Casing Stiffness Introduction
Investigation into Stiffener locations on Centrifugal Fan Casings using FEA.
Introduction.
Several fan casings were analysed to determine deflection and stress under internal pressure. The most important of these two will usually be deflection particularly around the shaft hole and inlet cone where excessive movement can cause performance or alignment problems.
The finite element analysis (FEA) software used was Autodesk SimStudio Tools and Autodesk Simulation Mechanical (SimMech). Note: These products are no longer available from Autodesk.
Deflection of Thin Circular Plates.
Centrifugal fan casing sides are similar to thin circular plates in that as deformation increases the stress and deflection are affected by diaphragm stresses. This becomes apparent when the deflection at the centre exceeds one half the plate thickness. (Source Roark).
Diaphragm stress can be visualised by observing a trampoline. A trampoline has no bending stiffness but does not collapse because of the surface tension in the mat and edge springs.
In order to analyse casings with these deflections a nonlinear analysis needs to be performed by the FEA software. This type of analysis can be a time-consuming process and is not always available within FEA software products.
In this report SimMech was used to determine the stress and deflection in the casing sides with or without stiffeners using linear and nonlinear analysis.
For most casings with stiffeners, static stress, linear material models were analysed. The pressure applied was not excessive to keep the deflection within approximately half the casing thickness.
Higher Pressures are also analysed with stiffener patterns recommended by Centrix.
It should be noted that, for a static analysis, stress and deflection are directly proportional to pressure.