CN203899484U - Tapered mixing equipment and damping structure thereof - Google Patents

Abstract

In order to solve a technical problem, the utility model aims to provide tapered mixing equipment and a damping structure thereof to reduce vibration generated during mixing and stirring of materials. The damping structure is characterized by comprising a tapered substance axially protruding the bottom at the shaft end of a long screw shaft; an included angle, which is smaller than 90 - Alpha degrees, is formed between the outer surface of the substance and the axis of the long screw shaft; the taper angle of the tapered mixing equipment is Alpha.

Description

Taper mixing apparatus and Vibration eliminating structure thereof

Technical field

The utility model relates to mixing apparatus and Vibration eliminating structure thereof.

Background technology

At present, the type of container fixed mixing apparatus is many, has helical-ribbon type mixing apparatus, plough coulter type mixing apparatus, instantaneous weight loss mix particles equipment, planet taper mixing apparatus.Wherein, horizontal mixing apparatus has horizontal ribbon mixing apparatus and colter mixing apparatus etc., and taper mixing apparatus is divided into three kinds of single-screw cone-type mixers, double helix cone-type mixer, three-screw tapered mixing machine by spiral number.

Conical twin-screw mixer is divided into again two kinds of symmetric double mixing screw and asymmetric double mixing screws, symmetrical and asymmetric spiral is all in mixing apparatus cylindrical shell, to make spiral when the axle around oneself rotates (being referred to as " rotation "), also around the central shaft of conical shell, and near cone wall, do again planetary motion (being referred to as " revolution ") by means of the revolution of pivoted arm.The whole equipment of conical twin-screw mixer makes material in cone, produce compound motion by public affairs, the rotation of spiral, thereby reaches mixed uniformly object.The direction of spiral rotation so that material from cone base to cone top, move as positive direction.It is more violent that this kind of conical double ribbon revolves equipment mixing intensity, but part powder material is had to the effect that destroys top layer.

Taper three-screw mixer is that symmetric double mixing screw adds a helical axis at medial axis position, and entity spiral is distributed in position, middle and lower part.Intermediate conveyor screw is the aspect to double helix bottom material lifting, then by double helix, completes the mixing to cylindrical shell upper materials.The identical branch respectively of rotation direction of two symmetrical spirals on the both sides of cylindrical shell and with intermediate conveyor screw switched in opposite, but three spirals are a material, from cone base, carry to cone top.Taper three-screw mixer mixture homogeneity is not so good as double worm mixer and is generally applicable to mix capacious equipment.

Double helix, along wall and the revolution of middle anchor ring, causes material to move in a circle along cone wall; Spiral rotation makes material rise along helicoid from boring the end.The public affairs of spiral, rotation compound motion are inhaled in the face of cylinder of spiral a part of material, are subject to again the effect of spiral rotation centrifugal force simultaneously, make the flow direction of material cone center at cone wall place.Thereby several motion stack circulations of material in cylindrical shell have formed complicated vortex movement and have produced convection current, shearing, diffusion like this, and the abundant convection current of material in mixing apparatus is mixed.

For solid particle material softer, that there is strain, in stirring, mixing of materials there will be the phenomenons such as vibration, heating.Vibration strengthens noise, has polluted environment around.

Utility model content

Technical problem to be solved in the utility model is to provide a kind of taper mixing apparatus and Vibration eliminating structure thereof, reduces the oscillation phenomenon that material occurs in mix and blend.

For solving the problems of the technologies described above, the utility model provides a kind of Vibration eliminating structure of taper mixing apparatus, the cone angle of this taper mixing apparatus is α, be characterized in being included in the axial entity that protrudes the convergent arranging in axle head bottom of long helical axis, in an angle, this angle is less than 90 degree-α for the outer surface of this entity and the axis of long helical axis.

Described Vibration eliminating structure, its further feature is that described entity is taper entity.

Described Vibration eliminating structure, its further feature is that α is 17 degree.

The taper mixing apparatus that the utility model provides, comprises long helical axis, is characterized in comprising described Vibration eliminating structure.

Described taper mixing apparatus, its further feature is that this taper mixing apparatus is that conical double ribbon revolves mixing apparatus.

