Worm reducer and electric power steering apparatus

Abstract

In a worm reducer for reducing the rotation transmitted to a worm and transmitting the reduced rotation to a worm wheel, the worm reducer being composed of the worm with screw faces formed on its outer periphery and the worm wheel with worm wheel teeth formed on its peripheral surface, the worm wheel teeth being in mesh with the screw faces of the worm, the worm wheel teeth are formed so that the lead angle of the worm wheel is smaller than that of the worm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining the schematic configuration of an electric power steering apparatus according to an embodiment of the invention;

FIG. 2 is a sectional view of the main part of the reducing mechanism in a power steering device according to an embodiment of the invention;

FIG. 3A is a view showing the worm of a worm reducer according to an embodiment of the invention;

FIG. 3B is a view showing the peripheral surface of the worm wheel of the worm reducer of the embodiment of the invention;

FIG. 4 is a top view showing the combined state of the worm and worm wheel of the worm reducer according to an embodiment of the invention;

FIG. 5 is a graph showing the relationship between the axis-crossing angle of the worm and the worm wheel and the module of the worm wheel where the reduction ratio of the worm reducer and the center distance between the worm and worm wheel are fixed;

FIG. 6 is a graph showing the relationship between the axis-crossing angle of the worm and worm wheel and the center distance therebetween where the reduction ratio of the worm reducer and the module of the worm wheel are fixed;

FIG. 7 is a view showing a machining tool for machining the worm wheel teeth of the worm wheel of the worm reducer of the invention;

FIG. 8A is a view showing the axis-crossing angle between the worm wheel and a worm in a worm reducer of the invention;

FIG. 8B is a view showing the axis-crossing angle between the worm wheel and an electro-deposition worm of the invention;

FIG. 8C is a view showing the axis-crossing angle between the worm wheel and a known hob-cutter;

FIG. 9A is a view showing a worm in the worm reducer of the related art;

FIG. 9B is a view showing the peripheral surface of the worm wheel in the worm reducer of the related art; and

FIG. 10 is a view showing a machining tool for machining worm wheel teeth on the worm wheel in a worm reducer of the related art.

Claims

1. A worm reducer comprising: a worm having a screw face on an outer circumferential surface thereof; anda worm wheel having worm wheel teeth formed on a circumferential surface thereof, the worm wheel teeth meshing with the screw face of the worm,wherein a rotation of the worm is reduced and transmitted to the worm wheel, anda lead angle of the worm wheel is smaller than that of the worm.

2. The worm reducer according to claim 1, wherein the worm wheel teeth are formed on the peripheral surface of the worm wheel by using a machining electro-deposition worm having a machining screw face, a shape of the machining screw face is the same as that of the worm, andabrasive grains are electro-deposited on the machining screw face.

3. The worm reducer according to claim 1, wherein the worm wheel teeth are formed on the peripheral surface by using a machining electro-deposition worm having a machining screw face, abrasive grains are electro-deposited on the machining screw faces,a tooth shape of the machining screw face is the same as that of the screw face of the worm, anda diameter of the tooth tip circle and a diameter of the tooth base circle of the machining screw face are larger than those of the screw face of the worm.

4. The electric power steering apparatus comprising: the worm reducer according to claim 1;an electric motor having a rotary shaft connected to the worm; anda steering shaft of which outer periphery is press-fitted to the worm wheel press-fitted,wherein a rotation of the electric motor is reduced and transmitted to the steering shaft through the worm and the worm wheel to thereby assist steering operation.