CN221269872U

CN221269872U - Sawing machine operating mechanism

Info

Publication number: CN221269872U
Application number: CN202323325418.3U
Authority: CN
Inventors: 俞大凤; 邱存兴
Original assignee: Sanming Yongxingsheng Construction Machinery Co ltd
Current assignee: Sanming Yongxingsheng Construction Machinery Co ltd
Priority date: 2023-12-07
Filing date: 2023-12-07
Publication date: 2024-07-05
Anticipated expiration: 2033-12-07

Abstract

The utility model relates to a sawing machine operating mechanism, which comprises a bottom plate, wherein a supporting plate is fixedly connected to the top wall of the bottom plate; the top wall of the supporting plate is fixedly connected with a first motor; the side wall of the supporting plate is provided with a moving plate, and the moving plate is connected with a first motor through a lifting assembly; the side wall of the movable plate is rotatably connected with a rotating rod; the end part of the rotating rod is fixedly connected with a band saw body; the side wall of the movable plate is fixedly connected with a mounting plate; the side wall of the mounting plate is fixedly connected with a second motor, and the second motor is connected with the rotating rod through an adjusting component; the utility model provides a sawing machine operating mechanism, which solves the problems that in the prior art, when the existing sawing machine is used, the included angle between the saw blade of the sawing machine and the surface of a workpiece with cutting is inconvenient to adjust, so that the sawing machine has certain use limitation.

Description

Sawing machine operating mechanism

Technical Field

The utility model belongs to the technical field of pipe pile sheet processing, and particularly relates to an operating mechanism of a sawing machine.

Background

In the production of tubular pile pieces, the steel material produced in an electric arc furnace is subjected to a series of processing steps, first wound into the shape of a string of tubular pile pieces by a winder, after winding and shaping, the tubular pile pieces need to be cut into individual, unsealed annular tubular pile pieces for subsequent processing and handling, and in order to achieve this step, a sawing machine is used for cutting.

In the prior art, the cutting angle of the saw blade refers to the included angle formed by the tooth surface of the saw blade and the processing surface of the workpiece, which is an important parameter affecting sawing capability and workpiece surface quality, and the included angle between the saw blade of the sawing machine and the workpiece surface with cutting is inconvenient to adjust when the existing sawing machine is used, so that the saw machine has certain use limitation.

Disclosure of utility model

In order to make up the deficiency of the prior art, solve the prior art, the saw blade of the existing sawing machine is inconvenient to regulate the included angle with the surface of the work piece cut by the belt when using, therefore have the problem of certain use limitation, the utility model provides a sawing machine operating mechanism.

The technical scheme adopted for solving the technical problems is as follows: the sawing machine operating mechanism comprises a bottom plate, wherein a supporting plate is fixedly connected to the top wall of the bottom plate; the top wall of the supporting plate is fixedly connected with a first motor; the side wall of the supporting plate is provided with a moving plate, and the moving plate is connected with a first motor through a lifting assembly; the side wall of the movable plate is rotatably connected with a rotating rod; the end part of the rotating rod is fixedly connected with a band saw body; the side wall of the movable plate is fixedly connected with a mounting plate; the mounting panel lateral wall rigid coupling has the second motor, and the second motor passes through adjusting part and is connected with the bull stick.

Preferably, the lifting assembly comprises a first chute; a first chute is formed in the side wall of the supporting plate; a first screw rod is rotatably connected between the upper side wall and the lower side wall of the first chute, and the output end of the first motor extends into the first chute and is fixedly connected with the top end of the first screw rod; the screw nut pair on the outer circular wall of the first screw rod is connected with a first sliding block, and the end part of the first sliding block is fixedly connected with the side wall of the moving plate.

Preferably, the adjustment assembly comprises a worm gear; the outer circular wall of the rotating rod is fixedly connected with a worm wheel; the output end of the second motor is connected with a worm, and the end part of the worm penetrates through the mounting plate to be meshed with the worm wheel.

