Ball screw

A ball screw is a mechanical linear-motion device that converts the rotary motion of a shaft into precise straight-line motion, or the reverse, by running a recirculating train of hardened steel balls between the matching helical grooves of a threaded screw shaft and a nut.^[1][2] Because the balls roll instead of slide, a ball screw carries load at a fraction of the friction of a plain threaded rod, which is why it has become a default way to turn rotation into linear push or pull across machine tools, aerospace actuators, and robotics.^[1][6] In humanoid hardware, ball screws are one of two dominant recirculating-rolling-element screw families used to build compact linear actuators for limbs, torsos, and hands, the other being the planetary roller screw, which trades some efficiency and cost for much greater tolerance of shock loads.^[3]

How it works

A ball screw has three main parts: the screw (also called the spindle or shaft), which carries a precision helical groove; the nut, which has a matching internal groove and rides on the screw; and a train of hardened steel balls that fill the helical channel formed between the two grooves.^[1][4] As the screw turns, or in some designs as the nut is driven around a stationary screw, the balls roll along the channel and push the nut down the length of the shaft, converting rotation into linear travel with very little sliding friction.^[4][19]

Because the ball channel is not endless, the balls must be recirculated. As the nut advances, balls exiting one end of the load-bearing thread are picked up and returned to the other end so they can re-enter the raceway and keep working. Manufacturers use three general recirculation designs: an external return tube that arcs over the outside of the nut and carries balls back through a stamped or cast tube; an internal deflector, sometimes called a button, that lifts balls out of the raceway and drops them into an adjacent turn without leaving the nut body; and an end-cap return, in which machined caps at each end of the nut route balls through transverse holes drilled into the nut.^[4][5] Each approach trades off compactness, noise, maximum ball speed, and manufacturing cost, and different suppliers favor different methods for different screw sizes.^[5]

In brief: a ball screw is a screw-and-nut pair with tiny ball bearings sandwiched in the threads instead of metal touching metal directly. The balls act like rollers under a heavy drawer, letting the nut move almost freely along the screw while still being driven precisely by every turn of the shaft. A small return path built into the nut continuously recycles the balls from one end of the thread back to the other, so the mechanism can travel any distance the screw is long enough to allow.

Origins

The idea of rolling balls inside a screw thread to cut friction dates to the end of the 19th century. American inventors H. M. Stevenson and D. Glenn were independently awarded United States patents for ball-bearing screw jacks in 1898 (patents 601,451 and 610,044), and a separate design credited to the Cleveland Machine Screw Co. was published in The Practical Machinist the same year.^[1][6][7] Early ball screws were held back by inconsistent ball sizing, which caused balls to jam in the return path; manufacturing precision improved enough after the Second World War to make the mechanism commercially practical.^[6] Its first major commercial success came in automotive power steering: General Motors' Saginaw steering gear, one of the first mass-produced ball screws, used dual recirculation tubes and shipped in cars such as the Ford Fairlane from 1956 to 1978 and the Chevrolet Corvette from 1962 to 1982.^[6] Ball screws were adopted for aircraft control-surface actuators and missile guidance around the same period, and by the late 20th century they had become the standard drive for computer-numerically-controlled machine-tool axes, a role they still hold; later, high-precision variants were specified for applications as demanding as the James Webb Space Telescope's deployable tower assembly.^[1][8]

Types and variants

Ground versus rolled manufacturing

Ball screws are made one of two ways, and the choice drives both precision and price.

Ground screws are cut with a grinding wheel after the shaft blank is hardened, with each thread pass individually machined relative to the part's centerline. Grinding is slow and requires specialized tooling, but it can hold the lead, the linear distance traveled per revolution, to within a few micrometers and produces a smooth, concentric raceway.^[9][10]

Rolled screws are formed by squeezing an unhardened blank between rotating dies in a single pass, similar to rolling a thread onto a bolt. No material is removed, so the process is fast and cheap, but the resulting thread is less concentric and has a rougher surface finish, which adds friction and typically limits achievable accuracy.^[9][11] Industry sources put rolled screws at 50 percent or more cheaper than an equivalent ground screw.^[10]

