CN1278046C - Turbine fan - Google Patents

Abstract

The present invention relates to a turbo fan. In the turbo fan, a plurality of blades are radially and vertically installed between a main plate and a baffle plate, the vertical projection planes of the blades form a wing surface shape, and a convex positive-pressure surface and a concave negative-pressure surface are formed at both side surfaces of a main body part; because the positive-pressure surface is convex, a vortex flow preventing part of the back end positive-pressure surface is formed at the side of the positive-pressure surface at the back end part of the main body part; because the negative-pressure surface is convex, a vortex flow preventing part of the partial back end negative-pressure surface is formed at the side of the negative-pressure at the back end of the main body part. Under the condition of the same power consumption or power consumption reduction, equivalent air quantity or more air quantity can be generated, and noise is reduced. Thus, the turbo fan made of the ultrathin blades can lower manufacturing expense, shorten manufacturing time, and lighten the weight of the whole fan.

Description

Turbofan

Technical field

The present invention relates to a kind of turbofan of field, particularly relate to a kind of under the situation of keeping identical air quantity and power consumption, the turbofan (TURBOFAN) of the ultra-thin blade of researching and developing in order to save manufacturing cost and production time.

Background technique

General blowing fan is by the rotating force force feed air of blade and rotor, and it is applied on the products such as refrigerator, air cleaner and vacuum cleaner widely.

Above-mentioned blowing fan can be divided into axial fan, centrifugal cutter and turbofan according to the suction of air and the different and shape of the mode of derivation.

In these fans, turbofan is that unique a kind of air that adopts flows into and by the blade mode, just adopted the mode by the radial derivation air in fan side from the direction of axle.Because this mode makes air can flow into the inner and derivation of blower fan naturally, therefore, do not need extra pipeline, relatively be suitable for large product (as the top suction type air cleaner).

Seeing also Fig. 1 to shown in Figure 3, is the structure of existing turbofan, and Fig. 1 is a planimetric map, and Fig. 2 is a sectional side view, and Fig. 3 is the planimetric map of expression blade vertical plane.

As shown in the figure, existing turbofan comprises main body 1, forms the bottom of main body 1 and the mainboard 2 of fan electromotor 5 is installed, along a plurality of blades (3) and the baffle plate 4 that along blade 3 upper ends be connected of these mainboard 2 inner side surfaces at a distance of certain intervals.

In order to suck air, suction port 7 has been formed at main body 1 top, and intermediate portion has formed a passage 6 for the direction of air being guided into discharge, and lateral parts has formed the exhaust port that inhaled air is discharged.

Therefore, after fan electromotor 5 drives, blade 3 rotations that main body 1 begins to rotate and drive and main body 1 link together, the suction port 7 of the air of outside below main body enters, and is discharged to exhaust port 8 along passage 6.

In addition, the vertical plane of blade 3 forms wing face (aerofoil), forms pressure surface 31 that protrudes and the suction surface 32 that caves in both sides, and radial being vertically mounted between mainboard 2 and the baffle plate 4.

Here said wing face is meant that nineteen fifty is wing by the streamline of NACA (US Airways Advisory Board) exploitation, and this is the blade shape that comes out according to following Design Theory.

Fig. 4 is the planimetric map of the existing turbofan blade vertical plane of expression, as the forward terminal O of blade inside end end points as initial point, and the virtual line of the aft terminal Z that connects initial point and blade outer ends end points formed a rectangular coordinate system as X-axis.

Described wing foliation opinion (NACA 4-Digit Aerofoil) is to utilize thickness function (the thickness function:y that describes vane thickness _t) and describe formed parabolical ogive 33 functions (the camberline function:y of its average thickness value _c), by following mathematical expression, be illustrated in the pressure surface of blade and the formed parabola of suction surface on the coordinate resulting.

