page_title: Roller screw
section_title: Principle of operation
hierarchical_section_title: Roller screw / Principle of operation
caption_attribution_description: English: Animation of a standard roller screw showing geared timing of the rollers to the nut.
context_page_description: A roller screw, also known as a planetary roller screw or satellite roller screw, is a low-friction precision screw-type actuator, a mechanical device for converting rotational motion to linear motion, or vice versa. Planetary roller screws are used as the actuating mechanism in many electro-mechanical linear actuators. Due to its complexity the roller screw is a relatively expensive actuator, but may be suitable for high-precision, high-speed, heavy-load, long-life and heavy-use applications.
  Roller screw mechanisms are commonly incorporated into motion/positioning systems in a variety of industries such as manufacturing and aerospace.
context_section_description: A roller screw is a mechanical actuator similar to a ball screw that uses rollers as the load transfer elements between nut and screw instead of balls.  The rollers are typically threaded but may also be grooved depending on roller screw type.  Providing more bearing points than ball screws within a given volume, roller screws can be more compact for a given load capacity while providing similar efficiency (75%-90%) at low to moderate speeds, and maintain relatively high efficiency at high speeds.  Roller screws can surpass ball screws in regard to positioning precision, load rating, rigidity, speed, acceleration, and lifetime.  Standard roller screw actuators can achieve dynamic load ratings above 130 tons of force (exceeded in single-unit actuator capacity only by hydraulic cylinders).  
  The three main elements of a typical planetary roller screw are the screw shaft, nut and planetary roller.  The screw, a shaft with a multi-start V-shaped thread, provides a helical raceway for multiple rollers radially arrayed around the screw and encapsulated by a threaded nut.  The thread of the screw is typically identical to the internal thread of the nut.  The rollers spin in contact with, and serve as low-friction transmission elements between screw and nut.  The rollers typically have a single-start thread with convex flanks that limit friction at the rollers' contacts with screw and nut.  The rollers typically orbit the screw as they spin (in the manner of planet gears to sun gear), and are thus known as planetary, or satellite, rollers.  As with a lead screw or ball screw, rotation of the nut results in screw travel, and rotation of the screw results in nut travel.
  For a given screw diameter and quantity of thread starts more rollers corresponds to higher static load capacity, but not necessarily to a higher dynamic load capacity.  Preloaded split nuts and double nuts are available to eliminate backlash.
caption: Standard roller screw timing