Both manufacturing methods are graded against the same accuracy scale, standardized as JIS B 1192 and closely aligned with ISO 3408, which runs from C0, the finest grade, to C10, the coarsest.^[1][12] C0 through C5 are generally called precision grades and C7 through C10 transport or general-purpose grades, though modern rolling processes can reach C5 and in some cases C3, blurring the old line between the two manufacturing methods.^[12][13] A C0-class screw is typically specified to hold travel variation to roughly 15 micrometers over any 300-millimeter stroke, while a C7 to C10-class rolled screw is built for coarser positioning tasks that do not need machine-tool-grade accuracy.^[13]

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Sources: ^{[9][10][11][13]}

Preload and backlash

A ball screw with no clearance between the balls and both raceways would still have a small amount of backlash, the free play that lets the nut shift slightly when the load direction reverses. Manufacturers remove this play by preloading the assembly, forcing the balls into simultaneous contact with both flanks of the thread in each raceway.^[14][15]

There are two common approaches. A single-nut design oversizes the balls slightly so each ball contacts the screw and nut raceways at four points; it is compact, lower-cost, and common in space-constrained applications such as robot joints, pick-and-place heads, and mobile equipment, though its preload is not easily adjustable after manufacture.^[14][16] A double-nut design couples two ball nuts together, preloaded against each other with a spring or a precisely sized spacer, so each nut contacts the raceway at only two points; it is more rigid and its preload can be tuned, which is why it is favored on CNC machine-tool axes that demand maximum stiffness.^[14][15] Preload is not free: increasing it also increases friction, heat, and torque ripple, and it can shorten the theoretical fatigue life of the screw, so manufacturers size preload to the application rather than maximizing it by default.^[15][16]

Tradeoffs and key evaluation criteria

Efficiency versus a leadscrew

The main advantage of a ball screw over a plain-threaded leadscrew, also called an ACME or trapezoidal screw because it relies on sliding contact between the nut and shaft threads rather than rolling elements, is efficiency. Because the balls roll instead of slide, a typical ball screw converts on the order of 90 percent or more of input torque into linear force, with quoted ranges spanning roughly 70 to 95 percent depending on preload, size, and lubrication.^[2][17] A same-size ACME leadscrew is commonly quoted at 20 to 45 percent efficiency, with some sources putting the low end as far down as 20 percent.^[17][18] THK, a major manufacturer, states that a ball screw needs only about one-third of the drive torque of a comparable sliding screw to move the same load.^[19]

That efficiency cuts both ways. A leadscrew below roughly 35 percent efficiency is self-locking: friction in the sliding threads is high enough that an external load on the nut cannot back-drive the motor, so the mechanism holds its position without power or a brake.^[18][20] A ball screw's low friction means it will generally not self-lock; a vertically mounted ball-screw actuator tends to drop its load if power is removed unless the system adds a brake or the drive electronics hold position actively.^[20] This is a real design tradeoff and not just an efficiency number: it is one reason cost-, weight-, and safety-sensitive designs sometimes choose a leadscrew over a much more efficient ball screw.

Shock loads and brinelling

Because a sphere touches a grooved raceway at a single point in theory, and in practice over a very small elliptical patch under load, each ball in a ball screw carries its share of load across a tiny contact area. That concentrates stress: a sudden overload, such as an impact, a jammed mechanism, or a hard stop at the end of travel, can exceed the raceway material's elastic limit at that contact patch and leave a permanent dent, a failure mode called brinelling. Engineers distinguish "true brinelling," caused by a single severe overload, from "false brinelling," a related but mechanically different wear pattern caused by vibration while the screw is stationary.^[21][22] A dent does not usually stop a ball screw from turning, but every time a ball rolls across the damaged spot it produces a small vibration and a momentary friction spike, degrading positioning accuracy and smoothness and accelerating further wear until the screw needs replacement.^[21][23]

This point-contact geometry is the ball screw's central mechanical weakness relative to line-contact alternatives: because ball screws concentrate load on points rather than lines, they are markedly less tolerant of shock than the planetary roller screw, which spreads contact along the length of each roller (see the comparison below). Application guides consistently recommend staying within a manufacturer's static load rating, avoiding hard mechanical stops, and, for applications with repeated impact loading, choosing either an oversized ball screw or a roller screw instead.^[21][23]

Ball screw versus planetary roller screw versus leadscrew

The three screw-drive families used in robotics and machine design trade off contact geometry, load capacity, efficiency, cost, and shock tolerance in different ways.

