Machine Design #105: Keyways on Drawings — Transmit Torque and Still Remain Assemblable
Keyway on a drawing must preserve function, intent, manufacturability, inspectability, release quality, and serviceability when the boundary changes.
Evidence before release
Write the input, expected result, acceptance limit, failure symptom, and measurement method before changing a drawing or releasing a package. Assign an owner to every assumption, value, and change.
Core checks
- Torque function and keyway standard: record value, source, method, owner, and pass/fail evidence.
- Datum, orientation, width, and depth: record value, source, method, owner, and pass/fail evidence.
- Shaft stress and root radius: record value, source, method, owner, and pass/fail evidence.
- Hub fit, clearance, and assembly access: record value, source, method, owner, and pass/fail evidence.
- Inspection and supplier capability: record value, source, method, owner, and pass/fail evidence.
- Revision and service replacement: record value, source, method, owner, and pass/fail evidence.
| Failure mode | Symptom | Verification |
|---|
| Intent missing | Correct file, wrong function | Review requirements and evidence |
| Package incomplete | Supplier or inspection stops | Run a release completeness check |
| Documents out of sync | Correct number, wrong revision | Baseline every reference |
Dimensioning a keyway: four dimensions and where to put them
A keyway has only a few dimensions, but each of them can be written in more than one way, and an ambiguous one means the shop reads it differently from the designer with nobody noticing until assembly.
| Dimension | On which view | Note |
|---|
| Keyway width | Cross-section through the keyway | The tolerance on the shaft and on the hub are two different values |
| Depth | Cross-section | The measuring origin has to be stated — see below |
| Keyway length | Longitudinal view | Measured to where, when the slot end is an arc? |
| Slot bottom radius | Cross-section or a general note | Against stress concentration |
Depth: two ways to state it, choose one and be explicit
This is the most ambiguous point. Keyway depth can be stated in two entirely different ways, giving two different numbers on the same part:
| Way of stating it | What is measured | Advantage |
|---|
| Keyway depth | From the cylindrical surface down to the slot bottom | Intuitive, easy to understand |
| Remaining shaft dimension | From the slot bottom across to the opposite cylindrical surface | Measured directly with a micrometer, no subtraction |
The second is preferred in production because it is measured directly: put the micrometer across the slot bottom and the opposite surface and read the number. The first is easier to picture but has to be measured indirectly and then subtracted, and every subtraction is a chance to err.
The hub is the same: state the keyway depth, or state the dimension from the slot bottom across to the opposite bore wall. Either is acceptable, but it has to be consistent throughout the drawing set and explicit enough that the inspector does not have to guess.
Where the keyway length is measured to
A keyway milled with an end mill has arcs at both ends with the radius of the cutter. "Keyway length" can then mean the distance between the two arc centres or between the two outermost edges — differing by exactly one cutter diameter.
The way to prevent a misreading: dimension to the arc centre and state the end radius alongside, or state explicitly that the dimension is the overall length. For a through-milled or broached keyway the question does not arise.
Angular position of the keyway
If the part has only one keyway and no other feature needs an angular relationship to it, the angular position does not need to be stated. But it does need stating when:
- There are two or more keyways — their angular relationship decides correct orientation on assembly.
- The keyway has to line up with another feature (an oil hole, a flat, an assembly mark).
- The mating part only fits one way round.
When needed, state it with a basic angular dimension and a position requirement against the datum system — not with "equally spaced" or "as drawn".
Choosing the fit, the trade-off between key and interference, and the three failure modes of a keyed joint are in Machine Design #56 — Keys and keyways.
Keyway tolerances and the root fillet: the most stressed spot
Stating the four keyway dimensions is not enough; the width fit and the root fillet decide whether the key rattles or the shaft cracks.
The keyway width fit
The keyway width decides whether the key is tight or loose, chosen by load:
| Fit | Keyway tolerance (per standard) | Suits |
|---|
| Free (guided) | Shaft slot H9, hub slot D10 | Key slides along the shaft, or frequent assembly |
| Normal | Shaft slot N9, hub slot JS9 | One-direction torque, little reversal |
| Tight | Shaft slot P9, hub slot P9 | Heavy load, reversing, resisting key rattle |
A loose key hammers under reversing load and widens the slot over time; a tight key is hard to fit and replace. Choose by the real load regime, do not default one fit to every shaft.
