mech intro2 14.0 l04 joints

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2011 ANSYS, Inc. August 26, 20131 Release 14.0

14. 0 Release

Introduction to ANSYS

Mechanical Part 2

Lecture 4

Joints, Springs and Beams


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Chapter Overview

In this chapter we introduce the use of joints, springs and beams in Mechanical:

A. The Joint FeatureB. Joint Definitions

C. Joint Coordinate Systems

D. Joint Configuration

E. Modifying Joint Coordinate Systems

F. Joint Stops and Locks

G. Springs and Beams

H. Workshop 4, Using Joints


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The joint feature in Mechanical can provide a fast and simple alternative to

contact when simulating the interaction between bodies or constraints toground:

While used extensively in rigid body analysis joints are not limited by body type

and can be used in flexible and mixed rigid/flex models.

Joints are defined in terms of their degrees of freedom with respect to a specific

coordinate system (e.g. translation in the X direction or rotation about the Zaxis).

Joints are attached to bodies by scoping to a specific region of the part, a surface

for example, just like contact.

Contact pairs are defined as contact and target while joints use the terms

reference and mobile to describe each side of a joint (for body to ground

joints the ground is assumed to be the reference).

Note, because this material is meant to be an introduction to the joint

feature, not all aspects of it will be covered in detail. Advanced courses innonlinear, dynamic and rigid body analysis are available.

A. The Joint Feature


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In the example shown here, a body to ground

revolute joint is scoped to a cylinder: The legend shows the RZ or rotation about Z is free.

Degrees of freedom shown in grey are constrained.

The Reference Coordinate System listed in the

details is shown at the origin of the joint. This is the

joints line of action.

. . . The Joint Feature


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There are 9 joint types available in Mechanical which can be

either body to body or body to ground.In the revolute joint example below notice the reference andmobile regions are color coded.

The legend displays the joints behavior with respect to the

reference coordinate system. Colored DOF are free, grey

indicates a fixed DOF.

B. Joint Definitions


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All joints are defined in terms of 2 coordinate systems, the reference and

mobile CS. The CS are associated with each part scoped to the joint. Its the

relationship between the CS that controls the joints motions.

By default the mobile coordinate system is assumed to be coincident with

the reference and is not displayed.

If the mobile coordinate system is set to Override it will be displayedboth graphically and in the tree.

C. Joint Coordinate Systems


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D. Joint Configuration

Configuring a joint allows the initial relationship

between the reference and mobile coordinatesystems to be changed:

Begin by highlighting the joint to be configured in

the tree.

Now click the Configure icon in the context

menu.

When a joint is in configure mode its position can

be changed by dragging the DOF handle shown

below.



. . . Joint Configuration

Joint configuration may be used to merely test the effect of the joints

motion. Toggle off the configure tool and the joint will return to its originalconfiguration.

A joint can be locked into a new position if desired:

After setting a new configuration for the joint, choose Set from the context menu.

The new configuration becomes the starting position when solved.

The Revert icon can be used to cancel the operation.

In addition to manually configuring a joint,

a value can be entered into the field next to

the configure icon.



. . . Joint Configuration

Example:

When configuring angular joints, holding the control key will cause the joint

to move in increments set in WB > Tools > Options > Graphics interaction

(e.g. 10 degrees).



E. Modifying Joint Coordinate Systems

Recall that a joints motions are determined according the joints coordinate

systems. In some instances it will be necessary to reorient these systems toobtain the correct joint behavior.

Click in the Coordinate System field in the details to bring up the

apply/cancel buttons and place the coordinate system in edit mode. Notice

the CS graphically expands while editing.


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. . . Modifying Joint Coordinate Systems

While in edit mode click on the CS axis to be

modified: With that axis active you can click on another

axis, edge, face, etc. to establish a newdirection.

Note the negative axes show as well while

editing directions. Complete the change by pressing the Apply

button in the joint details.


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In the example below, the original orientation of the

translational joints X axis is incorrect for the desired motion: With the CS in edit mode, the X axis is selected.

The local negative Y axis is then selected as the new

direction for the X axis.


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In addition to manually reorienting a joint

coordinate system the same transforms usedin creating and modifying local coordinate

systems are available.


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F. Joint Stops and Locks

For the Revolute and Cylindrical joint types a

torsional stiffness and/or damping can bedefined in the joints details.

Most joints can also employ stops and/or

locks to limit the range of joint motion (see

table below).


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G. Springs and Beams

A longitudinal spring can be defined as another form

of connection. Springs can be of the body to body orbody to ground type:

Springs are assumed to be in their free state (unloaded)by default.

A preload may be added using either a free length or

load value.

Damping may be added to the springs definition.

For grounded springs it is often useful to create local

coordinate systems to control the ground location.

Note, for flexible bodies, a springs behavior is always

tensile and compressive (both). Rigid body analysesallow a spring to be tensile, compressive or both.


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. . . Springs and Beams

As mentioned before a spring represents a

remote boundary condition. Many of the samefeatures discussed earlier are available when

defining springs:

Springs are defined in terms of reference and

mobile sides.

The behavior can be rigid or deformable.

A pinball region can be defined to limit the

creation of constraint equations.


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The Beam feature allows connections from body to body or body to ground.

Although not limited to this purpose, beams are often useful in simulatingvarious fasteners (e.g. bolts).

As with springs, beams share many of the same features as the other remote

boundary conditions already discussed:

Beams are defined in terms of reference and mobile sides.

The behavior can be rigid or deformable.

A pinball region can be defined to limit the creation of constraint equations.


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In the beam details choose both a material and

radius for the section (note current beams are onlydefined using a circular cross section).

As with springs, beams share many of the same

features as the other remote boundary conditions

already discussed.

Again for body to ground beams local coordinate

systems allow the ground location to be specified.


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Workshop 4

Workshop 4, Using Joints

mech intro2 14.0 l04 joints

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