Planetary roller screw

A planetary roller screw is a precision mechanical component that converts rotary motion into linear motion by running a ring of threaded steel rollers between a central screw shaft and a surrounding nut, with each roller spinning on its own axis while orbiting the screw the way a planet orbits a sun. It belongs to the same family of screw drives as the far more common ball screw and the plain leadscrew, but its rollers contact the screw and nut along continuous helical lines rather than at the discrete points a ball screw's balls produce. That difference lets a planetary roller screw carry far higher loads, run stiffer, and survive many more shock cycles than a ball screw of the same size. The combination of strength and durability has made planetary roller screws the preferred linear-actuation element for the highest-force joints in humanoid robots, including hips, knees, and ankles, even though the part remains expensive, difficult to manufacture at scale, and, as of 2026, a recognized bottleneck in the humanoid-robotics supply chain.

In brief: picture a ball screw's steel balls replaced by a dozen small threaded rods that both spin in place and orbit the central shaft, like a miniature planetary gearbox built directly into a screw. Because each roller touches the shaft along a line instead of at a single point, load is spread across far more surface area, so the assembly can push and pull much harder, resist denting from sudden impacts, and last much longer, at the cost of being one of the hardest and most expensive linear-motion components to manufacture.

How it works

A planetary roller screw has three main parts: a central screw shaft cut with a multi-start helical thread, a ring of several cylindrical rollers arranged around it (each roller also threaded, usually with a single-start thread matched to the screw's lead), and an outer nut that encloses the rollers and engages them through its own internal thread. As the screw turns, each roller both rotates on its own axis and orbits the screw, similar to how a planet gear orbits a sun gear in a planetary gear train, and that combined spin-and-orbit motion carries the nut linearly along the screw's axis ^[1][2]. In most standard designs, small gear teeth machined into the ends of the rollers mesh with fixed ring gears inside the nut. This timing gear keeps every roller's spin-to-orbit ratio synchronized with the screw's lead so the rollers neither climb nor slip along the thread, while spacer rings hold each roller's pivot ends in place ^[1].

The detail that matters most is contact geometry. A ball screw carries load through a recirculating chain of hardened steel balls that touch the screw and nut raceways at single points. A leadscrew, commonly cut with an ACME thread, has no rolling elements at all; its screw and nut threads slide directly against each other. A planetary roller screw's threaded rollers, by contrast, engage the screw and nut along continuous helical lines, and because several rollers, commonly six to twelve depending on size, share the load at the same time, the total contact area inside a given envelope is far larger than the handful of instantaneous point contacts in a ball screw ^[1][3]. Spreading force over lines instead of points lowers the peak contact stress for a given load, which is the underlying engineering reason roller screws tolerate higher static and dynamic loads, resist denting under shock, and last longer before their raceways fatigue ^[4].

That extra strength is not free. Leadscrews are the cheapest and mechanically simplest of the three designs, but sliding friction limits their efficiency to roughly 20 to 40 percent, low enough that many leadscrew and nut combinations are self-locking (they cannot be back-driven by an axial load), a useful safety property in some machines and a source of wasted motor power in others ^[5]. Ball screws trade that simplicity for rolling-element efficiency commonly quoted around 80 to 95 percent. Planetary roller screws sit close to ball screws on efficiency, generally cited between about 75 and 90 percent depending on the variant and preload, while pulling well ahead on load capacity, stiffness, and service life ^[1][4].

The mechanism itself is not new. French engineer Carl Bruno Strandgren developed the concept and filed a French patent in February 1942, granted as patent No. 888,281 in August 1943. The first commercial roller screw, built under his supervision, went into service in 1949 driving a narrow-gauge locomotive in a French coal mine ^[1]. Strandgren's later United States patents through the 1950s and 1960s, including a recirculating-roller design patented in 1965, established the basic variants still in use today ^[1].

Types and variants

Manufacturers and the trade press generally describe five commercially available families of planetary roller screw, distinguished mainly by how the rollers move relative to the screw and nut, and by whether they need to periodically "recirculate," resetting their axial position, or stay fixed in place through gear timing ^[1][6].

