Maximum Compression and Set: Preventing Spring Loss
Short answer: Maximum compression is the shortest length a spring can reach without taking permanent set — normally 85–90% of solid height for static duty, 95%+ only for rare, slow events. "Set" (or "loss") is the permanent length change left after the first over-compression: a spring compressed past its elastic limit comes back shorter and weaker. On a typical music-wire compression spring with a 10 mm free length, a 4% set means roughly 0.4 mm of free length gone and a proportional drop in load. BQUQ controls this by calculating solid height, stress at solid, and scragging (presetting) before shipment — quoting custom springs in 12 working hours from one Dongguan factory.
What Is Maximum Compression in a Compression Spring?
Maximum compression is the design limit on travel — the shortest loaded length you allow a spring to reach in service. It is not the same as solid height, although the two are related. Solid height is a geometric fact: the length of the spring when every coil touches, calculated from wire diameter and total coils. Maximum compression is an engineering decision: how far you are willing to push the spring while keeping stress below the material's elastic limit and leaving margin for fatigue, tolerance stack-up and temperature.
Confusing the two is the single most common cause of "spring loss" complaints in production. A spring that is geometrically capable of reaching solid height may be metallurgically incapable of doing so without setting, cracking or losing load.
The three lengths every spring drawing should state
| Length | Symbol on drawing | What it means | Typical rule |
|---|---|---|---|
| Free length | Lf | Unloaded length, no force applied | Reference dimension |
| Loaded / working length | L1, L2 | Length at installed and max working load | Defines rate and preload |
| Solid height | Ls | All coils in contact | Absolute mechanical floor |
A fourth value matters just as much: the maximum compression length you permit, often written Lmax or Lmin. This is the number the spring maker must design to, and it should always be greater than solid height — never equal to it, unless the application genuinely bottoms out and the spring is designed for it.
How Does a Spring "Lose" Length — What Is Set?
Set is permanent deformation. When a compression spring is loaded beyond the yield strength of its wire, some of the compression becomes plastic rather than elastic. Remove the load and the spring does not return to its original free length. It comes back shorter. Engineers call this set, loss of free length, or "taking a set."
The consequences cascade:
- Free length drops, so installed preload drops.
- Rate (spring constant) usually changes slightly because effective active coils change.
- Load at a given installed height drops by roughly the same percentage as the length loss.
- In severe cases, coils interfere, the spring binds, and fatigue life collapses.
Set typically appears after the first few compression cycles, which is why a spring can pass inspection at the factory and still fail on the customer's assembly line. The first actuation does the damage.
Elastic vs plastic: where the line sits
For most spring steels, the usable elastic range is defined by allowable torsional stress, not by the wire's tensile strength directly. A common design rule is to keep working stress at or below roughly 40–45% of ultimate tensile strength for fatigue duty, and up to about 60% for static or rarely cycled springs. Push past those bands and set becomes likely.
These percentages are indicative design guidance, not a guarantee — they vary with wire grade, diameter, surface condition and shot peening. The point is that "maximum compression" is a stress question, and stress depends on geometry.
How Do You Calculate the Compression Limit?
Two calculations run in parallel: geometry (can the coils physically reach that length?) and stress (will the wire survive it?).
Step 1 — Solid height
For a compression spring with squared and ground ends:
Ls = (Nt) × d
where Nt is total coils and d is wire diameter. For plain or open ends, add a fraction of a wire diameter — typically Ls = (Nt + 1) × d for plain ends, and Ls = (Nt + 0.5) × d for plain and ground ends. Always confirm the end condition on the drawing, because a 0.5d error on a small spring is a real percentage of travel.
Step 2 — Deflection to the limit
Deflection at maximum compression:
f = Lf − Lmax
Then stress at that deflection, using the standard Wahl-corrected torsional stress formula:
τ = K × (8 × F × D) / (π × d³)
where F is the load at that length, D is mean coil diameter, d is wire diameter, and K is the Wahl correction factor that accounts for curvature and direct shear. Compare τ against the allowable stress for the material and duty cycle.
Step 3 — Check the margin
| Duty | Suggested max working stress (% of UTS) | Suggested Lmax vs Ls |
|---|---|---|
| Static / installed only | ~55–60% | Lmax ≥ 1.10 × Ls |
| Infrequent cycling (<10⁴ cycles) | ~45–50% | Lmax ≥ 1.10–1.15 × Ls |
| Fatigue duty (>10⁵ cycles) | ~35–45% | Lmax ≥ 1.15–1.25 × Ls |
| Critical / safety-related | Project-specific | Lmax ≥ 1.25 × Ls or redesign |
The rightmost column is the practical answer to "how close to solid can I go?" For most industrial applications, designing maximum compression to 85–90% of solid height leaves a workable margin. Going to 95% or beyond is possible for slow, rare events, but it demands tighter tolerance control and a preset operation.
What Is Scragging (Presetting) and Why Does It Prevent Loss?
Scragging — also called presetting, setting, or stress-relieving to length — is a controlled over-compression performed at manufacture. The spring is compressed to a length at or slightly beyond its intended maximum compression, held briefly, and released. The plastic deformation that would have happened in the field happens in the factory instead.
After scragging, the spring has:
- A stable free length that will not drift further under normal service loads.
- Residual stresses redistributed in a way that usually improves fatigue performance.
- Predictable load at installed height, because the first-cycle set has already been absorbed.
This is why a good spring supplier will ask for maximum compression, not just free length and rate. If the design sits close to the stress limit, scragging converts a field failure into a controlled process step.