The root fillet: where fatigue starts
The keyway root is an inside corner, a strong stress raiser when the shaft takes reversing torque. A sharp corner is where a fatigue crack tends to begin. To reduce it: radius the keyway root to the standard value for the key size (state it on the drawing, do not leave a sharp default), and prefer a disc-cutter keyway (rounded bottom) over an end-mill keyway (square bottom) for a fatigue-loaded shaft.
The depth datum and the effect on shaft strength
- The depth datum: give the hub keyway depth measured from the opposite bore face (a fittable
dimension), not just the slot depth — because fitting the key needs the right top clearance, and that depends on the datum.
- Effect on strength: a keyway cuts away section and adds a stress raiser, lowering the shaft's torsional
capacity. A heavily loaded shaft should have its diameter re-checked with the keyway accounted for, or use a spline or another joint when a single keyway is not enough.
MINATA release checklist
- [ ] Function, intent, boundary, and failure symptom are written.
- [ ] Ownership, interfaces, process, and mistake-proofing are clear.
- [ ] Six topic checks have evidence and pass/fail limits.
- [ ] Manufacturing, assembly, inspection, release, and maintenance were tried.
- [ ] Revision, supplier, package, and configuration records agree.
Good engineering ends with questions that can be verified. For keyway on a drawing, evidence that function and intent remain reliable is the MINATA standard.
Frequently asked questions
Which way should keyway depth be stated?
The directly measurable one — from the slot bottom across to the opposite cylindrical surface — is usually more convenient in production, because a micrometer gives the number directly. Stating the keyway depth is also acceptable, but it has to be explicit and consistent across the drawing set, because the two give different numbers.
Where is the length of an end-milled keyway measured to?
It has to be stated: to the centres of the two end arcs, or the overall length including both arcs. The two differ by exactly one cutter diameter. The safe approach is to dimension to the arc centre and state the end radius.
Are the keyway width tolerances the same on the shaft and on the hub?
No. The two parts play different roles in the joint, so their tolerance zones differ, taken from the applicable key standard. Giving one value for both discards what decides the side clearance.
When does the angular position of a keyway have to be stated?
When there are two or more keyways, when the keyway has to line up with another feature, or when the mating part only fits one way round. Otherwise it is unnecessary, and a superfluous callout only adds a dimension to inspect.
Does the slot bottom radius need to be stated?
It should be. A sharp bottom corner is a stress raiser exactly where the shaft section has already been reduced — under a reversing load, that is where the fatigue crack starts. Leaving it blank hands the decision to whatever cutter the shop happens to use.
Frequently asked questions, continued
Should a keyway be a loose or tight fit?
By the load. A reversing, heavily loaded shaft wants a tight fit (P9 slot) so the key does not hammer and widen the slot; a guided key that slides along the shaft wants a free fit (H9/D10). Do not default one fit to every shaft.
Why does a keyway root tend to crack?
Because it is an inside corner, a strong stress raiser under reversing torque. Radius the keyway root to the standard value for the key size, and prefer a disc-cutter keyway (rounded bottom) for a fatigue-loaded shaft.
Where do I measure hub keyway depth from?
From the opposite bore face (a fittable dimension), not just the slot depth. Fitting the key needs the right top clearance, and that depends on the datum; the wrong datum gives the key the wrong clearance.
A quick table for the shop floor
A keyway is easier to set when the situation, the way it is given and the purpose are lined up:
| Situation | How to give or control it | Purpose |
|---|
| Groove width | Follow the key standard and the fit | Transmit torque correctly on the sides |
| Depth | Split it between shaft and hub | Do not lift the hub |
| Groove ends | Suit the milling cutter or broach | Do not create a corner that cannot be machined |
A worked case
A keyway does not transmit torque through the top of the key. If the depth errors add up, the key can bottom out before the two sides work, causing looseness and crushing. The drawing should call the key by its standard before placing its own dimensions.
Conclusion
Keyways on Drawings is not paperwork done to make a file look tidy. It is how intent becomes a result that can be manufactured, assembled and measured repeatedly. A good drawing does not need the designer standing beside it to explain it; the structure of the information has to do that work.
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