Described taper mixing apparatus, its further feature is, this taper mixing apparatus comprises cylindrical shell, reductor, long curved boom, short curved boom, described long helical axis and short helical axis, cylindrical shell is shaped as back taper, the top cover on cylindrical shell top is that flat head and cylindrical shell are integral type structure, cylindrical shell top arranges charging aperture and access hole, reductor is installed in the middle of top cover, cylindrical shell center arranges liquid-jet device, reductor has hollow shaft and the axis of rotation, hollow shaft connects long curved boom by adpting flange, short curved boom, rotating long helical axis is installed in the elbow of the end of long curved boom, rotating short helical axis is installed in the elbow of short curved boom end, the axis of rotation by bevel gear set respectively with length, power transmission shaft in short curved boom connects, power transmission shaft in long curved boom connects by bevel gear set the worm drive axle that long helical axis connects again, power transmission shaft in short curved boom connects by bevel gear set the worm drive axle that short helical axis connects again.

Described taper mixing apparatus, its further feature is that long curved boom is monoblock cast structure.

Described taper mixing apparatus, its further feature is, the axis of rotation of reductor is hollow-core construction and forms hydrojet passage, so that spray enters cylindrical shell to the liquid of material from reductor top, from shower nozzle, sprays.

Described taper mixing apparatus, its further feature is, and the blade of short helical axis is larger than the blade diameter of long helical axis, and long and short helical axis is all hollow shaft thin-wall construction.

The utility model is inner at cone-type spiral mixing apparatus, and the vibration that can produce while solid particle material softer, that have strain being carried out to married operation under the stirring action of mechanical stirring device is particularly effective, can eliminate vibration, makes equipment operation steady.Reach energy-conservation, reduce the object of the noise pollution of environment.

Accompanying drawing explanation

The above and other feature of the present utility model, character and advantage become more obvious by the description by below in conjunction with drawings and Examples, wherein:

Fig. 1 is that the assembling that the conical double ribbon of the utility model one embodiment revolves mixing apparatus (configuration Vibration eliminating structure before) is always schemed.

Fig. 2 is the sectional structural map of the conical twin-screw mixer drive disk assembly of the utility model one embodiment.

Fig. 3 be in Fig. 1 reductor to connecting in cylindrical shell and the structural map of engagement.

Fig. 4 is the partial enlarged drawing of curved boom middle gear engagement in Fig. 1.

Fig. 5 is the partial enlarged drawing at outlet valve device 22 places in Fig. 1.

Fig. 6 is pivoted arm structure enlarged drawing in Fig. 1.

Fig. 7 is the enlarged drawing of top cover flat head structure in Fig. 1.

Fig. 8 is force analysis figure when long helical axis reverses in Fig. 1.

Fig. 9 is long helical axis time rotational bottom axle head force analysis figure in Fig. 1.

Figure 10 is the stressed analysis chart of material when long helical axis revolves round the sun in Fig. 1.

Figure 11 is that in Fig. 1, long helical axis bottom axle head arranges the force analysis figure after Vibration eliminating structure.

Figure 12 is that the extruding force that the material of extruding produces is applied to the schematic diagram that cone produces counter-force P.

Figure 13 is the partial view that in Fig. 1, long helical axis bottom axle head arranges Vibration eliminating structure.

The specific embodiment

Below in conjunction with specific embodiments and the drawings, the utility model is described in further detail; set forth in the following description more details so that fully understand the utility model; but the utility model obviously can be implemented with the multiple alternate manner that is different from this description; those skilled in the art can do similar popularization, deduction according to practical situations without prejudice to the utility model intension in the situation that, therefore should be with the content constraints of this specific embodiment protection domain of the present utility model.

Fig. 1 to Fig. 7 has shown the structure of the taper mixing apparatus before additional Vibration eliminating structure; these and follow-up other accompanying drawing are all only as example; it is not to draw according to the condition of equal proportion, and should not using this as the protection domain of the utility model actual requirement is construed as limiting.

