Bolts (and certain types of threaded fasteners) are commonly tightened to levels producing very high preload forces. Preloading bolts to about 75% of their proof strength is typical. The bolt preload capability in NX Nastran allows you to predict stresses in the bolts and the bolted medium that arise from bolt preload forces alone or bolt preload forces and service loads.
Historically, bolt preload was modeled using either an equivalent thermal load approach or a multipoint constraint (MPC) approach. Both methods are capable of providing accurate results. However, both methods are labor intensive requiring multiple solutions, manual capture of data, and hand calculations.
The NX Nastran approach is much more efficient because the entire run is automated and allows for direct entry of the bolt preload forces. During the run, the model is solved twice. The first solution calculates the deformed shape of the bolted medium resulting from bolt preload forces. The software then performs an intermediate calculation. The appropriate compressive forces to apply to the bolts during the second solution are calculated correcting for differences in the length of the bolt model and the loaded length of the actual bolt. Solving the model a second time gives the stress state resulting from bolt preload forces and, optionally, service loads.
Bolts can be modeled using CBEAM and CBAR line elements or CHEXA, CPENTA, and CTETRA solid elements. Bolt preload is supported in SOLs 101, 103, 105, 107 through 112, and 601.
Blas Molero Hidalgo, Ingeniero Industrial, Director IBERISA • 48004 BILBAO (SPAIN) WEB: http://www.iberisa.com Blog Femap-NX Nastran: http://iberisa.wordpress.com/