DE202017100169U1 - Crumple element for a safety steering column - Google Patents

Abstract

Crumple element (1) for a safety steering column (10), with axially end-side first and second connecting portions (2, 3) for connection to adjacent steering column parts (16, 17), and an interposed broken crush section (4), which by an axially acting swelling force (F) is deformable, wherein the crushing element (1) is made in one piece by additive manufacturing of metal.

Description

The invention relates to a crush element for a safety steering column.

Safety steering columns are used in motor vehicles to increase the driver's safety during head-on collisions. The safety steering column is intended to prevent parts of the steering column and in particular the steering wheel from being pushed into the driver's compartment. To realize this, different, usually passive, mechanisms are known. One approach is to form the steering column as a telescopic steering column, in which a part of the steering column engages in another part of the steering column and telescopically inserted into these when exceeding a predetermined threshold force, thereby reducing the overall length of the steering column. The two parts can, for example, be connected to one another via a connecting element which is designed such that it yields when the threshold force is exceeded.

Alternatively or in addition to this, crushing elements can be provided which are deformed by a force which is above a threshold value and compressed in the longitudinal direction of the steering column. As a result, in addition to an effective shortening of the steering column and an energy absorption is achieved. While a corresponding crush element should yield reliably when exposed to a sufficiently strong axial force, a transmission of torque within the steering column must be ensured at the same time. For this, either the crushing element itself must be sufficiently torsionally rigid with respect to torques, or the torque transmission must take place via other components.

Modern safety steering columns are functionally effective, but often complex and consist of a plurality of cooperating parts. This increases the cost and time required for the construction. In addition, the high number of parts and the complexity of a possible repair of the steering column is expensive and expensive.

The EP 0 091 671 A2 discloses a safety steering column having a rigid steering column section and a compliant upon collapse of the steering column, a grid structure having pipe part. The pipe part is formed integrally with the steering column section made of fiber-reinforced plastic. The grid structure may in particular be formed by webs which cross each other and extend at an angle of 55 ° to the longitudinal axis of the tubular part.

In the US 4,465,301 A discloses a safety steering column, in which an approximately tubular security element is inserted, which consists of intersecting strands of fiber-reinforced plastic and, for example, may have a round or octagonal cross-section. In the event of an accident, it is envisaged that the safety element will be compressed longitudinally while resisting torque during steering movements.

The GB 1 125 206 A shows a safety steering column, in which a total tube-like security element is provided with a perforated structure made of metal. Here, either helically opposite wound metal strips are welded together or it is a tube made of a perforated plate. A similar safety steering column is in the US 3,500,698 A shown.

The US 4,634,399 A discloses a component for transmitting torques between two shafts, for example within a steering column. In this case, a grid-like jacket is arranged at a distance around a continuous core. The jacket is formed of cross-strands of fiber reinforced plastic.

In the WO 2015/053940 A1 is disclosed a component that can be used, for example, in aircraft or motor vehicles. In order to achieve a reduction of stresses and an improved energy absorption, it is provided that the component is manufactured in an additive manufacturing process with a broken internal structure. This inner structure is made in one piece with the outer parts of the component.

The WO 2015/164663 A1 discloses an energy absorbing cell having a first structural member and a second structural member spaced therefrom and aligned in parallel. The structural elements are connected by intermediate elements, which are arranged at an angle to each other. The energy absorption is based primarily on a deformation of the intermediate elements. By repeating this cell structure, energy absorbing parts of different sizes can be produced, in particular by additive manufacturing processes.

Furthermore, various safety steering columns are known in the prior art, which are based essentially in the manner of a telescopic steering column on two telescopically interlocking steering column parts, wherein the telescoping of the two parts is a deformation of an energy absorbing element.

That's how it shows US 3,401,576 A a safety steering column with two telescopically interlocking Parts which are generally enclosed by a metal sleeve, which is shaped like a wave tube and is compressed in an accident while absorbing energy.

In the US 3,461,740 A a safety steering column is described with a coupling arrangement in which a handlebar remote first part engages with a frusto-conical end in a handlebar-side second part. If the axial force between the two parts exceeds a certain threshold value, the first part is pressed into it with deformation of the second part, whereby the steering column is shortened.

