Leadscrew
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A leadscrew (also written lead screw, and sometimes called a power screw) converts rotary motion into linear motion using a threaded shaft and a matching internally threaded nut that slide directly against each other. That direct sliding contact is what separates it from a ball screw or a planetary roller screw, the other two mechanisms commonly used inside a linear actuator, both of which carry the load on rolling balls or rollers instead of sliding thread flanks. Leadscrews are the oldest and cheapest of the three families, and they remain common in 3D printers, low-cost linear stages, and medical instruments, even though humanoid robotics has largely bypassed them in favor of their rolling-contact cousins.
In brief: picture an ordinary bolt and nut. Spin the bolt while holding the nut so it cannot rotate, and the nut travels along the bolt instead of spinning with it. A leadscrew is that same idea refined for repeated, precise use: a specially shaped thread, a matching nut, and usually a motor to turn one of the two. It costs little to build, because the threads simply slide against each other, but that sliding contact is also why a leadscrew loses more energy to friction and wears faster than the ball- and roller-based screw types described below.
How it works
A leadscrew assembly has two parts: the screw, a shaft with a helical thread machined or rolled into it, and the nut, a mating internally threaded collar. Rotating the screw while preventing the nut from rotating drives the nut, and whatever is mounted to it, along the screw's axis; a motor typically turns the screw while the nut is fixed to a carriage or slide that is constrained, usually by separate guide rails or rolling element bearings, to move only in a straight line. The arrangement can also run in reverse, with the nut rotating and the screw held fixed to translate. Because there is no separate rolling element between the two threaded parts, the screw and nut bear directly on each other, and friction at that sliding interface produces both the leadscrew's main weakness (energy loss and wear) and one of its most useful properties (self-locking, covered below)[1][4].
Small leadscrew assemblies are very often paired with a stepper motor, since a stepper's discrete step angle combined with the screw's fixed lead gives repeatable positioning without necessarily needing a rotary encoder for closed-loop feedback. That open-loop simplicity is a large part of why leadscrew and stepper motor combinations are inexpensive to build into a product[19][21]. Higher-performance systems instead pair a leadscrew with a brushless DC motor or an AC servo motor and closed-loop position feedback, trading some of that simplicity for speed and accuracy.
Lead, pitch, and multi-start threads
Two dimensions describe a leadscrew's thread and are frequently confused with each other. Pitch is the axial distance from one thread crest to the next, while lead is the axial distance the nut travels in one full revolution of the screw[16]. For a single-start thread, one continuous helical groove, lead and pitch are the same number. Many leadscrews instead use a multi-start thread, in which two or more parallel helical grooves are cut into the same shaft; lead then equals pitch multiplied by the number of starts, so a double-start thread moves the nut twice as far per revolution as a single-start thread of the same pitch[16][18]. Multi-start threads let a designer raise linear speed for a given motor speed while keeping the individual threads fine and easy to cut, rather than cutting one very coarse, deep single-start thread. As explained below, however, a larger lead also pushes the screw toward the point where it can no longer hold a load on its own.
The angle the thread helix makes with a plane perpendicular to the screw's axis is called the lead angle. It equals the arctangent of the lead divided by pi times the thread's pitch diameter, and it is the single most important number for predicting a given leadscrew's efficiency and whether it will self-lock[15].
Thread forms
Leadscrews are cut with one of a handful of standardized thread profiles, each trading off machining cost, friction, and strength differently.
