SolidWorks Motion 21 July 2011

Professor Erik Spjut, Engineering Clinic Director, HMC

Ayyappa Vemulkar, HMC 2013

Objective

By completing this tutorial, you will learn to model, simulate and analyze the motion of

mechanical parts in SolidWorks, using the Motion add-in.

Introduction

SolidWorks motion 2010 is an add-in module that comes with SolidWorks 2010. Apart from

being a 3-D CAD Design Software, SolidWorks also allows you to evaluate and analyze your

design in motion prior to moving on to the prototyping stage. It allows you to determine specific

aspects related to design such as the interference of moving parts; power consumption of motors;

sizes of springs, dampers and motors; and the forces related to surfaces in contact.

SolidWorks Motion allows one to analyze two major types of problems pertaining to the motion

of solid bodies. The first is kinematic, which refers to the study of the motion of a rigid body

without considering the forces that result in the motion of the body. The second is dynamic,

which refers to the study of the motion of a rigid body as a result of the applied external forces

on the body.

SolidWorks motion allows you to answer the following questions:

1. Will the various assembled components of your design move as you intended?

2. Will the various assembled components of your design collide when in motion?

3. How much force or torque would you require to drive the system?

4. What are the magnitudes of the forces between two parts in contact with one another?

5. What is the path followed by a particular end or joint?

Before you begin

As most people with experience with SolidWorks will agree, it is important that you create a

single folder (preferably on Charlie) to save all your parts and assemblies to. Also consistently

saving your document will help save time and effort if SolidWorks crashes unexpectedly. What

is very important with using SolidWorks motion as a tool is that it is only a tool. You must

already understand how you expect the parts of your assembly to move, prior to using motion. It

is also imperative that you verify the results output by SolidWorks Motion.

The Analysis Process

It is advisable to break up the analysis of motion into three main sections, namely creation of the

model, analysis of the motion, and verification of the results.

Starting a Motion project

Prior to analyzing the motion of your design there are two components of the design process that

need to be taken care of. First begin by drawing the various parts associated with the design.

Second create a New Assembly and assemble the various parts of the design. It is important to

assemble the parts using particular mates that allow for specific kinds of motion of certain parts

(more on choosing the right mate options will be stressed later on in the tutorial). Once the parts

have been assembled, you are ready to begin motion analysis. Select Tools » Add-ins »

SolidWorks Motion. Checking the box on the left opens the SolidWorks Motion module for

your current session, checking the box on the right however opens SolidWorks motion

permanently for every session you begin.

User Interface and Common Tools/Terms

The interface for SolidWorks Motion is similar to that of SolidWorks itself, with one or two

added toolbars for motion analysis, as seen in Figure 1: SolidWorks Motion user interface.

Figure 1: SolidWorks Motion user interface

When SolidWorks Motion is activated there will be a tab, normally titled ‘motion 1’ on the

bottom left end of the screen. This tab allows you to toggle between the assembly and the motion

model you wish to develop. Hovering your mouse over a button gives you a brief description of

the function of that button.

There are certain terms or entities one must keep in mind prior to beginning a motion project.

They are:

1. Ground Part:

Refers to the part of the assembly that will be considered as the fixed reference frame for

the motion simulation. It is usually the first part you place on the assembly.

2. Moving Part:

Refers to the parts of the assembly that will move during the motion simulation.

3. Constraints:

Refers to a joint or contact that restricts the relative motion between two parts. Note that

SolidWorks Motion directly converts Mates into Joints or Constraints.

4. Degrees of Freedom:

Any unconstrained body has six degrees of freedom, three translational and three

rotational. When mates are created as parts of an assembly, constraints are imposed that

restrict relative motion between the two mating surfaces, often restricting motion to one

degree of freedom.

For more information on how SolidWorks Motion calculates degrees of freedom of

moving parts based on the mates placed in an assembly, refer to Motion Simulation and

Mechanism Design with SolidWorks Motion 2009 by Kuang-Hua

SolidWorks motion ignores redundancies in degrees of freedom for kinematic analysis

but asks the user to verify redundancies in degrees of freedom when computing dynamic

calculations.

5. Forces:

In SolidWorks Motion, motors, springs, dampers or gravity produce forces. When

creating a force using the Force tool , you will have the option of creating a linear

force or a torque.