The US 3,590,655 A discloses a safety steering column with different energy absorption units. In a first energy absorption unit, a plurality of balls are arranged between two concentrically arranged cylindrical elements. In a second energy absorption unit, a cylindrical member has a lattice-like section constructed of intersecting strip portions.

The US 3,740,068 A shows a safety steering column, in which also two parts telescopically interlock, wherein an externally arranged energy absorbing element is constructed of partially perforated, angled sheet metal parts.

One in the US 3,835,725 A disclosed safety steering column comprises an upper and a lower portion, which telescopically engage and are interconnected via a concentrically arranged metal tube. If an axial force between the two sections exceeds a limit value, the metal tube is compressed under deformation and the sections are pushed into each other.

The US 4,411,167 A shows a safety steering column, in which two parts of the steering column are coupled to each other via a plurality of elements, which partially mesh telescopically and are interconnected by plastic rods. When a certain axial force is exceeded, the respective plastic rod breaks and the elements are pushed into each other. Finally, even the complete separation of the two parts from each other is possible.

At one in the US Pat. No. 7,644,951 B2 shown safety steering column engages an inner tube part in an outer tube part. The inner tube part has an expanded portion at an axial distance from the outer tube part. Between this section and the outer tube part, the inner tube part is surrounded by a plastic tube, which is deformed when an axial force exceeds a threshold, wherein the inner tube part is pushed into the outer tube part.

The US 5,342,091 A shows a safety steering column, wherein a solid inner part engages in a tubular outer part. Through a transverse bore of the inner part of a pin is pushed, which projects beyond both sides and a tubular energy absorbing element is supported. When the inner member is inserted into the outer member, the energy absorbing member comes into contact with the outer member and is thereby stopped, and the pin is displaced on the inside thereof with deformation of the energy absorbing member.

In view of the state of the art, the provision of a reliable, structurally simple safety steering column still offers room for improvement.

The invention has for its object to provide a reliable, structurally simple safety steering column available. In particular, the number of items should be kept as low as possible.

According to the invention the object is achieved by a crushing element for a safety steering column with the features of claim 1, wherein the dependent claims relate to advantageous embodiments of the invention.

It should be noted that the features listed in the following description as well as measures in any technically meaningful way can be combined with each other and show other embodiments of the invention. The description additionally characterizes and specifies the invention, in particular in connection with the figures.

The invention provides a crush element for a safety steering column. Instead of a crushing element could also speak of a crash element or security element. Of course, such a safety steering column is normally provided in a motor vehicle, in particular a passenger car. As already discussed above, should be prevented by the safety steering column that are pressed in a head-on collision parts of the steering column or the steering wheel in the driver's compartment.

The crushing element has axially end-side first and second connecting sections for connection to adjoining steering column parts, as well as a perforated crushing section arranged therebetween. The axial direction corresponds in this case in the assembled state Extension direction of the steering column, at least in the section in which the crush element is installed. By the axial direction and the below-mentioned radial and tangential direction are fixed. The crushing element may be formed in sections at least approximately symmetrical to the axial direction. As a rule, it has its greatest extent along the axial direction or is elongated in the axial direction. The first and the second connecting portion are used for connection to other parts of the steering column, which connect in the assembled state in the axial direction on both sides. A corresponding compound can, for. B. are made by screwing, including the corresponding connection portion may have an internal or external thread and approach surfaces for a wrench or other tool. In general, at least one connecting section may have means for positive connection with an adjacent steering column part.

While the connection sections serve for connection to other parts, the crumple section located between the connection sections serves in an accident to contribute to the shortening of the steering column on the one hand and to absorb energy by deformation on the other hand. For this reason, the crushing portion is deformable by an axially acting threshold force. The threshold force is in this case an axially acting force corresponding to a prescribed threshold, which is selected so that it is not exceeded during normal operation of the vehicle, but only in a head-on collision, which affects the steering column. If an axially acting force exceeds this threshold value, a deformation of the crushing section takes place. Of course, this deformation includes compressing the crushing portion in the axial direction. This is realized in that the crushing section is broken through, thus having a perforated structure with recesses. Through the recesses, the rigidity of the crumple section in the axial direction is reduced so that it yields according to plan when the threshold force is reached or exceeded. In contrast, the connection sections are normally predominantly or completely resistant to deformation with respect to the threshold force. D. H. they are not (or not in relevant way) deformed when the threshold force is exceeded.

