Spring Solid Height and Compression: Design Limits
Short answer: Solid height is the length of a compression spring when every coil touches, calculated as total wire diameter times the number of coils (active coils plus the two closed end coils). A spring must never be compressed to solid height in service — the working deflection should stop at 80–85% of the deflection to solid, leaving a residual gap. For a spring with 8 mm outer diameter, 1 mm wire, and 10 total coils, solid height is 10 mm; if free length is 40 mm, total travel to solid is 30 mm, so the usable stroke is roughly 24–25.5 mm. Exceeding this limit causes coil clash, stress spikes, and premature fatigue failure.
What Is Spring Solid Height?
Solid height is the physical stack length of a compression spring when all coils are pressed flat against each other. It is a hard geometric limit, not a soft guideline. No compression spring can be shorter than its solid height, and any design that requires travel beyond it will either bind, deform, or break.
The formula is simple:
Solid height = (total number of coils) × (wire diameter)
"Total number of coils" includes the active coils that do the work plus the end coils that are closed and ground. For most compression springs, the ends are closed and ground square, which adds approximately two dead coils to the active count. If a spring has 8 active coils and closed-ground ends, the total is about 10 coils.
For a spring wound from 1.2 mm wire with 10 total coils, solid height is 12 mm. That number never changes with load, temperature, or time — it is fixed by the wire and the coil count. What changes is how close you dare to run to it.
Solid Height vs. Free Length vs. Installed Length
Three lengths define every compression spring application:
| Term | Definition | Example (mm) |
|---|---|---|
| Free length | Unloaded length, standing on the bench | 50.0 |
| Installed length | Length when seated in the assembly at rest | 40.0 |
| Working length | Length at maximum service load | 28.0 |
| Solid height | All coils touching | 12.0 |
The gap between working length and solid height is your safety margin. In the example above, the spring has 16 mm of unused travel at maximum load — comfortable. If working length were 14 mm, only 2 mm would remain, and any tolerance stack-up, thermal growth, or overload event would drive the spring into coil clash.
How Do You Calculate Maximum Compression?
Maximum usable deflection is the difference between free length and the minimum allowable working length. The minimum allowable working length is not solid height — it is solid height plus a clearance margin.
Maximum deflection = Free length − (Solid height + clearance)
Clearance is typically 10–15% of the travel to solid, or a minimum of 1–2 mm for small springs. This margin absorbs:
- Manufacturing tolerances on wire diameter and coil count
- Set/creep loss after the spring is cycled
- Thermal expansion in the housing
- Load spikes from shock or misalignment
Worked Example: 10 mm OD Spring
Take a compression spring with 10 mm outer diameter, 1.2 mm wire, 8 active coils, closed and ground ends (10 total coils), free length 45 mm.
| Parameter | Value |
|---|---|
| Wire diameter | 1.2 mm |
| Total coils | 10 |
| Solid height | 12.0 mm |
| Free length | 45.0 mm |
| Travel to solid | 33.0 mm |
| Recommended max deflection (85%) | 28.0 mm |
| Minimum working length | 17.0 mm |
| Spring rate (typical, indicative) | ~4.5 N/mm |
At 28 mm deflection, the spring carries roughly 126 N. Push it to 33 mm and it sits on solid — the rate effectively becomes infinite, the load path turns into a rigid column, and the coils take the full force as a solid block. That is how springs get flattened, cracked, or permanently shortened.
Why Is Coil Clash Dangerous?
When coils touch, two things happen at once. First, the spring stops behaving like a spring — stiffness jumps to that of a steel tube, so any additional travel transfers shock directly into the housing and the mating components. Second, the contact points between coils become stress concentrators.
Fatigue life in compression springs is governed by the maximum shear stress at the inside of the coil. At solid height, the effective active coil count drops and the stress per coil rises sharply. A spring rated for 500,000 cycles at 28 mm deflection may fail in a few thousand cycles if regularly driven to 33 mm.
Coil clash also damages the protective surface. Plating, powder coat, or passivation layers wear through at contact points, exposing bare steel to moisture. In outdoor or marine assemblies this accelerates corrosion and shortens service life even if the spring never fractures from fatigue.
If your design regularly approaches solid height, consider a resonance and surge analysis — high-frequency cycling near coil clash often excites surge waves that compound the stress problem.
What Design Rules Keep Springs Safe?
Experienced spring designers apply a small set of rules that keep compression springs out of trouble. These are conventions, not laws, but they reflect decades of field data.
The 80–85% Deflection Rule
Never use more than 80–85% of the available travel to solid. For a spring with 33 mm of travel, the working stroke should stay at or below 28 mm. This leaves a visible gap at maximum load and keeps stress within the fatigue-safe range for most music wire and stainless steels.
Minimum Gap at Maximum Load
Aim for a minimum gap of 10–15% of travel to solid, or 1.5 mm, whichever is greater. For micro springs under 5 mm OD, even 0.5 mm of gap matters because tolerances are a larger fraction of the geometry.
Index and Slenderness Limits
The spring index (mean diameter ÷ wire diameter) should generally fall between 4 and 12. Below 4, coiling is difficult and stress concentration is severe. Above 12, springs buckle easily. Slenderness — free length ÷ mean diameter — should stay under 4 for unguided springs; beyond that, use a rod or bore to guide the spring.
