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Ball screws, linear guides & bearings
Smooth linear motion is a metallurgy-and-grinding art owned by a handful of Japanese precision houses.
Smooth linear motion inside a humanoid joint — the screw that turns motor rotation into the push of a knee or hip — is a metallurgy-and-grinding art, and the people who own it are not the robot companies. They are a handful of precision houses that have spent decades learning to grind a hardened-steel thread to single-digit-micron lead error.
Who owns it
Two distinct technologies sit in the joint. Ball screws and linear (LM) guides are the volume products of the Japanese majors: THK and NSK, where linear-motion systems and ball screws are a large share of THK's revenue. The harder, denser-load product — the planetary roller screw — is the part Tesla put in Optimus, 14 per robot, about 19% of the bot's cost. Here the merchant supply is even narrower: Tesla sources its Optimus roller screws from GSA of Switzerland, and GSA, Rollvis and Ewellix together hold over 70% of the planetary roller-screw market. The only one of those three inside a US-listed wrapper is Ewellix, owned by Schaeffler since 2022 — the cleanest listed proxy on the screw choke.
What breaks it
The constraint is grinding capacity at grade. Optimus-class joints want C3-grade lead error around 3 microns per 300mm, which demands ultra-precision CNC thread grinding and pushes part cost 30–40% above conventional screws. Chinese makers still largely sit at C5 and cannot yet mass-produce C3, so for now the precision tier is Swiss/Japanese/German. Tesla's own 2023 program saw roller-screw lead times balloon to 26 weeks — the canonical sign of a binding chokepoint. Bearings are the adjacent choke: a humanoid carries 25–30 actuated joints, each wanting compact, high-stiffness bearings, and Schaeffler has put its XZU double-row angular-contact needle bearing into that role.
What forces the reprice
The merchant story reprices when component makers convert demonstrated design positions into disclosed backlog. Schaeffler is furthest along: five ratified humanoid customer contracts, ~45 partners, a Nov 2025 Neura Robotics actuator partnership, and a Dec 2025 all-in-one 60–250 Nm planetary-gear actuator for hip/knee joints. NSK has gone vertical, taking >1/3 of voting rights in AI-robotics firm RT Corporation and targeting commercial humanoid actuators by 2028. Quarterly prints that begin to quantify robotics order intake — versus narrative — are the catalyst.
Be honest about size and earliness
This is the central caveat: robotics is a rounding error in these names today. Humanoids were about 0.2% of the 2025 robot market, and Schaeffler — the most committed of the group — says robotics was under 1% of group sales in 2025, with its hundreds-of-millions-of-euros order-book target dated to 2030 and contingent on ~1 million humanoid units shipping. THK and NSK are mostly machine-tool, semiconductor and factory-automation businesses; humanoid revenue is not separately material. Two structural hazards bound the upside. First, vertical integration caps the merchant story: Tesla intends to bring roller-screw manufacturing in-house to kill the 3–5x specialist markup, so the largest single program may never become a merchant order. Second, the purest pure-play is unlisted — GSA and Rollvis are private Swiss firms, so equity exposure to the actual Optimus screw runs only through Schaeffler's Ewellix. And beware robot-washing: several bearing names carry the narrative without the order book — RBC Bearings' CEO said on its Q2 FY2026 call he "hasn't thought that much about" humanoids, and Timken's robotics exposure is medical/automation drives via its CGI acquisition, not a named humanoid program. This is structure and catalysts, not advice.
Who owns the choke
Gewinde Satelliten Antriebe AG (GSA)
Swiss precision-screw maker (Gewinde Satelliten Antriebe AG), descended from thread-grinder Gewinde Ziegler AG. Named supplier of the 14 planetary roller screws in Tesla's Optimus humanoid and, with Rollvis, a majority of the global high-precision planetary roller-screw market. Privately held; no US listing.
- Bull
- GSA supplies the 14 planetary roller screws in Tesla's Optimus and, with Rollvis, holds a majority of the high-precision roller-screw market — the purest screw-choke pure-play.
- Bear
- Privately held with no US listing and no exposure vehicle, and Tesla intends to bring roller-screw manufacturing in-house to kill the specialist markup.
NSK Ltd. (ADR)
Makes precision ball screws, linear guides, monocarriers and bearings. Unveiled high-precision rotary and linear actuators for humanoid shoulder/elbow/hip joints targeting commercial integration by 2028, and in Oct 2025 took more than one-third of voting rights in AI-robotics firm RT Corporation to build a humanoid supply foundation.
