Numerical and Experimental Modal Analysis of Industrial Fan Bearing Supports

Abstract

Industrial fans are regularly composed of a supporting structure for the rotor and bearings as well as the casing. This support is also used to maintain a good alignment of the different rotating components. Since fans are subjected to vibrations, if the support is inappropriately designed, it can cause a misalignment and the resulting vibrations can be transmitted to the surroundings. Therefore, CBV has to comply with some ISO standards related to fan vibrations. One of these standards classifies the support flexibility based on the natural frequencies of the first mode-shape of the support in the axial, vertical and horizontal directions. This work aims to determine for CBV’s particular fans whether the support is rigid or flexible depending on its height. The influence of manholes in the support on the frequencies is also investigated. Finally, some suggestions for improvements are proposed. The first step consists in a numerical and experimental modal analysis of a fan in CBV’s factory in order to assess the correspondence between the numerical model and the real structure. It is found that the numerical model is reliable enough for the intended use. The support flexibility is then determined for 8 fans prescribed by CBV with a minimum and maximum height. They are all submitted to minimum and maximum values of the mass of the motor and rotor because it is found that the loading has a non-negligible impact on both the frequencies and mode-shapes. It is discovered that the central face of the support always deforms in the first mode because it has a free edge at the bottom. The effects of manholes are studied using various sizes, shapes and positions. It is concluded that a circular 500 mm diameter manhole in the centre of the face is the most adequate and it corresponds to what CBV currently designs. Finally, some ideas to avoid having natural frequencies of the support that could coincide with the rotation speed are suggested. It is noticed that adding a UPN at the bottom of the central face is greatly advantageous for all fans. For medium and large fans, adding one vertical stiffener on front, central and casing faces is beneficial. For small fans, shifting the horizontal stiffener beneath the motor mountings is wise and it does not add any cost. A thickening process of the faces of the support has a positive effect but it has the disadvantage of making the structure heavier and it is more expensive. For this reason, the thickness could be increased as a last resort.