Improved Cable-Drive Pretensioner

Lyndon B. Johnson Space Center, Houston, Texas

An improved mechanism has been devised to facilitate the adjustment of tension in a cable in a cable-and-pulley drive. Cable-and-pulley drives are being used increasingly in robots and other high-performance, computer-controlled machines. Typically, a cable is looped around various drive pulleys with its ends anchored in proximity to each other on two coaxial cylinders (see figure). During operation of the cable drive, the cylinders are locked against rotation relative to each other to maintain a preset tension. To adjust the tension, it is necessary to unlock the cylinders and rotate them relative to each other.

Tension in the Cable Is Adjusted by turning one cylinder relative to the other. In the improved mechanism, this is done by use of a worm-gear drive.

In a typical older cable-tensioning mechanism, lock between cylinders is maintained by setscrews. To adjust tension, it is necessary to loosen the setscrews, turn the cylinders slightly to obtain the desired tension, then tighten the setscrews. This procedure is tedious and labor-intensive; sometimes it requires as many as three technicians working simultaneously with three wrenches and a screwdriver. If one of the technicians slips, it is necessary to repeat the procedure and, depending on the design of the particular cable drive, it could be necessary to dismount and remount the cables.

The improved mechanism is both a locking and an adjustment mechanism that can be adjusted easily by one technician. In this mechanism, the cylinders are turned relative to each other by use of a worm-gear drive. The outer cylinder houses the worm, while the mating worm gear is attached to the inner cylinder. A technician turns the worm by use of a hand tool. There is no need to provide explicitly for locking against inadvertent or undesired relative rotation: the friction inherent in the worm-gear drive prevents backdriving under cable tension.

This work was done by William Townsend of Barrett Technology, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to