Ver!ftcation q1f a Finite ElementModel q1f a
Rotating Tippler Structure W Means q1f Strain
Gauge Measurements
P.f .A. van Zyl, N.D.L. Burger and P.R. de Wet
The complex rotational working of a tippler structure complicates the analytical
lngo
side
Outgo side
evaluation of the structure. A further
complication is the ever-changing boundary conditions while the structare rotates,
together with the weight reduction of the
coal in the wagons when the wagons are
ffioaded. Both these factors need to be
taken into account when determining the
Conveyor system
stress levels in the structure while operaFigure 1: Tippler process layout
tional. To verify the accuracy of a finite
element simulation of a ttpping cycle, strain gauge
bined with changing boundary conditions during each incremeasurements obtained from the actual tippler
ment of the tip cycle. These load variations and changing
structure was compared with stress resalts obtained
boundary conditions, induce varying stress levels in the structure while operating. To accurately simulate these stress levels
from linear static finite element analyses of the
in the structure by means of a finite element model, the model
structure, simulating different ttp positions at set
would need to allow for the changing loads and boundary
time intervals. The results obtained from the comconditions for each of an infinite number of positions. However,
parison indicated an accurate simulation of the tipthe cost and time consumed by such an analysis would render
ping cycle by means of the finite element simulation.
the benefits of the analysis inappropriate. For this reason, it was
1. lntroduction
Tippler structures experience varying load or force inputs comDepartment of Mechanical Engineering, University of Pretoria,
South Africa, Email: [email protected]
decided to investi gatethe possibility of simulating the complete
tippler load and tip cycle by means of a small number of linear
static finite element models, each model representing a 10degree interval of the tip cycle, and then verifying the stress
results obtained from the analysis by means of comparison with
calculated stress results determined from strain gauge readings
k
lngo cage assembly
1. Clamp gear
2. Ring gear
5. Pinion
8. lngo outgo ring
11 . Support rollers
14. Clamp
17. Cross beam
4. Platform
7. Base
10. Side beam
13. Counterweight
16. Tre rod
19. Counterweight
Figu
re 2: Components of
lngo cage clamp mechanisnr
the
3. lngo ingo ring
6. Support rollers
9. Cross beam
12. Base
15. Clamp arm
18. Clamp mechanism
ingo cage
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
3
Verification of a Finite Element Model of a Botating Tippler Structure
obtained from the actual tippler structure. The measured results
were obtained by applying strain gauges at selected posjtions
and measuring the strain levels and calculating the stress levels
during consecutive tip cycles. By comparing the finite element
results and the measured results the accuracy of the analysis
method was established.
2. Tipplef tefminOlOgy
A tippler structure consists of two drumlike
cages resting on
eighi support roller assemblies through which the coal wagons
*! driu"n, clamped and then rolled over or tipped to offload the
coal. The coal falls onto a conveyor system which transports it
away. Figure 1 shows the process layout.
Each cage consists of two end rings, a platform
structure, a cross beam at the back and a side beam at
the front. Mounted on the cross beam is a clamp assembly that clamps the wagon onto the rail during the tip
cycle. The layout of the ingo cage and its clamp detail
is shown in Figure 2.
The ingoing and outgoing cages are of similar construction and are referred to as the ingo cage and the
outgo cage. Each cage has two end rings, which are
referred to as the ingo ingo end ring for the ingo side
end tittg on the ingo cage and the ingo outgo end ring
for the outgo side end ring on the ingo cage. Similarly,
the outgo cage's end rings are referred to as the outgo
ingo end ring for the ingo end ring on the outgo side
cage and the outgo outgo end ring for the outgo side
end ring of the outgo cage.
The clamp system consists of two clamps mounted
to two clamp affns which are in turn mounted to the back
of the cross beam. The clamp arrn is further connected
Figure 3: Strain gauges applied to the bottom of the platform structure
by means of a tie rod to a clamp mechanism. This clamp
mechanism incorporates a counterweight. The clamping
process is completely mechanical and there are no outside
forces (hydraulic or electrical) that contribute to the clamp-
ing action.
