Making Joints Flexible

When you make joints flexible, the mechanism will update to have bushing representations of the basic joint types instead of rigid constraints. If the mechanism contains entities that were obtained from physical simulation, then the default setting will be for joint flexibility to be active.

Joint motion and friction are not affected by using flexible joints.

Example.

Limitations:

• In some models, using bushings will slow down the solve time because of induced dynamic effects.

• We are not accounting for the stiffness of the part in the solution. Therefore, the distribution of loads due to part stiffness may differ from the bushing constraint solution. This bushing approach will ensure that force results are obtained at all joint locations.

• Advanced mates do not support joint flexibility and are ignored when you make joints flexible.  See the table below for a list of joints that are made flexible.

• If the mechanism starts in a dynamic condition, there may be a spike in initial forces as the model reaches initial equilibrium (that you will not see with rigid joints). The spike is generated by initial conditions of the parts not balancing and the bushings resisting rapid changes in force/acceleration. If the model started with enforced motions (for example, constant velocity), try ramping up motions from zero to the desired value over a time range to eliminate or minimize this (for example, use a step function to ramp velocity from zero to a certain value over a time range).

 You will not see individual joint compliance. This setting is taken into account during the solve and also for the display of the symbols in COSMOSMotion (The on screen symbols for all joints that are made flexible appear as bushings). It does not affect the individual joint dialog boxes.

You still need to apply joint motions, even though the joint does not exist. Use the markers on the bushing representing the joint for the underlying motion statement.

For each joint type, only specific degrees of freedom will have bushing properties, as shown next.

To make joints flexible:

1. Display the Simulation tab (Motion -> Options -> Simulation tab or right-click Motion Model -> Simulation Parameters).

2. Select Make Joints Flexible.
   
   COSMOSMotion displays the default translational and rotational stiffness and damping. You can change these values as needed.

3. Select OK.