If your drawing says "blue die spring" and your supplier reads it against a different standard, you will get a spring with a different load rate in the same physical envelope. The mould assembles. The tool runs. It just does not behave the way the designer intended, and you find out several thousand cycles later.
This is not a rare edge case. Two colour systems are in wide circulation and they disagree — and for two of the colours they are exact opposites.
The two systems side by side
| Colour | JIS system | ISO 10243 |
|---|---|---|
| Yellow | Extra light | Extra heavy |
| Blue | Light | Medium |
| Red | Medium | Heavy |
| Green | Heavy | Light |
| Burgundy | Extra heavy | — |
The two that will actually hurt you
Yellow. On the JIS system yellow is the lightest spring in the range. On ISO 10243 it is the heaviest. These are opposite ends of the same catalogue. A yellow spring ordered by colour alone can arrive several times stiffer or softer than the one it replaces.
Green. Same inversion, the other way round. JIS green is heavy duty; ISO green is light duty.
Blue and red are less dramatic — each shifts by one class between the systems — but a spring that is one class off still changes your ejection force and your stripper timing.
Maximum compression is not one number either
Load class also carries a maximum compression, and that figure moves with the cycle life you are designing for. On the JIS system:
- Extra light (yellow): 40% of free length at 1,000,000 cycles, up to 50% at 300,000 cycles
- Light (blue): 32% of free length at 1,000,000 cycles, up to 40% at 300,000 cycles
Catalogues usually quote the higher figure, because it is the more attractive number. If you are designing a tool for a long production run, the 1,000,000-cycle column is the one that applies to you. Designing to 50% on a yellow spring and then running a million cycles is a reliable way to produce a broken spring in a closed mould.
What to specify instead of a colour
Colour is a handling aid for the person at the rack. It is not a specification. On a drawing or an RFQ, give:
- Outside diameter and free length — the envelope
- Load class in words, not colour: light, medium, heavy
- The standard you are working to — JIS or ISO 10243
- The load you actually need at your working deflection, if you know it
- The cycle life you are designing for, which fixes the maximum compression
Any one of the first three, on its own, is ambiguous. All three together are not.
One question worth asking a supplier
Ask which colour system their springs follow. A supplier who answers immediately and can produce the max-load table for the outside diameter you are buying has thought about this. A supplier who says "our springs are good quality" has not answered the question, and that is information too.
The springs we supply are 65Mn to the JIS system — yellow is extra light, blue is light — and the full maximum-load tables by outside diameter are published on the product pages rather than kept in a PDF nobody can find.