10. How to model manufactured Ball/Socket assemblies in AutoPIPE?


 

Applies To    
Product(s): AutoPIPE,  
Version(s): all  
Area: Modeling  
Original Author: Bentley Technical Support Group  

Dec 2014, AutoPIPE V8i 09.06.01.10

Problem:

How to model manufactured Ball/Socket assemblies from:

1. Hyspan Barco Ball Joint

(Manufactures site) 

or 

2. EBAA Iron Sales, Inc. FLEX-TEND® Flexible Expansion Joints

(manufacture site)

Solution:

 The concepts mentioned AutoPIPE's help can be adapted to model most ball and socket assemblies manufactured. See the following AutoPIPE help section for modeling technique:

Help > Contents> Contents Tab> Modeling Approaches> Modeling Approaches> Flexible Joints> Ball and Socket Joint example in AutoPIPE's online help.

Actual ball and socket joints are limited in their range of angular rotation (shown below). AutoPIPE can model these joints with or without movement limitations. Therefore, they should be placed in the piping system and checked to confirm that ball Joint Manufacture limits are not exceeded.

Question:  In AutoPIPE, a ball-and-socket joint is modeled as a Flexible Joint. Which Flexible Joint stiffness values on the dialog correspond to the angular, torsional, and axial motion indicated by the image above?

Axial, Shear, Torsional, or Bending?

Answer: Correct...

Refer to KB article here for detail understanding of this 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 the 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 the ball joint with a breakaway stiffness that will be constant throughout the travel. This will be conservative. You can check the Forces and Moments report at the joint to see if this load is 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.  If exceeded, 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. (So set to rigid first to estimate the moment and if exceeded, set back to zero with constant moment).

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

Procedure:

a. Set stiffness 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 IDEA has has been logged to add a new feature to input Break-away torques based on Friction, add your Vote here. Development team takes notes to the number of votes when evaluating new project plans. 

Restrain Rotational angle:

As of 2023, AutoPIPE Rotational supports allow users to define rotational gaps. It is recommended to model a rotational support with the appropriate gap settings in conjunction with the flexible joint described above. Care must be taken to avoid duplicating rotational stiffness. Rotational stiffness should be defined either in the Flexible Joint dialog or in the Rotational Support dialog, but not in both.

Question #1: At which node point should a rotational support be inserted: A01 or A02?

Answer:

The rotational support should be placed at A02, Why? 

Because, A01 is the main housing of the ball and it is A02 that will actually rotate the angular distance. Therefore, A02 is the proper locations to insert the rotational support. 

Question #2: How many rotational supports are placed at one location ( ex. A02) to correctly model the angular rotation of the ball and socket joint?

Answer:

Minimum 2, Why?

Because, angular articulation happens in 2 planes, the Vertical Plane and the Horizontal plane perpendicular to the axis of the pipe. Make sure to enter both a clockwise and counterclockwise gaps for the amount of movement allowed by the manufacture.  

Specific to EBAA Iron Sales, Inc. FLEX-TEND® Flexible Expansion Joints, one would model back to back Flexible Joints and add the rotational supports as imaged below:

Axial expansion

Specific to EBAA Iron Sales, Inc. FLEX-TEND® Flexible Expansion Joints, the assembly includes a special expansion and contraction joint located at the center of the assembly. To properly account for this movement and restraining in AutoPIPE, model an additional Flexible joint whose length is equal to the expansion/contraction length of the joint (ex. A06 to A07). Set the shear and bending stiffness to Rigid, while keeping the axial and torsional stiffness low. Next, to restrain the axial movement, insert a line stop at the end of this new flexible joint (ex. A07) that is "Connected to" the opposite end of the joint (ex. A06). Be sure to input the gap settings specified by the manufacturer. This will correctly account for the axial moment allowed by the assembly.

See Also

Flexible Joint components

Bentley AutoPIPE