Lateral vibration is the most commonly measured type of vibration in machinery.However,lateral motion in the XY plane does not completely describe all the possible directions of motion of machine components.Torsional vibrations do not get the attention that they should.Since torsional vibration is more difficult to measure than lateral vibration,it is often ignored,due to an"out of sight,out of mind"attitude.In fact,torsional vibration can be quite severe and can produce damaging cyclic stresses that can cause a fatigue failure.Rotating machines are all involved in the delivery or transformation of power,and power delivery requires transmission of torque.The applied torque causes the shaft to twist about its rotational axis.Thus,instead of measuring displacement relative to an equilibrium position,torsion requires measuring the angle of twist of the shaft at different axial positions.All rotating machineries undergo some fraction of degrees of torsional vibration during operations.In many cases,the torsional vibrations are not as easily identified as the translational vibrations,due to lack of simple and direct measurement devices.Typical damages developed under excessive torsional vibrations include shaft cracks,coupling cracks,gear wear,gear tooth failures,key failures,shrink fit slippage,etc.Today torsional vibration detection and monitoring becomes an important step in rotating machinery condition monitoring [Eshleman 1977,Muszynska,A.G.1992,Naldi,L.B.(June 9-13,2008),Naldi,L.G.2011].Today the machinery diagnostic services team of Baker Hughes,Bently Nevada uses ADRE raw data along with self-developed Matlab executable [Huageng,L,Howard,B,Menon,A,Peton,N,Roengchai,C.2013,Luo,H.C.2013] application for diagnostic purposes based on phase demodulation process to the signals generated by Keyphasor~R or zebra tape of tooth wheels.In this article torsional issues will be illustrated with data and pictures using 6 real cases from the industry.
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