Patent Publication Number: US-2022235682-A1

Title: Methods of attaching a flat layer to a hub of an axial flow element

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/850,774, entitled “Methods of Attaching a Flat Layer to a Hub of an Axial Flow Element” and filed May 21, 2019, the entire disclosure of which is hereby incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to automated manufacturing techniques. 
     BACKGROUND 
     In various applications, it is generally desirable to minimize human interaction during a manufacturing process. Among other benefits, minimizing human interaction can reduce manufacturing costs and increase product throughput. However, many tasks that are simple for a human to perform are difficult to automate. For example, winding operations are particularly challenging to automate using existing manufacturing techniques. In a winding operation, a material is wrapped multiple times around a central mandrel (e.g., a bobbin, spool, reel, etc.). At the beginning of a winding operation, a leading edge of the material must be pressed against or otherwise secured to an outer surface of the mandrel in order to initiate the winding process. This operation can be time consuming when implemented manually by an operator of the winding system. Moreover, adhesives and other bonding agents that may be used to secure the material to the central mandrel are messy and may need time to cure before reaching full bond strength. 
     SUMMARY 
     In one set of embodiments, an axial flow element includes a hub, a groove, a locking member, and a flat layer. The hub includes a cylindrical outer surface. The groove is disposed in the outer surface. The groove extends in a substantially longitudinal direction along the hub from a first end of the hub to a second end of the hub. The locking member is disposed in the groove. The flat layer is disposed between the hub and the locking member. 
     In another set of embodiments, a hub includes a body, a groove, and a plurality of crush ribs. The body includes a cylindrical outer surface. The groove is disposed in the outer surface. The groove extends in a substantially longitudinal direction along the body 
     In another set of embodiments, a method of making an axial flow element includes providing a hub. The hub includes a groove disposed on an outer surface of the hub. The groove extends in a substantially longitudinal direction along the hub. The method also includes providing a flat layer and providing a locking member. The method further includes positioning the flat layer above the groove, positioning the locking member above the groove and the flat layer, and pressing the locking member into the groove. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the disclosure will become apparent from the description, the drawings, and the claims, in which: 
         FIG. 1  is a perspective view of a first example axial flow element; 
         FIG. 2  is a top view of a second example axial flow element; 
         FIG. 3  is a perspective view of a hub of the axial flow element of  FIG. 1 ; 
         FIG. 4  is a perspective view of a groove of the hub of  FIG. 3 ; 
         FIG. 5  is a perspective view of an example locking member; 
         FIG. 6  is a top view of a locking member disposed in the groove of  FIG. 4 ; 
         FIG. 7  is a perspective view of the axial flow element of  FIG. 1  near a groove; 
         FIG. 8  is a perspective view of an example hub for an axial flow element; 
         FIG. 9  is a flow diagram of a method of making an axial flow element; 
         FIG. 10  is a perspective view of a third example axial flow element; 
         FIG. 11  is a perspective view of a hub for the axial flow element of  FIG. 10 ; 
         FIG. 12  is a perspective view of the hub of  FIG. 11  near a catch member on the hub; 
         FIG. 13  is a perspective view of the axial flow element of  FIG. 10  near a catch member; 
         FIG. 14  is a perspective view of a fourth example axial flow element; 
         FIG. 15  is a perspective view of a hub of the axial flow element of  FIG. 14 ; 
         FIG. 16  is a perspective view of the axial flow element of  FIG. 14  during a heat staking operation; 
         FIG. 17  is a perspective view of the axial flow element of  FIG. 14  near a groove; 
         FIG. 18  is a perspective view of a fifth example axial flow element; 
         FIG. 19  is a perspective view of an inner surface of the axial flow element of  FIG. 18 ; 
         FIG. 20  is a side view of an example speed rivet; 
         FIG. 21  is a side cross-sectional view of the speed rivet of  FIG. 20 ; 
         FIG. 22  is a side view of another example speed rivet; 
         FIG. 23  is a perspective view of a sixth example axial flow element; 
         FIG. 24  is a perspective view of another example hub for an axial flow element; 
         FIG. 25  is a top view of the axial flow element of  FIG. 23  at the beginning of a winding operation; 
         FIG. 26  is a perspective view of another example hub for an axial flow element; 
         FIG. 27  is a perspective view of yet another example hub for an axial flow element; and 
         FIG. 28  is a perspective view of still another example hub for an axial flow element. 
