10th International Mine Ventilation Congress, IMVC2014
`© 2014, The Mine Ventilation Society of South Africa
Structural statics and dynamics on axial fan blades
T. Neff & A. Lahm
TLT-Turbo GmbH, Germany
ABSTRACT: The failure of a fan blade in heavy duty rotating equipment can have fatal consequences and
lead to a major loss of production. The static, dynamic and aero-elastic loads over the entire operating
range of a fan was analyzed to determine the design rules that must to be implemented for the different
types of axial fan blades in order to achieve the highest possible life and ensure the safety of the system.
1 MOTIVATION
1.1 Fan Type Selection
During the ordering phase for a main ventilation fan
a fan pre-selection process has to be followed,
similar to the process in Figure 1.
fans and impulse type fans would reveal that there is
not a robust, reliable design on either side
Beside aerodynamic performance differences
both fan solutions have their own strengths and
weaknesses in respect of static and dynamic
behavior.
2 LOADS ON AXIAL FAN BLADES
2.1 Static and quasi-static loads
The impeller blades of an axial fan is where
electrical energy from the electric motor is converted
to mechanical engineering, by the movement of the
gas through the fan impeller.
Impeller blades are subjected to various
mechanical loads of which a typical simplification is
given in Figure 2.
These loads are independent of the fan type.
Figure 1: Fan selction process, for the selection of the correct
fan technology for the required ventilation task.
This paper will focus on Axial fans and analyse
one of their major components, namely the impeller
blades, regarding safety aspects.
For heavy duty rotary equipment endurance is a
must, however commercial factors are becoming
more and more dominating, and it makes the
operating life of the equipment even more difficult.
The direct comparison between overpressure type
The most dominating force is the centrifugal
force FZ
FZ
m
r
m r
2
(1)
= generalized mass or a mass element
= radial distance of m from the rotation axis
= the angular frequency
Moreover there are other loads also acting on the
impeller blade, such as the aerodynamic force,
inertial force and the torque, all at the same time.
10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
fn,j = j n / 60
j = rotational multiples
(3)
In case the structural elements in front of the
impeller that are not equally distributed, each
i leads to a frequency fsup,i
fsup,i
i
i
Figure 2. Typical load vectors representing the static and
dynamic forces on an axial fan blade during operation.
As the impeller blades are typically twisted for
aerodynamic efficiency improvements, not all of the
blade mass is concentrated on a single plane or a
central axis. It is due to these offset masses that
additional inertial forces
subsequently increasing the internal blade material
stresses.
The aerodynamic forces are initiated by the
pressure distribution at the airfoil. They can be
fractionalized into a circumferential part, acting
against the rotation and a tangential part, acting
against the direction of flow. For more details, see
Eckert&Schnell (1980).
2.2 Dynamic Loads
In order to keep the hub in it s place in many cases,
it cannot be avoided that structural support ribs are
placed upstream of the fan impeller. Depending on
the form and thickness of the support ribs, the
intensity of vortices and wake turbulences
downstream will vary.
Each impeller blade with its own natural
frequencies fbl,i will be subjected to various other
disturbances, within a single rotation of the impeller
blade.
This blade passing frequency fpass is the most
important one
fpass = Zbl n / 60
(2)
Zbl = number of rotor blades
n = rotations per minute.
Another important and powerful excitation is the
rotational speed and its multiples fn,i
i
n / 60
(4)
= angle between two structural elements
= consecutive number
The downstream static guide vanes are also a
source for generating vibrational excitations within
the rotating blades, typically these guide vanes are
equally spaced and the excitation frequency due to
these guide vanes fgv can be described as:
fgv = Zgv n / 60
(5)
where: Zgv is the number of guide vanes
Furthermore the electrical power supply
frequency fel and 2 fel shall be checked for
resonances with fbl,i, too. It is the challenging task of
the blade design engineer to ensure that:
fbl,i
pass
n,i
fsup,i
gv
el
fel
(6)
To ensure that the above equation (6) holds true
the blade stiffness can be altered. It is therefore
advisable not only to tailor the blade from root to top
according to stress calculations, but moreover to also
increase the thickness locally that will have an
influence on the blade stiffness. A blade with a high
stiffness will have a high natural frequency and vice
versa.
