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Spring Creates Unbalanced Forces (Sometimes)

Question asked by John Willett on May 9, 2017
Latest reply on May 9, 2017 by John Willett

Novice (at springs at least) Question:  Why are the forces on the two ends of my spring balanced in this case

Balanced Forces.png

but not in this case?

Unbalanced Forces.png

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