The Little Screw Playing a Big Role in Humanoid Robot Joints

As humanoid robots move from research labs toward factory floors and potentially beyond, the focus often lands on sophisticated AI brains and complex sensor suites. However, the physical components enabling their movement are just as critical. Among these, a high-performance mechanical part—the planetary roller screw—is emerging as a key enabler for the next generation of humanoid robots.

From Ball Screws to Planetary Rollers

Actuators are fundamental to robot movement, converting electrical energy into physical motion. Many designs, particularly for precise linear movement, rely on screw mechanisms. Traditionally, ball screws, which use recirculating ball bearings to reduce friction between a threaded shaft and nut, have been common in industrial automation.

However, the demands placed on humanoid robots—requiring high force, precision, and durability within compact joints—are driving adoption of planetary roller screws (PRS). Unlike ball screws, PRS utilize threaded rollers arranged around the main screw shaft, significantly increasing the contact area. This design allows them to handle much higher loads and offer greater stiffness and longevity compared to ball screws of similar size.

According to a February 2025 research note from Morgan Stanley cited in recent reporting, planetary roller screws are expected to become the dominant type used in humanoid robots over time. This trend is already visible, with companies like Tesla (in its Optimus bot's calves), Figure AI, Agility Robotics, 1X Technologies, and numerous Chinese manufacturers reportedly incorporating PRS into their designs.

Why PRS Matters for Humanoids

"Actuators [often incorporating screws] are described as 'the crown jewel of the core moving parts of robots,'" notes Jonathan Aitken, a robotics expert at the University of Sheffield. He explains that screw mechanisms excel at converting rotary motion from a motor into precise linear motion. "They’re highly accurate as we get good precision in conversion from angle to linear distance travelled," Aitken adds, highlighting that PRS offers a way to achieve linear actuation capable of handling significant loads – crucial for tasks like lifting or stabilizing.

The demanding operational life expected of humanoid robots makes the durability of PRS particularly attractive. Their ability to withstand repeated high stresses is vital for joints that will be in constant motion.

Market Growth and Cost Hurdles

The shift towards PRS is creating a significant market opportunity. Jack Li, an R&D and product manager at Chinese screw manufacturer Nanjing Process Equipment, told investment bank Jefferies that the global market for these screws is already estimated at $1.8 billion and projected to grow at a compound annual rate exceeding 30% over the next five years.

However, this performance comes at a price. Manufacturing planetary roller screws requires specialized expertise and precision machinery, limiting the number of suppliers worldwide. This complexity translates to high costs. Researchers at J.P. Morgan estimate that reducers and roller screws together account for roughly 33% of a typical humanoid robot's bill of materials.

According to J.P. Morgan estimates, individual planetary roller screws can cost between $1,350 and $2,700. With experts like Scott Walter, chief technical advisor for Visual Components, suggesting that a single humanoid robot might require 40 or more actuators, the cumulative cost of these components is substantial. "The bulk of the cost of a humanoid will be in the actuators," Walter states, underscoring the significant supply chain and cost challenges.

While alternative designs exist, the performance characteristics of planetary roller screws currently make them a preferred, albeit expensive, choice for many leading humanoid developers. As the industry scales, driving down the cost and increasing the supply of these critical components will be essential for making advanced humanoid robots economically viable.

For further reading have a look at this article at Fast Company.

Read the Wikipedia article about PRS here

The Robot Report on PRS