TPT November 2016

AR T I C L E

Advanced Machine & Engineering/AMSAW

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.

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

AMSAW ® developed a unique device using rollers.

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N OVEMBER 2016