Rigid Body ModesThese modes exist for all objects and are the
modes of vibration where the object itself remains rigid and are
the rotation or translational modes of the object on its mounting
system.
A powertrain subsystem would be expected to develop six rigid
body modes on its mounts. Ideally, all six of these modes would
occur below the suspension modes, below approximately 11Hz.
However, the reality of vehicle and powertrain mount system design
dictates that some of these modes will occur at slightly higher
frequencies. Therefore, it is critical to maintain control of
certain powertrain modes within this region of the frequency
domain. The bounce and roll modes of a tranversely mounted, front
wheel drive powertrain may couple with front suspension vertical
modes (hop and tramp). Therefore, they should have separation
within the frequency spectrum. It is desirable to develop a lower
frequency powertrain pitch mode for optimal idle vibration
isolation. The other powertrain modes may occur at higher
frequencies as their respective mode shapes limit their ability to
couple with suspension resonances, even if they occur at similar
frequencies. A similar philosophy should be applied to other types
of powertrain systems.
A common way to remove the 6 rigid body modes is to fix all 6
DOF of a node. When lookingfor a node to fix, choose one that is
not in a high stress region. If the fixed point is locatedin a high
stress region, the optimization can be very slow. It is highly
recommended that the6 DOF of an independent node of a spider or
rigid element (usually used to model joints) beselected to be
fixed. On solid models, where the nodes do not have rotational
stiffness; if asolid model does not have a rigid element to
represent a joint, the next best way to removethe 6 rigid body
modes is to fix 3 translational DOF (123) of one node, 2
translational DOF(23) of another node, and 1 translational DOF (3)
of a third node. However, still make surethat the 3 nodes are not
in a high stress region, otherwise this method of removing 6
rigidbody modes does not work. One way or another, all of the 6
rigid body modes of each flexiblebody must be removed. SPC, SPC1,
or SPCADD (referenced in the subcase informationsection) can define
the fixed DOF(pg 233
ftp://ftp.altair.com.cn/priv/support/hw12.0/hw12.0_installation/hw12_docs/OptiStruct/OptiStruct_12.0_User_Guide.pdf)
How to remove them?Constraint all dof of the rubber bearing,
http://training.altairuniversity.com/wp-content/uploads/2013/08/modal_analysis_MG.pdf
Equivalent Static Load Method (ESLM)- Optimsation
