Development of lightweight precision ball screw shaft by swaging process

In recent years, the demand for lighter and more compact equipment has been increasing due to reduce product manufacturing time and the viewpoint of environmental load. Therefore, further lightweighting is demanded with the ball screw. The ball screw has step shape on both ends of shaft. Therefore, the lightweighting method of drilling to center of ball screw shaft makes it difficult to significantly reduce weight due to the thicker wall thickness to secure the strength of the shaft end. The lightweighting method devised is to form the step shape of the shaft end by swaging process to have a structure with a large space inside, which enables a significant weight reduction. The appearance and surface roughness of the lightweight ball screw shaft does not differ from that of conventional shaft. The weight of 0.70 kg is 0.55 times that of the conventional shaft, which enables a significant weight reduction. In the rotary bending fatigue test, the fatigue limit decreases with the ratio of wall-thickness, but the allowable torsional stress of the conventional shaft is satisfied. The axial rigidity is approximately 0.76 times that of the conventional shaft, which is smaller due to the reduction of the area moment of inertia in the hollow structure. The maximum torque is 0.02 Nm, satisfying the specified value of ±0.41 Nm. The torque fluctuation ratio is ±5 %, satisfying the best accuracy grade of ±15 %. The quietness is equivalent to number of revolutions above 2100 and improve at number of revolutions below 1000. Vibration characteristic is on the high-frequency side of resonance frequency in bending vibration, which increases the allowable rotational speed by approximately 1.2 times.