Abstract:
A method for stringing a first elongate element through a second elongate element is provided by placing the first elongate element in a channel and injecting compressed gas into the channel to propel the first elongate element therethrough. The channel has a first open end, and the second elongate element is sealed around the first open end. Compressed gas is injected into the channel towards the second elongate element, propelling the first elongate element through the second elongate element. Also disclosed is a system for performing such a method, including a source of compressed gas and a housing having a channel with a first end and a second open end. The first end is in fluid communication with the source of compressed gas. The channel has a tapered portion adjacent the open end of the channel, and the channel defines a straight longitudinal axis between the first end and the second open end.

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 12/025,358, filed Feb. 4, 2008, now U.S. Pat. No. 7,891,086, which is a continuation of U.S. application Ser. No. 11/172,282, filed Jun. 30, 2005, now U.S. Pat. No. 7,350,291, both of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to methods for assembling electrical cables. More specifically, the invention relates to devices and methods for stringing electrical cable through a tubular sheath. 
     BACKGROUND 
     Drawn brazed strand (DBS) is a type of cable characterized by good strength, stress resistance and conductivity properties that is frequently used in applications where cable failure is highly undesirable. One example is in the field of implantable medical devices, such as pacemakers, where the repair or replacement of electrical cables in the leads would require invasive surgery. 
     DBS typically includes a conductive element encased in a protective sheath. The conductive element is formed of a number of conductive strands twisted together. Each strand is formed from a plurality of individual alloy wires woven or wrapped about a core wire. The core wire is generally soft but highly conductive, and is usually made of silver, while the alloy wires are less conductive but stronger. The sheath is typically formed of a non-conductive material such as silicone or polyurethane. The sheath increases cable strength and also provides a protective electrical and environmental barrier around the conductive element. 
     Once the wires are formed into strands and the strands are twisted into the cable, the conductive element is inserted, or stringed, through one end of the sheath. Prior to assembling the conductive element with the sheath, a lubricant such as alcohol is injected into the sheath. The alcohol chemically interacts with the interior silicone wall of the tubing to provide a more lubricious surface. The conductive element is then pushed into and through the tube from one end. 
     This process has many drawbacks. First, despite lubricating the interior of the sheath, the conductive element has a tendency to become kinked within the sheath. Kinking degrades the conductive properties and strength of the cable such that kinked units are usually discarded. Second, alcohol is highly combustible and emits noxious fumes and odors bothersome to operators. Sometimes it is necessary to provide a venting system to maintain adequate air quality and additional fire control precautions must be employed. Third, residual alcohol must be removed from the stringed cable before further processing can be carried out. This is typically accomplished by placing the stringed cable into a furnace or near some other source of heat to evaporate the alcohol. Finally, the alcohol supply may become contaminated. Contamination can affect the lubricity between the conductive element and the sheath, and may cause particulates to be deposited within the sheath after the alcohol is evaporated. 
     Therefore, there exists a need for an improved method of stringing cables such as DBS type cable. There is a further need for a method that does not require the use of alcohol. 
     SUMMARY 
     In one embodiment, the present invention is a method for manufacturing a cable of the type including a conductive element disposed inside a tubular sheath. The conductive element is placed in an open channel terminating at a first end and is withdrawn through the channel a pre-determined distance from the first end. The channel is closed and a first end of the sheath is sealed to the channel first end. Compressed gas is injected into the channel towards the first end such that the conductive element is propelled through the channel into the sheath. 
     In another embodiment, the present invention is a method of manufacturing cable of the type having a conductive element and a hollow tubular sheath. The conductive element is inserted into a needle and at least a portion of the needle is inserted into the sheath. The sheath is sealed to the needle. An air bearing is formed on an inner surface of the sheath and the conductive element is propelled through the needle, over the air bearing and into the sheath. 
     In another embodiment, the present invention is a system for advancing a conductive element through a hollow tubular sheath. The system includes a source of compressed gas, a vacuum pump and an openable housing having a channel extending therethrough. The channel has a first end in fluid communication with the compressed gas and the vacuum pump and a second end that is open. The system also includes a holding area adjacent the first end of the housing. The holding area is sized and shaped to receive at least a portion of a conductive element and in fluid communication with the vacuum pump. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cable stringing system in an open position according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of the cable stringing system of  FIG. 1  in which the housing is in a closed position. 
         FIG. 3  is a perspective view of the system of  FIG. 2  in which the clamp is in the operating position. 
         FIG. 4  is a top view of the system of  FIG. 3  loaded with a sheath and conductive element. 