At first, thickness function y _tAs mathematical expression 1:

[mathematical expression 1]

$y_t\left(x\right)=\frac{t_c}{0.2}(0.2969\sqrt{x}-0.126x-0.35160x^2+0.2843x^3-0.1015x^4)$

Y wherein _tThe expression vane thickness, t _cBe the maximum ga(u)ge value.

Ogive function y _cAs mathematical expression 2:

[mathematical expression 2]

P＞x 〉=0 o'clock, $y_c\left(x\right)=\frac{M}{P^2}(2\mathrm{Px}-x^2)$

P＜x≤1 o'clock, $y_c\left(x\right)=\frac{M}{{(1-P)}^2}(1-2P+2\mathrm{Px}-x^2)$

Here, M represents the Y coordinate of maximum arch, and p represents the X coordinate of maximum arch.Therefore, the expression blade pressure surface 31 parabolical mathematical expression that forms such as mathematical expressions 3:

[mathematical expression 3]

x _u＝x-y _t(x)sinθ y _u＝y _c(x)+y _t(x)cosθ

Here,

P＞x 〉=0 o'clock, $\mathrm{\θ}=\arctan 2\frac{M}{\sqrt{P}}(P-x)$

P＜x≤1 o'clock, $\mathrm{\θ}=\arctan 2\frac{M.}{\sqrt{1-P}}(P-x)$

In addition, the expression blade suction surface 32 parabolical mathematical expression that forms such as mathematical expressions 4:

[mathematical expression 4]

x _l＝x+y _t(x)sinθ y _l＝y _c(x)-y _t(x)cosθ

Here, the θ value with described blade pressure surface 31 to form parabolical situation identical.

But, according to the blade of described mode made, because its thickness is thicker, therefore, problem such as the power consumption when same air quantity exist to take place is big, noise is big.

This shows that described existing turbofan still has many defectives, and demands urgently further being improved.

In order to solve the problem that described turbofan exists, relevant manufacturer there's no one who doesn't or isn't seeks solution painstakingly, but do not see always that for a long time suitable design finished by development, and common product does not have appropriate structure can solve described problem, and this obviously is the problem that the anxious desire of relevant dealer solves.

Because the defective that described existing turbofan exists, the inventor is based on being engaged in this type of product specification manufacturing abundant for many years practical experience and professional knowledge, actively studied innovation, in the hope of founding a kind of turbine wind (TURBOFAN) of new structure, can improve general existing on the market conventional turbofan structure, make its more practicability.Through constantly research, design, and after studying sample and improvement repeatedly, the present invention who goes out to have practical value finally.

Summary of the invention

Main purpose of the present invention is, overcomes the defective that described existing turbofan exists, and a kind of turbofan (TURBOFAN) of new structure is provided, and technical problem underlying to be solved is to make it make a kind of ultra-thin blade.This blade can produce equivalent or more air quantity and reduce noise under the condition of identical or minimizing power consumption.Therefore, the turbofan by this ultra-thin leaf production comes out can reduce manufacturing cost, shortens production time, and can alleviate the weight of whole fan.

Purpose of the present invention and to solve its technical problem underlying be to adopt following technological scheme to realize.Turbofan according to the present invention's proposition, radially between its mainboard and baffle plate be vertically installed with a plurality of blades, described blade, its vertical plane forms wing face shape, has the main body that forms the suction surface of protruding pressure surface and depression in both sides, in the rearward end pressure surface side of described main body, the rear end pressure surface turbulent flow with protruding formed part pressure surface prevents part; In the rearward end suction surface side of described main body, the rear end suction surface turbulent flow with protruding formed part suction surface prevents part; Above-mentioned wing face is to utilize the thickness function y that describes vane thickness _tWith the formed parabolical ogive function y of its average thickness value of description _c, the pressure surface of blade and the formed parabola of suction surface be illustrated in obtain on the coordinate,

Thickness function thickness function y wherein _tAs mathematical expression 1:

$y_t\left(x\right)=\frac{t_c}{0.2}(0.2969\sqrt{x}-0.126x-0.35160x^2+0.2843x^3-0.1015x^4)$

Y wherein _tThe expression vane thickness, t _cBe the maximum ga(u)ge value.

