Screw-driven linear actuators convert rotary motor torque into linear force through a mechanical advantage set by the screw lead and nut geometry, and that mechanical advantage alone does not buy a safety margin — the margin lives in how much torque the servo motor can deliver above the steady-state load, and how much of that surplus the screw and nut can transmit without back-driving.
Across a representative design sweep, ball-screw actuators reach roughly 2,500 N/kg of actuator mass while a lead screw sits near 600 N/kg (per [S10] actuator technical reference, 2026), and roller-screw actuators sit above that band where the duty cycle and shock load justify the larger nut envelope (per [S2] Tolomatic engineering blog, 2025; per [S4] Motion Control & Motor Association industry insight, 2025).
Torque margin defined against duty cycle, not peak load
Torque margin is the ratio between the continuous-rated motor torque available at the PLC demand output and the root-mean-square torque the actuator actually consumes over a working cycle, and it must be sized against the worst-case duty profile rather than the absolute peak force (per [S10] actuator sizing reference, 2026). When the calculated demand reaches 33% of a motor's continuous rating while the motor is only rated for 20% continuous duty, the motor overheats and the actuator stalls — the published catalog rating, taken as a peak number, is what fails first (per [S10] worked example, 2026). The fix is mechanical, not parametric: a high-duty BLDC-plus-ball-screw package rated for 50% continuous is the component that closes the loop, not a different controller gain (per [S10] sizing reference, 2026).
Ball-screw specifics: efficiency gains come with a back-drive tax
Ball screws sacrifice torque to friction in their construction, and they have an extremely low static load capacity — the same screw can become nearly free-floating once the drive motor loses power (per [S1] Wikipedia linear-actuator entry, accessed 2026-06-11). For vertical or inclined axes the consequence is direct: loss of motor torque puts a planetary roller-screw actuator into free fall, and a ball-screw actuator can back-drive the moment the holding torque drops below the load-induced axial force (per [S4] automate.org industry insight, 2025). Ball-screw designs also run cooler than lead screws in comparable duty because the lower friction reduces waste heat (per [S3] Progressive Automations, 2025), but the cooling gain does not buy back-drive safety, it only buys longer continuous-duty endurance.
Roller-screw and planetary roller: torque margin bought with envelope
Planetary roller screws multiply contact area by surrounding the central lead screw with several satellite rollers, which raises the thrust-force capacity per unit nut length and lets the designer widen the torque margin without increasing motor frame size (per [S4] automate.org, 2025). The same geometry forces a larger nut envelope than a ball screw of equivalent lead because the satellite rollers enlarge the nut outside diameter, and the trade-off is acceptable for repetitive, high-force, long-life applications where the additional rotating mass and inertia can still be absorbed by the drive (per [S2] Tolomatic, 2025; per [S4] automate.org, 2025). For high-force work the actuator and machine footprint will be materially larger than a ball-screw equivalent, which is a real cost in a constrained envelope (per [S8] Tolomatic resource, 2025).
Lead-screw and acme: brute-force margin, lower speed
Acme threads hold a "very high static load capacity" because sliding friction between the nut and the screw resists back-driving mechanically, while ball screws trade that resistance for efficiency and so collapse to near-zero holding load the instant the motor torque drops (per [S1] Wikipedia linear-actuator entry, accessed 2026-06-11). The static load capacity of any screw-driven actuator is fixed at design time by the thread pitch and nut geometry, and it cannot be dynamically adjusted mid-cycle without a secondary brake or lock mechanism (per [S1] Wikipedia linear-actuator entry, accessed 2026-06-11). Screw-driven actuators in general convert the rotary motion of a stepper, BLDC, servo, or DC motor into guided linear motion through a recirculating or sliding contact set (per [S7] igus Engineer's Toolbox, 2025), and the choice of contact set is what determines the back-drive penalty attached to each gain in efficiency.
Criteria comparison: ball, roller, and lead screw on torque-margin drivers
The four decision criteria that drive torque margin on screw-driven actuators are specific force per kg, back-drive resistance, duty-cycle tolerance, and envelope size, and the three screw families separate cleanly on each axis (per [S10] worked data, 2026; per [S1] Wikipedia, 2026-06-11; per [S2] Tolomatic, 2025; per [S4] automate.org, 2025; per [S5] Tolomatic, 2025). On specific force, ball screws hit ~2,500 N/kg, lead screws ~600 N/kg, and roller screws sit at the top of the band (per [S10] worked data, 2026). On back-drive resistance, lead/acme is highest because sliding friction is self-locking, ball screw is lowest and can be free-floating (per [S1] Wikipedia, 2026-06-11), and roller screw falls between but still back-drives the moment motor torque collapses (per [S4] automate.org, 2025). On duty-cycle tolerance, ball screws handle high duty cycles and moderate speeds with low heat rise (per [S3] Progressive Automations, 2025; per [S5] Tolomatic, 2025), while roller screws extend the same low-friction efficiency into higher force and longer life (per [S2] Tolomatic, 2025). On envelope, ball screw is the most compact, planetary roller is the largest because the satellite rollers enlarge the nut OD (per [S4] automate.org, 2025). The numeric specific-force band is the only column the research numbers explicitly; the other three columns remain qualitative because the sources do not assign percentage shares.
