Contact Connections - Part 2

Last blog we examined connectors in SolidWorks Simulation. Let’s review a few more commonly used contact sets; the bolts, pin and springs.

A Bolted connection is very common in connector design. Bolt connections are simulated in the software by a beam and rigid bar.  The beam resists only tension and allows pre-loading and the rigid bar connects the beam with the flange faces. Only a few parameters need to be defined for a bolt including the type (counterbore, counter sink, etc.), shank diameter, head diameter, pre-load conditions and bolt strength data.  The software does the calculations to determine the resultant axial force shear force and bending moments. The image below illustrates a typical Solidworks assembly that requires bolted connections:

Below is a detailed view of a piping bolted connection:

Pin connectors are also common connectors. In order to simulate pins, SolidWorks creates a beam element and allows rotation as well as translation between the bodies. In order to apply a pin connection you simply select the cylindrical face of the hole and enter the pin characteristics such as strength and resistance to rotation. The image below illustrates the pin connection of a pair of pliers:

Spring connectors can be difficult to model in traditional FEA. The spring’s stiffness has to be divided by the number of nodes on each face and uneven mesh distribution can affect the spring stiffness. SolidWorks makes spring connectors easy by allowing designers to apply the connector either to parallel faces, concentric cylinders or even between vertices. Springs appear only in common projected areas between faces to avoid unintended moments. Designers may define tension-only, compression-only or tension-compression springs. Examples for each respective definition would be a rope, bumper and a typical spring. For each case the user may define a stiffness value as well as a pre-load parameter. Below is an example of a spring connector between two parallel faces:

Applying the correct connection ensures the part reacts properly under loading conditions. Defining connections between parts is made easy using SolidWorks Simulation.  

Contact Sets in SOLIDWORKS SIMULATION - Part 1

CONTACT SETS

Contact sets are fundamental to defining how a part reacts under loading conditions.  We are going to review three contact sets: Rigid Connection, Link Connectors and Edge Weld.

Rigid Connections is quite simply making two faces rigid with respect to one another.  Please be aware that by applying this condition the gap between the two faces will remain fixed, be sure that this is what you want.  An example of rigid connection is as such:

We have two blocks with a rigid connection defined.  Next we can apply a load:

Under loading conditions the blocks have undergone significant deformation.  However the rigid connection between the two remains fixed as expected.

Link Connectors work in much the same way as rigid connections however the difference is they act on points rather than faces.  The distance between the two points remains fixed regardless of the force applied.  Note that link connectors do allow for point rotation.  An example of link connectors is as such:

Here we have two bars with a link connector at the top vertex, however the bottom does not have a link connector.  A small force is applied:

When the force is applied, the linked points at the top do not move, however the points at the bottom are free to move as expected resulting in higher displacement. 

Edge Welds are used to simulate how a weld would behave under loading conditions.  This works between a face and an edge as well as edge to edge.  The software does this by applying a bonding condition on the nodes between one set and the second set.  The weld size and material need to be set when defining an edge weld.  SolidWorks uses the throat shear method to evaluate edge welds instead of just using stresses.  The program calculates normal, bending and shear forces on the weld.  A good example to use edge weld is the following assembly: 

In this assembly the edge welds are applied along the connection points highlighted by the blue lines.

Once an analysis is complete we generate an edge weld size plot and ensure that the weld size defined at the connection is greater than the largest required thickness in the chart.

This refresher should help designers with setting up the correct connections in SolidWorks Simulation

SolidWorks Simulation 2016 - Contact Sets

Contact Sets - New in SolidWorks Simulation 2016

When a designer is putting together an assembly there may be a few missed mates which will cause the part to be free and unrestrained.  Usually the designer will need to go back and find the missing mate to fully restrain the assembly.  This is a similar concept to contact sets in simulation where a designer must define contact sets or the connection conditions between elements.  This was typically a difficult task that involved looking through the model to find the missing contact set. 

Consider a crane tower as shown:

We will setup a simulation study for this crane tower where it is fixed on the ground and experiencing horizontal and vertical loads at the top.  

Before running the study, the designer now has a new tool to help check for free moving bodies.  This tool is found by right clicking Connections then selecting “Find Underconstrained Bodies”

With the new tool selected, the designer may use the tool by selecting “calculate”.  This allows the software to run a simplified analysis to find whether there are under-restrained bodies.

The results return by isolating and clearly highlighting bodies of interest and showing all modes in which it is under-constrained.  In this case, Body 1 is shown as free floating and can move in every direction. 

This is a problem for the simulation analysis.

Upon closer inspection it was determined that this crane tower was originally modeled with a small gap between the elements to account for welding and tolerances. 

Usually the designer would need to create individual contact sets to account for the gap, however also new to SOLIDWORKS SIMULATION 2016 is the ability to accommodate small gaps.  This option is found under Connections> Component Contacts > Global Contact > Edit definition

Under the component contact option, the designer now has the ability to include “Non-touching faces” by selecting a gap size to treat as bonded.

This is one more tool to help designers be more efficient and effective when using SolidWorks Simulation by spending less time finding missing contact sets and more time analyzing the results.  

Contact Sets in SOLIDWORKS SIMULATION

Simulation Contact Set

SolidWorksSimulation of assemblies or multi bodies requires the application of contact sets to define the interface between two objects.  If the contact sets are not defined properly we will obtain incorrect results.

Consider a simple weldment framed structure with a steel plate sitting on top.  

This weldment is fixed at the end of each leg and has a load applied to the plate on top.

By default, SolidWorks Simulation treats weldments as beam elements and the plate on top as a solid element.  As such we have two different element types in this analysis; therefore we need to define a contact set.  As an option we can use the “automatically find contact set” function by selecting a beam element and the plate.  The software will find the connecting surface where you may apply a condition, in this case bonded.  This is repeated for all four faces.

Upon closer inspection, we find those contact sets to be a face to face contact

Following the creation of the contact sets we can mesh our model and run the analysis

However in doing so will result in an error message. 

This is because the contact set defined was incorrect.  When the contact sets were created they were treated as faces, however since this is a solid to a beam element, we need to select the beam element contact set under “Type”.  By redefining all four contact sets, remeshing and running the study, we now have the correct results.