6. Results:

In SolidWorks Motion, one may analyze different aspects pertaining to the motion model

one develops through animations, graphs and reports.

Tutorial

The tutorial will be divided into two parts. The first will be an introductory set of examples that

you will work through to understand the environment of and tools available in SolidWorks

Motion. The second will require you to understand problems present in a model of a single-

cylinder, two-stroke combustion engine, and fix them as directed.

Part 1: Introductory examples: applying forces and studying the motion of rigid bodies

To begin this part of the tutorial, download and open the file labeled

SolidWorks_Motion_Tutorial_2010.pdf from the SW Motion folder in the clinic folder. The

tutorial was developed by the higher education department of the McGraw-Hill companies.

Apart from guiding you through the SolidWorks Motion environment, the examples aim at

introducing you to three main tools in SolidWorks motion:

1. Simulation tools, such as Forces, Gravity, Friction and Rotary Motors

2. Analysis tools, which allow you to graph results and write data to spreadsheet files

3. Mating tools, which allow you to control the motion of parts in an assembly

The tutorial also emphasizes the importance of following the analysis process outlined earlier.

Complete the tutorial mentioned before moving on to the next section. One major concept to take

away from this section is the use of SolidWorks for dynamic analysis, i.e. the motion of a rigid

body as a result of the applied external forces on the body.

Things to keep in mind:

1. Try and save all the parts you create and assemblies in one folder, this will make

searching for them while building assemblies easy.

2. Remember to start drawing all parts in the front plane.

3. Many of the plots and results are directional. Thus when the tutorial asks you to

determine a certain result regarding the Z Component and you receive something

unexpected, remember to check the other directions.

4. Remember which plane you start drawing your part in. It may come in handy for mating

and arranging parts in an assembly, particularly for the roller on the ramp.

What to turn in:

1. Did you get the same graphs as depicted in the tutorial?

2. Did you get the same trace path as depicted in the tutorial?

Part 2: Debugging errors in design by simulating motion

The goal of Part 2 is not to give you step-by-step directions on the process, but to show you how

you would use SolidWorks to debug a design and make sure things work. If the instructions ask

you to do something, and you don’t know how, first check the SolidWorks help, and then ask a

proctor or Professor Spjut.

To get started, first copy the folder labeled SWmotion_part2 from the clinic SW Motion tutorial

folder to the folder you created for this tutorial. To begin this section open the assembly labeled

OneCylEngine and when prompted to find parts for the assembly, agree to do so and remember

that they are in the SWmotion_part2 folder. The assembly should appear as that in Figure 2.

Figure 2: Initial Assembly

The assembly is a simplified model of a single-cylinder four-stroke internal combustion engine.

If you don’t know how an internal combustion engine works review the Wikipedia article on it

http://en.wikipedia.org/wiki/Four_stroke_engine. Part of modeling is deciding what does and

doesn’t need to be modeled. The current model is useful for looking at the timing and the relative

motion of the parts, but it doesn’t model the force and energy dealing with the air intake,

compression, combustion, or exhaust output. While approximations for these four steps could be

added by adding time- or position-dependent forces, this particular model wasn’t designed with

those analyses in mind. It doesn’t have and exhaust or intake manifold or a carburetor or fuel

injectors. The current model also does not have friction, but it does have models for the mass and

moments of inertia of the components, and the behavior of the valve springs and the timing belt.

Friction can be added relatively easily by suppressing some of the mates and adding contact pairs

in their places. There are three issues with the current engine that you want to fix:

1. The compression ratio is far too high. A reasonable value for a modern car engine is 8:1.

2. The intake and exhaust valves are not opening and closing at the correct time, and the

valve throw may be too long.

3. The valves exhibit float at far too low a speed.

You are to fix the three problems and turn in a report explaining how you fixed them and the

values of the parameters you used. If you have time and desire, you can also modify the model to

account for friction and report on how much power goes into turning the mechanical

components.

This model has 21 mates of which 3 are suppressed. The suppressed mates are for setting initial

conditions in the model. Expanding the Mates tab on the pane on the left will allow you to

explore the various mates present in the assembly. As is important for all Motion assemblies it is

important to specify ground parts. In this particular example the Base acts as a ground piece.

