altair's student guides - instructor's manual - cae and multi body dynamics

Student Project Summaries CAE and Multi Body Dynamics

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Contents

Introduction.............................................................................................2 Installation Instructions: .......................................................................2

Dynamic Stresses in a Gear Assembly........................................................3 Description of the Problem ....................................................................3 Planning the Approach ..........................................................................4 Setting up the FE Model ........................................................................4 The MBD Model and Results ..................................................................5 Further Work........................................................................................6 Summary .............................................................................................6

Design of a Torque Limiter .......................................................................7 Description of the Problem ....................................................................7 Planning the Approach ..........................................................................8 Setting up the FE Model ........................................................................8 The MBD Model and Results ..................................................................9 Further Work........................................................................................9 Summary ........................................................................................... 10

Analysis and Design of a Valve Train ....................................................... 11 Description of the Problem .................................................................. 11 Planning the Approach ........................................................................ 12 Setting up the FE Model ...................................................................... 13 The MBD Model and Results ................................................................ 13 Further Work...................................................................................... 14 Summary ........................................................................................... 14

Design of a Wiper Subsystem ................................................................. 15 Description of the Problem .................................................................. 15 Planning the Approach ........................................................................ 15 Setting up the FE Model ...................................................................... 16 The MBD Model and Results ................................................................ 17 Further Work...................................................................................... 18 Summary ........................................................................................... 18


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Introduction This material is best used after reading the book CAE and Multi Body Dynamics. Access to HyperWorks software is not essential for you, the instructor. Of

course, if you choose to solve the problems yourself before working with your students, you will need HyperMesh, MotionView, MotionSolve,

HyperView, HyperGraph and OptiStruct.

This book describes 4 assignment problems that highlight different

applications of HyperWorks. Each problem is independent, and is complete in itself. Students may choose to do more than one, depending on their

interest.

To make best use of this material you will need a computer with a sound-

card and speakers. Your computer should have a media-player program (such as Windows Media Player) and an Internet Browser that supports

JavaScript. The material can be copied to a server and accessed by clients.

You can customize the HTML files to suit your requirements. After opening the file, double-

click on any text to edit it. Use the save changes link on the left of your Browser window when you are finished.

Installation Instructions: 1. Copy the folders to your computer or to your server. If you are

working on a server, it is a good idea to set the folders to “read

only” to prevent inadvertent modifications. 2. The videos are best played in full-screen at a resolution of 1024 x

768. You may need to install the CamStudio Codec to view video on your computer. To do this, right-click on the file camcodec.inf and choose Install from the popup menu. You may need administrator

privileges to do this. 3. Ensure that JavaScript is enabled on your browser.

4. Each folder contains one HTML file. Double-click on it to open the instructions.

5. Data files are provided as relevant – IGES files, HM files, etc.

6. In case you need support, contact your distributor or email [email protected]


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Dynamic Stresses in a Gear Assembly Areas covered:

• Use of an FE mesh to calculate mass and inertial properties

• Creation of Hexahedral and Shell meshes • Transfer between FE models and MDL models • The use of contact in transient analyses • Flexible Body Analysis – mixing the rigid-body and

flexible-body approaches in a single model Software used:

• HyperMesh • MotionView • MotionSolve • OptiStruct • HyperView • HyperGraph

Description of the Problem An actuator is being designed for use in a

position controller. The design is at the concept stage, and various alternatives are being worked

out. The design specification of the actuator

assembly calls for the design of gears, given the center-to-center distance is 106 mm and the

velocity ratio is 1.28.

The current proposal calls for the use of steel

gears, but these may later be replaced with plastic. Obviously various options exist, but the

easiest is to check if involute profile spur gears can do the job.

Hand-design for gears is well-established, but

FEA can provide higher precision calculations,

thereby allowing the designers to use a lower factor of safety. Conventional FEA’s

disadvantages are twofold. First, the problem setup can be fairly demanding since the loads

vary with time. Second, integrating the finite

element model into the assembly-design simulation is not very easy.


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Traditional rigid-body mechanics is limited by the

assumption that the bodies are rigid, but the option to use flexible models in a Multi-Body-

Dynamics (MBD) solver opens up new possibilities.

Planning the Approach The starting point for our design is a CAD model

of the two gears. The analysis involves contact between the teeth - as the gears rotate, teeth

engage and disengage. We use shell elements on the faces of the teeth to setup contact

between the two gears, since this is much more

efficient (computationally) than using the hexahedral-element model.

Further, to conduct a flexible body analysis, we

perform a Component Mode Synthesis, for which we need an acceptable model of the gear and

pinion. Since the proposal contains spur gears,

we use hexahedral elements for their greater accuracy. For further proposals, as the geometry

gets more complicated, we can use a larger number of tetrahedral elements if we are willing

to increase solver time in return for reduced

modeling time.