         FIG. 5  shows a flowchart detailing a method of assembling the cable according to an embodiment of the present invention. 
         FIG. 6  is a detailed view of a portion of a cable stringing system in accordance with another embodiment of the present invention. 
     
    
    
     While the invention is amenable to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and are described in detail below. The intention, however, is not to limit the invention to the particular embodiments described. On the contrary, the invention is intended to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
       FIGS. 1-4  show a cable stringing system  100  for advancing a cable (e.g., a conductive element) through a sheath, in accordance with an embodiment of the present invention, during various stages of system operation. As shown in  FIG. 1 , the system  100  includes a housing  102  for holding the conductive element (see  FIG. 4 ), a clamp  104  for holding the sheath (see  FIG. 4 ) and a compressed gas source  106 . In one embodiment, the system  100  further includes a vacuum pump  107 . In one embodiment, the housing  102  and the clamp  104  are positioned on a platform  108  at a convenient height for operator manipulation. The compressed gas  106  and the vacuum pump  107  are located nearby and are in fluid communication with the housing  102 . Multiple cable stringing systems  100  may be connected to the compressed gas source  106  and the vacuum pump  107 . 
     The housing  102  includes an upper housing member  110  pivotally hinged to a stationary lower housing member  112  at a hinge member  114 . The upper housing  110  is pivotable from an open position, as is shown in  FIG. 1 , to a closed position, as is shown in  FIG. 2 . 
     Open upper and lower channels  116  and  118  are located on the upper and lower housing members  110  and  112 , respectively. The lower channel  118  extends through a rear portion  119  of the lower housing member  112  (shown in dashed lines). In the closed position, the upper open channel  116  is aligned to the lower channel  118  to define a conductive element channel  120  extending through the housing  102  (See  FIG. 2  in dashed lines). A resilient seal member  121  is disposed alongside the upper channel  116  to seal the upper channel  116  to the lower channel  118  when the upper housing member  110  is in the closed position. The rear portion  119  of the lower housing member  112  has an angled upper surface  119   a  that is complementary to a lower surface  110   a  of the upper housing member  110 . When the upper housing member  110  is in the closed position, the surface  119   a  and  110   a  abut one another to seal the upper and lower channel  116  and  118  adjacent the rear portion  119 . 
     Both of the upper and lower channels  116  and  118  taper into upper and lower needle portions  122  and  124 , respectively. The upper and lower needle portions  122  and  124  form a hollow needle  125  protruding from the housing  102  when the upper housing member  110  is in the closed position. Each of the needle portions  122  and  124  forms approximately half of the circumference of the needle  125 . However, the upper needle portion  122  and lower needle portion  124  are slightly oversized such that when the upper housing member  110  is in the closed position, the upper needle portion  122  presses tightly against the lower needle portion  124  to form an air tight seal. 
     The upper and lower housing members  110  and  112  include locking pin receivers  126   a  and  126   b , respectively, that are aligned to one another when the upper housing member  110  is in the closed position. The rearwardly located locking pin receiver  126   b  is slightly larger than the forwardly located locking pin receiver  126   a . A locking pin  128  is insertable into the aligned locking pin receivers  126   a  and  126   b  to lock the upper housing member  110  to the lower housing member  112  in the closed position (See  FIG. 2 ). The locking pin  128  is cone-shaped and is sized relative to the locking pin receivers  126   a  and  126   b  to compress the upper housing member  110  and the lower housing member  112  together when engaged. The force exerted by the locking pin  128  is sufficient to cause the seal  121  around the channel  120  to be air tight, as well as to compress the upper and lower needle portions  122 ,  124  sufficiently to form an air tight seal. In one embodiment, as is shown in  FIGS. 1-3 , the locking pin  128  is engaged via a pneumatic actuator  130 . Other locking arrangements suitable for quickly and easily securing the upper housing member  110  to the lower housing member  112  are also contemplated. 
     The clamp  104 , shown in the lower, right quadrant of  FIGS. 1-3 , is bifurcated into two members  132  and  134 . The clamp members  132  and  134  are movable from a separated, open position, as is shown in  FIGS. 1 and 2 , to a closed position in which the clamp members  132 ,  134  are drawn inwardly adjacent one another, as is shown in  FIG. 3 . Each clamp member  132 ,  134  includes an inwardly facing, elongated, semi-hemispherical recess  136 , which are adapted to couple to the sheath (see  FIG. 4 ). 
     A locking arrangement is provided for locking the clamp members  132 ,  134  to one another in the closed position and for fixing the clamp  104  in the operating position. The clamp members  132 ,  134  are also movable from a retracted position that is spaced apart from the housing  102 , as is shown in  FIG. 1 , to an advanced, operating position adjacent the housing  102 , as is shown in  FIG. 3 . 
