TPT 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

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:

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. 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.

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

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.

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N OVEMBER 2016