Autodesk Inventor - Cam and Valve

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Cam and Valve

Cam and valve devices can transform a continuous rotational motion into an alternating translational motion. A spring under the valve

provides the force that maintains contact between the cam and the valve during complete rotations of the cam. But, because of contactfriction, this mechanism loses power over time, which can be calculated by Dynamic Simulation. With Autodesk Inventor

Professional for Simulation, you can run simulations that calculate the loss of power over time because of friction.

Information about the power loss due to friction is useful for designing elementary cam and valve mechanisms and, when generalized,

for designing multi-cam follower mechanisms.

Objectives

Insert joints you create manually.

Convert assembly constraints into joints.

Create springs.

Create 2D contact joints.

Impose motion on a joint.

Define gravity.Simulate the physical motion of a mechanism.

Analyze results with the Output Grapher.

Use the post-processing tools.

General Workflow

Add joints.1.

Apply forces and imposed motions as appropriate.2.

Run the simulation.3.

Analyze results.4.

Download the Dataset

cam_valve_dataset.zip(zip - 392Kb)

Open a Mechanism Created in the Assembly Environment

1. Open Cam_Valve.iam. The assembly can be downloaded from the attached zip file

2. Select File > Save Copy As.

3. On the Save Copy As dialog box, enterCam_Valve_sim.iam as the file name.

4. Close Cam_Valve.iam without saving so you or others can work through these steps later.

5. Open Cam_Valve_sim.iam.

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6. Select Applications > Dynamic Simulation.

If a message asks if you want to run the Dynamic Simulation Tutorial, click No.

In the Dynamic Simulation browser, all parts (cam, valve, and support) are in the Grounded group. Therefore, none of these parts has

any degrees of freedom.

7. In the graphics window, move the valve and the cam up and down.

Although none of the parts in this mechanism has any degrees of freedom, the software shows how they would move in the assembly

environment (due to the assembly mates). A true simulation shows the dynamic movement of the mechanism.

8. On the Simulation Panel toolbar, click Run or Replay Simulation.

The first box below the toolbar displays the End time for the simulation. By default, the end time is 1 s, which is fine for this simulation.

The second box displays the Frames (that is, the time steps). By default, the number of frames is 100, which is fine for this simulation.

There is no movement because there are no degrees of freedom.

Convert Existing Constraints to a Joint

Converting constraints converts assembly constraints into mechanical joints (for instance an insert constraint becomes a revolution

joint).

1. On the Simulation Panel toolbar, click Activate construction mode.

2. On the Dynamic Simulation panel, click Convert Assembly Constraints.

3. In the graphics window, select the support and then the valve.

4. On the Convert Assembly Constraints dialog box, select both mate constraints to turn them on.

Based on these constraints, the software automatically selects a prismatic joint.

5. Click OK.

In the browser, the prismatic joint is added to the Standard Joints group and the valve is removed from the Grounded group and put

under Mobile groups.

Define Gravity

You can simulate the movement of a component either by imposing motion or by applying internal or external forces that cause motion.

Both are appropriate in this simulation. Gravity is an external force that acts on all components of the mechanism.

1. In the browser, under External loads, right-click Gravity and select Define gravity.

2. On the Gravity dialog box, click Active to enable gravity.

3. Select a vertical edge of the support.

A yellow arrow denoting gravity appears along the selected edge.

4. If the arrow does not point down, click Invert normal to change direction.


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5. Click OK.

A simulation can show the effect of gravity.


The default values for End time and Frames are fine.

The support cannot move because it is a grounded part, and the cam cannot move because it has 0 degrees of freedom because we

have not created any standard joints yet. But the valve drops away because of gravity.

Insert a Spring

The spring/damper/jack joint creates a force that returns the valve to the top of the support.


2. On the Dynamic Simulation panel, click Insert Joint.

3. In the list of joints, select Spring/Damper/Jack.

4. Select the circular hole in the top circular edge of the bottom of the support.

5. Select the bottom circular edge of the top of the valve as shown.

6. Click OK.


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The software adds a spring damper

7. In the browser, right-click Spring/Damper/Jack and select Properties.

8. Click Active joint to turn it on.

By default, Spring/Damper/Jack joints are inactive until you turn them on.

9. In Stiffness, enter1 N/mm.

This determines the force exerted by the spring when it is compressed or extended beyond its free length.

In Free length, enter50 mm.

Free length is the length at which the spring is static and exerts no force.

11. Click More to see additional parameters.

12. In Radius, enter12 mm.

The Dimensions and Properties settings added in this section of the dialog box affect only the display of the spring.

13. Click OK.

A simulation can show the effect of the spring.



The valve moves up and down quickly because of the forces of gravity and the spring. The motion stays constant during the simulation

because there is no loss of energy due to friction or damping.

Modify the Initial Position of the Valve

Limiting the movement of the valve can change the effects of gravity and the force provided by the spring.

Note: The force provided by the spring is applied between 2 components, unlike gravity, which acts upon the entire mechanism.

