# SOLIDWORKS Simulation – Bolt Connector

Article by Irfan Zardadkhan, PhD, CSWE updated January 27, 2015

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A Bolt can connect two components, multiple components, or a component and the ground. You can define bolts through a mixed stack of solids, shells and sheet metal bodies. You can also define a bolt by selecting entities of the same component.

SolidWorks Simulation uses a beam element to represent the bolt shank and uses rigid bar elements to connect the beam to the flanges. It simulates the bolt preload using thermal expansion/contraction. The calculation of the thermal expansion/contraction is done automatically by the solver.

The simulation of bolt connectors involves the internal creation of rigid links which can lead to inaccurate results, particularly stress results, in the bolt areas. Error reduces away from the bolt regions. The level of accuracy is much higher for calculating the forces in the bolt regions. Therefore, it is always expected that stresses in the close vicinity of bolts are not to be taken at face value.

The bolt connector is defined by a beam element which does not have any resistance to torque because axial rotational degree of freedom is released. This is consistent with the physical model. In reality, the slippage between the connecting parts is resisted by the frictional force provided by the clamp force (preload).

In addition, it is important to note also that the beam used between the two ends is tension-resistant only (no resistance to compressive force). As a matter of fact this feature is the reason for the problem to become nonlinear (in the absence of any other nonlinearity), and simulate a contact behavior (but in opposite direction to no-penetration contact due to the tension-resistance feature of it). That is why the problem has to go through iterations and subsequently becomes slower (in the absence of any other nonlinearity such as actual contacts defined between the parts)

The number of rigid bars created for the bolt connector depend on the kind of bolt connectors and the mesh created.

For loose fit bolt: On each side of the modeled bolt, the number of rigid bars is equal to the number nodes on the face in contact with the nut and the screw head respectively. Therefore, it can only be determined after the model has been meshed.

For tight fit bolt: In addition to the rigid bar elements as generated for loose fit bolt, program generates the rigid bar elements between the nodes on the cylindrical surfaces in contact with the screw shank and the node from where spider arrangement originates.

Bolt pre-load in SolidWorks Simulation can be either calculated from a defined Axial force are from a combination of Torque and Torque coefficient. This combination helps calculate a tensile force.

In static studies, the definition of bolts induces two successive runs of the analysis:

1. The defined Pre-load value is used to pre-stress the Bolt Connectors. The analysis is run without any other load applied. Then, the tensile forces in the bolts are obtained, and they are compared with the Pre-load values for each bolt.
2. Based on the difference between the tensile forces in the bolts and the Pre-load values that was entered for each bolt, a new adjusted Pre-load value is internally calculated such that when used in a new similar run, the new tensile forces obtained in each bolt will match the initially desired Pre-load.
3. The analysis is run once again with all the loads and the internally adjusted Pre-load for each Bolt Connector

From an absolutely rigorous standpoint, the Pre-load should not be adjusted based on a first run with no load included at all. As a matter of fact, some loads could have been present when the bolts were actually tightened and they should be included in the first run. Such loads will typically include Gravity, but can also include all the loads representing the state of stress that was present when the bolts were tightened: this could potentially include some Forces, Pressures, thermal loads, Spring Pre-loads, etc. It is not currently possible to select which should be included.

In nonlinear analysis, the Pre-load is solved during the first time step of the analysis, and its value is taken as is and never adjusted by the program.

The bolt is prescribed the same reference temperature at zero strain as the rest of the assembly (T0), which is defined in the static study’s Thermal/Flow effects tab in the Properties dialog box. For calculating the thermal expansion or contraction of the bolt, a uniform temperature distribution is assumed for the entire bolt (T). If the tight fit option is not selected, then the bolt temperature is obtained by averaging the temperatures of the head and nut contact surfaces. If the tight fit condition is enabled, then the temperature of the cylindrical hole surface
is also taken into account for the calculation of bolt temperature. Since the bolt connector shank is modeled as a beam element, the bolt connector can only expand or contract in the axial (not the radial) direction.

The implementation of the Bolt Series option on middle plate for bolt connectors is much like the Tight fit option. The program creates rigid bars (light blue)to connect the bolt shank to the nodes of the cylindrical face of the middle plate (red) as shown below. The consequence is that the cylindrical face of the middle component is then rigid. It wont’s compress, and will
only show stress on the nodes belonging to the two circular edges.