Screw threads are one of the strongest and most flexible methods of mechanical adjustment, fastening, and coupling. However, cross threading, fastening, and unfastening screw threads can not only be time consuming, but difficult in some applications. For instance, nut rotation in space presents a host of problems: chiefly, astronauts have little means of bracing themselves to provide torque. Engineers working on the Hubble Space Telescope project needed a threaded fastener that astronauts could remove without unscrewing when disassembling Hubble components for later repair on Earth. What's called Double Zip technology from Thread Technology Inc., Sterling, Va., solved the torque problem.

The patented Double Zip "push-on/pull-off" design -- an improved version of standard threaded joints -- makes fastening and unfastening easy by using standard male and female screw threads that can be pushed on existing parts without cross threading or multiple rotation. And, after limited loosening, the nuts or couplings can be pulled off the bolt or male threads without rotation.

The Double Zip design is based on the company's standard ZipNut which consists of a top cap with a threaded segment inside an enclosure. The ZipNut simply pushes onto a bolt. Turn it one-revolution, and the ZipNut locks its threads into the bolt threads. The Double Zip, however, adds pull pins or other mechanical devices that connect the nut threads to a pull sleeve on the outer enclosure. By pulling the sleeve away, the female threads are retracted from the male threads. The nut or coupling can then be removed quickly without rotation. When the nut or coupling is under load force, load prevents retraction.