Application guide - Updated July 19, 2026
Design compact, impact-resistant linear actuators for humanoid and biped robots. Evaluate equivalent load and backdrivability for hip, knee, and ankle joints.
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.
Biped limb designs strictly constrain axial length. Inverted roller screws wrap the motor rotor over the nut, offering the shortest possible envelope.
Published running data commonly places vertical ground-reaction force around 1.5x-3.0x body weight. Robot jumps, hard stops, and falls need separate peak load cases before screw and bearing approval.
Screw lead directly affects mechanical impedance. A coarser lead (e.g., 5-10mm) reduces back-driving torque and reflected inertia, essential for force-controlled compliant walking.
Placing the actuator mass closer to the proximal joint (e.g., hip) reduces distal inertia, improving walking efficiency and control bandwidth.
Choosing between an inverted and standard planetary roller screw defines the shape and performance of the robot limb.
Use the calculator for first-pass sizing, then resolve these engineering decisions before freezing the actuator architecture.
| Condition | Conclusion | Evidence | Limit | Next Step |
|---|---|---|---|---|
| Axial envelope below 160 mm | Start with inverted planetary roller screw architecture. | Motor and nut integration can reduce actuator length while keeping the rod/push tube on the joint axis. | Bearing stack, encoder, brake, cable routing, and anti-rotation guide may consume the saved length. | Share motor OD, stroke, and joint lever geometry. |
| Repeated landing or jump load | Treat peak thrust as a separate shock case. | Running and landing loads are not the same as continuous walking load, so C rating and bearing life need their own peak profile. | A generic multiplier is only a screen; it is not a validation test. | Provide force-time trace, repetition count, and controller trip limits. |
| Force control or compliant walking | Review lead, friction, preload, and reflected inertia together. | Backdrivability and transparency are central actuator requirements for dynamic legged robots. | Very coarse leads increase motor torque and may require brakes or active holding logic. | Compare 5, 8, and 10 mm lead options against motor current. |
| Long stroke or high continuous thermal load | A standard planetary screw can be the better production fit. | Standard layouts provide more freedom for bearings, lubrication, and heat rejection when length is available. | Parallel motor layouts increase width and can add belt compliance. | Check limb width, belt path, and maintenance access. |
The same screw family behaves differently at the ankle, knee, and hip because stroke, cooling, and distal inertia are not equal.
| Scenario | Typical Inputs | Recommendation | Caution |
|---|---|---|---|
| Ankle push-off actuator | 50-90 mm stroke, high peak force, low continuous average | Prefer inverted layout, coarse lead review, strong sealing, and hard landing telemetry. | Reject if brake, bearing stack, or toe package cannot fit inside the ankle envelope. |
| Knee extension actuator | 120-180 mm stroke, balanced peak and duty cycle | Compare inverted and standard layouts; thermal duty and distal inertia usually decide. | Reject undersized screws selected only from peak thrust without equivalent-load history. |
| Hip pitch/roll actuator | 150-260 mm stroke, higher continuous force and better cooling path | Standard planetary screw may win if the hip shell can absorb added length or belt width. | Reject layouts that move too much mass into the thigh or reduce control bandwidth. |
These inputs turn the calculator output into a design review that can select diameter, lead, preload, and bearing support.
Lever ratio, joint range, stroke, and hard-stop positions.
Peak force, continuous force, landing pulse width, repetition count.
Linear speed, acceleration, dwell time, and gait duty cycle.
Motor OD, actuator length, limb width, bearing stack, cable exits.
Life hours, cycles, backlash, stiffness, temperature, inspection plan.
Key principles when selecting an actuator screw for bipedal robotics.
| Constraint | Implication for Roller Screw | Mitigation Strategy |
|---|---|---|
| Impact Loading | Published human running data commonly sits around 1.5x-3.0x body weight; robot landing and fall cases can differ materially from human gait. | Use measured or simulated force-time profiles. Apply 1.5-2.0x shock factors only for early screening, then verify screw rating, thrust bearing capacity, housing stiffness, and controller limits. |
| Force Control / Compliance | Robot must sense and yield to external forces without breaking. | Compare lead options, preload, sealing friction, brake strategy, and motor current. A 5-10 mm lead is a starting comparison range, not a universal answer. |
| Space Constraints | Actuator must fit inside a sleek leg profile. | Use inverted planetary architectures to wrap the motor around the nut. |
| Inertia Minimization | Heavy lower legs swing slowly and consume excess energy. | Move actuators proximally, near hips/knees, and use lightweight materials where possible. |
Usage: Supports separating walking, running, and landing load cases; running vertical ground-reaction force is commonly reported around 1.5-3.0x body weight.
