Crankshaft journal bearing pitting 1

[montgomeryjack]

Hi all

I have read a couple of threads related to this topic, but I can't put my finger on this specific problem -

We work with a diesel/electric drive system, diesel engine or electric standby motor driving a compressor via a clutch/belt drive system. We are experiencing electrical pitting on the main crankshaft bearing in the engine whilst operating in electric drive, i.e. crankshaft stationary.

Material of main bearing is aluminum alloy with tin and silicone, and its melting point is 230 deg C.Aluminium alloy layer is 0.3mm in main bearing. We are deducing from bearing analysis the pitting is electric static or electromagnetic in origin.

This issue is only occurring when operating on electric standby on baldy grounded mains - or on an isolated system on a ship. This means that the "GND" of our entire unit is now floating which is exaggerating the problem

Before you ask - Yes we have examined the belt static conductivity spec, but it only seems to be part of the problem

Any suggestions?

 

[itsmoked]

Skogsgurra will be along. He is an expert on this subject.

Is the motor in your setup ever run with a VFD?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[montgomeryjack]

No VFD - just a straight forward DOL start and run

 

[ScottyUK]

On much bigger machines (multi-MW turboalternators) we see windage-induced static problems when the shaft earthing gear fails. This causes problems with the white metal bearings if allowed to persist. Could you have something similar resulting from process conditions? Earthing brushes range from simple copper straps through carbon brushes to more complex shaft-riding brushes from the likes of Sohre Turbomachinery.

Other possibility is you have high earth currents flowing in the frame because of current leakage from the motor. Have you Meggered the motor winding to earth? What were the results?



----------------------------------

Sometimes I wake up Grumpy.
Other times I just let her sleep!

 

[Skogsgurra]

Hello,

Smoked invited me to look at this. I see that it has already been taken good care of. Grounding brushes are usually the best solution on bearings like these.

I have done lots of measurements (I guess that is why Smoked mentioned me) where return currents take "funny" and unexpected paths - sometimes via bearings. There are also the VFD/PWM problems. But you do not seem to have any here. Or, are there VFD:s on the same grid? Seen quite a lot of distant VFD:s "back-firing" on innocent machinery. Especially if the grid is impedance grounded or floating.

A sure way to find out what is going on is to use an oscilloscope with "derivative triggering" and have it look for the very fast edges (typically 0.02 - 0.05 microseconds) associated with EDM. But that may not be so easy to do. Such oscilloscopes are rare. Second best thing is to check to see what the voltage across the bearing is. If you see anything above one or two volts RMS, it usually means problems. But that means HF measurement. A Fluke scope with RMS measurement or peak voltage measurement can be used. Peak voltages above five volts are dangerous. Best is if peak voltage can be kept below one volt.

A rather unusual case is when there is a short circuit somewhere in the grid and fast transients travel all over the place. They have been shown to cause small pitting marks in bearings. A bonding strap between all machines involved is usually an effective solution.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

I believe that it's fairly common knowledge in the auto repair business that repeated or unexpected failures of front wheel bearings (typically on FWD minivans) can indicate a bad grounding strap from the engine block and/or transaxle to the body/frame.

 

[itsmoked]

Interesting.

Where is the current going out to rubber tire?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I understand that tires are conductive enough to discharge static buildup on a vehicle. They are not good conductors but not good insulators either.
I once saw some electricians use part of an old inner tube to help insulate the connections on a large 480 volt motor (300HP or so). About every 4 or 5 minutes the rubber would flash over with a bang inside the junction box and then heal itself. It didn't take out the fuses. The contractors men were in their truck heading for the exit and turned around to see what the noise was. They removed the rubber and sent for the varnished cambric they should have used in the first place. After that experience I can accept a static discharge through a tire.
Respectfully

 

[MikeHalloran]

There's a lot of carbon in tire rubber.



Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Yes,

I have actually seen a little spring that was/is used to short out the wheel bearings. I understand that it was placed so that it contacted the shaft center. A "Car Professor" used to install them in cars he took in as "patients". There may be something to it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

(Apologies for drifting slightly off-topic, but anyway...)

IS: "Where is the current going out to rubber tire?"

Obviously not there - even if the tires were conductive, the return path isn't much interested in a (planet) earth ground.

I assume that path is something like this: Out the half-shaft, through the wheel bearings (OUCH!), and perhaps up the coil spring to the frame. If not this exact path, then something similar. I'd be very surprised if the half-shaft didn't lead to chassis ground one way or another (at least in some cases, some of the time).

I once tried to count the number of links (of any sort) from one of my (RWD) car's rear wheels (the hub) and I lost count. Shock, five suspension links, sway bar, ABS signal cable, brake flex line, parking brake cable, drive axle, and several more. Plenty of opportunity for a ground path one way or the other.

 

[montgomeryjack]

Hi all

Some interesting points...I have another more specific question on the problem - so let's assume I am building up static charge on the crankshaft, and at the moment we have a badly grounded frame so a static discharge occurs..fact

now if I introduce a ground strap and ground the crankcase to the frame - won't this make it easier for the discharge to occur from crankshaft to bearing on crankcase to ground - now easier path to a solid electrical ground than previously....

Or are you assuming that the discharge current follows a different path like from crankshaft to piston to piston ring, cylinder, frame, ground

 

[itsmoked]

No no!! You have the crankshaft brush grounded to the ground/frame/earth/whatever. This provides a path bypassing the engine proper since the brush should provide a lower resistance than the bearings.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com