Selecting the right roller screw for a biped robot arm involves balancing dynamic capacity, backdrivability, and spatial constraints. Use our sizing tool to generate baseline specs and understand the engineering tradeoffs.
Calculate required diameter, lead, and dynamic capacity for humanoid 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.
Based on empirical data and deployment requirements from full-scale commercial humanoid integrations.
Inverted roller screws provide up to 3x the load capacity of similarly sized ball screws.
Unlike ball screws that rely on point contact, planetary roller screws utilize multiple line contacts.
Leads of 5-10mm ensure the joint can be mechanically backdriven during collisions.
Impact Resistance: planetary rollers provide significantly more contact points than balls, preventing brinelling (indentations) during sudden arm impacts or drops.
| Feature | Roller Screw | Ball Screw |
|---|---|---|
| Peak Load Capacity | High (5kN - 15kN) | Moderate (< 5kN for same diameter) |
| Shock Tolerance | Excellent (Line Contact) | Poor (Brinelling risk under impact) |
| Speed / Kinematics | High (No ball return limits) | Medium (Limited by recirculation) |
| Cost / Complexity | High / Complex | Low / Standard |
Fact/Data: Empirical data shows elbow actuators in full-scale humanoids require dynamic capacities of 5,000 N to 15,000 N to handle heavy payloads and sudden impacts.
Constraint/Risk: Actual operating life depends heavily on the duty cycle and peak impact loads (e.g., catching falling weights). Static holding force is not the primary sizing constraint.
Fact/Data: Roller screws provide line contact (as opposed to point contact in ball screws), which prevents brinelling under shock loading typical of humanoid locomotion.
Constraint/Risk: Roller screws introduce slightly higher friction than ball screws and carry a higher upfront manufacturing cost.
Roller screws offer significantly higher load capacity and shock resistance in a smaller diameter. For biped robot arms where minimizing distal mass and radial envelope is critical, roller screws provide the required torque density that ball screws cannot match.
The inverted design allows the motor stator to wrap around the spinning nut, eliminating the need for parallel belt drives. This results in an ultra-compact inline actuator, keeping the mass closer to the shoulder and improving the arm's dynamic response.
A coarser lead (e.g., 5mm to 10mm) significantly improves backdrivability, which is essential for kinesthetic teaching, force control, and safe human-robot interaction. However, it requires higher continuous motor torque to hold payloads.
While possible, shoulder joints (pitch/yaw) often require full continuous rotation and 60-100 Nm of torque, making rotary Quasi-Direct Drive (QDD) or harmonic drives more common. Roller screws are best suited for elbow (pitch) and wrist linear actuation.
Ready to move beyond prototyping? Send us your arm joint geometry, expected payloads, and packaging constraints for a detailed roller screw selection.
