rotating tippler structure w means q1f strain

Ver!ftcation q1f a Finite ElementModel q1f aRotating Tippler Structure W Means q1f Strain

Gauge Measurements

The complex rotational working of a tip-pler structure complicates the analyticalevaluation of the structure. A furthercomplication is the ever-changing bound-ary conditions while the structare rotates,

together with the weight reduction of thecoal in the wagons when the wagons are

ffioaded. Both these factors need to be

taken into account when determining thestress levels in the structure while opera-tional. To verify the accuracy of a finiteelement simulation of a ttpping cycle, strain gauge

measurements obtained from the actual tipplerstructure was compared with stress resalts obtained

from linear static finite element analyses of thestructure, simulating different ttp positions at set

time intervals. The results obtained from the com-parison indicated an accurate simulation of the tip-ping cycle by means of the finite element simulation.

1. lntroductionTippler structures experience varying load or force inputs com-

Department of Mechanical Engineering, University of Pretoria,South Africa, Email: [email protected]

P.f .A. van Zyl, N.D.L. Burger and P.R. de Wet

lngo side Outgo side

Conveyor system

Figure 1: Tippler process layout

bined with changing boundary conditions during each incre-ment of the tip cycle. These load variations and changingboundary conditions, induce varying stress levels in the struc-ture while operating. To accurately simulate these stress levelsin the structure by means of a finite element model, the modelwould need to allow for the changing loads and boundaryconditions for each of an infinite number of positions. However,the cost and time consumed by such an analysis would renderthe benefits of the analysis inappropriate. For this reason, it wasdecided to investi gatethe possibility of simulating the completetippler load and tip cycle by means of a small number of linearstatic finite element models, each model representing a 10-

degree interval of the tip cycle, and then verifying the stress

results obtained from the analysis by means of comparison withcalculated stress results determined from strain gauge readings

klngo cage assembly

1. Clamp gear

4. Platform

7. Base10. Side beam13. Counterweight16. Tre rod19. Counterweight

2. Ring gear

5. Pinion8. lngo outgo ring

11 . Support rollers14. Clamp17. Cross beam

3. lngo ingo ring

6. Support rollers9. Cross beam

12. Base15. Clamp arm18. Clamp mechanism

lngo cage clamp mechanisnr

Figu re 2: Components of the ingo cage

3R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering

Figure 3: Strain gauges applied to the bottom of the platform structure

Verification of a Finite Element Model of a Botating Tippler Structure

obtained from the actual tippler structure. The measured results 2. Tipplef tefminOlOgywere obtained by applying strain gauges at selected posjtions A tippler structure consists of two drumlike cages resting onand measuring the strain levels and calculating the stress levels eighi support roller assemblies through which the coal wagonsduring consecutive tip cycles. By comparing the finite element *! driu"n, clamped and then rolled over or tipped to offload theresults and the measured results the accuracy of the analysis coal. The coal falls onto a conveyor system which transports itmethod was established. away. Figure 1 shows the process layout.

Each cage consists of two end rings, a platformstructure, a cross beam at the back and a side beam atthe front. Mounted on the cross beam is a clamp assem-

bly that clamps the wagon onto the rail during the tipcycle. The layout of the ingo cage and its clamp detailis shown in Figure 2.

The ingoing and outgoing cages are of similar con-struction and are referred to as the ingo cage and theoutgo cage. Each cage has two end rings, which arereferred to as the ingo ingo end ring for the ingo sideend tittg on the ingo cage and the ingo outgo end ringfor the outgo side end ring on the ingo cage. Similarly,the outgo cage's end rings are referred to as the outgoingo end ring for the ingo end ring on the outgo sidecage and the outgo outgo end ring for the outgo sideend ring of the outgo cage.

The clamp system consists of two clamps mountedto two clamp affns which are in turn mounted to the backof the cross beam. The clamp arrn is further connectedby means of a tie rod to a clamp mechanism. This clamp

Measuring direction

Strain gauge positions k

mechanism incorporates a counterweight. The clampingprocess is completely mechanical and there are no outsideforces (hydraulic or electrical) that contribute to the clamp-ing action.