In Fig. 1, the cylindrical shell 19 of back taper and flat head top cover 12 weld together for overall structure, flat head top cover 12 center welding reinforcements, reducer pedestal adpting flange is welded on reinforcement and is connected with reductor 4, back taper cylindrical shell 19 outside weldings have 4 support ear framves 9 to be evenly distributed on back taper cylindrical shell 19 circumference, to strengthen connecting intensity and the static mixer of flat head top cover 12 and back taper cylindrical shell 19.Back taper cylindrical shell 19 lower part outlets carry out control appliance discharging by the switching of manual dislocation valve 21.The hollow shaft of reductor 4 connects with adpting flange 23, long curved boom 7 and short curved boom 11 are for integral solder structure and connect into an integral body with adpting flange 23, when revolution, by reductor 4, carry out input torque, by long curved boom 7 and short curved boom 11, drive long helical axis 13 and short helical axis 18 to complete revolution.Between a worm drive axle 8 and long helical axis 13 and short helical axis 18, adopt and be fixedly connected with, by power transmission shaft 10, be connected with bevel gear set moment of torsion is passed to long helical axis 13, long helical axis 13 adopts tubular structure with weight reduction and increases moment of flexure ability to bear, short helical axis 20 rotary tracks and long helical axis 13 are staggered in the design of short curved boom 11, short helical blade 18 diameters are larger than elongated helical blade 14 diameters, and the moment of by long curved boom 7 and short curved boom 11, biography being come during revolution causes long helical axis 13 and short helical axis 20 to do revolution motion at back taper cylindrical shell 19 centers.

In embodiment illustrated in fig. 2, the axis of rotation of reductor 4 connects a bevel gear by key and by being arranged on power transmission shaft 10 in long curved boom 7 and short curved boom 11 and one group of bevel gear engagement passes to a worm drive axle 8 moment, and a worm drive axle 8 again moment is delivered to long helical axis 13 and short helical axis 18 to realize rotation by flange.

The output of reductor 4 is comprised of hollow shaft output and axis of rotation output two parts, and sparge pipe 15 passes from the center of reductor 4 axis of rotation.The hollow shaft of reductor 4 is connected moment of torsion is passed to adpting flange 23 by key, by adpting flange 23, realize revolution, the axis of rotation of reductor 4 is also connected moment of torsion is passed to the gear on power transmission shaft 10 by key, moment of torsion is passed to long helical axis 13 in combination by one group of axle 10 and bevel gear and short helical axis 18 is realized rotation, makes overall structure seem compacter, reasonable.Swivel joint 1 is equipped with on the top of sparge pipe 15, can be connected with the pipeline of liquid-supplying system easily.

In the embodiment shown in fig. 4, the end of long curved boom 7 and short curved boom 11 is welded with respectively elbow welding, a worm drive axle 8 is fixed and carrys out transmitting torque by conical gear and key by the bearing in elbow, and a worm drive axle 8 is realized rotation with long helical axis 13 and short helical axis 11 by the fixed synchronous rotary that is connected of flange.

In the embodiment shown in fig. 5, dislocation valve 21 is connected by flange with back taper cylindrical shell 19, and the valve 21 that misplaces is plate armature, guarantees with elongated helical blade 14 nearestly, guarantees minimum residual material.During work, equipment guarantees the maximum ratio that is subject to mixed material in the combination of rotation and revolution, increases efficiency and the overall mixing of materials uniformity.

In the embodiment shown in fig. 6, double worm mixer top is flat head cap structure 12, a left side is charging aperture 6, the right side is for manhole 5 or be made as access hole, flat head top cover 12 center welding reinforcements, reductor adpting flange is welded on reinforcement and is connected by bolt with reductor 4 flanges, guarantees the intensity of whole transmission when load operation, because flat head top cover 12 is seldom cleared up also and facilitated for the structure event residual material with radian.Cleaning shower nozzle 17 can select gas or liquid to carry out the residual material on the long curved boom 7 of cleaning equipment or short curved boom 11, when reaching a fixed time cleaning shower nozzle 17, just can under the effect in power, rotate pressure, clean area is larger, and user also can be replaced by atomizer and uses when the needs spraying liquid cleaning shower nozzle 17.

In the embodiment shown in fig. 7, long curved boom 7, short curved boom 11 (containing the relatively-stationary elbow at two ends) are designed to cast member and welding formation box typed structure, all finishing steps are disposable to be completed, guaranteed the precision of transmission, well solved the Leakage of cone-type spiral mixing apparatus when heavy duty in the past, and residual material is still less cleared up also more convenient.