During the tip cycle the wagons are tipped towards the
side beam by means of two pinion gears that drive the two
ring gears situated on the ingo ingo end ring and the outgo
outgo end ring of the two cages. The drives of the two cages
are not mechanically coupled and the two cages can tip
separately. The tip angle is through 160 degrees and the
total tip cycle takes approximately 40 seconds. The complete load and tip cycle takes approximately 1 10 seconds to
complete. The terminology stated was used throughout the
Measuring direction
Strain gauge
positions k
Figure 4: Strain gauge applied to the top of the cross beam
study.
3. Analysis procedure
The model verification analysis was completed in two
components and the results of these components were
compared to determine the accuracy of the simulation
method. The first step in the analysis consisted of strain
measurements on the structure analysed. The stress results
calculated from the strain values were compared with the
stress results obtained from the finite element analysis for
the positions where the strain gauges were applied.
3.1 Strain gauge analysis
Figure 5: Strain gauge applied to the top of the clamp arm
4
The rotation of the tippler structure during operation complicates strain gauge measurements when using conventional wiring methods. It was therefore decided to make use
of wireless strain gauge amplifiers.
The positions selected for strain gauge application were
selected based on the expected stress patterns in the
structure. Only positions on main structural components
where one-directional stresses and no stress concentrations were expected, were selected for the study. The
R & D Joumql, 2006, 22 (3) of the South African Institution of Mechanical Engineering
Verification of a Finite Element Model of a Rotating Tippler Structure
selected strain gauge positions used for comparison purposes
are shown in Figures 3 to 5. Also note the direction of the strain
measurement. Some additional positions were strain-gauged to
determine the magnitude of internal stress levels in the structure during the tip cycle. These measurements were, however
not used in the model comparison. The positions of these strain
gauges are shown in Figures 6 and 7.
For the application, a half-bridge strain gauge arrangement
was used. This iurangement compensates for temperature
changes that may influence the strain gauge readings during
operation. In this application, the water sprayed in the air to
reduce coal dust during the tip cycle may have caused temperature fluctuations that could influence the readings. Note that
local bending on the strain-gauged plates was ruled out because of the section size of the structure where the strain
gauges were applied. The properties of the strain gauges used
Strain gauge
position
Measuring
direction
Figure 6: Strain gauge position on the ingo outgo end ring
are listed in Table 1.
lngo ingo front
support roller
Gauge type and
arrangement
Kwoya KFG 90o Rosette - applied
in a half bridge arrangement
Gauge type
Steel
Gauge resistance
120 Q with 5 mm grid length
Gauge factor
2.12
Table 1: Strain gauge properties
The strain gauge amplifiers' outputs were set to zero with
no wagons positioned on the platform. The sampling frequency used for the analysis was estimated from a sample
reading taken during set-up recording of a tip cycle as shown
in Figures 8 and 9. For the reading a sampling frequency of
50 Hz was used. This method was used as no known stress
frequency data for the structure was available and the accepted industry standard of using a sample frequency of at
Strain gauge
position
Measuring
direction
Figure 7: Strain gauge positionon the support assembly
Sample frequency evaluation
Ttp cycle
Load cycte
-
filro
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a
t7
n
E
A
- -
tStress fluctuations
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6
55
oooooo
cD o) o)
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50
70
90
sss
E
a-m
{r,
lll
Time [s]
Figure 8: Sample reading of a complete load and tip cycle
R & D Joumal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
5
Verification of a Finite Element Model of a Botating Tippler Structure
Sample frequency evaluation
lngo outgo clamp arm
lngo outgo rail
lngo platform back
o
rrr
o
[L
r410
=
@
Cn
o
*,lb
-2O
CN
I
L
J
I
Time [s]
Figure 9: Magnified sample of the measured stress fluctuations
material properties, geometry and boundary conditions should
be linear throughout the analysis. For the material properties,
this means that the stress levels should be of such nature that
no yielding takes place during the analysis. Furthennore, no
geometric stiffening should take place during the analysis and
the boundary conditions should not change from the original
application to the final deformed shape. The loads applied
should furthermore remain constant in magnitude, direction
and distribution2.