During the tip cycle the wagons are tipped towards theside beam by means of two pinion gears that drive the tworing gears situated on the ingo ingo end ring and the outgooutgo end ring of the two cages. The drives of the two cages

are not mechanically coupled and the two cages can tipseparately. The tip angle is through 160 degrees and thetotal tip cycle takes approximately 40 seconds. The com-plete load and tip cycle takes approximately 1 10 seconds tocomplete. The terminology stated was used throughout thestudy.

3. Analysis procedureThe model verification analysis was completed in twocomponents and the results of these components werecompared to determine the accuracy of the simulationmethod. The first step in the analysis consisted of strainmeasurements on the structure analysed. The stress resultscalculated from the strain values were compared with thestress results obtained from the finite element analysis forthe positions where the strain gauges were applied.

3.1 Strain gauge analysisThe rotation of the tippler structure during operation com-plicates strain gauge measurements when using conven-tional 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 thestructure. Only positions on main structural componentswhere one-directional stresses and no stress concentra-tions were expected, were selected for the study. The

Figure 4: Strain gauge applied to the top of the cross beam

4

Figure 5: Strain gauge applied to the top of the clamp arm

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

Strain gaugeposition

Measuringdirection

Figure 6: Strain gauge position on the ingo outgo end ring

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 todetermine the magnitude of internal stress levels in the struc-ture during the tip cycle. These measurements were, howevernot used in the model comparison. The positions of these straingauges are shown in Figures 6 and 7.

For the application, a half-bridge strain gauge arrangement

was used. This iurangement compensates for temperaturechanges that may influence the strain gauge readings duringoperation. In this application, the water sprayed in the air toreduce coal dust during the tip cycle may have caused tempera-ture fluctuations that could influence the readings. Note thatlocal bending on the strain-gauged plates was ruled out be-cause of the section size of the structure where the straingauges were applied. The properties of the strain gauges used

are listed in Table 1.

Gauge type andarrangement

Kwoya KFG 90o Rosette - appliedin 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 withno wagons positioned on the platform. The sampling fre-quency used for the analysis was estimated from a samplereading taken during set-up recording of a tip cycle as shownin Figures 8 and 9. For the reading a sampling frequency of50 Hz was used. This method was used as no known stress

frequency data for the structure was available and the ac-

cepted industry standard of using a sample frequency of at

lngo ingo frontsupport roller

Strain gaugeposition

Measuringdirection

Figure 7: Strain gauge positionon the support assembly

Sample frequency evaluation

Load cycte Ttp cycle- t7 -

n A - - tStress fluctuations

50 70 90

Time [s]

Ecu -go-oc$I-A- -

*rl

fi'E Eg, gE55 6ooo-ooocD o) o)ssslll

filro(L

e@0aE

{r,

a-m

Figure 8: Sample reading of a complete load and tip cycle

5R & D Joumal, 2006, 22 (3) of the South African Institution of Mechanical Engineering

Verification of a Finite Element Model of a Botating Tippler Structure

Sample frequency evaluation

Time [s]

lngo outgo clamp armlngo outgo raillngo platform back

I

L I

J

rrr o

o[L

=r410@Cnolb*, -2O

CN

Figure 9: Magnified sample of the measured stress fluctuations

least ten times the structure frequencyt could therefore not beapplied. The stress data obtained from the test readings showedthat no peak data values were lost during the recording, indicat-ing that the sample frequency was adequate.

Strain readings were then obtained for two complete loadingcycles, i.e. the firstpositioning of the wagons on the platform and

then for 25 consecutive tip cycles. The strain dataobtained wastranslated to stress values which were then compared with thestress data obtained from the finite element analysis.