     
    
    
     It will be recognized that some or all of the figures are schematic representations for purposes of illustration. The figures are provided for the purpose of illustrating one or more implementations with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
     DETAILED DESCRIPTION 
     Following below are more detailed descriptions of various concepts related to, and implementations of, methods, apparatuses, and systems for making an axial flow element for a liquid separation system. The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     I. Overview 
     In a winding operation, a material is wrapped in tension around a central mandrel to form a roll. The material may be a flat layer, sheet, or film that is applied to the central mandrel. Winding operations may be automated using a winding system that rotates the central mandrel while (at the same time) supporting the flat layer to maintain the material in tension during winding. At the beginning of the winding operation, the flat layer must be attached to the central mandrel. This starts the winding process and allows for the transmission of torque to the roll in order to achieve the proper tension in the flat layer. In certain applications, the amount of tension applied to the roll can be very important. For example, in winding systems used to manufacture axial flow elements for liquid separation systems, the amount of tension applied to the roll during the winding operation is an important factor in achieving the desired structural and dimensional requirements. To address these issues, winding systems may utilize adhesives or other bonding agents to form a connection between a leading ledge of the flat layer and the central mandrel. Alternatively, a leader strip may be used, which is pre-applied to the central mandrel before the winding operation. The leader strip may be easier to access by press tooling or other components of the winding system at the beginning of the winding operation. However, using a leader strip adds another step to the manufacturing process that must be manually performed. 
     Implementations herein relate to methods and systems for reliably securing the flat layer to the central mandrel at the beginning of a winding operation. Specifically, the methods disclosed herein facilitate automated winding of an axial flow element for a liquid separation system. The axial flow element includes a central mandrel or hub having a generally cylindrical outer surface and a flat layer disposed thereon. In one aspect, a pin-in-groove method is provided in order to connect the flat layer to the hub at the start of the winding operation. The method includes positioning the flat layer above a groove in the hub, positioning a locking member above the groove and the flat layer, and pressing the locking member against the flat layer and into the groove. The groove may be disposed in an outer surface of the hub. The groove may extend in a substantially longitudinal direction between a first end of the hub and a second end of the hub in order to secure the flat layer in tension along an entire width of the flat layer (e.g., a width of the flat layer in a direction parallel to a central axis of the hub). Among other benefits, the pin-in-groove method may be easily implemented in an automated manufacturing system; for example, by using a press tool to press the locking member into the hub, without any interaction from an operator or another user. 
     I. Example Axial Flow Element 
       FIG. 1  is a perspective view of a first example axial flow element, shown as element  100 . The element  100  is used in a liquid separation system for an internal combustion engine. The liquid separation system may be a crankcase ventilation system for a diesel engine that uses diesel fuel to drive the combustion process or another internal combustion engine system using a different type of fuel source. The crankcase ventilation system is used to separate combustion products (e.g., oil, soot, etc.) from blow-by gases that are introduced into the engine housing (e.g., crankcase). In the example embodiment of  FIG. 1 , the element  100  forms part of a rotating crankcase ventilation system which rotates the element  100  to improve liquid separation performance. The rotating crankcase ventilation system may be a hydraulically-driven rotating crankcase ventilation system (HRCV) (that utilizes hydraulic fluid or oil to rotate the element  100 ) or an electrically-driven rotating crankcase ventilation system (ERCV) (that utilizes an electrical motor to rotate the element  100 ). 
     As shown in  FIG. 1 , the element  100  includes a hub  102  (e.g., body, etc.), a flat layer  104 , and a locking member  106 . The locking member  106  is disposed within a groove  108  on an outer surface  110  of the hub  102  and secures the flat layer  104  in position relative to hub  102 . The flat layer  104  is “sandwiched” or otherwise disposed between the locking member  106  and the groove  108 . A leading edge  112  of the flat layer  104  is coupled to the hub  102  at a single circumferential position along the hub  102 . In other example embodiments, the flat layer  104  may be coupled to the hub  102  at a plurality of different positions along the outer surface  110 . 
