IN THE WORLD OF NUTS AND BOLTS
and other mechanical fasteners, things are usually flat,
parallel, perpendicular, and
rigid, just the way engineers
like it. But a professor at Cal
Poly in San Luis Obispo has
disrupted that notion.
Saeed Niku, a professor of me- chanical engineering at California Polytechnic State University, working
with students in one of his classes, has
developed the Flexible Fastener, a bolt
designed to bend.
At the heart of the fastener is a shank
made of a strong, yet flexible material
such as Kevlar, nylon, steel cable, or wire
rope. Threads made of steel coiled like
a spring wrap around the core and are
bonded to it along the length and at each
end, along with a head at one end.
The fastener is “axially rigid but
laterally compliant,” Niku said. “You can
tighten the screw like any screw, but it
can also move sideways.” It can carry
axial loads yet is flexible enough to connect non-parallel surfaces, go through
mismatched or misaligned holes, or
handle machine applications where small
lateral movements are required.
The idea for the Flexible Fastener came
from a project in Niku’s Philosophy of
Design class in 2000. Three students were
working on a project where there was a
question about putting screws into the
wooden frame of California houses for
earthquake protection. They wanted to bolt
the houses to the foundation. Niku came
up with the idea for a flexible bolt, and the
students took it and made samples.
After the class ended, Niku took over
the project and refined the design. The
National Collegiate Innovators and Inventors Alliance and the Lemelson Foundation Initially funded the effort.
The Flexible Fastener can be made in
many different sizes, from as small as
3/16 inch in diameter to as large as desired. According to Jim Dunning, program
manager in Cal Poly’s Office of Research
and Economic Development, which
engages industry through the California
Central Coast Research Partnership, the
fastener can be manufactured to various
degrees of rigidity.
Niku said the fastener can be used with
“anything basically that is not parallel
or aligned or needs to move sideways.”
In construction, it can attach parts with
non-parallel surfaces without a need for
countersinks, as in a truss. It can be used
in repairing and refurbishing products
and structures where holes do not line
up or are misplaced. You can actually
just bend a screw into the non-parallel
surfaces and attach them.
It can also be used in robotics ap-
plications where simultaneous lateral
compliance and axial stiffness is desired,
as in attaching a fixture to the robot
that can move sideways for assembly.
Similarly, the fastener can be used in
medical applications such as in artificial
knees, where bending is desired between
different parts while they remain attached
or where you need to move sideways
because the holes don’t match. And
Dunning adds, “An application I always
have in mind is a seismic retrofit project.”
The bolt’s lateral flexibility allows whole
structures to flex to better withstand the
jolt of an earthquake.
Niku received a patent on the Flexible
Fastener in 2005, and now the Research
Partnership, known as C3RP, is attempting to license it. “We seek a partner willing to invest the time and tooling to find
out exactly what the optimum manufacturing would be,” Dunning said. ME
TOM GIBSON, P.E. is a consulting mechanical engineer specializing in machine design and green building,
and a freelance writer. He publishes Progressive
Engineer, an online magazine ( www.ProgressiveEngi-neer.com).
Initial prototypes of the
Flexible Fastener were made
with an acme (square) thread.
Image: Dennis Steers/Cal Poly College of Engineering