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Joint Analysis
By D Cheshire^
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This tutorial looks at different ways of analysing bolted joints within ProMechanica. This is an important technique as correct analysis may havesignificant effects on the results for the whole part being analysed. Thetutorial^ uses^ a^ simple^ bracket
which^ can^ be^ downloaded^
from http://www.staffs.ac.uk/~entdgc/WildfireDocs/tutorials.htm
under^ the
bracket link. Bonded Analysis As a base point for our comparison of different methods of simulatingjoints we will analyse the bracket as though it was bonded (glued) to thewall (if you did the Introduction to Mechanica tutorial you should alreadyhave done this as a further exercise). Even this is a simplification and ifyou wanted to correctly analyse a bonded joint you would approach itdifferently to account for bond flexibility and other factors.You are now ready to start the analysis process so load the part into ProEngineer.^ Figure 1 : The Bracket for Analysis Choose^ APPLICATION^ >^ MECHANICA
now^ to^ take^ your^ model^ into analysis. Click OK on the box notifying you of the units of your model.From the MODEL TYPE dialog choose STRUCTURE and OK.As always the first step for this model is to define the constraints. We aregoing to roughly simulate the bracket being bonded to a wall so the backface needs to be fixed. INSERT > DISPLACEMENT (or you could just pickthe^ icon). Click on^ below Surface(s) in the constraint dialog then
pick the back surface of the bracket then OK to return to the constraintdialog. You have picked one surface to constrain and the
symbols show what movements are restricted – they all are so this surfaceis fully constrained. Click OK to finish.^ Figure 2 : Constraint Surface Next define the load on the bracket. Choose INSERT > FORCE/MOMENTLOAD or pick the^ icon to apply a load over a surface. Click on below^ Surface(s)^ in^ the^ Force/Moment
dialog^ then^ pick^ the^ surface highlighted in Figure 3 then OK to return to the Force/Moment dialog. Typea value of -10000 in the Y field below Force. Press PREVIEW – the arrowsshould point the same way as in Figure 3. Click OK in the Force/Momentdialog to finish.^ Figure 3 : Load Surface The final definition for this analysis is the material. Choose PROPERTIES> MATERIALS^ and the MATERIALS dialog will appear. Scroll downthe materials library in the Materials dialog to Find STEEL and double click
Joint Analysis
By D Cheshire^
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on it to transfer it to this model. Press ASSIGN > PART and click on thebracket and OK to assign the material. CLOSE the material dialog.That’s^ it^ you^ are^ ready^ to^ run
an^ analysis.^ Choose^ ANALYSIS
MECHANICA ANALYSES/STUDIES
and in the dialog that appears choose FILE > NEW STATIC and type the name BONDED and press OK.Choose the^ icon to run this analysis choosing yes for error detection.Press^ to watch the report of the analysis as it runs. After a fewseconds^ (longer^ on^ a^ slower^ machine!)
the report^ should^ state RUN COMPLETED. Close the REPORT dialog and the ANALYSES dialog.After the analysis completes choose ANALYSIS > RESULTS
menu. The main graphics window will go blank and the menus and icons will allchange. Choose INSERT > RESULT WINDOW or the
icon. In the RESULT WINDOW DEFINITION dialog that appears press
and click
(not double click) on the folder which is the same name as the analysisthat is BONDED. Make sure all the options are the same as in Figure 4then click OK AND SHOW.^ Figure 4 : Results Definition That’s the first analysis complete. Just note the stress distribution in thebracket for now and then try the next method. Choose FILE > EXITRESULTS before continuing. Creating the Holes Now let’s try to analyse a bolted joint – for which we need some holes!Modelling is done outside of Pro Mechanica so choose APPLICATION >STANDARD. Next choose INSERT > EXTRUDE from the menu. You
should see a new toolbar appear like the one in Figure 5. This is called thedashboard and contains all of the options for the type of feature you arecreating.^ Figure 5 : The Extrude Dashboard To^ start^ creating this feature choose
PLACEMENT^ >^ DEFINE^ in the dashboard and the Section dialog appears. Notice that this dialog hasmany fields^ but the sketch plane option is highlighted in pale
yellow awaiting your input. The sketch plane is a flat surface onto which you willdraw your shape. Choose the back surface of the bracket (the one youconstrained in Figure 2) by clicking on it in the graphics window. The otherfields in the Shape dialog are filled in automatically so you don’t need toworry about them at the moment – just click on the SKETCH button.The graphics screen will change to a black background looking directly onto the sketch plane, and the drawing icons described will appear. You canCLOSE the References dialog.You^ are^ now^ ready^ to^ use^ sketcher.
