01
Linkage geometry sets thrust
For a static pose, axial force is joint torque divided by the effective perpendicular moment arm. That arm changes with elbow angle, so check the full range of motion.
Robotics application · Screening tool and design guide
Biped Robot Elbow Actuator Screw
Estimate screw thrust, torque, and speed from elbow torque and linkage geometry; check limits before design review.
Screen a linkage caseScreen an elbow linkage
Use a peak joint torque, effective perpendicular moment arm, actuator speed, candidate screw lead, and estimated screw efficiency. The calculator shows the conversion for one pose.
Static single-pose relation: joint torque = actuator force × r⊥.
What the result means
It estimates force and screw drive requirements for a single steady-speed case. It does not choose a diameter, predict life, or certify the actuator for impact or human contact.
Elbow actuator screw screening calculator
Convert a joint torque target at one linkage pose into estimated screw thrust, screw torque, and speed. The prefilled values are an editable example, not a recommended elbow design.
Enter the peak torque required for the pose being checked.
Measure perpendicular to the actuator line of action in the same pose.
Use the required speed along the screw axis for this motion case.
Use the lead of the candidate screw, not a nominal pitch assumption.
The 90% example is illustrative. Efficiency varies with screw design, load, speed, lubrication, and operating condition; replace it with supplier data when available.
Estimated axial thrust
312.5 N
Joint torque ÷ effective moment arm
Estimated screw torque
0.442 N·m
Includes the entered screw efficiency
Screw / nut speed
750 rpm
Does not include any separate gearbox ratio
Estimated input mechanical power
34.7 W
Steady speed estimate; excludes motor and drive losses
Screening only. Check the full force and speed profile, changing linkage leverage over range of motion, motor torque-speed and thermal limits, screw life, critical speed, buckling, bearings, and alignment before selecting hardware.
Request an engineering review
Opens an email draft containing the current inputs and screening result.
Worked example: convert one elbow pose
This reproduces the calculator's editable example inputs: 25 N·m joint torque, an 80 mm effective moment arm, 100 mm/s linear speed, 8 mm lead, and an estimated 90% screw efficiency. It is arithmetic for a hypothetical case, not a hardware test or product rating.
- Axial thrust
- 25 N·m ÷ 0.080 m = 312.5 N
- Screw torque
- 312.5 N × 0.008 m ÷ (2π × 0.90) ≈ 0.442 N·m
- Screw speed
- 100 mm/s ÷ 8 mm/rev × 60 = 750 rpm
- Estimated input power
- 312.5 N × 0.100 m/s ÷ 0.90 ≈ 34.7 W
Use the candidate screw's efficiency and check the full pose range, motion profile, motor losses, temperature, life, buckling, critical speed, bearings, and alignment before selecting hardware.
Key conclusions for an elbow screw actuator
Start with the joint and linkage requirements; choose the screw only after the full operating envelope is known.
02
Lead is a conversion input
Lead maps screw rotation to linear travel and contributes to drive torque. It does not by itself specify the assembled actuator's backdrivability.
03
Ratings are model-specific
Use the chosen supplier's dynamic and static ratings, efficiency, life, speed, and support conditions. Generic force thresholds cannot select a screw diameter.
04
Compare complete installed mass
Include the motor, screw, bearings, guides, linkage, and cable routing. A rotating-nut layout can help some packages, but does not guarantee a lighter or smaller elbow.
From elbow requirement to design check
These checks connect the calculator output to the engineering evidence needed for a candidate assembly.
| Design question | Implication | Verify next |
|---|---|---|
| Linear actuator drives a rotating elbow | Convert joint torque to axial thrust using the effective perpendicular moment arm at each pose. | Linkage geometry, full range of motion, force direction, acceleration, gravity, payload, and any near-zero moment arm. |
| Choosing a screw lead | Lead maps screw speed to linear speed and contributes to the torque needed to produce axial thrust. | Candidate lead, supplier efficiency data, required speed, motor torque-speed curve, and any separate reduction stage. |
| Backdrivability or contact control matters | Treat it as a property of the complete actuator and controller, not a lead-size shortcut. | Measured backdrive force or torque, motor cogging, transmission friction, sensing, controller, and safety strategy. |
| Long stroke, compression, or high cycle count | A force-to-torque estimate does not establish screw capacity or life. | Dynamic and static ratings, load spectrum, buckling, critical speed, lubrication, temperature, support bearings, and duty cycle. |
Compare actuator architectures by the same requirements
The tradeoffs below are screening questions, not universal rankings. Cost, life, and performance need supplier quotes and project data.
