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This section gives specific examples on how to build an assembly tolerance model. Several examples will be given listing the exact steps to model each assembly correctly.

CRANK SLIDER MODEL

The crank slider is a simple one loop problem that we will use to explain the basics of tolerance modeling. The CATIA geometric model is shown in Figure 5.1.

Figure 5.1 Crank Slider

Defining Parts

There are four parts associated with the crank slider. Their corresponding datum reference frames should be modeled as:

• Base : rectangular DRF
• Crank : cylindrical DRF
• Slider : rectangular DRF

Starting with the base, we will show the process of creating a DRF.

A selection window will appear in the lower left corner listing all the datum types.

Select "rectangular" from the window as the datum reference frame type.

Figure 5.2 Locations of DRFs

Select the center point for the DRF. This is the inside corner on the lower left of the drawing as seen in Figure 5.2.

The selected geometry will be highlighted. If at any time a mistake was made press the No key (F12) to redo the selection.

Select the x-axis as a DRF axis and give 0 for the tolerance.
Repeat selecting the y-axis and the z-axis giving 0 tolerance for each.
Press the Yes key (F11) to finish entering axes.
Select the base solid entity.
Enter the part name "Base"

The other parts are defined in a similar manner as shown in Figure 5.2. Use the appropriate datum type for each as listed above.

Defining Joints

There are four joints in this assembly between the four parts. These joints should be modeled as:

• Base - Crank : revolute
• Crank - Link : spherical
• Link - Slider : spherical
• Slider - Base : prismatic

Starting with the base-crank interface, we will show the process of creating a joint.

A selection window will appear in the lower left corner listing all joint types.

Select "revolute" from the list of joint types.

Figure 5.3 Locations of Joints

Select the center point as a point along the axis of rotation between the base and crank as seen in Figure 5.3.

The selected geometry will be highlighted. If at any time a mistake was made, press the No key (F12) to redo the selection.

Select the joint primary direction along the axis of rotation. (x-axis)
Select another axis direction that is perpendicular to the primary axis. (y-axis or z-axis)
Select the base as the first solid.
Create a rectangular feature datum at the intersection of the axis of rotation with the left plate of the base.
Use the x, y & z axes with 0 tolerance to define datums.
Select the DRF of the base to end the datum path.
Select the crank as the second solid.
Select the DRF of the crank to end the second datum path.

The other joints are specified in a similar manner and their locations are shown in Figure 5.3.

Defining Loops

Select the joint between the base and the crank.
Next select the joint between the crank and the link.
The vector path between joints will be shown.
Next select the joint between the link and the slider.
Next select the joint between the slider and the base.
Finally select the first joint to finish the loop.
Enter yes (F11) to accept the loop.

The complete loop path should look like Figure 5.4.

Figure 5.4 Locations of Loop

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