Aim:
find out the critical frequencies of a rotating shaft in Solidworks and Find out 5 mode shapes and list the resonant frequencies
Objective:
To carry out the frequency analysis of a rotating shaft.
To determine its natural frequency and corresponding mode shapes.
theory :
Frequency analysis is performed to find out the natural frequency and corresponding mode shapes of the component.
Natural frequency is an inherent characteristic property of the component, The Natural Frequencies of a structure are the frequencies at which the structure naturally tends to vibrate if it is subjected to a disturbance. This vibration will cause the structure harm and sometimes the structure might even fail
Mode shapes of the component give the direction in which component is vibrating at that corresponding natural frequency.
The deformed shape of the structure at a specific natural frequency of vibration is termed its normal mode of vibration.
Damping effects are neglected in frequency analysis when finding the 'Natural frequency' of the component.
Components will have an infinite number of natural frequencies but the lowest natural frequency is known as the fundamental frequency.
Natural frequency is determined to avoid the resonance of the component.
Resonance occurs when the external frequency is equal to the natural frequency, in this state resonance amplitude causes the failure of the component. In physics, resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force is equal or close to a natural frequency of the system on which it acts. At resonance, the vibration amplitudes are so high that there is a chance that the system might fail
Solver: We use SolidWorks which provides us with two types of solvers:
FFEPLUS - It is an iterative solver. An initial solution is assumed and from this solution through iterations, the final solutions can be reached.
Direct Sparse - It is a direct solver which does not use any approximations.
Options in Frequency analysis in SolidWorks:
Number of frequency: it allows the user to specify the number of mode shapes or natural frequency that is needed to be considered.
Frequency closest to: It allows the study to be focused on a particular frequency. It is only available with direct solver.
Upper Bound Frequency: It allows the user to set a particular frequency above which the solver will not calculate.
Incompatible Boundary Options: if the body is having several contacts between the parts inside the model, the nodes may not be perfectly matched with each part in order for the force to be transferred.
These problems can arise due to several reasons: 1.The geometric size is not uniform; 2.The mesh size is not uniform throughout.
SolidWorks uses the default mode of surface to surface bonding contact, if results do not converge properly, the node to surface contact is used.
To create a model of a rotating shaft in Solidworks. The dimensions used are as follows:
Smaller Shaft Diameter = 40mm
Extrude = 120mm (from midplane)
Shaft Diameter = 60mm
Extrude = 140mm (from midplane)
Rotating Shaft diameter = 20mm
Extrude = 10mm (from midplane)
BOUNDARY CONDITIONS:
The rotating shaft is considered as Homogeneous, isotropic bodies
Shaft materiall is taken as Alloy steel.
Applied a bearing fixture on the stepped face of the rod at the two ends.
Mesh conditions.
Mesh type : Solid mesh
Mesher used : standard mesh
To perform the Frequency Analysis on the rotating shaft, enter the simulation tab with frequency study.
to perform the Frequency Analysis on the rotating shaft, enter the simulation tab with frequency study.
Here, Alloy material is provided as the material for the rotating shaft.
material properties :
then Apply a bearing fixture on the stepped face at the two ends of the rotating shaft to stop rotation of shaft.(as shown a fig)
now create a mesh here we are selecting default option (as shown belown)
and run the frequency analysis study to get the different mode shapes of the shaft.
Save the results and find out the 5 mode shapes of the shaft and listing the resonant frequency.
Amplitude1 :
Amplitude2 :
Amplitude3 :
Amplitude4 :
Amplitude5 :
list of resonant frequency of the rotating shaft
The shaft can be operated apart from the above resonant frequencies. Whenever the natural frequencies becomes equal to the frequencies of the applied force the shaft vibrates with high vibration amplitude whicn ultimately fails the system.
Conclusion: -
The resonance frequencies which is obtained from the amplitude Apart from these frequencies, the shaft can be operated without any problem. But whenever the frequency is equal to any of the above-mentioned ones, the system will go in resonance and high vibrations amplitude are obtained. This fails in the system. This can easily be prevented by providing Dampers in the system. The material also plays an important role in the determination of the resonant frequency. If a more suitable material is used, the resonant frequency can be increased such that it can never be reached under working conditions.