ac 80 ch10(bolt pretension)
TRANSCRIPT
Bolt Pretension
Chapter Ten
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Training ManualBolt Pretension
• Whenever you model a bolted structure, it might be important to include the pretension (or preload) in the bolt caused by the tightening of the bolt.
• ANSYS provides a convenient way to simulate bolt pretension:
– Pretension elements PRETS179
– Automatic pretension mesh generation
– Load management for pretension sequence of multiple bolts
Stresses due to specified
pretension in bolt
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Training Manual…Bolt Pretension
• Although most pretension analyses involve contact elements to simulate contact between the bolt and the surrounding structure, we will avoid the contact issues here and focus on how to "pretension the bolt."
• In this Chapter, the following Topics are covered:
A. PRETS179 Element
B. Typical Procedure
C. Workshop
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Surrounding Structure
Preloaded Bolt
Pretension Node
Pretension Section(brick or tet meshes)
Bolt Pretension
A. PRETS179 Element
• Pretension elements (PRETS179) apply the specified preload across a pretension section.
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Bolt Pretension
…PRETS179 Element
• Features of the pretension element:
– A set of pretension elements is identified as a “section”.
– 2-D or 3-D line element that acts like a "hook" connecting two halves of a bolt.
– Nodes I, J are the end nodes, usually coincident.
– Node K is the pretension node:
• Location is arbitrary.
• Has one DOF: UX.
• Used to define the preload, as an FX force or UX displacement.
• Actual line of action is in pretension load direction
Preload direction
J
K
Node I
Pretension section
Bolt — solids, shells, or beams
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Bolt Pretension
…PRETS179 Element
• Features of the pretension element (continued):
– Preload direction is constant – it does not update for rotations
– No material properties or key options
– Underlying bolt elements may be solids, shells, or beams, lower or higher order.
– Elements created automatically using GUI-based procedure.
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Bolt Pretension
B. Typical Procedure
• Five main steps:
1. Import or create the geometry, including the bolt(s) and the surrounding structure.
2. Mesh all parts.
3. Create the pretension elements.
4. Apply loads and solve using multiple load steps:
• Load step 1 for the bolt preload
• Load step 2 to “fix” the bolt length
• Load step 3 for other loads on the structure
5. Review results.
• We will expand on steps 3 and 4 next.
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Bolt Pretension
...Typical Procedure
Creating the Pretension Elements
• After all parts of the structure, including the bolt(s), have been meshed, the next step is to create the pretension elements.
• Two options:
– Using PSMESH
• Fastener must be meshed as one piece.
• PSMESH will cut the fastener in two and generate the pretension section together with the pretension elements.
– Elements at coincident nodes, EINTF (not discussed here)
• The fastener must be meshed in two separate pieces.
• Requires a matching node pattern.
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Bolt Pretension
...Typical Procedure
• The menus provide a wide variety of methods to create the pretension mesh.
– Preprocessor > Modeling > Create > Elements > Pretension > Pretensn Mesh >
• We will illustrate the procedure using the With Options > pick.
ANSYS splits the bolt shown and inserts the necessary pretension elements.
ANSYS splits the bolt shown and inserts the necessary pretension elements.
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Bolt Pretension
...Typical Procedure
• Select the option to – Divide at Node – Picked Elements +
– Follow the menu prompts
Pick a node to indicate where to section the bolt.
A pretension node will be created at this location.
Using the “Box”options, select the elements where the bolt should be split.
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– Next, fill in the dialog box
– Optional specifications include:• TOL= allows nodes locate slightly above/below the separation to be grouped
properly
• TYPE = type number for pretension elements (automatically defined if it is not supplied)
• ECOMP = name of component to be composed if new pretension elements
• NCOMP = name of component to be composed of nodes in new pretension elements
– Pressing OK will separate the elements of the bolt into two unconnected groups, tied together with pretension elements.
Bolt Pretension
...Typical Procedure
Assign a section number and name
Specify the preload direction (Y-axis points along the bolt axis in this case)
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Bolt Pretension
...Typical Procedure
– The output window or file will list a summary of important pretension specifications after each PSMESH execution
A Plot of the FE model will show a visible split along existing element boundaries representing where the pretension elements have been added
A Plot of the FE model will show a visible split along existing element boundaries representing where the pretension elements have been added
Pretension Section ID
Pretension direction
Pretension node number
9 pretension elements created
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Bolt Pretension
...Typical Procedure
• You can “undo” a PSMESH operation as follows:
– Preprocessor > Modeling > Delete > Pre-tens Elemnts
• The GUI automatically deletes the pretension elements and merges the nodes at the pretension section.
– However, the pretension node (K node) is not deleted.
– Note: if you don’t pick the pretension node, only individual pretension elements attached to that node will be deleted.
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Bolt Pretension
...Typical Procedure
Pretension Load Application
• When a physical bolt is pretensioned:
– Turning the nut reduces the unstretched grip length of the bolt, thereby inducing pretension
– When the desired pretension is achieved and the wrench is removed, the new unstretched grip length becomes ‘locked’
• Typical ANSYS pretension loading procedure represents this same sequence
– First, apply the specified pretension (usually a specified force) in one load step
– Then, lock the pretension section displacement (lock the shortened grip length) in a subsequent load step.
– Once all bolts are pretensioned and locked, apply external loads in the final load step
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Bolt Pretension
...Typical Procedure
• Pretensioning and locking a large number of bolts in a specified sequence is made easy with the load-management tool.
– Solution > Define Loads > Apply > Structural > Pretnsn Sectn …
Section to be preloaded first
Semi-automated ‘locking’ of pretension displacement in the next load step
Pretension load to be applied to this bolt
Stiff plate
Soft gasket
bolt1 bolt2 bolt3
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Bolt Pretension
...Typical Procedure
• Continue the loading sequence for the other two sections:
Incrementing the loadstep number controls the pretensioning sequence
Semi-automatic restraint of the free-body bolt pieces– Tied together - applies a displacement constraint of zero until preload is applied– Axial sliding - offers no resistance in axial direction until preload is applied– 0.1% of force - applies 0.1% of used defined force until preload is applied
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• Multiple loadings (up to 15) can also be applied to the same pretension section(s) simulating a torquing sequence.
– Or by Command Input:SLOAD,1,pl01,TINY,FORC,1000,2,3
SLOAD,1,pl02,,FORC,10000,6,7
Multiple loads on highlighted section “bolt_1”
Bolt Pretension
...Typical Procedure
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Bolt Pretension
...Typical Procedure
• In solution, ANSYS automatically applies and locks the loads in the specified load step every time you issue a SOLVE.
– For the three bolts shown in the previous example, you would need to solve seven times to apply the specified pretension sequence.
• SOLVE (LS1, initial untorqued assembly)
• SOLVE (LS2, applies the preload to bolt1)
• SOLVE (LS3, fixes the preload displacement for bolt 1 and tightens bolt2)
• SOLVE (LS4, locks bolt 2 and tightens bolt3)
• SOLVE (LS5, locks bolt3)
• SOLVE (LS6, applies 2nd preload to bolt1)
• SOLVE (LS7, fixes the new preload displacement for bolt 1)
– With the preload applied, apply external loads acting on the overall structure.
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Bolt Pretension
...Typical Procedure
• Deformed geometry plot and bolt forces for the first pass of bolt preloads (LS1 thru LS6) in previous example
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Bolt Pretension
C. Workshop
• Please refer to your Workshop Supplement for instructions on:
• W10A. Bolt Pretension.