According to the invention, the crushable element is manufactured in one piece from metal by additive manufacturing. The term "metal" also includes alloys which contain not only metals but also semimetals or non-metals. In the context of additive manufacturing, metallic powder is usually applied layer by layer to a base surface, partially melted and thereby connected, whereby the crumple element is gradually formed. The layer thickness can be, for example, between 10 .mu.m and 500 .mu.m. The first layer is applied directly to the base surface, after which the further layers are successively applied one above the other. In particular, the joining of the powder can be effected by selective laser melting (SLM) or selective electron beam melting (SEBM). Of course, the melting or the radiation takes place according to a specific pattern. It could also be said that a predetermined area is heated or irradiated. It is possible, for example, that the area is scanned by a narrowly focused beam or that a specific radiation pattern is projected at once. It is understood that the spatial radiation pattern may be controlled in accordance with predetermined data (e.g., CAM data) of the deformable element to be fabricated. The irradiated surface corresponds to a (usually flat) cross section of the object.

With such an additive manufacturing, almost any three-dimensional shapes can be generated, so that it is possible to use the three o.g. To manufacture sections in one piece, each of the sections can be optimized in terms of its function (reliable connection to other steering column parts or energy absorption) and at the same time ensures a reliable connection of the sections with each other due to the one-piece production, which corresponds to a prototypes. As will be explained in more detail, advantageous shapes can also be produced here without further ado which, for example, can not be produced by casting or (machining or non-cutting) machining or only with disproportionately great effort. The crushing element can be easily integrated into the steering column by connecting the connection sections with other parts, for example by screwing. Likewise, after an accident in which the crumple section has been deformed, the crumple element can be replaced without problems, since it is only a single component. Basically, the entire safety function of the steering column can be realized by the crushable element according to the invention, whereby the total number of parts required can be kept low. However, other parts of the steering column can also contribute to the safety function.

In addition to a compression of the crumple section in the axial direction, it can be advantageously provided that the crumple section bends transversely to the axial direction or bends. As a result, parts of the steering column can escape laterally, which under safety aspects may have advantages over a simple compression. Such a kinking can in particular by the design of the region in which the crumple section merges into one of the connection sections are initiated or influenced. According to a preferred embodiment, an end region of the first connection section facing the crush section has a first section which projects axially in relation to a second section lying transversely to the axial direction. The end region could also be referred to as the transition region from the first connection section to the crush section. In any case, this is the area in which the connecting area ends and where the crumple section adjoins. This end region is designed asymmetrically, with the first section projecting axially (and the crush section correspondingly axially receding there), in comparison with the second section, which lies opposite the first section transversely to the axial direction. Insofar as an axially extending central axis of the first connection section or of the entire deformation element can be defined, the first and the second section lie opposite one another with respect to this central axis. The result of this embodiment is that, when the crumple section is axially compressed, the part associated with the first section is fully compressed earlier than the part associated with the second section. That is, the compression and thus the effective restoring force through the crush section are asymmetric. This, in turn, causes the crush section to bend or bend toward the side of the second section.

The above-described projection of the first portion can be realized in different ways, for example. Step-shaped. That is, either only the two o.g. Sections or additional intermediate sections projecting stepwise different distances in the axial direction. In accordance with another preferred embodiment, the end section has an end face extending obliquely to the axial direction towards the crushing section. Again, one could speak of a transition surface instead of an end face, which marks the transition from the first connection section to the crumple section. Ie. at least parts of the end surface do not correspond to a physical surface, since there immediately and integrally adjoins the crushing section. Said end surface runs obliquely to the axial direction (that is, neither parallel nor orthogonal thereto), which automatically results as described above, an asymmetrically projecting first section. The end surface may in this case be designed in particular as a plane, although it is conceivable that it is curved or angled in itself.

In order to effectively initiate the above-described kinking of the crushing portion, the angle to the axial direction should not be too large (that is, not too close to 90 °), since in this case the first region protrudes only slightly. Even too small an angle is generally not advantageous. According to an advantageous embodiment, the end face extends at an angle between 20 ° and 70 °, preferably between 40 ° and 50 °, to the axial direction. In particular, the angle in about 45 °, ie between 43 ° and 47 ° or between 44 ° and 46 ° preferably be exactly 45 °.