       FIG. 2  is a perspective view of a second example axial flow element, shown as element  200 . Several components of the element  200  of  FIG. 2  are similar to the components of the element  100  of  FIG. 1 . To that extent, similar components have been labeled with similar reference numbers. As shown in  FIG. 2 , the element  200  includes a hub  202  and three grooves  108  (not shown) disposed on an outer surface  210  of the hub  202 . The element  200  also includes three locking members  106 , each locking member  106  disposed in a respective one of the grooves  108 . The grooves  108  (and the locking members  106 ) are spaced approximately equally along an outer perimeter of the hub  202  (e.g., circumferentially along the outer surface  210 , at equal angular intervals along the outer surface  210 , etc.). In the example embodiment of  FIG. 2 , an angle  105  between each of the locking members  106  is approximately 120°. Among other benefits, using a plurality of locking members  106  to attach the flat layer  104  to the hub  102  increases the torque that may be applied to the hub  202  (and the tension applied to the flat layer  104 ) during a winding process. Moreover, because the locking members  106  are equally spaced along the outer perimeter of the hub  202 , the hub  202  remains rotationally balanced. 
       FIG. 3  is a perspective view of the hub  102  of  FIG. 1  in isolation from the flat layer  104  and the locking member  106 . As shown, the element  100  includes an opening  113  disposed centrally on the hub  102  and extending in a longitudinal direction, parallel to a central axis  114  of the hub  102 . The opening  113  is structured to receive a rod, post, or other support member therein in order to couple the hub  102  to a winding system. In this way, the winding system may exert a torque on the hub  102  during the winding operation. 
     As shown in  FIG. 3 , the groove  108  extends in a substantially longitudinal direction along the hub  102 , parallel to the central axis  114  of the hub  102 , from a first end  116  of the hub  102  to a second end  118  of the hub  102 . The groove  108  is integrally formed into the outer surface  110  of the hub  102  as a single unitary structure. In some example embodiments, the hub  102  is formed from a plastic material such as polypropylene, nylon, glass reinforced nylon, or another suitable plastic via an injection molding process. In other example embodiments, the groove  108  may be machined or otherwise formed into the outer surface of the hub  102 . 
       FIG. 4  is a perspective view of the groove  108  near the first end  116  of the hub  102 . The groove  108  is sized and shaped to receive the locking member  106  (see also  FIG. 1 ) therein. In the example embodiment of  FIG. 4 , the groove  108  is a recessed area within the outer surface  110 . The groove  108  includes a lower wall  120  and two side walls  122  in substantially perpendicular orientation relative to the lower wall  120 . Together, the lower wall  120  and the side walls  122  define a generally C-shaped channel. The hub  102  additionally includes a plurality of crush ribs  124  disposed within the groove  108  and extending along a length of the groove  108  in the longitudinal direction (e.g., parallel to the central axis  114  of the hub  102 ). The crush ribs  124  extend outwardly from the side wall  122  in a substantially perpendicular orientation relative to the side wall  122 . In the example embodiment of  FIG. 4 , the crush ribs  124  are integrally formed with the hub  102  (e.g., into the side wall  122 ) as a single unitary structure. The crush ribs  124  are spaced at approximately equal intervals along the length of the groove  108  in the longitudinal direction. In other example embodiments, the spacing between the crush ribs  124  may be different and/or non-uniform along the length of the groove  108 . 
       FIG. 5  is a perspective view of the locking member  106 . In the example embodiment of  FIG. 5 , the locking member  106  comprises a pin  126  and/or wire having a generally circular cross-section normal to a central axis through the pin  126 . The pin  126  may include a magnetic material (e.g., iron) structured to engage with a permanent magnet or an electromagnet in a press tool (e.g., a press tool in a winding system for the element  100 ). In some example embodiments, the pin  126  is an inexpensive copper coated weld wire or steel wire pin. In other embodiments, the locking member  106  may be shaped differently. As shown in  FIG. 1 , the locking member  106  is sized and shaped to nestably mate with the groove  108  in the hub  102 . A length of the locking member  106  in the longitudinal direction is approximately the same as the length of the groove  108  in the hub  102  such that the tension applied to the flat layer  104  is distributed evenly along the length of the flat layer  104  (e.g., a length of the flat layer  104  that is oriented perpendicular to a winding direction for the hub  102 ). 
