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 electromechanical linear actuators. Due to its complexity, the roller screw is a relatively expensive actuator (as much as an order of magnitude more expensive than ball screws), 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.
Principle of operation
A roller screw is a mechanical actuator similar to a ball screw, but which 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, the nut, and the 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 correspond 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.
Planetary roller screw types Carl Bruno Strandgren developed some of the earliest effective forms of roller screws and applied for a patent in Nice, France in February 1942. The French patent #888.281 was granted in August 1943 and published in December of the same year. The first commercial roller screw was designed and manufactured under his supervision in 1949 and was mounted on a narrow-gauge locomotive which operated in a northern France coal mine. Subsequent units were produced and mounted on machine tools and (starting in 1955) on aircraft. At that time, Strandgren applied for a new patent incorporating detailed calculations and detailed manufacturing considerations, for which he was awarded US patents for such a “Screw-Threaded Mechanism” in 1954, and “Nut and Screw Devices” and the "Roller Screw" in 1965. Roller screw types are defined by the motion of the rollers relative to the nut and screw. The four commercially available types of roller screw are standard, inverted, recirculating, and bearing ring. Differential roller screws, typically variants of the standard and recirculating types, are also commercially available. Differential roller screws modify the rotational speed ratios between the rollers and the screw by varying the flank angles and contact points of the threads or grooves. In that way differential roller screws change the effective lead of the screw. William J. Roantree received a US patent for the "Differential Roller Nut" in 1968. Now that the core patents have expired, an open source roller screw was developed in OpenSCAD to be 3-D printed for food processing applications. The planetary roller screw can be fabricated in dishwasher-safe polyethylene terephthalate glycol (PETG) on any desktop 3-D printer. The maximum force is more than doubled for the roller screw actuator using the same materials when compared to a direct screw press, making them adequate for some food processing techniques.
Standard planetary roller screw
The standard planetary roller screw is also known as the non-recirculating roller screw. The lack of axial movement of the roller relative to the nut, and the gearing of rollers to nut, are definitive of the standard type of roller screw. The nut and screw have identical multiple-start threads. The rollers have a single-start thread with an angle matching the nut thread. The matched thread angle prevents axial movement between the nut and the roller as the rollers spin. The nut assembly includes spacer rings and ring gears that position and guide the rollers. The spacer rings, which rotate within the ring gears, have equidistant holes that act as rotary bearings for the smooth pivot ends (studs) of the rollers. The ring gears time the spinning and orbit of the rollers about the screw axis by engaging gear teeth near the ends of the rollers. The spacer rings rotate on axis with the screw in unison with the orbit of the rollers. The spacer rings float relative to the nut, axially secured by retaining rings, because they spin around the screw at a lower frequency (angular velocity) than the nut.
Configuration Standard roller screws are typically identified by screw diameter (typically ranging from 3.5mm – 200mm) and lead (1mm – 62mm). The threading of the screw (3 – 6 starts) is either rolled (lower capacity) or ground (higher capacity). The diameters of the nut and rollers (7 – 14 in quantity) are simple functions of the screw diameter and lead. Where:
s d {\displaystyle s_{d}} is the effective screw diameter, or pitch diameter
r d {\displaystyle r_{d}} is the effective roller diameter
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