- Bull
- NSK makes precision ball screws and linear guides and went vertical — taking >1/3 of RT Corporation's votes and targeting commercial humanoid joint actuators by 2028.
- Bear
- Its book is dominated by machine-tool, semiconductor and factory-automation businesses; humanoid revenue is not yet material, and NPSKY is a thin OTC ADR.
RBC Bearings Incorporated
Manufactures thin-section ball bearings (to >40-inch OD) used in industrial automation and robots. On its Q2 fiscal 2026 earnings call, management indicated no meaningful revenue from humanoid programs and framed the opportunity as speculative and distant.
- Bull
- RBC's thin-section ball bearings (to >40-inch OD) used in industrial automation give it an adjacent-component foothold if humanoid joint-bearing demand materializes at volume.
- Bear
- Management said on its Q2 FY2026 call there is no meaningful humanoid revenue, framing the opportunity as speculative and distant.
Schaeffler AG (ADR)
Owns Ewellix, one of the top planetary roller-screw makers (GSA/Rollvis/Ewellix together >70% market share). Has five ratified humanoid customer contracts and ~45 partners, signed a Nov 4 2025 actuator partnership with Neura Robotics, and unveiled a 60-250 Nm all-in-one planetary-gear actuator for hip/knee joints on Dec 16 2025. Targets a hundreds-of-millions-of-euros humanoid order book by 2030, though robotics was under 1% of 2025 group sales.
- Bull
- Schaeffler owns Ewellix (with GSA/Rollvis >70% of roller screws) and is furthest along — five ratified humanoid contracts, ~45 partners, a Neura deal and an all-in-one hip/knee actuator.
- Bear
- Robotics was under 1% of 2025 group sales, the order-book target is dated to 2030 contingent on ~1M units, and SFFLY is a thin OTC ADR.
THK Co., Ltd. (ADR)
Japanese precision house; linear-motion systems and ball screws are roughly 60% of revenue. Actively developing next-generation precision ball screws and compact LM guides for humanoid actuators (e.g., the SDA-VZ ready-to-install ball-screw line). Reported FY2025 (Dec-year) results on Feb 12, 2026 following its Automotive & Transportation divestiture; the book is still dominated by industrial machinery, not humanoids.
- Bull
- THK is a Japanese precision house (~60% revenue from linear motion) actively developing next-gen ball screws and compact LM guides like the SDA-VZ line for humanoid actuators.
- Bear
- The book is still dominated by industrial machinery, humanoid revenue is not separately material, and THKLY is a thin OTC ADR.
The Timken Company
Timken owns Cone Drive (100% subsidiary), a U.S.-based strain-wave/harmonic gear maker that explicitly markets harmonic joints for humanoid hips, knees, ankles, shoulders and wrists with U.S.-manufacturing supply-chain positioning. Timken also acquired CGI Inc. (precision drives for robotics/automation) in Q3 2024. Robotics is a small, undisclosed share of the ~$1.16B/quarter company; humanoid claims are merchant marketing, not named design wins.
- Bull
- Timken owns Cone Drive, a US-based strain-wave maker explicitly marketing humanoid hip/knee/wrist joints with domestic-supply positioning, plus CGI precision drives for robotics.
- Bear
- Robotics is a small undisclosed share of a ~$1.16B/quarter company, and the humanoid claims are merchant marketing, not named design wins.
Catalyst calendar
- 2026-08-03NSK Ltd Q1 FY2026 resultsmediumFirst print after NSK's RT Corporation stake and 2028 humanoid-actuator roadmap; watch for any quantification of robotics order intake versus narrative on the ball-screw/bearing choke.
- 2026-08-05Schaeffler AG H1 2026 resultsmediumKey checkpoint on whether Schaeffler's five humanoid contracts and Ewellix roller-screw/actuator pipeline convert into disclosed backlog progress toward the 2030 order-book target.
- 2026-08-31THK Co. Q2 FY2026 resultsmediumTHK's Q2 FY2026 print (quarter ending ~June 2026) is the read on whether linear-motion orders are inflecting on humanoid and semiconductor demand; THK reported Q1 FY2026 on May 11, 2026.
- 2026-11-03Schaeffler AG nine-month 2026 resultsmediumUpdates the humanoid order-book trajectory and any new customer contracts on the bearings-plus-roller-screw chokepoint heading into 2027.