Once the finite element models are set up we transfer the data to the MBD modeler. Here we

define joints, apply the motion, define the contact, and solve. We choose to use contact

instead of defining a "gear" joint, since our

interest is in the calculation of tooth stresses.

Setting up the FE Model We use HyperMesh to create the Finite Element Models. As described earlier, there are two sets

of Finite Elements we need to create: one for

the flexible body analysis, the other for contact.


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The process involves

• import of CAD data from an IGES file • creation of material collectors so that

mass properties can be calculated

• generation of hexahedral elements by

“dragging” shell elements • creation of nodes at locations where the

MDL model will place joints

• translation of the FE data into an MDL

Model

• generation of shell elements for contact-

surface definitions

Several files are generated by this stage – the MDL statements themselves, the H3D file for the

graphics images and the mass and inertial

properties of the model.

The MBD Model and Results Once the FE model is ready, the MBD setup is relatively easy.

We follow the incremental approach in model-building. This involves checking that the model is

behaving as expected at each step, before adding the next level of abstraction or

complexity.

The MDL file created from the HyperMesh model – this contains the “links” (or “bodies”), with the

associated graphics and mass-properties. We add revolute joints for the gear-rotation and

setup contact using the surfaces defined by the

shell meshes. Since the bodies already have H3D graphics associated with them, we create

dummy bodies, locate these in the correct place, and assign the surface definitions to these.

For Component mode synthesis we use rigid elements to connect the joint-center to the finite

element mesh and export this FE model using


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the utility FlexPrep that interfaces with OptiStruct to generate the flexible body data.

After the analysis we use HyperView and HyperGraph to view the results of interest -

forces at various locations, animation of the motion, stresses, etc.

Further Work There are several aspects that can make the

project more complete. You may choose to

assign these to your students based on their level of proficiency on programming, the time

available, etc.

Some of the areas for further work include

• tracking the variation of tooth-forces

with contact parameters

• running the analysis for plastic gears

• plotting the change in peak-stress with

the change in rotation-speed of the gears

Summary By the end of this assignment, the student will know how to

• create MDL models from CAD data

• create hexahedral and shell elements

• create revolute joints

• assign graphics to various bodies

• define contact and assign friction

parameters • setup Component Mode Synthesis

• assign prescribed motion

• assign solver parameters

• animate the assembly’s motion

• plot parameters of interest


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Design of a Torque Limiter Areas covered:


• Creation of tetrahedral meshes • Transfer between FE models and MDL models • The use of contact to investigate friction • Preloading springs to generate contact forces

Software used: • HyperMesh • MotionView • MotionSolve • HyperView • HyperGraph

Description of the Problem A torque limiter is similar to a friction clutch,

except that it works "backwards". Where a clutch transmits motion, the job of the torque

limiter is to permit free relative motion once the design torque has been exceeded.

The case at hand is at the concept-design stage. The designers are trying to achieve the specified

functionality within the given package space, which is tightly limited.

The shaft, supported by bearings at both ends, has a nut which carries an axial load. The gear,

mounted on the shaft, receives the drive. If the axial load exceeds the design value (90 dN) the

gear must slip - that is, the shaft should not rotate. As long as the axial load is less than the

design load, the shaft must rotate with the gear.

To achieve this, the designers have chosen to

rely on friction. They want to use springs to exert a frictional-force between the gear and the

shaft. Using power-screw calculations, the gear-

torque can be related to the axial load on the nut. If the spring-preload is set correctly, drive

will be transmitted as long as the friction-load is


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more than the gear torque. When the gear

torque exceeds the friction-forces, the gear will

slip.

The design requirement is to establish the required spring-load to ensure that slip occurs at

the design load.

Planning the Approach We start with the IGES geometry of the sub-assembly. We could build an MBD model directly

from this geometric data, with links and joints only. However, 3D graphics can dramatically

improve visualization and results-interpretation.

Accordingly, we use the FE pre-processor to

address two tasks: calculation of mass properties of the components, and generation of 3D

graphics images for subsequent use.

After that, it's plain sailing: once the joints have

been defined and motion assigned, we run the analyses for a series of different frictional-loads

on the bearing-surfaces.

Setting up the FE Model We start with the IGES geometry of the

assembly, and delete components that are not

essential for the analysis. The design uses Belleville springs, which we can dispense with in

the MBD model, for example.

The first step is to assign material data so that the mass and inertial properties are calculated

correctly.

