Anti-backlash planetary gearing for optic rotary joint

The invention includes a precision planetary gear system which requires 2:1 gear ratio. The planetary gear system consists of three, or more planet gear assemblies. By adding a local degree of freedom, the planet gear assembly could be tilted relative to the carrier by a spring-loaded annular ring through a spherical feature thus force the three, or more planet gear assemblies symmetrically mesh into the sun gear and internal gear. A couple of elastomeric sleeves are introduced into the connection between the planet housing and the carrier so as to balance the load evenly among the three planet assemblies and to reduce the speed fluctuation of transmitted motion as well as the noise and vibration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to epicyclic gear trains, and in particular to anti-backlash mechanism of precision planetary gear systems for the application of multi-channel fiber optical rotary joint in telecommunication industry.

2. Description of Related Art

In a gear transmission system, when two gears mesh each other, there is usually a clearance, or backlash between the teeth of two gears due to manufacturing errors and assembly errors. At a reversal transmission, the direction of rotation changes and the output shaft of gear transmission system would turn a slight angle due to the above named clearance, or backlash. That would cause a motion loss, or kinematic transmission error and dynamically also cause noise and vibration.

Precision planetary gear systems has a unique application in multi-channel fiber optical rotary joint. U.S. Pat. No. 5,442,721 illustrates one such rotary joint which includes a stationary bundle of fiber optical collimators and a rotational bundle of fiber optical collimators. The light beams from rotational bundle of fiber optical collimators are passed through a de-rotating prism and directed onto individual collimators of stationary bundle. In such an application, the kinematic requirement is that the rotational bundle of fiber optical collimators should rotate at twice the speed of rotation of the prism coaxially. To reduce the optical loss and variation of loss with rotation, the kinematic transmission accuracy should be less than 4 to 8 arc minutes.

U.S. Pat. No. 4,189,951 discloses an anti-backlash twin gear with a spring bias for rotating the gears relative to each other about a common axis. When the twin gear is intermeshed with a driving pinion engaging both half gears thereof, the springs are then tensioned to cause the half-gears to exert pressure on opposite tooth flanks of the pinion, thereby avoiding lost motion. The disadvantages of the anti-backlash twin gear are that the diameter of the twin gear need to be large enough for installing the spring bias apparatus and an open space should be big enough for assembly of further gears.

A more sophisticated approach can be found in U.S. Pat. No. 4,072,064. Here, the anti-backlash gear assembly includes a hydraulic piston that automatically maintains clearance filling positioning of two circumferentially and axially adjustable segments of a split gear. However, this device is complicated and expensive when compared to simple spring biasing system.

SUMMARY OF THE INVENTION

The first object of the present invention is to provide an anti-backlash apparatus in a planetary gear train for multi-channel fiber optic rotary joints.

Another object of the present invention is to increase the accuracy of kinematic transmission and to reduce noise and vibration in a planetary gear train.

A further object of the preset invention is to provide a very low-profile and compact design of a planetary gear train with 2:1 gear ratio for multi-channel fiber optic rotary joints.

DETAILED DESCRIPTION OF THE INVENTION

As shown inFIG. 1, a typical design of a multi-channel fiber optical rotary joint comprises rotor1, fiber bundles2, stator6, Dove prism7and prism holder5. The stationary fiber bundles2is mounted along the axis of stator6through threaded plate9. While the rotational fiber bundles2is fixed in the central hole of rotor1. When the rotor1rotates, the prism holder5should also rotates at half the speed of rotor1and in the same rotational direction to ensure the light beam from the collimator11of the rotational bundle are transmitted onto the correspondent collimator11of the stationary bundle. Vise versa. The 2:1 gear ratio is realized by the planetary gear system which includes sun gear3, first planet gear4(3 pieces), second planet gear18(3 pieces), internal gear8and carrier5(also called prism holder). The first planet gear4is coaxially fixed with the second planet gear18.

Kinematic joints, i.e., three revolute pairs are constituted through a pair of bearings20, a pair of bearings21and a pair of bearings22between rotor1and stator6, planet gears4(with planet gear18) and carrier5, carrier5and stator6. As shownFIG. 2, two gear pairs are formed between sun gear3and planet gear4, as well as planet gear18and internal gear8. Based on Gruebler's equation, the degree of freedom for the said planetary gear system is one.

InFIGS. 4A & 4B, the carrier, or prism holder5has three symmetrically placed through bores (51). Correspondingly, three slots (40) are symmetrically designed on the two side plan of carrier5.

FIG. 5illustrates the planet gear assembly which includes planet gear4, second planet gear18, planet housing14and17, a pair of bearing21, elastomeric sleeves15and16. In the present invention, planet housing14and17are coaxially fixed together. On planet gear housing17, a feature (42) is deliberately designed as a spherical shape which is loose fitted in the bore (51) of carrier5. When a force is applied on the tapered feature (141) of planet housing14, relative to carrier5, the whole planet gear assembly will tilt around the spherical feature (42).

In a preferred embodiment, 3 or more said planet assemblies are symmetrically arranged encircling the sun gear and internal gear. A spring-loaded elastomeric annular ring13is tightly placed on the tapered feature (141) of planet housing14and is centered with carrier5. The annular ring13is seated in the annular slot of steel holder12(FIG. 1andFIG. 3), and is connected with carrier5through three screw19(FIG. 3) so that a force is applied on the tapered feature (141) of planet housing14. As shown inFIG. 2, the three planet gear4are symmetrically meshed with sun gear3and are hold tightly against the sun gear by the spring-loaded elastomeric annular ring13. At the same time, the three planet assemblies are tilted so that the three second planet gear18are forced to tightly mesh with the internal gear8. The backlashes between the meshed teeth are eliminated due to the applied force from elastomeric annular ring13. By adjusting the screw19, the applied force from elastomeric annular ring13can be changed.

As shown inFIG. 1˜5, the elastomeric sleeves15and16are mounted on the shaft of planet gear housing14and17respectively, and fitted within the slots (40) of carrier5. The function of the three pairs of elastomeric sleeves15and16are to balance the load evenly among the three planet assemblies and to reduce the speed fluctuation of transmitted motion as well as noise and vibration.