Plastic gear
A plastic gear having a gear peripheral portion including a tooth portion is constituted by a plastic material, and a gear inner circumferential member with an annular shape or a disk shape is provided at the inner circumferential side of the gear peripheral portion. The gear peripheral portion includes an inner core and a surface layer member, the inner core being covered with the surface layer member and made of plastic with high elastic modulus and high strength, and the surface layer member being made of plastic with relatively low elastic modulus. The inner core is revolvably fitted around the gear inner circumferential member so as to revolve around the gear revolving axis, and an engaging portion is provided at an interface of the surface layer member and the gear inner circumferential member for preventing relative revolution around the gear revolving axis.
1. A plastic gear comprising:
a gear peripheral portion made of plastic, including gear teeth; and
an inner circumferential member with an annular shape or a disk shape, positioned at an inner circumferential side of the gear peripheral portion,
wherein the gear peripheral portion includes an inner core having inner tooth portions corresponding to respective gear teeth, and a surface layer member covering the inner core and having obverse tooth portions which cover respective inner tooth portions, wherein the inner core and the surface layer member are made of plastic, the plastic of the inner core having a higher elastic modulus and strength than the plastic of the surface layer member, in which the inner core is fitted and slidably engaged around a periphery portion of the inner circumferential member so that the periphery portion extends over the entire periphery of the inner circumferential member with an even cross sectional shape so as to allow relative displacement of the inner core in the circumferential direction thereof, and an engaging portion is provided at an interface between a lateral layer portion of the surface layer member and the inner circumferential member so as to prevent relative displacement in the circumferential direction thereof.
2. The plastic gear according to claim 1 , wherein a fitted portion defined by the interface between the inner core and the periphery portion of the inner circumferential member is comprised of at least one pair of a circumferential groove and a circumferential ridge, both extending in a circumferential direction of the inner core and the gear inner circumferential member.
3. The plastic gear according to claim 2 , wherein the gear inner circumferential member is made of metals or ceramic materials, the inner core is formed by an insert injection molding with the gear inner circumferential member as an insert material around the gear inner circumferential member, and the surface layer member is formed by insert injection molding with the gear inner circumferential member and the inner core as insert materials around the gear inner circumferential member and the inner core.
4. The plastic gear according to claim 1 , wherein the gear inner circumferential member is made of metals or ceramic materials, the inner core is formed by an insert injection molding with the gear inner circumferential member as an insert material around the gear inner circumferential member, and the surface layer member is formed by insert injection molding with the gear inner circumferential member and the inner core as insert materials around the gear inner circumferential member and the inner core.
5. The plastic gear according to claim 1 , wherein the periphery portion of the inner circumferential member is provided as a large diameter portion located intermediately in an axial direction thereof and small diameter portions at both sides of the large diameter portion, the inner core is fitted around the large diameter portion, respective lateral layer portions of the surface layer member overlap respective lateral faces of the large diameter portion and each inner periphery edge portion of the lateral layer portions is adjacent to each small diameter portion; the engaging portion is provided at each of the interfaces thereof.
6. The plastic gear according to claim 5 , wherein the gear inner circumferential member is made of metals or ceramic materials, the inner core is formed by an insert injection molding with the gear inner circumferential member as an insert material around the gear inner circumferential member, and the surface layer member is formed by insert injection molding with the gear inner circumferential member and the inner core as insert materials around the gear inner circumferential member and the inner core.
7. The plastic gear according to claim 5 , wherein circumferential concavities are formed in respective lateral faces of the large diameter portion extending in the circumferential direction along the small diameter portion, and the interfaces of respective lateral layer portions and small diameter portions are widened inwardly to the thickness of the large diameter portion.
8. The plastic gear according to claim 7 , wherein the gear inner circumferential member is made of metals or ceramic materials, the inner core is formed by an insert injection molding with the gear inner circumferential member as an insert material around the gear inner circumferential member, and the surface layer member is formed by insert injection molding with the gear inner circumferential member and the inner core as insert materials around the gear inner circumferential member and the inner core.