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