Ogive function y _cAs mathematical expression 2:

P＞x 〉=0 o'clock, $y_c\left(x\right)=\frac{M}{P^2}(2\mathrm{Px}-x^2)$

P＜x≤1 o'clock, $y_c\left(x\right)=\frac{M}{{(1-P)}^2}(1-2P+2\mathrm{Px}-x^2)$

Here, M represents the Y coordinate of maximum arch, and p represents the X coordinate of maximum arch.

Therefore, expression blade pressure surface forms parabolical mathematical expression such as mathematical expression 3:

x _u＝x-y _t(x)sinθ y _u＝y _c(x)+y _t(x)cosθ

Here,

P＞x 〉=0 o'clock, $\mathrm{\θ}=\arctan 2\frac{M}{\sqrt{P}}(P-x)$

P＜x≤1 o'clock, $\mathrm{\θ}=\arctan 2\frac{M}{\sqrt{1-P}}(P-x)$

In addition, expression blade suction surface forms parabolical mathematical expression such as mathematical expression 4:

x _l＝x+y _t(x)sinθ y _t＝y _c(x)-y _t(x)cosθ

Here, the θ value is with to form parabolical situation at described blade pressure surface identical;

And, described rear end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface, described rear end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface.

The object of the invention to solve the technical problems can also adopt following technical measures further to realize.

Aforesaid turbofan, the rear end turbulent flow of wherein said blade prevent that the length of the string of part from being below 40% of length of described main body string.

Aforesaid turbofan, wherein said blade, in the front end pressure surface side of its described main body, the front end pressure surface turbulent flow with protruding formed part pressure surface prevents part; Front end suction surface side in its described main body, front end suction surface turbulent flow with protruding formed part suction surface prevents part, on the vertical plane of wherein said blade, described front end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, and the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface; On the vertical plane of described blade, described front end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface; The front end turbulent flow of described blade prevents that the length of the string of part from being below 40% of length of described main body string.

Aforesaid turbofan, wherein said blade, in the front end pressure surface side of its described main body, the front end pressure surface turbulent flow with protruding formed part pressure surface prevents part; Front end suction surface side in its described main body, front end suction surface turbulent flow with protruding formed part suction surface prevents part, on the vertical plane of wherein said blade, described front end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, and the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface; On the vertical plane of described blade, described front end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface; The front end turbulent flow of described blade prevents that the length of the string of part from being below 40% of length of described main body string.

The present invention compared with prior art has tangible advantage and beneficial effect.By above technological scheme as can be known, turbofan of the present invention has following advantage at least:

Blade provided by the present invention can produce equivalent or more air quantity and reduce noise under the condition of identical or minimizing power consumption.Therefore, the turbofan by this ultra-thin leaf production comes out can reduce manufacturing cost, shortens production time, and can alleviate the weight of whole fan.

In sum, the turbofan of special construction of the present invention, under the situation of keeping identical air quantity and power consumption, can save manufacturing cost and production time, it has described many advantages and use value, and in like product, do not see have similar structural design to publish or use, no matter and it structurally or bigger improvement all arranged on the function, have large improvement technically, and produced handy and practical effect, and have the effect of enhancement really, thus be suitable for practicality more, really be a new and innovative, progressive, practical invention.

Described explanation only is the general introduction of technical solution of the present invention, for can clearer understanding technological means of the present invention, and can be implemented according to the content of specification, below with preferred embodiment of the present invention and conjunction with figs. describe in detail as after.

The specific embodiment of the present invention is provided in detail by following examples and accompanying drawing thereof.

Description of drawings

Fig. 1 to Fig. 4 is the schematic representation of existing turbofan.

Fig. 1 is a planimetric map.

Fig. 2 is a sectional side view.

