TPT November 2016

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76 NOVEMBER 2016
AR T I C L E Advanced Machine & Engineering/AMSAW
by Willy Goellner, chairman and founder – Advanced Machine & Engineering/AMSAW
Minimising the damaging effect of vibration
and resonance with stabilisers and dampers
by Willy Goellner, chairman and founder – Advanced Machine & Engineering/AMSAW
Even with a well-designed carbide saw, blade vibrations will
still occur. That is because it is nearly impossible to take into
account every possible cause of vibrations. Understanding
stabilising and damping best practices is the key to minimising
the inevitable vibrations created during the sawing process.
From a structural dynamics standpoint, the main parameters
of mass and stiffness are considered to be conservative. This
means that they can store, or conserve energy. Damping is the
parameter that dissipates energy by converting mechanical
energy into heat resulting in the reduction of mechanical
energy.
Where can you see damping?
Dampingwithin the structure and thematerial: For example,
the damping in conventionally jointed metal structures is partly
due to hysteresis within the metal itself, but primarily due to
friction at bolted or riveted joints and the fluid (sometimes just
air) that pumps in the joints or even from micro slip.
Discrete units, usually using fluids, such as vehicle
suspension dampers and viscoelastic damping layers on
panels that are often added to a mechanism or structure to
suppress unwanted oscillations.
This third and final article in the current series from AME
focuses on how best to minimise the damaging effects of
vibrations and resonance with stabilisers and dampers.
As part of the team that invented the first billet saw
using carbide-tipped circular saw blades and the founder
of AMSAW machines, my design team has learned
throughout the past 50 years that success in carbide
sawing comes from a solid understanding of four factors:
vibration, resonance, damping and stabilisation.
Fluid around the structure: Fluid around the structure can
also be a dampener.
Damping can be generated by magnetic fields . The
damping effect of a conductor moving in a magnetic field
is often used in measuring instruments. Moving coils can
develop surprisingly large damping forces. One common
application is electromagnetic brakes in a gear train.
Now that we know the different kinds of
damping, we will explore ways to quantify
the effect:
a) Logarithmic decrement
Damping is the loss of energy and in the case of any single
degree of freedom vibrations it can be quantified with the
logarithmic decrement (reduction of amplitude per cycle).
When you know the amplitudes of two successive peaks
and the number of cycles in between them, the logarithmic
decrement can be calculated as followed:
b) Force-displacement diagram
In the force-displacement diagram you can see the energy
loss as a mechanical hysteresis because the area underneath
the force-displacement diagram is proportional to energy. The
load and unload curve don’t match as you would assume for
an ideal elastic material, but you get a hysteresis loop which
is proportional to the energy loss. Between the stress and
the strain you’ll see a temporal phase shift. The end value
of the strain will be reached after a relaxation time, which
depends on a time dependent processes taking place in the
material. In the same fashion you can observe a remaining
strain after unloading which will reach zero after a longer
period. This is due to the reason that atoms will change to
an energetically favourable position and withdraw energy.
When using ferromagnetic materials the strain will cause