3.2 Finite element analysisIn order to obtain comparative stress data for the analysis, linearstatic finite element models were constructed, representing each

l0-degree interval of the tipping cycle. It was decided to use thelinear static analysis method for the analysis as this method issimpler, faster to complete, and the software used is readilyavailable and less costly. For a linear analysis to hold true, the

material properties, geometry and boundary conditions shouldbe linear throughout the analysis. For the material properties,this means that the stress levels should be of such nature thatno yielding takes place during the analysis. Furthennore, nogeometric stiffening should take place during the analysis andthe boundary conditions should not change from the originalapplication to the final deformed shape. The loads appliedshould furthermore remain constant in magnitude, directionand distribution2.

The method used in which the tippler's tip action is brokendown into seventeen intervals and where each interval is dealtwith as a linear static analysis with its own set of staticboundary conditions therefore meets the criteria of a linearstatic analysis.

3.2.1 Finite element model preparationThe surface and solid components of the tippler structure wereconstructed using IDEAS NX software and each componentwas meshed separately. A shell model was constructed for the

main structural components and a solid model for the primarycompensating beam and support rollers. Based on an evaluationof some of the curved surface edges in the model, the decisionwas made to use second-order elements as these elements havethe advantage of providing more accurate results on curvedgeometries3. Fewer elements could therefore be used and accu-rate results would still be obtained from a smaller model size. Forthe cage assembly , asecond-order or parabolic quadrilateral thinshell 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 40millimetres was used. The rollers were map-meshed with second-

order 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. Thedifferent colours applied to the model indicate the differentplate

6

Figure 10: Surface mesh applied to the cage structure

R & D Joumal, 2006, 22 (3) of the South African Institution of Mechanical Engineering


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 railindicated on the structural drawings were used. The rail was tiedto the platform structure by means of rigid elements to simulatethe rail on platform interface. No relative movement is possiblebetween the rail and the platform.

All pins, shafts and damping springs were simulated bymeans of rigid elements to reduce model set-up times. Thisassumption was made as the effect of shaft or pin-bending or thestress levels obtained in these components would have noinfluential effect on the stress levels in the tippler structure.

The main advantage of building a model of the complete cageassembly lies in the accurate weight distribution and stiffnessrepresentation that the model provides. Each of these factorscould influence the stress results obtained with the modelsduring the rotation simulation. Where two plates are bolted

Figure 1 1: Solid and surface mesh applied to the roller assembly

Figure 12: Finite element model of tippler in the 60-degree position

together in the assembly the connection was simulated as one

plate with the combined thickness of the two plates. The differ-ence in model stiffness created by simulating the bolted connec-tions as a single plate of representing thickness would notinfluence the stress results as these connections are situated farfrom the strain gauge positions. Where possible all short sur-faces, broken edges and scared surfaces were removed from themodels to reduce the possibility of generating badly shaped

elements during the meshing process.

The next step in the model construction process was tocombine the different structural meshes into one assembly meshfor each of the lO-degree tip intervals. To reduce model construc-tion time, the I-DEAS "mesh from assembly" function was used.

This function allows the user to mesh all assembly componentsseparately and then combine all the separate meshes into oneassembly mesh that represents the assembly orientation used.

This process sped up the mesh-generation process for all thetipping positions investigated. In total 17 models wereconstructed. Figure 12 shows the assembly mesh of thetippler structure in the 60-degree position.

The element thicknesses selected for the wagon donot represent the actual construction of the wagonstructure, but provide an accurate estimation of thewagon with its centre of gravity at aheight of 933 mmabove the rail as indicated in the wagon specification.Additional stiffness was added to the wagon structureby means of rigid elements that do not contribute to theweight of the wagon. The main functions of the wagonmodel are to simulate the weight of the empty wagon,provide the force transfer points from the wagon to thetippler structure and provide clamping areas for theclamps on the wagons.

A11 access covers in the structure were left open as

the bolt connections on these covers are normally notpreloaded and the cover is sealed with water-resistantputty which is applied between the cover and thestructure. The covers would therefore not provide anystructural stiffness to the tippler structure. Further-more, no handrail, walkway structures or piping on thestructure allowed for. The structural weight contribu-tion of these components is negligible.

3.2.2 Bou nd ary cond itionsTo accurately simulate component interfaces in themodels, 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 offreedom, which is a set of nodes linked in specificdirections and rotations. No frictional forces can, how-ever, be simulated by these connections and weretherefore not allowed for in this analysis.