The method used in which the tippler's tip action is broken
down into seventeen intervals and where each interval is dealt
with as a linear static analysis with its own set of static
boundary conditions therefore meets the criteria of a linear
static analysis.
3.2.1 Finite element model preparation
Figure 10: Surface mesh applied to the cage structure
least ten times the structure frequencyt could therefore not be
applied. The stress data obtained from the test readings showed
that no peak data values were lost during the recording, indicating that the sample frequency was adequate.
Strain readings were then obtained for two complete loading
cycles, i.e. the firstpositioning of the wagons on the platform and
then for 25 consecutive tip cycles. The strain dataobtained was
translated to stress values which were then compared with the
stress data obtained from the finite element analysis.
3.2 Finite element analysis
In order to obtain comparative stress data for the analysis, linear
static finite element models were constructed, representing each
l0-degree interval of the tipping cycle. It was decided to use the
linear static analysis method for the analysis as this method is
simpler, faster to complete, and the software used is readily
available and less costly. For a linear analysis to hold true, the
6
The surface and solid components of the tippler structure were
constructed using IDEAS NX software and each component
was meshed separately. A shell model was constructed for the
main structural components and a solid model for the primary
compensating beam and support rollers. Based on an evaluation
of some of the curved surface edges in the model, the decision
was made to use second-order elements as these elements have
the advantage of providing more accurate results on curved
geometries3. Fewer elements could therefore be used and accurate results would still be obtained from a smaller model size. For
the cage assembly , asecond-order or parabolic quadrilateral thin
shell mesh was used with an average element length of 150
millimetres. Where needed the element length was reduced and
triangular elements were used. For the solid model components,
a second-order tetrahedral element with an element length of 40
millimetres was used. The rollers were map-meshed with secondorder solid parabolic bricks.
The meshed surface model of the cage structure is shown in
Figure 10 and that of the roller assembly in Figure 11. The
different colours applied to the model indicate the differentplate
R & D Joumal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
Verification of a Finite Element Model of a Rotating Tippler Structure
thicknesses used in the construction of the structure.
To simulate the rail section mounted to the platform structure,
beam elements with the same cross-sectional profile as the rail
indicated on the structural drawings were used. The rail was tied
to the platform structure by means of rigid elements to simulate
the rail on platform interface. No relative movement is possible
between the rail and the platform.
All pins,
shafts and damping springs were simulated by
means of rigid elements to reduce model set-up times. This
assumption was made as the effect of shaft or pin-bending or the
stress levels obtained in these components would have no
influential effect on the stress levels in the tippler structure.
The main advantage of building a model of the complete cage
assembly lies in the accurate weight distribution and stiffness
representation that the model provides. Each of these factors
could influence the stress results obtained with the models
during the rotation simulation. Where two plates are bolted
together in the assembly the connection was simulated as one
plate with the combined thickness of the two plates. The difference in model stiffness created by simulating the bolted connections as a single plate of representing thickness would not
influence the stress results as these connections are situated far
from the strain gauge positions. Where possible all short surfaces, broken edges and scared surfaces were removed from the
models to reduce the possibility of generating badly shaped
elements during the meshing process.
The next step in the model construction process was to
combine the different structural meshes into one assembly mesh
for each of the lO-degree tip intervals. To reduce model construction time, the I-DEAS "mesh from assembly" function was used.
This function allows the user to mesh all assembly components
separately and then combine all the separate meshes into one
assembly mesh that represents the assembly orientation used.
This process sped up the mesh-generation process for all the
tipping positions investigated. In total 17 models were
constructed. Figure 12 shows the assembly mesh of the
tippler structure in the 60-degree position.
The element thicknesses selected for the wagon do
not represent the actual construction of the wagon
structure, but provide an accurate estimation of the
wagon with its centre of gravity at aheight of 933 mm
above the rail as indicated in the wagon specification.
Additional stiffness was added to the wagon structure
by means of rigid elements that do not contribute to the
weight of the wagon. The main functions of the wagon
model are to simulate the weight of the empty wagon,
provide the force transfer points from the wagon to the
tippler structure and provide clamping areas for the
clamps on the wagons.
