Excessive backlash and audible grinding from Joint 4 gearbox on R-100 robot

Description

We are experiencing a performance issue with one of our R-100 series robots on the assembly line. For the past week, the robot has been failing its final position quality check, exceeding the +/- 0.5mm tolerance. Our maintenance team has isolated the issue to the Joint 4 axis. During manual manipulation in teach mode, we can feel a significant amount of play or backlash, which we measured to be approximately 15 arcmin, well above the spec sheet's <3 arcmin for the NXB-GBX-PLN122-005 gearbox. There is also a distinct, low-pitched grinding noise coming from the gearbox housing when the joint is under load, especially during deceleration. The servomotor itself seems to be functioning correctly, and encoder readings are stable, but the controller intermittently throws a position tracking error. We suspect premature failure of the planetary gear set inside the gearbox.

Symptoms

• Audible grinding noise from robot Joint 4 during operation.
• Inconsistent positioning and path deviation.
• Increased backlash measured at Joint 4 output flange.
• Robot controller throwing E-4115 Position Tracking Error alarms.

Resolution

The investigation confirmed that the NXB-GBX-PLN122-005 gearbox on Joint 4 had failed prematurely. Upon removal and inspection, the lubricant was found to be contaminated with fine metal shavings, indicating significant internal gear wear. The input shaft bearing also exhibited signs of spalling. The root cause was determined to be excessive shock loading from a previous programming error that was not caught in time. The faulty gearbox was replaced with a new unit. Following the hardware replacement, a full robot mastering and calibration procedure was performed to ensure accurate positioning. The robot passed all subsequent quality checks and has been returned to production.

Resolution Steps

1. Power down the robot and apply LOTO (Lockout/Tagout) procedures.
2. Remove the Joint 4 axis covers to access the servomotor and gearbox assembly.
3. Disconnect the servomotor from the PLN122-005 gearbox input.
4. Unbolt and carefully remove the faulty gearbox from the robot arm structure.
5. Install the new NXB-GBX-PLN122-005 gearbox, ensuring proper alignment and torquing mounting bolts to the specified 45 Nm.
6. Reconnect the servomotor and all associated cabling.
7. Power on the robot, clear all alarms from the teach pendant.
8. Perform a zero position mastering and full axis calibration for the R-100 robot to update the controller with the new component's precise position.