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16.0 Release

Lecture 4:

Bolt Pretension

ANSYS Mechanical

Advanced Connections

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Bolt 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.

Mechanical provides a convenient way to simulate bolt pretension

Automatically disconnects the nodes about midway thru shank of meshed bolt body (3D solid and line bodies only)

Reconnects nodes with constraint equations

Uses constraint equations to define a mathematical offset with the expressed purpose of generating the user defined preload.

Stresses due to specified pretension in bolt

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Features of the pretension element:

A set of pretension elements is identified as a section. 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 (always aligned with bolt shank axis via local coordinate system).

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 or beams

Bolt Pretension

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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 Mechanical 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

Bolt Pretension

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Pretension Load Application (contd)

Bolt pretension load can be applied to a cylindrical face, to a straight edge of a line body, to a single body, or to multiple bodies.

Line of action of the bolt load applied to a cylindrical surface will be along cylinder axis by default.

Line of action of bolt load applied to a line body is always parallel to line.

If you apply the Bolt Pretension load to a body, you will need to have a local Coordinate System object in the tree.

The application of the load will be at the origin and along the z-axis of the local coordinate system.

You can place the coordinate system anywhere in the body and reorient the z-axis.

Bolt Pretension

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There are a number of options under the Define By Column of the Bolt Pretension Tabular Data.

The most common procedure involves a minimum of two Load Steps where a user defined Load is applied in LS1 followed by Lock in subsequent LS2,3,etc.

Bolt Pretension

Load: Applies a force as a preload. A Load field is displayed where you enter the value of the load in force units.

Lock: Fixes all displacements. You can set this state for any step except the first step.

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Adjustment: Applies a user defined length as a pre-adjustment (for example, to model x number of threads).

An Adjustment field is displayed where you enter the value of the adjustment in length units.

Multiple Adjustments can be applied over multiple Load Steps.

An Adjustment can also follow or precede a Load

Bolt Pretension

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Bolt Pretension Incremental load

Conventional: Load - Lock

User defined adjustment (displacement) is ramped from zero at LS1. Same displacement value is parameterized to %_FIX% and locked at LS2.

User defined preload ramped from zero at LS1. Corresponding displacement required to achieve preload is calculated, parameterized to %_FIX% and locked at LS2.

From ds.dat files

Conventional: Adjustment - Lock

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Increment: applies user defined offset as an incremental sequential adjustment for challenging bolted assemblies

Value gets added to the solved deformation from the previous step

Is ramped on from previous step

Can follow Load, Adjustment or Open after LS1

Can be applied multiple times in sequence

Mechanical: Bolt Pretension Incremental load

Supports Restarts

If a solution restart is performed from a substep of a load step including an Increment, the increment value gets added to the solved deformation value at the beginning of the selected restart sub-step.

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Bolt Pretension Incremental load

Conventional: Load - Lock

. Increment - Load - Lock

Displacement is ramped from the current solved value to the newly defined value

User defined preload ramped from the reaction force of previous step. Prior displacement constraint removed.

From ds.dat files

Alternative:

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Open: Effectively suppresses the load for the step so that the load has no effect on the applied step

Note that in order to avoid convergence issues from having under-constrained conditions, a small load (0.01% of the maximum load across the steps) will be applied. You can set this state for any step.

Bolt Pretension

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If you try to apply a preload on the same face more than once, all definitions except the first one are ignored.

Be sure that a sufficiently fine mesh exists on a face or body that contains Bolt Pretension loads so that the mesh can be correctly partitioned along the axial direction (that is, at least 2 elements long).

For simulating one Bolt Pretension through multiple split faces, you should apply only one Bolt Pretension load to one of the split faces, as the Bolt Pretension load will slice though the whole cylinder even though only part of the cylinder is selected.

Bolt Pretension

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Care should be used when applying a Bolt Pretension load to a cylindrical face that has bonded contact. There is a possibility that if you apply a Bolt Pretension load to a cylinder that had a bonded contact region, the bonded contact will block the ability of the Bolt Pretension to deform properly.

The Bolt Pretension load should be applied to cylindrical faces that contain the model volume (that is, do not try to apply the Bolt Pretension load to a hole).

Use caution when defining bolt loads by bodies and a coordinate system because the entire body is sliced along the local XY plane (Z=0).

Bolt Pretension

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Body Scoping

Body scoping of a Bolt Pretension load can be to more than one body. In this case all the scoped bodies will be cut.

There is still only a single Bolt Pretension load created but this feature allows you to apply a bolt load to a bolt that has been cut into several bodies. This feature is illustrated in the following figure.

Bolt Pretension

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Bolt Pretension Reactions

Use the Probe tool in Solution branch to confirm reaction in bolt

Bolt Pretension

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Visualizing Bolt Pretension Elements

The PRET179 elements used for bolt pretension are actually just constraint equations. You can visualize these constraints on the FE model as follows:

Highlight the Solution Information Branch. In the Details Window, FE Connection Visibility:

Activate Visibility =Yes

Display = All FE Connectors

Bolt Pretension

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The solver output will also list a summary of important pretension specifications for each bolt pretension section that was created for additional confirmation

Pretension Section ID

Pretension direction

Pretension node number

9 pretension

elements created

Bolt Pretension

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Bolt Pretension via Joint Technology

Joint technology as an alternate

Supports Large rotation based pretension and pretorque loads

Supports locking in subsequent loadsteps

The preloaded Bolt needs to be sliced in advance of analysis.

The two cut surfaces are connected by a cylinder joint.

The pretension loading is applied via Joint load (FJ,,FZ)

The loading can be locked via DJ,,UZ,%_FIX% and DJ,,ROTZ,%FIX%

For large deflection applications where the bolt itself is going thru a large rotation, the CEs might become invalid. In such cases consider using a bolt pretension option available in

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Using cylindrical joint the stress

appear without significant bending

Using conventional bolt pretension,

the stresses appear with significant

bending with rotation

Bolt Pretension via Joint Technology

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Please refer to your Workshop Supplement on

WS3A-bolt.wbpz

Workshop Bolt Pretension Modeling

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Produce the bolt thread stress profile without meshing the threads

Contact Normals computed internally based on user defined bolt thread parameters.

Supports 3D and 2D(MAPDL only) axisymmetric models

Frictional, Frictionless, Rough and No Separation

Applicable only for standard straight threads

Small strain formulation, small rotation

Much easier to set up

Improved run time efficiency

In this test case:

True thread model solves in 22167 sec.

Virtual thread model solves in 9142 sec.

Bolt Thread Modeling

True thread Virtual Thread

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Build conventional surface to surface asymmetric contact between cylindrical faces.

Define thread parameters in contact details window

Mean Pitch diameter

Thread Pitch distance

Thread angle

Starting/ending orientation points (Program Controlled defaults to top and bottom of scoped cylindrical bolt body center).

In MAPDL, use SECTYPE and SECDATA commands

Bolt Thread Modeling

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Solver output records thread specifications as defined:

Bolt Thread Modeling

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With sufficient mesh refinement, general stress profiles match very closely

Bolt Thread Modeling