Worked sizing: humanoid knee actuator and the 33% / 20% failure
A standard sizing pass on a 70 kg humanoid robot with a 2.5x impact load and a 2.0 kg per-actuator mass budget starts at a peak force of 3,000 N (150 Nm torque ÷ 0.05 m arm), then divides by 2.0 kg to set a 1,500 N/kg minimum specific-force floor — a lead screw at ~600 N/kg fails the check outright, a ball screw at ~2,500 N/kg passes the force check, and the impact-load question is what pushes the selection toward a roller screw (per [S10] technical reference, 2026). The other half of the margin check is thermal: a motor whose continuous duty rating is 20% cannot be loaded to 33% RMS demand, and the high-duty BLDC-plus-ball-screw package rated for 50% continuous is the component that closes the loop (per [S10] worked example, 2026). The two checks interact: a screw that survives the static specific-force number can still stall the motor on the duty cycle, which is why "margin" on a screw-driven actuator has to be reported as two numbers, not one.
Application fit: who picks which screw family
Ball-screw linear actuators are the default for lab automation, robotics, and precision assembly where smooth, repeatable positioning and the ability to stop at exact coordinates on every cycle matter more than peak holding force (per [S9] JLCMC comparison guide, 2025), and they are also the popular pick for moderate-speed, high-duty-cycle industrial axes where the price-to-performance ratio is the deciding factor (per [S5] Tolomatic, 2025). Roller-screw and planetary roller-screw actuators are the fit for high-force, repetitive, long-life industrial axes such as press rams, valve actuation, and heavy material handling, where the larger nut envelope is acceptable (per [S2] Tolomatic, 2025; per [S4] automate.org, 2025; per [S8] Tolomatic, 2025). Lead-screw and acme actuators keep their slot where the duty cycle is light, the speed is low, and self-locking behavior under power loss is worth more than efficiency (per [S1] Wikipedia, accessed 2026-06-11; per [S3] Progressive Automations, 2025). A linear-motor alternative enters the picture when the application can tolerate harder sealing and the contamination profile is aggressive, because a properly sealed linear-motor bearing set can withstand more contamination than a ball-screw assembly (per [S6] linearmotiontips.com, 2025).
Limits, failure modes, and signals to track
The dominant failure mode on a ball-screw-driven vertical axis is back-driven free fall the moment the holding torque collapses, and on a roller-screw axis the same collapse initiates a free-fall condition because reduced resistance to motion is inherent to the geometry (per [S4] automate.org, 2025; per [S5] Tolomatic, 2025). The dominant thermal failure is motor stall when RMS demand exceeds the motor's continuous-duty rating, which is what the 33% / 20% mismatch in [S10] demonstrates for a standard BLDC package. Trackable signals: catalog continuous-duty percentage versus measured RMS demand, the screw family's published static load capacity against the worst-case vertical load, and a back-drive holding test on every prototype axis before the safety brake is approved. The pressure transmitter reading on any hydraulic load-sense line, the flow-meter pulse on the actuator cooling or seal-leak circuit, and the industrial-valve position feedback on a downstream hydraulic actuator are the cross-domain signals worth wiring into the same alarm log during commissioning, because each one independently flags a margin problem on the screw-driven axis that drives it.
Two trackable signals worth wiring into the next design review: (a) a continuous-duty percentage line item on every motor datasheet used in the BOM, matched against the calculated RMS torque demand for the actual motion profile; (b) a static-load-capacity line item from the screw vendor, matched against the worst-case vertical or inclined load with a documented back-drive holding test result. Both numbers are now standard columns in the sizing worksheets published by actuator OEMs (per [S2] Tolomatic, 2025; per [S10] technical reference, 2026), and both should land on the engineering release before any safety brake sizing is finalized — a back-drive holding test result without a continuous-duty percentage is half a margin check, and a continuous-duty percentage without a static-load capacity is the other half.