A11 access covers in the structure were left open as
the bolt connections on these covers are normally not
preloaded and the cover is sealed with water-resistant
putty which is applied between the cover and the
structure. The covers would therefore not provide any
Figure 1 1: Solid and surface mesh applied to the roller assembly
structural stiffness to the tippler structure. Furthermore, no handrail, walkway structures or piping on the
structure allowed for. The structural weight contribution of these components is negligible.
3.2.2 Bou nd ary cond itions
To accurately simulate component interfaces in the
models, the boundary conditions applied should be
able to transfer all translations and rotations needed
from the one component to the other and vice versa.
This is made possible by using coupled degrees of
freedom, which is a set of nodes linked in specific
directions and rotations. No frictional forces can, however, be simulated by these connections and were
therefore not allowed for in this analysis.
All pinned connections were simulated by
means
of
coupled degrees of freedom. Where the connection
pins are not able to transfer moments the rotational
constraints around the pin centrelines were disabled
allowing the components to rotate freely around these
centrelines. The support roller shafts were constrained
by means of rigid elements and were not allowed to
rotate around their centrelines. This would have no
Figure 12: Finite element model of tippler in the 60-degree position
effect on the results, as the rollers are free to slide on the
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical
Engineering
7
verification ol a Finite Element Model of a Rotating Tippler structure
rail interface in the directions allowed for. The rails ffe, however,
not allowed to slide in the horizontal direction on the grooved
rollers but can slide on the non-grooved rollers. Any sliding on
the non-grooved rollers would simulate play that exists in the
support roller assemblies of the tippler structure. It would
furthermore simulate relative slip that occurs between the rail
and the rollers during the rotational motion of the cagewhen the
static friction coefficient is overcome.
For the raiUroller interface, acoupled degree of freedom was
applied that simulates the perpendicular reaction force that
would be generated by the rollers on the rail. The applied coupled
degrees of freedom are shown in Figure 13. The cage is free to
rotate around its own centreline to allow for twisting during the
analysis.
The wagon wheel interface on the platform rail was also
simulated with coupled degrees of freedom. This method only
transfers the vertical load to the rail and the side force generated
by the wheel flange on the rail when the cage is rotating. The
Figure 13: Coupled degrees of freedom applied at the roller assembly interfaces
Figure 14: Constraints applied at the wagon wheel/platform-rail interface
I
R & D Joamal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
Goal load remaining in wagon
9UXt0
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78000
7An0
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66000
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50000
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24000
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18m0
1AFo
\
6000
0
0
10
m
30
40
50
60
70
80
90
100 110 1m
130 140 150
150
Tip angle [Degrees]
Figure 15: Coal weight in wagon for different tip angles
Speed / time graph for tippler cage
15
3,
11
.428
14, 11.428
10
FI
o
ctt
Otr
E\,
H
!to
o
oo
*0
-G-
15
5
L-
,0
20
E-5
s
to
-10
-15
Time [s]
Figure 16: Rotational speed / Time graph for the tippler cage
Time at each tip angle
180
ii-lr-e-i
L:_-_tt1-_l
---,- -.----- . 17:00::!:!:::::: =!! a a?30 -..-
160
14.71
\25.29
140
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12.83
27.O7
;95--
120
11.08
28.E3
10;20*
gfl.'oo
30.57
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32.33
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35.83
20
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\37.71
0
0.00
10.00
lsoo
30_00
40.00
40.00
rim6'llr
Figure 17: Time steps calculated for stress comparison
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
9
verification of a Finite Element Model of a Rotating Tippler structure
constraints would not affect the bending pattern of the platform
structure. The constraints used on the wagon assembly are
shown in Figure 14. Note, however, that these constraints
change when the cage rotates. From an inspection of the wear
plate on the side beam during the strain gauge installation
process, it was clear that the wagons lean against the plates
during the tip cycle. This would suggest that the wheels on the
back rail of the platform would reduce their reaction force on the
rail or even lift from the rail when the wagon leans against the
wear plate.