What is the magnitude of stresses at the rotating
blades, caused by aerodynamic excitation from
structural support elements upstream and
downstream of the impeller? As shown by Staiger
(1991) the stress amplitudes can be considerably
high and careful positioning can improve the load
situation of the impeller significantly.
The blades of an axial test rotor, with 12 blades
were equipped with strain gauges and the influence
of different strut configurations on the blade stresses
were tested.
In the worst case, the comparison between no
struts and three equally distributed, radial struts of
cylindrical shape gave a stress amplification factor
of eight.
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10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
When a fan is operated in
conditions the gas might contain elements which
may cause abrasion and adhesion effects on the
impeller blades. Abrasion is the removal of original
blade material (weight reduction) by the working gas
while adhesion is increasing the blade mass due to
particles in the working gas sticking to the original
blade material.
Both abrasion and adhesion will alter the natural
frequency of the blades and can cause unwanted
resonances additional to the original designed
resonances.
To complete the picture regarding important
loads on axial fan blades, the driving torque
variances have to be mentioned as well.
These torque forces act as foot excitations to the
blades and should not coincide with the natural
blade frequencies. When a variable speed drives is
used, the pulsations in rotational speed can correlate
with e.g. the torsional resonance of the motor / fan
impeller system. This could increase the amplitudes
of the above mentioned blade foot excitations.
It is however very unlikely that the blade will fail
earlier than the shaft system.
one value per section - the maximum of stress or
strain.
Other methods like FEA can be used when
stress distribution details are essential. This would
also enable stress values to be for any point on the
relevant component.
Figure 3. Sketch about sections (A-A I-I) for conventional
blade calculation
3.2 Determination of natural frequencies
3 DESIGN RULES
3.1 Conventional engineering approach
Three decades ago, when computers and Finite
Element Analysis (FEA) tools were much less
powerful than today, the engineers had to simplify
complex blade geometry in order to calculate the
various blade stresses.
The sectional approach
the most common
calculation method during that time. This method is
still in use today. The blade is split from foot to tip
into portions (Fig 3) and each of these is loaded
according to its radial position. While at the base (AA) the centrifugal forces of all elements above are
responsible for the sectional stresses, at the top (HH) there is only the element itself that contributes to
the load.
In case the blade has a circular foot to be
connected to the impeller, it can be assumed that
within an angle of 45° the flux of force is
developing.
Accordingly, the loaded section is reduced (Fig 3,
A-A, B-B and C-C).
By approximation, the section modulus can be
derived from the geometry together with the bending
moment, the bending stresses per section can be
superimposed to the tensional stresses.
Usually von Mises combined stresses are
compared with the selected material properties. This
kind of conventional stress calculation develops only
Most ambitious was - and still is - the estimation of
the blade natural frequencies. The influence of the
supporting structure and consequently the fixing of
the impeller blade to the hub is essential.
Blade to
shaft
connection
Figure 4: Typical impeller of a blade pitch axial fan with the
pivot-mounted blade shaft and the bolted connection to the
blade
It is obvious that a bolted blade (Fig 4) will
behave different from a welded blade (Fig 6).
Nevertheless the bolted connection is also very rigid
during operation when the very high centrifugal
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10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
force FZ stiffens the angular ball bearing of the blade
shaft to a remarkable extent. For a fan impeller of
Ø3.7m and operated at 750rpm, FZ of one single
casted steel blade can reach values of 350kN or even
more.
In the case of individually mounted blades,
measurements of natural frequencies can be
conducted on a workbench. The blade is bolted on a
big mass when a hammer is used to excite an
oscillation and a vibration pick-up sensor records the
response.
