91 - force moment using total load at point – exercise

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StructuralLoadsForceMomentUsingTotalLoadatPointExercise

Exercise: Force/Moment Using Total Load at Point

Objectives

After successfully completing this exercise, you will be able to:

Identify when using a total load at a point can be useful. Apply a Force/Moment with the Total Load At Point option selected.

ScenarioThere are instances where a part is being designed to carry a specific load, but the loadsorigin lies outside the part. It could be the weight of a book that a shelf is carrying, or a

moment arm exerted by a person on a bolt through a wrench or similar tool. In these

instances, you have no interest in modeling the book (any generic payload) or thewrench, but you do want to include the load. An engineer would draw a free body diagramand attempt to transpose the force, adding whatever moments may be needed to create

the equivalent load on the shelf or on the bolt.

However, in Mechanica, the user need not get into complex calculations or draw free bodydiagrams. Here the user can make use of the Total Load At Point distribution option. The

model for this assembly is a bracket that holds a battery in place. The battery will be

removed from the model and its weight applied in its place.

TotalLoadatPoint battery_asm.asm

Task 1. Create a Datum Point at the Battery C.G.

1. Click Datum Point Tool to create a point. Select PNT_COG as the reference,

then click OK to create the point.

The point PNT_COG created for the BATTERY.PRT is the location of the C.G.

Since this point will be excluded along with the battery from the assembly, anew point needs to be created.

The center of gravity is the virtual point where the weight of an object may

appear to act, or about which all moments are equal to zero.


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Task 2. Removing the Battery from the Assembly.

1. Create a simplified representation that excludes the battery from the assembly.

Select BATTERY.PRT in the model tree. Click View > Representation > Exclude. Click View Manager Select the Simp Rep tab if necessary, then click Edit > Save Click OK > Close to complete the simplified representation

This will exclude the battery from the model, and hence it will not be usedby Mechanica in any subsequent analysis.

2. The battery is absent from the model, and a load can be applied. A parameter hasalready been created with the value of the batterys weight. Click Tools >Parameters to view the value of the BATTERY_WEIGHT parameter.

3. Click OK to close the Parameters dialog box.

4. The model is now ready for use in Mechanica. Click Applications > Mechanica.


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Task 3. Apply a Force/Moment Load.

1. Click Force/ Moment Load . Select the top surface of the bracket where thebattery would have rested as shown in the figure.

2. Click Advanced and select Total Load At Point from the Distribution drop-down

menu. Select APNT0 for the reference point. Type -battery_weight in the force Ycomponent field as shown in the figure.

This will place the negative value of the BATTERY_WEIGHT parameter as the

force in the Y direction.

3. Click Preview to view how the load will act.


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4. Click OK to create the load and close the Force/Moment Load dialog box.

Task 4. Verify the Load Application.

1. Click Info > Review Total Load...

2. Click Select Reference under Loads, select the load created in the previous taskfrom the display window, and click OK to complete selecting loads.

3. Click Select Reference under Evaluation Point and select APNT0 from thedisplay window.

4. Click Compute Load Resultant. The dialog will then report the resultant as shown

in the figure.

The resultant is in agreement the battery weight parameter's value beingapplied in the negative Y direction. Furthermore, the Moment registered

around the evaluation point is zero which corroborates the load beingapplied at the point APNT0.

Values that are expressed in terms of 10e-16 and 10e-12 can more or lessbe considered zero.

5. Click OK to close the Load Resultant dialog box when you are through reviewing it.


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Task 5. Investigate the effects of using the default Total Load option instead of the

Total Load at Point option.1. Select the load created in Task 3, right-click and select Edit Definition.

2. Select Total Load from the Distribution drop-down menu and click OK to close the

dialog box.

3. Click Info > Review Total Load...

4. Click Select Reference under Loads, select the load created previously from the

display window, and click OK to complete selecting loads.

5. Click Select Reference under Evaluation Point and select APNT0 from thedisplay window.

6. Click Compute Load Resultant. The dialog will then report the resultant as shownin the figure.

Note that the applied load is creating moments about the X and Z axes. This

scenario does not match the real world load because the battery's weight

should not be exerting moments about its C.G.

7. Click OK to close the Load Resultant dialog box when you are through reviewing it.


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Task 6. Save the model and erase it from memory.

1. Return to the Standard Pro/ENGINEER mode by clicking Applications > Standard.

2. Click Save from the main toolbar and click OK to save the model.

3. Click File > Erase > Current > Select All > OK to erase the models frommemory.

This completes the exercise.

91 - force moment using total load at point – exercise

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