     The crush ribs  124  are structured to engage with the locking member  106  (e.g., indirectly via the flat layer  104 ) and to secure the locking member  106  in position with respect to the hub  102  (e.g., within the groove  108 ) during a winding process.  FIG. 6  is a top view of the element  100  isolated from the flat layer  104  (see also  FIG. 1 ). As shown, the crush ribs  124  are arranged in diametrically opposed pairs along the length of the groove  108 . In other embodiments, the crush ribs  124  may be staggered such that the crush ribs  124  on either side of the groove  108  are located at different longitudinal positions along the length of the groove  108 . 
     The crush ribs  124  are structured to deform in response to an applied force from the locking member  106 . A width  128  of the groove  108  between the diametrically opposed pair of crush ribs  124  (e.g., between an innermost edge of the crush ribs  124 ) is less than a diameter  129  of the locking member  106 . In the example embodiment of  FIG. 6 , each one of the crush ribs  124  includes a beveled portion  130  (e.g., a taper, etc.) proximate to an outer edge  132  of the groove  108 . Among other benefits, the beveled portion  130  provides a lead-in for the locking member  106  and facilitates positioning of the locking member  106  within the groove  108  by a winding system. As shown in  FIG. 6 , a depth  134  of the groove  108 , in a substantially radial direction (e.g., relative to the central axis  114  of the groove  108  as shown in  FIG. 3 ), from the lower wall  120  to the outer surface  110 , is approximately the same as the diameter  129  of the locking member  106 . In this way, the locking member  106  remains substantially flush with the outer surface  110 , which reduces the risk of bumps or protrusions where the flat layer  104  overlaps the locking member  106 . 
       FIG. 7  is a perspective view of the element  100  of  FIG. 1  in an area near the groove  108 . As shown, the flat layer  104  is “sandwiched” or otherwise disposed between the hub  102  (e.g., the groove  108 ) and the locking member  106 . The leading edge  112  of the flat layer  104  is secured in position relative to the groove  108  by a frictional force resulting from the contact force between the locking member  106  and the crush ribs  124 . In other words, the groove  108  is sized to receive the locking member  106  in an interference fit arrangement (e.g., press fit, force fit, etc.). In some example embodiments, the flat layer  104  may include or be made from a thin metal foil (e.g., aluminum, etc.). In alternative embodiments, the flat layer  104  may be directly coupled to the locking member  106 ; for example, via a welding operation, to secure the flat layer  104  in position relative to the hub  102 . 
       FIG. 8  is a perspective view of another example hub  302  for an axial flow element. The hub  302  includes a groove  108  and a balancing member  304  disposed on an opposite side the hub  302  as the groove  108  (e.g., an opposite side of an outer surface  310  of the hub  302 ). The balancing member  304  may be sized and shaped to counter balance (e.g., rotationally) the locking member (not shown). In the example embodiment of  FIG. 8 , the balancing member  304  is a solid piece of material that extends along the length of the hub  302 . In other embodiments, the balancing member may include a filler material embedded within the outer surface  310  of the hub  302  or another suitable balancing feature. 