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Sources: ^{[2][3][17][20][24][25]}

Use in humanoid robots

Ball screws are one of the standard ways to build a compact linear actuator for a humanoid robot: a motor spins the screw, often through a small gear or belt stage, and the nut's linear travel is coupled to a limb segment either directly or through a lever arm and rod end that convert straight-line motion back into joint rotation.^[26] That conversion step is itself a tradeoff. Driving a rotary joint with a linear screw actuator adds linkage hardware and rod-end joints that can introduce compliance and looseness compared with mounting a motor and gearbox directly at the joint, so a humanoid's mechanical designers weigh actuator efficiency and packaging against added linkage complexity on a joint-by-joint basis.^[26][27]

A division of labor between the two rolling-element screw types has emerged across most humanoid programs. Roller screws' higher load capacity and shock tolerance make them the preferred choice for hip, knee, and ankle joints, which absorb the impact of walking, running, and landing; trade press covering Tesla Optimus, which uses planetary roller screws across its legs, elbows, and wrists, reports a similar leg-joint approach at Figure AI and 1X Technologies, among other humanoid developers.^[27][28] Ball screws, in turn, tend to appear in linear degrees of freedom that see lower peak shock, such as torso lift or lean mechanisms, arm extension, and the small linear actuators that drive dexterous-hand fingers.^[27][29] Trade coverage has also reported that Tesla's 2025-to-2026 Optimus Gen-3 update introduced C3-precision ball screws in some joints, with lead error held to roughly 3 micrometers per 300 millimeters of travel, alongside the platform's established use of planetary roller screws elsewhere; this specific figure comes from industry trade reporting rather than a first-party Tesla specification sheet, and is presented here with that caveat.^[27][28]

Ball screws are especially well suited to the miniature linear actuators used inside a dexterous hand's forearm housing, where a pencil-sized ball screw actuator pulls a tendon-driven cable to curl a finger. Screws roughly 3 to 4 millimeters in diameter, with about a 1-millimeter lead and C3 to C5 precision, are commonly specified for this role, since finger joints need repeatable, low-backlash positioning inside a very small envelope.^[29] The approach has academic precedent: a 2021 study in Nature Communications describing an integrated linkage-driven anthropomorphic robotic hand used a commercial miniature ball screw as the core of its finger actuation mechanism.^[30]

The use of ball screws in legged humanoid research predates the current commercial wave. Virginia Tech's THOR, built for the 2013-to-2015 DARPA Robotics Challenge, used ball-screw-driven linear series-elastic actuators, pairing a ball screw with a titanium leaf spring, in its legs, delivering up to roughly 2,000 newtons of force in a compact, lightweight package, an early demonstration that a rolling-element screw could substitute for a direct-drive rotary joint in a bipedal robot.^[31]

Suppliers and market landscape

Market size and share

Estimates of the global ball screw market vary widely depending on how narrowly "ball screw" is defined and which research firm is asked. One industry analysis put the global market at roughly 1.75 billion US dollars in 2021, growing to about 1.86 billion dollars in 2022 at a compound annual growth rate near 6 percent, with China representing about a fifth of global volume by value.^[32] Broader market-research estimates, which may bundle a wider range of linear-motion products under the same heading, range from roughly 5.7 billion dollars in 2025 growing toward 10.8 billion dollars by 2034 in one report, up to figures in the mid-teens of billions of dollars for 2023 with projected growth toward 40 billion dollars by 2032 in another.^[33][35] Given this spread, any single market-size figure should be treated as one research firm's estimate rather than a settled number, and the humanoid robot market is widely cited as a fast-growing new source of demand layered on top of the long-established machine-tool base.

