BIZOMAT MAKEOS · Power Transmission
Sprocket Bore and Key Plan (Sprocket Diameters, Shaft Keyseat Readings, Wall Under the Teeth)
For a roller chain sprocket bored and keyed to a shaft, what are its diameters and keyseat readings, and is there enough metal left between the keyway and the tooth roots?
Plan of a roller chain sprocket blank and its shaft key: the pitch, bottom, caliper and outside diameters of the sprocket (as the sprocket diameters tool), the chordal height, keyseat measurement S and measurement over the key T for the shaft (as the keyseat depth tool), and the wall left between the top of the key (where the hub keyway reaches) and the bottom diameter of the teeth, against the least wall you will accept.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Chain pitch | mm | Pitch of the roller chain. |
| Number of teeth | teeth | Number of teeth on the sprocket. |
| Roller diameter | mm | Roller diameter of the chain. |
| Shaft and key | ||
| Shaft (bore) diameter | mm | Shaft diameter, equal to the sprocket bore. |
| Key width | mm | Width of the key. |
| Shaft keyseat depth at the side | mm | Depth of the shaft keyseat measured at its side. |
| Key height | mm | Height of the key. |
| Least wall under the teeth | mm | Smallest radial wall you will accept between the hub keyway and the tooth bottoms. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Chain pitch
- What it is
- The centre-to-centre distance between neighbouring rollers of the roller chain the sprocket runs on, from the chain designation or maker’s catalogue. It sets every sprocket diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15.875 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All sprocket diameters rise with the pitch, leaving more room under the teeth.
- Get it right
- Chain sizes are often named in inches; convert the pitch to millimetres (1 in = 25.4 mm).
What this means: Number of teeth
- What it is
- The number of teeth round the sprocket. With the chain pitch it fixes the pitch diameter, and an odd count changes how the caliper diameter is measured.
- Units and format
- Enter a number in teeth: a whole number of teeth. The form accepts 5 teeth to 500 teeth. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 17 teeth as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More teeth enlarge the sprocket and leave more wall under the teeth.
- Get it right
- An odd tooth count has no tooth directly opposite another, so its caliper diameter is less than its bottom diameter.
What this means: Roller diameter
- What it is
- The outside diameter of the chain rollers, from the chain maker’s data. The bottom diameter of the teeth is the pitch diameter minus this.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10.16 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger roller lowers the bottom diameter and so the wall under the teeth.
- Get it right
- Use the roller diameter, not the pin or bush diameter.
Shaft and key
What this means: Shaft (bore) diameter
- What it is
- The diameter of the shaft, equal to the bore of the sprocket hub. The keyseat readings and the top of the key are worked from it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore raises the top of the key and thins the wall under the teeth.
- Get it right
- The bore must be smaller than the bottom diameter of the teeth, or the tool rejects the inputs.
What this means: Key width
- What it is
- The width of the parallel key across the shaft, from your key standard for the shaft size. It sets the chordal height of the keyseat edge.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The key width must be smaller than the shaft diameter, or the tool rejects the inputs.
What this means: Shaft keyseat depth at the side
- What it is
- The depth of the keyseat cut in the shaft, measured at the side of the keyseat from where it meets the round surface down to its floor, from your key standard.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A deeper shaft keyseat lowers the reading S and the top of the key, leaving more wall.
- Get it right
- This is depth at the side, not from the top of the full round; the tool adds the chordal height to get the total depth.
What this means: Key height
- What it is
- The overall height of the key from its bottom face to its top face. The top of the key marks where the hub keyway reaches.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A taller key raises the top of the key and thins the wall under the teeth one for one.
- Get it right
- Use the key’s actual height, not the depth of the hub keyway.
What this means: Least wall under the teeth
- What it is
- The thinnest radial wall of metal you will accept between the top of the hub keyway and the bottom of the tooth spaces, from your design rules or engineer.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- This is the wall over the keyway, not the general hub wall; it is usually the weakest point of a small sprocket.
Method and formulas
- Pitch diameter = p ÷ sin (180° ÷ N); bottom = PD − D_r; caliper = PD cos (90° ÷ N) − D_r for odd teeth; outside = p (0.6 + cot (180° ÷ N)) (as the sprocket diameters tool).
- Chordal height Y = (D − √(D² − w²)) ÷ 2; S = D − Y − t; T = S + h (as the keyseat depth tool).
- Top of the key from the bore centre = T − D ÷ 2; wall = bottom diameter ÷ 2 − (T − D ÷ 2).
- Example: bottom diameter 76.235 mm; keyseat reading S 25.457 mm; 20.661 mm of wall under the teeth.
Limitations
- Plain hub without a raised boss; a hub boss adds metal beside the teeth but not under them.
- Hub keyway depth taken to the top of the key; add the keyway clearance from your key standard.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check the largest bore for a small sprocket
- Set the keyseat readings for a sprocket shaft
- Rebore a stock sprocket for a bigger shaft
Who it is for
- Machinists
- Maintenance fitters
- Design engineers
What you get
- Sprocket diameters
- Shaft keyseat readings
- Wall under the teeth
Interpreting the result
- A thin wall under a keyway is where small sprockets crack; go up a tooth or two or use a smaller key with a set screw.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2537–2550, 2460–2486. Topic reference; no source tables or recommended settings are reproduced.
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