If the measured natural frequency would coincide
with one of the possible excitation frequencies, the
blades would need a tuning e.g. by mass reduction
(grinding, drilling holes from the blade tip), leading
to a higher natural frequency. This method allows
only small steps in the range of up to 5Hz.
When measured frequencies are compared to the
calculated excitations during operation, the
stiffening influence of the centrifugal force must be
considered. More details about the analytical
calculation are given in Traupel (1982). It might be a
challenging task to find suitable blade dimensions,
especially for the first three natural frequencies of
the rotating blade fbl,1, fbl,2 and fbl,3 so that no
concord with the known excitations (see section 2.2)
exists. Figure 5 illustrates it graphically.
Figure 5. Graphical overview of possible blade excitation
frequencies and the first 3 blade natural frequencies (example).
Conservative, safe intervals between fbl,1, fbl,2 and
fbl,3 to some of the important excitation frequencies
are given in Table 1.
Table 1. Recommended safety margin between first three blade
natural bending frequencies and selected excitations.
fn1
%
+30
fn2
%
±20
fn3
%
±10
fsup,i
%
±15
fpass
%
±10
The analytical calculation of blade frequencies
requires a lot of experience and tests for method
verification. But there is no compromise possible
when safety is of the essence.
The impeller of an axial impulse fan (Fig 6) is a
completely welded structure and its dynamical
behaviour can be compared to that of
means, that there is one very dominant base mode
which even with a strong punch can be excited,
leading to an audible tone.
Hub
Blades
Figure 6. Typical impeller of a mixed flow fan with welded
blade to hub connection.
And there are the harmonics where packages of
blades together are moving contrary to other
packages. Although this fan type is commonly
known as robust and easy to handle, care has to be
taken to avoid any kind of excitation, which may
lead to fatigue cracks, Section 5 describes impulse
impellers in greater detail. Blade flutter is a serious
concern which has to be addressed by the blade
engineer as well. This aero-elastic phenomenon is
especially critical for long, slender, thin blades. By
altering the blade geometry, e.g. the blade thickness
in the root portion of the blade, the blade stiffness
will be increased and blade flutter would be
minimized or avoided all together.
3.3 State of the art calculations
The state of the art method for the determination of
blade natural frequencies is using F.E.A. simulation
software, which is an integrated part of modern,
three dimensional mechanical design software. The
F.E.A method is an appropriate way to get numerical
and animated results regarding various vibrational
modes of the blade under investigation.
Figure 7 shows typical results of a FEA
simulation for a fan blade, where different colors
indicate the displacement at the given natural
frequency, at various point on the blade surface.
When applying the operation loads on the blade, the
same model can be used for stress determination as
well.
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10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
influence the required safety factor include, but are
not limited to: material of construction, the quality
of surface, the ductility of the blade material, the
operation temperature of the fan and the gas quality.
Typical values of safety factors against mechanical
yield point can vary from 2.0 to 3.0, but may be
increased subject to the above mentioned factors.
3.5 Manufacturing boundaries and restrictions
While the blade of a mixed flow fan usually is
fabricated from a single thickness, formed steel
plate, the manufacturing of an airfoil profile of an
overpressure fan is usually more complex.
There are numerous fabrication methods available
for the manufacturing of impeller blades, however a
detailed discussion regarding these processes would
fall outside the scope of this paper.
The preferred method of fabrication for airfoil
blades of heavy duty fans would be casting, forging
or hot forming. These casted or forged blades must
be accurately machined after casting and various
quality checks must be performed to verify that the
blades are acceptable.
4 QUALITY REQUIREMENTS
4.1 Welded parts
Figure 7. Examples of first 3 natural frequency modes with and
without the influence of centrifugal force. Red color = largest
displacements
Impellers of mixed flow fans as well as some small
axial fan impellers have their blades directly welded
to the hub. This crucial connection should be done
with the highest care and checked with the same care
afterwards.
The quality of the welding seams must be
specified and evaluated according to ISO 5817
Group B or another comparable welding standard.