The estimated angle at which the wagon would start to lean
over was calculated from the available data for the wagons at
approximately IJ" . To simulate this situation the coupled de-
Figure 18: Platform stress
-
Case 1 (Cage empty)
grees of freedom were removed between the rail and the wagon
and applied between the wagon and the side-beam wear plates
for all positions after the 2}-degree rotation interval.
The support roller assembly bases were constrained in all
directions on the surface interfacing with the concrete foundation. Furthermore, the cage was constrained against rotation at
the pinion/ring gear interface on the ingo side end ring.
The main forces contributing to the stress in the tippler
structure are the gravitational force and the forces introduced to
the structure by means of the wagon and coal load. A gravitational acceleration value of 9.81 m/st was used for analysis.
To simulate the reduction in the weight of coal in the wagon
during the tip cycle a constant load curve was assumed as shown
in Figure 15. This approach was selected to eliminate the
complexity of estimating the weight of coal in the wagon
at each tip angle simulated. From video material taken of
the tip cycle and the angle of repose of coal of between
30 and 40 degreeso, it was estimated that the first coal
would start dumpin gata tip angle of between 30 and 40
degrees. The lower value of 30 degrees was selected for
analysis purposes to allow for all possible angles of
repose. The weight of coal in the wagon was reduced by
6 000 kg for each lO-degree interval rotated up to the I 60degree interval. For the return cycle the wagon was
simulated as empty.
The weight of the coal as obtained from the graph
was applied as a point load at the CG position of the
wagon. Although this boundary condition could influence the structural stresses for certain tip intervals,
applying this condition to all the tip intervals the error
introduced is constant for all tip intervals. The data was
therefore still valid for evaluatin.-e stress trends in the
structure during the tip cycle.
The seventeen FEA models simulating the different
tip intervals were solved, each model solution taking
approximately 40 minutes on a Windows-based workstation. An additional analysis was also done on the
ingo cage with no wagon positioned on the platform.
The results of this analysis were used to determine the
mean stress in the structure caused by .-gravity alone.
The strain gauge data obtained earlier does not take
into account the stress in the structure caused by
gravity and can therefore not directly be compared to
Figure 19: Platform stress
-
Case 1 1 (Empty wagon)
the FEA results.
4. Results comparison
As previously described, a finite element model was
constructed and solved for each lO-degree interval of
the tipping cycle. The finite element results for a specific
tip interval should, however, be compared with
the
strain gauge data for the exact time step when the tippler
cage rotates through the set angle used in the finite
element analysis. To be able to perform this comparison
the time steps at the different tip angles had to be
determined. Further, note that the cage will pass each
interval angle twice during the tip cycle, the first time
with a loaded wagon and the second time with an empty
Figure 20: Platform stress
10
-
Case111 (Full wagon)
wagon.
From the strain gauge results, the total tip cycle time
was determined as approximately 40 seconds. This
R & D Journal, 2006, 22 (3) of the South African Institution of Mechonical Engineering
Verification ol a Finite Element Model of a Rotating Tippler Structure
FEA result
Calculated value
Measured value
Position
Front
Back
Front
Back
Front
Back
Full wagon being
loaded onto platform
14.89 MPa
14.97 MPa
14.00 MPa
14.30 MPa
15.54 MPa
15.54 MPa
Full wagon replacing
empty wagon
10.05 MPa
10.95 MPa
11
.2 MPa
12.4 MPa
Table 2: Comparative stress values for tippler platform
Comparative stress levels - wagon loading
45
40
35
30
Full wagon in
iil r'
o-
3ro
a
E1s
l+t
A10
5
0
5
295
315
-5
Time [s]
lngo platform back
lngo platform front
Figure 21: Full wagon being loaded onto platform
Comparative stress levels - empty and full wagon
50
40
Full wagon in
Empty wagon in
30
F-l
20
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(L
10
r--l
a
a
o
f-
0
+-,
0
-10
-20
-30
40
Time [s]
lnoo latform back
lngo platform front
Figure 22: Full wagon replacing empty wagon
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
11
verification of a Finite Element Model ol a Rotating Tippler Structure
Figure was verified by means of short video clips recorded on
the day the strain gauge analysis was done. Furthermore, the
tippler cage ramp-up and ramp-down intervals were set at 3
seconds. No further cycle detail was, however, available. Figure
l6 shows the speed / time graph for the cage calculated for a 160degree tip angle to be completed in 17 sec with the 3-sec rampup and ramp-down intervals included. The area under graph
represents the 160 degrees rotated. From this graph, the time
intervals at each l0-degree tip angle were calculated from the
slope of the graph and the area underneath the graph.