The simulation can be set to detect collisions or contact between parts. In the existing model

there are four Solid Body Contacts specified, one each between the camshaft and each of the

valves, and one each between the cylinder and each of the valves. Until we get the model

working properly, we don’t want to set solid body contacts between the valves and the piston, or

the piston and the cylinder because forcing two parts together beyond simple contact will cause

the simulation to run extremely slowly or crash. To account for friction in the bonus exercise,

you will remove some of the concentric constraints and let the parts (such as the crankshaft and

the pushrod) have a solid body contact.

Let us determine the first two flaws by observing the motion of the model. Make sure Motion is

active by selecting Tools » Add-ins » SolidWorks Motion and checking the appropriate box.

Next toggle over to the Motion Study 1 tab. Click on the pop-up menu and set it to Motion

Analysis (the other two choices are Animation and Basic Motion). Always make sure the time-

scrolling bar has been moved all of the way back to the left before making any changes.

Otherwise the change will be made at the location of the time-scrolling bar, and the first part of

the simulation won’t change. Right-click on any motors currently present on the bottom left pane

and select Delete. Also remove any contact forces or friction that may be applied between

surfaces.

Add a rotary motor to the long piece of the crankshaft, as shown in Figure 3. Set the motion to

Constant Speed and 60 RPM (1 per second).

Figure 3: Adding a motor to the simulation

The frame rate determines the time intervals used in modeling the motion. A high frame rate

gives you the smoothest and most detailed motion, but also takes the longest to calculate. For this

simulation set the frame rate somewhere between 60 frames per second (fps) and 120 fps, by

clicking on the Motion Study Properties box, indicated by the arrow in Figure 4. Be sure to click

the check mark when you finish.

Motion Study Properties

Figure 4: Adjusting the Frame Rate

Calculate the motion study and observe the motion of the parts, particularly the top of the piston

and the two valves. You should see two issues:

1. The piston actually extends a little beyond the face of the cylinder at the top of the stroke.

2. The timings of the valves are off. They sometimes penetrate into the piston (very

destructive in a real engine), and they don’t open and close exactly as you would expect

from the Wikipedia article.

Clearly we need to make a few adjustments. There are multiple ways to adjust things in the

model, and in your own work you need to decide which adjustment is best. For this exercise, we

want to adjust the piston position by adjusting the tie rod (called the PistonRod in the model).

Toggle back to the Model tab. Follow the steps below:

1. Determine the maximum and minimum distances between the top of the piston and the

bottom of the top plate of the cylinder, either with the Tools»Measure…, or more

productively, by plotting the distance between the piston face and the cylinder face using

the Results and Plots tool.

2. Expand the PistonRod assembly on the pane on the left. Expand the PistonRod part »

Right Click on the feature Boss Extrude and select Edit Sketch as shown in Figure 5

Figure 5: Editing the sketch of the crankshaft

3. Adjust the length of the tie rod by double-clicking on the appropriate dimension and

entering a new value. Your measurement of the distance of the initial simulation should

vary between –0.1 inch and 1.4 inches. You want to adjust the length of the tie rod until

the distance is always positive and the ratio of the maximum to the minimum is

approximately 8:1 (or whatever you would like the compression ratio to be). You will

want to report the new length and the compression ratio.

4. Ok the change and select Edit Component towards the top left of the screen to confirm

changes to the assembly.

5. Re-calculate the simulation and make sure the compression ratio is correct. Toggle over

to the Motion Study 1 tab, and if prompted to update the initial positions of the

animation, select Yes. Drag the slider by the play button back to the initial position, and

press the calculate button to begin the analysis.

Once you are satisfied with the compression ratio, toggle back to the Model tab. We will now

correct the timing and stroke of the valves. To do so you will need to edit features of the

camshaft. The relevant features are the sketches of Boss-Extrude2 and Boss-Extrude4. In the

sketch, leave the .700 diameter alone (it is the base diameter of the cam for the valve), but you

may adjust the other three dimensions. When properly adjusted the intake valve should begin to

open just as the piston is moving down, not run into the piston, and close just as the piston is

moving up on the compression stroke. The exhaust valve should begin to open just as (or just

before) the piston begins to move up on the exhaust stroke, not run into the valve, and just close

as the piston reaches top dead center. In real engines, the exhaust and intake valve strokes may

overlap slightly, with the exhaust valve just closing as the intake valve is just opening. You will

want to report which valve is the intake and which is the exhaust, and to what value you set each

of the three dimensions in each of the two sketches. Verify your values by re-calculating the

simulation after you save your changes.

