Official Answer
Answer
Friction $\mu$ introduces a 'hysteresis' effect. When the spring is compressed (loading), the measured load $P_{load} = P_{theoretical} + F_{friction}$. When the spring is released (unloading), $P_{unload} = P_{theoretical} - F_{friction}$. The friction force $F_{friction} = \mu P_{radial}$ is caused by the radial expansion of the spring against the housing. In precision instruments, this hysteresis can be as much as $5-10\%$ of the total load. To minimize this, engineers specify polished housing walls or apply PTFE coatings to the spring to ensure the actual preload remains within the designed tolerance.
