1. Field of the Invention
This invention relates generally to a system and method for efficiently threading a tube-nut onto a threaded connector and, more particularly, to a system and method for threading a tube-nut associated with a high pressure fuel line in a diesel engine onto a threaded connector using an integrated control strategy that coordinates a robot employing force control and a tube-nut runner tool employing torque and angle control.
2. Discussion of the Related Art
Diesel engines, such as the 6.6 liter V-8 Duramax diesel engine, offer improvements in efficiency over similar sized conventional internal combustion engines. This efficiency is mainly due to employing compression-ignition that uses the heat of compression to initiate the ignition to burn the fuel injected into the engines combustion chambers instead of using spark-ignition employing a spark plug. Because a diesel engine uses compression for ignition, the fuel is provided to the fuel injectors at high pressure. Because the fuel is provided to the injectors at high pressure, the nuts and other fittings that couple the fuel line to a fuel rail and to the injector require high integrity sealing. Typically, the nut that attaches the fuel line to the threaded connector on the fuel rail is a tube-nut, where the tube-nut is attached to an end of the fuel line and is rotatable relative thereto, and where threading the tube-nut onto the threaded connector seals the end of the fuel line to the orifice in the fuel rail.
During assembly of a diesel engine of this type, an operator will often use a tube-nut runner to thread the tube-nut onto the threaded connector. A tube-nut runner is a power tool similar to a nut runner, but with an open-ended socket. The open end allows the socket to slip onto nuts with concentric tubes running through their center. Advanced nut-runners and tube-nut runners can have capabilities to control the torque and/or angle displacement of the socket. They can also feature capabilities of monitoring the angle and/or torque profiles to detect failure modes.
Because the operator is required to rotate the tube-nut onto the threaded connector with high torque, the assembly operation has high physical demands on the operator, especially the operator's arm joints, which limits the number of tube-nuts the operator can tighten during a particular time period. Further, because the fuel is delivered at high pressure requiring a high integrity seal provided by the tube-nut, a quality issue could arise because of the complicated nature of threading the tube-nut onto the threaded connector and the wear on the operator. Further, because of the torque requirements on the tube-nut for tightening purposes, even after the tube-nut is tight using the tube-nut runner, one and possibly two more tube-nut tightening steps are often employed using a manual torque wrench to ensure that the tube-nut is properly tightened on the threaded connector, thus increasing the cost of assembly of the engine.