| Thread form | Thread angle | Friction and efficiency | Strength and typical use |
|---|---|---|---|
| Square | 0 degrees (flanks parallel to the screw axis) | Lowest friction and highest efficiency of the common forms | Weakest at the root, and the hardest and most expensive to cut, since the flanks cannot be ground with a standard angled wheel; largely displaced by ACME and trapezoidal threads outside specialty uses[5][12][13] |
| ACME (trapezoidal, US/ASME) | 29 degrees | More friction than square; commonly cited around 20 to 40 percent efficiency, higher at steep lead angles | Angled, thicker flanks make it stronger at the root than a square thread and much easier to cut and repair; carries load well in either direction, which is why it is the default choice for a general-purpose leadscrew[3][5][12] |
| Metric trapezoidal (Tr, DIN 103) | 30 degrees | Functionally similar to ACME | The European/ISO counterpart to ACME; similar geometry but not directly interchangeable with ACME threads because of differing angle, pitch series, and tolerances[14] |
| Buttress | Load flank about 3 to 7 degrees off perpendicular to the axis; clearance flank about 33 to 45 degrees | Low friction on the loaded flank, close to square-thread efficiency | Asymmetric profile with a wide, nearly radial load flank gives roughly double the shear strength of a square thread, but only in one axial direction; used in jacks, presses, and other equipment loaded in a single direction[12][13] |
A fifth option, the standard 60-degree V-thread used on ordinary fasteners, occasionally appears in miniature leadscrews for lathes and mills, but it is a poor general choice: V-threads are shaped to wedge tight and resist loosening, which is the opposite of what a leadscrew's thread and nut want to do[5].
Efficiency and self-locking
Leadscrew efficiency, the ratio of the ideal frictionless torque needed to move a given load to the torque actually required, is consistently the lowest of the three common screw-drive types. It is a function of lead angle, thread angle, and the coefficient of friction between the screw and nut materials, not a single fixed number[1][2][4]. Published figures vary with the specific assumptions behind them, and sources do not fully agree on the typical range: linear-motion supplier PBC Linear cites 20 to 80 percent for leadscrews generally, against about 90 percent for ball screws[1]; actuator maker Tolomatic narrows this to 20 to 40 percent for ACME leadscrews, against about 90 percent for ball screws and about 85 percent for roller screws[3]; and an engineering summary citing the textbook Shigley's Mechanical Engineering Design puts square-thread leadscrew efficiency at 25 to 50 percent over typical lead angles[5]. The consistent pattern across these sources is that a larger lead angle raises efficiency, up to a point.
That "up to a point" is the other defining leadscrew property: self-locking. A leadscrew is self-locking (meaning an axial force pushing on the nut cannot spin the screw backward on its own) when the coefficient of friction between screw and nut is greater than or equal to the tangent of the lead angle, or equivalently, when the lead angle is smaller than the friction angle (the arctangent of the coefficient of friction)[2][4][5]. Linear-motion vendor Roton illustrates the tradeoff using a bronze nut with a coefficient of friction of 0.15: at an 8-degree lead angle the screw is self-locking (45 percent forward efficiency, and active torque is needed just to lower a load), while at a 20-degree lead angle with the same materials the screw is no longer self-locking (65 percent forward efficiency, and it back-drives under load)[2]. PBC Linear gives a similar illustration with a 10 mm diameter screw: a 2 mm lead yields about 41 percent efficiency and holds its position, while a 25 mm lead on the same screw diameter reaches about 83 percent efficiency but back-drives[1]. As a rule of thumb, screws with forward efficiency above roughly 50 percent tend to back-drive, so most catalog leadscrews are deliberately specified with a low enough lead angle to stay self-locking[1].
Self-locking is a genuine advantage wherever a mechanism must hold position without continuous power: jacks, vertical lift stages, and clamps have relied on it for well over a century. It also means that, unlike a ball screw or planetary roller screw, a leadscrew driven without a separate gearbox or brake generally cannot be back-driven by pushing or pulling on the load side, since ball and roller screws use rolling friction and are not self-locking at ordinary lead angles[2]. That same friction, however, caps the leadscrew's efficiency and, over enough cycles, wears the thread flanks and the nut.
Nut materials and backlash
The nut's material has a large effect on a leadscrew's friction, load rating, and service life. Bronze, and less often other brass or bronze alloys, is a traditional choice: it is self-lubricating and wear resistant, and it handles higher loads and temperatures than plastic, but it has a higher coefficient of friction than plastic and generates more heat in continuous use[17][18]. Plastic nuts, most commonly acetal (POM) or nylon and sometimes filled with PTFE for lower friction, are common in lighter-duty and cost-sensitive designs: they are cheap, quiet, and self-lubricating enough to run dry, but they carry less load and tolerate less heat than metal[17][18][20].
