TPT November 2016

AR T I C L E

Advanced Machine & Engineering/AMSAW

an alignment of the magnetic elementary and causes local eddy currents. The amount of the dissipated energy is driven by the load and velocity, which goes along with amplitude and the frequency of an oscillation. Furthermore it will also be influenced by the temperature. By dividing the amount of energy dissipated in a harmonically excited structure by the work applied the specific damping capacity ψ can be calculated.

advantage when the mode dynamically deforms the part and due to lack of stiffness you have a dynamical weak spot. b) Damping of joints The damping of joints and interfaces is about one to two decimal powers bigger than material damping. Main driving factors for this kind of damping are:

Influence of different contact conditions for damping Higher surface roughness, less pressure and fluids with higher viscosity can cause favourable damping conditions. The mechanism in the joints is driven by many parameters, which are difficult to collect by measurements. Practical experience shows that the values are subject to a huge variance. The resonance frequencies as demonstrated in the last article can be calculated pretty accurately due to mass and stiffness distribution, whereas the resonance amplitude can show bigger deviations in practice.

c) Damping ratio The Damping ratio D is a dimensionless parameter that describes the amount of damping relative to the critical damping. A critically damped system converges to zero as fast as possible without oscillating. When the system is excited with resonance frequency, the amplitude q max and resonant frequency fo is measured. Then the exciting frequency is changed until the amplitude will be

With the two frequencies f1 and f2 the damping ratio can be calculated. This is the so called half-power band width method.

c) Squeeze film damping of guide ways (box ways) Squeeze film dampers use the speed-proportional shear forces in viscous liquids for damping. The damping effect of squeeze film dampers is based on the displacement of liquid between two surfaces, which have a relative motion normal to the surface. The calculation of the damping due to an oscillating movement in y–direction assumes that the gap is filled completely with oil at any time. Parallel to the track longitudinal direction the oil is displaced or sucked in again. The flow creates a pressure gradient between the track centre and the outer edge. The integration of the resulting pressure gradient results in the damping force F:

Where do you experience damping on a machine tool? a) Damping of machine bases In big structures, damping forces will dominantly occur where relative motion exists (eg coupling points such as bolt connections and guide ways). In order to increase the friction within the machine base, machine builders use core sand in the hollow areas to cause dry friction. In case of rigid body vibrations the material damping is nearly non-existent. The material damping of cast iron is better than that of steel, but the welding joints of steel structures and interfaces outweigh this disadvantage. On the other hand reaction resin concrete has a considerably higher damping factor, but high damping material only has an

The damping factor d is the force derived by the velocity:

η=Dynamic viscosity y. =velocity

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N OVEMBER 2016