Abstract:
A connector for connecting a pull string to a tube or cable for pulling the tube or cable through a duct. When an obstructing edge stops progress, the connector converts additional pull string tension into a rotation force that enables the connector header to pivot on the obstructing edge until it slips and passes by the obstructing edge.

Description:
BACKGROUND OF INVENTION 
     In zone climate control systems for residential forced air HVAC systems, airflow control valves are installed in air ducts. The airflow control valves can be pneumatically operated through small air tubes or powered and controlled by electrical signals through wires in a cable. When zone systems are installed in existing systems, it is often difficult to find a path for the air tubes or cable from an airflow control valve to the central controller because the ducts are behind walls and ceilings or in the attic or crawlspace. 
     For retrofit installations, the inside of the existing ducts can provide a path for the tube or cable. The tube or cable needs to be pulled from the airflow control valve through the air duct to a central location such as the discharge plenum of the HVAC blower. This is accomplished by connecting a strong installation blower to the duct system at the central blower plenum and blocking all airflow paths except one so the only airflow path is through the one unblocked air duct to the installation blower. The blower is connected so that air flows from the air duct toward the blower. A parachute about twice the diameter of the air duct is connected to a strong and flexible string and placed in the airflow. The airflow inflates the parachute and quickly pulls the parachute and string through the duct path to the installation blower. The string is then used to pull the tube or cable from the air vent to the central plenum. 
     The airflow control valves can be installed at the air vents where the air ducts terminate in a room. FIG. 1 illustrates three typical paths from air vents  102 ,  103 ,  104  through air ducts to the blower plenum  100 . Air ducts  101  can have a round or rectangular cross section and all cross section dimensions are greater than 3″. Duct paths can be over 100 feet long and have several sharp bends such as  114  and  113  where the ducts transition from horizontal to vertical and/or connect to main trunks. 
     The tubes or cables  110 ,  111 ,  112  are typically no more than ¼″ in diameter and sufficiently flexible and strong to be pulled through the duct path using the pull string. However, air ducts are often poorly installed and have sharp edges where ducts make turns and connect to trunks. 
     FIG. 2A illustrates the problem caused by the sharp edge  204  formed by duct  202  making an off-center connection with trunk  203 . The pull string  201  has a small diameter and is very flexible, so when pull string  201  pulls tube or cable  200 , the pull string makes tight contact with the edge  204 . Referring to FIG. 2B, when tube or cable  200  reaches position  211 , the tube or cable is obstructed by the edge. Applying additional tension on the pull string can not generate a force that can lift the tube or cable over the edge. Using a rigid device or flexible material to transition from the pull string to the tube or cable does not prevent obstruction because the obstructing edge is sharp. The edge deforms the pull string as the pull string is pulled over the edge. The obstructing edge catches any discontinuity in diameter or change in flexibility. 
     Using residential HVAC air duct as conduits for tubes or cables is unusual and there is little prior art to teach solutions to passing an obstructing edge. The electrical and communication industry has the most applicable prior art, but the ducts and conduits are designed for cables to be pulled, so obstructing edges are uncommon. Access ports at bends and corners are often provided to limit the length of the pull. Therefore the prior art for cable pulling does not teach how to pass by an obstructing edge. For example, U.S. Pat. No. 5,654,526 issued Aug. 5, 1997 to Sharp describes connectors for conduit sections that provide access for lubrication to reduce the friction when pulling. Patent U.S. Pat. No. 5,029,817 issued Jul. 9, 1991 to Tamm describes a device that includes a roller for installation at a bend in the conduit so the cable can pass by the corner when pulled, but this does not provide a method to pass an obstruction edge. U.S. Pat. No. 5,310,294 issued May 10, 1994 to Perkins describes a connector for connecting a boring devise to a cable for pulling the cable through the hole made by the boring device, but this connector is not adaptable to connecting a pull string to a cable. U.S. Pat. No. 4,078,767 issued Mar. 14, 1978 issued to Battaglia describes a connector for connecting a multi-wire cable to a pull wire. The connector provides a strong, quick, and non-damaging connection, but does not provide a way of passing by an obstructing edge. U.S. Pat. No. 4,552,338 issued Nov. 12, 1985 to Lindgren describes a devise for pushing or pulling a cable through a conduit comprised of many beads with axial holes and helical springs and a connector that connects sections of beads together. This devise is not adaptable for use in a HVAC air duct and provides no method for passing by an obstruction. 
     SUMMARY OF THE INVENTION 
     The invention is a connector to connect a pull string to a tube or cable. The connector converts the tension force in the pull string to a rotation force about a pivot point between the connector and an obstructing edge so that increasing the tension on the pull string causes rotation about the pivot point until the pivot point becomes unstable and the connector slips by the obstructing edge so that a tube or cable can be pulled by the obstructing edge. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagram of typical air duct paths in a residential forced air HVAC system. 
     FIG. 2 is a diagram showing how a sharp edge obstructs pulling a tube. 
     FIG. 3 is a cross section drawing of the invention. 
     FIG. 4 is a diagram showing how the invention enables a tube to pass an obstruction. 
     FIG. 5 is a diagram showing two alternative embodiments of the invention. 
