If anyone in the community is interested in using Translational Motors, 
the Spring Constraint (Generic6DofSpringConstraint) and the ConstraintDemo in JBullet, 
you can pull my fork of JBullet from:
https://github.com/affogato/JBullet-QIntBio-Fork

This was added for use in a cellular simulation project called QIntBio.

The spring constraint was added to Bullet _after_ the major refactoring of the constraint solver in revision 1542 
involving SIMD support. The translational motor and the way the spring constraint uses it to simulate the effect 
of a spring, is written for the newer software design (with getInfo() functions and optimised SIMD functions).
Therefore, there was no code to solve the linear spring constraint, that can be ported in a straightforward manner.

New code was written following the old template.