 is concave, and the edge of the diaphragm 5 is arranged in the annular groove 32; preferably, be provided with the joint portion that has the through-hole on the diaphragm 5, have the joint groove in the joint portion, the joint inslot is provided with the through-hole, the lower extreme of pull rod 10 is provided with the joint arch, the protruding card of joint of pull rod 10 is gone into the joint groove of diaphragm 5. Preferably, the membrane 5 is made of an elastic material. The diaphragm 5 moves upwards or downwards along with the change of the gas pressure, so as to drive the rack 22 to move up and down, namely, the pressure change is transmitted to the gear rack transmission mechanism through the change of the diaphragm 5, and the valve port is closed and opened.

As shown in fig. 4, the rack 22 includes a rack portion 22-1 engaged with a pinion 23, a tie rod 22-2 connected to the rack portion 22-1, and a connecting portion 22-3 connected to the tie rod 22-2; the rack 22 is connected with the pull rod 10 through the fixed connecting piece 7, a sealing gasket 6 is arranged between the rack 22 and the pull rod 10, the lower end of the fixed connecting piece 7 is provided with a clamping protrusion, the clamping protrusion of the fixed connecting piece 7 is clamped into the connecting portion 22-3, the other end of the fixed connecting piece 7 penetrates through the diaphragm 5 and then is fixedly connected with the pull rod 10, preferably, the fixed connecting piece 7 is a screw, and the connecting portion 22-3 is a connecting groove.

As shown in fig. 1, 6, 7 and 8, the flow limiting assembly 2 is arranged in a channel at the air inlet end of the valve body, the flow limiting assembly 2 is used for limiting the flow of gas to work at a rated flow, and comprises a sliding piece 25, the channel of the valve body 1 between the rack and pinion transmission mechanism and the flow limiting assembly 2 in the valve body 1 is provided with a sliding piece channel 1-2, the sliding piece 25 is arranged in the sliding piece channel 1-2, the sliding piece 25 comprises a wing plate 25-1 and an end plate 25-2, the end of the wing plate 25-1 is provided with a stepped groove, the groove of the sliding piece 25 is abutted against the push rod 24, the size of the groove is slightly larger than the outer diameter of the push rod 24, one end of the sliding piece 25 close to the end plate 25-2 is provided with an annular groove, and a; the flow limiting assembly 2 further comprises a limiting support 29, and a flow limiting shaft plate 28 and a spring support 27 which are connected with the limiting support 29, wherein two ends of the flow limiting shaft plate 28 are respectively arranged in central through holes of the spring support 27 and the limiting support 29 in a sliding mode, a plurality of limiting protrusions 27-1 and a plurality of support channels 27-2 are arranged on the spring support 27, the support channels 27-2 are used for allowing gas to pass through, the limiting protrusions 27-1 of the spring support 27 are used for being connected with one end of a return spring 3, and the other end of the return spring 3 is connected with the small diaphragm 26.

One end of the limiting support 29 is provided with a plurality of radial plates 29-1, a plurality of wedge bodies 29-2 arranged at intervals along the circumferential direction and a plurality of limiting support channels 29-3, and the wedge bodies 29-2 are arranged in clamping grooves 1-1 in the inner wall of the air inlet end of the valve body 1. The spacing bracket channel 29-3 is used for passing gas.

The working principle of the current limiting assembly is as follows: the sliding piece 25 and the small diaphragm 26 at the front end of the flow limiting component 2 are under the reciprocating action of the push rod 24 and the action of the return spring 3, so that the opening and closing of the inlet valve port under different pressures are realized. In addition, the flow limiting assembly 2 can realize the flow limiting closing after the flow passing through the valve body exceeds a preset value, as shown in fig. 1, in the rated flow, gas flows into the cavity 33 of the valve body 1 through the passages on the limiting bracket 29 and the spring bracket 27, and the self-closing valve works normally; when the rated flow is exceeded, the airflow acts on the shaft plate of the flow-limiting shaft plate 28 and pushes the flow-limiting shaft plate 28 to slide along the central through holes of the spring bracket 27 and the limiting bracket 29, and under the shielding effect of the flow-limiting shaft plate 28, the air passing through the bracket channel 27-2 on the spring bracket 27 is reduced, so that the flow limitation in the over-flow state is realized.

