For this purpose, a nitride layer having {HEAD ~ a hardness} of Hv 650 or more at a thickness of 3 mum or more and 15 mum or less is formed on {TAIL ~ the surface} of a cage 
For this purpose , a nitride layer having { HEAD ~ a hardness } of Hv 650 or more at a thickness of 3 mum or more and 15 mum or less is formed on { TAIL ~ the surface } of a cage
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH