Analysis of Vibration in Machines and Structural Strength Stillage

This case study examines vibration issues in a centrifugal fan system used for drying malt and a structural failure in a stillage table. The vibration analysis identifies potential causes such as imbalance, misalignment, improper speed loads, and foundation issues, recommending solutions like bearing adjustments, rigid mountings, and proper lubrication to prevent recurrence. The structural failure analysis of the stillage focuses on the collapse of legs due to a sloped floor and inadequate safety factors. The redesign includes material selection, load calculations, buckling load consideration, and adjustments for floor tilt, along with recommendations for safe lifting and mechanical handling. The student document is available on Desklib, which provides a platform for students to access solved assignments and study resources.

Case Study: Vibration on Machineries
And
Structural Strength of Stillage
Table of Contents
Task2A: Vibration in Machineries................................................................................................................2
References:..............................................................................................................................................6
Appendix A..............................................................................................................................................7
Task2 Part B: Structural Failure in stillage...................................................................................................8
References:............................................................................................................................................14
Appendix A:...........................................................................................................................................15
Appendix B............................................................................................................................................17

Task2A: Vibration in Machineries
Case Study brief:
A case study was done on a Centrifugal fan on a foundation which is used to force the hot air on
a drying malt. The specifications for the centrifugal fan are given, the problems observed and the
failures occurred are also listed.
Measured Vibrations:
The vibration on pedestal and foundation are measured using an accelerometer. The graph is
given at figure1. From the provided graph we can decipher the places in which the vibration
occurrence is very high.
The vibration graph:

Figure1. Illustrates the graph of Vibration rate.
1. What are the likely causes of the vibration Problem?
The major likely causes for the vibration can be found out using the observed data, graph and
the impact that the vibrational problem left on the machine. We observed that there are several
problems and failures occurred due to this vibration. The primary cause for the pulsating
vibration which is experienced when closing the inlet vane is due to the fact that there will be a
partial imbalance in the mass of the fluid that is being processed by the centrifugal fan, as the
area of the fluid inlet experience a sudden reduction there will be an imbalance in the mass of
fluid so that the mass becomes heavier at the middle portion and lower at the end portion which
paved a way for pulsating vibrational motion.
The second problem noted was the excessive vibration even though the impeller is balanced
carefully. This problem is not due to the impeller this problem might have been due to the
Misalignment of the shafts (see Appendix A for force analysis on shafts). The shafts of any
mechanical system should be aligned in a proper manner. Misalignment could load to the
resonance which will further cause heavier and more vigorous vibrations. As we have noted from
the failure that there are two broken 24mm bolts, which might have been occurred due to the
vibration in shaft. The same vibration might have also lead to the damage in the surfaces where
the bearings are mounted (Trebuňa, F., et.al 2014). There are many types of misalignments such
as angular and parallel which are explained in the figure 2.
Figure2. Illustrates the types of misalignments(Simm et.al 2016)
Improper or fluctuating speed loads might also cause vibrations which may arise due to the slack
in the belt of the V belt drive system. The shaft speed depends upon the speed of the pulley if the
speed fluctuates due to slack in the pulley belt then the shaft will also experience fluctuating
loads which will cause pitching vibrations. The foundation mounting is also the most noticeable
cause for the problem as we can see that there are only two bearings to hold the whole assembly
of the drive shaft and there are also only few grouting between the pedestal and the foundation.

Also, the reason for excess vibration might be due to loose bolt and nuts or any other parts of the
whole assembly which might have not been set at its proper torque criteria (Panovko, G., et.al
2015).
The maintenance person might have not to check if all the connections are made as per the
instructed torque level and the connecting nut and bolt are of good shape and form. The problem
might also be due to running the impeller at a higher speed than that of its rated speed. The RPM
(Rotation per Minute) of the impeller might be higher than that of the rated RPM will cause an
excessive vibration problem.
The third problem noticed is the occurrence of second and third harmonic vibration in the
frequency spectrum which might have occurred to the excessive loading of the impeller i.e
allowing more amount of fluid than it can handle. Which in turn creates pitching and rocking
vibrations across the impeller end and the drive shaft end which will pass through the shafts. The
bearing might have been damaged due to these kinds of vibrations.
Due to the unequal lubrication such as lubricating a certain part properly and not taking care of
another part will also lead to vibration. The unequal lubrication will set the particular part in
vibratory motion. The to faulty bearings and worn out bearing which were not check properly
before the operation of the machine is also a major cause.
Figure3. Illustrates types of faults in a bearing
The irregular knocking noise might be due to the worn-out parts and improper lubrication while
running a machine. The machine must be properly lubricated in order to work efficiently and to
carry away the heat. The misalignment in shaft might also lead to knocking. The straightness of
the shaft and the impeller blade mountings may also be the potential cause for this problem
(Wright Jeremy).

2. What would you do as a plant engineer to prevent recurrence?
In order to prevent the reoccurrence of these kind of vibration problem:
The bearing diameter can be increased in order to facilitate more load-bearing capacity and the
meanoptimal position of the bearing, in general bearings must be placed close to the load and the
spot near to the drive system. Also the bearings can be placed on positions where the forces
acting are large (See Appendix A).
The fixing of the pedestal and the foundation shall be made the more rigid and proper size of the
nut and bolts shall be used (Ohashi, H. 2016).
The mountings on which the pedestal and the foundation are joined together might be provided
with rubber bush mountings.
The general nuts can be replaced with wingnuts, which will hold the nut in position in spite of
heavy vibrational problems.
The V belt of the powertrain is set in its mean tension and it is of good form and shape.
Ensuring that all the partssuch as bearings, impeller hub, pulley hub, etc. are lubricated properly.
All the maintenance for the machine is done without any delay and as per the manual.
All the machine components are set in its mean set position.
There is no other unwanted component present in the machine bed which may induce a harmonic
vibration.
The machine parts are uniformly lubricated.
The bearing of the machine parts must be check periodically in order to ensure that the bearing
and its inner race are of the proper condition.
The ambient conditions must be set or maintained at an optimal level which is prescribed by the
manufacturer of the machine.
Critical parts such as drive system and impeller should be operated at its optimal speed level so
that the generation of vibration and noise will be within the limits.

References:
Ohashi, H. (2016). Vibration and oscillation of hydraulic machinery. Routledge.
Panovko, G., Shokhin, A., Eremeykin, S., &Gorbunov, A. (2015). 1644. Comparative analysis of
two control algorithms of resonant oscillations of the vibration machine driven by an
asynchronous AC motor. Journal of Vibroengineering, 17(4).
Simm, Anthony & Wang, Qing & Huang, Songling & Zhao, Wei. (2016). Laser Based
Measurement for the Monitoring of Shaft Misalignment. Measurement. 87.
10.1016/j.measurement.2016.02.034.
Trebuňa, F., Šimčák, F., Bocko, J., Huňady, R., &Pástor, M. (2014). A complex approach to the
Vitro diagnostic analysis of excessive vibration of the exhaust fan. Engineering Failure
Analysis, 37, 86-95.
Wright, Jeremy. “Causes of Machine Vibration.” What Causes Machinery Vibration?, Retrieved
from www.machinerylubrication.com/Read/25974/signs-tips-machinery-vibration.

Appendix A
The force acting on the shaft can be calculated by:
Radial Force,
F= W tan ϕ
cos θ N
Here,
W is thetangential force excerted on the gears which are mounted onthe shaft .
ϕis the pressure angle( Normal) for the gears .
θ is thehelix angle of gear .
W is calculated by:
W = P ( 60000 )
π n D
D isthe gear pitch diameter
P isthe power∈KW
n is the Speed ∈RPM

Task2 Part B: Structural Failure in stillage.
Fig1. Stillage and the pressure vessel.
A solution for questions:
1. Why did the leg collapse? What effect did the designer ignore?
It is observable from the case study that the legs of the stillage collapsed as soon as the
pressure vessel started to tilt on one side which happened due to the slope in the floor on
which the whole assembly is mounted.
The detail or effect that the designer missed during the design of the stillage is the tilt that
is present on the floor and the Factor of Safety, the floor on which the whole assembly
will be placed must be visited and analyzed by the designer before starting the design and
some corrections might have been done as per the condition of the floor. (Farshchin, M,
et.al 2016)
The main reason is that the designer failed to provide Factor of safety for the legs. The
designer should have provided a proper safety factor to which the legs can withstand
loads. Every component should be designed for some safety factor which is nothing but it
can withstand the more amount of load than that of its required load carrying capacity
(Jamal Haseeb)

2. Redesign the stillage to avoid a similar incident in future:
The Stillage must be designed with the same dimensions and should be designed with
proper Factor of Safety (FOS). The tilt that is present on the floor should also be
considered while designing.
Fig2.Stillage and Pressure vessel
Dimensions of stillage required:
The above image shows the dimension of the stillage.
Design Load per leg: 25KN (see Appendix A) of load needed to be carried by each
legs. The calculation for which is given at the Appendix A
Material Selection for Stillage:
Assumed material: Carbon Steel: C30
Tensile Strength: 500 Mpa

Yield Stress: 300Mpa
Type of frame: L-shaped(Assuming)
Area cross section: 50 X 50 X 5mm thick (Taken for commercial availability for testing
strength) (Available at: http://www.buildmantra.com/productdescription/SAIL-L-Shaped-
Mild-Steel-Angle--50-x-50-x-5mm-?product_id=5022)
Now, Actual load on the stillage per leg is calculated as:
σ = P
A
Where,
σ is the Stress
P is the Load
A is the area
Now,
The design Stress is the tensile strength of the chosen material which is 500Mpa
Now, we need to calculate if the selected cross section of angle bracket can bear the load:
Area of the L bracket:
A = (50 X 5 + (45 X 5)) = 475 mm2 = 4.75e-4 m2
σ = 25000
A
P: 25000 (See Appendix A)
σ =0.5267 e 8 N/m2

It is found that the Actual Stress on the stillage leg is less than that of the design stress so
that the design is safe for impact loading conditions. Also, it is designed to withstand
sudden high loadings.
Fig3. Shows the Cross section area of the angle bracket.
Now the design will be tested for buckling load safety
Buckling load Consideration:
Considering Pin-Pin configuration
Buckling can be checked as:
Pcr= π 2 EI
L2
Now,
E is Young's modulus of the material: 200 Gpa
L is 2.4 m

I- Moment of inertia
Buckling: BuckL= π2 EI
L2
Where,
E is the Elastic modulus (200Gpa)( General for steel)
I- is the moment of inertia. I =0.3469e-6 m4 (Calculation for which is given in Appendix
B)
L is the length of the leg 2.4 m
Applying all the known values and solving the equation for BuckLwe will get,
BuckL = 11.88 KN
The buckling Load is less than that of the load to be applied thus the structure is safe for
Buckling.
Now finally,
Considering the tilt on the floor The 2 legs on the left of the stillage is made shorter than
the right leg:
Assuming the Tilt in the floor make a right angled triangle, then
tan θ= opposite side
adjuscent side
tan5= A
2
A=¿0.174 m