Ball screw device and linear motion device

A ball screw device comprising:

• a screw shaft of which an outer peripheral surface is formed with a helical screw groove;
• a nut of which an inner peripheral surface is formed with a helical screw groove corresponding to the helical screw groove of said screw shaft;
• a helical circulation path defined by the two helical screw grooves;
• a multiplicity of balls so disposed in said helical circulation path as to be capable of rolling;
• and
• a plurality of spacers,
• wherein each spacer is disposed between two adjacent balls and has two concave surfaces facing respectively to said two balls, and
• wherein when all said balls and all said spacers inserted into said helical circulation path are converged to be in successive contact, a total gap spacing (S1) formed between a leading ball and a tailing spacer is larger than zero (S1>0) and a distance (S2) between said leading ball and a tailing ball is smaller than a 0.8 times of a diameter (ds) of said tailing spacer (S2<0.8×ds).

A linear motion device comprising:

• an outer member;
• an inner member facing to said outer member via a gap;
• a multiplicity of balls disposed between said outer member and said inner member;
• and
• a spacer interposed between adjacent balls,
• wherein said spacer has such a configuration that said adjacent balls come only into line contact with said spacer and said line contact is at outer edges of said spacer or portions vicinal to the outer edges.