Choose^ SKETCH^ >^ CIRCLE^ > CONCENTRIC and draw the circles with two clicks – the first click on theradius in the corner of the bracket, the second click to determine the sizeas shown in Figure 6 – press the middle mouse to finish drawing eachcircle. You should be able to get all four circles to lock on to the same sizeand showing an R1 symbol.^ Figure 6 : Sketch for Four Holes Your window should now look like
Figure 6^ but the diameter of the circle will be different. To set the size of the circle to the correct value, choose
Joint Analysis
By D Cheshire^
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accurate comparison we can show the two analyses side by side with thesame legend values.Choose INSERT > RESULT WINDOW or the
icon. In the RESULTS WINDOW^ DEFINITION^ dialog that appears press
and click^ (not
double click) on the folder for the BONDED analysis then click OK ANDSHOW. You will see the two analyses together. Click on each window inturn then choose FORMAT > LEGEND. Type 2 as the minimum value and20 as the maximum value so that you can get a fair comparison.^ Figure 9 : Comparison 2 Choose FILE > EXIT RESULTS before continuing. Surface Constraint We are going to improve the simulation of a bolted joint by holding thesurfaces of the holes in position. Follow the procedure for defining a newconstraint as you did with the edge constraints. This time create a surfaceconstraint (INSERT > DISPLACEMENT) and create a new constraint set(this will be CONSTRAINTSET3). Pick the internal surfaces of the holes.^ Figure 10 : Constraint Surfaces
Choose^ ANALYSIS^ >^ MECHANICA
ANALYSES/STUDIES^ and^ EDIT
COPY^ a^ new^ analysis^
called^ SURFACES^ which^
uses
CONSTRAINTSET3. Run the analysis and show the results of the threeanalyses side by side with the same legend values.^ Figure 11 : Comparison 3Accurate Analysis of Bolted Joints In many situations where the area around the bolt is not critical the surfaceconstraint^ technique^ would^ be
acceptable^ (ignoring^ the^ stress concentrations around the holes). For greater accuracy a more complextechnique is required.To understand the technique it first important to understand how a boltedjoint works. It may seem straight forward but it is not! A good source ofresearch for this is^ http://www.boltscience.com/
where there is a tutorial on bolted joints.We will need to define the area where the washer contacts the face of thebracket. Currently this is a single surface so we need to split it into whatare^ called^ regions.^ This^ is^ done
in^ Pro^ Mechanica^ using^ INSERT
VOLUME REGION > CREATE > EXTRUDE > DONE (we use volumeregion rather than surface region as we can split the front and backsurfaces in one command). Pick the front face of the bracket as the sketchplane then OKAY > DEFAULT to enter sketcher. Draw 4 equal diametercircles concentric to the four holes. After exiting sketcher choose THRUALL > DONE then OK. What this has done is created an imaginaryextrusion. Where this extrusion passes through the bracket it has split thesurfaces. This will only be visible if you are in a non shaded display.
Joint Analysis
By D Cheshire^
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Figure 12 : Volume Regions We will also need to create 8 datum points – 1 at each end of the fourholes. Choose INSERT^ >^ MODEL^ DATUM^ >^ POINT
^ POINT.^ The DATUM POINT dialog appears. Click on the edge of one of the holes anda datum point is created where you pick. We want the datum point to be atthe centre of the arc so in the DATUM POINT dialog click on the word ONand change it to CENTRE then click on NEW POINT. Repeat this for the 8points. Don’t forget to click on new point after each point is defined. Closethe dialog with OK.^ Figure 13 : Datum Points That is all of the preliminary geometry defined. Next we will simulate thebolt with a special type of element known as a beam. This will appear likea simple line in the analysis but will have all the same properties as thebolt shank. Choose INSERT > BEAM. In the BEAM DEFINITION dialogclick on^ below Reference(s) then pick the point at either end of onehole. Press the MORE button next to MATERIAL to set the material to
STEEL. Press the MORE button next to SECTION. Choose NEW then setthe type to SOLID CIRCLE with a radius of 6 (this represents the size ofthe bolt shank). Click all the OK buttons to finish defining this beam.Repeat it for all four holes (you won’t need to define material or section asthey are now the defaults).^ Figure 14 : Four Beams defined Now the bolt needs to be connected to the bracket. If the bolt is designedand fitted correctly it should not move relative to the bracket so we will userigid connections. These don’t allow any movement. Choose INSERT >CONNECTION > RIGID CONNECTIONS > CREATE and in the RIGIDCONNECTION dialog click on^
icon then pick the edges of the volume region and the adjacent datum point IN THAT ORDER. Click OK thenclose the dialog with OK. Repeat for each end of the four holes (8 times).^ Figure 15 : Rigid Connection
