Author: Michael Boudreau, Master of Engineering, Application Engineering Manager

As a relatively young technology, and its initial application is in heavy industries such as injection molding, roller screws are generally considered to be the most suitable replacement for hydraulic cylinders or other high-load drive applications.

And, yes, some roller screw designs are great for heavy-duty applications, but roller screws also provide solutions to other design challenges, such as the need for precise sub-micron positioning or special nut designs and customer-specified materials.

Here, we will study the design of four types of roller screws, explain their unique working principles and performance characteristics, and provide examples of applications that each type of roller screw is good at.

Planetary roller screw: the "main force" of the roller screw family

The planetary roller screw design uses five main components-screw, nut, satellite roller, synchronous gear and guide ring. The guide ring and the synchronizing gear are located at each end of the nut, and the end of each roller has a cylindrical pivot and gear teeth.

The pivots of the rollers are installed in the holes of the guide ring to keep the rollers equidistant. The gear teeth at the end of the roller mesh with the synchronizing gear and guide the roller parallel to the axis of the screw. The well-designed geometry and precise manufacturing ensure that there is no axial movement between the roller and the nut.

These very stable kinematics enable planetary roller screws to achieve higher speeds and accelerations than ball screws, and ensure that the screws continue to function even under adverse operating conditions. The use of rollers to support the load makes the roller screw have a higher load capacity and better rigidity than a ball screw of similar size, and have the same or similar efficiency.

Inverted roller screw: inverted nut, customized shaft

In the inverted roller screw design, the synchronizing gear is directly cut into the shaft, which makes the outer diameter of the nut smaller than the nut of the planetary assembly with the same size screw shaft. Inverted designs usually have three starts, while standard planetary roller screws usually have five or six starts. Due to the less number of starts, the inverted design can use a larger thread form, thereby providing a higher load capacity than the standard planetary design.

Since the screw shaft of the inverted roller screw is threaded only on a section equal to the length of the roller, the rest of the shaft can be customized, so the inverted screw is very suitable for integration into systems such as linear actuators.

Inverted roller screws in linear actuators. Offered by Diakont

Recirculating roller screw: precise positioning with low torque

Recirculating roller screws are designed for small, precise movements with high rigidity and applications where the available motor torque is limited.

The core components of the recirculating roller screw are the same as those used in the planetary roller screw. However, the rollers in the recirculating roller screw have circular grooves (no helix angle) instead of the threads on traditional rollers.

The roller is fixed in a cage with a groove slightly longer than the length of the roller. Because the roller is slotted instead of threaded, when the screw shaft rotates, the roller moves axially along the length of the slotted cage in the nut. This axial movement means that the rollers need to be recirculated. Once the roller rotates a full circle around the screw shaft, the cams at both ends of the nut will disengage the roller from the screw shaft and return it to the initial position, but advance the lead of the screw.

The cyclic design only requires single- or double-start threads, which means it can be manufactured with very fine or fractional millimeter leads, such as 0.25 mm-even with large diameter screws. Smaller lead means less input torque is required. This makes the recirculating roller screw ideal for applications that require high rigidity and precise positioning, such as grinding equipment, semiconductor manufacturing, and lens focusing.

The ultra-high vacuum wafer handling robot adopts the RVR recirculating roller screw, and the positioning accuracy can reach /-2μm. Provided by Trust Automation

Differential roller screw: sub-micron positioning

Differential roller screws are so named because they operate based on differential motion between internal components. This difference allows the screw to have a very small lead or linear movement distance per revolution. How small? The lead of the differential roller screw can be as small as 0.03 mm. When used with linear rulers, they can move precisely in increments of less than 0.001 mm.

Differential roller screws combine sub-micron positioning with the ability to provide high thrust and stiffness. Differential designs also have high efficiency, which allows them to be driven in reverse. This unique combination of performance characteristics makes them ideal for professional designs, such as high-precision scientific equipment and satellites.

The RVD differential roller screw is used as part of the electron beam bending assembly in the Swiss light source SLS. Provided by PSI Institute

Which roller screw technology is right for you?

Planetary roller screws are suitable for a wide range of applications requiring high speed, high load capacity and high rigidity. The inverted roller screw has the same advantages, but has a better force-to-size ratio and the ability to easily customize the screw shaft, making it an ideal choice for integration into actuators and other linear motion systems.

Recirculating roller screws provide micron-level positioning capabilities for applications where positioning accuracy and rigidity are critical. Differential roller screws provide a unique combination of sub-micron positioning, good thrust and high rigidity for the most challenging high-precision applications.

Through a variety of design changes-from planetary to differential type-roller screws can meet a wide range of application requirements. But all these changes have two things in common: high thrust capability and high rigidity.

Since 1970, as an expert in roller screw technology, Rollvis has manufactured planetary, inverted, circulating and differential roller screws, with a highly skilled labor force and the most modern equipment. Our roller screws are the first choice for the most demanding and complex applications-from industrial presses to semiconductor manufacturing and space telescopes.

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