Fig. 3 is the planimetric map of expression blade vertical plane.

Fig. 4 is for fastening the planimetric map of expression blade vertical plane at right angled coordinates.

Fig. 5 is the blade vertical plane planimetric map of turbofan of the present invention.

Fig. 6 is the planimetric map of the thicker blade of diagram vertical projection face thickness.

Fig. 7 is the planimetric map of the blade of diagram vertical plane thinner thickness.

Fig. 8 is embodiments of the invention, is illustrated in the blade vertical plane planimetric map that front end and rearward end form projection.

The label declaration * * of main parts size in the * accompanying drawing

O: forward terminal Z: aft terminal

C: string 40: main body

41: pressure surface 42: suction surface

50: front end pressure surface turbulent flow prevents part 51: the part pressure surface

60: front end suction surface turbulent flow prevents part 61: the part suction surface

70: rear end pressure surface turbulent flow prevents part 71: the part pressure surface

80: rear end suction surface turbulent flow prevents part 81: the part suction surface

Embodiment

Below in conjunction with accompanying drawing and preferred embodiment, to its embodiment of turbofan, structure, feature and the effect thereof that foundation the present invention proposes, describe in detail as after.

See also shown in Figure 5, be expression according to the present invention the planimetric map of the blade vertical plane of turbofan.As shown in the figure, blade in according to the present invention, its vertical plane becomes the wing face, two sides of main body 40 are formed with protrudes pressure surface 41 and depression suction surface 42, and, in rearward end pressure surface 41 sides of this main body 40, the rear end pressure surface turbulent flow with protruding formed part pressure surface 71 prevents part 70; In rearward end suction surface 42 sides of main body 40, the rear end suction surface turbulent flow with protruding formed part suction surface 81 prevents part 80.

Here, the thickness of blade is thinner than original thickness in the main body 40, and just the rearward end pressure surface 41 of main body 41 and the turbulent flow that suction surface 42 is protruded prevent that part 70,80 from having kept original thickness.Like this, when reaching the purpose that the present invention will reduce vane thickness, the adverse factors such as formation of eddy current have also been prevented to greatest extent.

For being described in detail as follows of the principle of the invention.

Described mathematical expression 3 and 4 is to try to achieve the formula that forms wing face shape blade pressure surface and suction surface parabolic coordinates, and these are according to thickness function y _tThe value of gained is come.Variable t according to the described mathematical expression 1 of any change _c, we can obtain the pressure surface and the formed parabolical coordinate of suction surface of any thickness blade.

On the other hand, in order to save raw material, reduce the purpose of power consumption, preferably reduce the thickness of blade, but the thickness of blade is thin excessively as far as possible according to the weight that reduces blade, can eddy generation in its front-end and back-end portion.

Therefore, supposing to have maximum ga(u)ge shown in Figure 6 is the blade of thick wing face shape of a and the blade of the wing face shape that maximum ga(u)ge shown in Figure 7 is b, we can imagine that a kind of shape of blade is, described deathtrap rearward end is made as shown in Figure 6 thick shape, and remaining part is made thin shape as shown in Figure 7.So just can obtain the blade structure that very economical vertical plane is arranged shown in Figure 5.

Such theory represents with original wing foliation opinion mathematical formulae, can be as follows:

The same with original situation, as shown in Figure 4, forward terminal O as initial point, and the virtual line that connects initial point and aft terminal Z as the rectangular coordinate system of X-axis as standard, suppose that the blade vertical plane is placed on described rectangular coordinate system.

In the blade vertical plane, forward terminal O is meant blade inside end end points, and aft terminal Z is meant blade outer ends end points.

Form the pressure surface 41 of main body 40 and the parabola of suction surface 42, each the free above-mentioned mathematical expression 3 and the coordinate of mathematical expression 4 gained are formed.According to the thickness of the profile of the blade of these parabola gained, within the scope that satisfies wing foliation opinion, with thin being advisable as far as possible.