Standard (planetary) type. The baseline design described above: the nut is relatively short, the rollers ride on the outside of a screw shaft, and ring gears keep every roller synchronized so none of them ever needs to recirculate. It offers a broad, well-rounded balance of load capacity, speed, and manufacturability, and is the configuration most often used in industrial and robotic linear actuators ^[1][3].

Inverted type. The geometry is turned inside out. Instead of rollers orbiting a thin screw inside a compact nut, the rollers orbit the outside of a central threaded rod and travel inside a longer, hollow outer tube that acts as the moving element, with only a limited section of that tube's internal thread engaged at any given position. That limits inverted screws to relatively short strokes, but it packages high force into a compact, self-contained unit, which is one reason the type shows up in space-constrained robot limb joints ^[2][6].

Recirculating type. Instead of a thread that matches the screw's lead, the rollers carry plain grooves and behave more like small wheels. Because they do not advance in lockstep with the screw the way a gear-timed roller does, they must be periodically recirculated back to the start of their travel, similar in principle to how a ball screw recirculates its balls, and recirculating designs dispense with ring-gear hardware entirely ^[1]. Having more rollers engaged at once gives this type high rigidity and load capacity for its size, at the cost of coarser achievable leads and somewhat lower speed, which is why it is often used in high-stiffness precision equipment such as medical devices and optical positioning stages.

Differential type. Each roller is cut with two helical threads of very slightly different pitch, one meshing with the screw and the other with the nut. Because the two pitches are almost, but not quite, identical, a full turn of the screw produces only the small difference between them as net linear travel, giving an effective lead far finer than the physical thread pitch would suggest ^[7]. That lets a differential screw deliver very high resolution and a large force multiplication from a compact package. Schaeffler, for example, markets a small-diameter (5 to 30 mm) unit in this family under the PWG name, which the company's own literature describes as capable of generating substantially more force than a same-size ball screw, aimed at applications such as replacing hydraulic cylinders ^[8].

Bearing-ring type. Patented by Oliver Saari in 1986 and commercialized under the Spiracon trademark, this variant places an intermediate grooved bearing ring between the rollers and the nut housing instead of timing each roller with its own gear teeth. The rollers key into this ring, and the ring transfers load to the nut through a set of thrust bearings. That arrangement removes the need for gear teeth on every roller and somewhat simplifies roller manufacture, at the cost of adding the bearing-ring and thrust-bearing parts themselves ^[1].

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Tradeoffs and key evaluation criteria

Engineers choosing between a leadscrew, a ball screw, and a planetary roller screw are mainly trading cost and efficiency against load capacity, stiffness, and durability under shock.

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Table figures per ^[1][4][5]; see discussion below for the life and cost claims specifically.

Load capacity and life. Because load is spread along lines rather than concentrated at points, a planetary roller screw's peak contact stress for a given force is much lower than a ball screw's, so large industrial roller screws can be rated to dynamic load capacities exceeding 130 tonnes ^[1]. The same geometry makes roller screws far more resistant to brinelling, the permanent denting of a raceway that occurs when a ball screw absorbs a sudden shock load, such as a robot's foot striking the ground ^[4][9]. Manufacturer engineering literature illustrates the resulting life gap with worked examples rather than a single universal number: Exlar, a roller-screw actuator brand owned by Curtiss-Wright, publishes a technical note calculating that a roller screw's fatigue life can come out to roughly 15 times that of a comparably sized ball screw under the same load, based on Hertzian contact-stress calculations for a specific example case ^[10]. Other manufacturers and the trade press describe the advantage more loosely as "several times" to "an order of magnitude" longer service life ^[4][9]. That range brackets the roughly 10 to 15 times figure commonly quoted for roller screws in high-impact, high-duty-cycle robotics use, though the exact multiplier depends heavily on the specific load, speed, and screw size being compared, and should be read as an illustrative range rather than a fixed constant.

