DETERMINATION OF THE CRITICAL SPEED OF SHAFT

Abstract

This thesis gives a demonstration of critical speed of shaft. An approach of theoretical, experimental & modal analysis of a shaft with various properties is used in this body of work. A shaft is connected with motors to detect the critical speed. Centrifugal Single Phase AC motor HC84-14 is the main motor used here to rotate the shaft. This observation is done in Simply Supported Beam with a uniformly distributed load. A 25kg steel channel was used a base for this project. Mild, Stainless & Hollow Carbon Fiber shaft of various diameters & lengths was used to measure the speed. Whirling/Critical speed is a very serious factor to consider while designing any mechanical product. It is a must known for safety or to prevent disaster form happening in any situation. Ignoring the effect of critical speed will result in the bending or dislocation of shaft. Due to transverse & horizontal vibration the whirling of shaft happens. If shaft is under vibration from outside forces & the frequency of it matches with natural frequency then amplitude of the shaft reaches maximum & creates instability for any engine. By modal or Eigenvalue analysis we can see the perfect critical RPM for any types of shaft. Different types of shaft will give different result based on shape, geometry or property of the materials. The result of the experiment indicates that critical speed decreases with the increase of the shaft length. Also if we increase the diameter of shaft & keep length the same then critical speed increases. We are assessing the various mechanical conditions that take place when shaft is rotating. A particular response of a shaft while rotating can be seen from here. Stainless steel performs better than mild steel & carbon fiber shaft performs better than both of them as it has better weight to strength ratio. For the theoretical calculation we used the Dunkerley's Formula which provides a lower range on the lowest natural frequency of the system meaning detection of critical speed can be done very early by user. The prediction of critical speed can help an engineer to great extent while selecting engine of a product, to understand failure mode in dynamic condition & counter measures can be developed to minimize it.