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When your CNC router, 3D printer, or automation rig demands slip-free, repeatable motion, a mismatched or poorly machined pulley becomes the weakest link in the entire drivetrain. Engineers and makers who need a reliable S3M timing belt pulley with a keyed bore — not a plain bore that spins under load — will find this AF-type series purpose-built for the job.
Each pulley in this series is machined from aluminum alloy with an anodized finish, covering tooth counts from 18T to 40T and bore diameters from 8mm to 25mm (approximately 0.31" to 0.98"). The 3mm pitch (S3M standard) matches standard S3M/HTD-3M open or closed timing belts. Drive width is 7mm, and every variant ships with a precision-cut keyway — sizes range from 3×1.4mm up to 8×3.3mm depending on bore — ensuring positive torque transfer without set-screw slippage.
Typical use cases include desktop CNC axis drives requiring backlash-free positioning, DIY robotic arm joints where keyway engagement prevents rotational slip under load, and compact conveyor or pick-and-place automation systems using S3M belt loops. The wide tooth-count range (18T–40T) lets you dial in your gear ratio without sourcing multiple pulley families.
| Belt Standard | S3M (HTD-3M compatible) |
|---|---|
| Pitch | 3mm |
| Drive Width | 7mm |
| Tooth Count | 18T, 19T, 20T, 22T, 24T, 25T, 26T, 27T, 28T, 30T, 32T, 34T, 35T, 36T, 38T, 40T |
| Bore Diameter | 8mm, 10mm, 12mm, 12.7mm, 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, 25mm |
| Keyway Size | 3×1.4mm / 4×1.8mm / 5×2.3mm / 6×2.8mm / 8×3.3mm (varies by bore) |
| Hub Type | AF-type (flanged hub with keyway) |
| Material | Aluminum alloy |
| Finish | Anodized |
| Outer Diameter | See variant options |
| Flange Diameter | See variant options |
| Quantity per Order | 1 piece |
The tooth count determines your pulley's pitch diameter and, combined with your driven pulley's tooth count, sets the drive ratio. A smaller tooth count (e.g., 18T) on the motor gives a speed reduction when paired with a larger driven pulley, increasing torque. A larger tooth count (e.g., 40T) on the motor increases output speed. Match the tooth count to your required belt loop length and center distance as well — use a belt length calculator to confirm compatibility before ordering.
Measure your motor or drive shaft diameter with calipers and select the matching bore. Available bores are 8mm, 10mm, 12mm, 12.7mm (1/2"), 14mm, 15mm, 16mm, 17mm, 18mm, 19mm, 20mm, and 25mm. The 12.7mm bore is designed for imperial 1/2" shafts common in some stepper motors. If your shaft is metric, choose the exact metric bore — do not use an oversized bore with a sleeve unless you have a precision adapter.
The keyway is a rectangular slot machined into the bore that accepts a matching key (a small rectangular bar) inserted between the shaft and pulley. This creates a positive mechanical lock that prevents the pulley from rotating on the shaft under torque — something a set-screw alone cannot reliably do under reversing or high-cycle loads. If your shaft has a matching keyway cut, use the key. If your shaft is plain (no keyway), the pulley can still be mounted using the set screw, but for high-torque or reversing applications, a keyed shaft is strongly recommended.
S3M and HTD-3M share the same 3mm pitch, so these pulleys are generally cross-compatible with both belt profiles. However, the tooth flank geometry differs slightly between the two standards. For light-to-medium duty applications the difference is negligible, but for high-load or precision positioning systems, use the belt and pulley from the same standard (S3M with S3M, HTD-3M with HTD-3M) to ensure optimal tooth engagement and belt life.
Aluminum alloy pulleys are well-suited for light-to-medium duty CNC, 3D printing, robotics, and small automation applications. The S3M belt standard itself is rated for relatively modest torque loads (typically up to a few Nm depending on belt width and speed), so the aluminum pulley is not the limiting factor in most S3M drive systems. For heavy industrial applications with high shock loads, consider steel pulleys instead. If you are unsure, calculate the belt drive load using your motor torque, pulley diameter, and safety factor before selecting materials.
The anodized aluminum finish provides good resistance to oxidation and mild corrosion in typical indoor environments. Timing belt drives are dry-running systems — do not apply grease or oil to the belt or pulley teeth, as lubricants attract debris and degrade belt rubber, reducing belt life and causing slippage. For outdoor or high-humidity environments, inspect the pulley periodically for surface oxidation and ensure the keyway and bore remain clean and free of moisture ingress.
Choose the AF-type keyway pulley when your shaft has a matching keyway cut and you need reliable torque transfer under reversing loads, high cycle counts, or moderate-to-high torque. Choose a plain-bore pulley only for very light-duty, unidirectional, low-torque applications where a set screw is sufficient. For CNC axis drives and robotics joints — the primary use cases for S3M pulleys — the keyed version is the correct choice in most scenarios.