Once you have the compression ratio adjusted and the valve timing correct, it is time to examine

the valve float. Make sure you have dragged the motion cursor all the way to the left, to the start

of the simulation. Right Click on the motor and adjust the RPM to 2000. Most cars idle at 500 to

800 RPM and shift gears around 2000 RPM under gentle driving. Adjust the Motion Studies

Properties frame rate to 1000 fps. Calculate a few cycles of the simulation and observe the

motion of the valves (calculating the full 12 seconds takes too long, and the simulation may

crash before you reach the end). It is possible that the simulation may fail after a few cycles

because of the motion and collision of the valves. Just close the error box if it does and look at

the portion of the simulation that calculated properly. Are the valves staying in contact with the

camshaft? Are they running into the piston? The phenomenon is known as valve float and limits

the maximum speed of most internal combustion engines. In our case the problem is that the

springs are not stiff enough. Right click on the springs (did you remember to reset the cursor?),

and adjust the spring constants upwards until the valves just follow the camshafts without much

float, and report the values of the spring constants. In a real engine, you would probably adjust

both the spring constant and the mass of the valves. Real physical springs can only be made so

stiff before they are too massive to let the valve have its full range of motion. If you enjoy the

challenge, adjust the spring stiffness and the valve mass so that the valves just begin to float at

6000 RPM, the red line for most consumer engines. Report on the value(s) of the spring constant

to prevent float.

Bonus Problem

Finally, if you have time, you will simulate friction between the piston and the cylinder,

crankshaft and block, crankshaft, tie rod, and piston, and camshaft and block. You will calculate

the power needed just to overcome the friction. Open the Mates in the top window and suppress

all of the concentric mates. Do not suppress the Distance or Coincidence mates that haven’t

already been suppressed (they keep the crankshaft, camshaft, and tie rod from sliding sideways).

Then click on the Contact icon . Set up contact pairs between the Camshaft and Base,

Crankshaft and Base, Crankshaft and PistonRod, PistonRod and Pin, and Piston and Cylinder.

You will need to select materials to use preset values of friction available in SolidWorks. On the

options pane on the left scroll down and change the material from Acrylic to Steel (Greasy) as

shown in Figure 6.

Figure 6: Setting the material to Steel (Greasy)

Once you’ve made the changes, click on calculate to re-run the simulation. To plot the motor

power consumption (used to overcome inertia and friction), follow the steps in the SolidWorks

Help:

You can plot the power required by a motor to move a part.

1. From a Motion Analysis study, click Results and Plots .

2. In the PropertyManager, under Result, for Category, select

Momentum/Energy/Power.

3. For Subcategory, select Power Consumption.

4. For features , select a motor.

Select the Plot Results options you require, and click . The result appears in the MotionManager tree. The results should show some periodicity as the engine runs. Run the simulation with a motor

speed of 60 RPM, 600 RPM, and 2000 RPM. Turn in the plots of the power for each speed. The

maximum power output from a typical cylinder in an internal combustion engine is about 20 HP

or 15 kW. What fraction of that power output is consumed by friction and inertia?

For Part 2 what to turn in:

1. The new dimension in the PistonRod sketch (or a screen capture of the PistonRod sketch)

and the calculated compression ratio.

2. The new dimensions in each of the Camshaft sketches (or screen captures of the relevant

Camshaft sketches)

3. The new spring constants that prevent float and the RPM for that spring constant.

4. Bonus: The plots of power at 60 RPM, 600 RPM, and 2000 RPM, and assessment of the

importance of friction at each rotation rate.

References/Good Sources:

1. Kuang-Hua Chang,Ph.D, Motion Simulation and Mechanism Design with SolidWorks

Motion 2009,SDC Publications 2010, ISBN: 978-1-58503-595-3

2. A number of tutorial available on youtube are helpful

Feedback:

As this is the first time we are using tutorials as part of the clinic program, we would really value

your feedback.

How many hours did you spend on the tutorial?

Did you face any problems during the tutorial? Please explain

What would you like to see added to the tutorial? Please explain

What do you feel is not required in the tutorial? Please explain