Machine builders are beginning to gain sound technical and commercial
benefits from the use of Festo's new pneumatic actuator technology, the
'fluidic muscle'. Why use conventional cylinders when all that is needed
is a little muscle flexing?
Middlesex-based Hamlyn Engineering faced a tough challenge when a
customer wanted to automate a shearing process to crop copper strips in
an electrical starter motor manufacturing application. During the
production process, some 27 of these strips, constituting part of the
rotor assembly, need to be cut to length in situ. Hamlyn duly turned to
it sister company, Camberley Auto-Mech, for the one-off bar cropper
machine design. Head of the design team, Mac Ghadially wanted to crop all
27 conductors simultaneously, but with a combined cross-sectional area of
1.125in2, this was going to require some power and precision (the shear
strength of copper is between 22,000 and 30,000 lbf/in2).
Mr Ghadially initially thought of using hydraulics, but in the end-user
application environment, a hydraulic service supply was not readily
available. The cost of providing this in addition to the machine was not
justifiable, he explains. Of course, all manufacturing facilities have
an electrical supply, so Mr Ghadially also contemplated an electrically
powered system driving through a gearbox. The design concept had some
efficiency merits but was not as cost effective as a pneumatic solution.
The customer's site already had air prep and supply, says Mr Ghadially,
so he designed a rotary guillotine mechanism comprising two circular dies
with 27 wire-eroded slots to take the copper strips. The task then became
one of calculating the linear force required at the edge of the die to
shear the copper strips at the 27 load points, plus the linear travel
required to effect a full cut. Once determined, these suggested a
specification for the pneumatic cylinders (125mm diameter units with 50mm
travel), which was passed on to the local pneumatic distributor, Festo
Premier Stockist, Blackson & Kenridge. The cylinders were to be arranged
as two rotary couples and drive in unison to operate the rotary
guillotine mechanism. Blackson & Kenridge sales consultant, Peter Howes,
however, had another idea. He proposed a radical pneumatic solution that
not only reduced costs but also the size of the system.
Instead of eight cylinders, each 125mm in diameter, he suggested eight of
the new Festo 'Fluidic Muscles' (FMs), each only 40mm in diameter. Rather
than using air to drive a piston, the fluidic muscle has a mesh of
interwoven fibres surrounding a hollow flexible core, through which air
flows to deform the structure, generating movement and an axial force.
The FM's main advantages are exceptional power-to-weight ratio and
smoothness of operation. The FM weighs just one tenth that of a metal
cylinder of equal inner diameter, yet the force is typically ten times as
great.
The economics were also in favour of the Blackson & Kenridge solution. At
around £230 each, the 125mm cylinders would have cost £1,840. The minimum
axial deflection of a 40mm diameter fluidic muscle is one quarter of its
length. So to achieve the 30mm movement, muscles of 120mm length were
required. These cost just £74 each - a total of £592.
It was an innovative and compelling idea - helped by the fact that Peter
Howes made sure I got everything I needed at the drop of a hat to
evaluate the proposition, says Mr Ghadially, who eventually went with
the fluidic muscle solution for the bar cropper. During assembly, the
muscles revealed another advantage. They have superb accuracy upon
return in the relax mode, Mr Ghadially observes. This attribute was on
my wish list because it eliminates the need for a stopping cylinder to
reposition the cutter at the start point, but I didn't expect to get it
from the muscle.
The fluidic muscle succeeded in offering a sound c