Roller screws, also commonly known as planetary roller screws or satellite roller screws, convert rotary to precise linear motion in a similar manner to ball screws or lead screws. Roller screw devices have a greater number of contact roller points compared to balls or thread-on-thread contact, providing higher stiffness and smooth motion. For this reason, roller screws are used in applications that require high stiffness or thrust outputs. They are also commonly used in electro-mechanical linear actuators.
Roller screws are similar to ball screws, except that the load transfer elements between the nut and screw are rollers, instead of balls. The additional bearing points allow roller screws to provide increased precision, higher load rating and rotational speed, high stiffness and longer lifetime. Roller screws may also be more compact than ball screws, while exhibiting similar efficiency.
Common elements in roller screws are the screw, nut, and a roller. The screw provides a V-shaped helical space for radially-arrayed threaded rollers around the screw and encapsulated by a threaded nut. The threading of the screw matches the internal threading of the nut. The rollers spin in contact with the screw and the nut, while provide low-friction transmission between the two.
Types of Roller Screws
Types of roller screws include standard roller (planetary) screws, inverted roller screws, and recirculating roller screws. Standard roller screws are also known as non-recirculating roller screws, due to the lack of axial movement of the roller in relation to the nut.
Inverted roller screws, also known as reverse roller screws, lack axial movement much like the standard roller screw as they were developed together. Inverted roller screws share other similarities with standard roller screws, except that the function of the nut and screw is reversed in relation to the rollers. In these roller screws, the rollers move axially within an elongated nut. The threaded part of the screw shaft can only travel as far as the threaded length of the rollers.
Recirculating roller screws, or recycling roller screws, provide a high degree of accuracy with minimal thread leads. The rollers in these screws re-circulate axially within the nut until they are reset after one orbit around the screw. Recirculating roller screws have very fine, identical single or two-start threads. The rollers are grooved, opposed to threaded, which moves them axially with the angled threads of the nut and screw. The rollers shift up and down by one lead of thread after an orbit. After a roller completes an orbit, the nut is released into a groove in the nut, which disengages from the nut and screw and is pushed between the cams at the midpoint of the assembly.