Abstract: A lubricant composition, which is low in friction, does not have an attacking property against polyamide resin, etc., is excellent in heat resistance, and especially does not undergo deterioration or loss of the respective characteristics in a short term when used under a high temperature environment, is provided along with a speed reduction gear and an electric power steering apparatus that use the lubricant composition. The lubricant composition has blended therein a synthetic hydrocarbon oil, a urea thickener, zinc stearate, and a sulfur-based antioxidant. A speed reduction gear has a portion of engagement of a small gear and a large gear filled with the lubricant composition. In an electric power steering apparatus, an output of a steering assisting electric motor is speed reduced and transmitted to a steering mechanism via the speed reduction gear.

Abstract: A lubricant composition, which is low in friction, does not have an attacking property against polyamide resin, etc., is excellent in heat resistance, and especially does not undergo deterioration or loss of the respective characteristics in a short term when used under a high temperature environment, is provided along with a speed reduction gear and an electric power steering apparatus that use the lubricant composition. The lubricant composition has blended therein a synthetic hydrocarbon oil, a urea thickener, zinc stearate, and a sulfur-based antioxidant. A speed reduction gear has a portion of engagement of a small gear and a large gear filled with the lubricant composition. In an electric power steering apparatus, an output of a steering assisting electric motor is speed reduced and transmitted to a steering mechanism via the speed reduction gear.

Abstract: A power transmission shaft which can be evaluated on torsional fatigue strength based on a management indicator different from compressive residual stress in an outer surface. In a power transmission shaft having outer circumferential serrations 1a, 1b, torsional fatigue strength is evaluated by using compressive residual stress at a depth of 30 ?m (and 50 ?m) from an outer surface of a rising portion of each concave of the outer circumferential serrations 1a, 1b as a management indicator. A high torsional fatigue strength can be secured by having a compressive residual stress of 1150 MPa or more at a depth of 30 ?m (and 50 ?m) from the outer surface.