     I. Example Manufacturing Method for the Example Axial Flow Element 
       FIG. 9  is a flow diagram of a method  400  of making an axial flow element. The axial flow element may be the same or similar to the axial flow element  100  of  FIG. 1  (or, alternatively, the axial flow element  200  of  FIG. 2 ). At  402 , a hub  102  is provided. Block  402  may include forming the hub  102  from a plastic material using an injection molding process. Alternatively, or in combination, block  402  may include forming a groove  108  into an outer surface  110  of the hub  102 . At  404  and  406 , both a flat layer  104  and a locking member  106  are provided. At  408 , the flat layer  104  is positioned above the groove  108  on the hub  102 . Block  408  may include inserting the hub  102  onto a mandrel, post, spool, bobbin, reel, or another suitable support for a winding system (e.g., via opening  113  as shown in  FIG. 1 ). Block  408  may additionally include advancing (e.g., feeding) the flat layer  104  (e.g., the leading edge  112  as shown in  FIG. 7 ) toward the hub  102 ; for example, by using a feeding system and a series of guides. Block  408  may further include aligning the flat layer  104  with a portion of the groove  108  and/or the outer surface  110  of the hub  102 , for example by advancing the flat layer  104  until the leading edge  112  of the flat layer  104  extends to just beyond a trailing edge of the groove  108 . 
     At  410 , the locking member  106  is positioned above the groove  108  and the flat layer  104 . Block  410  may include providing a press tool including a magnetic holder and engaging the magnetic holder with the locking member  106 . The magnetic holder may include a permanent magnet (e.g., a rare earth magnet) structured to magnetically couple the magnetic holder to the locking member  106 . Alternatively, the magnetic holder may include an electromagnet. Block  410  may include activating the electromagnet to magnetically couple the magnetic holder to the locking member  106 . A length of the magnetic holder may be approximately the same as the length of the locking member  106  in order to more accurately position the locking member  106  above the groove and to provide an equal force along the length of the locking member  106  during the assembly process. Block  410  may additionally include aligning the magnetic holder with the groove  108  (e.g., with a central axis passing through the groove  108 ). This may include determining an angular position of the groove  108  relative to the magnetic holder. The angular position of the groove  108  may be determined by using a sensor such as an optical sensor directed toward the hub  102 . 
     At  412 , the locking member  106  is pressed into the groove  108 . Block  412  may include extending the magnetic holder of the press tool toward the groove  108  and applying a radial force to the magnetic holder (and locking member  106 ) to press the locking member  106  into the groove  108 . The method  400  may additionally include retracting the magnetic holder away from the locking member  106 . In other example embodiments, the method may include additional, fewer, and/or different operations. 
     I. Additional Example Axial Flow Elements 
       FIG. 10  depicts a third example axial flow element, shown as element  500 . The element  500  includes a hub  502  and a flat layer  504  coupled thereto. The hub  502  includes a plurality of catches  536  (e.g., catch lugs, etc.) structured to secure a flat layer  504  in position along an outer surface  510  of the hub  502 .  FIG. 11  is a perspective view of the hub  502  isolated from the flat layer  504  (see also  FIG. 12 ). As shown, the catches  536  extend along the length of the hub  502  in a longitudinal direction. The catches  536  are spaced at approximately equal intervals along an outer surface  510  of the hub  502  between a first end  516  of the hub  502  and a second end  518  of the hub  502 .  FIG. 12  is a perspective view of the hub  502  near one of the catches  536 . The catches  536  extend substantially radially outward from the outer surface  510  of the hub  502 . Each one of the catches  536  includes a step portion  538  and a transition portion  540 . An inner surface  542  of the step portion  538  forms an acute angle with the outer surface  510 . In other embodiments, the inner surface  542  of the step portion  538  may be substantially perpendicular to the outer surface  510 . Among other benefits, engaging the flat layer  504  with the angled surface reduces the risk of the flat layer  504  detaching from the catch  536 . As shown in  FIG. 12 , the transition portion  540  is disposed on an opposite end of the catch  536  as the step portion  538  and sets a roll direction for the element  500  (e.g., ensures that the same winding direction is used for each element  500 ). 
       FIG. 13  is a perspective view of the element  500  that shows an area near the catch  536  at the beginning of a winding operation. As shown in  FIGS. 10 and 13 , the flat layer  504  includes a plurality of generally rectangular-shaped perforations  544  in the flat layer  504 . The perforations  544  may be die cut or otherwise formed into the flat layer  504  in a separate processing operation or at the beginning of the winding operation (e.g., by a laser, etc.). In the embodiment of  FIG. 13 , a flap  546  of excess material is included at each perforation  544 . As shown in  FIG. 13 , the catches  536  are received within the perforations  544 , and press upward against the flaps  546 . Among other benefits, the flaps  546  may provide a visual aid to assist an operator in identifying an engagement condition between the catches  536  and the perforations  544 . As the winding operation continues, each of the catches  536  are pushed against an edge of a corresponding one of the perforations  544 , thereby preventing separation of the flat layer  504  from the hub  502 . 