All pinned connections were simulated by means ofcoupled degrees of freedom. Where the connectionpins are not able to transfer moments the rotationalconstraints around the pin centrelines were disabledallowing the components to rotate freely around thesecentrelines. The support roller shafts were constrainedby means of rigid elements and were not allowed torotate around their centrelines. This would have noeffect on the results, as the rollers are free to slide on the

Institution of Mechanical Engineering 7R & D Journal, 2006, 22 (3) of the South African

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 groovedrollers but can slide on the non-grooved rollers. Any sliding onthe non-grooved rollers would simulate play that exists in thesupport roller assemblies of the tippler structure. It wouldfurthermore simulate relative slip that occurs between the railand the rollers during the rotational motion of the cagewhen thestatic friction coefficient is overcome.

For the raiUroller interface, acoupled degree of freedom was

applied that simulates the perpendicular reaction force thatwould be generated by the rollers on the rail. The applied coupleddegrees of freedom are shown in Figure 13. The cage is free torotate around its own centreline to allow for twisting during theanalysis.

The wagon wheel interface on the platform rail was alsosimulated with coupled degrees of freedom. This method onlytransfers the vertical load to the rail and the side force generatedby 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

9UXt0

E4000

78000

7An0

66000

6fin0

r-r 50000tf,-Y €ooo

E moooO 3flxloJaxn0

24000

18m0

1AFo

6000

0

Goal load remaining in wagon

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

10FIocttOtrE\,

H

!too*0-G-L-

E-5sot

,0

-10

-15

Time [s]

3, 11 .428 14, 11.428

oo5 15 20

Figure 16: Rotational speed / Time graph for the tippler cage

Time at each tip angle180

160

140

6o 120

of-

fl.'oogo80o)E60

40

20

0 40.000.00 10.00 lsoo

rim6'llr30_00 40.00

ii-lr-e-iL:_-_tt1-_l---,- -.----- . 17:00::!:!:::::: =!! a a?30 -..-

14.71 \25.29

12.83 27.O7

;95--

11.08 28.E3

10;20*

9.33 30.57

7.57 32.33

5.83 t 34.08

4.07 \ 35.83

2.A \37.71

Figure 17: Time steps calculated for stress comparison


verification of a Finite Element Model of a Rotating Tippler structure

constraints would not affect the bending pattern of the platformstructure. The constraints used on the wagon assembly areshown in Figure 14. Note, however, that these constraintschange when the cage rotates. From an inspection of the wearplate on the side beam during the strain gauge installationprocess, it was clear that the wagons lean against the platesduring the tip cycle. This would suggest that the wheels on theback rail of the platform would reduce their reaction force on therail or even lift from the rail when the wagon leans against thewear plate.

The estimated angle at which the wagon would start to leanover was calculated from the available data for the wagons atapproximately IJ" . To simulate this situation the coupled de-

grees of freedom were removed between the rail and the wagonand applied between the wagon and the side-beam wear platesfor all positions after the 2}-degree rotation interval.

The support roller assembly bases were constrained in alldirections on the surface interfacing with the concrete founda-tion. Furthermore, the cage was constrained against rotation atthe pinion/ring gear interface on the ingo side end ring.

The main forces contributing to the stress in the tipplerstructure are the gravitational force and the forces introduced tothe structure by means of the wagon and coal load. A gravita-tional acceleration value of 9.81 m/st was used for analysis.

To simulate the reduction in the weight of coal in the wagonduring the tip cycle a constant load curve was assumed as shown

in Figure 15. This approach was selected to eliminate thecomplexity of estimating the weight of coal in the wagonat each tip angle simulated. From video material taken ofthe tip cycle and the angle of repose of coal of between30 and 40 degreeso, it was estimated that the first coalwould start dumpin gata tip angle of between 30 and 40degrees. The lower value of 30 degrees was selected foranalysis purposes to allow for all possible angles ofrepose. The weight of coal in the wagon was reduced by6 000 kg for each lO-degree interval rotated up to the I 60-degree interval. For the return cycle the wagon wassimulated as empty.