As already described above, due to the additive manufacturing, the individual sections can be configured independently of one another in almost any desired manner. In one embodiment, a radial outer dimension of at least one of the terminal portions is greater than a radial outer dimension of the crush portion. In this case, the term "radial outer dimension" in each case means the maximum extent in the radial direction. In the case of a cylindrical element, this would be, for example, the outer radius. It could also be said that the respective connection section protrudes radially beyond the crush section or protrudes with respect thereto. Such a dimensioning can, for example, serve to make the connection section more stable than the crushing section. It can also be prevented in this way that the crumple section, when it is severely deformed, moves laterally (ie transversely to the axial direction) past the respective connection section. This could be disadvantageous in that then the further deformation of the crumple section would only be difficult to control through the connection sections.

Normally, it is preferred that the connection sections are designed to be as stable as possible and, to this extent, in contrast to the crumple section, they also do not have to be broken through. However, each of the terminal portions may have a radially inner recess which serves to receive a portion of another steering column part. In this case, the interior of the recess could have means for a positive connection, for example an internal thread. Apart from this, it is preferred that at least one of the connection sections has a tangentially and axially closed jacket. The jacket may at least partially have the shape of a cylinder jacket, wherein, as mentioned above, flat approach surfaces for a wrench or the like may be provided.

The crumple section preferably has a lattice-like sheath which surrounds a radially inwardly lying, axially continuous recess. The said recess passes through the crush section, but not necessarily through the adjacent connection sections. If an above-mentioned recess of a Terminal portion is formed continuously, however, it is connected to the recess of the crushing portion. In particular, the jacket can be designed as a cylinder jacket, with which it can be described by an inner and outer cylindrical surface. The jacket is grid-like, that is, it has a plurality of continuous through holes in the radial direction.

According to one embodiment, the jacket has a plurality of obliquely extending to the tangential direction, crossing strip portions. Of course, recesses are formed between the strip sections, d. H. the strip sections are partially spaced from each other. They run obliquely to the tangential direction, d. H. its course also has an axial component. In particular, the strip portions may be helical, with a first group of straight-sided strip sections extending along the shell and a second group forming intersection regions with the first group extending in a left-handed direction. Under certain circumstances, the metal in the intersection regions may not be unambiguously assigned to a specific strip section, provided that two strip sections there seamlessly merge into one another due to the one-piece production. Depending on the view, it is also possible to speak of a correspondingly larger number of shorter strip sections, which in each case run only from one crossing area to the next, instead of several times circumferential strip sections which intersect each other. Overall, a lattice-like structure results, which is torsionally rigid with respect to axial torques (in which case the vector of the torque is axially aligned) behaves, but is reliably compressible at an axially acting threshold force. Even a kinking described above can be realized with this structure.

The angle of the strip sections to the tangential direction can in principle assume different values and can be, for example, between 5 ° and 80 °, wherein the angle need not be the same for all strip sections. In particular, at least a portion of the strip portions may extend at an angle of at most 20 ° to the tangential direction. Related to a helical course, this corresponds to a relatively small slope. As a result, on the one hand, the torsional stiffness is increased compared to axial torques, on the other hand, the crushing portion in the axial direction is easier to compress. Nevertheless, this can achieve a bending stiffness that is low enough to allow the above-mentioned bending.

From the aspect of increased torsional stiffness, it is also preferred that at least a portion of the strip portions have a radial thickness greater than an axial thickness. D. h., The corresponding strip portions are formed thicker in the radial direction than in the axial direction. So you can call the corresponding strip sections also as more or less flattened. One could also speak of a rib-like structure of the strip sections. It is understood that this can increase the torsional rigidity without increasing the stiffness against axial forces in a similar manner. Also, the flexural rigidity can be kept low enough to allow the Knautschabschnitts to bend. It should be noted that this configuration, in particular in connection with a small angle of the strip sections described above with respect to the tangential direction, can hardly be realized in one piece without additive manufacturing.