       FIG. 14  is a perspective view of a fourth example axial flow element, shown as element  600 . The element  600  includes a hub  602  and a flat layer  604  that is attached to the hub  602  using a heat staking operation. As shown in  FIG. 15 , the hub  602  includes a recessed area  648  including a plurality of extension pieces  650  in the form of cylindrical posts spaced approximately evenly along an outer surface  610  of the hub  602 . In other embodiment, the hub  602  does not include a recessed area  648  (e.g., the flat layer  604  may be affixed directly to an outer surface of the hub  602 , etc.). The extension pieces  650  are alignable with a plurality of openings  652  that are cut or otherwise formed into the flat layer  604 , proximate to a leading edge  612  of the flat layer  604 . The openings  652  are sized and shaped to receive the extension pieces  650  therein. In some example embodiments, the openings  652  are laser cut (e.g., by a laser cutting tool) into the flat layer  604  at the beginning of the winding operation. 
       FIGS. 16 and 17  depict the element  600  during the heat staking operation and after the heat staking operation, respectively. As shown in  FIG. 16 , once the flat layer  604  is positioned within the recessed area  648  (and/or an area along the outer surface of the hub  602  at which the extension pieces  650  are located), a first press tool at elevated temperature is brought into contact with the extension pieces  650  (see also  FIG. 16 ). The first press tool  651  melts or otherwise deforms the extension pieces  650  onto the flat layer  604 , thereby securing the flat layer  604  in position with respect the hub  602 . In other embodiments, the flat layer  604  is staked directly to the surface of the hub  602 , without the extension pieces  650 . 
     In some embodiments, the element  600  also includes other layers in addition to the flat layer  604  that are spirally wound onto the hub  602 . As shown in  FIG. 16 , the element  600  includes a second layer  654  that is affixed to an outer surface of the hub  602 , at a location that is opposite to where the hub  602  is connected to flat layer  604  (e.g., spaced approximately 180° from a position along the outer surface at which the flat layer  604  is staked to the hub  602 ). The second layer  654  may have different material properties from the flat layer  604 . In other embodiments, the second layer  654  may have similar material properties as the flat layer  604 . In one embodiment, the second layer  654  is a corrugated layer that includes corrugations  656  that are embossed or otherwise formed into the corrugated layer. As shown in  FIG. 16 , the second layer  654  is attached to the hub  602  at the same time as the flat layer  604 . The heat staking operation used to connect the second layer  654  to the hub  602  may be the same as or similar to the heat staking operation used to connect the flat layer  604  to the hub  602 . As shown in  FIG. 16 , the second layer  654  is positioned between the outer surface of the hub  602  and a second press tool  658 , which stakes the second layer  654  onto the hub  602  and/or deforms a second plurality of extension pieces (not shown) over the second layer  654 . 
       FIGS. 18 and 19  depict a fifth example axial flow element, shown as element  700 , in which a flat layer  704  is attached to a hub  702  using a plurality of fasteners  754 . The fasteners  754  may include bolts, screws, or anther suitable fastener. In the example embodiment of  FIGS. 18 and 19 , the fasteners  754  are rivets that extend through the flat layer  704  and an outer wall  756  of the hub  702 . In some example embodiments, the rivets are speed fastening rivets that are installed using a reusable mandrel (e.g., Neospeed® Speed Fastening® Rivets, etc.). An example speed rivet  758  is shown in  FIGS. 20 and 21 . The speed rivet  758  includes a plurality of ribs  760  extending radially outward from an outer surface of the speed rivet  758  in substantially parallel orientation with respect to a central axis of the speed rivet  758 . During installation, a reusable mandrel presses radially outward against an inner surface  762  of the speed rivet  758  to deform the ribs  760  and thereby secure the speed rivet  758  to the hub  702  and the flat layer  704  (as shown in  FIGS. 18-19 ). Among other benefits, using a speed rivet  758  to secure the flat layer  704  to the hub  702  eliminates metal debris and/or the risk of any metal pieces coming loose during installation.  FIG. 22  shows another example speed rivet  764 . A leading end  766  of the speed rivet  764  is tapered (e.g., pointed, etc.), which, advantageously, allows for installation of the speed rivet  764  into an unperforated flat layer. During installation, the leading end  766  punches a hole through the flat layer on an incoming stroke and secures the speed rivet  764  in position on an outgoing stroke. In other example embodiments, the shape of the leading end of the speed rivet may be different. For example, the leading end may include a tapered/sharp edge that extends along a perimeter of an internal opening of the speed rivet. The tapered/sharp edge punches through the flat layer, leaving behind a small piece of material from the flat layer, which may be discarded after joining the flat layer to the hub. 
