7th Axis Rack and Pinion DESTROYED (Pictures Within)

  • I've attached a few photos of what our group ran into over the weekend. This is the "rack and pinion" from underneath one of our vision robots that runs on a track to load completed parts into a rack. For the most part the robot sits stationary during the job with only short runs back and forth on the track until it goes to load it's first part into the rack and during this the robot runs the full length of the track into the rack, sets the part, and then runs the full length of the track to exit the rack once the drop has been completed. It does this ten times per rack but the travel gets shorter and shorter as the rack fills up. The major wear areas are at the two points where the robot is "accelerating" and "deccelerating" at a pretty quick speed.


    So my question is what would be more beneficiary to correct this issue?


    Would slowing down the ACCEL/DECCEL of this axis be a good start? (I would like to avoid slowing the line down but you can't win them all)


    Was this maybe caused by a misalignment or lack of proper lubrication? *EDIT* the more I think about it the more I believe the gears teeth are not properly “meshed” with the track and maybe there is a gap between the teeth. This is really hard to see being that it’s underneath the robot but I’m going to take some photos with my phone and see if I can get a clear shot. We may just need to shim the servo a bit to get this closed in.


    The linear bearings all seem perfectly fine as well at the rails.


    We have a second line that is literally identical except it was setup by a different integrater/programmer so all the equipment is the same to a "T" but the programs are entirely different and that line is showing minumal wear. The line in question uses one taught position and relys on Voffset to find the other 9 spots whereas our line that is still in good condition uses PR's for every point and then the vision adjusts X and Y. So this sticks out to me that maybe it could be something as a simple program change as far as acceleration limits or something along those lines?


    Our first shift guys replaced the rack and then when I got here I replaced the drive gear so I could begin reteaching the jobs. So everything is running currently but I want to know what you guys think of this and what we could try to do to prolong the life of this new track without effecting tact time to much. If you need to know anything else about the program or setup please ask.


    When I get in to work tomorrow I'm going to start trying to compare some variables and stuff and see if I can find any differences so if you guys have some you think I should check let me know! Thank you all in advance.

  • This is a setup issue. The backlash was inadequate. Make sure the backlash is set at the highest point on the gear rack. Most rail manufacturers have a fixture where an indicator can be mounted to measure this.

  • With my minimum mechanical experience, I would certainly be looking at the logs for overload occurrences of the drive motor, it is clear this damage has been accruing over a period of time and errors may have occurred, but successfully just been reset and carried on and therefore no intervention has been applied.


    The other thing to consider, is the inertia's involved during the track transitions, chances are inertias going in a different direction to track travel during dynamic changes, incorporate this with a foreign object intervening, too much force between rack and pinion, backlash, misalignment of pinion in rack (maybe due to a warped/worn drive shaft) then I can see how this damage could go un noticed until it completely fails in the way that it has.


    Just my 2 cents worth......hope you can get to the root cause, but I'd be surprised if this anything code related......

  • The last time I saw something like that, the metal in the rack&pinion was below spec. IIRC, there was a bad batch from the steel foundry that slipped past quality inspections.


    The fit of the pinion to the rack could also be an issue.


    On the software side, the only thing I can think of is that the accel/decel limits might be too generous. Or, possibly, the steel in the rack&pinion was spec'd for a lower stress level. It all depends on how much mass you're trying to move, and how quickly, vs the strength of the metal. Not all rack&pinion materials are created equal.

  • Thank you all for your input! I also believe this is a backlash issue as well as foreign contaminates and lack of lubrication. We have since greased the track and gear and are now doing so on a regular bases. Our foreign contamination issue is a bit harder to take care of though. In our process we friction stir weld aluminum to steel and then eliminate the burrs leaving bits of aluminum everywhere. These bits of aluminum fall off in different areas one of which is around the material handler which runs on this track. So far everything is looking good now and I did go through and slow some of the moves down just a bit because it was moving relatively fast. I did notice that the holes in the gear where the bolts hold it to the drive motor were quite worn out and I don't know if that is a result of the track failing or a reason the track failed.

    "I could tell that my parents hated me. My bath toys were a toaster and a radio."

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    The mounting holes for the pinion gear (to the driveshaft of the motor) were worn out? Like, oval?


    That suggests that the pinion gear was not mounted correctly to the drive shaft. Normally, the interface between the gear and drive shaft should not be just bolts -- there should be a spline, a keyway, or a dowel, something with a tight fit that makes it nearly impossible for the gear to rotate relative to the shaft. By contrast, the clearance holes for bolts are always rather loose, and no matter how tightly torqued, they will always allow the gear to "wiggle" a bit on the shaft, which will oval-out the holes, which will allow the gear to wiggle more, which...


    If the gear was not properly mounted to the drive shaft, that could be the root cause of your wear issues.

  • Yes, if you look at the first picture closely you will see what i'm talking about with the holes being worn out into an oval shape. There is no spline, key way, or dowl pins holding our gear in a fixed place so what you say makes perfect sense and it's not really anything i had thought about. Now that you've mentioned it I honestly can't believe I hadn't already thought about it. Thank you for pointing that out to me! I will see what we can do about having something done to correct this issue because we now have four robots that are rail mounted and they are all done the same way.

    "I could tell that my parents hated me. My bath toys were a toaster and a radio."

  • I would strongly suggest considering a more robust mechanical solution (something like Nexen's rack and roller pinion system) rather than what looks to be a standard straight rack (14 1/2° pressure angle?)


    At the very least, I'd suggest adding an oiler to the system with a visible/monitored reservoir - something like a way oiler from a CNC machine, for example.

  • Quote

    The mounting holes for the pinion gear (to the driveshaft of the motor) were worn out? Like, oval?


    That suggests that the pinion gear was not mounted correctly to the drive shaft. Normally, the interface between the gear and drive shaft should not be just bolts -- there should be a spline, a keyway, or a dowel, something with a tight fit that makes it nearly impossible for the gear to rotate relative to the shaft. By contrast, the clearance holes for bolts are always rather loose, and no matter how tightly torqued, they will always allow the gear to "wiggle" a bit on the shaft, which will oval-out the holes, which will allow the gear to wiggle more, which...


    If the gear was not properly mounted to the drive shaft, that could be the root cause of your wear issues.

    Totally agree, part of installation (Kawasaki usually do this) would be to add screwlock/Threebond (not that its good for high torque) and 'pen' them after 1st install so you can make a visual inspection of movement during preventative maintenance periods and maybe torque adjust.

    Maybe something to consider............

    But absolutely agree with SkyeFire, this would result in small amounts of wear initially, which would only get worse overtime.

  • I thank you all again for your suggestions and I will get with our engineer about possibly making a change. I would like to see some dowl pins or something installed here because I do agree that it started with the drive gear shifting and causing wear inside the bolt holes. Then over time, like you guys mentioned it got worse and worse until it then started destroying the track itself.

    "I could tell that my parents hated me. My bath toys were a toaster and a radio."