As shown in fig. 9, 10 and 11, guide blocks 22-4 are disposed on both sides of a rack portion 22-1 of a rack 22, the rack-and-pinion transmission mechanism is disposed in a cavity 33 of a self-closing valve body 4 through a housing assembly, the housing 20 and the housing 21 are clamped with each other, a plurality of fastening grooves 20-1 and positioning posts 20-3 are disposed on an edge of the housing 20, a pin shaft for mounting a pinion 23 is further disposed in the housing, a stop pin (not shown) for mounting the second spring 31 and a guide groove 20-6 slidably engaged with the guide block 22-4 of the rack 22 are disposed inside the housing 20, a positioning fastening groove (not shown) connected with the valve body 1 is disposed outside the housing 20, a plurality of fasteners 21-2 engaged with the fastening grooves 20-1 and a positioning hole 21-4 engaged with the positioning posts 20-3 are disposed on an edge of the housing 21, the inner part of the second shell 21 is provided with a stop pin 21-1 for mounting the first spring 30 and a guide groove in sliding fit with a guide block 22-4 of the rack 22, the outer part of the second shell 21 is provided with a positioning clamping groove 21-3 connected with the valve body 1, the bottoms of the first shell 20 and the second shell 21 are both provided with a positioning bulge 20-2, and the positioning bulge 20-2 is matched with a positioning groove 1-3 at the bottom of a cavity 33 of the valve body 1 and used for being matched with the positioning clamping groove 21-3 together to fix the gear and rack transmission assembly in the valve body 1.

The specific operation method of the gas self-closing valve of the utility model is as follows: when the normal air pressure is 2kpa, the button is manually pulled up and drives the push rod to open the sliding part of the flow limiting assembly through the gear rack transmission mechanism, so that the small diaphragm does not block the passage of the sliding part, and the valve is opened; when the gas pressure is 0.8kpa, the valve is automatically closed when the pressure is reduced; when the pressure rises to 8kpa, the valve is automatically pulled up and closed, and the valve can be opened only by manually pressing a button after the valve is closed.

The working principle of the gas self-closing valve is as follows: the upper part of a rack of the gear transmission mechanism is connected with the diaphragm, the diaphragm can move up and down under the action of gas pressure, so that the rack is driven to move up and down, the rack drives the gear to rotate, the balance contact state between the gear and the push rod is changed when the gear rotates, the push rod in contact with the gear moves forward, the sliding part moves, and finally the small diaphragm on the sliding part closes the sliding part channel, so that the valve is closed.

Specifically, when the gas pressure is abnormally reduced to be below 0.8kpa, the low-pressure spring is always in a contact state with the limiting part of the gear, at the moment, the limiting part of the gear is not in contact with the high-pressure spring, the low-pressure spring pushes the gear to rotate anticlockwise, the supporting force of gas on the diaphragm is lower than the torsion of the low-pressure spring on the gear, the diaphragm moves downwards, the reset spring pushes the push rod to push the gear to move anticlockwise after the critical point is missed, at the moment, the rack is lowered to the bottom, the small diaphragm moves leftwards, the sliding piece channel is blocked, and the valve is completely closed.

When the gas pressure rises to more than 8kpa from normal, the supporting force of the gas on the diaphragm exceeds the torsion of the low-pressure torsion spring and the high-pressure torsion spring, so that the diaphragm moves upwards, the return spring drives the push rod to drive the gear to rotate clockwise to move the diaphragm upwards after the critical point is missed, the gear rotates clockwise, the limiting part of the gear continuously presses the low-pressure spring and contacts with the high-pressure spring to continue rotating, at the moment, the small diaphragm moves leftwards to block the sliding part channel, and the valve is completely closed.

When the diaphragm moves upwards, the valve is automatically closed, and the valve can be opened only by manually pressing down the handle after the valve is closed.

