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.
