Greasy Brakes

    J2 axis on an M-16 robot is slipping when the brakes engage. The arm drops/droops for a split second before the brakes actually grab and stop/hold the robot as expected, but at that point, the controller has faulted for Stop Error Excess (SRVO-023).

    My robot guru suggested grease on the brakes and sent me the procedure for replacing J2. No problem with that, but where exactly is the brake? Is it internal to the motor? Do I need to open the motor up to look for grease leakage? I am not completely mechanically inept, but neither am I a servo surgeon. I've delayed the inevitable by increasing the servo_off_time variable, but I am going to have to address this sooner than later, I guess. Just trying to decide if I should order a replacement while I have this one repaired or if this is something I can handle in house with minimal down time... any advice is welcome.

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    yes brake is part of the motor. servo brakes use friction to hold position. adding lubricant would reduce friction and make things worse. most likely brake is worn out and motor has to be replaced.

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    As panic mode said, the motor should be replaced. The brakes in Fanuc motors are inside the front of the motor assembly where any lubricant getting past the front seal can affect the brake rather quickly. If the robot hasn't had the grease exchanged in a while, I would consider doing that while it's down for the motor R&R. If the old grease has thinned out, it's easier to get past the front seal. Brakes also wear out from being engaged too many times while the robot is in motion.

    Be careful installing the input gear on the new motor so its bearings are not stressed.

    Make sure to use a new o-ring for the motor.

    When installing the motor, make sure it seats flat to the casting once you find the sweet spot centered in the 3 gears of the RV reducer. A worn RV & input shaft can sometimes allow enough play for the input shaft to engage only 2 of the 3 gears and essentially lock the RV.

    Regardless of the reason for the brake slippage, taking apart the motor is risky if not done right. There are videos of Fanuc motors being disassembled although some seem to lack the detail needed to show how to do it right.

    If it is lubricant in the motor, it can tough to remove as it will contaminate the brake friction pad, hide in narrow areas, soak into the porous surface of the castings. The friction pad should at least be replaced. Brake manufacturer Ogura suggests replacing the whole brake.

    • Best Answer

    Motors as a 'rule' should effectively be 'dry' inside.

    The seals/O-rings associated at the shaft end should prevent this from happening.

    When ingress occurs, you can suffer from internal electrical problems around the stator and damage to the rotor assembly and also possible encoders/pulse coders attached.

    Most time ingress occurs is due to maintenance routines using too much pressure to add lube/grease, or the process of using incorrect equipment or incompetence when removing/replacing the motors damaging the seals, on rare occasions the seal has degraded.

    Irrespective of the cause, always replace motor.

    Brake replacement can be done, but should really be done in a clean room and with the specialist equipment to avoid damage and contamination and being able to check correct holding torque as outlined by the motor manufacturer.

    Therefore, unless you are a motor specialist, always replace the motor.

    As far as the statement 'grease on the brakes', I would never term this situation that way, I would always say the brake is 'worn', just like your brake pads on your car, as this would be the most common cause as panic mode and Skooter mentioned.

    Irrespective, this issue is nearly always localised to the motor assembly.

    Brake wear is a normal evolution for a robot, exacerbated by frequent emergency stops or forced stops as opposed to controlled stops.

    If you are replacing a specific motor frequently for the same reason as brakes, then I would investigate further of the frequency of uncontrolled stops, resting position when the servo's are not in use and load associated with the rest position.

    I have seen lots of brake wear occur, due to the 'park' position the robot has been programmed with when the servos are off.

    This position is concentrating the COG to pivot around this specific motor, where a simple adjustment of the park position to equalize this across multiple motors (usually JT2 and JT3) can assist with increasing the lifetime of brakes.

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