Preferably, the top wall of the bottom plate is fixedly connected with symmetrically distributed U-shaped plates; both sides of the side wall of the pair of movable plates penetrate through and are connected with guide rods in a sliding manner; a clamping plate is fixedly connected to the end part of one group of guide rods; fixing plates are fixedly connected to two side walls of the bottom plate; a pair of adjacent side walls of the fixed plate are fixedly connected with hydraulic cylinders; the output ends of the pair of hydraulic cylinders are fixedly connected with connecting plates, and the side walls of the connecting plates are fixedly connected with the end parts of the guide rods.

Preferably, the bottom wall of the bottom plate is fixedly connected with a third motor; the top wall of the bottom plate is provided with a placing plate, and the placing plate is connected with a third motor through a transmission assembly; a pair of fixed blocks are fixedly connected to two sides of the top wall of the placing plate; and a carrier roller is rotationally connected between the two pairs of side walls of the fixed block.

Preferably, the transmission assembly comprises a second chute; the bottom plate is provided with a second chute in a jacking mode; a second screw rod is rotatably connected between the two side walls of the second chute; the screw rod nut pair of the outer circular wall of the second screw rod is connected with a second sliding block, and the top end of the second sliding block is fixedly connected with the bottom wall of the placing plate; the side wall of the bottom plate is rotationally connected with a rotating shaft, and one end of the rotating shaft extends into the second sliding groove and is fixedly connected with the end part of the second screw rod; the other end of the rotating shaft is fixedly connected with a driven sprocket; and the output end of the third motor is fixedly connected with a driving sprocket, and the driving sprocket is connected with a driven sprocket through a chain belt.

Preferably, both sides of the top wall of the bottom plate are provided with guide grooves; mounting grooves are formed in two sides of the bottom wall of the placement plate, and the mounting grooves are located right above the guide grooves; a pair of guide wheels are rotatably connected between two side walls of the mounting groove, and horizontally move in the guide groove.

Preferably, the inner diameter of the driving sprocket is smaller than the inner diameter of the driven sprocket.

The beneficial effects of the utility model are as follows:

1. According to the sawing machine operating mechanism, the second motor is started, the rotating rod is driven by the second motor through the adjusting component to rotate, and the band saw body at the end part of the second motor is driven by the rotating rod to rotate, so that the inclined angle of the saw blade can be adjusted.

2. According to the sawing machine operating mechanism, a workpiece to be cut is placed below a band saw body, two connecting plates are driven to move oppositely through two hydraulic cylinders, and the connecting plates drive clamping plates to move oppositely through guide rods, so that two ends of the workpiece to be cut are clamped and fixed; the two pairs of clamping plates are independently arranged, and a gap is reserved in the middle of the clamping plates, so that the situation that the saw blade cannot be reset after cutting is completed is avoided.

The foregoing summary is merely an overview of the present utility model, and may be implemented according to the text and the accompanying drawings in order to make it clear to a person skilled in the art that the present utility model may be implemented, and in order to make the above-mentioned objects and other objects, features and advantages of the present utility model more easily understood, the following description will be given with reference to the specific embodiments and the accompanying drawings of the present utility model.

Drawings

The drawings are only for purposes of illustrating the principles, implementations, applications, features, and effects of the present application and are not to be construed as limiting the application.

In the drawings of the specification:

FIG. 1 is a perspective view of the present utility model;

FIG. 2 is an enlarged view of a portion of FIG. 1 at A;

FIG. 3 is a second perspective view of the present utility model;

FIG. 4 is a schematic view of the structure of the present utility model;

Fig. 5 is a partial structural view in the present utility model.

Reference numerals referred to in the above drawings are explained as follows:

1. A bottom plate; 2. a support plate; 3. a first motor; 4. a moving plate; 5. a rotating rod; 6. a band saw body; 7. a mounting plate; 8. a second motor; 9. a first chute; 10. a first screw rod; 11. a first slider; 12. a worm wheel; 13. a worm; 14. a U-shaped plate; 15. a guide rod; 16. a hydraulic cylinder; 17. a connecting plate; 18. a third motor; 19. placing a plate; 20. a fixed block; 21. a carrier roller; 22. a second chute; 23. a second screw rod; 24. a second slider; 25. a rotating shaft; 26. a driven sprocket; 27. a drive sprocket; 28. a guide groove; 29. a mounting groove; 30. a guide wheel; 31. a fixing plate; 32. and a clamping plate.

