Fail-safe, continue-to-operate concept for jackscrews
John F. Kennedy Space Center, Florida
A fail-safe, continue-to-operate design concept for machine jackscrews calls for the incorporation of a redundant follower nut that would assume the axial jack load upon failure of the primary nut. Heretofore, the way to design for increased reliability of jackscrews has been to provide for multiple jackscrews operating in unison. The present fail-safe, continue-to-operate design concept offers an alternative for preventing catastrophic failures in jackscrews, which are used widely in aeronautical, aerospace, and industrial applications.
A conventional jackscrew contains only one nut made of a material softer than that of the threaded shaft. With prolonged use, the thread in the nut wears away. If not inspected and replaced when wear becomes excessive, the nut eventually fails by shearing of the thread under load. A typical jackscrew according to the present failsafe, continue-to-operate concept would include a redundant follower nut in addition to the primary nut. The follower nut is mechanically attached to the primary nut and free to move axially relative to the primary nut. The follower nut would bear no axial load and would have negligible wear as long as the primary nut continued to function normally.
In the absence of thread wear and play, the follower nut would be axially separated from the primary nut by a distance comparable to the thread pitch. Increasing wear would cause a change in this distance that would be taken as an indication of the amount of wear prior to failure of the primary nut. The redundant follower nut assumes the axial load in the event of primary nut wear and subsequent thread shear failure. Hence, the jackscrew would continue to operate with the follower nut bearing the load until a repair could be made.
Unlike the case of a conventional jackscrew, it would not be necessary to relieve the load to measure axial play or disassemble the nut from the threaded shaft to inspect for wear. Instead, wear could be determined by measuring the axial gap between the primary and follower nut. This could be accomplished by visual inspection, or possibly with the help of a simple measuring tool. Another option could incorporate electronic or mechanical wear indicators to monitor the gap during operation and assist during inspection. These devices would be designed to generate a warning when the thread was worn to a predetermined thickness. Note: A half-thickness value is the wear tolerance recommended by major manufacturers of jackscrews.
The fail-safe, continue-to-operate concept applies to all types of machine jackscrew designs. It can be applied equally well to ball screw jacks.
This work was done by John G. Fraley, Ivan Velez, and Charles G. Stevenson of Kennedy Space Center and Richard T. Ring, Jr., and Ralph Webber of United Space Alliance. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech. com/tsp under the Machinery/Automation category.
Copyright Associated Business Publications Sep 2002
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