Leader arm · Step 8 of 26 · L-014

Add the shoulder motor holder

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Put the shoulder motor holder on top of the shoulder. It has no screws of its own: motor 2's screws will clamp it in the next step.
src
- lerobot/docs/source/so101.mdx#joint-1 — "Add the shoulder motor holder."
- instructions — No screws of its own: motor 2's four side screws clamp it in the next step (on each side one enters the shoulder, one the holder).
- check — Holder seated on the shoulder, ready to receive motor 2 from the top.

Done when The holder sits on the shoulder, open at the top for motor 2.

Upstream numbers the follower's steps; this leader step is the same build step (extends F-014).

lerobot/docs/source/so101.mdx#joint-1"Add the shoulder motor holder."

waveshare.comFollower step 5: put F4 on F3

SO-ARM100/STEP/SO101/SO101 Assembly.stepplacement check (viewer/public/geometry/placements.json): Motor_holder_SO101_Base encloses 42% of servo 2, above the shoulder: the shoulder motor holder

SO-ARM100/STEP/SO101/SO101 Assembly.stepplacement check (pipeline hole test): no fastener passes through this part until motor 2 is fitted; two of motor 2's tab screws then go through Motor_holder_SO101_Base holes

Notes: No screws of its own: motor 2's four side screws clamp it in the next step (on each side one enters the shoulder, one the holder).

Check: Holder seated on the shoulder, ready to receive motor 2 from the top.

Motor slot: shoulder_lift · ID 2 · STS3215 1:345 · 7.4V servo on a 5V supply;

How we placed it: Part role confirmed by placement in the upstream assembly STEP: Motor_holder_SO101_Base encloses 42% of servo 2, above the shoulder: the shoulder motor holder.

Motor 2 · shoulder lift

The motor you labelled 2: STS3215 motor,

1:345

Gear ratio. The number on the motor's label, such as 1:345. The motor inside turns 345 times for one turn of the output. The follower arm uses 1:345 at all six joints. The leader arm mixes 1:191, 1:345 and 1:147, so check the label before fitting a motor.Source: lerobot/docs/source/so101.mdx#step-by-step-assembly-instructions · SO-ARM100/README.md#sourcing-parts

gear, code C001. details

You need

Shoulder motor holderMotor holder (base side) motor-holder-so101-base details
ShoulderRotation/pitch (shoulder) rotation-pitch-so101 details

Sources (4)

Upstream numbers the follower's steps; this leader step is the same build step (extends F-014).

lerobot/docs/source/so101.mdx#joint-1 — "Add the shoulder motor holder."
waveshare.com (retrieved 2026-09-07) — Follower step 5: put F4 on F3
SO-ARM100/STEP/SO101/SO101 Assembly.step — placement check (viewer/public/geometry/placements.json): Motor_holder_SO101_Base encloses 42% of servo 2, above the shoulder: the shoulder motor holder
SO-ARM100/STEP/SO101/SO101 Assembly.step — placement check (pipeline hole test): no fastener passes through this part until motor 2 is fitted; two of motor 2's tab screws then go through Motor_holder_SO101_Base holes

How we placed it

Part role confirmed by placement in the upstream assembly STEP: Motor_holder_SO101_Base encloses 42% of servo 2, above the shoulder: the shoulder motor holder.