Detailed Description

In order to describe the possible application scenarios, technical principles, practical embodiments, and the like of the present application in detail, the following description is made with reference to the specific embodiments and the accompanying drawings. The embodiments described herein are only for more clearly illustrating the technical aspects of the present application, and thus are only exemplary and not intended to limit the scope of the present application.

Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the application. The appearances of the phrase "in various places in the specification are not necessarily all referring to the same embodiment, nor are they particularly limited to independence or relevance from other embodiments. In principle, in the present application, as long as there is no technical contradiction or conflict, the technical features mentioned in each embodiment may be combined in any manner to form a corresponding implementable technical solution.

Unless defined otherwise, technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application pertains; the use of related terms herein is for the purpose of describing particular embodiments only and is not intended to limit the application.

In the description of the present application, the term "and/or" is a representation for describing a logical relationship between objects, which means that three relationships may exist, for example, X and/or Y, representing: x, Y, and both X and Y are present. In addition, the character "/" herein generally indicates that the front-to-back associated object is an "or" logical relationship.

In the present application, terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual number, order, or sequence of such entities or operations.

Without further limitation, the use of the terms "comprising," "including," "having," or other like terms in this specification is intended to cover a non-exclusive inclusion, such that a process, method, or article of manufacture that comprises a list of elements does not include additional elements but may include other elements not expressly listed or inherent to such process, method, or article of manufacture.

As in the understanding of "review guidelines," the expressions "greater than", "less than", "exceeding" and the like are understood to exclude this number in the present application; the expressions "above", "below", "within" and the like are understood to include this number. Furthermore, in the description of embodiments of the present application, the meaning of "a plurality of" is two or more (including two), and similarly, the expression "a plurality of" is also to be understood as such, for example, "a plurality of" and the like, unless specifically defined otherwise.

In the description of embodiments of the present application, spatially relative terms such as "center," "longitudinal," "transverse," "length," "width," "thickness," "up," "down," "front," "back," "left," "right," "vertical," "horizontal," "vertical," "top," "bottom," "inner," "outer," "clockwise," "counter-clockwise," "axial," "radial," "circumferential," etc., are used herein as a basis for the description of the embodiments or as a basis for the description of the embodiments, and are not intended to indicate or imply that the devices or components referred to must have a particular position, a particular orientation, or be configured or operated in a particular orientation and therefore should not be construed as limiting the embodiments of the present application.

Unless specifically stated or limited otherwise, the terms "mounted," "connected," "affixed," "disposed," and the like as used in the description of embodiments of the application should be construed broadly. For example, the "connection" may be a fixed connection, a detachable connection, or an integral arrangement; the device can be mechanically connected, electrically connected and communicated; it can be directly connected or indirectly connected through an intermediate medium; which may be a communication between two elements or an interaction between two elements. The specific meaning of the above terms in the embodiments of the present application can be understood by those skilled in the art to which the present application pertains according to circumstances.

As shown in fig. 1 to 5, the sawing machine operating mechanism comprises a bottom plate 1, wherein a supporting plate 2 is fixedly connected to the top wall of the bottom plate 1; the top wall of the supporting plate 2 is fixedly connected with a first motor 3; the side wall of the supporting plate 2 is provided with a moving plate 4, and the moving plate 4 is connected with the first motor 3 through a lifting assembly; the side wall of the movable plate 4 is rotatably connected with a rotating rod 5; the end part of the rotating rod 5 is fixedly connected with a band saw body 6; the side wall of the movable plate 4 is fixedly connected with a mounting plate 7; the side wall of the mounting plate 7 is fixedly connected with a second motor 8, and the second motor 8 is connected with the rotating rod 5 through an adjusting component; during operation, through start second motor 8, second motor 8 drives bull stick 5 through adjusting part and rotates, and bull stick 5 rotates band saw body 6 that drives its tip rotates to can adjust the angle of saw blade slope, among the fine prior art of having avoided of above-mentioned technical scheme, current sawing machine is when using, is inconvenient for adjusting the saw blade of sawing machine and the work piece surface contained angle of taking the cutting, consequently has the problem of certain use limitation.