Further advantageous details and effects of the invention are explained in more detail below with reference to an embodiment shown in the figures. Show it:

1 a perspective view of a safety steering column with a crushable element according to the invention;

2 a perspective view of the crumple from 1 ;

3 a sectional view of a part of the crumple from 2 ;

4 a sectional view corresponding to the line IV-IV in 2 ;

5 a sectional view corresponding to the line VV in 4 ; such as

6 a perspective view of the crumple from 1 - 5 in a deformed state.

In the different figures, the same parts are always provided with the same reference numerals, which is why these are usually described only once.

1 shows a perspective view of a safety steering column 10 for a car, as an angled steering column with two universal joints 11 . 12 is trained. With regard to these cardan joints 11 . 12 you can the safety steering column 10 in three steering column sections 13 . 14 . 15 which remain rigid during normal operation of the vehicle. In a first steering column section 13 which extends along an axial direction A is a crushing member 1 arranged to be upset on the one hand in a head-on collision and on the other hand kinked to thereby penetrate parts of the safety steering column 10 to prevent in a driver's compartment of the car. The upsetting and kinking is initiated when an axially acting threshold force F (more precisely, a pair of forces, shown in 2 . 5 and 6 ) on the crushing element 1 and thus on a crumple section 4 acts. The deformable element 1 is here with adjacent steering column parts 16 . 17 connected, preferably screwed.

2 shows the crushing element 1 individually in perspective. One can total a first connection section 2 , a second connection section 3 and the crumple portion disposed along the axial direction A therebetween 4 detect. The deformable element 1 with the three sections 2 . 3 . 4 is for example made in one piece from metal by selective laser melting (SLM). It is constructed predominantly symmetrically to an axially extending central axis M. The two connection sections 2 . 3 are largely massive and wise, as in the sectional view in 5 can be seen, only one inner recess in each case 2.2 . 3.2 on, which is provided with a (not shown) internal thread. The respective internal thread engages in the assembled state with corresponding external threads of the adjacent steering column parts 16 . 17 one. To facilitate screwing, are on a tangential and axially closed shell 2.1 of the first connection section 2 as well as on a likewise closed coat 3.1 of the second connection section 3 level approach surfaces 2.3 . 3.3 intended for a wrench. Apart from that, the coats are 2.1 . 3.1 shaped like a cylinder. While the connection sections 2 . 3 Thus, all in all predominantly closed and solid, is the intermediate crumple section 4 formed open. Its shape is roughly cylindrical, with its outer diameter is smaller than the two connection sections 2 . 3 , The crumple section 4 has a coat 4.1 on, the one inside, in the axial direction A continuous recess 4.2 surrounds.

The structure of the coat 4.1 which, in particular, in the detail view of 3 can be characterized as lattice-like. The coat 4.1 has a plurality of strip portions 4.3 . 4.4 on, which run in each case helically. A first group of strip sections 4.3 in this case runs with an angle of inclination of approximately + 15 ° with respect to the tangential direction (or 75 ° with respect to the axial direction A) in the right direction of rotation, while a second group of strip sections 4.4 with an inclination angle of about 15 ° with respect to the tangential direction with the left direction of rotation runs. The strip sections 4.3 . 4.4 of the two groups often intersect each other in intersection areas 4.5 and thus form a coherent, grid-like structure. In 3 are in the intersection areas 4.4 Dividing lines between those strip sections 4.3 . 4.4 drawn, but only to illustrate the course. Due to the one-piece production there is no physical separation.

Already the connection of the strip sections 4.3 . 4.4 contributes to the crumple section 4 relative to axial torques that arise during steering, is substantially torsionally rigid. This is further enhanced by the comparatively small angle of inclination with respect to the tangential direction, and by the fact that - as in 3 clearly visible - a radial thickness of each strip section 4.3 . 4.4 is significantly larger than an axial thickness. Due to the comparatively small axial thickness of the strip sections 4.3 . 4.4 as well as interspaces between them 4.6 can the crumple section 4 be compressed by the axially acting threshold force F, which is exceeded in a frontal collision.

In addition to the axial compression buckling of the crushing section 4 to reach, has the crumple section 4 facing end region 2.4 of the first connection section 2 an oblique to the axial direction A extending end surface 2.5 on. In the present example, the end surface runs 2.5 at an angle of 45 ° to the axial direction A. Thus, the end region 2.4 a first section 2.6 on that in relation to a second section 2.7 projecting axially, the first section 2.6 transverse to the axial direction A opposite. One can also speak of the two sections 2.6 . 2.7 lie opposite each other with respect to the central axis M.

