The main screw shaft is coupled to the stepper motor by a bellow coupler (bellow #1) and the motor to the encoder by a second bellow coupler (bellow #2). If for any reason (encoder change, motor change, one of the bellows slept) the angular relationship between the screw and the encoder is lost, then:
-these two objects must be linked again in good angular agreement (see detail)
-the theodolite survey of the bench must be redone (see detail)