Bipedal robot arm joints—shoulder, elbow, and wrist—demand extreme compactness and low distal mass. Use this tool to evaluate inverted roller screw sizing, check backdrivability tradeoffs, and identify architecture risks before ordering prototypes.
Estimate screw sizing and identify risks for your bipedal robot arm joints.
Joint context affects load priorities.
Usually 3-10 mm. Drives backdrivability vs torque.
Maximum required force during high acceleration or payload handling.
Average force during typical arm manipulation tasks. Drives thermal and life limits.
Active travel length. Drives actuator envelope.
While leg actuators focus on surviving jump impacts and carrying full body weight, arm actuators prioritize dexterity, speed, and transparency.
| Joint | Typical Peak Torque | Preferred Actuator Type | Inverted Roller Screw Role |
|---|---|---|---|
| Shoulder (Pitch/Yaw) | 60 – 100 Nm | Rotary (Harmonic/QDD) | Limited. Requires multi-axis rotation where linear actuators hit singularities. |
| Elbow (Pitch) | 40 – 80 Nm (600N – 4000N Thrust) | Linear (Inverted Roller Screw) | Primary. Provides high torque density, excellent shock absorption, and fits inside the upper arm envelope. |
| Wrist (Pitch/Yaw) | 10 – 30 Nm | Micro Linear / Cable Driven | Possible, but extremely tight spatial constraints limit standard planetary screw sizes. |
Inverted Layout Advantage: The heaviest component (motor stator) is fixed to the proximal joint structure, while the lighter screw shaft extends distally. This reduces reflected inertia and improves arm dynamics.
If: Axial envelope below 120 mm
Then: Start with inverted planetary roller screw architecture.
Why: Motor stator wraps around the spinning nut, forcing the central screw shaft to extend linearly without a parallel belt drive.
If: Requirement for Force Transparency / Kinesthetic Teaching
Then: Select a coarse lead (>5mm) or integrate a series force sensor.
Why: A fine lead (e.g. 2-3.5mm) approaches self-locking. Coarser leads (5-6.5mm+) back-drive under load, improving interaction safety.
If: Peak Joint Torque Requirement > 60 Nm (e.g. Shoulder)
Then: Evaluate if a rotary quasi-direct drive (QDD) is more suitable.
Why: Shoulders often require continuous 60-100 Nm torque and full rotation. Linear actuators are typically better suited for 40-80 Nm elbow joints (600N-4000N thrust) where moment arms are fixed.
Application: Demonstrates baseline requirements for humanoid arm linear actuators: 600N to 8000N thrust, 3.5mm to 6.5mm lead, and up to 400 mm/s speed.
Constraint: Actual thermal duty cycles vary based on controller implementation.
Application: Confirms that inverted planetary roller screws are ideal for compact linear packaging (stator remains fixed while rotor spins nut). Outlines typical peak torque profiles (e.g., 60-100 Nm for shoulder, 40-80 Nm for elbow).
Constraint: Rotary actuators are often preferred for the shoulder to avoid kinematic singularities; inverted screws are primarily for elbow/bicep/tricep and ankle joints.
Application: Explains the fundamental backdrivability tradeoff: fine leads (e.g., 2-3.5mm) approach self-locking to save holding current, while coarse leads (5-6.5mm) enable force transparency but increase motor thermal load.
Constraint: Most linear actuators still require a series force sensor for safe human interaction, regardless of lead.
Understand what this screening tool can and cannot do.
Pre-RFQ architecture choice, rough dynamic capacity (up to 8000N), stroke/speed limits (up to 400 mm/s), and lead tradeoff analysis (2mm - 6.5mm) for humanoid elbows and arms.
Final bearing life calculations, detailed thermal duty cycle analysis, or series-elastic force sensor tuning.
Failure modes that should be addressed before prototype freeze.
Trigger: Using a coarse lead (>5mm) for backdrivability while continuously holding a payload at full elbow extension.
Mitigation: Integrate a mechanical brake, use a finer lead (approaching self-locking), or limit the maximum holding duration.
Trigger: Dropping heavy payloads or robot falling, transmitting sudden linear shocks through the arm.
Mitigation: Planetary roller screws distribute shock across multiple planets (unlike ball screws). Preloaded nuts remove backlash but increase stiction.
Trigger: Mounting heavy linear actuators far from the shoulder.
Mitigation: Use the inverted architecture to place the heaviest component (the motor stator) closer to the proximal joint.
Inverted roller screws allow the motor rotor to wrap around the nut, significantly reducing the axial length of the actuator. This compact design is critical for fitting inside robot arm limbs without protruding, while keeping the center of mass close to the torso.
Arm actuators typically handle lower peak forces since they do not carry the full body weight or absorb jump impacts. However, they require higher backdrivability, extreme compactness, and very low distal mass to maintain high control bandwidth and dexterity.
Yes, extremely. Backdrivability allows for force control, compliant interaction with the environment, and safety when working around humans. A sufficiently coarse lead (e.g., 5mm to 10mm) coupled with a high-efficiency inverted roller screw improves this.
Ball screws can be used in arms where forces are low, but roller screws offer higher load capacity and stiffness in a smaller diameter. If radial space is at a premium, an inverted roller screw provides unmatched torque density.
Lead choice is a tradeoff. 3-5mm offers high mechanical advantage and holding force but lower backdrivability. 8-10mm offers excellent transparency and backdrivability but requires a motor with higher continuous torque.
No. It is an early screening tool for architecture and load magnitude. Final sizing must account for full manipulation cycle loads, thermal limits of the motor, bearing life, and housing stiffness.
Explore the core roller screw technologies that enable high-performance robotic joints.
Send the calculator output with arm joint geometry, payload, package envelope, and target life so diameter, lead, preload, and bearing support can be reviewed together.
