The invention is particularly useful when an existing coiled cable cannot meet the retraction requirements without increasing the outer diameter of the coil by adding high retraction components inside the cable itself.
The invention is also very useful in applications requiring high shielding effectiveness which cannot be met by commonly used shielding methods in coiled cables, such as those devised by Takebe, in U.S. Pat. No. 4,683,349. The invention will allow additional shielding material to be added without degrading the retraction ability of the coiled cable.
In addition, the invention is useful in applications where materials needed for the conditions of the operating environment have poor retraction characteristics, therefore, not meeting the retraction requirements.
Several methods have been used to achieve retractable coiled electrical cables. Many forms of retractable coiled cables rely upon a jacket surrounding the wires of the cable which may be heat-set in coil form while the cable is in coiled configuration. Such methods are typically found in U.S. Pat. No. 3,993,860 to Snow, et al., U.S. Pat. No. 3,318,994 to Perrone, et al.,, U.S. Pat. No. 3,324,229 to Ingamson, U.S. Pat. No. 3,694,279 to Rohrig, et al. and U.S. Pat. No. 2,609,417 to Cox, et al.
Shielded cables are also known where a strength member or a drain wire is included inside the shield of the cable, such as in U.S. Pat. No. 3,278,329 to Timmons; however, inclusion of a strength member or drain wire within the shield usually leads to a larger diameter cable and requires more material to make it.
A conductor is spiralled around an insulated electrical conductor, such as devised by Takebe, in U.S. Pat. No. 4,683,349. The spiralled outer conductor provides shielding around the center conductor when the cable is in coil form, or not stretched, but leaves wide gaps in the shielding, which greatly reduces the effectiveness of the shield, when the cable is extended or stretched. Metal strips have been imbedded within the insulation of a cable in a longitudinal manner so that the springy strips would coil the cable into a flat coil to act as a spring to support a hanging lamp at the end of the coiled cable, as shown in U.S. Pat. No. 3,042,742, to Foster. The spring strips did not provide significant shielding coverage of the center conductor so as to be useful for a coaxial cable for electronic signals.