| Architecture | Potential fit | Tradeoff to evaluate |
|---|---|---|
| Roller screw with elbow linkage | Useful when a linear actuator can be placed remotely or along the arm and the linkage can provide the required motion. | Needs linkage design, guides, bearings, and pose-by-pose force and stroke checks. Ratings are model-specific. |
| Ball screw with elbow linkage | Can be screened with the same force, speed, package, and life requirements when a candidate product fits. | Compare the selected model’s dynamic and static ratings, life, speed, lubrication, and impact limits; do not infer performance from the screw type alone. |
| Rotary motor and gearbox at the joint | Avoids the linear actuator linkage and its changing moment arm. | Compare joint package, complete mass, reflected inertia, friction, efficiency, sensing, and impact behavior for the actual transmission. |
Evidence and method
The calculator uses basic linkage statics and screw kinematics. The sources below support the limits and checks; none provides a universal biped elbow design. Page published October 11, 2026; page and sources reviewed October 12, 2026.
Nook Industries — Linear Motion Engineering Calculators
Reviewed October 12, 2026
Lists manufacturer calculators for horsepower, critical speed, column strength, and unit conversion that support early screw screening.
Limit: These tools do not select a roller-screw model or validate this elbow. Confirm efficiency, ratings, support conditions, and operating limits with the chosen supplier.
Wensing et al. — Proprioceptive Actuator Design in the MIT Cheetah
Published 2017; reviewed October 12, 2026
Analyzes reflected rotor inertia and actuator transmission tradeoffs in dynamic legged robots; the paper notes reflected inertia scales with the square of rotary gear ratio.
Limit: This is legged-robot research on rotary transmissions, not a test of an elbow roller-screw actuator. A screw lead is not the same thing as a rotary gear ratio.
Moog — Planetary Roller Screw product information
Reviewed October 12, 2026
Shows that roller-screw lead, diameter, length, and load capacity are model and product-family specific.
Limit: Manufacturer family data is not a recommendation for this elbow. Obtain the candidate model drawing, ratings, and operating limits.
Risks and mitigations
Resolve these checks against the actual elbow layout and duty cycle before freezing the actuator design.
Peak load mistaken for a continuous rating
Supply peak and RMS force or torque over time, cycle frequency, and ambient conditions; check motor and screw thermal limits separately.
Linkage approaches a small moment arm
Sweep the geometry through the full range of motion and check peak thrust, required stroke, and any kinematic singularity.
Misalignment or transverse load reaches the screw
Use an appropriate guide and bearing arrangement, check alignment over stroke, and confirm allowable loads with the supplier.
Screw capacity inferred from this calculator
Select from supplier ratings and verify life, static load, buckling, critical speed, lubrication, and support conditions.
Evidence boundary: Published load ratings do not establish suitability for a particular robot. Use the force-time profile, CAD geometry, and supplier limits for the final decision.
Biped robot elbow actuator FAQ
Answers for first-pass sizing and supplier discussions.
Does screw lead alone determine elbow actuator backdrivability?
No. Lead sets travel per revolution and contributes to the force-to-torque relationship. Backdrive behavior also depends on screw friction, motor cogging, bearings, any gearbox, sensing, and control. Use measured data for the assembled actuator.
How do I convert elbow torque into linear screw thrust?
For a static single-pose estimate, divide the torque about the elbow by the effective perpendicular distance from the pivot to the actuator line of action. The moment arm changes with linkage pose, and a dynamic design also needs gravity, acceleration, payload, and losses.
Does this calculator select a screw diameter or service life?
No. It converts one pose and steady-speed case into first-pass thrust, screw torque, screw speed, and mechanical power. Diameter, dynamic life, static safety, buckling, critical speed, bearing life, and thermal limits require the selected supplier data and the complete operating profile.
Are inverted roller screws always lighter or more compact?
No. The rotating nut and translating shaft can fit some linear actuator layouts, but total installed mass and envelope depend on the motor, bearings, guides, linkage, stroke, and service access. Compare complete assemblies in CAD.
Can the screw carry the elbow linkage side load?
Do not assume it can. Design the mechanism and guides to control alignment and transverse loads, then confirm allowable loads with the selected screw and bearing supplier. A linkage drawing is needed to evaluate those loads.
When should I compare a ball screw or a rotary actuator?
Compare actual candidate assemblies against the same force or torque profile, speed, duty cycle, life, package, mass, and cost requirements. No technology is the default winner for every elbow layout.
Prepare an elbow actuator review
Include the force and speed profile, linkage CAD or effective moment-arm curve, stroke, duty cycle, candidate screw lead and diameter if known, mounting constraints, accuracy or backlash requirements, material or process constraints if known, and target service life.