Efficiency. Planetary roller screws are not the most efficient screw drive. Independent comparisons put ball-screw actuators around 80 percent mechanical efficiency and roller-screw actuators a few points lower, in the 75 to 80 percent range, for broadly similar hardware, while other manufacturer literature quotes standard-type roller screws as high as 90 percent, with finer-lead variants such as the differential and recirculating types running lower, down toward 60 to 70 percent, because achieving very fine resolution trades away some mechanical efficiency ^[1][4]. In practice, the efficiency gap between a well-preloaded ball screw and a similarly sized roller screw is usually a secondary concern next to load capacity and durability.

Cost and manufacturing complexity. A planetary roller screw typically costs several times, and by some estimates as much as an order of magnitude, more than a same-size ball screw ^[1]. Analysts at J.P. Morgan estimated in 2025 that a single humanoid-grade planetary roller screw sells for roughly $1,350 to $2,700, and that reducers and roller screws together account for around 33 percent of a typical humanoid robot's bill of materials, making the roller screw one of the single most expensive component categories in the robot ^[11]. The underlying reason is manufacturing difficulty: the screw and every roller are made from case-hardened alloy steel, precision-ground after heat treatment so that the screw's helical thread and its mating rollers stay within tolerances of only a few micrometers. One account of the process for a Tesla Optimus-grade screw puts the thread-lead error tolerance at roughly 3 micrometers, about one-twentieth the width of a human hair, achieved on specialized multi-axis thread grinders that relatively few factories in the world own ^[12][13]. That combination of expensive material, dedicated grinding equipment, and a small pool of experienced toolmakers is the practical reason planetary-roller-screw output has not scaled nearly as fast as demand from the humanoid-robotics industry.

Use in humanoid robots

Planetary roller screws are used almost exclusively in the joints that carry the most force and absorb the most repeated impact: hips, knees, and ankles, and in some designs the shoulders and elbows, where a linear actuator built around a roller screw converts a motor's rotation into the push or pull needed to extend or flex the joint. That role puts them in direct competition with rotary actuators built around a gear reducer such as a harmonic drive, and most current humanoid platforms use a mix of both: rotary, reducer-driven actuators for joints that mainly need range of motion, and linear, roller-screw-driven actuators for joints that need to support and absorb body weight ^[3][13].

Tesla's Tesla Optimus is the most widely documented example. The Optimus Gen 2 version Tesla unveiled in December 2023 uses 28 custom-designed actuators: 14 rotary actuators pairing a frameless torque motor with a harmonic-drive reducer, and 14 linear actuators pairing a frameless torque motor with a planetary roller screw, distributed through the arms and legs ^[13][14]. Industry teardown analysis and Chinese supply-chain research have reported that Optimus's linear actuators use inverted-type roller screws supplied by Switzerland's GSA, deployed across the upper arm, lower arm, thigh, and lower leg ^[3][15]. Trade press has also reported planetary roller screws in linear actuators built by Figure AI, Agility Robotics, and 1X Technologies, and the technology is now widely used across Chinese humanoid-robot programs as well, according to Jefferies analyst commentary relayed by a Nanjing Process Equipment research and development manager ^[11].

That demand has been enough to make planetary roller screws a genuine supply bottleneck rather than just an expensive line item. Jefferies estimated the global planetary-roller-screw market at roughly $1.8 billion, projecting more than 30 percent compound annual growth over five years as humanoid-robot programs scale from prototypes toward production runs that each need dozens of screws ^[11][16]. Analysts at Morgan Stanley have separately argued that, as manufacturing matures and costs fall, planetary roller screws should eventually displace ball screws as the majority screw type used in humanoid robots, reversing today's mix in which cost and supply constraints still push some designs toward cheaper ball screws where the load case allows it ^[16].

One reason China has been able to scale roller-screw production quickly, according to industry analyses, is that it already had a dense base of small precision-actuator manufacturers built up to supply its drone industry, sitting inside a broader electric-motor and precision-component supply cluster concentrated in the Yangtze River Delta. That installed base of tooling, skilled labor, and component suppliers gave Chinese firms a head start in retooling for humanoid-robot actuators once demand appeared, even though the most established roller-screw specialists remain concentrated in Switzerland, Sweden, and the United States ^[17][16].

