Roller screw
Planetary roller screws are screws in the drive technology, which honed planetary rollers, in a nut between two hole rims widened to rotate a special roller screw spindle, whereby the nut moves linearly along the spindle. This system, there are also as an inverted version in which a correspondingly designed nut is moved linearly in a cylinder with internal threads. Advantage over the system with internal spindle is a compact design that allows for easy integration into hollow shaft motors. The push rod is smooth and the external appearance resembles a pneumatic or hydraulic cylinder.
Operation
The name planetary roller screw is derived from the type of rotation of the rollers around the spindle, which revolve like planets around a central star. This spindle, roller and nut diameters are selected so that the peripheral speeds of the spindle and roles match. The synchronization is performed by a built- in ring nut with an internal toothing which drives the rollers. Since you do not move the rolling elements in contrast to the ball screw or roller screw with recirculating rollers relative to the nut, no feedback mechanism is necessary. This allows higher speeds than for example with ballscrews would be possible.
The high load ratings of roller screws arising from the large number of bearing contact surfaces. A typical roller screw has about 10 rolls, each about 15 to 25 threads available. Thus arise 100-200 points of contact. In order to achieve the same load rating with a ball screw, either a large number or very large balls would be necessary. A thread roll thus fulfills the function of a large number of superimposed balls on a much smaller space.
Another advantage of the roller screw is the ability to achieve very small pitches. For ball screw drives the incline must be at least larger than the sphere diameter, as their careers otherwise overlap. When roller screw this restriction does not apply, however. However, for large gradients increases the sliding portion in the rolling motion, whereby the efficiency decreases.
Therefore, these linear actuators are suitable for technical, in particular for heavy loads and medium speeds.
Life
The calculation of the lifetime ( ) of a roller screw is as for rolling bearings and ball screws on the dynamic load rating. Since only axial forces ( ) are in any case permitted, accounting for radial and Axialbeiwert. For the characteristic fatigue life of the planetary roller screw drive is valid:
As with roller bearings, these are to the life that is with a probability of 90 % is reached or exceeded.
Applications
Especially in combination with servo motors and the corresponding control technology provides various applications. Basically a replacement of hydraulic and pneumatic cylinders is possible for example. This can be achieved due to the better efficiency, energy savings often. In comparison to hydraulics and the hydraulic fluid and the associated environmental and safety problems is eliminated. The position and speed control is much simpler than for the hydraulic or pneumatic drive systems due to the high rigidity of the overall system in general. Typical applications include motion simulators, presses, robot welding guns, dosing, mounting stands and valve actuators. This usually forces are up to about 200-300 kN and speeds up to about 1.5 m / s is required.