Gear

Abstract

The problem of the present invention involves providing a gear that has high tooth-root bending strength and for which there is no chipping of the tips of the teeth. Accordingly, the surface of the gear is carburized and the gear is strengthened by imparting residual stress, with the residual stress in the region with a surface depth of 5 μm to 20 μm being −1000 MPa or less, and the residual stress in the region with a surface depth of 50 μm to 150 μm being −1000 MPa or greater.

1 . A gear including a carburized surface and strengthened by application of residual stress to the gear,
 wherein the residual stress in a region at a depth of 5 μm or more but 20 μm or less from the surface is −1400 MPa or less, so that when a tooth of the gear is subjected to repeated stress, the repeated stress is canceled out by compressive stress to prevent surface cracks in the surface of a tooth root and improve a strengthening rate of the gear,   the residual stress in a region at a depth of 50 μm or more but 150 μm or less from the surface is −1000 MPa or more to prevent inner cracks and restrain tooth-tip chipping, and   the residual stress in a region at a depth of more than 150 μm but 230 μm or less is −800 MPa or less so that the residual stress cancels out hertz stress caused by contact of the teeth of the gears.   
     
     
         2 . The gear according to  claim 1 , wherein the region at a depth of 50 μm or more but 150 μm or less from the surface is subjected to a shot peening treatment using shot particles, and
 the region at a depth of 5 μm or more but 20 μm or less from the surface is subjected to a shot peening treatment using shot particles with a smaller particle diameter and a larger hardness than said shot particles.