It’s generally seen as the least glamorous part
of the design cycle. Developers of heavyduty
data-center, industrial, commercial,
and other critical equipment that resides
in large cabinets and enclosures face the challenge
of ensuring that these enclosures and their contents
stay in place and in one piece under adverse
conditions.
Often having to work with soft materials
such as copper, zinc, and plastics, the possibilities
abound for screws stripping during
assembly and loosening during shipping,
operation, and acts of nature. As per senior
engineer Paul Parker at the electrical division of Eaton Corp.,
“Vibration and internal thread stripping can be the biggest
challenges for soft materials like copper. Cross-country shipping
can be even more brutal to equipment than seismic tests.”
Traditional assembly strategies include lock washers, adhesives,
torque rings, and the like. Though fairly reliable for a
time, these techniques often incur further costs across the
lifespan of a product in terms of replacement and servicing. In
some designs, they may even prove inadequate.
Assembly-hardware maker Spiralock Corp. offers a simple
solution to these reliability concerns in the form of a unique
internal thread form that combats the effects of seismic, shipping,
and operational forces while working in soft materials.
Providing a viable alternative to traditional fastening methods,
the internal thread form also promises to encourage design
innovation while streamlining assembly and inventory-management
chores.
TRADITIONAL PROBLEMS AND A SOLUTION
According to Spiralock, most locking fasteners
tend to overlook the gap that exists
between the crest of male and female threads.
Apparently this is a common problem associated
with industry-standard, 60° thread forms,
a problem conducive to thread loosening in
the presence of vibration.
The company also points to the first
few engaged threads of a fastener as a
reliability risk. This area exhibits a high
level of mechanical stress, and it adds to thread
shearing due to axial loading. Obviously,
expansions and contractions of the fastener
due to extreme temperature variations present
further compromises to structure integrity.
Resolving these issues, Spiralock’s self-locking
fasteners employ a re-engineered thread
with a 30° wedge ramp at the base of each
thread of the female half of the component (Fig.
1). Mating with standard 60° male fasteners,
the ramp enables the bolt to spin
freely until tension builds in the male
fastener, the threads of which tighten
against the wedge ramp.
This eliminates radial clearances
and creates continuous contact along
the mated threads, spreading clamp
force evenly over all engaged threads.
The overall result is significantly higher
resistance to vibration, axial loading, joint fatigue, and temperature
extremes.
Another benefit of the fastener is a locking feature integrated
within the thread form that compensates for variations
in manufacturing tolerances. In addition to boosting joint reliability,
the locking feature eliminates the need for extra threadlocking
devices or procedures, opening the door for more
design options in terms of layouts and configurations.
DESIGN OPTIONS
With reliability number one, the next two concerns surrounding
these large installations are space savings and ease of
access for upgrades and maintenance. Spiralock says its fasteners
easily enable front access, which eliminates the need to allot
space behind the cabinet.
One example is Eaton’s electrical Cutler-Hammer brand
metal enclosed front (MEF) accessible switchgear. After testing
traditional fastening strategies, Eaton created front-accessible
and maintenance-free MEF switchgear using the Spiralock
thread forms (Fig. 2).
“The self-locking threaded fasteners and internal thread
form are capable of holding the MEF switchgear’s bus joints
secure for decades,” said Eaton’s Paul Parker. “By helping us
attain a no-maintenance bus design, the fasteners and internal
thread form opened up new design possibilities.”
As yet another benefit, upgrading existing designs to the
self-locking fasteners usually entails exchanging nuts, wire
inserts, or drilling and re-tapping existing parts.
Spiralock corp. • www.spiralock.com