Suppliers and the market landscape

Planetary-roller-screw manufacturing has historically been far more concentrated than ball-screw manufacturing, which is dominated by large Japanese and European bearing makers. For decades a small number of Swiss, Swedish, German, and American specialists effectively defined the industry, and Chinese entrants have only begun closing the gap since the early 2020s, as humanoid-robot demand justified new investment.

Switzerland's GSA and Rollvis are consistently reported as the two largest suppliers by volume. GSA (formally Gewinde Satelliten Antriebe AG), founded in 1982 and linked to the century-old Swiss thread-grinding firm Gewinde Ziegler AG, and Rollvis SA, founded in Geneva in 1970, are commonly cited together as holding more than half of global planetary-roller-screw supply ^{[3][18][19][20]}. Industry and financial research reports describe Rollvis as having been acquired by GSA around 2016, bringing the two Swiss makers under common ownership ^[3][15]. Neither company's own public materials disclose the details of that transaction, so it is reported here as it appears in trade and financial press rather than as independently confirmed corporate fact.

That Swiss consolidation is a useful correction to a claim that circulates in some market write-ups, including the industry report this article originally drew on: available evidence does not support GSA and Rollvis being part of the Schaeffler Group. The company that Germany's Schaeffler Group did acquire is Ewellix, the Swedish maker of linear actuators, lifting columns, and ball and roller screws that began as SKF's linear-motion division before being sold to the investment firm Triton in 2018 and rebranded as an independent company. Schaeffler announced its agreement to acquire Ewellix in July 2022, for a reported price of about 582 million euros, and completed the purchase later that year ^[21][22][23]. Separately from the Ewellix deal, Schaeffler has long sold its own in-house roller-screw lines, including the RGT roller-screw drive and the PWG planetary/differential screw drive, under its own Schaeffler and INA brands. Schaeffler is therefore a genuine multi-pronged presence in the market through its own INA-branded products and its Ewellix acquisition, but the available evidence indicates it does not own GSA or Rollvis ^[8].

Bosch Rexroth sells its own planetary screw assemblies, marketed as PLSA, and integrates them into complete linear actuators such as its EMC HD line, with dynamic load ratings up to about 544 kilonewtons and speeds up to 50 meters per minute in its largest catalog sizes ^[24]. Moog Inc., the American aerospace and motion-control company, manufactures ball screws and both standard and inverted planetary roller screws at a dedicated facility in Presezzo, near Bergamo, Italy, expanded in 2013, giving it an established European production base independent of the Swiss cluster ^[25][26]. An earlier version of the industry report behind this article claimed Moog's Italian roller-screw business came from acquiring a company referred to as "VCS." No such acquisition could be verified in Moog's own corporate history or in trade press, so that specific claim is omitted here pending better sourcing. Tolomatic, a US linear-actuator specialist, builds planetary roller screws into several of its own rod-style and heavy-duty actuator lines, including the RSA, RSH, IMA, and RSX platforms, rather than selling bare screws ^[27].

On the Chinese side, Nanjing Process Equipment has emerged as the most frequently cited domestic producer with genuine mass-production capability, alongside Shandong-based Bote Precision. In October 2024, auto-parts maker Shanghai Beite Technology announced a roughly 1.85 billion yuan (about $260 million) investment in a dedicated planetary-roller-screw production base, signaling how much new Chinese capital is chasing the category ^[17]. Several other Chinese firms, including Wuzhou New Spring, Jiangsu Leili, and Zhenyu Technology, are reported to be in sampling or trial-production stages ^[15]. Even so, Chinese industry research describes the domestic market as still roughly 80 percent import-dependent as of the mid-2020s, with GSA, Rollvis, and Ewellix together holding roughly 70 percent of China's own planetary-roller-screw market ^[3][15], underscoring how much of the supply chain remains outside China even as Chinese humanoid-robot makers are among the technology's biggest customers.

