Using a series of straight mineral oils with a variety of molecular weight distributions failure loads of spur gears with gear ratios 27:25 and 1:1 were investigated. The failure loads in the case of the synchronous engagement 1:1 were considerably higher than those in the asynchronous engagement 27:25 as a result of imprinting of asperities of harder teeth on softer engaged teeth. The correlation among the results was found to be quite regular, showing the beneficial effect of wide-ranged oils in the molecular weight distribution. Summarizing the obtained results, it is concluded that the failure loads of narrow-ranged oils were conditioned by their viscosity grades, and that those of wide-ranged oils increased further with the aid of oxide film formation on teeth. Results of four-ball tests and two-disk tests previously carried out under mild conditions using the same narrow-ranged and wide-ranged oils show the similar feature to those of the gear tests.

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