Using Contact Interfaces with Infinite Friction in Pro/ENGINEER Wildfire 4.0

In previous releases of Pro/ENGINEER Mechanica, contact regions were entirely frictionless surfaces. This meant that additional constraints needed to be added to prevent rigid motion tangential to the contact region. With the release of Pro/ENGINEER Wildfire 4.0, contact regions have the option to add infinite friction. As such, axial constraints for pin-hole combinations or awkward spring setups for ball bearings will no longer be required.

In addition, you will be able to view traction and slippage results to get a rough idea if the model will slip under the conditions imposed in Pro/ENGINEER Mechanica. If you want to view results for the slippage indicator, you will have to enter the value of the coefficient of static friction between two parts. You must determine the value of this coefficient. The values used in this example are arbitrary and for the sake of demonstration.

The UI for contacts have changed as well, and this document will attempt to cover those changes in addition to demonstrating infinite friction. Chief among the changes is that Contact has been moved to the interface menu. As such, contact will be referred to as a "Contact Interface" instead of the usual "Contact Region" term used in previous releases. Free and bonded interfaces can also be created from the interface menu and are covered in other documents.

Model Setup Download and unzip the model available here and open friction_asm.asm. The model is displayed below (Figure 1). The model consists of two parts, mounted_bracket.prt which is fully constrained and block.prt which sits on top of it with no constraints defined. The loads are applied to block.prt and consist of a force acting into the contact region and another pulling tangential to it.


Figure 1

The contact interface is set by selecting Insert > Connection > Interface. In the interface dialogue, set the Type field to Contact, as shown in Figure 2 below. The coefficient of friction in this situation is entered as 0.0015. Note that the full name for this field is Coefficient of Friction for Slippage Indicator Measures. This means that this value of the coefficient will be used only to calculate the slippage indicator measure and nothing else. The model will not see any added stress due to the friction. 


Figure 2

The slippage indicator is calculated by deducting the friction force from the traction force across the contact region. A positive value indicates that slipping will occur, a negative value indicates no slip, and zero indicates a force balance or equilibrium that straddles the boundary of slip and no slip. A more detailed explaination can be found in the Help Center installed locally with Pro/ENGINEER (Not to be confused with documents on PTC's website). If <Pro/E_Load_point> is the directory where Pro/ENGINEER was installed, document address would be <Pro/E_Load_point>/html/usascii/proe/promec/modstr/measures/reference/slippage_measures.htm

Analysis & Results

If the current model were solved in a previous release it would fail because the model is insufficiently constrained. However, with infinite friction this model can be analyzed without further additions. The analysis itself remains unchanged over previous releases, no additional options need to be selected. The first analysis to be run will be CONTACT_FRICTION_0015, shown below (Figure 3).


Figure 3

The analysis should run relatively quickly. Note how default contact measures Contact Area and Contact Force are now referred to as Interface1_area and Interface1_force in Figure 4 below.


Figure 4

The results can now be reviewed. Instead of analyzing the usual stress and displacement results, the results quantity will be set to "Contact Slippage Indicator" as shown below (Figure 5).


Figure 5

The results for slippage can be seen below (Figure 6). The red area shows where slipping will occur, and grey represents a slippage indicator of zero. So the forces applied in this model would likely force the block to move off of the mounting bracket.


Figure 6

Now edit the contact interface and set the friction coefficient to 0.15 instead of 0.0015. Now run the analysis named CONTACT_FRICTION_15. When the analysis complete, look at the measures and not how changing the friction coefficient did not alter displacements or stresses reported. Now look at the contact slippage results for the new analysis, shown below.


Figure (7)

As seen in Figure 7, in some areas the friction force is greater than the traction force. If the goal was to preventing slipping all together, then the entire area would need to be green/gray.

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