sensors & systems - Industrial Technology Magazine

DESIGN COMPONENTS
Springs, Gas Springs & Dampers
Four spring rates in one part
A medical manufacturer thought it was testing spring capabilities to
the limits with a requirement for a unique characteristic. But Abssac
was able to meet the company’s needs with a machined spring
Probably one of the most demanding machined
spring applications to date for Abssac involved a
medical equipment manufacturer wanting to
utilise precision compression, torsion, lateral
bending and lateral translation rates – all within one
individual spring. In addition to this, the springs required
easy attachment and would have to be lightweight and
relatively inexpensive – something that previously was
unobtainable from a standard wound spring format.
To add to the difficulty, Abssac’s customer also
requested three other springs, identical in size but with
different spring rates. But that was easy compared with
the real challenge of being able to accurately supply not
just one spring elastic rate, but all four in the single part.
Precise linear deflection
Machined springs are quite different from conventional
wound products. Rather than deforming a wire around a
set form to produce the spring, machined springs are – as
the name suggests – machined from a solid piece of
material. In use, two main advantages are immediately
apparent. Machined springs can provide very precise,
linear deflection rates because virtually all residual
stresses are eliminated. And the machined spring also
enables the designer to incorporate the way in which the
spring attaches into a single piece design, often
incorporating parts of the spring assembly into the single
part construction.
But what is most compelling about machined springs
is the fact that it is possible to have more than one spring
coil in the single piece construction or multi start
configurations, which deliver quite outstanding
performance advantages. In fact, the machined spring’s
capability to be supplied in multiple start spring coil
configurations takes the performance and reliability to
levels not achievable by the traditional wound spring
format. Naturally, the most common configuration is the
single start spring, which consists of a single continuous
coil element, which starts at one end and terminates at
the other end, much like its wire wound counterpart. The
double start or indeed a triple start spring has two or three
intertwined continuous coil elements, still within the
same single part construction.
In effect, this puts multiple independent helixes in the
same cylindrical plane, which provides totally enhanced
spring performance. On multiple start machined springs,
virtually all internal moments are resolved within the
spring itself and the double start machined spring
configuration is the closest single Cartesian co-ordinate
deflection part available with today’s technology.
Five start configuration
Pulling on extensive product knowledge it was known that
the triple start Heli-Cal beam configuration delivered very
high lateral stiffness. Combining this with the requirement
of a high lateral translation rate became the starting point
for analysis. Next, came the remaining three rates. Using
first-class finite element analysis software, these three
were then quantified. A trend also appeared, showing that
as the number of starts increased, the ratio of the
remaining three rates converged. In fact, at five starts, the
ratio was near perfect. At six or more, divergence was
experienced, hence no benefit.
All that was left was to select a coil thickness, and
outer diameter to inner diameter ratio that would best fit
the customer’s needs. Combine this with the use of
7075-T6 aluminium for light weight and lower machining
cost, and the design was successfully completed.
The advantages of the machined spring product are
numerous, having proven itself in medical, aerospace,
semiconductor and motor-sport industries to name but a
few. Wherever a wound spring is not able to meet your
performance criteria or a new spring design requires
ultimate accuracy and repeatability, the machined spring
is the ideal partner. So the next time you require a spring
in a critical or high duty cycle environment, the machined
spring from Abssac may have the answer to your
design problems.
MORE INFORMATION:
Enter D345 on the enquiry card or visit ‘latest issue stories’ at
www.industrialtechnology.co.uk for further details. You can also find
more stories from Abssac and more news on springs and dampers
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INDUSTRIAL TECHNOLOGY • May 2008