Satellite roller screws and their inherent advantages

With a traditional ball screw design, the captive ball bearings transfer the load between the screw and nut as they recirculate in the ball nut housing. Higher load capacities can be achieved either by increasing the number of ball bearings in the nut housing - for example, by doubling the number of ball returns - or increasing the number of times the ball bearings recirculate within the ball nut housing. Another option is to use a larger and more expensive ball screw. In all three cases, however, there is still a limitation based on the load-bearing surface area per ball bearing.

This is where the satellite roller screw (sometimes called a planetary roller screw) has a major advantage, as the matched rollers to rotate (or 'satellite') around the screw during actuation instead of ball bearings. This results directly in an increase in the number of contact points on the screw for supporting the load compared with that of a ball screw of the same diameter. The admissible static and dynamic load capacities are therefore considerably higher than that of a ball screw of the same diameter. In fact the static load can be three times greater than that of ball screws and their lifespan is 15 times longer.

The many points of contact also give a satellite roller screw greater rigidity and shock tolerance than a ball screw without compromising friction or efficiency.

Another key feature of satellite roller screws is that they can rotate twice as fast as a ball screw and support much greater accelerations, as there is no recycling of the ball bearings.

Smaller leads

Taking the product to another level, it is possible to supply satellite roller screws with smaller leads compared to ball screws. As the lead is a function of the pitch of the planetary roller screws, the lead can be very small (0.5mm or less). The lead can therefore be chosen and supplied without any geometrical changes to either the screw shaft or nut body. In a ball screw the lead is limited by the diameter of the ball bearings.

The Abssac satellite roller screw product has two standard designs directly aimed at replacing ball screw designs. A full design capability enables satellite roller screws to be developed to upgrade linear designs around the world.

Applications for satellite roller screws have so far included aerospace, medical, exploration and machine tools.

As standard, satellite screws can be supplied with outside diameters as small as 3.5mm and up to 200mm, and leads can be as fine as 0.25mm. Both steel and stainless steel versions are available in both right- and left-hand threads. The product can be finitely accurate and can be adapted for specialist and demanding linear applications.

Roller screw nut types

In the RV and BRV types of roller screw nut, the satellite rollers move as the lead on the screw. In this execution the rollers are in constant contact with the thread of the screw and the thread of the nut. The rollers do not move axially relative to the nut and are not recycled. Due to its design this execution is particularly suitable for applications where high axial loads, high speeds, and high feed rates are required. RV and BRV types may be distinguished from each other by the execution of the screw; the RV screw has a ground thread whereas the BRV has a precision rolled thread.

The RVR execution is designed for small leads and is used where high positioning precision is required in association with high stiffness and heavier axial loads. As the rollers are recycled after each of their revolutions, this design is not suitable for high speeds.

Standard nut styles are cylindrical, flanged cylindrical (with a central or end flange), plus custom designs can be manufactured. In addition, nuts can be preloaded to give zero backlash.

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