Because a leadscrew nut needs some clearance to thread onto the screw at all, an unmodified nut has backlash, a small amount of axial play before the threads re-engage when the direction of travel reverses. That may not matter for a hobby project, but it degrades repeatability in any application that reverses direction and must return to the same position, such as a 3D printer or a lab instrument. Anti-backlash nuts solve this with two nut sections, commonly both plastic, preloaded against each other, usually by a spring, so that opposing thread flanks stay in contact regardless of direction[17][20].
Common applications
Leadscrews driven by stepper motors are a mainstay of desktop fused-deposition 3D printers, most often on the vertical Z axis and occasionally on the X and Y axes as well, where their higher precision and lower backlash compared with a belt-and-pulley drive translate into better layer alignment and surface finish, at some cost to top speed[19]. In laboratory and bench automation, leadscrews position stages, syringe pumps, and dosing systems; because a leadscrew's lead can be made very fine, down to roughly 1 millimeter of travel per revolution or finer, they are used to meter drug delivery down to microliter volumes and to move medical imaging components, in X-ray, CT, and similar equipment, with sub-millimeter precision[20]. They also remain the default mechanism in vises, jacks, presses, and other manually or lightly motorized equipment where self-locking is wanted and the load or duty cycle does not demand a ball or roller screw's efficiency and lifespan[5][12][13].
Use in humanoid robots
Of the three screw-drive families used in robotics, the leadscrew is the least significant for humanoid robots' primary structural joints, and by a wide margin. Humanoid legs and arms need linear actuators that survive large, repeated shock loads (a heel strike can deliver two to three times body weight in a fraction of a second) at high duty cycles of thousands of steps per hour, while keeping actuator mass down[8]. Sliding thread contact is a poor match for that combination. One actuator maker's engineering comparison put typical specific force (load capacity per unit of actuator weight) at roughly 300 to 800 newtons per kilogram for a leadscrew, well behind the roughly 800 to 2,000 newtons per kilogram cited for a ball screw and the roughly 3,500 to 5,000 or more newtons per kilogram cited for a planetary roller screw, and it rated leadscrews unsuitable for humanoid structural joints on that basis[8]. The friction that makes a leadscrew self-locking also concentrates wear at the sliding thread interface under repeated load, unlike a ball or roller screw's rolling contact[1][6][8].
Even ball screws face a related problem in this setting: their recirculating balls touch the raceway at a single point, so a sudden shock load like a heel strike can locally exceed the material's yield strength and leave a permanent dent, a failure mode called brinelling. Repeated impacts accumulate these dents over thousands of steps, degrading accuracy and eventually the raceway itself[6][8]. A planetary roller screw's threaded rollers instead contact the nut and screw along a line rather than a point, spreading the same load over a much larger area and keeping peak stress below the damage threshold[6][7][8]. That shock tolerance, combined with higher load capacity and longer service life, is why manufacturers of high-end humanoid platforms have converged on planetary roller screws for leg and arm joints despite their higher manufacturing cost. Tesla presented planetary roller screws as its choice for the 14 linear joints in early Optimus prototypes, alongside 14 harmonic-drive rotary joints, and industry reporting describes that configuration continuing into later Optimus generations[9][10]. Trade press covering humanoid actuator design generally describes other developers, including Figure AI and Boston Dynamics, as following the same pattern of pairing rolling-element screws with electric linear actuators for leg and arm joints, rather than adopting sliding-contact leadscrews for that role[7][8][10].
