# Spring and lock washers (DIN 137 A / DIN 6797 A)

Curved spring washers (DIN 137 form A, M2 to M10) and external tooth lock washers (DIN 6797 form A, M2 to M8).
Presets cover the catalogue's metric curved spring washers and external tooth lock washers.

## Geometry

- **curved** (DIN 137 A): the annulus **d1** × **d2**, **s** thick, curved about an axis parallel to X so that it
  stands **h** (max, the free height) tall: the part of the annulus between two coaxial cylinders of radii R and
  R + s, R chosen so that the inner (upper) surface rises h − s at the rim (R = ((d2/2)² + (h − s)²) / (2 (h − s))).
  Its lowest point is on the XY plane at the origin. The standard gives d1, d2, s and h only; this is the curved form
  those four numbers define.
- **tooth** (DIN 6797 A): envelope, the flat blank, an annulus d1 × d2, s thick, lying on the XY plane. The standard
  gives the number of teeth (6 up to M3, 8 from M4) and the overall height (at least 2 s) but not the teeth's shape,
  width or twist, so the teeth are not modelled.

## Parameters

| Parameter | Meaning |
|---|---|
| `size` | Screw size: M2 to M10 (curved), M2 to M8 (tooth) |
| `style` | `curved` (DIN 137 A) or `tooth` (DIN 6797 A) |
| `material` | `spring-steel` or `stainless` |

## Sources

- **DIN 137** (1985; withdrawn without replacement), *Spring washers, curved or waved*, form A, as reproduced by
  fasten.it (https://fasten.it/en/Norms/Norm/DIN%20137, table "source DIN 137 A"). Cross-check: McMaster-Carr's DIN
  137A washer specifications (https://mcmaster.com/products/washers/specifications-met~din-137a) agree on d1, d2, s
  and h max for M2, M3, M4, M5, M6, M8 and M10.
  Disagreement: the fasten.it page's text calls form A "waved" and form B "curved"; its figure, DIN 137 itself
  (form A gewölbt, form B gewellt) and McMaster have form A curved. The table used here is the one with h max 1.0 (M2)
  to 4.0 (M10), which McMaster sells as DIN 137 A curved washers.
- **DIN 6797:1988-07** (withdrawn), *Toothed lock washers*, form A (external teeth), as reproduced by fasten.it
  (https://fasten.it/en/Norms/Norm/DIN%206797). Cross-checks: Bossard BN 787 "Toothed lock washers type A, external
  teeth" (28 Apr 2017) agrees on every d1, d2, s; Duratool/Farnell datasheet V1.1 (21/07/17,
  https://www.farnell.com/cad/2337766.pdf) agrees on the teeth (M3 6, M4 8). The rows "for hexagon screws only" (M5
  5.1/9, M8 8.2/14) are not used.

| Size | Curved d1 | d2 | s | h max | Tooth d1 | d2 | s | teeth |
|---|---|---|---|---|---|---|---|---|
| M2 | 2.2 | 4.5 | 0.3 | 1.0 | 2.2 | 4.5 | 0.3 | 6 |
| M2.5 | 2.8 | 5.5 | 0.3 | 1.1 | 2.7 | 5.5 | 0.4 | 6 |
| M3 | 3.2 | 6 | 0.4 | 1.3 | 3.2 | 6 | 0.4 | 6 |
| M4 | 4.3 | 8 | 0.5 | 1.6 | 4.3 | 8 | 0.5 | 8 |
| M5 | 5.3 | 10 | 0.5 | 1.8 | 5.3 | 10 | 0.6 | 8 |
| M6 | 6.4 | 11 | 0.5 | 2.2 | 6.4 | 11 | 0.7 | 8 |
| M8 | 8.4 | 15 | 0.5 | 3.4 | 8.4 | 15 | 0.8 | 8 |
| M10 | 10.5 | 18 | 0.8 | 4.0 | – | – | – | – |

The catalogue's entries carry DIN 125 plain-washer dimensions (for example M3: 3.2 / 7 / 0.5), not a curved or
toothed washer's; the presets follow the standards. Not covered: M12 curved (DIN 137 A has no M12); wave washers
(DIN 137 B gives no number of waves).

Preset list: sizes of the step.parts catalogue (MIT), see `library/NOTICE.md`.