Flat strain wave gearing
A relieving portion is formed between a first external tooth portion and a second external tooth portion in the external teeth of a flexible externally toothed gear of a flat strain wave gearing. The length L 1 of the relieving portion in the tooth trace direction is within the range of 0.1 to 0.5 of the tooth width L of the external teeth. The maximum relieving amount t from the tooth top land of an external tooth in the relieving portion is 3.3×10 −4 <t/PCD<6.3×10 −4 where the PCT is defined as the pitch circle diameter of the external teeth. The tooth face load distribution of the external tooth in the tooth trace direction can be equalized, and a flat strain wave gearing having a high transmitted-load capacity can be achieved.
1. A flat strain wave gearing comprising:
a first rigid internally toothed gear provided with first internal teeth having a first number of teeth;
a second rigid internally toothed gear disposed adjacent to and coaxially with the first rigid internally toothed gear, and provided with second internal teeth having a second number of teeth different from the first number of teeth;
a cylindrical, flexible externally toothed gear which is disposed coaxially with inner sides of the first and second rigid internally toothed gears, which is provided with external teeth capable of meshing with the first internal teeth and the second internal teeth, and which is capable of flexing in a radial direction; and
a wave generator which is disposed on an inner side of the flexible externally toothed gear, and which makes the flexible externally toothed gear flex into a non-circular shape, causing the external teeth to partially mesh with the first and second internal teeth;
wherein the external teeth are divided into a first external tooth portion that faces the first internal teeth, and a second external tooth portion that faces the second internal teeth, by a relieving portion, the relieving portion being formed in a portion of the external teeth located in a center portion along a tooth trace direction of the external teeth, and the relieving portion being formed through a predetermined length in the tooth trace direction;
wherein the relieving portion is subjected to relieving so that a tooth depth thereof is lowest in a tooth-trace-direction center of the relieving portion;
wherein the wave generator is provided with a first wave bearing for supporting the first external tooth portion and a second wave bearing for supporting the second external tooth portion;
wherein a relieving length L 1 of the relieving portion in the tooth trace direction satisfies a conditional expression
0.1 L<L 1<0.5 L
where L is a total tooth width of the external teeth; and
wherein a maximum relieving amount t in a tooth depth direction from a top land of the external teeth in the relieving portion satisfies a conditional expression
3.3×10 −4 <t /PCD<6.3×10 −4
where PCD is a pitch circle diameter of the external teeth.
2. The flat strain wave gearing according to claim 1 ,
wherein, when Lo is a distance between support centers of the first and second wave bearings,
the distance Lo between the support centers increases and decreases as the relieving length L 1 increases and decreases; and
the distance Lo between the support centers satisfies a following conditional expression:
0.35 L<Lo< 0.7 L.
3. The flat strain wave gearing according to claim 2 ,
wherein, the relieving length L 1 satisfies a following conditional expression, in relation to the distance Lo between the support centers:
0.3 Lo<L 1<0.7 Lo.
4. The flat strain wave gearing according to claim 3 ,
wherein, where a tooth width L 2 is a tooth width of each of the first and second rigid internally toothed gears, the tooth width L 2 satisfies a following conditional expression, in relation to the total tooth width L of the external teeth:
0.3 L<L 2<0.5 L ; and
wherein the tooth width L 2 decreases as the relieving length L 1 increases, and increases as the relieving length L 1 decreases.