When a retaining ring lets go, the ring is usually blamed and the groove is usually at fault. A ring that dishes out of a shallow groove, a groove corner radiused by a worn tool, a groove cut too close to a shaft end - all of these fail at a fraction of the ring's catalogue thrust rating. The ring is a bought-in standard part; the groove is the bit you control.
The Three Groove Dimensions
Every circlip chart gives three groove figures. They are not independent - they describe one feature from three directions:
- d₂ - groove diameter. The diameter at the bottom of the groove. On a shaft it is smaller than the nominal; in a bore it is larger. This is the dimension the ring grips.
- m - groove width. Measured axially. It is slightly wider than the ring thickness s, so the ring drops in without being pinched. Quoted as a minimum with a plus tolerance.
- t - groove depth. The radial wall the ring bears against. It follows directly from the other two: t = (d₁ − d₂) / 2 on a shaft, and (d₂ − d₁) / 2 in a bore.
Depth is the dimension that carries load. The ring only resists thrust through the part of its section standing proud of the groove. Cut the groove 0.1 mm shallow on a small size and you may have given away a quarter of the depth - and with it, a large share of the thrust capacity.
Tolerances That Matter
Three tolerances do the real work:
- Groove diameter d₂: h11 on a shaft, H11 in a bore. This is the one to hold. Too large on a shaft and the ring sits proud with no grip; too small and you cannot get the ring in without overstressing it.
- Groove width m: a plus-only tolerance. Never cut it undersize - a ring forced into a tight groove is pre-stressed before it has taken any load. But do not be generous either: excess width lets the ring tilt under thrust, which is how rings dish and pop out.
- Groove-to-shoulder position: if the ring is locating a bearing or gear, the axial position of the groove sets the end float of the whole assembly. Tolerance it from the same datum as the shoulder, not from the shaft end.
One more that is easy to forget: the groove must be square and concentric to the shaft or bore axis. A groove cut in a second setup, out of concentricity, loads one side of the ring and unloads the other.
Representative Dimensions
A cross-section of common sizes, for both external and internal rings:
| Nominal Ø | Ring thickness s | Groove Ø d₂ (shaft) | Groove Ø d₂ (bore) | Groove width m | Groove depth t |
|---|---|---|---|---|---|
| 10 | 1 | 9.6 | 10.4 | 1.1 | 0.2 |
| 12 | 1 | 11.5 | 12.5 | 1.1 | 0.25 |
| 16 | 1 | 15.2 | 16.8 | 1.1 | 0.4 |
| 20 | 1.2 | 19 | 21 | 1.3 | 0.5 |
| 25 | 1.2 | 23.9 | 26.2 | 1.3 | 0.55 |
| 30 | 1.5 | 28.6 | 31.4 | 1.6 | 0.7 |
| 35 | 1.5 | 33 | 37 | 1.6 | 1 |
| 40 | 1.75 | 37.5 | 42.5 | 1.85 | 1.25 |
| 50 | 2 | 47 | 53 | 2.15 | 1.5 |
| 60 | 2 | 57 | 63 | 2.15 | 1.5 |
| 70 | 2.5 | 67 | 73 | 2.65 | 1.5 |
| 80 | 2.5 | 76.5 | 83.5 | 2.65 | 1.75 |
| 100 | 3 | 96.5 | 103.5 | 3.15 | 1.75 |
All dimensions in mm. Groove width m is a minimum and carries a plus tolerance; groove diameter d₂ is h11 on a shaft and H11 in a bore. The full 78-size external and 73-size internal tables are in the size-chart guides linked below.
Corners and Edge Margin
Two details ruin more grooves than any tolerance:
The groove corner must be sharp. The ring bears against the load-side wall, and it must bear against a flat face right down to the root. A worn parting tool leaves a radius that lets the ring ride up and out under thrust - the ring appears undamaged afterwards, which is why the failure gets misdiagnosed. Keep the corner radius at the load-side wall as small as the tooling allows, and regrind or index before it degrades.