From this data the time increments for stress data comparison
were calculated and are shown in Figure 17. These time steps
were used as reference to compare the stress values calculated
from the strain gauge data to the stress values obtained from the
FEA results angles.
Two data verifications were done to verify the accuracy of the
FEA method used. For the first verification, the tippler results for
an empty and loaded cage were compared. For the second
verification, the strain gauge data and FEA data for the different
tip intervals were compared. From these results, the accuracy of
the FEA method was determined.
a)
Loaded and unloaded tippler structure
The stress results obtained from the finite element models of the
empty and loaded cages were compared with the strain gauge
results obtained for the same load cases and with values ob-
lngo Platform Back (1)
-
lngo cage - platform stress
---;;il"r*
Ingo Platform Front (1)
lngo Platform Back (5)
lngo Platform Front (5)
Ingo Platform Back (10)
Platform Front (10)
-lngo
lngo Platform Back (15)
Platform Front (15)
-lngs
-__- lngo Platform Back (20)
lngo Platform Front (20)
lngo Platform Back (25)
_
6-
Ingo Platform Front (25)
3'o
8o
c)
l+t
a-n
Figure 23: Measured platform stress values for non-consecutive tip
lngo back support roller - inner and outer stress comparison
5
Offset caused by
bending in support
4
2
6"0
TL
l-l
a
CII
o
l-
_t
+t
A+
back inner support (1)
-8
-10
-12
-14
6 H
Compressive
stess increase
-lpg6 back inner support (11)
-lngo back inner support (13)
-lngo
Ingo back inner support (20)
- Ingo back outer support (1)
- Ingo back outer support (11)
- lngo back outer support (13)
- Ingo back outer support (20)
-
Time [s]
Figure 24: Stress measurements obtained from the support roller assembly
12
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
Verification of a Finite Element Model of a Rotating Tippler Structure
tained from a basic calculation done on the platform structure.
The comparative data is shown in Table Z.TheFEA values used
for the comparison are shown in Figures 18 to 25.
The largest stress difference between the measured and FEA
results is I 1 .l Vo. This is for the back strain gauge where the full
wagon replaces the empty wagon on the platform.
Figures 18 to 20 show the stress results obtained from the
FEA for three load cases, i.e.:
tr The tippler cageempty with only gravitational forces applied
tr An empty wagon positioned on the platform
tr A loaded wagon positioned on the platform
The results used for the strain gauge comparison are shown
in Figures2I and22.
To compare the stress values with the calculated and stress
levels from strain gauge readings the Case I stress was deducted
from the Case III stress simulating a full wagon being loaded onto
the platform. The Case II stress was deducted from the Case III
stress to simulate the difference in stress for an empty and loaded
wagon on the platform. Note for all three Figures the stress scale
was kept the same.
The data for the comparison between the FEA model and the
stress levels determined from strain gauge readings differs by
Il.7 7o at most. This indicates the model is representative of the
actual conditions when the tippler is loaded with wagons.
lngo cage - outgo ring
'o I
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o-
a
a
q)
-r
uu
^-r4;
-5
!b
.l-,
ai0
lngo outgo rail
Time [s]
Figure 25: Stress readings obtained from the outgo ring strain gauge
Stress comparison - platform front
10
5
0
-5
6o-
=aa
-10
-15
o
l-
+-,
a
-20
-25
-30
-35
-40
Time [s]
Figure 26: Stress comparison for front strain gauge on platform structure (Tip cycle)
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
13
verilication ol a Finite Element Model of a Rotating Tippler structure
b) Stress comparison for full tip cycle
results of these readings are shown in Figures 24 and 25.