The weight of the coal as obtained from the graphwas applied as a point load at the CG position of thewagon. Although this boundary condition could influ-ence the structural stresses for certain tip intervals,applying this condition to all the tip intervals the errorintroduced is constant for all tip intervals. The data wastherefore still valid for evaluatin.-e stress trends in thestructure during the tip cycle.

The seventeen FEA models simulating the differenttip intervals were solved, each model solution takingapproximately 40 minutes on a Windows-based work-station. An additional analysis was also done on theingo cage with no wagon positioned on the platform.The results of this analysis were used to determine themean stress in the structure caused by .-gravity alone.

The strain gauge data obtained earlier does not takeinto account the stress in the structure caused bygravity and can therefore not directly be compared tothe FEA results.

4. Results comparisonAs previously described, a finite element model wasconstructed and solved for each lO-degree interval ofthe tipping cycle. The finite element results for a specifictip interval should, however, be compared with thestrain gauge data for the exact time step when the tipplercage rotates through the set angle used in the finiteelement analysis. To be able to perform this comparisonthe time steps at the different tip angles had to bedetermined. Further, note that the cage will pass eachinterval angle twice during the tip cycle, the first timewith a loaded wagon and the second time with an emptywagon.

From the strain gauge results, the total tip cycle timewas determined as approximately 40 seconds. This

Figure 18: Platform stress - Case 1 (Cage empty)

Figure 19: Platform stress - Case 1 1 (Empty wagon)

Figure 20: Platform stress - Case111 (Full wagon)

10 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 Measured value Calculated value

Position Front Back Front Back Front Back

Full wagon beingloaded onto platform

14.89 MPa 14.97 MPa 14.00 MPa 14.30 MPa 15.54 MPa 15.54 MPa

Full wagon replacingempty 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

Full wagon in

45

40

35

30

iil r'o-3roaE1sl-

+tA10

5

0

-5

5 295 315

Time [s] lngo platform backlngo platform front

Figure 21: Full wagon being loaded onto platform

Comparative stress levels - empty and full wagon50

40

30

20

10

0

-10

-20

-30

40

Empty wagon in Full wagon in

F-l

$(L

r--l

aaof-

+-,0

lnoo latform backTime [s] lngo platform front

Figure 22: Full wagon replacing empty wagon


verification of a Finite Element Model ol a Rotating Tippler Structure

Figure was verified by means of short video clips recorded onthe day the strain gauge analysis was done. Furthermore, thetippler cage ramp-up and ramp-down intervals were set at 3seconds. No further cycle detail was, however, available. Figurel6 shows the speed / time graph for the cage calculated for a 160-degree tip angle to be completed in 17 sec with the 3-sec ramp-up and ramp-down intervals included. The area under graphrepresents the 160 degrees rotated. From this graph, the timeintervals at each l0-degree tip angle were calculated from theslope of the graph and the area underneath the graph.

From this data the time increments for stress data comparisonwere calculated and are shown in Figure 17. These time stepswere used as reference to compare the stress values calculated

from the strain gauge data to the stress values obtained from theFEA results angles.

Two data verifications were done to verify the accuracy of theFEA method used. For the first verification, the tippler results foran empty and loaded cage were compared. For the secondverification, the strain gauge data and FEA data for the differenttip intervals were compared. From these results, the accuracy ofthe FEA method was determined.

a) Loaded and unloaded tippler structureThe stress results obtained from the finite element models of theempty and loaded cages were compared with the strain gaugeresults obtained for the same load cases and with values ob-

lngo cage - platform stress

---;;il"r*

6-3'o8oc)l-+ta-n

- lngo Platform Back (1)

- Ingo Platform Front (1)


lngo Platform Front (5)

- Ingo Platform Back (10)

-lngo Platform Front (10)


-lngs Platform Front (15)

-__- lngo Platform Back (20)

lngo Platform Front (20)

lngo Platform Back (25)

_ Ingo Platform Front (25)