1. In the browser, right-click the Prismatic joint and select Properties.

2. Click the dof 1 (T) tab.

By default, Edit initial conditions is selected, which is correct for this example.

3. In Position, enter8 mm.

The valve moves so that the 2 reference frame origins are separated by 8 mm. This preloads the spring because its free length is 50

mm.

4. Click OK.

A simulation can show the change in motion.



The valve now oscillates gently because the spring preload is now less than before because we modified the initial position of the valve.

Insert a Revolution Joint

The software cannot automatically create the proper joint between the cam and the support because there are no constraints between

the 2 components. Since there is only 1 rotational degree of freedom required for the joint between these 2 components, a revolution

joint is necessary.


2. Click Insert Joint.

3. In the list of joints, select Revolution.


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4. Make sure that Out of Place is selected.

5. Rotate the model to view the back of the cam and support.

You can select the components for a joint in any order. However, the software determines the extent of the motion by comparing the

position of the joint coordinate system of the second component selected to the position of the joint coordinate system of the first

component selected.

6. Select the circular edge on the support.

A joint coordinate system appears on the support. Because of this selection of a circular edge, the origin of the coordinate system is at

the center of the circle and the X and Y axes are in the plane of the circle with the Z axis perpendicular to the plane of the circle.

7. On the Insert Joint dialog box, under Component 2, click Select.

8. Select the circular edge on the cam.

9. Click OK.

The software moves component 2 to component 1 so that the 2 coordinate systems are aligned and there is 1 rotational degree of

freedom between the 2 Z axes.

10. Return to an isometric view.


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Insert a Contact Joint

Well create a contact disc curve joint. Before you start, make sure the sketches are displayed.

1. From the View menu on the main toolbar, select Object visibility > Sketches.

2. Click Insert Joint.

3. In the list of joints, select Ct Disc Curve.

4. Select one edge of the cam.

5. On the Insert Joint dialog box, under Component 2, click Select.

6. Select the circle on top of the valve stem.

7. Click OK.


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In the browser, the Ct DiscCurve joint appears under a new group, Contacts.

8. View the model from the front.

9. In the browser, right-click the Ct Disc Curve joint and select Properties.

By default, Activate joint is selected to make the software consider this joint in all calculations.

10. Click Invert normal so that the Z axis points away from the cam.

This tells the software that this is the outer surface of a part rather than the inner surface of a hole. The Z axis must always point away

from the material into the air surrounding it or surrounded by it.

By default, the restitution coefficient is .8, which is fine for this simulation. Restitution is the ratio of the normal relative velocity between

the 2 components after contact to the normal relative velocity between the 2 components before contact. (Because it is a ratio, restitution

has no units.) Remember that this ratio uses only the normal velocities, that is, the part of the velocities that is perpendicular to a tangent

on the curve at the point of contact.


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The value of restitution is always between 0 and 1. Restitution applies only to non-penetration joints such as the 2D contact joints (that is,

not to the 3D contact joint).

By default, the friction coefficient is .15, which is fine for the first simulation. Friction is the ratio between the tangential force and the

normal force. Like restitution, friction is without units, but friction usually has a value between 0 and 2.

11. Click More to see additional parameters.

12. Under Display Force, click both Normal and Tangential.

This causes the software to display both the normal force and the tangential force.

13. For the normal vector, keep the default (0.01) for Scale.

14. For the tangential vector, change the Scale value to 0.1.

15. Click OK.

Add Imposed Motion to Drive the Camshaft

Add an imposed motion to indicate the required rotation of the cam.

1. In the browser, right-click the Revolution joint and select Properties.

2. Click the dof 1 (R) tab.

3. Click Edit Imposed Motion.

4. Click Enable Imposed Motion to turn it on.

5. Under Driving, select Velocity.

6. Click the arrow next to the velocity input box and select Constant Value.

7. Enter360 deg/s.

8. Click OK.

View the First Set of Simulation ResultsThe amount of power lost to friction is:

PL = Pf Pw,

Where

PL is power in watts lost due to fric tion,

Pfis power in watts required to drive the mechanism when friction is considered, and

Pw is power in watts required to drive the mechanism when friction is not considered.

Rotational power (driving or resistive force) is:

P = T x,

Where

P is power in watts,

T is torque in Nm, and

is rotational velocity in rad/s.

To find the power lost because of friction, you need the torque loss because of friction. This first simulation displays the torque required

when considering friction.


The End time and Frames are fine for this simulation.

2. On the Dynamic Simulation panel, click Output Grapher.


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3. On the Output Grapher toolbar, click Deselect All.

4. In the Output Grapher browser, under the Revolution joint, in the Driving Force folder, select Ukin[3.1].

A curve of the rotational force (torque) on the cam appears. This torque takes friction into consideration.

View Larger

5. Select File > Save.

This saves the current simulation with the name of the assembly file used and the extension .iaa. If you do not save a simulation, the

software overwrites it the next time you run a simulation. If you want to save 2 or more simulations during the same session, use the

Windows Rename feature to rename each immediately after saving it.

