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 fail-safe, 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.
Follower Nuts would add protective
redundancy. Upon shearing of the thread in the primary
nut, the primary nut would push against one of the follower
nuts, causing that nut to bear the load.
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.