Figure 23: Measured platform stress values for non-consecutive tip

5

4

2

6"0TL

l-l

aCII _tol-+tA+

lngo back support roller - inner and outer stress comparison

Offset caused bybending in support

-8

-10

-12

-14

-lpg6 back inner support (1)

-lngo back inner support (11)

-lngo back inner support (13)

- 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)6 H Compressive stess increase

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


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 FEAresults is I 1 .l Vo. This is for the back strain gauge where the fullwagon 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 appliedtr An empty wagon positioned on the platformtr 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 IIIstress 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 byIl.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-

-r -5

aaq)!b

.l-,

ai0

'o I uu ^-r4;

lngo outgo rail

Time [s]Figure 25: Stress readings obtained from the outgo ring strain gauge

Stress comparison - platform front

6-o-

=aaol-+-,a

10

5

0

-5

-10

-15

-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 cagepositions where the strain gauges were applied were compared the stress values used for the comparison were calculated bywiththestressvaluescalculatedfromthesffaingaugereadings. averaging the stress readings obtained from non-consecutiveThecomparisonwasdonepertimeintervalascalculatedearlier. 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 showfor non-consecutive tip cycles. Note the difference in stress the calculated stress comparison for the two strain gaugeslevels between the different tip cycles. These variations are applied to the platform structure. The deviation between thecaused by internal forces generated in the structure during the stress values at the maximum stress values is approximatelyrotational motion of the cage. The existence of these forces was ll.|Vo for the front strain gauge and 5.OVo for the back strainconfirmed 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

6-(L> -10

H

aao -20l-+to

.---a i 1'flql 1 75 80

\

E5 90 95 100 105 1

i

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


Stress comparison - clamp arm

I

-10

l-l

$

3.*aE.rol-+,

U)40

O

a

Clamp armFEA ]

Time [s]

Figure 29: Stress comparison for the clamp arm structure

in Figure 28. There is a slight deviation in the stress patternbetween the two data sets. This is caused by a difference in the

time of contactbetween the clamps and wagon, in the FEA modeland the actual occuffence. The maximum deviation atthe higheststress for the cross beam is approximately 9.47o.

The last comparison is between the stress values calculatedfrom the strain gauge readings on the clamp arm and the FEAresults obtained for the similar position. The results are shownin Figure29 and have a maximum difference in value of approxi-mately 8.87o.

The difference between the measured and FEA results can be

contributed to effects such as differences in the boundaryconditions applied, ramp-up and ramp-down speeds of thetippler structure, weight distribution or other effects not simu-lated in the FEA model. The largest difference in the measuredand FEA data is seen for the cross beam data. This may be caused

by the fact that the spring assembly in the clamp arm mechanismwas simulated by means of a rigid element. The deviation is,

however, only seen in the shape of the signal and not themaximum stress levels obtained. The data therefore indicatesthat the method applied to simulate the tip cycle by means ofmultiple linear static finite element analyses does provide an

accurate representation of the actual stresses obtained duringthe tip cycle.

5. Results discussionThe FEA model results compare well with the strain gauge

readings obtained from the tippler structure. The maximum errorbetween the readings and the model is approximately Il.|Vo atthe front strain gauge position on the platform structure. Thisindicates that the rotational motion of a tippler structure can besimulated accurately by using linear static finite element modelssolved for set intervals. The method would provide a goodestimation of the stress values observed during the tip processand would therefore be suitable for the calculation of structuraldesign stresses or fatigue life estimations.

(Tip cycle)

ReferencesI. Mercer I, Melton G and Draper J, The Effect of User Deci-sions on the Accuracy of Fatigue Analysis from FEA. 2003ABAQUS Users' Conference, 2003.

2. Adams V and Askenazi A, Building Better Products withFinite Element Analysis. 1" ed. Santa Fe, NM: OnWord Press( 104), 1999.

3. Adams V and Askenaz, A. Building Better Products withFinite 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.

R & D Journal, 2006, 22 (3) of the South African Institution of Mechanical Engineering 15

rotating tippler structure w means q1f strain

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