Calculating Stringer Inverse Bending Lines

Inverse Bending Lines – Concepts

Stringer inverse bending lines are used to assist in the production of the correct shape of a stringer from a straight piece of stringer material (for example flat bar).  To calculate the inverse bending lines select a stringer or stringers and choose Calc Inverse Bending Lines from the Stringer menu.

Note : Calculating stringer inverse bending lines

It is recommended to calculate the stringer at highest precision before calculating the inverse bending lines. See Workshop Precision on page 10 for more information.

Inverse Bending Lines – Video

* View a video showing how to calculate Inverse Bending Lines for stringers in Workshop.

Inverse Bending Lines – Procedures

The following dialog box will appear:

The “Neutral Axis” dimension is the height from the base of the stringer section shape to the neutral axis of the stringer.  The “Overlap” dimension is the distance that each of the subsequent inverse bending lines will overlap by.  The “Buffer” line signifies the area inside which the inverse bending line contours will be drawn.  The “Offset spacing”  denotes the distance between offset readings of the inverse bending line dimensions in the display (see output diagram below).  To better illustrate the inverse bending lines in the output the vertical scale may be exaggerated, to do this check the exaggerate scale check box and the desired scaling factor

 

If there are no currently selected stringers and the active view is the part view with a stringer displayed then the inverse bending lines for that stringer will be calculated.  To display the Inverse Bending Lines, go to the parts window, select the stringer and click on the Inverse Bending Lines Button from the Parts toolbar: .

The bottom figure is the developed shape of the stringer, the middle figure is th shape of the stringer when the inverse bending lines are straight (i.e the shape required after bending) and the top figure is the straight piece of stringer material prior to bending with the inverse bending lines drawn on (note the middle and top figures have a vertical exaggeration of 2.0).  The measurements in grey show the dimensions from the baseline to the edge of the curved stringer (in the middle figure) and from the edge of the stringer to inverse bending line for the mark up of the stringer prior to bending.  Dimensions are spaced according to the value entered in the dialog box.  To turn off the inverse bending line diagram, click on the part button : .

Note: double curvature of stringers

Inverse bending line information is only useful if the stringer bending is planar (i.e. in one plane).  In practice, however, a small amount of double curvature in the stringer will not invalidate the inverse bending line data.  For this reason Workshop allows you to calculate the inverse bending lines of the stringer even if it has double curvature.  This is done by first projecting the stringer shape onto the “closest” arbitrary plane.  All the inverse bending line calculations are then done on this projected shape.  A good way to check how much double curvature is in the stringer is to compare the stringer shapes on the expanded stringer (bottom figure) and the projected stringer (middle figure) in the inverse bending line expansion drawing.  For a stringer with no double curvature the shapes will be the same.