Run the Results of the Simulation Without Friction

Now you must calculate the torque necessary without friction. First, change the cam valve model to remove friction from the contact disc

curve joint connecting the cam and the support.

1. On the simulation panel, click Activate construction mode.

2. In the browser, under Contacts, right-click Ct Disc Curve and select Properties.

3. Enter0 in the Friction box.

The default restitution coefficient is fine for this simulation.

4. Click OK.


The End time and Frames are fine for this simulation.

6. On the Output Grapher toolbar, click Deselect All.

7. In the Output Grapher browser, under the Revolution joint, in the Driving Force folder, select Ukin[3.1].

A curve of the rotational force (torque) on the cam appears. This torque does not consider friction.

View Larger

Compare the 2 Simulations

The torque lost to friction is the torque required when friction is considered minus the torque required when friction is not considered.


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1. On the Output Grapher toolbar, click Import simulation.

2. On the Dynamic Simulation - Load File dialog box, browse to the directory containing the first simulation.

3. Select the file containing the first simulation.

4. Click Open.

The Output Grapher now contains 2 simulations.

5. In the Output Grapher browser, under the Revolution joint for the simulation considering friction, in the Driving Force folder, selectUkin[3.1].

A second curve of the rotational force (torque) on the cam appears. One considers friction (the upper curve) and 1 ignores friction (the

lower curve).

The difference between the 2 curves is the torque lost due to friction.

Create a Curve of the Torque Lost to Friction

1. On the Output Grapher toolbar, click New curve.

The New curve dialog box has 2 text boxes, 1 for a name for the new curve and 1 for the equation that defines the new curve.

2. In the Name box, enterTorque_diff.

3. In the Equation box, enter:

Ukin[3.1]{} - Ukin[3.1]

To specify torque driving force considering friction, click Ukin[3.1] in the Driving force folder under Revolution in the Cam Valve sim.iaa

folder in the Output Grapher browser. The software automatically enters Ukin[3.1]{} in the Equation box. Path name is the

path to Cam_Valve sim.iaa on your system.

To specify torque driving force without considering friction, click Ukin[3.1] in the Driving force folder under Revolution in the Cam Valve

sim folder in the Output Grapher browser. The software automatically enters Ukin[3.1] in the Equation box.

Note: You must enter a space before and after the minus sign (-).

4. Click OK.

The software calculates and displays the new curve superimposed on the first 2 curves. The new curve is, by definition, the difference

between the first 2 curves. It shows the torque lost to friction throughout the simulation time.


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View Larger

The new curve displays the difference between the other 2 curves, that is, the torque lost due to friction. The new variable is included in

the User variable folder in the current simulation browser.

Create a Curve of the Power Lost due to Friction

Remember, rotational power (driving or resistive force) is:

P = T x,

Where

P is power in watts,

T is torque in Nm, and

is rotational velocity in rad/s.

Therefore, the power lost to friction is a function of the torque lost to friction. Since the dataset uses Nmm, it is necessary to convert T

(torque in Nm) to t (torque in Nmm).

PL = (tf tw)/1000

Where

PL is power in watts lost due to fric tion,

is rotational velocity in rad/s,

tfis torque in Nmm required to drive the mechanism when friction is considered, and

tw is torque in Nmm required to drive the mechanism when friction is not considered.

To convert to degrees to radians, multiply by /180 deg.

= V /180

Where

V is rotational velocity in deg/s

Therefore,

PL = V /180 (tf tw)/1000

From the comparison of the 2 simulations performed above, you can calculate the power lost due to fr iction.

1. In the Output Grapher toolbar, click Deselect all.

2. Click New curve.

3. In the Name box, enterPower_lost.

4. In the Equation box, enter:

Torque_diff/1000*v[3.1]*PI/180

PI is . To specify torque lost due to friction, click Torque_diff in the User variables folder under the Cam Valve sim folder in the Output

Grapher browser. The software enters Torque_diff in the Equation box.

5. Click OK.


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The software calculates and displays the new curve showing the power lost to friction throughout the simulation time.

View Larger

Export the Results to Excel

1. On the Output Grapher toolbar, click Export Data to Excel.

A dialog box asks if you want to export the selected curves to Excel.

2. Click Yes

Excel automatically opens and displays the chart and data imported from the dynamic simulation.

3. In Excel, view the chart and data.

4. Select File > Save and enterPower lost to friction.xls in the File Name field of the Save As dialog box.

5. Close Excel.

6. Close the file without saving changes.

You have simulated a cam lobe, valve, and spring and determined the contact forces between the cam and valve, the forces in the

spring, the torque required to drive the cam, and the power lost to friction.

This quick start to simulating dynamic motion in models gives you an overview of the process. Try applying what you have learned to

models you create.

Copyright 2010 Autodesk, Inc. All rights reserved.


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Autodesk Inventor - Cam and Valve

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