When to specify scragging
- Springs designed to operate above roughly 45% of UTS.
- Any spring where installed preload tolerance is tight.
- Springs that will be compressed to near solid height during assembly or shipping.
- Long, slender springs prone to buckling and uneven coil contact.
For springs operating well below the elastic limit, scragging is optional. It costs a process step, so it should be justified by the application rather than applied by default.
What Are the Common Causes of Spring Loss in the Field?
Set is not the only way a spring loses length or load. When a customer reports "the spring went soft," the actual root cause is often one of these:
1. Over-compression beyond design. The assembly presses the spring to solid, or a jam condition does. This is a specification problem, not a manufacturing defect.
2. Wrong end condition. Plain ends on a spring designed as squared and ground reduce effective solid height and change load.
3. Heat. Operating temperature above the material's recommended range relaxes stress and causes creep. Standard music wire is not suitable for hot environments; stainless and alloy grades have their own ceilings.
4. Fatigue cracking. A crack reduces the effective wire cross-section, which raises stress and accelerates further loss. See our analysis of how compression springs fail in service.
5. Corrosion. Pitting removes material and creates stress risers, especially on springs used outdoors or in wash-down equipment.
6. Tolerance stack-up. A spring at the low end of rate tolerance installed into a cavity at the high end of depth tolerance can sit at a stress level nobody designed for.
The fix in most cases is to revisit the solid height calculation and confirm the spring was specified with a real maximum compression figure, not just a free length and a rate.
How Should You Specify Maximum Compression on a Drawing?
A complete compression spring specification removes ambiguity. Include:
| Parameter | Why it matters | Example |
|---|---|---|
| Free length ± tolerance | Sets installed preload | 25.0 ± 0.3 mm |
| Wire diameter ± tolerance | Drives rate and stress | 1.20 ± 0.02 mm |
| Outside diameter ± tolerance | Fits the bore or rod | 10.0 ± 0.1 mm |
| Total coils / active coils | Drives rate and solid height | 8.5 total / 6.5 active |
| End condition | Changes solid height | Squared and ground |
| Rate | Core performance | 2.5 N/mm ± 10% |
| Max compression length | Prevents set | 12.0 mm |
| Load at max compression | Verifies performance | 32.5 N ± 10% |
| Duty cycle | Sets allowable stress | 500,000 cycles |
| Material and finish | Fatigue and corrosion | Music wire, zinc plated |
| Scragging required | Stability | Yes / No |
If you supply only free length, rate and diameter, the spring maker has to guess at the service condition. That guess is where loss comes from.
A note on tolerance and cost
Tighter tolerances on rate, free length and squareness all raise cost, because they require more inspection and more sorting. The cheapest way to hit a tight installed-load window is usually to specify a slightly looser rate and a controlled free length, or to add scragging — not to demand ±2% on every dimension.
Where Does BQUQ Fit In?
BQUQ runs four production lines in one ISO9001 factory in Dongguan, covering CNC machining to ±0.005 mm, metal stamping, custom springs and heat sink production. Spring work includes compression, extension and torsion designs in music wire, stainless, oil-tempered and alloy grades, with in-house coiling, end grinding, stress relieving and scragging where the design calls for it.
Because the springs, stampings and machined housings come from the same floor, we can check the spring against the actual cavity it will sit in rather than against a drawing alone. MOQ is flexible, and quotes go out in 12 working hours.
Send a drawing with maximum compression, duty cycle and installed height marked, and we will come back with a stress check at solid height, a recommended Lmax, and whether scragging is needed. Browse custom compression springs or custom extension springs for reference designs, or email sc@bquq.com.
Frequently Asked Questions
Q: What is a safe maximum compression for a compression spring?
A: A practical starting point is 85–90% of solid height for static or lightly cycled springs, and 75–80% for fatigue duty. This keeps working stress within the elastic range with margin for tolerance stack-up and temperature. If your design must go closer to solid, specify scragging and confirm the stress calculation with your spring supplier before release.
Q: How much free length loss is normal after scragging?
A: Scragging deliberately removes the first-cycle set, so expect a controlled permanent change — often in the range of 1–3% of free length for springs operating near their stress limit, and less for lightly loaded springs. After scragging, free length should be stable. If a spring continues to shorten in service, the working stress is too high.
Q: Does maximum compression affect spring rate?
A: Rate is set by wire diameter, mean coil diameter and active coils, so it does not change with compression within the elastic range. However, if the spring takes a set, active coils can change and rate shifts. Once coils begin to touch near solid height, the effective rate rises sharply because fewer coils remain active.
Q: Can I design a spring to compress fully to solid height?
A: Yes, but only deliberately. Springs used as rigid stops or in bottom-out applications are designed with stress at solid below the allowable limit, usually with scragging and a generous safety margin. For most load-bearing applications, designing to solid is poor practice because it leaves no room for tolerance, wear or overload.
Q: How do I know if my spring has taken a set?
A: Measure free length before and after a full compression cycle. A measurable drop means plastic deformation occurred. In production, load-test the spring at installed height rather than relying on free length alone, because load is what the assembly actually feels. See our notes on spring overload and yield for test methods.
Related Resources
- About BQUQ and our Dongguan factory: /about/
- Custom compression springs: /compression-springs/
- Custom extension springs: /extension-custom-springs/
- Industry trends in spring and metal component sourcing: /industry-dynamics/
- Technical articles on spring design and manufacturing: /bquq-blog/
- Frequently asked questions: /faq/
- Case studies: /case/
- Contact the engineering team: /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
