Objectives Upon successful completion of this lesson, you will be able to:
I Build proper SolidWorks Motion models for kinematic simulation.
I Create local mates for a SolidWorks Motion study.
I Create and modify plots for post-processing.
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Creating Local Mates
In the previous lesson, the mates created in SolidWorks were used as joints in SolidWorks Motion. If the components are not mated in SolidWorks, or if we wish to examine different connection types in SolidWorks Motion, mates can be added or modified in the Motion Analysis.
Case Study:
Crank Slider Analysis
In this lesson, we will setup the mechanism for the crank slider model.
We will use SolidWorks mates that most closely represent the real mechanical connections. The crank slider model is used in a variety of engineering applications, such as a steam engine or the cylinder of an internal combustion engine. Therefore, we will apply a motor on the crank part, run the simulation, and then postprocess some results to estimate the required torque.
Problem Description
The crank is driven at a constant angular velocity of 60 RPM.
Determine the torque required to rotate the crank part.
Stages in the Process
I Create a motion study.
I Preprocessing.
Add local mates to the assembly with the motion study active.
I Run the simulation.
Calculate the motion.
I Post-processing.
Plot and analyze the results.
1 Open an assembly file.
Open 3dcrankslider from the Lesson02\Case Studies folder.
Arm Mount
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2 Examine the assembly.
SolidWorks Motion assumes that all components that are fixed in SolidWorks are considered to be grounded parts, and all components that are floating are assumed to be moving parts.
However, the movement of these parts is constrained by the SolidWorks mates.
There are no mates in this assembly, but three parts are fixed. The collar_shaft, arm_mount and crank_housing are fixed as these are parts that would be connect to ground and will have no motion in the assembly.
The remaining parts will need mates to constrain their motion to that expected of the mechanical system.
Mates Mates are used to constrain the relative motion of a pair of rigid bodies by physically connecting them.
Note A rigid body acts and moves as a single unit. SolidWorks components situated at the root level are considered rigid bodies. This means that SolidWorks and SolidWorks Motion treat subassemblies as single rigid bodies.
Mates can be classified into two main types:
I Mates used to constrain the relative motion of a pair of rigid bodies by physically connecting them. Examples: Hinge, Concentric, Coincident, Fixed, Screw, Cam, etc.
I Mates used to enforce standard geometric constraints. Examples:
Distance, Angle, Parallel, etc.
Below are some descriptions of some of the most commonly used mate types. For a comprehensive understanding of all the other mates, please refer to the SolidWorks help.
No
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Concentric Mate The concentric mate allows both relative rotation as well as relative translation of one rigid body with respect to another rigid body. The concentric mate origin can be located anywhere along the axis about which the rigid bodies can rotate or slide with respect to each other.
Example: Piston sliding and rotating inside a cylinder.
Hinge Mate Hinge mate is essentially concentric mate with the restricted translation between the two components.
In SolidWorks Motion, the hinge mate is used rather than a
combination of concentric and coincident because the mechanical joint is a hinge. Hinge mates are found in the Mechanical Mates tab of the Mate PropertyManager.
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Point-to-Point Coincident Mate
This type of mate permits free rotation about a common point of one rigid body with respect to another rigid body. The origin location of this mate determines the point about which the rigid bodies can pivot freely with respect to each other. Example: Ball and Socket joint.
Lock Mate The lock mate locks two rigid bodies together so they may not move with respect to each other. For a lock mate, the origin location and orientation does not affect the outcome of the simulation. A real world example of a lock mate is a weld that holds two parts together.
Two Face-to-Face Coincident Mates
This mate allows one rigid body to translate along a vector with respect to a second rigid body. The rigid bodies may only translate, not rotate, with respect to each other.
The location of the origin of a translational joint with respect to its rigid bodies does not affect the motion of the two bodies but does affect the reaction or the bearing loads.
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Universal Mate A universal mate permits the transfer of rotation from one rigid body to another rigid body. This mate is particularly useful to transfer rotational motion around corners, or to transfer rotational motion between two connected shafts that are permitted to bend at the connection point (such as the drive shaft on an automobile).
The origin location of the universal mate represents the connection point of the two rigid bodies. The two shaft axes identify the center lines of the two rigid bodies connected by the universal joint. Note that SolidWorks Motion uses rotational axes parallel to the rotational axes you identify but passing through the origin of the universal mate.
Screw Mate The screw mate constrains one rigid body to rotate as it translates with respect to another rigid body.
When defining a screw mate, you can define the pitch. The pitch is the amount of relative translational displacement between the rigid bodies for each full rotation of the first rigid body. The displacement of the first rigid body relative to the second rigid body is a function of the rotation of the first rigid body about the axis of rotation. For every full rotation, the displacement of the first rigid body along the translation axis with respect to the second rigid body is equal to the value of the pitch.
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Point-on-Axis Coincident Mate
This type of mate permits one translational and three rotational motions of one part with respect to another. The translational motion between the parts is confined to the orientation axis. The point defines the initial pivot location on the axis.
Parallel Mate A parallel mate permits only translational motion of one part with respect to another. No rotation is allowed.