Leadscrews have not disappeared from humanoid robots entirely; they simply retreat to places where the force is small and cost or size matters more than efficiency. The clearest example is finger and tendon actuation inside a dexterous hand. Trade press reported in 2026 that Tesla's Optimus Gen 3 hand redesign was moving toward a combination of a gearbox, a leadscrew, and a tendon-drive stage, specifically to pull tendons in a straight line rather than winding them onto a spool, which had caused tendon fatigue from repeated tight bending in earlier versions. The same reporting described a hand bill-of-materials target of under 3,000 dollars, down from about 12,000 dollars, and a durability target of 2,000 operating hours versus roughly 500 hours measured on an earlier design[11]. That is a natural fit for a leadscrew's strengths: finger forces are a small fraction of a hip or knee joint's, self-locking can help a gripping end effector hold force with the motor turned off, and low part count and cost matter more than squeezing out extra efficiency. The Gen 3 hand upgrade also roughly doubled per-hand degrees of freedom, from 11 to 22, in the same generation[10].
| Property | Leadscrew | Ball screw | Planetary roller screw |
|---|---|---|---|
| Contact between screw and nut | Sliding, thread flank on flank[1][6] | Rolling, point contact via recirculating balls[6][7] | Rolling, line contact via threaded rollers[6][7] |
| Typical efficiency | About 20 to 40 percent, up to roughly 80 percent at high lead angles[1][3][5] | Roughly 80 to 95 percent[1][3][6] | Roughly 75 to 98 percent, depending on source and preload[3][6][7] |
| Relative load capacity for a given size | Lowest of the three[3][8] | Higher than a leadscrew; limited by ball size at small diameters and short leads[7] | Highest; roller-screw actuators are rated to several times the thrust of similarly sized ball-screw actuators in supplier catalogs[25] |
| Behavior under repeated shock load | Not rated for repeated shock; wear accelerates | Point contact is prone to brinelling (denting) under repeated impact[6][8] | Line contact spreads load and resists brinelling; preferred for humanoid gait loads[6][8] |
| Self-locking / back-drivable | Often self-locking; generally will not back-drive if the lead angle is below the friction angle[2][4][5] | Not self-locking; back-drives freely[2] | Not self-locking; back-drives freely[2] |
| Relative manufacturing cost | Lowest[1][3] | Medium[3] | Highest[3][8] |
| Typical role in a humanoid robot | Rare; at most small finger or tendon actuators[10][11] | Occasional lower-cost or lower-shock designs | Preferred for primary leg, arm, and torso joints in high-end platforms[8][9][10] |
Suppliers and product landscape
Because leadscrews are simple, well-standardized parts, they are made both by large, diversified motion-control manufacturers and by smaller firms that specialize in leadscrews and their nuts. THK sells leadscrew nuts built to the 30-degree metric trapezoidal standard alongside its better-known ball screw and linear guide lines[22]. HIWIN, likewise best known for ball screws and linear guideways, also lists trapezoidal leadscrews in its catalog[26]. Ewellix, the former SKF linear-motion division that became an independent company in 2018 and was acquired by Schaeffler in 2023, sells electric cylinders that can be configured with either a ball screw or a leadscrew depending on the load and cost target[23][24].
Alongside these larger suppliers, a cluster of specialist manufacturers, including Thomson Industries, PBC Linear, Nook Industries, Roton Products, and Helix Linear Technologies in the United States, and igus in Germany, which sells plastic-on-metal leadscrew and nut combinations under its DryLin brand, build leadscrews, nuts, and pre-assembled linear actuators as their primary business. They compete mainly on precision, backlash specification, and lifetime rather than raw thrust[1][2][17][19].