The utility model discloses a gas self-closing valve can realize the under-pressure and close when 0.8 0.2kpa, realizes the superpressure and closes when 8 ± 2kpa, and various operating condition are specifically as follows:

the state shown in fig. 1 is the working state of the self-closing valve by normal air pressure: when the air pressure is 2kpa, the valve is manually pulled up and kept in an open state; when the pressure is 0.8kpa, the pressure reduction valve automatically falls and is closed; when the pressure is 8kpa, the pressure rises, and the valve automatically pulls up and closes.

Fig. 13 shows the low-pressure critical closing state of the self-closing valve at 0.8 kpa: when the gas pressure is abnormally reduced to 0.8kpa, the supporting force of the gas on the diaphragm and the torsion of the low-pressure torsion spring on the gear are balanced, the pressure is reduced again, the torsion is larger than the supporting force, the diaphragm moves downwards, the reset spring pushes the push rod to push the gear to move upwards anticlockwise after the critical point is missed, and at the moment, the rack descends to the bottom, and the valve is completely closed.

Fig. 14 shows the high-pressure critical closing state of the self-closing valve 8 kpa: when the gas pressure rises to 8kpa from normal, the supporting force of the gas on the diaphragm is balanced with the low-pressure torsion spring and the high-pressure torsion spring, if the gas rises again, the supporting force exceeds the torsion, the diaphragm moves upwards, and after the gas misses a critical point, the return spring drives the push rod to drive the gear to rotate clockwise, the small diaphragm moves left, and the valve is closed.

Fig. 15 shows the overpressure closing state above the self-closing valve 8 kpa: after the air pressure exceeds 8kpa, the diaphragm continuously moves upwards to close the valve automatically, and the valve can be opened only by manually pressing a button after the valve is closed.

The above description is only exemplary of the present invention and should not be taken as limiting the scope of the present invention, as any modifications, equivalents, improvements and the like made within the spirit and principles of the present invention are intended to be included within the scope of the present invention.

Claims

1. A gear rack transmission assembly comprises a gear rack transmission mechanism and a spring assembly, wherein the gear rack transmission mechanism comprises a gear (23) and a rack (22) which are meshed with each other, the gear (23) comprises a gear tooth part (23-1) and a contact part (23-3), the gear rack transmission assembly is characterized in that limiting parts (23-2) are arranged on two sides of the contact part (23-3), the spring assembly comprises a first spring (30) and a second spring (31), stiffness coefficients of the first spring (30) and the second spring (31) are different, and the limiting parts (23-2) can be abutted to the first spring (30) and the second spring (31).

2. A rack and pinion transmission assembly according to claim 1, characterised in that the rack (22) has a rack portion (22-1), a tie rod (22-2) connected to the rack portion (22-1), and a connecting portion (22-3) connected to the tie rod (22-2).

3. A rack and pinion drive assembly according to claim 2, wherein the outer surface of the contact portion (23-3) is arcuate and the centre of the toothed portion (23-1) is not coincident with the centre of the gear wheel (23).

4. A rack and pinion drive assembly according to claim 1, characterised by a push rod (24), one end of the push rod (24) abutting the contact portion (23-3).

5. A rack and pinion transmission assembly as claimed in claim 4, characterised in that the push rod (24) is provided with a first projection (24-1) at one end and a second projection at the other end.

6. A rack and pinion drive assembly as claimed in claim 5, characterised in that the first projection (24-1) is of a size smaller than the outer diameter of the push rod (24).

7. A rack and pinion drive assembly as claimed in claim 5, characterised in that the outer surfaces of the first and second projections (24-1, 24) are spherical surfaces.

8. A rack and pinion drive assembly as claimed in claim 4, characterised in that the gear wheel (23) and the push rod (24) are made of a highly wear resistant material.

9. A gas self-closing valve, comprising a valve body (1), wherein a self-closing valve body (4) and a ball valve assembly (12) are arranged on the valve body (1), a gear rack transmission assembly as claimed in one of claims 1-8 is arranged in the self-closing valve body (4), and a flow limiting component (2) is arranged at the air inlet end of the valve body (1).

10. A gas self-closing valve according to claim 9, further comprising a manual control mechanism, wherein the manual control mechanism comprises a pull rod (10) connected with the rack and pinion transmission mechanism, a push button (9) connected with the pull rod (10), and a pull ring (8) matched with the push button (9).