Feetech STS3215 vs STS3250: Which Servo for Which Joint of a LeRobot Arm
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The STS3215 is the standard servo of the SO-101 arm; the STS3250 is its metal-case, 50 kg·cm big brother on the same bus protocol. Both are made by Feetech, both use 25 T splines and 3-pin 5264 cables, and both are driven by the same USB adapter. This article compares them and tells you where each belongs.
Specifications side by side
| STS3215 (C018, 12 V) | STS3215 (C001, 7.4 V) | STS3250 (C002/C001, 12 V) | |
|---|---|---|---|
| Stall torque | 30 kg·cm | 19 kg·cm | 50 kg·cm |
| No-load speed | ~45 rpm | ~52 rpm | 75 rpm |
| Case | Plastic | Plastic | Full aluminium |
| Motor | Brushed | Brushed | Coreless |
| Encoder | 12-bit magnetic, 360°, multi-turn | ||
| Footprint | 45.2 × 24.7 mm, ~35 mm tall | ||
| Weight | 55 g | 55 g | 75 g |
| Price | from $24.99 | from $22.99 | from $74.99 |
Joint-by-joint recommendation for an SO-101 follower
- Shoulder pan (base): STS3215-C018. Load is mostly inertia, not gravity.
- Shoulder lift: the joint that stalls first. STS3215-C018 handles blocks and cups; upgrade to STS3250 if you add a heavy gripper, a camera and payloads above ~300 g.
- Elbow flex: STS3215-C018; STS3250 only together with the shoulder upgrade, otherwise the shoulder becomes the weak link.
- Wrist flex, wrist roll, gripper: STS3215-C018. Weight here costs torque at the shoulder; never put an STS3250 on the wrist.
The leader arm should stay on 7.4 V STS3215 in the 1:345 / 1:191 / 1:147 mix: the point of a leader is to be light to move by hand, and 50 kg·cm servos make it stiff.
What else changes with an STS3250 upgrade
- Power: budget 2–3 A per STS3250 under load. Two of them plus four STS3215 on a 12 V 5 A adapter is the limit.
- Printed parts: the STS3250 is the same footprint but its mounting holes and metal case differ, so the shoulder part needs a remixed pocket. ArmDojo's free print files include an STS3250 shoulder option.
- Horns: unchanged, both take the same 25 T metal horns.
FAQ
Can I mix STS3215 and STS3250 on the same bus?
Yes. Both use the Feetech half-duplex TTL protocol at the same baud rate; give each servo a unique ID and LeRobot sees them as one chain.
Does the STS3250 fit the SO-101 printed parts?
Not in the stock shoulder pocket: the STS3250 is the same footprint but has a metal case and different end-face hole positions, so it needs a modified shoulder part. The standard SO-101 files are designed around the STS3215.
Is the STS3250 worth it for a desktop arm?
Only if you load the arm beyond about 300 g at the gripper or run long autonomous sessions where the shoulder servo gets hot. For standard LeRobot tasks with wooden blocks, six STS3215 are enough.