Many different devices are defined as “linear actuators”. All of them convert the rotary movement given by the motor into a linear movement, applied on the rod or on the saddle that moves the object. Be careful not to confuse the screw-jacks with the electric cylinders. Whatever could be its shape, the screw-jack (force multiplier), is based on the coupling between a motor and a screw; this coupling generates a liner movement. This happens both with trapezoidal acme screw or with ball screw, through a group “endless screw and gear”, with its incorporated reduction gear. The reduction values of this device can be very high, but its transmission efficiency is very low, about 0.3 overall, and having a running frequency usually about 30% of time.