The lifting assembly comprises a first chute 9; the side wall of the supporting plate 2 is provided with a first chute 9; a first screw rod 10 is rotatably connected between the upper side wall and the lower side wall of the first chute 9, and the output end of the first motor 3 extends into the first chute 9 to be fixedly connected with the top end of the first screw rod 10; the screw-nut pair on the outer circular wall of the first screw rod 10 is connected with a first sliding block 11, and the end part of the first sliding block 11 is fixedly connected with the side wall of the moving plate 4; when the sliding block type sliding block is in operation, the first motor 3 is started, the first motor 3 rotates to drive the first screw rod 10 at the output end of the first motor to rotate, and the first screw rod 10 rotates to drive the first sliding block 11 to move up and down along the first sliding groove 9; the first sliding block 11 drives the band saw body 6 to move up and down through the rotating rod 5, so that the spiral tubular pile sheet is cut.

The adjustment assembly includes a worm gear 12; the outer circular wall of the rotating rod 5 is fixedly connected with a worm wheel 12; the output end of the second motor 8 is connected with a worm 13, and the end part of the worm 13 penetrates through the mounting plate 7 to be meshed with the worm wheel 12; during operation, through starting the second motor 8, the output end of the second motor 8 drives the worm 13 to rotate, the worm 13 rotates to drive the worm wheel 12 to rotate, and the worm wheel 12 rotates to drive the rotating rod 5 to rotate.

The top wall of the bottom plate 1 is fixedly connected with a symmetrically distributed U-shaped plate 14; both sides of the side walls of the pair of movable plates 4 are penetrated and connected with guide rods 15 in a sliding manner; a clamping plate 32 is fixedly connected to the end part of one group of guide rods 15; the two side walls of the bottom plate 1 are fixedly connected with fixing plates 31; the adjacent side walls of the pair of fixing plates 31 are fixedly connected with a hydraulic cylinder 16; the output ends of the pair of hydraulic cylinders 16 are fixedly connected with a connecting plate 17, and the side walls of the connecting plate 17 are fixedly connected with the end parts of the guide rods 15; during operation, a workpiece to be cut is placed below the band saw body 6, two connecting plates 17 are driven to move in opposite directions by two hydraulic cylinders 16, and the connecting plates 17 drive clamping plates 32 to move in opposite directions by guide rods 15, so that two ends of the workpiece to be cut are clamped and fixed; the two pairs of clamping plates 32 are independently arranged, and a gap is reserved in the middle, so that the situation that the saw blade cannot be reset after cutting is completed is avoided.

The bottom wall of the bottom plate 1 is fixedly connected with a third motor 18; the top wall of the bottom plate 1 is provided with a placing plate 19, and the placing plate 19 is connected with a third motor 18 through a transmission assembly; a pair of fixed blocks 20 are fixedly connected to two sides of the top wall of the placing plate 19; a carrier roller 21 is rotatably connected between the side walls of the two pairs of fixed blocks 20; during operation, the workpiece to be cut is placed between the two carrier rollers 21 on the surface of the placement plate 19, the placement plate 19 is driven to move by the third motor 18 through the transmission assembly by starting the third motor 18, so that a tool is moved to the position right below the saw blade, and a user can conveniently feed and discharge the workpiece.

The transmission assembly includes a second runner 22; the bottom plate 1 is provided with a second chute 22; a second screw rod 23 is rotatably connected between the two side walls of the second chute 22; the outer circular wall screw nut pair of the second screw rod 23 is connected with a second sliding block 24, and the top end of the second sliding block 24 is fixedly connected with the bottom wall of the placing plate 19; the side wall of the bottom plate 1 is rotatably connected with a rotating shaft 25, and one end of the rotating shaft 25 extends into the second chute 22 and is fixedly connected with the end part of the second screw rod 23; the other end of the rotating shaft 25 is fixedly connected with a driven sprocket 26; the output end of the third motor 18 is fixedly connected with a driving chain wheel 27, and the driving chain wheel 27 is connected with a driven chain wheel 26 through a chain belt; when the automatic feeding device works, the third motor 18 is started, the third motor 18 drives the driving sprocket 27 at the output end of the third motor to rotate, the driving sprocket 27 drives the driven sprocket 26 to rotate through the chain belt, the driven connection drives the second screw rod 23 to rotate through the rotating shaft 25, and the second screw rod 23 rotates to be placed along the second sliding groove 22 to horizontally move through the second sliding block 24.

