Application guide - Updated July 25, 2026
Design ultra-compact, impact-resistant linear actuators for humanoid ankles. Navigate the trade-offs of short strokes, massive push-off forces, and critical backdrivability.
Estimate screw sizing and identify risks for your bipedal robot joints.
Range: 1-60 kN. Includes impact force during landing or jumps.
Continuous force for steady walking/standing. Must not exceed peak thrust.
10-400 mm
1-25 mm/rev
Screening output for robot actuator trade studies. Results update from the current inputs after validation.
Inverted architecture is strongly recommended for biped joints to minimize mass and axial length.
Use these values as a first RFQ baseline and confirm stroke, lead, mounting length, and anti-rotation details.
The calf provides very little axial space. Inverted roller screws, which wrap the motor around the nut, are usually the most practical choice for ankle integration.
The ankle absorbs the brunt of ground reaction forces during jumps and foot strikes. Thrust bearings and screw thread flanks must be sized for high impact.
Ankle strokes often sit in a short-stroke band; use 40-90mm as a screening assumption until the linkage geometry is frozen. Short travel concentrates wear and complicates lubrication distribution.
Compliant walking requires the ankle to be backdrivable. Coarse leads, often 5-10mm in early screens, improve this but raise the motor torque required for push-off.
Because the lower leg (calf) is typically slender, standard parallel-mount actuators are too wide, and inline standard actuators are too long.
Resolve these engineering decisions before freezing the ankle actuator architecture.
| Condition | Conclusion | Evidence | Limit | Next Step |
|---|---|---|---|---|
| Ankle envelope length < 150mm | Mandates an inverted roller screw architecture. | Inverted designs integrate the motor over the nut, providing the only viable high-force solution in extremely short envelopes. | Requires custom motor winding integration and tight thermal management. | Define absolute maximum length from clevis to clevis. |
| Force-controlled walking (compliant ankle) | Start with a coarse lead screen, often 5-10mm. | Coarser leads reduce the reflected inertia and friction felt at the joint, improving ground interaction transparency and backdrivability. | Increases the current/torque required from the ankle motor to hold position. | Check motor torque-speed curve against peak push-off requirement. |
| Jumping or running gait planned | Do not apply a universal ankle shock number; size from simulated or tested force-time traces, then apply an explicit shock margin. | Public humanoid ankle load data does not transfer cleanly across robot mass, foot compliance, gait controller, lever geometry, and landing task. The screen therefore treats peak thrust as a project input. | A 2.0x screening margin is only an early sizing guardrail and does not replace time-domain multibody simulation. | Simulate the drop-landing case and extract the peak force-time trace. |
Different ankle topologies require different actuator optimizations.
| Scenario | Typical Inputs | Recommendation | Caution |
|---|---|---|---|
| Ankle Pitch Actuator (Dorsiflexion/Plantarflexion) | Short stroke, often 40-90mm during early screens; peak force entered from the gait or landing force-time trace. | Compact inverted roller screw selected from a catalogue-supported diameter and lead family; coarse lead around 5-10mm when backdrivability matters; load cell near the rod end. | Watch for thermal buildup during continuous standing if the lever arm is short. |
| Ankle Roll Actuator (Inversion/Eversion) | Very short stroke (30-60mm), moderate force, balancing duty cycle. | Smaller inverted roller screw candidate selected from the same catalogue family. A finer lead can be acceptable if roll-axis transparency is less critical than pitch. | Ensure the two actuators (if used in a parallel platform) do not mechanically interfere inside the calf shell. |
Prepare these details to receive accurate custom sizing and CAD for your ankle actuator.
Distance from ankle pivot to actuator mount, required angular range of motion.
Peak push-off force, continuous standing force, maximum expected drop-landing shock force.
Maximum allowable diameter in the calf, maximum pin-to-pin length at full retraction.
Desired back-driving torque threshold, required linear speed during swing phase.
Key principles when selecting an actuator screw for bipedal ankles.
| Constraint | Implication for Roller Screw | Mitigation Strategy |
|---|---|---|
| Actuator is too long to fit in the calf | Selecting a standard planetary screw or failing to account for the encoder and brake in the length budget. | Commit to an inverted architecture early. Integrate the brake and encoder radially if axial space is exhausted. |
| Screw brinelling from drop landings | Using a ball screw or undersizing the roller screw based only on average walking loads. | Size the screw and thrust bearings from the maximum expected shock load and require a documented shock margin, not only an average walking load. |
| Thermal failure during continuous standing | Short lever arm requiring high continuous force to keep the robot upright, combined with a sealed calf shell. | Increase the lever arm if possible. Ensure the motor stator has a conductive heat path to the outer aluminum shell. |
Calculator values are screening outputs. Peak thrust is supplied by the user from simulation or test data, equivalent load uses duty-cycle weighting, and the dynamic-capacity target applies an early shock guardrail. Public sources support architecture and rating constraints, not a universal ankle load number.
Frames the humanoid ankle as a ground-interaction joint where actuator availability, mass distribution, compliance, and task requirements must be optimized together.
Constraint: This is a mechanism-design framework, not a validated load table for this buyer’s robot; force-time traces still need simulation or test data.
Supports the packaging claim that inverted roller screws reduce actuator dimensions by using the nut/roller set as the rotating assembly and the screw or push tube as the translating output.
Constraint: Catalogue ranges are product-family data, not humanoid ankle qualification. Confirm lead, stroke, bearings, and thermal path against the actual calf envelope.
Explains why roller screws are selected for high-load electromechanical actuators and why ratings must be checked across dynamic, static, life, and duty-cycle conditions.
Constraint: Catalogue static and dynamic ratings do not prove shock survival. Landing loads require the project force-time trace and bearing-stack validation.
Common inquiries regarding biped robot ankle actuators and roller screw integration.
Ankle actuators operate in the most confined envelope of the robot (the lower leg/calf) while being subjected to the highest direct impact loads from foot strike. They require a very short stroke but massive peak thrust, making standard actuators too long or too weak.
By wrapping the motor rotor directly around the elongated nut and using the screw shaft as the translating push-rod, the inverted roller screw eliminates the parallel motor mount and belt drive, fitting cleanly within the slender profile of a robotic lower leg.
Ankle joints typically require a very short stroke, often between 40mm and 90mm, depending on the lever arm to the foot and the required range of motion (dorsiflexion/plantarflexion).
Planetary roller screws distribute thrust across multiple threaded rollers, which makes them a stronger candidate than ball screws for shock-prone axes. However, the thrust bearings, housing, anti-rotation guide, and rod-end structure must still be validated against the project force-time trace.
The ankle is the primary interface with the ground. High backdrivability allows the robot to feel ground reaction forces, adapt to uneven terrain, and absorb small shocks compliantly without relying entirely on high-bandwidth active control.
A coarse lead (e.g., 5mm to 10mm) is typically preferred to improve backdrivability and reduce reflected inertia. However, this increases the torque demand on the motor during push-off, requiring a careful trade-off.
Because the screw shaft translates, it must be prevented from rotating. In ankle designs, this is often handled by integrating an anti-rotation guide into the lower leg structure or utilizing the linkage geometry itself if it provides sufficient torsional stiffness.
Use these adjacent pages to validate the ankle screw architecture, manufacturing path, and RFQ package before freezing the lower-leg design.
Send the stroke, force-time trace, calf envelope, lead target, and CAD constraints so engineering can return a focused sizing path.