     In the closed position, the recesses  136  are aligned with one another to define a tubular passageway  138  for receiving a portion of the sheath  160 . The recesses  136 , in one embodiment, are sized such that a circumference of the passageway  138  is only slightly larger than a circumference of the needle  125  when the clamp members  132 ,  134  are moved into the closed position. The recesses  136  may have a length or depth of several centimeters to increase the surface area and frictional engagement between the sheath  160  and the clamp  104 . Furthermore, the clamp  104  may be provided with a non-skid coating or be formed with a surface texture at the recesses  136  that is adapted to increase frictional engagement between the sheath (see  FIG. 4 ) and the clamp  104 . In the operating position, the clamp  104  is positioned such that the needle  125  is inserted into the passageway  138 . 
     In one embodiment, a guide  140  extends in a lateral direction over the platform  108  for accommodating opening and closing movement of the clamp  104  and for guiding the clamp  104  towards the housing  102 . The clamp portions  132 ,  134  are movably coupled to the guide  140  and each clamp portion  132 ,  134  includes a guide recess  142  for capturing the guide  140 . In other embodiments, the platform  108  may include rails, tracks, grooves, rollers or other means for guiding the movement of the clamp members  132 ,  134  between the retracted and operating positions and between the open and closed positions. In still other embodiments, the clamp  104  is movably suspended above the housing  102 . 
     In the present embodiment, the retracted position of the clamp  104  is spaced apart from the housing  102  along a longitudinal axis a aligned with the channel  120  and parallel to the plane of the platform  108 . However, in other embodiments the clamp  104  is movable along other axes or even within other planes. For example, in other embodiments, the clamp  104  is lowered from a position above the housing  102  into the operating position. Likewise, in the present embodiment, the clamp members  132  and  134  are movable along an axis b perpendicular to the axis a within the plane of the platform  108  between the open position and the closed position. In other embodiments, however, the clamp members  132 ,  134  are movable from the open position to the closed position along other axes or even within other planes. For example, in other embodiments, the clamp portions  132  and  134  are raised and lowered between the open and closed positions. Furthermore, while in the present embodiment both of the clamp members  132 ,  134  move approximately equal distances from their respective open positions to the closed position, as is shown in  FIGS. 1-3 , in other embodiments one of the clamp members  132 ,  134  moves from an open position to a closed position while the other is stationary, or their relative movements are otherwise unequal. 
     Movement of the housing  102  and clamp  104  into respective closed positions and into the operating position may be automated, manual, or power-assisted, or any combination thereof. 
     The compressed air  106  and vacuum pump  107  are both in fluid communication with the housing  102  via a fluid or gas line  144 . The gas line  144 , in one embodiment, is detachably couplable to the housing  102  via a quick-connect adaptor  146 . The adaptor  146  is positioned at a rearward end  148  of the lower channel  118 . The adaptor  146  is preferably configured to both direct compressed air  106  and draw a vacuum via the vacuum pump  107  parallel to or in line with the longitudinal axis a of the channel  120 . As is shown in  FIGS. 1-3 , a portion  150  of the gas line  144  immediately adjacent the housing  102  is straight or slightly arcuate. In one embodiment, the portion  150  of the gas line  144  has a length of up to about 40 inches. In another embodiment, the portion  150  of the gas line  144  has a length of about the length of the conductive element  164 . The portion  150  of the gas line  144  serves as a holding area for holding all or a portion of the conductive element  164  without deforming the conductive element  164 . 
     The system  100  further includes a sensor  152  operationally coupled to the vacuum pump  107 . The sensor  152  is located in the rearward end  148  of the lower channel  118  within the lower housing member  112 . The sensor  152  is configured to sense the presence of the conductive element  164  when loaded into the lower channel  118 . The sensor  152  provides a signal to either or both of the vacuum pump  107  and compressed gas source  106  indicating the presence or absence of the conductive element  164  in the lower channel  118  and may further provide a signal indicating the position of the conductive element  164  relative to a reference features, such as an end of the channel  120 , the needle  125  or the adaptor  146 . This signal may be used to control at least a part of the operation of either or both of the vacuum pump  107  and compressed gas source  106 . 
       FIG. 4  shows the system  100  in an intended operating position. As shown in  FIG. 4 , a tubular sheath  160  is coupled to the clamp  104  between the clamp members  132  and  134 , and a cable or conductive element  164  is pre-loaded into the gas line  144 . As further shown, a distal end of the sheath  160  is positioned against the housing  102  and over the tip of the needle  125 . 
