Simulating these assemblies using traditional finite-element techniques, however, takes a great deal of time and experience. "And when a design model contains several pins, such as a scissors lift, using contact in the simulation taxes computer resources. Sometimes it's downright impractical," says Ramesh Ramalingam, product manager with SolidWorks' Cosmos division in Los Angeles. "What's more, it may not provide the most needed information." Ramalingam says his team of developers has recognized the shortcoming and added features that make it easier to analyze pin connections.
To show how the software works, consider a set of pliers. The traditional analysis setup includes meshing the pin, and defining contact between the pin and cylindrical faces of the pliers' hands. "Users are often interested in the effect the pin has on parts next to it or on the overall assembly, rather than the stress distribution on the pin. In these cases, FEA software such as CosmosWorks has virtual pin connectors that make it easier and faster to analyze assemblies with pins. This lets users find forces and moments acting on the pin, and use the information to calculate an appropriate pin size," he says.
To determine the inputs needed to calculate the pliers pin, "the user first identifies the faces contacting the pin. The virtual pin connector in CosmosWorks simulates pins that rotate, slide back and forth, or both. Those possibilities mean users must identify the required movement. For pliers, the user assumes the faces freely rotate against the pin," he adds.
Ramalingam says the software models the pin with a beam element, and connects it to the cylindrical faces of the two parts using bar elements. Users must specify axial and rotational stiffness for the pin connector. The relative axial movement of the faces depends on the axial force developed in the joint and the specified axial stiffness. Similarly, relative rotation depends on the moment that develops in the joint and the specified rotational stiffness. If users want to model a pure-pin connection with free rotation, they should choose the no-translation option. Then the software will define axial stiffness so there is no translation while still allowing free rotation.
In addition to simplifying modeling, virtual-pin connectors make it easy for novice analysts to work with assemblies connected with pins. They don't have to worry about placing beam elements or what stiffness to use for bar elements that would constrain rotation or translation.
If users manually model pin connections, they have to define the beam and bar elements manually. For the relatively simple pliers, engineers would have to create one beam element and 160 bar elements, and then repeat the steps each time the design, geometry, or mesh changes. If errors creep in when defining the beam or bar elements, isolating them would be difficult. Virtual-pin connectors eliminate that problem and make follow-on iterations much easier.