Eccentric rocking type reduction gears include a crank shaft. The crank shaft includes an eccentric cylindrical cam in order to perform eccentric rocking on an external gear. In general, the crank shaft is utilized as an input shaft, and rotates at a fast speed. Hence, fluctuating load acts on a bearing that supports the crank shaft due to centrifugal force produced inherently to the unbalanced shape of the eccentric cylindrical cam. For example, according to a reduction gear disclosed in Patent Document 1, in order to reduce such fluctuating load, the eccentric cylindrical cam is formed with a balancer weight. This suppresses the unbalance originating from the weight of the eccentric cylindrical cam.
Moreover, there are reduction gears that include two external gears. According to the reduction gears of this type, a crank shaft is provided with two eccentric cylindrical cams to support the two external gears, respectively. The respective eccentric cylindrical cams are disposed around the axial line of the crank shaft with respective phases shifted by 180 degrees from each other. This structure cancels translational force.
As explained above, according to the reduction gear of Patent Document 1, the balancer weight is formed inwardly of the eccentric cylindrical cam. This balancer weight eliminates the unbalance around the axial line of the crank shaft. However, the unbalance around the axial line orthogonal to the axial line of the crank shaft still remains unaddressed. Accordingly, couple is produced around the axial line orthogonal to the axial line of the crank shaft due to centrifugal force.
According to the reduction gears having the two external gears, the two eccentric cylindrical cams are disposed around the axial line of the crank shaft with respective phases being shifted by 180 degrees from each other. According to such a structure, the unbalance around the axial line of the crank shaft can be also addressed. However, the unbalance around the axial line orthogonal to the axial line of the crank shaft still remains unaddressed. Hence, couple is still produced around the axial line orthogonal to the axial line of the crank shaft.
The couple around the axial line orthogonal to the axial line of the crank shaft also applies fluctuating load to the bearing supporting the crank shaft. This often results in the shortage of the lifetime of the bearing. Moreover, the eccentric rocking type reduction gear is likely to generate vibration.