Compression springs are the default helical spring — cylindrical, coiled from round wire, loaded axially. Every engineer knows the force equation (F = kx). What separates a spring that works from one that binds, buckles, or takes a set is the geometry that lives outside Hooke's law.

The four lengths you must track:

• Free length (L_f): Uncompressed length sitting on the bench.
• Installed length (L_i): Length after seating in the assembly (usually with a small preload).
• Working length (L_w): Length at maximum operating compression.
• Solid height (L_s): Length when every coil touches its neighbor and the spring cannot compress further. Any load beyond solid transfers directly through the coil stack as a rigid column — a fast way to destroy whatever pushed it there.

End conditions change solid height and active-coil count. The four types: plain (coil ends left as-cut), plain-ground, squared (last coil bent flat but not ground), and squared-and-ground. Squared-and-ground ends sit flat and buckle less — worth the cost for anything precision. Rule of thumb: total coils N_t vs. active coils N_a differ by 2 for squared ends. Solid height for squared-and-ground ≈ N_t × d, where d is wire diameter.

Buckling. A compression spring is a slender column. If free length exceeds ~4× mean coil diameter (D), it will buckle sideways under load unless guided by a rod inside or a hole around it. This catches designers who scale a working prototype up in stroke without adding a guide.

Set (permanent deformation). Compress a spring past its elastic limit and it comes back shorter forever. Manufacturers pre-set springs by compressing them to solid once during production — this raises the effective yield point. A spring that has never been pre-set will take set the first time it hits solid in service, throwing off your installed preload.

Quick sizing example. You need 50 lb at 1.5" installed length and 100 lb at 1.0" working length. Rate k = (100 − 50)/(1.5 − 1.0) = 100 lb/in. Deflection from free to working = 100/100 = 1.0", so free length = 1.0" + 1.0" = 2.0". Now check: is solid height less than 1.0"? With d = 0.100" wire and 8 total coils, solid ≈ 0.8" — safe, with 0.2" clash allowance. Designers target at least 10–15% clash between working length and solid to survive tolerance stack-up and vibration overshoot.

Real-world example: Automotive valve springs run at ~30% of solid height clearance and are shot-peened to resist fatigue at 100 million+ cycles. Skip the peening and they fail in weeks.