Market share is reported just as inconsistently, and a commonly repeated headline figure needs a direct qualification. One industry analysis found the top five global manufacturers holding a combined share of about 46 percent, and Japanese and European manufacturers together holding roughly 70 percent of the global market by value.^[32] That 70 percent figure describes Japanese and European makers as a regional group, a set that includes NSK, THK, SKF, Bosch Rexroth, Schaeffler, and others, rather than a specific three-company total. Other summaries of the competitive landscape describe a narrower "top three" of THK, NSK, and Hiwin, without giving SKF a top-three place.^[34][36] Precision ball screw manufacturing is genuinely concentrated in Japan, Taiwan, and Europe, and NSK, THK, SKF, and Hiwin are all named consistently among the leading suppliers, but the specific "top three" and its combined share depend on which market report is consulted, so this article attributes the figures to their sources rather than stating one number as settled fact.

Established suppliers

NSK, THK, and SKF are the companies most consistently named as the largest global ball screw manufacturers, with secondary market reporting describing NSK as the largest-volume producer by unit output.^[34][36] Hiwin, founded in Taichung, Taiwan, in 1989, grew from a linear-guide and ball-screw specialist into a top-tier global player after acquiring European thread-grinding capability, and it is now frequently grouped alongside NSK and THK in "tier one" rankings of the industry.^[37][38] Bosch Rexroth and Schaeffler are other large European suppliers with established ball screw product lines, alongside a longer tail of specialists such as Steinmeyer and Kuroda; Schaeffler's own linear-motion business is more strongly associated with planetary roller screws, through brands including GSK, Ewellix, and INA, than with ball screws specifically.^[15][34]

Chinese entrants

A wave of Chinese manufacturers, several with roots in automotive-parts supply, has moved into ball screw and roller screw production over the last several years, drawn by the precision-manufacturing overlap between steering, suspension, and brake components and robot actuator screws.^[39][40]

Shanghai KGG Robots Co., Ltd., founded in 2008, specializes specifically in miniature and rolled ball screws and markets them directly to makers of humanoid dexterous hands and 3C-electronics micro-actuators; one industry analysis rated its miniature ball screw quality as comparable to that of Japan's KSS.^[32][41] Jiangsu Hengli Hydraulic, long known as a manufacturer of hydraulic cylinders for construction equipment, formed a dedicated precision subsidiary in 2022 to design and produce ball screws for electric cylinders and injection-molding machinery, illustrating how established heavy-industry suppliers are diversifying into the segment.^[42] Wuzhou Spring, listed as Wuzhou New Spring Technology (603667.SH), an automotive precision-parts maker, has reported breaking through a 1.8-millimeter micro-ball-screw manufacturing barrier alongside a reverse-engineered planetary roller screw line, and has entered the supply chains of Tesla and Bosch; the company announced an investment plan in 2025 of roughly 1.5 billion yuan spanning planetary roller screws, micro ball screws, and automotive steering, brake, and suspension screw components.^[40][43]

One name commonly grouped with Chinese ball-screw entrants deserves a direct correction. Shanghai Beite Technology (603009.SH), a Shanghai-listed automotive chassis-parts maker, is widely reported in Chinese financial media for an investment of roughly 1.85 billion yuan, about 260 million US dollars, announced in 2024, but that investment specifically targets planetary roller screw research and production, with a stated capacity target of 2.6 million units per year, not ball screws.^[44][45] Beite is a real and notable supplier to the humanoid supply chain, but its publicized bet is on the roller-screw side of the business, so it is more accurately described alongside planetary roller screw suppliers than as a ball-screw example.

Chinese suppliers remain concentrated in coarser accuracy grades, roughly C7 to C10 and occasionally C5, rather than the C0 to C3 grades that machine-tool and top-tier humanoid-hand applications demand, but the cost gap is real. Rolled ball screws and adjacent micro-screw products from Chinese suppliers are frequently cited by industry sources as substantially cheaper than Japanese or European ground screws of comparable size, which is driving increased interest from humanoid-robot developers building at scale against tight per-unit cost targets.^[39][40]

See also

• Planetary roller screw
• Leadscrew
• Actuator
• Rolling element bearing
• Dexterous hand
• Humanoid robot

References

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