Answer
Shim ends, which are flat 360-degree circular ends added to a multi-turn spring, provide a more uniform distribution of the load across the contact surface compared to plain (wavy) ends. In a plain-end spring, the load is concentrated at the crests of the waves, which can lead to localized indentations in softer mating materials like aluminum. With shim ends, the load $P$ is spread over the entire $360^{\circ}$ circumference, reducing the contact stress $\sigma_c = \frac{P}{A}$. This is critical in high-precision assemblies where parallelism is required, as shim ends eliminate the 'point-loading' effect that can cause the assembly to tilt. However, shim ends increase the solid height $H_s$ by $2 \cdot t$, which must be accounted for in the space envelope calculation.