The stress values obtained from finite element models for the
To compensate for the internal stress variations in the cage
positions where the strain gauges were applied were compared the stress values used for the comparison were calculated by
withthestressvaluescalculatedfromthesffaingaugereadings.
averaging the stress readings obtained from non-consecutive
Thecomparisonwasdonepertimeintervalascalculatedearlier. tip cycles, i.e. the results as shown in Figure 23.
Figure 23 shows a comparison of the stress readings obtained The first two sets of data as shown in Figures 26 and 27 show
for non-consecutive tip cycles. Note the difference in stress the calculated stress comparison for the two strain gauges
levels between the different tip cycles. These variations are applied to the platform structure. The deviation between the
caused by internal forces generated in the structure during the stress values at the maximum stress values is approximately
rotational motion of the cage. The existence of these forces was ll.|Vo for the front strain gauge and 5.OVo for the back strain
confirmed by the measurements taken with the strain gauges gauge.
applied at the positions as indicated in Figures 6 and 7. The
The data for the strain gauge on the cross beam is indicated
Stress comparison - platform back
.---a
6(L
> -10
a
a -20
'flql i 1
1
75
80
E5
90
95
100
105
\
i
H
o
o
l+t
o
Time
[sJ
Figure 27: Stress comparison for back strain gauge on platform structure (Tip cycle)
Figure 28: Stress comparison for cross beam structure (Tip cycle)
14
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
1
Verification of a Finite Element Model of a Rotating Tippler Structure
Stress comparison - clamp arm
I
-10
l-l
$
O
3.*
a
E.ro
l+,
U)
a
40
Clamp arm
FEA
]
Time [s]
Figure 29: Stress comparison for the clamp arm structure (Tip cycle)
in Figure 28. There is a slight deviation in the stress pattern
between the two data sets. This is caused by a difference in the
time of contactbetween the clamps and wagon, in the FEA model
and the actual occuffence. The maximum deviation atthe highest
stress for the cross beam is approximately 9.47o.
The last comparison is between the stress values calculated
from the strain gauge readings on the clamp arm and the FEA
results obtained for the similar position. The results are shown
in Figure29 and have a maximum difference in value of approximately 8.87o.
The difference between the measured and FEA results can be
contributed to effects such as differences in the boundary
conditions applied, ramp-up and ramp-down speeds of the
tippler structure, weight distribution or other effects not simulated in the FEA model. The largest difference in the measured
and FEA data is seen for the cross beam data. This may be caused
by the fact that the spring assembly in the clamp arm mechanism
was simulated by means of a rigid element. The deviation is,
however, only seen in the shape of the signal and not the
maximum stress levels obtained. The data therefore indicates
that the method applied to simulate the tip cycle by means of
multiple linear static finite element analyses does provide an
accurate representation of the actual stresses obtained during
the tip cycle.
References
I. Mercer I, Melton
G and Draper J, The Effect of User Decisions on the Accuracy of Fatigue Analysis from FEA. 2003
ABAQUS Users' Conference, 2003.
2. Adams V and Askenazi A, Building Better Products with
Finite Element Analysis. 1" ed. Santa Fe, NM: OnWord Press
( 104), 1999.
3. Adams V and Askenaz, A. Building Better Products with
Finite Element Analysis. 1" ed. Santa Fe, NM: OnWord Press
( 141). reee.
4. Conveyor knowledge and information technology, 2005.
Available online: http:// www.ckit.co.za. Last accessed:
September 2005.
5. Results discussion
The FEA model results compare well with the strain gauge
readings obtained from the tippler structure. The maximum error
between the readings and the model is approximately Il.|Vo at
the front strain gauge position on the platform structure. This
indicates that the rotational motion of a tippler structure can be
simulated accurately by using linear static finite element models
solved for set intervals. The method would provide a good
estimation of the stress values observed during the tip process
and would therefore be suitable for the calculation of structural
design stresses or fatigue life estimations.
R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering
15