In the picture below, the blue x part can move relative to the ground in the X direction. The red y part can move relative to the x part in the Y direction. The z part can move relative to the y part in the Z direction.
Finally, the red/yellow/blue cube on the z part has a curvilinear motion relative to the ground but always stays parallel.
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Perpendicular Mate
The perpendicular mate allows both translational and rotational motion of one part with respect to another. It imposes a single rotational constraint on the components so that the component axes remain perpendicular. This allows relative rotations about either z-axis, but does not allow any relative rotation in the direction perpendicular to both z-axes.
It is recommended that the mates are representing the real mechanical connections as closely as possible, i.e. mechanical hinge should be modeled using the hinge mate and not using a combination of coincident and concentric mates.
Local Mates Mates created in SolidWorks can be transferred to the Motion Analysis and used as mechanical joints. If there are no mates in the SolidWorks assembly or if we wish to define the connections differently than the SolidWorks mates, we can add local mates directly to the motion study.
Local mates only apply to the study to which they were added.
To add local mates, make a motion study active and add the mates.
With a motion study active, any mate added is only applied in that motion study.
3 Verify the document units.
Verify that the document units are set to MMGS (millimeter, gram, second).
4 Create a Motion Study.
Right-click the Motion Study 1 tab and click CreateNew Motion Study.
Make sure that the Motion Analysis is selected as the Type of Study in the MotionManager.
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5 Move components.
Move the components that are not fixed to separate the assembly. We are doing this only to make it easier to select faces and to keep track of what components are mated.
6 Create a local mate.
Add a mate and select Hinge from the Mechanical Mates section. For Concentric Selections, select the two cylindrical faces of the shaft and hole shown with red arrows. For Coincident Selections, select the end face of the shaft and crank housing shown with the blue arrows.
Click OK.
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7 Warning.
Because the timeline is active, the mate changes the position of the crank at the starting position of the animation. This is OK for what we are doing.
Click Yes.
8 Examine the mate.
Notice that this mate is only located in the MotionManager and not in the FeatureManager design tree.
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9 Add additional mates.
Add a concentric mate between the two spherical surfaces shown on the parts Link1 and crank.
10 Mate arm to arm_mount.
Add a hinge mate to connect the arm to the arm_mount.
11 Mate Link1 to the arm.
This connection requires two hinge mates, one between Link1 and cardian, and a second hinge mate between cardian and arm.
Concentric
Link1
crank
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12 Mate Link2.
Link2 will use a hinge mate to connect the arm. As there is no pin going through the two holes, the coincident selections will be the two touching faces.
Mate the other end of Link2 to the pin on the collar with just a
concentric mate.
13 Mate collar to collar_shaft.
Add a concentric mate between a cylindrical surface on each part.
14 Test the assembly.
Rotate the crank and make sure the components move as expected.
Check the FeatureManager design tree and the Motion Study tree. All the mates should just be in the Motion Study tree.
15 Add gravity.
Add gravity in the negative Y direction.
16 Calculate.
Adjust the assembly key to 5 seconds. Click Calculate Simulation . 17 Play the simulation.
Play the simulation at 25% speed.
The crank will rock back and forth as gravity affects the components and potential and kinetic energy are exchanged. As there is no friction, the parts will continue to move without end.
18 Set the timebar to 0s.
To add a motor at time 0s, the timebar needs to be set to 0s.
19 Add a motor.
Create a Motor that drives the crank. Click Motor on the Motion Manager.
Under Motor Type, select Rotary Motor. Under Motor Location, select the cylindrical face of the crank part (as shown in the figure).
The default selection for the Motor
Direction is correct for this analysis. Make sure that the motor is oriented as shown in the figure.
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Under Motion, select the Motor Type as Data Points. The command invokes the Function Builder window.
Make sure that Value (y) and Independent variable (x) are set to Displacements (deg) and Time (s).
Function Builder Function Builder can be used to construct functional expression for motors and forces.
Introducing:
Function Builder
Function Builder can build functional expressions using predefined Segments, imported set of discrete Data Points or mathematical Expressions.
The figure below shows the segment view of the Function Builder window.
I Segments
In Segment view, user select both the independent (typically time) and dependent variable (displacements, velocity or acceleration).
For each specified interval, the transition from the initial to final value is controlled using one of the predefined profiles curves. The following profile curves have been implemented: Linear, Cubic,
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Quarter-Sine, Half-Sine, 3-4-5 Polynomial and others. As the function is constructed, the graph windows show the corresponding variation of displacement, velocity, acceleration and the jerk (time derivative of acceleration). Note that it is possible to save and retrieve function from stored location.
I Data Points
The discrete set of data points can be imported from a *.csv file or entered manually. The functionality as well as the options are similar to the Interpolated option of the input and are explained in this lesson.
I Expression
Expression enables the construction of functions with the help of predefined mathematical functions, variables and constants, and existing motion study results. As in both previous cases, the function can be saved at a specific location. This procedure will be used in this lesson.