In the aforementioned embodiment, the direction of the spiral rotation of the blade of long helical axis 13 so that material from cone base to cone top, move as positive direction.The direction of spiral rotation is so that material moves from cone base to cone top as positive direction, and the helical axis of positive direction motion is subject to axial tensile force, the safe handling of equipment energy.If in the other direction equipment can not safe handling, as shown in Figure 8, because material promotes to cause extruding downwards, the extruding force producing when the material of extruding is applied to cone and produces counter-force P and act on helical axis.

Because semi-cone angle is α, when the extruding force producing is F, cone wall is F ' to the counter-force of axle.Generation is P to the axial compression power of axle.

${\stackrel{\→}{F}}^{\′}=\stackrel{\→}{P}+\stackrel{\→}{N}$

? $\stackrel{\→}{P}={\stackrel{\→}{F}}^{\′}-\stackrel{\→}{N}$

Cos17 °=0.9563F ' of P=F ' when α=17 °

When material is constantly piled up by helical feed, extruding force F ' constantly increases.Once extruding force F ', reach enough large, axle bending maybe may may be destroyed top cover, reductor, cause serious consequence.Therefore the reversion of equipment refusal increases safeguard measure and ensures safety in production in the electrical equipment control of equipment operating.In use procedure, there are indivedual enterprises in actual production, make because of carelessness the direction of spiral rotation for operating the phenomenon that has occurred that top cover arches upward in the other direction, cause device damage.

The utility model people finds that spiral different material in normal rotation and revolution situation there will be following phenomenon in practice.

For the solid particle material of harder, nonelastic distortion in mixing of materials stirs compared with smooth operation, vibration and the noise of generation are also very little.But lower to some intensity, more crisp broken solid material can be squeezed into powder.Cause the waste of material and the energy.

2. for solid particle material softer, that there is strain, in stirring, mixing of materials there will be the phenomenons such as vibration, heating.Revolution there will be pulsed stepping phenomenon in process of production.Moment before and after pulsed stepping phenomenon occurs that curent change is larger.Large electric current can may exceed normal range (NR), and the energy consumption of equipment strengthens.May cause motor overload and burn out.Therefore, some enterprises even suspect that power of motor is inadequate, require expanded motor power to meet the running of equipment.Pulsed stepping vibration effect the safe operation of equipment.Vibration strengthens noise, has polluted environment around.Heating can make material melting or rotten.

When material is in mixing process, material is risen progressively with spiral, and is centrifuged and dishes out.Aloft by gravity, declined.The stressing conditions of the material of the bottom axle head of the long helical axis that positive direction is rotated, as shown in Figure 9, in Fig. 9, only considers the stressing conditions of long helical axis time rotational material, in Figure 10, and the stressing conditions of material while considering the rotation simultaneously of long helical axis and revolution.

In bottom, because Yu Zhui bottom surface, axle head bottom surface becomes, α's tiltedly carve angle, when mix and blend, material is rubbed, squeezed, can produce certain axial compression power, because the rotation of spiral produces material, rub, squeeze power P, can be decomposed into the inclined-plane thrust N of material and the pressure F to material and baseplane ".Add the thrust F of the revolution of spiral to material ₂".What tiltedly carve o place, be Q with joint efforts.

In Fig. 9, $\stackrel{\→}{P}={\stackrel{\→}{F}}^{\′\′}-\stackrel{\→}{N}$

In Figure 10, $\stackrel{\→}{Q}=\stackrel{\→}{P}+{{\stackrel{\→}{F}}_2}^{\′\′}$

When the characteristic of material is harder solid material, material is not yielding.Tiltedly carving the generation of o place, to the Q that makes a concerted effort of material, be the power that variation is very little.So smooth operation, vibration and the noise of generation are also very little.But active force certainly exists.Lower to some intensity, more crisp broken solid material can be squeezed into powder.Cause the waste of material and the energy.

