A linear motion actuator is disclosed. The linear motion actuator can include a double-shafted motor having a first motor shaft with a first screw thread and a second motor shaft with a second screw thread. The first screw thread can have certain characteristics that are different from that of the second screw thread. The linear motion actuator is disclosed for use in an arm assembly of a laparoscopic manipulator, but can also be used in other types of devices.