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See also

• Ball screw
• Leadscrew
• Harmonic drive
• Actuator
• Tesla Optimus
• Humanoid robot
• Schaeffler
• Ewellix
• Bosch Rexroth
• Moog

References

1. Wikipedia, "Roller screw," accessed July 2026. https://en.wikipedia.org/wiki/Roller_screw ↩
2. Design World Online, "What is a planetary roller screw?" https://www.designworldonline.com/what-is-a-planetary-roller-screw/ ↩
3. KGG, "The Application of Planetary Roller Screws in Humanoid Robots and Market Development." https://www.kggfa.com/news/the-application-of-planetary-roller-screws-in-humanoid-robots-and-market-development/ ↩
4. Linear Motion Tips / Tolomatic, "How roller-screw and ball-screw actuators compare in high-force applications." https://www.linearmotiontips.com/how-roller-screw-and-ball-screw-actuators-compare-in-high-force-applications/ ↩
5. Helix Linear Technologies, "The Unique Engineering Characteristics of a Precision Acme Screw." https://www.helixlinear.com/blog/engineering-characteristics-of-a-precision-acme-screw ↩
6. Linear Motion Tips, "What is an inverted roller screw and how does it work?" https://www.linearmotiontips.com/what-is-inverted-roller-screw-how-does-it-work/ ↩
7. Machine Design, "Creating Larger Forces with Differential Roller Screws." https://www.machinedesign.com/fastening-joining/article/21832121/creating-larger-forces-with-differential-roller-screws ↩
8. IndustryArena / Schaeffler Technologies, "PWG Planetary Rolling Screw Drive." https://en.industryarena.com/schaeffler/products/drive-elements--1112/pwg-planetary-rolling-screw-drive--20188 ↩
9. Linear Motion Tips, "How to account for shock and vibration loads in ball screw drives." https://www.linearmotiontips.com/how-to-account-for-shock-and-vibration-loads-in-ball-screw-drives/ ↩
10. Exlar (Curtiss-Wright), "Roller Screw Basics," technical note. https://actuation.curtisswright.com/sites/default/files/Resources/Roller-Screw-Basics.pdf ↩
11. Fast Company (via Yahoo Finance), "This tiny screw is powering the humanoid robot revolution." https://finance.yahoo.com/news/tiny-screw-powering-humanoid-robot-100000283.html ↩
12. EMAG, "Precision manufacturing of planetary roller screws: WPG 7 ensures tight tolerances and top quality." https://www.emag.com/industries-solutions/workpieces/planetary-roller-screws/ ↩
13. Firgelli, "How Tesla Bot Actuators Work" and "Humanoid Robot Actuators: The Complete Engineering Guide." https://www.firgelliauto.com/blogs/actuators/how-do-tesla-bot-actuators-actually-work ↩
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15. Sina Finance, "人形机器人高壁垒环节：行星滚柱丝杠市场格局梳理," January 12, 2025. https://finance.sina.com.cn/roll/2025-01-12/doc-ineetfvv3140948.shtml ↩
16. Humanoids Daily, "Planetary Roller Screws: The High-Stakes Component Powering Humanoid Robot Motion." https://www.humanoidsdaily.com/news/planetary-roller-screws-the-high-stakes-component-powering-humanoid-robot-motion ↩
17. Interesting Engineering, "Tiny humanoid robot screw puts China ahead of US, allies in bot race." https://interestingengineering.com/innovation/china-grip-on-humanoid-robot-future ↩
18. GSA AG (Gewinde Satelliten Antriebe AG), "About us." https://gsascrews.com/en/ueber-uns/ ↩
19. Rollvis SA, "About us." https://rollvis.com/about-us/ ↩
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21. Schaeffler Group, "Schaeffler further strengthens industrial business with purchase of Ewellix Group," press release, July 25, 2022. https://www.schaeffler.com/en/media/press-releases/press-releases-detail.jsp?id=87843779 ↩
22. Power & Motion, "This Week in Power & Motion: Schaeffler Completes Ewellix Acquisition." https://www.powermotiontech.com/news/article/21259306/this-week-in-power-motion-schaeffler-completes-ewellix-acquisition ↩
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26. Automazione Plus, "Moog, i segreti del successo." https://automazione-plus.it/moog-i-segreti-del-successo_127078/ ↩
27. Tolomatic, "Planetary Roller Screws" product page. https://www.tolomatic.com/products/product-details/roller-screw/ ↩