       FIG. 5  is a flowchart illustrating a method  200  of stringing or inserting the conductive element  164  through the tubular sheath  160  with the system  100 , according to one embodiment of the present invention. A first end of the conductive element  164  is placed in the lower channel  118  and inserted into the rearward end  148  of the lower channel  118  (block  202 ). The sensor  152  senses that the conductive element  164  is in the lower channel  118  and communicates with the vacuum pump  107  (block  204 ). In one exemplary embodiment, upon receiving input from the sensor  152  that the conductive element  164  is loaded into the lower channel  118 , the vacuum pump  107  exerts negative pressure sufficient to withdraw the conductive element  164  from the housing  102  into the portion  150  of the gas line  144  immediately adjacent the housing  102  (block  206 ). 
     The strength and duration of the vacuum exerted by the vacuum pump  107  is preferably pre-determined or calculated to bring the conductive element  164  to a particular position within the gas line  144  relative to a reference feature, such as the needle  125 . In this manner, regardless of the length of the conductive element  164 , or how far the operator manually inserts the conductive element  164  into the lower channel portion  118 , the conductive element  164  is moved into a consistent position relative to the needle  125  for stringing into the sheath  160 . In one embodiment, the vacuum pump  107  operates until the sensor  152  indicates that the conductive element  164  is no longer positioned in the channel portion  118 . 
     In other embodiments, the vacuum pump  107  is not included. Rather, either the operator is responsible for consistently positioning the conductive element  164  within housing  102  or the system  100  is provided with additional sensors to determine when the conductive element  164  is fully stringed through the sheath  160 . 
     The upper housing member  110  is pivoted downward into the closed position (block  208 ) and secured with the locking pin  128  and locking pin receivers  126   a  and  126   b , forming the sealed channel  120  (block  210 ). To load the sheath  160  into the clamp  104 , the operator manually places an end  158  of the sheath  160  over the needle  125  (block  212 ) and brings the clamp portions  132 ,  134  forward to the operating position on either side of the needle  125  (block  214 ). The first and second portions  138  and  140  are moved into the closed position to clamp the sheath  160  into position over the needle  125 , as is shown in  FIG. 4  (block  216 ). As stated above, the passageway  138  is only slightly larger than the needle  125  to facilitate forming a seal over the needle  125 . 
     After the conductive element  164  and the sheath  160  have been loaded into the housing  102  and clamp  104 , the compressed air  106  is released or injected into the channel  120  (block  218 ), propelling the conductive element  164  through the needle  125  and into the sheath  160  (block  220 ). The force at which the compressed air  106  is released as well as the duration is calculated to advance the conductive element  164  a pre-determined distance into the sheath  160 . Typically, the conductive element  164  is stringed all the way through to an opposite end of the sheath  160 . According to one embodiment, the system  100  is configured to string a conductive element  164  having a length of up to about 40 inches through a sheath  160  having a length of up to about 40 inches. In other embodiments, the system  100  is configured to string longer or shorter lengths of conductive element  164  and sheath  160 . 
     It may be necessary to adjust the position of the conductive element  164  with respect to the sheath  160  the initial stringing process described above. More compressed air  106  may be injected into the sheath  160  to “nudge” the conductive element  164  forward. Once the conductive element  164  is in a satisfactory position within the sheath  160 , the compressed air  106  is de-activated and the clamp portions  132  and  134  are opened, releasing the assembled sheath  160  and conductive element  164 . Alternately, so as to withdraw or back out the conductive element  164  from the sheath  160 , the partially stringed sheath  160  and conductive element  164  are released from the clamp portions  132  and  134  a re-assembled or reloaded into the tool  100  in the reverse direction. The opposite end of the sheath  160  is inserted over the needle  125  and the compressed air  106  is activated to propel the conductive element  164  in the opposite direction in as the initial stringing process. 
     The above-described process may be partially automated, in which the operator merely loads the conductive element  164  into the lower channel  118  and places the sheath  160  over the needle  120  as described. Alternately, the operator can also be responsible for opening and closing the housing  102  and clamp  104  and for engaging the various locking mechanisms. The amount of the time the compressed gas  106  and vacuum pump  107  are activated may be automated or subject to the controls of additional sensors, or may be engaged and disengaged under operator control. Various additional safety features can also be employed to prevent injury to the operator. For example, sensors may be employed to allow the compressed air  106  to engage only when either or both of the housing  102  and clamp  104  are in closed positions. 