     FIG. 6 is a diagram showing the invention adapted for pulling a cable. 
    
    
     DETAILED DESCRIPTION 
     FIG. 3A is a cross section drawing of the connector. The pull string  300  passes through an axial hole in the header  302 . The pull string is typically a high quality fishing line 0.015″ to 0.030″ in diameter with a tensile strength of at least 100 lbs. The header  302  is cylindrically symmetric, approximately 0.25″ in diameter to match the diameter of the tube  305 . The header presents a spherical surface in the direction of pulling. The header is made of a rigid and hard material such a steel, and is polished smooth so there are no sharp edges that might abrade the pull string and so it is easy to thread the pull string through the hole. 
     A compressible elastic cylinder  303  connects to the header  302 . The compressible cylinder  303  is approximately 0.25″ in diameter to match the diameter of the tube  305  and approximately 1″·2″ long. The cylinder material and length is selected so that the cylinder is axially unstable as it is compressed. As the cylinder is compressed, the cylinder axis deforms into a “c ” or “s” shape while the cross section remains substantially circular. Surgical tubing made of silicon rubber is an example of suitable material. 
     A joiner  304  mates the compressible cylinder  303  to the tube  305 . The joiner is a plastic or metal tube approximately ½″ to ¾″ long and selected to press fit to the inside of the compressible cylinder and the inside of the tube. 
     To make the connection between the pull string and tube, the pull string  300  is threaded through the header  302  and compressible cylinder  303  and joiner  304 . A loop  306  is tied at the end of the pull string and the loop is inserted into the tube  305 . A pin or nail  307  with a sharp point is pushed through a side of tube  305  approximately ½″ from the end of the tube so that the pin passes through the loop  306  in the pull string and the pin then passes through the opposite side of the tube. FIG. 3C is a cross section end view showing the pin passing through the loop in the pull string the pin is perpendicular to the axis of the tube and passes through the axis of the tube. The pin end  308  is cut off so that the pin end is flush with the outside of the tube. The pin is held in place by friction between the pin and the sides of the tube. The tube is pushed onto the joiner and the pull string is pulled while holding the header. This completes the connection. 
     FIG. 3B is a cross section diagram of the connector and completed connection with the pull string  300  under tension and the header  302  obstructed by an edge  301 . The pull string passes freely through the hole in the header so the tension is transferred to the tube by the pin  307 . As pull string tension is increased, the compressible cylinder deforms and the walls take a “c” or “s” shape as illustrated by  311  and  312 . The contact point  310  between the header and the obstructing edge is a pivot point. Since the pivot point is below the axis where the tension is applied, the compressible cylinder will preferentially deform upward. The upward deformation of the compressible cylinder causes the header to rotate about the pivot point. An increase in tension in pull string  313  causes an increase in deformation and additional rotation. 
     FIG. 4 illustrates the behavior of the connector  400  as the tube  404  is pulled by pull string  401  to the obstructing edge  403 . Referring to FIG. 4A, a the tube is pulled, the header  402  is obstructed. Referring to FIG. 4B, a tension in pull string  411  increases, the connector  410  begins to deform. Additional tension  421  causes more deformation and more rotation of the header of the connector  420 . Referring to FIG. 4C, a the header rotates and tension increases, the pivot point between the obstruction edge and the spherical surface of the header becomes unstable and slips. Referring to FIG. 4D, te header passes the obstructing edge and the connector  430  returns to a substantially cylindrical shape and the pull string  431  can pull the tube  432  past the obstructing edge  433 . 
     FIG. 5A illustrates using a coil spring  502  as part of the compressible cylinder  500 . One end of the spring presses against the header  502  and the other end presses against the Joiner  501 . The spring can provide a wider range of compression and instability characteristics than possible with rubber or plastic alone. 
     FIG. 5B illustrates an alternate embodiment where the compressible cylinder  510  has an integrated joiner  511  that mates with the tube  512 . This shape is practical if the compression cylinder is made by injection molding or by chemically joining together two separate plastic or rubber cylinders of the appropriate size. 
     FIG. 6A illustrates the connector  600  adapted for pulling a thin cable  602 . A short section of tube  601  approximately 2″ -4″ long is connected to the pull string as described above. The cable is fastened to the tube in a similar way the pull string is fastened to the tube. A loop is made in the end of the cable by twisting or other known process. A pin  603  is pushed through the side of tube  601 , through the loop in the cable, and into the opposite side of the tube. 
     FIG. 6B illustrates an alternate embodiment of the connector  610  where the joiner is adapted to receive the cable  612 . A thicker, multi-wire cable is illustrated where the cable sheath has sufficient strength so that the loop in the cable can be omitted. Pin  613  is pushed through the cable and cut flush with the other side of the joiner. 
     The forgoing describes illustrative examples of how to construct and use the invention, and should not be interpreted to limit or restrict the generality of the invention. Other methods of converting pull string tension into a rotational force can be devised by those with ordinary skills in the art of mechanical design. 
     From the description and the figures described above, the function and benefit of the connector can be understood and practiced by those ordinarily skill in the art of pulling tubes and cables through ducts.