Tom Shelley reports on a superficially simple looking
family of devices likely to revolutionise much of mechanical
engineering
By forcing to make solid material deform as it passes round a
bend in a closed channel, it is possible to construct compact,
low cost and infinitely re-usable devices that absorb large or
small amounts of mechanical energy.
The ideas are derived from metal forming, but working materials
already studied also include gels, polymers and complex mixtures.
Applications investigated range from multi use car impact crash
absorbers through rotary mechanical and building earthquake
dampers to protective materials for sportsmen.
The base idea is the brainchild of Dr Fayek Osman, a lecturer in
the University of Bath Department of Mechanical Engineering. He
recently told us that his inspirations came from a combination of
his long industrial experience of metal forming in industry, and
an article on an energy absorbing "Collision protection
fuse" originally published in Eureka around 20 years ago.
The fuse was a collapsible tube with side slots to ensure that it
collapsed in a predictable manner and direction. This led him to
consider how it might be possible to devise a device that would
absorb equal or preferably greater amounts of energy but which
could be re-used.
Since he was familiar with metal forming, and was aware of how
metals could be made to undergo large amounts of deformation, he
hit on the idea of forcing metal through a round channel with a
bend in it. Provided the input cross section of the tube is the
same for entry and exit, what comes out is of exactly the same
geometry as what goes in, allowing the device to be used again
and again.
The substance being forced around the bend does not have to be
metal, and the bend can be of almost any geometry. The device can
be used as is, or in combination with mechanical mechanisms to
spread a reduced absorption force over a longer movement
distance. Speed of operation can be from mm per minute, up to the
speed of sound in the solid material. The devices produce no
rebound after impact, and are maintenance free, requiring no
lubrication.
The idea and many of its potential configurations and
applications are protected by patent, and a wide range of devices
employing different materials and configurations either have been
or are being researched by Bath University students.
For the research studies, some of the early work has been with
plasticine in devices made of acrylic, a technique often used for
research studies of metal forming. Dr Osman considers, however,
that plasticine is not a good material for practical devices
because it tends to dry and change its mechanical properties if
left in contact with air. Other relatively deformable materials
being studied include paraffin wax and 'Polymorph'. The idea is
also being studied with devices made of tool steel and with
aluminium and lead as working materials. Here, establishing
performance parameters become more complicated because metals
work harden when deformed. Soft, low melting point metals such as
lead recrystalise and revert to their original mechanical
properties if left for a while. Aluminium, on the other hand only
recrystalises when heated, but recrystalisation temperatures can
easily be reached during the deformation process if it occurs at
speed. Even if the metal does not have a chance to recrystalise,
Dr Osman points out that work hardening occurs only up to a
certain limit, and experimental results have been obtained
showing the resistance of such devices rising until they reach a
plateau force.
The first and simplest device to be made consisted of two
connecting holes drilled into a block at right angles. Even using
such a simple configuration, 120mm on each side, with 25mm
diameter channels, it is possible to achieve an absorption force
of 100kN.
Less force is required if the channel is made to bend by less
than 90 degrees. The channel can also be made 'U' shaped so that
the pushed out slug emerges on the same side as the pushed in
slug. If the device is then rotated, it can be made to present
the pushed out slug in its original position, ready for the
device to be re-used. The slug of material can be pushed in by a
captive punch, presenting a larger cross section to the outside
world. It may then be used to push out another captive punch at
the other end of the channel. If the input and/or output punch
resides in a screw thread, the device may be made to absorb
energy from a rotating force attached to the punches.
If the device includes a 'dog leg' section, the emerging slug or
material or punch can be moved along the same axis as the input
force.
A large number of potential applications have already been
identified and more are constantly being added. One of the most
obvious applications is to employ a series of through acting
devices behind car bumpers. After an impact, they could be turned
round ready for use again, instead of having to be thrown away as
at present. Because of the very large energy density of the
device, it is possible to place units between inclined parallel
inclined ramps to absorb linear vibration. Such a configuration
is being considered for absorbing earthquake shocks in buildings.
Another arrangement achieving a similar effect is to place a
through acting device on rockers attached to linearly moving
elements.
By engaging punches of successively larger diameters, or having
them act on slugs of material of increasing stiffness, it is
possible to make devices that become progressively stiffer. Such
a configuration has been put forward for absorbing impact at the
bottom of a lift shaft.
Other proposed applications include end stops for machine tools,
aircraft undercarriage impact safety devices and energy absorbers
in artillery pieces. The devices may also be expected to be found
in future joints in bridges, cranes, building trusses, train
buffers and automation equipment. As well as re-usable car bumper
mounts, the devices have been proposed for door safety devices,
and incorporation into body members, dampers and steering
columns. A 'bubble' structure has been devised, but not tested,
in which slugs of soft material are pushed through channels
between spherical cavities for use in protective sports wear,
trainers and limb and joint prosthetics. Most areas of
engineering are likely to have a use for the technology. Cost and
weight savings are expected to be significant.
Analytical methods have been developed to assist design, based
on the Upper Bound Method traditionally used to study metal
forming as well as using ANSYS finite element analysis. The
complexity of the processes involved, however, require
experimental verification of designs in all cases.
For more information, email Dr Fayek Osman at Bath
University
Eureka says: The devices look to be able to
totally change just about everything in the field of energy
absorption in mechanical engineering design. The Victorians could
have thought of it but didn't, proving once again that even the
most basic mechanical components often still have considerable
room for innovative improvement.
Pointers
* Simple to use
* Low cost and easy to manufacture
* Negligible fatigue effects
* No rebound after impact
* Very large range of possible force ratings
* Energy absorption rates can be tailored to design requirements
* Maintenance free
* Re-usable
* Single action to initiate energy dissipation
* Compact, easy to integrate into any structure
For more technical
developments see www.eurekamagazine.co.uk