Do-it-yourselfers know the problem
almost as well as do mechanics and manufacturers of heavy machinery.
With too much vibration, bolts screwed into blocks of metal come
loose. Tighten the bolt too much, though, and you strip the threads.
The usual solution has been to drill out the damaged
hole and use a device that cuts new threads to fit a thicker bolt.
But that bolt, too, can suffer the same fate.
The newest solution is a cleverly redesigned bolt
that is simply screwed into the damaged hole. Like conventional
self-tapping bolts, it cuts its own threads. But unlike others, this
one contains a hollow space that collects the metal shavings instead of
pushing them ahead of the bolt.
The reservoir is sized so that, when the bolt is
fully screwed in, the metal shavings pack the cavity just tightly enough
to push back on the metal block, keeping the bolt from loosening with
vibration. Because the packed shavings are not rigid like the bolt,
they can flex with the stresses of vibration and spring back to keep
pushing on the inside wall of the hole.
Lock washers serve the same vibration-resisting
function with bolts that go all the way through the pieces being fastened
and take a nut. The new bolts are intended for applications where
there is no nut. The packed shavings are the internal equivalent of
a lock washer.
The invention, called the Ultra-lok bolt, was named
one of the best products of 1995 by Design News magazine, a trade
publication that covers new product designs in a wide variety of
industries. The bolt, designed by Blake McKinney, who ran a truck
repair business, is manufactured by Ultra-lok Fasteners of Busy, Ky.
Considerably more expensive than ordinary bolts, the
new ones come in sizes a few hundredths of an inch larger than standard
sizes. The bolt's threads match those in the hold it is repairing,
but because its metal is a special alloy stronger than conventional bolt
metal, it cuts more deeply into the block to make new threads.
According to the manufacturer, Ultra-lok bolts now
are used to repair machinery ranging from bicycles to tanks. Auto
manufacturers are considering the bolts for use in places where it is
vital that a bolt not come loose, such as in anchoring seat belts.
