Guest Blog Post | Making Rotating Assemblies with a Hinge Joint

In Virtual Crash you have the ability to take solid objects (.3ds, .dxf), separate out components, and add joints to manipulate movement. In this post, we will look at using a hinge joint to create a rotating assembly. If you've wondered how the following two animations were created in Virtual CRASH, read on, and you'll learn how they were done. 

First we will start by bringing in a helicopter in .3ds format. Note that we have already colorized our helicopter using the “Elements” selection type in the top menu bar (this is illustrated in the User's Guide (VC5 | VC4 | VC3) and in this video).

Next we will need to separate the rotor and blade assembly from the helicopter fuselage. We will do this by choosing “Elements” from the top menu, selecting all of the blade and rotor polygons (using the lasso feature) and selecting “detach” under the “faces” menu on the left control panel. Note that selecting entire assemblies is usually easier in an orthographic view. 

After selecting “detach”, you’ll notice that Virtual Crash makes a separate entry into the objects list in the left side control panel. Since we now have two separate objects, we can connect them with a hinge joint and make one rotate.

First you will want to move the pivot point of the rotor and blade assembly. You’ll notice that when you use the “detach” feature and create separate objects, the pivot point for the new object will remain in the same place as the original object. Select your new object, then select the pivot point icon from the top menu. Using the orthographic views will ease the exact placement of the pivot point. The picture below shows the original placement of the pivot point for the rotor and blade assembly.

And the pivot point after being placed in the correct position. You can now switch back to “object” mode.

Now you will want to select the rotor and blade object and make them a “Rigid Body” from the top menu. Now you can edit the weight of your object under the “mass properties” portion of the left control panel. Note that if you forget to select “Make Rigid Body”, you will not be able to attach a hinge to the rotor and blade assembly.

You will notice that as soon as you select “Make Rigid Body”, the laws of physics will begin to act upon the rotor and blade assembly, causing them to fall. This may interfere with the following steps, so now would be a good time to “Stop Simulation.” As the name implies, this will stop the simulation from running and prevent any of our objects from moving.

Now it is time to attach our two objects using a hinge joint, which can be found in the top menu.

After selecting the hinge joint, we will hover our cursor over the rotor and blade assembly until it turns blue, indicating that we have selected it.

Now left click and hold, while moving your mouse down to highlight the helicopter fuselage.

Release the left mouse button and you should see a hinge joint has been added between the rotor and blade assembly and the helicopter fuselage. As with the pivot point, you’ll want to make sure that the hinge is properly aligned to ensure smooth operation. You can do this by selecting the joint from the project menu, then selecting “parent space” from the top menu. Remember that orthographic views are your friend when doing precise placement.

Here we now have our hinge in the correct location. Note that using the “Contours” view (top window) is often helpful when placing hinges because it allows you to see through objects.

Once your hinge is properly placed, restart the simulation by deselecting “stop simulation” from the top menu. Now select your joint from the project menu and deselect “use” from the “limits” portion of the left control panel. This will allow our hinge joint to spin freely.

Now we are ready to put our rotor and blade assembly into motion! Select the assembly from the project menu and open the “dynamics” tab on the left control panel. Now put in a value for “omega-z”. You should see the blade and rotor assembly rotating. Note that putting in a positive value for “omega-z” will cause the assembly to rotate counterclockwise and putting in a negative value will cause the assembly to rotate clockwise.

Sometimes hidden polygons within the separate assemblies will interact with each other and cause undesirable effects with respect to rotation. If this happens, select the hinge joint under the project menu. Then select “disable before break” under the “contact” tab. Your assembly should now rotate smoothly. If not, double-check to make sure your pivot point and hinge are placed correctly.

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