On the other hand, form rear end pressure surface turbulent flow and prevent that part 70 formed part pressure surfaces 71 and rear end suction surface turbulent flow from preventing the parabola of part 80 formed part suction surfaces 81, also is made up of the coordinate of mathematical expression 3 and mathematical expression 4 gained.This situation with described main body 40 is consistent.As shown in Figure 5, this is in order to prevent the formation of eddy current, to have formed a projection in the blade rearward end, therefore, and according to the blade profile thickness of parabola gained, as long as be enough to prevent the formation of eddy current.

Therefore, relevant rear end pressure surface turbulent flow prevents the Y-axis coordinate figure of part 70 formed part pressure surface 71 parabolic coordinates, and is bigger than the Y-axis coordinate figure of relevant main body 40 pressure surfaces 41 parabolic coordinates.

In contrast, relevant rear end suction surface turbulent flow prevents the Y-axis coordinate figure of part 80 formed part suction surface 81 parabolic coordinates, and is as far as possible littler than the Y-axis coordinate figure of the parabolic coordinates of relevant main body 40 suction surfaces 41.

In addition, when the vane thickness attenuation, the air flows that eddy current etc. are harmful does not occur over just the rearward end of blade, can occur in front end yet, therefore, can utilize described principle, imagines that a kind of front end and rearward end all have the blade of protruding structure.

Fig. 8 is illustrated in the planimetric map of vertical plane that front end and rearward end all have the blade of lug boss.

As shown in the figure, in the blade in the present embodiment, front end pressure surface 41 sides of main body 40, front end pressure surface turbulent flow with protruding formed part pressure surface 51 prevents part 50 and in front end suction surface 42 sides of main body 40, the front end suction surface turbulent flow with the projection part suction surface 61 that forms prevents part 60.

Meaning on the engineering science that this protruding structure comprised and effect prevent that with described rearward end turbulent flow part 70,80 is the same.

In addition, when selected front end pressure surface turbulent flow prevents that part 50 formed part pressure surfaces 51 and front end suction surface turbulent flow from preventing that the formed part suction surface of part 60 61 is parabolical, also prevent that with described rearward end turbulent flow the situation of part is the same, because be the convex portion that has just formed the blade front end for the generation that prevents eddy current etc., therefore, according to the thickness of the blade profile that its parabola obtained, as long as be enough to prevent the formation of eddy current.

Therefore, front end pressure surface turbulent flow prevents part 50 formed part pressure surface 51 parabolical Y-axis coordinate values, be as far as possible greater than main body 40 pressure surfaces 41 parabolical Y-axis coordinate figures; Front end suction surface turbulent flow prevents the parabola Y-axis coordinate figure of part 60 formed part suction surfaces 61, be as far as possible less than main body 40 suction surfaces 42 parabolical Y-axis coordinate figures.

On the other hand, prevent that the string B of part is long, can desalinate the aim of the present invention that will reduce leaf weight if the front end turbulent flow prevents the string D of part and rear end turbulent flow.

Therefore, according to experimental result, the length that the front end turbulent flow prevents the length of string D of part and the string B that the rear end turbulent flow prevents part is advisable below 40% with the length of described main body string C.

The technological innovation of the turbofan of the present invention that described structure like this constitutes all has many saving graces for technician of the same trade now, and the progressive that possesses skills really.

The above, it only is preferred embodiment of the present invention, be not that the present invention is done any pro forma restriction, any those skilled in the art may utilize the technology contents of described announcement to be changed or be modified to the equivalent embodiment of equivalent variations, but every technical solution of the present invention content that do not break away from,, all still belong in the scope of technical solution of the present invention any simple modification, equivalent variations and modification that above embodiment did according to technical spirit of the present invention.

