One of the main causes of increased vibration can be considered soft foot which, due to distortion induced in to the machine structure (pump casing, compressor casing or motor stator, etc.) which leads to a mix of failure modes like shaft misalignment, mechanical looseness, internal rubbing, uneven air gap issue, incipient failure of anti-friction bearings, that negatively impact long-term equipment operation.
A simple maintenance best practice for identifying and correcting soft-foot issue will save maintenance team time and money. Thus, when performing precise shaft alignment or soft foot has already been revealed by vibration analysis, repair team should follow a straight-forward alignment procedure which consistently considers soft foot detection at an early stage (by using feeler gauges, dial indicators or any laser devices, etc.) in order to correct it (either by shimming or machining the machine feet).
The allowable value for soft foot identification should not exceed 0.05 mm (but there are some cases where this value is considered maximum 0.03 mm). Anything above this value needs to be carefully corrected!In some articles and books, pipe stress is also considered as soft foot, but for pipe stress detection is measured coupling displacement (permissible value is 0.05 mm). But this issue will be tackled in the next article.
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