Just much complicated for the softer/solid particle material situation with strain.Now, produce the Q that makes a concerted effort of material will be rubbed material, be squeezed tiltedly carving o place, make material produce distortion, heating.Form deformation energy and heat energy and be stored in material body.Along with whipping process constantly continues, material is proceeded to rub, squeezed, make material produce larger distortion.Deformation energy gathers in a large number in material body.After acquiring a certain degree, material deformation force surpasses the Q that makes a concerted effort.Now rubbed, crowded material can flared, sudden outburst deformation energy.Return to original state.Being rubbed, crowded material can flared, while discharging deformation energy, will produce vibration.The mix and blend of material is carrying out continuously, rubbing, squeezing and be out of shape, gather deformation energy, finally outburst release deformation energy due to material.As periodically constantly circulating.Therefore there will be in process of production pulsed stepping phenomenon.In this process, current strength also there will be periodically constantly circulation change.When deformation energy gathers in a large number in material body, current strength is also assembled increasing, and energy consumption is increased.When current strength that What is more surpasses the rated value of motor, motor is burnt out.Vibration can make the noise of surrounding environment increase.Owing to material being proceeded to rub, squeezing, make material produce more heat energy, gathering of heat energy raises temperature of charge.When being low-melting-point material, material melting conglomeration.Destroyed the production technology of mixing of materials.What is more, and at high temperature the change of material properties, causes quality accident.

Based on the aforementioned force analysis to material characteristic and material, it seems, the centrostigma of contradiction is Yu Zhui bottom surface, the axle head bottom surface Cheng Xieqie angle α of long helical axis.As long as the existence of axle head bottom surface, Yu Zhui bottom surface, axle head bottom surface becomes the angle of tiltedly carving of α to certainly exist, as long as tiltedly carve angle α, exists, and, to the rubbing, squeeze of material, makes material produce distortion and also certainly exists.Therefore the angle of tiltedly carving of Yu Zhui bottom surface, axle head bottom surface one-tenth α is the breach of dealing with problems.

As shown in figure 13, the axle head at long helical axis increases a taper entity 30.

When the semi-cone angle of conical twin-screw mixer is α:

(1) when axle head bottom surface is plane, Yu Zhui bottom surface, axle head bottom surface Cheng Xieqie angle α;

(2) if axle head bottom surface is the conical surface while being β, the axle head conical surface and cone bottom surface bevel θ, angle θ and stressing conditions are as shown in figure 11.

θ＝(α+β/2)-90°。

When β=180 °,

θ＝α。

Be that axle head bottom surface is plane, Yu Zhui bottom surface, axle head bottom surface becomes that α's tiltedly carve angle.

When 90 ° of (alpha+beta/2) >, 0 ° of θ=(alpha+beta/2)-90 ° >.

θ angle more approaches α value, and the effect of P power is more obvious.

When (alpha+beta/2)=90 °, θ=(alpha+beta/2)-90 °=0 °.

Now P power is vertically bored bottom surface.The plane contacting with material is a cambered surface, and P power is to the effect of material not obvious (as shown in figure 11).

When 90 ° of (alpha+beta/2) <,

θ＝(α+β/2)-90°＜0°。

P power obviously declines to the effect of material, works hardly.

By the conical double ribbon of previous embodiment, revolving mixing apparatus is example, its semi-cone angle α=17 °.

17°+β/2＜90°

β＜(90°-17°)×2＝146°。

So the cone angle beta of getting in the embodiment shown in fig. 13 taper entity 30 is 100 ° of left and right.

Like this, mixer is when work, double helix is along wall and the revolution of middle anchor ring, cause material to move in a circle along cone wall, spiral rotation makes material rise along helicoid from boring the end, the public affairs of spiral, rotation compound motion are inhaled in the face of cylinder of spiral a part of material, are subject to again the effect of spiral rotation centrifugal force simultaneously, make the flow direction of material cone center at cone wall place.Thereby several motion stack circulations of material in cylindrical shell have formed complicated vortex movement and have produced convection current, shearing, diffusion like this, cause the convection current between each component of material in whole mixing apparatus to mix.The original feature that has kept double worm mixer.

Embodiment shown in Figure 13, has solved the problem of vibration, and has had following beneficial effect:

1. improvement expenses is few, only changes the bottom flush end plate of former long helical axis into taper entity, and operation principle etc. all do not change;

2. reduce inner useless energy loss (as the fragmentation of material, heating etc.);

3. in producing, material heating minimizing has guaranteed hybrid technique, has guaranteed product quality;

4. rotation is steady with revolution, and operating current is stable, without peak point current, occurs, reduced power consumption, motor is not fragile;

5. do not produce the stepping phenomenon in revolution, reduced the vibration in equipment operating process, eliminated unsafe factor, particularly revolve round the sun reductor and revoluting motor are effectively protected, and have increased service life;

6. the noise being caused by vibration is effectively controlled, and has improved working environment.

