Presetting and Scragging Springs: Settling Load Before Delivery
Short answer: presetting (also called scragging) means compressing a spring to solid height once or several times before delivery so that it takes its permanent "set" in the factory instead of in your assembly. A preset compression spring delivers its designed load at working height from day one and keeps it stable over life; an unpreset spring can lose 5-15% of its load in the first cycles as the wire settles. Preset when load accuracy, space, or fatigue life matters — which is most production springs. Do not preset springs that must hold a precise free length, because presetting shortens the free height permanently.
Every coiled spring, no matter how well designed, settles a little when first compressed to a high stress: the wire yields plastically in tiny local zones and the spring takes a permanent reduction in free length. If that settling happens inside your product during the first cycles, your mechanism's load changes after assembly. Presetting moves the settling to the spring factory. This guide explains what presetting does in measurable terms, when it is mandatory, and how it changes the numbers on your drawing.
What Presetting Actually Does
When a compression spring is compressed to solid (all coils touching) at a stress above the elastic limit, the outer fiber of the wire yields and the spring's free length shortens permanently — engineers call this "set." Presetting compresses the spring to solid once or more during manufacturing so this permanent set is taken deliberately. After presetting, the spring's load at any working height is higher than an unpresetted spring of the same free length (because the same wire now sits at a shorter free height, meaning more deflection at the same working position), and critically, it is stable — subsequent cycles produce almost no further set.
| What you measure | Unpreset spring | Preset spring |
|---|---|---|
| Load at working height, first cycle | Lower (set not yet taken) | Designed load immediately |
| Load after 10,000 cycles | Drops 5-15% typical | Stable (within 1-3%) |
| Free length | As-wound (longer) | Shorter (permanent set taken) |
| Fatigue life at same working stress | Lower (extra stress from set cycling) | Higher (operates below yield) |
| Solid height | Same | Same |
When Presetting Is Required
Four situations call for presetting as a standard operation. High load accuracy: if your assembly tolerance depends on spring load at a given height, preset or the load drifts after assembly. Limited working space: a preset spring carries more load in the same envelope because it starts from a shorter free length — often the only way to fit the force into the space. Dynamic or high-cycle service: springs that cycle continuously must be preset, or the early-cycle set adds stress and shortens fatigue life. Minimum-solid-height designs: when the spring bottoms out (compresses to solid) in service, presetting prevents it from growing shorter during those events and changing the mechanism. For prototype and non-critical springs, presetting can be skipped — but state that on the drawing so the manufacturer does not surprise you with either a shorter free length or a load that settles in the field.
The Load and Length Trade-off
Presetting changes the geometry numbers, which surprises buyers who compare quotes against an unpresetted sample. After presetting a spring wound to a given length, the free length is shorter by the amount of set — often 3-10% of free length, more with high stress and thin wire. If the drawing specifies free length AND load at working height, the spring maker winds it longer, presets it, and delivers it at the specified free length with the specified load — the winding length and the delivered free length are different numbers. Always give the delivered free length and load; let the maker manage winding and presetting. If you instead give only a free length and no load requirement, you will receive a spring that is whatever it settles to — and you may find the load is not what your mechanism needs.
Presetting and Fatigue Life
The fatigue benefit is mechanical: a spring that is preset operates below its yield point through its entire working range, because the material's residual stress state was set during presetting. An unpreset spring in the same duty starts above its shakedown limit and cycles down into set, adding plastic strain cycles that consume fatigue life. Springs preset and then shot-peened (where practical) show the best fatigue performance of any coiled spring construction. For high-cycle applications — engine valves, suspension, actuators, switch mechanisms — presetting is not an option, it is the baseline, and your spring supplier should state the preset condition and expected residual set on the datasheet.
How to Specify Presetting on Your Drawing
| Drawing callout | What it means |
|---|---|
| "Preset (scragged) to solid" | One or more compressions to solid; then free length measured |
| "Preset at load X N" | Compressed to the specified load rather than solid (used when solid is too high a stress) |
| "Load at height H: F N after presetting" | The acceptance load; verify at working height after preset |
| "Max set after test: X mm" | Bound the remaining set you will accept |
| "Do not preset" | Free length is functional; expect normal set in early service |
Note that preset is applied after stress relief but before any final grinding and finishing, and the measurement basis is the preset condition. If you buy prototype springs without a preset requirement and later scale to production, ask for the preset version in the production RFQ — the production parts will have a different (shorter) free length than your prototypes unless you specify it.
Frequently Asked Questions
Q: What is the difference between presetting and scragging a spring?
A: None — they are the same operation under two names. Presetting (or scragging) compresses a spring to solid or to a specified high load during manufacturing so it takes its permanent set before delivery, giving stable load from the first service cycle.
Q: Does presetting reduce spring load?
A: No — presetting makes load at working height higher and stable. The spring's free length shortens permanently, so at the same working height the deflection is greater and the load is higher than an unpresetted spring of the same free length. Over service life, a preset spring holds its load while an unpresetted spring loses 5-15%.
Q: Should all compression springs be preset?
A: Preset whenever load accuracy, limited working space, or cyclic/fatigue service matters — the majority of production springs. Springs where free length itself is functional (such as some detent and safety applications) should not be preset, or must be wound and preset to meet a specified delivered free length.
Q: How much does presetting shorten a spring?
A: Typical permanent set is 3-10% of free length, depending on stress level, spring index and wire diameter. The spring maker winds to a longer length, presets, and delivers at your specified free length and load — so specify the delivered dimensions, not winding dimensions.
Q: Does presetting improve fatigue life?
A: Yes, significantly. Preset springs operate below their yield point throughout the working range and avoid the plastic strain cycles that consume fatigue life during early service. Combined with shot peening, presetting gives the best fatigue performance available in coiled springs.
Authored by the BQUQ Engineering Team. BQUQ is an ISO9001-certified source factory in Dongguan, China, running CNC machining, metal stamping, custom springs, heat sink and collet lines under one roof. Send drawings to sc@bquq.com or WhatsApp +86 13713157787 for a quote within 12 working hours. www.bquq.com