In the case of an axially acting force F is a compression of the crushing section 4 initially approximately symmetrical, but from a certain point, the compression approaches on the side of the first section 2.6 a possible maximum value while doing so on the part of the second section 2.7 not yet the case. As a result of the resulting asymmetric restoring forces results in cooperation with the oblique end face 2.5 a bending moment on the crumple section 4 leading to a kinking towards the second section 2.7 leads. A corresponding state is in 6 shown. From the synopsis with 1 becomes clear that this not only the crushing element 1 but also the adjacent steering column parts 16 . 17 laterally, so can dodge transversely to the axial direction A. This puts in addition to the compression of the crumple section 4 that effectively shorten the first steering column section 11 causes an additional mechanism, with the penetration of the safety steering column 10 can be prevented in the driver's compartment.

A like in 6 deformed crushing element 1 can be easily solved by loosening the screw with those steering column parts 16 . 17 be replaced.

LIST OF REFERENCE NUMBERS

1

 deformable element

2, 3

 connecting section

2.1, 3.1, 4.1

 coat

2.2, 3.2, 4.2

 recess

2.3, 3.3

 approach surface

2.4

 end

2.5

 end face

2.6

 first area

2.7

 second area

4

 crash portion

4.3, 4.4

 strip section

4.5

 crossing area

4.6

 gap

10

 safety steering column

11, 12

 universal joint

13, 14, 15

 steering column section

16, 17

 steering column part

A

axial direction

M

 central axis

F

 threshold force

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0091671 A2 [0005]
• US 4465301 A [0006]
• GB 1125206 A [0007]
• US 3500698A [0007]
• US 4634399 A [0008]
• WO 2015/053940 A1 [0009]
• WO 2015/164663 A1 [0010]
• US 3401576A [0012]
• US 3461740 A [0013]
• US 3590655 A [0014]
• US 3740068 A [0015]
• US 3835725A [0016]
• US 4411167 A [0017]
• US Pat. No. 7,644,951 B2 [0018]
• US 5342091 A [0019]

Claims (10)

Crushing element ( 1 ) for a safety steering column ( 10 ), with axially end-side first and second connecting sections ( 2 . 3 ) for connection to adjacent steering column parts ( 16 . 17 ), as well as a perforated crumple section arranged therebetween ( 4 ), which is deformable by an axially acting threshold force (F), wherein the deformable element ( 1 ) is manufactured in one piece from metal by additive manufacturing. Crumple element according to claim 1, characterized in that a crumple section ( 4 ) facing end region ( 2.4 ) of the first connection section ( 2 ) a first section ( 2.6 ) in relation to a transverse to the axial direction (A) opposite the second section ( 2.7 ) protrudes axially. Crumple element according to claim 1 or 2, characterized in that an end section ( 2.4 ) to the crumple section ( 4 ) an obliquely to the axial direction (A) extending end surface ( 2.5 ) having. Crumple element according to claim 3, characterized in that the end surface ( 2.5 ) at an angle between 20 ° and 70 °, preferably between 40 ° and 50 °, to the axial direction (A). Crumple element according to one of the preceding claims, characterized in that a radial outer dimension of at least one of the connecting sections ( 2 . 3 ) is greater than a radial outer dimension of the crumple section ( 4 ). Crushing element according to one of the preceding claims, characterized in that at least one of the connecting sections ( 2 . 3 ) a tangentially and axially closed shell ( 2.1 . 3.1 ) having. Crumple element according to one of the preceding claims, characterized in that the crumple section ( 4 ) a grid-like jacket ( 4.1 ), which has a radially inner, axially continuous recess ( 4.2 ) surrounds. Crumple element according to claim 7, characterized in that the jacket ( 4.1 ) of the crumple section ( 4 ) a plurality of obliquely to the tangential direction extending, crossing strip portions ( 4.3 . 4.4 ) having. Crumple element according to claim 8, characterized in that at least a part of the strip sections ( 4.3 . 4.4 ) at an angle of at most 20 ° to the tangential direction. Crumple element according to claim 8 or 9, characterized in that in at least a part of the strip sections ( 4.3 . 4.4 ) a radial thickness is greater than an axial thickness.

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