Leave enough material beyond the groove. The land between the groove and the end of the shaft (or the end of the bore) has to resist being sheared off by the ring under thrust. As a working rule, keep at least three groove depths of material beyond the groove - and more in a soft material. Grooves cut close to a shaft end are a classic failure: the ring holds, and the land in front of it peels away.
If you cannot get the edge margin, the fix is a different retention method - a shouldered shaft, a spacer, or a ring with a much lower thrust requirement - not a deeper groove.
Surface Finish
The load-bearing wall of the groove is a bearing surface under high contact stress. A rough, torn finish becomes a stress raiser and a crack initiation site, particularly under dynamic or reversing load. Aim for a clean turned or ground finish on the load-side wall; the groove root matters less. Deburr the groove edges after machining - a raised burr at the groove mouth holds the ring off its seat and can be enough to stop it entering fully.
How to Cut the Groove
- Single-point turning (external) or boring with a grooving tool (internal): the usual route, and the most flexible. Use a full-form grooving insert ground to the groove width where you can, so width, depth and corners come from one tool.
- Plunge grooving with a form tool: best repeatability, ideal for volume. The whole groove is one feature, so concentricity and squareness come free.
- Grinding: for hardened parts, or where the groove wall finish is critical.
- Rolling: used on some high-volume shafts. It work-hardens the groove surface, but the profile is less crisp - confirm the ring seats fully before committing.
Whichever method, cut the groove after heat treatment where the part allows it, or allow for the size change if you cut before.
Inspecting the Groove
- Diameter: a groove micrometer or ball-anvil micrometer on a shaft; a bore gauge with a groove attachment internally. Do not try to read d₂ with calliper jaws - you will measure the mouth, not the root.
- Width: pin gauges or a groove gauge. A ring of the correct thickness should drop in and have perceptible but small axial play.
- Corner condition: look at it under magnification, or take a silicone impression on larger sizes. This is the check that gets skipped and catches the most problems.
- Fit test: install a ring and check that it seats fully round its circumference with no gap at the groove root, and that it does not rock axially.
If a ring will not seat and you are confident in the groove diameter, measure the ring itself before recutting - a sprung or wrong-series ring behaves exactly like a mis-machined groove.
Rings for Every Standard Groove
DIN 471 and DIN 472 circlips, Ø4 to Ø300 mm, in spring steel and stainless - ex-stock Mumbai.
View Circlip RangeFrequently Asked Questions
What tolerance should a circlip groove diameter have?
h11 for a groove on a shaft and H11 for a groove in a bore, per DIN 471 and DIN 472 respectively. This is the tolerance to hold tightly - it controls whether the ring grips at all. Groove width carries a plus-only tolerance, and should never be cut undersize.
How do I calculate circlip groove depth?
Groove depth t is derived from the nominal and groove diameters: t = (d₁ − d₂) / 2 for an external groove on a shaft, and (d₂ − d₁) / 2 for an internal groove in a bore. It is the radial wall the ring bears against, so a shallow groove directly reduces thrust capacity.
How much material should be left beyond a retaining ring groove?
As a working rule, at least three groove depths of material between the groove and the end of the shaft or bore, and more in soft materials. Too little edge margin and the land in front of the ring shears away under thrust while the ring itself stays intact.
Does the groove corner radius matter?
Considerably. The ring must bear on a flat wall right down to the groove root. A radius left by a worn grooving tool lets the ring ride up and dish out under thrust, and because the ring usually survives, the failure is often blamed on the ring rather than the groove.
Should the groove be wider than the ring thickness?
Slightly - that is why groove width m is quoted as a minimum with a plus tolerance, a little above the ring thickness s. The ring needs to drop in without being pinched. But excessive width lets the ring tilt under load, which is a common cause of rings dishing out of the groove.
Can I machine the groove before heat treatment?
You can, but you must allow for the dimensional change through hardening and tempering, and re-check the groove diameter afterwards. Where the part allows, cutting the groove after heat treatment - or grinding it - gives much better control of diameter and wall finish.