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Unread 02-13-2018, 01:00 PM
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Snapping 775 Pro Shaft

So my team somehow was able to snap the shaft off while running a 775 Pro. We had an AM 12t Pinion on it when it snapped, connected to our drivetrain. For those teams that ran 775 drivetrains last year (2767, 2451, 3357, etc.), did you ever discover any failures similar to this?
We at first believed it was due to a lot of sudden forward and then reversing while driving but later we remembered our gearboxes became loose at one point and we are wondering if the sag of the motors put too much stress on the pinion, later causing it to fail. The pinion has an unusual wear pattern on it (flat teeth in a few spots but we have an enclosed gesrbox so could be due to getting wedged somewhere after failing).

And by the way, the motor still runs.
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Unread 02-13-2018, 01:10 PM
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Re: Snapping 775 Pro Shaft

We blew through a Versa Planetary last night...
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Unread 02-13-2018, 01:20 PM
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Re: Snapping 775 Pro Shaft

Can you post a close up picture of the end of the shaft while it is not spinning? From your description of the sagging motors, my guess is bending fatigue.
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Unread 02-13-2018, 01:20 PM
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Re: Snapping 775 Pro Shaft

Quote:
Originally Posted by Type View Post
And by the way, the motor still runs.
Was it running when that picture was taken? Most of the image is very clear, but the broken shaft surface looks blurry. A better look at that surface might help identify the failure mode.

When you said the gearboxes became loose, did you mean the motor screws?
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Unread 02-13-2018, 01:29 PM
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Re: Snapping 775 Pro Shaft

Quote:
Originally Posted by Type View Post
So my team somehow was able to snap the shaft off while running a 775 Pro. We had an AM 12t Pinion on it when it snapped, connected to our drivetrain. For those teams that ran 775 drivetrains last year (2767, 2451, 3357, etc.), did you ever discover any failures similar to this?
We at first believed it was due to a lot of sudden forward and then reversing while driving but later we remembered our gearboxes became loose at one point and we are wondering if the sag of the motors put too much stress on the pinion, later causing it to fail. The pinion has an unusual wear pattern on it (flat teeth in a few spots but we have an enclosed gesrbox so could be due to getting wedged somewhere after failing).

And by the way, the motor still runs.
Do you have a picture of your gearbox or at least how the gearing is laid out?
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Unread 02-13-2018, 01:30 PM
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Re: Snapping 775 Pro Shaft

I'll take a picture later of it not spinning. I took some without it spinning but they were blurry so I screenshotted a video.

Quote:
Originally Posted by Richard Wallace View Post
When you said the gearboxes became loose, did you mean the motor screws?
With our current setup, the screws holding the gearbox together holds the motors in, so technically the motor screws became loose.
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Unread 02-13-2018, 01:32 PM
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Re: Snapping 775 Pro Shaft

Were these motors in a custom gearbox? Were the pinions double-loaded?

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Unread 02-13-2018, 01:45 PM
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Re: Snapping 775 Pro Shaft

Quote:
Originally Posted by Jon Jack View Post
Do you have a picture of your gearbox or at least how the gearing is laid out?
This is how the gearbox is currently setup. We are using AM DeCIMates connected to modified WCP SS (little smaller than normal configuration). If I can remember off the top of my head, we had a 26.775:1 reduction (total final reduction to the wheel, 6" wheel).
Since the picture was taken, plastic covers were added to keep grease in and dirt out.
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Unread 02-13-2018, 01:46 PM
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Re: Snapping 775 Pro Shaft

It looks like either ductile torsional failure:



Or bending+rotating failure:



Possible that you got a motor without a heat-treated shaft, but perhaps more likely that the bending load from having the motor 'sag' induced a fracture that grew as the motor continued in use.

Edit: looks just like the fracture pattern for 'severe stress riser' in rotating+bending fatigue failure. Perhaps something to do with how the pinon was pressed on or made.

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Unread 02-13-2018, 01:48 PM
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Re: Snapping 775 Pro Shaft

I've had a RS550 fail like this when the motor mount to the GB got loose.
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Unread 02-13-2018, 02:27 PM
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Re: Snapping 775 Pro Shaft

In the first picture; the lower mounting screw location looked damaged, and turned to an angle. It appears this motor bolt could have been cross-threaded off center.

There is also no indication of any red or blue traces on the threads, indicating no Loctite was used.
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Unread 02-13-2018, 03:47 PM
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Re: Snapping 775 Pro Shaft

I would venture to guess it was the side loading from the gearbox coming loose. You could also have picked something up between the spur and pinion gear. A screw, rivet, washer, etc. The motor just doesn't have the torque to snap a shaft like that. If you started and stopped super quick, the armature would reverse direction before you could snap a shaft. The only other thought that comes to mind is the pressing of the pinion onto the shaft might have damaged the shaft which then broke.

2451 has never had a 775Pro shaft failure.
The motors we installed at the beginning of last season are still on our comp bot.
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Unread 02-13-2018, 04:26 PM
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Re: Snapping 775 Pro Shaft

Yeah in our off season 775pro drivetrain and in the history of abusing 775pros we've never seen a failure like that.
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Unread 02-13-2018, 11:48 PM
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Re: Snapping 775 Pro Shaft

Here is a picture of the shaft snapped while not moving. If it's blurry, I'll try to re-upload. CD didn't like the original pictures size.
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Unread 02-14-2018, 07:17 AM
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Re: Snapping 775 Pro Shaft

All of the flaked-off plating around the motor mounting holes makes me think that motor was moving around a lot.
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