See also
References
- PBC Linear, "What Is Lead Screw Efficiency in Linear Motion?" https://pbclinear.com/blogs/blog/what-is-lead-screw-efficiency-in-linear-motion ↩
- Roton Products, "Screw Backdriving Efficiency," Screw University. https://www.roton.com/screw-university/screw-actions/screw-backdriving-efficiency/ ↩
- Tolomatic, "Selecting the Optimal Screw Technology" (ACME, ball, and roller screw comparison). https://www.tolomatic.com/info-center/resource-details/acme-ball-roller-screw-selection/ ↩
- RoyMech, "Power Screw Torque and Efficiency Equations." https://www.roymech.co.uk/Useful_Tables/Cams_Springs/Power_Screws_1.html ↩
- Wikipedia, "Leadscrew," citing Shigley's Mechanical Engineering Design and Bhandari's Design of Machine Elements. https://en.wikipedia.org/wiki/Leadscrew ↩
- Machine Design, "What's the Difference Between Roller and Ball Screws?" https://www.machinedesign.com/mechanical-motion-systems/article/21175536/whats-the-difference-between-roller-and-ball-screws ↩
- The Robot Report, "Roller Screws Unlock Peak Performance in Robotic Applications." https://www.therobotreport.com/roller-screws-unlock-peak-performance-in-robotic-applications/ ↩
- Firgelli, "Humanoid Robot Actuators: The Complete Engineering Guide." https://www.firgelli.com/pages/humanoid-robot-actuators ↩
- KGGFA, "Another Look at the Tesla Robot: The Planetary Roller Screw." https://www.kggfa.com/news/another-look-at-the-tesla-robot-the-planetary-roller-screw/ ↩
- RoboticsTomorrow, "Linear Actuators vs Rotary Actuators: The Core Choice for Humanoid Robot Joints." https://www.roboticstomorrow.com/article/2025/10/linear-actuators-vs-rotary-actuators-the-core-choice-for-humanoid-robot-joints/25703 ↩
- RobotToday, "Tesla's Optimus Hand Undergoes Redesign: Gen-3 Robot May Adopt Gearbox, Lead Screw, and Tendon Drive." https://robottoday.com/article/tesla-s-optimus-hand-undergoes-redesign-gen-3-robot-may-adopt-gearbox-lead-screw-and-tendon-drive ↩
- Regal Cutting Tools, "Where Acme and Buttress Thread Forms Are Used and Why." https://www.regalcuttingtools.com/learning-center/articles/where-acme-buttress-thread-forms-are-used-and-why ↩
- WayKen, "Types of Threads and Their Geometric Parameters." https://waykenrm.com/blogs/types-of-threads/ ↩
- Rolled Threads, "What Is the Difference Between Acme and Trapezoidal Threads?" https://www.rolledthreads.com/what-is-the-difference-between-acme-and-trapezoidal-threads/ ↩
- Helix Linear Technologies, "Formula for Lead Angle," Engineering Calculator. https://www.helixlinear.com/engineering-calculator/formula-for-lead-angle/ ↩
- Thomson Linear, "Ball Screws: Lead vs Pitch." https://www.thomsonlinear.com/en/training/ball_screws/lead_vs_pitch ↩
- SDP/SI, "Anti-Backlash Supernuts, Lead Screw Nuts." https://shop.sdp-si.com/products/linear-guide-systems/lead-screws-lead-screw-nuts/lead-screw-nuts-anti-backlash-supernutsr.html ↩
- iQS Directory, "Types, Materials, and Benefits of Lead Screws." https://www.iqsdirectory.com/articles/ball-screw/lead-screws.html ↩
- Helix Linear Technologies, "How Are Lead Screws Used in a 3D Printer?" https://www.helixlinear.com/blog/how-are-lead-screws-used-in-a-3d-printer ↩
- PBC Linear, "Lead Screw Precision in Medical Applications." https://www.pbclinear.com/Blog/2019/November/Lead-Screw-Precision-in-Medical-Applications ↩
- AutomationDirect, "Linear Actuator Options." https://library.automationdirect.com/linear-actuator-options/ ↩
- THK, "Lead Screw Nut," Other Power Transmission Elements. https://www.thk.com/?q=us%2Fnode%2F5213 ↩
- DirectIndustry, "Electric Servo-Cylinder, CASM Series, Ewellix." https://www.directindustry.com/prod/ewellix/product-183264-1814158.html ↩
- Schaeffler Group, "Schaeffler Further Strengthens Industrial Business With Purchase of Ewellix Group," press release. https://www.schaeffler.com/en/media/press-releases/press-releases-detail.jsp?id=87843777 ↩
- Tolomatic, "How Roller-Screw and Ball-Screw Actuators Compare in High-Force Applications." https://www.tolomatic.com/info-center/resource-details/how-roller-screw-and-ball-screw-actuators-compare-in-high-force-applications/ ↩
- HIWIN Corporation, product and catalog resources. https://hiwin.com/resources/catalogs/ ↩
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