Gang Deng

Effect of Surface Treatments on the Strength of Carburized Gears

This paper deals with effects of surface treatment on the bending fatigue strength of SCM415 carburized spur gears. The test gears are treated by the combination of shot peening, chemical polishing and electropolishing after carburization. The fatigue tests demonstrate that the strength is sensitive to the surface condition of tooth fillet and the removal of the nonmartensitic layer caused by decarburization is considerably effective in enhancing the strength. In the first part of this paper, the influence of surface treatments such as shot peening, chemical polishing and electropolishing on the strength enbancement for carburized gears are summarized and discussed. In the second part, the crack lengths are calculated from the fatigue test results for the carburized and surface-treated gears, and the effect of surface treatments is discussed from the view point of fracture mechanics.

Initial Temperature Evaluation for Flash Temperature Index of Gear Tooth

In the AGMA standard for the scoring resistance evaluation of gears, flash temperature is used as an index indicating the risk of scoring. The flash temperature is expressed as the sum of flash rise of temperature and initial temperature. In this research, the initial temperatures are measured for various gear sets in running by thermocouples which are mounted just beneath the working surface of pinion. The effects of gear dimensions, rotating speed, and lubrication conditions on the initial temperature are clarified. An experimental formula is proposed for estimating the initial temperature, which depends on the temperature of oil inlet, the frictional power loss, and the running conditions such a rotating speed, contact position, and the oil supply rate. The estimated initial temperature are in good agreement with the measured values. The proposed method is available for the scoring resistance evaluation of the vehicle transmissions

Size Effect in Bending Strength of Carburized Gear Teeth

This paper deals with the bending strength and the size effect of carburized fine module gears. The fine module test gears (m = 1–1.5) are made of a low-carbon alloy steel, and they are carburized to have various effective case depths. The bending fatigue tests are performed using these gears. From the test results, the AGMA recommendation of the effective case depth is confirmed to be valid in view of the bending strength. The SN curves are obtained and they are expressed by the experimental formulas. The fatigue strength of fine module gears are compared with the strength of gear of m = 5, and the size factor is evaluated. In order to make clear the size effect, the fracture mechanics is applied to the fatigue test results. The fatigue life is assumed to be the crack propagation process, and the maximum initial length of crack is evaluated. This crack length is proportional to the thickness of nonmartensitic layer at the fillet. The size effect is discussed on the basis of stress intensity factor and the thickness of nonmartensitic layer.

Effect of CBN Grinding on the Bending Strength of Carburized Gears

The surface condition of a carburized gear is improved by CBN grinding, in addition to the generation of a compressive residual stress at the ground surface. To clarify the effect of these merits on the strength enhancement, a bending fatigue test is performed for the carburized and the CBN-ground gears. The fillet as well as involute tooth surface is ground by an electroplated threaded CBN wheel. The low axial feed rate and the spark-out treatment are effective in improving the surface finish and increasing the compressive residual stress, and the strengths of CBN-ground gears are increased from 100 MPa to 160 MPa in comparison with the carburized gears. Moreover, the test results are discussed on the basis of the fracture mechanics-based strength evaluation which has been proposed by the authors. This analysis demonstrates that the influence of CBN grinding on strength is smaller than the effects of surface-treatments such as shot peening and chemical polishing.

Influence of Spin and Skew on Non-Newtonian EHL Traction

The traction characteristics for a point contact are evaluated with the model of the limiting shear stress of lubricants. The influences of spin and skew on the initial slope of the traction curves are also investigated. The traction coefficient will drop severely for a very small skew angle, but it not so sensitive to the spin rate. With the skew angle or spin rate, the initial slope of the traction curves decreases at the heavy loads. The approximate formulas for the with-spin traction coefficient and for the with-skew traction coefficient are proposed based on the examination of the calculated shear stress.