FCeng84,
I have acquired the following information regarding "cable stretch".
Using a 3/16th, 7x19 strand Carbon Steel Tin-Zinc Coated cable for example. Upon manufacture, the cable is "proof loaded" to 2520 +125/-0lbs. The load must be applied within 3secs and held for a minimum of 5secs.
Before installation into, for example, the elevator system, to ensure you have the correct cable tension, permit a min of 1hr at constant ambient temp (+/- 5F or +/- 3C) for aircraft temp to stabilise. Install the cables and operate the system for several cycles at TWICE the working tension per the applicable system tension/temp chart. Then back off tension (via the turnbuckles) to the correct operating tension and cycle the system again. If all rig pin checks are good then lock the applicable turnbuckles. This must definitely prove the cable "stretch/spring" theory.
Machinbird,
The answer to your question "What keeps the PCU input torque tube from back driving the jammed control column side and therefore jamming the free side. How is the necessary free motion generated?" is the "Elevator Breakout Mechanism" located between the control columns under the cockpit floor.
The breakout mechanism is attached to the forward input torque tube between the control columns. The mechanism separates the left and right sides of the input torque tube. This allows elevator control if one column has a jam.
The breakout mechanism is a cam-roller type. The cam connects to the Capts torque tube and the roller connects to an arm on the F/O's torque tube. 2 springs hold the roller in the cam detent position (both control columns move together). The pilot must apply 31lbs of additional force to "breakout" of the cam detent and extend the springs during a jam. When 100lbs is applied, the elevator moves4 degrees. (hence FCeng84's cable stretch, albeit minimally)
During normal operation, a control column moves and operates both torque tubes together. If one column has a jam, the other column can still move after the pilot overcomes the breakout force of the 2 springs.
Rgds McHale.