What do you get when you combine planetary gears with a ball screw? The answer: a Planetary Roller Screw.
Replacing the recirculating balls of a ball screw with grooved rollers creates a precision drive mechanism capable of extremely high load capacity, acceleration, and speed, all within a compact package. Planetary roller screws are ideal for applications requiring higher performance than a ball screw can offer, yet needing the speed or positioning accuracy that hydraulic actuators cannot provide. But how exactly does a planetary roller screw work? This article breaks down its operating principles.
A planetary roller screw assembly consists of a precision-threaded screw shaft and a nut, similar to a ball screw or lead screw. However, its key components and their interaction are distinct:
● The Screw: The screw shaft and nut have threads cut at a 90-degree angle, with multiple thread starts to accommodate the rollers.
● The Rollers (The "Planets"): Instead of balls, the load-bearing elements are threaded rollers. A roller's thread profile matches that of the nut. Crucially, for a given size, rollers provide significantly more contact points than balls.
● The "Planetary" Action: The ends of the rollers are geared. These gears mesh with internal gear rings at both ends of the nut. Guide rings support the roller end shafts, ensuring proper spacing. This setup allows the rollers (the "planets") to rotate on their own axes while also revolving around the nut, hence the "planetary" name.
The rollers are synchronized with the nut via their threads, and the gearing enables their rotation around the screw. This design, with its multiple contact points compared to a similarly sized ball screw, grants the planetary roller screw superior load capacity, speed, and rigidity.
Since the fundamental motion principles are the same, sizing and selecting a planetary roller screw is almost identical to the process for a ball screw. Formulas for bearing life, critical speed, maximum allowable rpm, buckling load, and required torque are identical or based on the same fundamentals, making design integration straightforward.
● Higher Load Capacity & Durability: The inherent increase in contact points provides a much higher load capacity, making it suitable for high-load or high-duty-cycle applications that challenge traditional ball screws.
● High Precision & Rigidity: The numerous contact points also reduce deflection, offering positioning accuracy equal to or better than ball screws.
● Higher Speed & Acceleration: The absence of recirculating elements allows for higher rotational speeds and accelerations and eliminates a common failure point found in ball screws.
● Smoother & Quieter Operation: With lower friction and less noise—thanks to rollers that do not contact each other (unlike balls in a ball screw)—operation is smoother. Noise levels also do not increase significantly with speed, making them much quieter in high-speed applications.
● Long Stroke & High Accuracy: Stroke lengths and accuracy are comparable or superior, with typical products offering strokes up to 3 feet and accuracy within ±0.003 inches. It's important to note that while load capacity can be up to 15 times greater, buckling and critical speed limits still apply.
Compared to hydraulic cylinders and actuators, the primary advantage of planetary roller screw systems is their elimination of hydraulic fluid. This translates to less maintenance, no risk of environmental contamination from leaks, and a simpler overall system with no need for valves, pumps, or filters. From a performance standpoint, they also offer superior positioning accuracy and efficiency.
● Comparable Force: They generate forces equivalent to similarly sized hydraulic systems—typically ranging from 20,000 lbf to 800,000 lbf depending on size and design—thanks to the high thrust capacity from numerous, relatively large (line) contacts between the nut's rollers and the screw.
● Higher Efficiency: A roller screw-based actuator can be up to 90% efficient, compared to roughly 50% for a hydraulic cylinder.
● High Speed & Acceleration: Speeds and accelerations can rival pneumatic systems, with speeds potentially exceeding 40 inches/sec and accelerations reaching 3g. This high output is possible because the load-bearing rollers do not contact each other, avoiding the collisions, impact forces, and heat generation seen in ball screws. This makes them useful for replacing pneumatic systems even where high force isn't the primary need, significantly improving a design's ability to withstand continuous operation and frequent cycling.
In short, a planetary roller screw is a screw assembly that uses threaded rollers instead of balls. These rollers move along the screw's length and are gear-driven to rotate with the nut, eliminating the need for recirculation.
Whether compared to ball screws actuators or hydraulic actuators, planetary roller screws deliver higher performance in a smaller footprint while reducing maintenance and operational costs.
As one of several screw-drive types (alongside lead screws and ball screws), planetary roller screws—sometimes called satellite roller screws due to the rollers ("satellites") revolving around the drive shaft axis—are increasingly used in electromechanical actuators. Their geometry provides greater internal contact area between components than ball screws, increasing rigidity and dynamic load capacity. Furthermore, the fine pitch of the grooves/threads typically offers an excellent mechanical advantage, reducing the input torque needed to move a given load.
Two drawbacks include higher cost (potentially double that of an equivalent ball screw) and lower market prevalence, which can mean more effort sourcing manufacturers and longer lead times.
Nevertheless, planetary roller screws excel in machine tool and packaging applications, as well as in welding torches, injection molding equipment, and presses. They are also ideal for axes traditionally driven by hydraulics (e.g., in sawmills and woodworking machinery), as they eliminate hydraulic oil to prevent product staining while maintaining the power needed for high-strength, large-scale processing. Other suitable applications include environments with significant temperature variations or those requiring sustained high forces.