End Condition Matters
Closed and ground ends give the flattest bearing surface and the most predictable solid height. Closed-not-ground ends add roughly one wire diameter of uncertainty. Open ends are only suitable for springs that never approach solid.
How Do Tolerances Affect Solid Height?
Solid height is not exact. Wire diameter carries a tolerance — often ±0.02 mm for small music wire, larger for heavy gauge — and coil count is an integer, but the end coil positions vary. The stack-up means solid height typically varies by ±3–5% in production.
| Source of Variation | Typical Effect on Solid Height |
|---|---|
| Wire diameter tolerance | ±1–2% |
| Coil count rounding | ±1 wire diameter |
| End grinding depth | ±0.3–0.8 mm |
| Set/creep after cycling | 1–3% permanent shortening |
This is why the clearance margin exists. If your assembly can only tolerate 0.5 mm of gap at maximum load, you are designing at the edge of what production tolerances can hold. BQUQ's spring length tolerance guide covers how free length, solid height, and load tolerances interact in a real drawing.
Set Removal and Presetting
Most quality compression springs are preset (scragged) at the factory — compressed to solid or near-solid once to induce controlled plastic deformation. This stabilizes free length and reduces creep in service. Presetting does not change the theoretical solid height formula, but it does mean the as-delivered spring has already lost its initial 1–3% of length. Design your clearance around the post-set dimensions.
When Should You Choose a Different Spring Type?
If your application demands travel that a compression spring cannot deliver within the available envelope, the geometry is telling you something. Options include:
- Longer free length — increases travel but risks buckling; add a guide rod.
- Smaller wire, more coils — lowers rate and stress but reduces solid height margin only slightly.
- Die springs — heavy-duty rectangular wire for high force in short strokes.
- Belleville washers — very high force in a tiny axial space, though with limited travel.
- Gas springs — constant force over long strokes, but with sealing and temperature limits.
For extension springs, the analogous limit is the maximum safe extension before the body yields or the hooks deform — a different calculation with the same underlying principle: never design past the elastic limit. BQUQ's extension spring end types article covers hook geometry and its effect on allowable travel.
How Does BQUQ Control Solid Height in Production?
BQUQ runs four production lines in one Dongguan factory — CNC machining, metal stamping, custom springs, and heat sinks — under ISO9001. Spring production covers compression, extension, and torsion types in wire from 0.1 mm to 8 mm, with CNC coiling for tight index control and in-house heat treatment and presetting.
Solid height is verified on first article and at in-process inspection using calibrated height gauges and load testers. Because quoting, tooling, and production sit in the same building, tolerance questions get answered before the order ships, not after. Quotes issue in 12 working hours, and MOQ is flexible — prototype quantities and production volumes run on the same lines.
If your design is close to the solid height limit, send the drawing with free length, solid height, rate, and maximum working deflection marked. The engineering team will flag any margin issues at quote stage. Review compression spring capabilities or custom extension springs for starting points.
Frequently Asked Questions
Q: What is the formula for spring solid height?
A: Solid height equals total coil count multiplied by wire diameter. Total coils include active coils plus the dead end coils — typically two for closed and ground ends. For a spring with 1 mm wire and 10 total coils, solid height is 10 mm. Always confirm the end configuration, because open ends and closed-not-ground ends change the effective count.
Q: Can a compression spring be compressed to solid height?
A: It can physically, but it should not be in normal service. At solid height the spring becomes a rigid column, stress rises sharply at coil contact points, and fatigue life drops dramatically. Design working deflection to 80–85% of travel to solid, leaving a visible gap at maximum load to absorb tolerances and overload.
Q: How much clearance should I leave above solid height?
A: Leave 10–15% of the travel to solid, or at least 1.5 mm, whichever is greater. Small springs under 5 mm OD should keep a minimum 0.5 mm gap because tolerances are a larger fraction of the geometry. The margin absorbs wire tolerance, coil count rounding, set loss, and thermal growth.
Q: Does presetting change solid height?
A: Presetting compresses the spring to near-solid once, inducing controlled plastic deformation that stabilizes free length. It does not change the theoretical solid height formula, but the delivered spring is 1–3% shorter than the as-coiled length. Design your clearance around post-set dimensions, which is what the spring will hold in service.
Q: What happens if a spring is overloaded past solid height?
A: The coils clash and the spring acts as a solid block, transferring shock directly to the housing and mating parts. Contact points become stress concentrators, plating wears through, and the spring may take a permanent set, crack, or fracture. Overload past solid is one of the most common causes of premature compression spring failure.
Related Resources
- About BQUQ and the Dongguan factory: /about/
- Compression spring manufacturing capabilities: /compression-springs/
- Custom extension spring design and production: /extension-custom-springs/
- Industry trends in spring and metal component sourcing: /industry-dynamics/
- Technical articles on spring design and tolerances: /bquq-blog/
- Frequently asked questions about ordering and quoting: /faq/
- Case studies from precision manufacturing projects: /case/
- Contact the engineering team for a 12-hour quote: /contact/
Authored by the BQUQ Engineering Team. BQUQ (Dongguan) runs CNC machining (±0.005 mm), metal stamping, custom springs, and heat sink production in one ISO9001 factory. Source-direct from Dongguan, China — quote in 12 hours: sc@bquq.com | WhatsApp +86 13713157787 | www.bquq.com