       FIG. 23  depicts a sixth example axial flow element, shown as element  800 , in which a catch-and-fold technique is used to attach a leading edge  812  of a flat layer  804  to a hub  802 . As shown in  FIG. 24 , the hub  802  includes a lip  858  defining a slot. The slot is configured to receive the leading edge  812  of the flat layer  804  therein. In the example embodiment of  FIG. 24 , the lip  858  is an upper one of a plurality of lips that further includes a lower lip  862 . Together, the upper lip  858  and the lower lip  862  define a generally U-shaped slot  860 . The slot  860  may be sized and shaped to receive a folded portion of the flat layer  804  therein. Alternatively, as shown in  FIG. 25 , the flat layer  804  may be received in only an upper portion of the slot  860 . In  FIG. 25 , the winding operation begins by positioning the leading edge  812  of the flat layer  804  into the slot  860 . This operation may be performed by rotating the hub  802  in a clockwise direction as indicated by arrow  864 . Once the leading edge  812  is positioned within the slot  860 , further rotation folds the flat layer  804  over the upper lip  858 . The frictional force between the flat layer  804  and the upper lip  858  prevents the flat layer  804  from detaching or otherwise separating from the hub  802 . 
       FIG. 26  is a perspective view of another hub  902  for an axial flow element in which a welding tape  966  is used to secure a flat layer to the hub  902 . As shown, the welding tape  966  is applied over a small angular portion of the hub  802  in a longitudinal direction from a first end  916  of the hub  902  to a second end  918  of the hub  902 . In other embodiments, the welding tape  966  may cover a greater portion of an outer surface  910  of the hub  902 . For example,  FIGS. 27 and 28  are perspective views of example hubs, shown as a first hub  1002  and a second hub  1102 , including welding tapes (e.g., first welding tape  1066  and second welding tape  1166 ) that cover different portions the outer surfaces (e.g., first outer surface  1010  and second outer surface  1110 , respectively) of the first hub  1002  and the second hub  1102 . The welding tape may be a stainless steel or aluminum tape that is bonded to the hubs using adhesive product or another suitable bonding agent. 
     IV. Construction of Example Embodiments 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed but rather as descriptions of features specific to particular implementations. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can, in some cases, be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     As utilized herein, the terms “approximately,” “substantially” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims. 
     The terms “coupled,” “attached,” and the like, as used herein, mean the joining of two components directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two components or the two components and any additional intermediate components being integrally formed as a single unitary body with one another, with the two components, or with the two components and any additional intermediate components being attached to one another. 
     The term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list. Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, Z, X and Y, X and Z, Y and Z, or X, Y, and Z (i.e., any combination of X, Y, and Z). Thus, such conjunctive language is not generally intended to imply that certain embodiments require at least one of X, at least one of Y, and at least one of Z to each be present, unless otherwise indicated. 
     It is important to note that the construction and arrangement of the system shown in the various example implementations is illustrative only and not restrictive in character. All changes and modifications that come within the spirit and/or scope of the described implementations are desired to be protected. It should be understood that some features may not be necessary, and implementations lacking the various features may be contemplated as within the scope of the application, the scope being defined by the claims that follow. When the language a “portion” is used, the item can include a portion and/or the entire item unless specifically stated to the contrary.