Find enclosed a gluten mill that lost a blade 3-4 years ago and now one axial knocktest on 1 blade indicate a change of more than 1Hz lower it´s a blade of the same generation as the broken. Since some of the other blades keep the same within 3 decimal places from year to year I will do another test in 6 months to see if the change is continuous, hope it will not accelerate to fast if it´s a crack starting. Do you think 1Hz is significant? If it did 1Hz in 1year, will it break within 6 months? Not so much data to extrapolate... Olov

gluten.doc (1,274 KB, 28 downloads)

Olov,

I have not worked on a Glutin Mill. Before commiting action to further investige a possible crack, I would want to be sure that boundry conditions have not changed instead. Things like fouling (added mass), support stiffness reduction, or bearing condition may affect the blade natural frequencies. Just my 2-cents (US$)!

Walt

True. I was expecting a real change like 5-10% to be worried so I will shorten the interval to half and see. Would expect that also a small crack would change the freq. significant. I am not so sure about the mounting but the other 61 blades have not had that change in 3 years. Blades are "cleaned" and look like they do in the picture every time. Gluten stuff is from wheat and this is part of the drying process from the gluten paste so the material is like dust. We will see in early December. Olov

Even with changed boundary conditions, for instance support stiffness, would blade resonance frequency change? Pictire a 2 degree freedom system modeled as m1, k1 and m2, k2.
Wouldn't f1 and f2 be independent? ( trying to recall the math expession for this model).

In this particular case, since other blades stayed the same, the boundary conditions ( if they are having an effect ) apparently have not changed.