Irregularities such as excessive peaking, linear offset
or spatters must be avoided at all cost.
Regardless of the welding standard utilized a
visual inspection followed by a non-destructive
(ND) test procedure should be employed to
determine any hidden failures (e.g cracks).
4.2 Casted pieces
3.4 Safety factors
In order to make fans cost effective, high tip speeds
of 190 m/s and even higher are desired. Blade
mechanical design limits are defined by the material
properties in combination with the necessary safety
factors.
Each fan supplier may have its own philosophy
regarding the required safety factors which would be
applied to the blade design.. Other important factors
that would typically be considered and which would
Typically larger blades are more difficult to cast
without flaws. Large casted blades, regardless
whether they are made from aluminum, steel, or cast
iron should therefore always be subjected to
intensive testing. The test methods are defined by
the relevant original equipment manufacturer
inspection plan may prescribe that from each casting
batch at least one blade should be tested with
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10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
destructive methods to confirm the correctness of the
material properties, both chemical and mechanical.
Visual inspection, dye penetrant or magnetic
particle testing is the first choice here due to the
relative low cost. Cavities, cracks and other
irregularities must however be ruled out by
additional ND-testing on each blade.
On thick section blades it is also worthwhile to
employ radiographic or ultrasonic testing procedures
as these methods are able to detect failures deep
within the material structure. Figure 8 shows an
example.
there is a high load of acid-forming contents
together with moderate amount of medium-sized
particles in the air, stainless steel with leading edge
protectors would usually be a good choice to
maintain the expected lifetime of the impeller
blades.
However if this crucial step is misinterpreted by
the fan engineer, or in-sufficient details from the end
user were received, the wear protection might not
selected correctly the predicted blade lifetime will be
dramatically reduced. Even small defects, which are
tolerable during the quality assurance process, will
very quickly develop into larger cracks or craters
and the blade may fail prematurely.
4.4 Operational experience
4.4.1 Mixed flow fans
Figure 8. Colored blade foot regions for zones, indicating
different quality requirements (red = higher; green = lower).
For each of the sections the blade design engineer
has to find the best compromise between acceptable
failure sizes and the risk of a blade failure during
operation.
4.3 Material selection
Mine ventilation is one of the most complex
applications in terms of fan blade material selection.
In order to achieve long life-cycles, operational
considerations regarding gas quality (dusty or
abrasive) as well as mechanical considerations have
to be made. Dust load and humidity, together with
the underground and surface temperature conditions
may cause serious problems for the durability of the
blades. At worst, the blade material will be subjected
to not only particle impacts, but also chemical
erosion may occur, by the formation of acids due to
the presence of various corrosive elements in the
process gas.
Due to the variety of mines and the specific
operating conditions of each mine, there is no
general applicable wear protection method, the
selection of a suitable wear protection system is
done on a case by case study, which takes into
account the specific details for every mine site.
This first step in selecting the appropriate surface
protection is in the correct selection of a blade base
material. Aluminum, SG iron, stainless steel and
composite materials are all proven on mining fans.
Depending on the primary threat, the base
material should be selected and an additional
protective coat has to be applied. For example if
In Section 3.2 it was mentioned that the operating
conditions and the built-in situation has to be
considered in order to avoid damages. At low
volume flows when the inlet guide vanes are nearly
closed, mixed flow fans can deliver only low
pressure. The stall line is low and during start-up or
part load conditions the operating points may cross
that line (Fig 9a).
The impeller will subject to severe resonances
during this time, and continued operation within the
stall zone may lead to premature impeller failure,
concentrated at the welds between the impeller blade
and hub, due to fatigue cracks caused by the
resonance.
The installation of a performance curve
stabilization ring can raise the stall line at low flow
volumes and avoid such dangerous difficulties (Fig
9b).
Figure 9. Comparison of mixed flow fan without (9a) and with
(9b) performance curve stabilization.
