12. How to model a Ball joint in AutoPIPE?

Applies To
Version(s):2004, XM, & V8i
Environment: N/A
Area: Modeling
Original Author:Bentley Technical Support Group

Dec 2014, AutoPIPE V8i


My specific Ball Joints have a particular torque which allows them to move. For example I am modeling a 14" ball joint with a friction torque of 9,000 FT-LBS. Once this moment is developed in the system it remains constant and the ball joint rotates.


First, start by modeling a ball and socket joint as specified by AutoPIPE help.

Help > Contents> Contents Tab> Modeling Approaches> Modeling Approaches> Flexible Joints> Ball and Socket Joint example

Actual ball and socket joints are limited in their range of angular rotation (shown below). AutoPIPE will not limit this range. Therefore, they should be placed in the piping system so that these limits are not exceeded. User are responsible for manually checking angular movement does not exceed manufacture limits.

Question: In AutoPIPE, a ball and socket is modeled as a Flexible Joint, what Flexible Joint stiffness values on the dialog represent the angular motion shown in the image above?

Axial, Shear, Torsional, or Bending?

Answer: Correct... Torsional and Bending Stiffness values correctly represent this type of movement (see wiki here to understand Flexible joint dialog).

Model Break-away / Resisting Moment:

Generally,there is a limit load reached before the joint begins to move or stiffness values goes to 'zero' stiffness. A ball and socket manufacture will provide these break-away torques for their ball joint assembly. As of Dec 2019, AutoPIPE models these break-away with torsional and Bending “stiffness” resistance but the ball joint friction is more of a constant “friction torque” during movement and not proportional to the angular displacement (note, AutoPIPE units for Flexible Joint torsional and bending input values are Ft-lbs / deg or N*m / deg).

Use one of the following modeling options to model break-away torque:

Option #1

Suggest modeling ball joint break-away stiffness that will be constant throughout the travel. This will be conservative. One can check the Forces and Moments output report at the joint to see if this load has been reached. If the load has not been reached, increase the corresponding stiffness and recheck. If load has been exceeded, decrease stiffness accordingly and recheck until breakaway forces have been approximated.

Option #2

Estimate the amount of angular displacement of the ball joint and scale the stiffness value accordingly so that the friction torque specified above is not exceeded.

Option #3

Calculate the moment in the joint and if the moment exceeds the limit friction moment apply a constant moment (using force/moment) on one end of the joint to counter the friction and set rotational stiffness to zero. If the moment is not exceeded,  set the stiffness to rigid. 

The problem with this approach is that you cannot set stiffness to zero for one load case and rigid for different load case.


a. Set stiffnesses to rigid.

b. Analyze to calculate moment in the joint (for all basic load cases, not combinations)

c. If moment does not exceed friction moment, you are done, rigid is valid for that load case

d. If moment exceeds friction. Set stiffness to zero (unless one is provided) and apply a constant moment equal to friction moment on one end of the joint. The moment is applied in the load case evaluated. Make sure the moment sign is correct.

An enhancement has been logged (TFS-108453, CAE-TR-4779) to add a feature to input Break-away torques. 

See Also

Flexible Joint - Modeling Approaches, Tips, Techniques

Bentley AutoPIPE