TPT November 2016

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78 NOVEMBER 2016
AR T I C L E Advanced Machine & Engineering/AMSAW
by Willy Goellner, chairman and founder – Advanced Machine & Engineering/AMSAW
The formula shows that the damping by the oil viscosity
depends in highly disproportionate extent on the surface
area and the gap height.
How can you measure the damping of a
structure
1) The natural frequency without damping is determined
by using an FEA package, in our case Solid Works
Simulation.
2) A suitable suspension for experimental modal analysis is
chosen to keep the effect of the suspension small and to
be close to a free boundary condition. The effect should
be little for the natural frequency as well as damping.
Since you always have to support the load you can just
do this approximately, with suspension cords, springs or
other means. As a rule of thumb the natural frequency of
the rigid body mode and suspension should be 1:10 of the
lowest elastic mode. Helical steel springs usually have a
negligible damping factor as well, and would be a good
suspension.
3) The structure is excited with an impact hammer and
the transfer function between an accelerometer and the
hammer and the modal damping is calculated. We use a
DataPhysics ® Data Acquisition System for this step. The
modal damping is the damping ratio D for a particular
mode.
Vibration reducing gadgets for saw blades
Today more or less effective damping devices are on the
market. AME has developed and tested stabilisers and
dampers for the last 50 years; we started with a broom
stick, and pushed it against the blade, while supported by a
machine structure. As silly as it may sound, it proved to be
very effective.
Besides designing a saw, using circular carbide or cermet-
tipped saw blades with the best damping characteristics, it is
important to stabilise the blade to obtain acceptable surface
finish and tool life. A saw blade vibrates 90° to its plane
and its amplitudes will cut a wider slot if not contained. This
was discussed in more detail in our first article, “Effect and
Prevention of Vibrations in Carbide Sawing”. This results in
larger chip volume, which will increase the sawing torque and
could create torsional vibrations in the gear box. Torsional
vibrations are very damaging to the carbide teeth and the
gears, and must be avoided.
a) Stabilisers
To minimise vibration-related problems, carbide saw
manufacturers developed gadgets that they mistakenly called
“damping devices”, but in truth they operate as a stabilising
device. These mechanisms usually consist of hardened
plates, which can be adjusted to suit the blade body thickness.
These so-called damping devices actually do not dampen
the blade, but only stabilise by constraining the vibration
amplitudes of the blade by limiting vibration amplitudes. In
fact, stationary hardened plates do not absorb any energy
caused by vibration. True damping is only accomplished
through dissipating the energy produced by the vibrating
blade. The friction between the rotating blade and the
stationary stabilising plates will create heat. Generally the saw
blade has to be properly tensioned for stability purposes. Heat
within the blade reduces the blade tension and, therefore,
must be cooled by air. In order for the air to cool the blade
there must be a small clearance between the stabilising plates
and blade body for air flow, which unfortunately diminishes the
effect of stabilisation.
b) Dampers
Damping requires absorption of the energy created by the
forces of the amplitudes of the lateral blade vibrations. This
can be done by using vibration-absorbing material for the
dampers, using fluids as energy absorbers or other means.
AMSAW® developed a unique device using rollers.