The FE model is only for the calculation of mass

properties and for 3D graphics. There is no stress analysis involved, so we do not need to

use hexahedral elements – they are more

accurate for stress analysis but harder to generate. Instead, we use the easily generated

tetrahedral meshes and translate this to the


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MBD model using a TCL program to do this

automatically.

We also use shell meshes to define the contact-

surface. Since contact-calculations are critical, the mesh is chosen to closely follow the contours

of the bearing-surfaces.


We follow the incremental approach in model-

building. This involves checking that the model is

behaving as expected at each step, before adding the next level of abstraction or

complexity.

We start with the MDL file created from the

HyperMesh model – this contains the “links” (called “bodies”), with the associated graphics

and link-properties (center of gravity, moments

of inertia, coordinate systems, and mass). We then add revolute joints for the gear and shaft

rotation, and create springs at the bearing-surfaces.

Next, we setup contact using the surfaces

defined by the shell meshes.

Finally, we assign the uniform angular velocity to

the gear and run the analysis for the period of interest, after which we use HyperView and

HyperGraph to view the results of interest – force-vs.-time plots, animation of the motion,

etc.

Further Work There are several aspects that can make the project more complete. You may choose to

assign these to your students based on their level of proficiency, the time available, etc.


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• creating a screw joint between the nut

and the shaft and applying the axial load • generating graphs showing the variation

of slip-load vs. spring-preload for

different surface finishes of the bearing surfaces

• using flexible body analysis to check the

strength of the shaft

• a study of the effect of friction at the

shaft-nut interface • check the affect of uneven spring-loads

Summary By the end of this assignment, the student will

know how to


• create tetrahedral and shell elements




parameters • use springs and assign spring

parameters

• use screw-joints

• assign prescribed motion





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Analysis and Design of a Valve Train Areas covered:


• Transfer between FE models and MDL models • The use of joints to mimic the effects of contact • Application of simple-harmonic motion • Generation of the profile of the cam


Description of the Problem There are several different valve-train designs

that are tried-and-tested in IC engines. The problem at hand, related to the design of

components of the valve-train, addresses two different aspects.

First, the engine designers have started working with a particular configuration that has to be

detailed. The design is currently undergoing stress analysis, and the cam-profile has to be

generated for the specified valve-movement.

Second, a proposal has been received for an

alternate valve-design. The details are sketchy, but the designers have been asked to verify

whether the proposed design functions as claimed.

We start with the finite element models of the current configuration, and calculate the required

profile of the cam. Once this has been done, we analyse the alternative mechanism to check

whether it performs as specified. Note that the

two models are totally unrelated: our task is to provide a comparative analysis of the results for

both designs.


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Details on the alternative design can be found in

a patent-document. Variable timing valves, of course, are one way to enhance engine

performance, but not the only way. As the Further Work section describes, there are a number of options that can be worked on

towards design improvement.

Planning the Approach We start with the Finite Element model of the

current configuration, and use this to build our MBD model - the elements help us calculate the

mass properties and to define graphic images,

should we choose to use them.

Our design problem is: given the time-displacement profile of the valve (a simple-

harmonic function has been specified) we need to derive the cam profile.

To do this, we take the reverse approach: we apply the given motion to the valve and track

the displacement of the point-of-contact between the cam and the rocker arm. If we plot

this displacement, as measured from the center

of the cam, we have the profile.

The second part of the assignment is similar, except that we start with the IGES geometry.

Further, our interest here is not to calculate the cam profile, but to calculate how the valve

moves when the cam is actuated.

In both cases, we can work with "traditional"

MBD data - links, properties of bodies and joints. However the availability of 3D graphics has a

dramatic impact on our ability to appreciate the

results of the analyses, so we prefer to use the "solid" (shaded images) bodies in our models.


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Setting up the FE Model Since we have the FE model already, our work is

relatively easy: all we need to do is calculate the

mass properties of the various bodies and transfer these to the MBD model.

We review the FE data to ensure that material

data has been specified correctly (both values

and units), and create nodes at locations where we want to create joints in the MBD model.

We then use a TCL program to do this

automatically, as shown in the 6 minute video.



building. This involves checking that the model is behaving as expected at each step, before

adding the next level of abstraction or

complexity.



and link-properties (center of gravity, moments

of inertia, coordinate systems, and mass).

We then add a revolute joint for the rotation of

the camshaft, and a translational joint for the reciprocating movement of the valve. Since we

want to track the locus of the cam-rocker contact point, we define a dummy body here.

We then use in-plane joints to ensure that the

bodies move as designed. Note that this approach assumes that the components are

always in contact – it does not provide, for example, for valve-float. Also, we do not need to

model the spring.


