This is where you go wrong : you try to animate the sweep (loft, pipe,…) whereas you have to animate the elements which are used to create the sweep (loft, pipe, …). A clear view on the basic structure is obtained by deleting the sweep (the surface but not the curves) in the sweep.3dm model. There is the helix and 2 circles, one moving.
The top circle is made moving by applying a Simple constraint: LookAlongZUp with its Y-axis as ConstraintHeadingVector and the helix as path. Initially this circle coincides with the bottom one.
Installing the Simple Constraint initiates 2 keyframes (at 0 and at the end of the timeline – which can easily be moved). The Constrain Parameter in the first keyframe is then edited to shift the circle one helix-turn up.
This Simple Constraint puts everything in action. Now all that has to be done is make the Sweep with History Recording, the helix as rail and both circles as cross sections (Natural seam point Option, and Roadlike Top Style).
Same technique in “Truck Path Trace 001.3dm”. A curve-path and 2 lines. One line is made moving by Simple Constraining (LookAlongZUp) it to the path-curve. Then with History, a Sweep is made with the path as rail and both lines as cross sections. An similar approach can be found in “Experiment Spooling 001.3dm” that I posted in Extruding (flexible) pipe along a path
The same basic idea when you want to animate a loft.
Not the loft itself is animated but the curves that are used to create the loft. Thanks to History the loft follows.
To be clear, the impetus isn’t necessarily created by a Simple Constraint. Moving, rotating and/or scaling do just as well.
Luc
PS. Please not that we live in a different time zone, hence a slow conversation.