Sheared bolts on generator

[morford49]

MEP-003a . Hi. I just purchased my first generator from GL. Looked like new with 32 hours on the meter. Did all the startup maintenance and she started right up. No output. Got the TM's and started doing the checks. No output. Put 12 volts on the exciter and no output. Started to check the diodes in the rotating rectifier assembly and found the problem. No rotation.

The eight bolts that connect the disk to the rotor and thus to the flywheel had sheared off. My question is; what would cause this? The only strange thing I could find was three dicks (part 27 Fig. 8-1) were used instead of one shown in the manual.

Any help would be much appreciated. This might be another thing to check before someone buy's a generator.

 

[gungearz]

The possibilities are endless for the cause of that. Maybe some pictures would help.

 

[uscgmatt]

You have this thread posted twice.

 

[storeman]

I'm sure Speddmon will spot it soon.
Jerry

 

[Speddmon]

Which bolts are sheared off, item #21 or item #24 in the figure you referenced in your post. Also, as far as the disks are concerned, I believe that is a laminated disk, so you are probably seeing the laminations and not 3 separate disks. That being said, I am going to guess that the bolts you are talking about are item #24.

It sure sounds like you have a locked up generator head.

 

[ETN550]

I wonder if the wrong grade bolts were used or if not tightened properly, or maybe anti-sieze or some other thread compound was used where it should not have been. The alignment is determined by the machining of the parts in these single beraing generators so it is unlikely there is an alignment problem.

If there is no damage to the flex disc or the flywheel then replace the bolts and see what it does. May want to check the generator end bearing too as it all has to come apart to check out and re-assemble.

Watch for any heavy vibration when it first runs again.

 

[morford49]

Thanks

Thanks to everyone. The bolts that were sheared were #24 on Fig. 8-1. Numbers 21 through 28 were attached to the flywheel and the entire rotor assembly was lying on the bottom side of the stator. I will send pictures tomorrow. Again, Thanks to all of you.

 

[morford49]

Pictures of sheared off bolts.

Here are the pictures. I agree with Speddmon that at some time the generator must have locked up. I don't know if the damage to the rotor and stator was a cause or effect of the bolts shearing. Any guesses.

 

[gungearz]

No clue.... That's weird. Never seen bolts break off like that before. Maybe gremlins?

 

[Ken_86gt]

I see contact wear between the rotor and stator- not good. It could have happened when the bolts sheared.

 

[morford49]

Sorry about the flawed photography. These may be better. I think that the bolts were the correct grade.

 

[Isaac-1]

I am trying to picture how this could happen, it seems a seized end bearing would likely be spun by the force before these bolts snapped. The only thought I have is perhaps this generator was mounted on a dual generator trailer and the transfer switch was wired wrong. Then when the second generator was started, and the transfer switch thrown it caused the second generator to be wired directly to the output of the first generator with opposite phase rotation instantly turning both into a motors turning against the prime driver diesels. This is just a guess, but I suspect if could happen so fast that there would be little or no sign of overheating. This could explain how something like this would happen to such a low hour unit.

 

[RichardR]

I think Ike correctly diagnosed how this happened -- the generator electrical output was closed on an out-of-phase bus. Something you don't see very often, but that has instantly wrecked even very large generators. Good diagnosis!

 

[derf]

Do the windings test good? It may still work.

 

[ETN550]

Sorry about the flawed photography. These may be better. I think that the bolts were the correct grade.

The bolt at 11:00 O'clock shows markings of fatigue indicating that at least that one bolt did not fail suddenly but rather developed a hairline crack and propogated. IF what I see is the bolt surface and not the tapped hole.

I think this is a fatigue failure. The bearing could have been bad and caused extra stress on the drive plate if it rotated off center.

As far as an electrical failure causing this... Is it possible to get this type of damage if the output goes through a breaker?

I bet the engine either ran or was restarted and ran some before they abandoned it. that would account for the smearing of metal. The contact between the rotor and stator looks minor just scuffing paint.
But the bolts are the failed part here.

I'm going to check my torn down unit to see if it has more than one flex plate. I only recall seeing one used.

 

[ODdave]

Bolts Too tight, fracturing them?

 

[Isaac-1]

I could see something like this happening through a breaker, even with a massive overload a magnetic trip breaker is still going to take a non zero amount of time to break the connection which would be more than long enough to sheer these bolts when you consider the whole assembly is turning at 1800 rpms (30 revolutions per second). While it is possible we are looking at a fatigue failure, I find that hard to accept in under 40 hours of run time, massive over torquing is also possible, but would we see such even amounts of it, unless someone used a torque wrench and read off the wrong values. If it was over torqued we should see thread elongation in the bolts. Of course at this point all this is academic, the thing failed and something caused it, do we really need to know more?

Ike

 

[Speddmon]

Having 3 flex plates in the machine might have something to do with the bolts shearing off. You could probably bolt 3 plates up just fine, but I see a few problems in doing so.

1, The bolts are going to have to be longer than normal to get the proper thread into the holes...unless they used longer bolts (which I doubt they did) there is going to be a much reduced load bearing capacity of the bolts since they would not be threaded into the holes as far as needed.

2, Once you assembled the head to the engine flywheel, the added thickness of the extra plates would create tremendous axial loading on the single bearing, possibly enough to complete bind it in position...this would make it even easier to shear the bolts off

 

[uscgmatt]

Is it possible to get this type of damage if the output goes through a breaker?

Yes, on equipment designed for paralleling there is a device called a reverse power relay that will open if it sees back feeding, I think its about 5%. This prevents the generator from becoming a motor.

 

[steelypip]

I'm thinking it was overtightened bolts. I've seen similar failures on flexplate/flywheel mountings to car or truck engine crankshafts. Once the first one let go the others were under more stress. If they were all overtightened to the same torque, crack propagation is a definite possibility.