Linear motion device

A linear motion device comprising:

• a shaft;
• a linear motion body fitted to said shaft and guided by said shaft so as to be linearly movable in an axial direction of said shaft;
• a plurality of rolling elements disposed between said shaft and a rolling element groove formed in an inner circumferential surface of said linear motion body so as to roll freely;
• a circulation path formed in said linear motion body, for circulating said rolling elements from one end side of said rolling element groove to the other end side thereof;
• and
• retaining pieces each disposed between adjacent ones of said rolling elements and having two concave surfaces facing said adjacent ones of said rolling elements respectively
• wherein at least one kind of said shaft, said linear motion body and said rolling elements is made of steel containing 0.4% by weight to 0.9% by weight, both inclusively, of carbon (C), 2.5% by weight to 8.5% by weight, both inclusively, of chromium (Cr), 0.1% by weight to 2.0% by weight, both inclusively, of silicon (Si), 0.1% by weight to 2.0% by weight, both inclusively, of manganese (Mn), and a residual part of iron (Fe) or inevitable impurities while satisfying a relation:
• C content (wt %)≦−0.05×Cr content (wt %)+1.41 (wt %).

A ball screw comprising:

• a screw shaft having a helical male thread groove formed in its outer circumferential surface;
• a nut having a helical female thread groove formed in its inner circumferential surface;
• balls interposed in a rolling path formed between said male thread groove and said female thread groove so as to roll freely;
• and
• retaining pieces each disposed between adjacent ones of said balls and having a pair of concave surfaces facing said adjacent ones of said balls,
• wherein a section taken along an axial line, of at least one of said male thread groove and said female thread groove has a curvature radius not smaller than 52.5% and not larger than 55% as large as a diameter of each of said balls.

A ball screw comprising:

• a screw shaft having a helical male thread groove formed in its outer circumferential surface;
• a nut having a helical female thread groove formed in its inner circumferential surface so that said female thread groove is opposed to said male thread groove;
• balls interposed between said male thread groove and said female thread groove so as to roll freely;
• and
• retaining pieces each disposed between adjacent ones of said balls and having a pair of concave surfaces facing said adjacent ones of said balls,
• wherein a grease composition containing 0.1% by mass to 10% by mass of an electrically conductive substance is enclosed.

A ball screw comprising:

• a screw shaft having a helical male thread groove formed in its outer circumferential surface;
• a nut fit on said screw shaft and having a helical female thread groove formed in its inner circumferential surface to oppose said female thread groove to said male thread groove;
• balls interposed between said male thread groove and said female thread groove so as to roll freely;
• and
• retaining pieces each disposed between adjacent ones of said balls and having a pair of concave surfaces facing said adjacent ones of said balls,
• wherein a sulfonate-free grease composition is enclosed.