Guide grooves 28 are formed in two sides of the top wall of the bottom plate 1; mounting grooves 29 are formed in two sides of the bottom wall of the placement plate 19, and the mounting grooves 29 are located right above the guide grooves 28; a pair of guide wheels 30 are rotatably connected between the two side walls of the mounting groove 29, and the guide wheels 30 horizontally move in the guide grooves 28; in operation, the guide wheel 30 at the bottom of the placement plate 19 moves in the guide groove 28, so that the placement plate 19 moves more labor-saving.

The inner diameter of the driving sprocket 27 is smaller than that of the driven sprocket 26; in operation, the drive sprocket 27 is rotated relatively slowly by the inner diameter of the drive sprocket 27 being smaller than the inner diameter of the driven sprocket 26.

The working principle is that by starting the second motor 8, the second motor 8 drives the rotating rod 5 to rotate through the adjusting component, and the rotating rod 5 drives the band saw body 6 at the end part of the rotating rod to rotate, so that the inclined angle of the saw blade can be adjusted; by starting the first motor 3, the first motor 3 rotates to drive a first screw rod 10 at the output end of the first motor to rotate, and the first screw rod 10 rotates to drive a first sliding block 11 to move up and down along a first sliding groove 9; the first sliding block 11 drives the band saw body 6 to move up and down through the rotating rod 5, so that the spiral tubular pile sheet is cut; the workpiece to be cut is placed below the band saw body 6, two connecting plates 17 are driven to move in opposite directions by two hydraulic cylinders 16, and the connecting plates 17 drive clamping plates 32 to move in opposite directions by guide rods 15, so that two ends of the workpiece to be cut are clamped and fixed; the two pairs of clamping plates 32 are independently arranged, and a gap is reserved in the middle, so that the situation that the saw blade cannot be reset after cutting is completed is avoided; the workpiece to be cut is placed between the two carrier rollers 21 on the surface of the placing plate 19, the placing plate 19 is driven to move by the third motor 18 through the transmission component by starting the third motor 18, so that a tool is moved to the position right below the saw blade, and a user can conveniently feed and discharge the workpiece; the guide wheel 30 at the bottom of the placement plate 19 moves in the guide groove 28, so that the placement plate 19 can move more labor-saving; by the inner diameter of the drive sprocket 27 being smaller than the inner diameter of the driven sprocket 26, the drive sprocket is caused to rotate more slowly.

In this embodiment, the power mechanism or power unit includes, but is not limited to, an engine, an electric motor, a pneumatic tool, a hydraulic pump, and the like. The power unit also comprises a direct power source and an indirect power source, wherein the direct power source can provide power for the power unit, such as an engine, a motor and the like, and the indirect power source comprises a cylinder, a hydraulic cylinder and the like. The power mechanism or the power unit can drive the execution unit to do linear reciprocating motion through the cooperation of a gear and a rack, the cooperation of a sliding block and a sliding groove, the cooperation of a screw rod and a nut, and the like.

In this embodiment, the transmission mechanism or transmission unit includes a speed reducer, a gearbox, a worm gear mechanism, a link mechanism, a compound mechanism, and the like. The transmission mechanism or the transmission unit is used for transmitting power from the power mechanism or the power unit to the execution mechanism or the execution unit.

In this embodiment, the actuator or actuator unit includes, but is not limited to, a compression mechanism, a rotation mechanism, a swing mechanism, a vibration mechanism, a lift mechanism, a cutting mechanism, and the like.

Those skilled in the art will appreciate that while some embodiments herein include some features but not others included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the application and form different embodiments. For example, in the claims, any of the claimed embodiments may be used in any combination.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present application, and not for limiting the same; although the application has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical scheme described in the foregoing embodiments can be modified or some or all of the technical features thereof can be replaced by equivalents; such modifications and substitutions do not depart from the spirit of the application, and are intended to be included within the scope of the appended claims and description. In particular, the technical features mentioned in the respective embodiments may be combined in any manner as long as there is no structural conflict. The present application is not limited to the specific embodiments disclosed herein, but encompasses all technical solutions falling within the scope of the claims.