     The force exerted by the compressed gas  106  traveling through the sheath  160  radially expands the sheath  160 , increasing the ease with which the conductive element  164  is propelled through the sheath  160 . However, injection pressure in excess of about 110 psi may cause the sheath to over-expand and rupture. Generally, the mechanical properties and characteristics of the sheath  160  material will determine the maximum injection pressure and the minimum injection pressure necessary to sufficiently radially expand the sheath  160 . For example, if the sheath  160  is constructed of a more rigid material, such as polyurethane, a higher injection pressure may be necessary to expand the sheath  160  to a chosen radius. Furthermore, the differential between the inner diameter of the sheath and the outer diameter of the conductive element will also impact the pressure necessary to string the conductive element. 
     The compressed gas  106  is preferably released or injected into the channel  120  at a pressure of from about 90 to about 110 psi. Peripheral fixtures, such as the gas line  144 , adaptor  146  and other such features between the compressed gas  106  and the channel  120  reduce the actual injection pressure. Therefore, the compressed gas  106  is maintained at a sufficiently elevated pressure to achieve the necessary actual injection pressure. Alternately, a pressure booster as is known in the art may be employed with a lower pressure compressed gas  106  to increase the actual injection pressure to adequate levels (not shown). According to one embodiment, the source of compressed gas  106  is maintained under a pressure of about 60 psi and is employed in conjunction with a pressure booster to approximately double the pressure of the compressed gas  106  to 120 psi. 
     The following is merely one example of system settings for stringing a conductive element through a sheath. For a conductive element having a diameter of approximately 0.200″+/−0.0015″ and a length of approximately 40″ and a sheath having an interior diameter of approximately 0.022″+/−0.001″ and a wall thickness of approximately 0.008″+/−0.001″, approximately 106 to approximately 120 psi of compressed air is applied for 3 to 10 seconds to fully string the conductive element. 
     In one embodiment, the compressed gas  106  is injected into the channel  120  in pulses. The pulses serve to increase the propellant force and reduce the likelihood of the conductive element  164  becoming kinked within the sheath  160 . However, the compressed gas  106  may be injected into the channel  120  in any other pattern or at a constant rate of flow. Pulsing or other variations in injection of the compressed gas  106  may be automated or may be accomplished by manually engaging and disengaging the compressed gas  106 . 
     The gas flow creates an air bearing between the interior of the sheath  160  and the conductive element  164 . The air bearing serves to reduce friction between an inner surface  161  of the sheath  160  and the conductive element  164 , further facilitating the insertion of the conductive element  164  through the sheath  160  (See  FIG. 4 ). 
     Any type of gas may be employed to propel the conductive element  164  through the sheath  160 . According to one embodiment, either of air or nitrogen is employed. Both air and nitrogen are inexpensive, commonly available gases relatively safe for use under pressure. 
       FIG. 6  shows a portion of a device  300  according to another embodiment of the present invention. The device  300  includes a housing  302  and a clamp  304  similar to the embodiment shown generally in  FIGS. 1-3 , and like parts are given like numbering. According to the present embodiment, however, a plurality of upper and lower needle portions  360   a  and  360   b  extend from the upper and lower channels  316  and  318 , respectively. When the upper housing member  310  is in the closed position, the upper and lower needle portions  316 ,  318  form a plurality of needles arranged for insertion into a sheath  160  divided into multiple inner lumens. According to various embodiments, the device  300  includes 2, 3 or 4 sets of needle portions  316 ,  318  for stringing  2 ,  3  or  4  lumens within a single sheath  160  simultaneously. 
     In order to ensure that each conductive element advances through separate needles, the conductive elements are not fully withdrawn into the gas line. Rather, a forward end of the conductive elements is positioned in the needle and the upper housing is closed. The pre-loaded conductive element is then stringed through the individual lumens of the sheath. 
     In the embodiment of  FIG. 6 , the needle portions  316 ,  318  are permanently affixed to the housing  302 . In other embodiments, however, all or some of the needle portions  316 ,  318  are detachable from the housing  102  individually or as a unit. This allows interchangeability of variously arranged needle units, increasing the versatility of device  300 . 
     While the present invention is described generally in terms of manufacturing DBS cable, the methods and devices of the present invention are suitable for any number of applications. For example, the present invention may be used, but is not limited, for stringing non-DBS cables, coil cables, stylets and plastic beats. 
     Various modifications and additions can be made to the exemplary embodiments discussed without departing from the scope of the present invention. Accordingly, the scope of the present invention is intended to embrace all such alternatives, modifications, and variations as fall within the scope of the claims, together with all equivalents thereof.