As shown by Maddox (1991) there are important
features of welds in relation to fatigue. The
difference between welded and unwelded materials
is significant in respect of endurance and fatigue
limits. Maddox (1991) showed that for a BS 4360
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10th International Mine Ventilation Congress, IMVC2014
© 2014, The Mine Ventilation Society of South Africa
Grade 50B steel specimen the stress range for 106
cycles is approx. 300 MPa. When at an equivalent
specimen additional material is welded from both
sides, the stress range drops down to 100 MPa. At
107 cycles the ratio becomes even worse, namely
270/45.
Consequently the quality acceptance have to be
of the highest grade for all welds, however the
presence of micro-cracks cannot be avoided.
Therefore procedures for the fatigue design of
welded structures, e.g. according to the European
standard EN 1993-1-9 (2005) or equivalent are not
negotiable.
When mixed flow fans are operated with variable
speed drives, the possibility that structural, welded
elements of the impeller fall into a resonance zone is
realistic. Even when the loads at resonance speeds
might be low, the number of cycles will reach
millions within some days. The addition of load
cycles is like adding something to a reservoir which
have limited capacity, eventually it will be full and
failure cannot be avoided.
Alternatively these fans can be fitted with inlet
guide vanes to modulate the airflow at constant
driving speed.
4.4.2 Overpressure fans
The general arrangement for this type of fan is to
have individually mounted airfoil blades which are
fixed to a central impeller hub. Duty modulation and
energy efficiency would be optimized by blade pitch
adjustments. Blade pitch adjustments alter the angle
of attack of the blades and thus the airflow can be
modulated. Blade pitch adjustments are available in
manual or automatic versions. A manual method
would mean an artisan will have to stop the fan,
adjust all the blades individually and then restart the
system, while an automatic system would adjust the
blades all at once while the fan is online.
Further optimization by using variable
speed\frequency drives is possible as well, however
these variable speed\frequency drives are still
relatively expensive and it is a matter of weighing up
investment
expenditure
versus
operational
expenditure.
Reversal of airflow is also possible with blade
adjustments\reversal so that at least 70% of the
flow in the reverse direction can be
achieved.
The advantage of a constant speed drive is
evident in respect of rotor dynamics. The risk to run
through an aerodynamic unstable condition (stall) is
relatively low, because the characteristic curve is
similar to Figure 9b.
In-flight variable pitch fans can be started with
closed blades. This will ensure that the starting
torque is kept on the lowest possible level and the
flow is started smoothly.
For less demanding requirements, simpler fans
with blades adjustable at rest are chosen. Auxiliary
mining fans are usually constructed using this
principle. Designed for onerous and robust
conditions they can be used where flow control or
energy efficiency are not the dominating factors.
5 CONCLUSION
There is no other part of a fan which is of similar
importance as its blade. It is the aerodynamic
performance which the buyer of a fan is specifying
and where his primary focus is on. He will expect
that the fan supplier did his utmost to diminish the
failure probability of his rotating machinery to an
absolute minimum.
specification complies with both the aerodynamical
and mechanical characteristics of the selected fan
type.
In this context, the article aimed to provide an
insight into how loads, design rules and quality
aspects are interwoven in a typical fan design.
Following a well-proven engineering approach, will
ensure a safe and reliable product, free of failures
and with the longest possible service life.
6 REFERENCES
Eckert, B. & Schnell, E. 1980. Axial und Radial-kompressoren.
Berlin: Springer-Verlag
European Committee for Standardisation 2005. EN 1993-1-9:
Eurocode 3: Design of steel structures - Part 1-9 Fatigue
Maddox, S.J. 1991. Fatigue Strength of Welded Structures. 2nd
Edition. Cambridge: Woodhead Publishing Ltd.
Staiger, M. et al. 1991. Beanspruchung der Laufschaufeln von
Axialventilatoren bei gestörter Zuströmung: 125-145. VDIBerichte Nr. 872. Düsseldorf: VDI-Verlag
Traupel, W. 1982. Thermische Turbomaschinen: 405-409.
Band II, 3.Auflage. Berlin: Springer-Verlag
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