Where to Find It I In the Motor or Force/Torque PropertyManagers, under Motor Type or Force Function dialog select Segments, Data Points or Expression.
20 Import data points.
Rather than type the individual values into the table, we can load them from a file. In this case, we have an Excel file. Locate the file crank rotation.csv in the Case Studies folder and examine the file. It is just two columns of numbers representing the time and displacement.
Click the Import Data button. Navigate to and select the crank rotation.csv file and click Open. The values from the file are now inserted into the Time and Value columns.
Select Akima as the Interpolation type.
Note The Function Builder graph windows automatically updates the plots for displacement, velocity, acceleration and jerk. The data points describe linear increase of the angular displacement in time, a harmonic motion
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.
Click OK to complete the definition of the profile and close the Function Builder.
Click OK to complete the definition of the Motor feature.
Rename this motor feature to Motor-crank.
Importing Data Points
Using imported data points, you can use your own motion data to control the displacement, velocity, or acceleration of the motion. The data points that can be imported into SolidWorks Motion must be in a text file (*.txt) or comma separated file (*.csv) format. The file should contain one data point per line. The data point consists of two values, the time and the value at that time. Commas or spaces can be used as separators between the values. The file is essentially free format aside from these restriction. SolidWorks Motion allows for unlimited number of data points to be used. The minimum number of data points to be defined is four.
The first column, Independent variable (x), in the data point template is typically time, but other parameters such as cycle angle, angular displacement and others can be used as well. The second column, Value (y), is the displacement, velocity, or acceleration. These values can be manually entered or imported.
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Besides Linear interpolation, two spline-fitting options are available to smoothen out the data: the Akima spline (AKISPL) and cubic curve spline (CUBSPL). It is recommended that you use a cubic curve because it will work well even if you data points are not evenly spaced.
An Akima curve is fast, but will not work as well if you points are not evenly spaced.
21 Run the simulation.
Click Calculate to run the simulation for 5 seconds.
22 Plot the torque.
Create a plot for the torque required to turn the mechanism.
Define the plot by Forces, Motor Torque and Magnitude.
Select the Motor-crank for the Simulation element.
Click OK.
23 Examine the plot.
The plot may be improved by recording more data points by increasing the Frames per second option in the Motion Study Properties.
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24 Plot the power.
Create a plot for the power required to turn the mechanism.
Define the plot by Momentum/Energy/Power, and Power Consumption.
Select the Motor-crank for the Simulation element.
Click OK.
Note Knowing the operating RPM, torque and/or power we can select the appropriate motor to drive our system.
Power Power is the rate at which work is performed, or the amount of work conducted in one second. Forces conduct work on distances, moments then on the angular displacements. For rotating motors the following relationship therefore holds:
The power plot in the previous figure can be easily verified. The maximum torque is 10 N-mm = 0.01 N-m
Students can easily verify this by creating the plot of the angular velocity.
The resulting maximum power is then:
The graph of the power indicates 0.06 W because two significant digits precision is used by default.
Power [W] = Torque [N-m] ×Angular velocity [rad/sec]
Angular velocity = 360 deg/sec = 2π rad/sec
Power = 0.01×2π = 0.063W
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Alternative Units Often times the rating of the electric motors is expressed in maximum power and torque. Alternative units are used frequently.
If rpm is used for the angular velocity, then:
If horsepower is used instead, the following conversion can be used:
A useful formula when computing power using mechanical horsepower in the English system of units is the following:
While mechanical horsepower is common in some industries in the United States (automotive industry, for example), similar measure called metric horsepower is used in Europe and Asia. Metric horsepower is then defined as:
Because of this ambiguity in the definition of horsepower, its use today is not recommended.
25 Add a mate.
When we added mates to this motion study, we only added mates essential to describing the motion. Depending on how the assembly is built, a mate preventing the collar from rotating around the collar shaft could be defined. This mate would represent the mechanical function of the keyway.
Add a coincident mate between one side face of the key and the corresponding face on the keyway.
Power [W] Torque [N-m]×2π×Angular velocity [rpm]
---60
=
Mechanical horsepower = 33,000 lb-ft/min = 745.7W
Power [hp] Torque [lb-ft]×2π×Angular velocity [rpm]
33,000
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26 Run the study.
Change the Frames per Second to 100, then re-calculate the study.
27 Review results.
The torque values are essentially the same as when we did not have the coincident mate. The plot is now smoother as we have four times more data points.
Following the recommendations that all mates should represent the real mechanical connections for the kinematic analyses, this mate defining the keyway could be defined, even if it is not required for the actual motion analysis.
28 Plot reaction force.
Create a new plot to show the reaction force on the motor.
Define the plot by Forces, Reaction Force, and Magnitude.
In this assembly, the first hinge we defined was between the crank and crank_housing. As the crank_housing is fixed, the mate must transmit the reaction force.
Select the first hinge mate as the Simulation element.
Because the selected mate connects two parts, there are two equal and opposite forces acting in the mate. One of the two parts must be selected for the plot of this force.
Select any face on the crank-1 part as the second component in the
Select any face on the crank-1 part as the second component in the