Claims

1. The sawing machine operating mechanism is characterized by comprising a bottom plate (1), wherein a supporting plate (2) is fixedly connected to the top wall of the bottom plate (1); the top wall of the supporting plate (2) is fixedly connected with a first motor (3); the side wall of the supporting plate (2) is provided with a moving plate (4), and the moving plate (4) is connected with the first motor (3) through a lifting assembly; the side wall of the movable plate (4) is rotatably connected with a rotating rod (5); the end part of the rotating rod (5) is fixedly connected with a band saw body (6); the side wall of the movable plate (4) is fixedly connected with a mounting plate (7); the side wall of the mounting plate (7) is fixedly connected with a second motor (8), and the second motor (8) is connected with the rotating rod (5) through an adjusting component.

2. A sawing machine handling mechanism according to claim 1, wherein the lifting assembly comprises a first chute (9); a first chute (9) is formed in the side wall of the supporting plate (2); a first screw rod (10) is rotationally connected between the upper side wall and the lower side wall of the first chute (9), and the output end of the first motor (3) extends into the first chute (9) and is fixedly connected with the top end of the first screw rod (10); the screw-nut pair on the outer circular wall of the first screw rod (10) is connected with a first sliding block (11), and the end part of the first sliding block (11) is fixedly connected with the side wall of the movable plate (4).

3. A sawing machine operating mechanism according to claim 2, wherein the adjustment assembly comprises a worm wheel (12); the outer circular wall of the rotating rod (5) is fixedly connected with a worm wheel (12); the output end of the second motor (8) is connected with a worm (13), and the end part of the worm (13) penetrates through the mounting plate (7) to be meshed with the worm wheel (12).

4. A sawing machine operating mechanism according to claim 3, characterised in that the top wall of the base plate (1) is fixedly connected with a symmetrically distributed U-shaped plate (14); guide rods (15) penetrate through two sides of the side walls of the pair of movable plates (4) in a sliding manner; the end parts of one group of guide rods (15) are fixedly connected with clamping plates (32); both side walls of the bottom plate (1) are fixedly connected with fixing plates (31); a pair of adjacent side walls of the fixed plates (31) are fixedly connected with hydraulic cylinders (16); the output ends of the pair of hydraulic cylinders (16) are fixedly connected with connecting plates (17), and the side walls of the connecting plates (17) are fixedly connected with the end parts of the guide rods (15).

5. A sawing machine operating mechanism according to claim 4, wherein the bottom wall of the bottom plate (1) is fixedly connected with a third motor (18); a placing plate (19) is arranged on the top wall of the bottom plate (1), and the placing plate (19) is connected with a third motor (18) through a transmission assembly; a pair of fixed blocks (20) are fixedly connected to two sides of the top wall of the placing plate (19); and a carrier roller (21) is rotatably connected between the side walls of the two pairs of fixed blocks (20).

6. A sawing machine operating mechanism according to claim 5, wherein the transmission assembly comprises a second chute (22); a second chute (22) is formed in the top of the bottom plate (1); a second screw rod (23) is rotatably connected between the two side walls of the second chute (22); the screw-nut pair of the outer circular wall of the second screw rod (23) is connected with a second sliding block (24), and the top end of the second sliding block (24) is fixedly connected with the bottom wall of the placing plate (19); the side wall of the bottom plate (1) is rotatably connected with a rotating shaft (25), and one end of the rotating shaft (25) extends into the second sliding groove (22) to be fixedly connected with the end part of the second screw rod (23); the other end of the rotating shaft (25) is fixedly connected with a driven sprocket (26); the output end of the third motor (18) is fixedly connected with a driving chain wheel (27), and the driving chain wheel (27) is connected with a driven chain wheel (26) through a chain belt.

7. A sawing machine operating mechanism according to claim 6, characterised in that guide slots (28) are provided on both sides of the top wall of the base plate (1); mounting grooves (29) are formed in two sides of the bottom wall of the placement plate (19), and the mounting grooves (29) are located right above the guide grooves (28); a pair of guide wheels (30) are rotatably connected between two side walls of the mounting groove (29), and the guide wheels (30) horizontally move in the guide grooves (28).

8. A sawing machine operating mechanism as claimed in claim 7, characterized in that the inner diameter of the driving sprocket (27) is smaller than the inner diameter of the driven sprocket (26).