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Finally, we assign the simple-harmonic motion to

the valve and run the analysis for the period of

interest, after which we use HyperView and HyperGraph to view the results of interest –

displacement-vs.-time plot, animation of the motion, etc.

The locus of the cam-tip gives us the cam-profile, which can be exported as a text file for

use in a CAD program to create the 3D model of the cam.


project more complete. You may choose to assign these to your students based on their

level of proficiency, the time available, etc.


• using the flexible-body analysis

capabilities of MotionSolve and

OptiStruct to calculate stresses • use a contact model to simulate motion

• include a valve-spring in the model and

check for valve-float as the rpm changes

Summary By the end of this assignment, the student will know how to



• create in-plane joints • create translational joints


• assign prescribed motion using the

function-generator





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Design of a Wiper Subsystem Areas covered:


• Creation of tetrahedral meshes • Transfer between FE models and MDL models • The use of contact to transmit motion


Description of the Problem Windscreen wipers for vehicles represent an

interesting problem: there are several statutory aspects that must be complied with, such as the

time of operation, coverage of the windscreen etc. In addition, the power-consumption of the

assembly must lie within the specifications laid

out by the OEM.

The wiper itself must, obviously, follow the shape of the windshield itself. This is usually

done by spring-loading the wiper blade. But if

the frictional force is too high, the motor required to drive the wiper blade must generate

more power.

Our problem concerns a design that is to being used as the basis for an initial quotation. In

order to verify that the motor power

consumption is adequate, the designers want to carry out an MBD analysis - they wish to

generate a graph showing the change in motor-power rating vs. the frictional force on the wiper

blade.

Planning the Approach We start with the IGES geometry of the sub-


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assembly. We could build an MBD model directly

from this geometric data, with links and joints

only. However, 3D graphics can dramatically improve visualization and results-interpretation.

Accordingly, we use the FE pre-processor to

address two tasks: calculation of mass properties

of the components, and generation of 3D graphics images for subsequent use.

After that, it's plain sailing: once the joints have

been defined and motion assigned, we run the

analyses for a series of different frictional-loads on the blade.

The sub-assembly uses a worm-drive, but we

can ignore it in our model - assuming no losses between the gear and the worm, it is enough to

track the forces at the gear as a function of

time.

Further, we choose to use a contact model instead of a gear-joint. for improved

visualization. Since the assembly is small, there

is little computational penalty.

Setting up the FE Model The FE model is only for the calculation of mass

properties and for 3D graphics. There is no stress analysis involved, so we do not need to

use hexahedral elements – they are more accurate for stress analysis but harder to

generate. Instead, we use the easily generated

tetrahedral meshes.

Starting with the IGES model of the assembly, we assign material properties – the density is

critical, since we want to calculate the mass

properties from the elements. We also create nodes at points where the MBD model will need

to locate joints, then translate this FE model to the MBD model using a TCL program to do this

automatically.


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We also use shell meshes to define the contact-

surface, to transfer motion between the sector-

gear and the link.

The MBD Model and Results Once the FE model is ready, the MBD setup is

relatively easy.


building. This involves checking that the model is behaving as expected at each step, before

adding the next level of abstraction or complexity.



and link-properties (center of gravity, moments of inertia, coordinate systems, and mass).

We then add revolute, cylindrical and ball joints, as dictated by the designer. It’s important to

remember that the different joints impose

different constraints on the motion. The ball-joint and cylindrical joint, for instance, are

essential for the design to function as required – other joints may over or under-constrain the

assembly.

Rather than use gear joints, we use contact to ensure that the sector gear drives the link. The

geometry that defines the pinion-surfaces is attached to the link, so that motion is

transmitted as desired.

We also apply an “action-reaction” force a the wiper-blade to mimic the affect of friction. The

value of the force can be linked to the rpm of the drive gear if we assume that the frictional

force between the blade and the windshield can be taken as a “constant” value.

Finally, we apply a uniform angular velocity to

the drive gear and run the analysis for the


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period of interest, after which we use HyperView

and HyperGraph to view the results of interest –

force-vs.-time plot, animation of the motion, etc.


project more complete. You may choose to assign these to your students based on their

level of proficiency, the time available, etc.


• use of a contact model to simulate

friction between the blade and the

windhsield

• use of gear-joints to capture the motion

of the pinion • inclusion of the worm in the model, with

an assigned friction-loss characteristic

• application of a non-uniform rotation

speed to the drive to calculate power-requirements

• investigation of the impact of different

stiction / transition velocities on the startup torque of the motor

Summary By the end of this assignment, the student will

know how to


• create tetrahedral and shell elements


• create ball joints

• create cylindrical joints



parameters • assign prescribed motion

• use action-reaction forces


• animate the mechanism’s motion


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