Novice (at springs at least) Question: Why are the forces on the two ends of my spring balanced in this case
but not in this case?
Both cases are compression/extension springs between plane parallel faces with total compression preload -- 1 lb in the top case and 18 lb in the bottom -- but no displacement (so far), modeled in Solidworks Premium 2017 SP2.0 (linear static) simulation running under Window 7 SP1. It doesn't seem to matter which solver I choose.
The top case is the simplest I could think of and appears to behave as expected. The bottom case is slightly more complex: a tube sliding over the shaft of a bushing, pushed against one end by the spring's preload acting on the other. The bushing does not need a fixture since it is a slip fit (no-penetration contact) in the fixed slider and pulled into another no-penetration contact against its end by the spring.
I must be missing something obvious... -- John Willett