Limitation: Human running data is not a robot qualification load; foot geometry, control policy, and fall recovery can move robot peaks outside this range.
Usage: Documents planetary roller screw strengths such as high load ratings, long life, high cycling, and shock-load resistance.
Limitation: Catalogue ratings do not replace project-specific load history, lubrication, preload, mounting, or thrust bearing verification.
Usage: Supports inverted architecture claims around compactness, high load capability, rotor integration, and direct push-tube use.
Limitation: Product-note language still needs custom envelope review for the motor, bearings, encoder, seals, and anti-rotation guide.
Usage: Supports the robotics-side priorities of torque density, high backdrivability, transparency, and variable impedance in dynamic legged locomotion.
Limitation: The MIT example is a rotary actuator design method; screw lead and friction assumptions must still be validated for each linear actuator.
The tool is intentionally conservative for early engineering triage, but it is not a release calculation.
Pre-RFQ architecture choice, rough dynamic capacity, peak torque order-of-magnitude, and risk discovery.
Final life rating, safety certification, bearing stack approval, buckling approval, or controller stability validation.
Joint lever ratio, force-time history, velocity profile, duty cycle, life target, package envelope, motor limits, lubrication, and environment.
These are the common failure modes that should be closed before a biped actuator screw moves from screen to sample order.
| Risk | Trigger | Mitigation |
|---|---|---|
| Landing peak exceeds screw or bearing margin | Peak thrust above 30 kN, jump mode, fall recovery, or hard stops | Use measured/simulated force-time data, apply shock factor only as a screen, and verify thrust bearing impact capacity. |
| Low backdrivability hurts force control | Fine lead, high preload, high friction seals, or large reflected inertia | Compare multiple leads, use motor current limits, review brake strategy, and validate impedance response on a rig. |
| Thermal limit reached before load-life target | High continuous force, high walking duty cycle, or sealed limb shell | Model motor copper loss, screw friction, grease temperature, and cooling path before prototype freeze. |
| Packaging conflict erases inverted-layout benefit | Late encoder, load cell, bearing, cable, or guide changes | Reserve radial and axial integration zones in CAD before committing to screw diameter and lead. |
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 limbs without protruding.
Planetary roller screws are a strong candidate because load is shared across many rolling contacts, but survival is never automatic. The screw, thrust bearings, housing, lubrication, and controller limit logic all need to be checked against measured or simulated landing loads.
Backdrivability is the ability of external forces on the joint to drive the motor backwards. In biped robots, this is essential for force control, compliance, and safe interaction with the environment. Using a screw with a sufficiently coarse lead (e.g., 5mm to 10mm) and high efficiency improves backdrivability.
Ball screws can be appropriate for lighter laboratory prototypes, but roller screws are usually preferred when peak load, compactness, shock margin, and high duty cycle dominate the trade. Final selection should compare life, preload, bearing support, cost, and availability.
Proper preloading, thread locking compounds on static mounts, and integrating anti-rotation mechanisms directly into the linear guide system are standard practices for high-vibration robotic applications.
No. It is an early screening tool for architecture and load magnitude. Final sizing still needs equivalent-load history, rotational speed, L10 or project life target, lubrication, preload, thrust bearing life, housing stiffness, and thermal checks.
No. Coarser leads can reduce reflected mechanical impedance, but they also raise motor torque demand and may reduce holding capability. The useful lead range depends on motor constant, gear ratio, brake strategy, controller bandwidth, and safety requirements.
A standard screw is often better when axial length is available, heat rejection is more important than minimum package length, or the actuator can use a parallel belt or gearbox layout without increasing distal limb inertia.
Provide peak and continuous force, stroke, speed, duty cycle, joint lever ratio, expected gait modes, fall or jump cases, target life, package envelope, environment, backlash target, and motor or drive limits.
Treat them as separate load cases. Use force-plate testing, multibody simulation, or prototype telemetry to define peak thrust, load duration, repetition count, and controller trip limits before approving a production screw.
Yes, but integration has to be planned early. Encoder routing, load-cell placement, anti-rotation guidance, seals, and bearing stack access can consume the space saved by the inverted architecture if left until late design.
Explore the core roller screw technologies that enable high-performance robotic joints.
Send the calculator output with joint geometry, load profile, package envelope, and target life so diameter, lead, preload, and bearing support can be reviewed together.