Claims (6)

1, a kind of turbofan is characterized in that:

Radially between its mainboard and baffle plate be vertically installed with a plurality of blades, described blade, its vertical plane form wing face shape, have the main body of the suction surface of the pressure surface that forms projection in both sides and depression,

In the rearward end pressure surface side of described main body, the rear end pressure surface turbulent flow with protruding formed part pressure surface prevents part;

In the rearward end suction surface side of described main body, the rear end suction surface turbulent flow with protruding formed part suction surface prevents part;

Above-mentioned wing face is to utilize the thickness function y that describes vane thickness _tWith the formed parabolical ogive function y of its average thickness value of description _c, the pressure surface of blade and the formed parabola of suction surface be illustrated in obtain on the coordinate,

Thickness function thickness function y wherein _tAs mathematical expression 1:

$y_t\left(x\right)=\frac{t_c}{0.2}(0.2969\sqrt{x}-0.126x-0.35160x^2+0.2843x^3-0.1015x^4)$

Y wherein _tThe expression vane thickness, t _cBe the maximum ga(u)ge value.

Ogive function y _cAs mathematical expression 2:

P＞x 〉=0 o'clock, $y_c\left(x\right)=\frac{M}{P^2}(2\mathrm{Px}-x^2)$

P＜x≤1 o'clock, $y_c\left(x\right)=\frac{M}{{(1-P)}^2}(1-2P+2\mathrm{Px}-x^2)$

Here, M represents the Y coordinate of maximum arch, and p represents the X coordinate of maximum arch.

Therefore, expression blade pressure surface forms parabolical mathematical expression such as mathematical expression 3:

x _u＝x-y _t(x)sinθ y _u＝y _c(x)+y _t(x)cosθ

Here,

P＞x 〉=0 o'clock, $\mathrm{\θ}=\arctan 2\frac{M}{\sqrt{P}}(P-x)$

P＜x≤1 o'clock, $\mathrm{\θ}=\arctan 2\frac{M}{\sqrt{1-P}}(P-x)$

In addition, expression blade suction surface forms parabolical mathematical expression such as mathematical expression 4:

x _l＝x+y _t(x)sinθ y _l＝y _c(x)-y _t(x)cosθ

Here, the θ value is with to form parabolical situation at described blade pressure surface identical;

And described rear end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, and the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface,

Described rear end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface.

2, turbofan according to claim 1 is characterized in that the rear end turbulent flow of wherein said blade prevents that the length of the string of part from being below 40% of length of described main body string.

3, turbofan according to claim 1 is characterized in that wherein said blade, and in the front end pressure surface side of its described main body, the front end pressure surface turbulent flow with protruding formed part pressure surface prevents part;

In the front end suction surface side of its described main body, the front end suction surface turbulent flow with protruding formed part suction surface prevents part;

On the vertical plane of wherein said blade, described front end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, and the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface;

On the vertical plane of described blade, described front end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface.

4, turbofan according to claim 3 is characterized in that the front end turbulent flow of wherein said blade prevents that the length of the string of part from being below 40% of length of described main body string.

5, turbofan according to claim 2 is characterized in that wherein said blade, and in the front end pressure surface side of its described main body, the front end pressure surface turbulent flow with protruding formed part pressure surface prevents part;

In the front end suction surface side of its described main body, the front end suction surface turbulent flow with protruding formed part suction surface prevents part,

On the vertical plane of wherein said blade, described front end pressure surface turbulent flow prevents that the formed part pressure surface of part parabola from being made up of the coordinate of mathematical expression 3 gained, and the Y-axis coordinate figure of its coordinate is bigger than the parabolical Y-axis coordinate figure of described main body pressure surface;

On the vertical plane of described blade, described front end suction surface turbulent flow prevents that the formed part suction surface of part parabola from being made up of the coordinate of mathematical expression 4 gained, and the Y-axis coordinate figure of its coordinate is littler than the parabolical Y-axis coordinate figure of described main body suction surface.

6, turbofan according to claim 5 is characterized in that the front end turbulent flow of wherein said blade prevents that the length of the string of part from being below 40% of length of described main body string.

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