7. with conical twin-screw mixer, carrying out, in the similar technique of mixed material production, effectively solving the problems such as stepping and vibration.

In Figure 13, the conehead of taper entity 30 is clipped, but the part of being clipped is very little, to such an extent as to taper entity 30 plays in fact the effect of a complete cone.Taper entity 30 can be cone or multiaspect cone, and its bus can be the sweep of straight line or certain radian, as long as can prevent that power P from progressively increasing with the rotation of helical axis according to aforementioned principles.Taper entity 30 can also be hollow, can be also solid.Taper entity 30 can add and form on the basis of original helical axis, can be also integrally formed with helical axis.The cone angle of taper entity 30 can also be not limited to one, can also be a plurality of.The axis conllinear of taper entity 30 center and long helical axis 13 or have certain departing to be all fine.As shown in figure 10, taper entity 30 Main Functions are to become an angle that is less than 90 degree-α by its conical surface with the axis of helical axis 13, make the directed force F to material " be no longer parallel to the axis of helical axis 13; from this angle; the bottom axle head that taper entity 30 can replace with long helical axis 13 protrudes any entity arranging, as long as the outer surface angle of this entity and the axis of helical axis 13 become an angle that is less than 90 degree-α.

Although previous embodiment be take conical double ribbon and is revolved mixing apparatus as example, but the Vibration eliminating structure of previous embodiment can have been used long helical axis for other, (it approaches the bottom of conical shell as far as possible, so that bottom material also can be mixed) mixing apparatus, single-screw mixing apparatus for example, triple helical mixing apparatus.

Although the utility model with preferred embodiment openly as above, it is not for limiting the utility model, and any those skilled in the art, within not departing from spirit and scope of the present utility model, can make possible change and modification.Therefore, every content that does not depart from technical solutions of the utility model, any modification, equivalent variations and the modification above embodiment done according to technical spirit of the present utility model, within all falling into the protection domain that the utility model claim defines.

Claims (9)

1. the Vibration eliminating structure of a taper mixing apparatus, the cone angle of this taper mixing apparatus is α, the axial entity that protrudes the convergent arranging in axle head bottom that it is characterized in that being included in long helical axis, in an angle, this angle is less than 90 degree-α for the outer surface of this entity and the axis of long helical axis.

2. Vibration eliminating structure as claimed in claim 1, is characterized in that, described entity is taper entity.

3. Vibration eliminating structure as claimed in claim 1, is characterized in that, α is 17 degree.

4. taper mixing apparatus, comprises long helical axis, it is characterized in that, comprises the Vibration eliminating structure as described in claims 1 to 3.

5. taper mixing apparatus as claimed in claim 4, is characterized in that, this taper mixing apparatus is that conical double ribbon revolves mixing apparatus.

6. taper mixing apparatus as claimed in claim 5, it is characterized in that, this taper mixing apparatus comprises cylindrical shell, reductor, long curved boom, short curved boom, described long helical axis and short helical axis, cylindrical shell is shaped as back taper, the top cover on cylindrical shell top is that flat head and cylindrical shell are integral type structure, cylindrical shell top arranges charging aperture and access hole, reductor is installed in the middle of top cover, cylindrical shell center arranges liquid-jet device, reductor has hollow shaft and the axis of rotation, hollow shaft connects long curved boom by adpting flange, short curved boom, rotating long helical axis is installed in the elbow of the end of long curved boom, rotating short helical axis is installed in the elbow of short curved boom end, the axis of rotation by bevel gear set respectively with length, power transmission shaft in short curved boom connects, power transmission shaft in long curved boom connects by bevel gear set the worm drive axle that long helical axis connects again, power transmission shaft in short curved boom connects by bevel gear set the worm drive axle that short helical axis connects again.

7. taper mixing apparatus as claimed in claim 6, is characterized in that, long curved boom is monoblock cast structure.

8. taper mixing apparatus as claimed in claim 6, is characterized in that, the axis of rotation of reductor is hollow-core construction and forms hydrojet passage, so that spray enters cylindrical shell to the liquid of material from reductor top, from shower nozzle, sprays.

9. taper mixing apparatus as claimed in claim 6, is characterized in that, the blade of short helical axis is larger than the blade diameter of long helical axis, and long and short helical axis is all hollow shaft thin-wall construction.