Damage by cavitation ?

[21121956]

Hello everybody:

Let us start in this manner: Webster´s Dictionary defines the word cavitation as the rapid formation and collapse of cavities in a flowing liquid in regions of very low pressure.

I do understand the phenomenon of cavitation associated to hydraulic turbines and centrifugal pumps.

But, in medium speed (514 rpm) stationary Diesel engines we are experiencing some problems in the crankpin journal bearings (Big End Bearings). Besides of circumferential scratching, on the tri metal material, signs of erosion appear on the back side of the bearing back of the lower half.

The eroded (pitted) area is a small spot at the outer edge of one of the lubricating holes. This situation is the same in, at least, 6 engines.

Some colleagues say that this damage is a typical product of the cavitation phenomenon, even when the lube oil leaving the engine to the oil cooler has a temperature of 76º C.

The question is: at that speed of rotation, this temperature of oil can the cavitation process be developed?

Thanks in advance for your comments.

 

[Tmoose]

Are they all running as supplied by the OEM , or have they been rebuilt?
Why are the engines apart?

I can't picture the operating oil pressure varying much in an oil passage, as in the crank. But you say these are the rod bearings.

What is the purpose of the oil hole in the rod bearing?
To feed oil up to the wrist pin, or to a squirt hole?

I'm wondering about pressure variations on the working side of the bearing insert causing insert flex, and a pressure variation on the insert-to-rod side. If the rods were rebuilt I'd wonder about incorrect insert crush by the rod bore size and profile (taper/barrel/bell-mouth) being correct. Some inspection with a bore gage of the assembled rod, with and without insert might be revealing,

 

[21121956]

Hello everybody:

Tmoose, thanks for your reply.

Well, these engines have around 27.000 hours of operation. They have been already overhauled and, for overhaul and routine maintenance, as an strategic decision, ALL the spare parts used are acquired from the engines manufacturer and/or from their original suppliers.

No engine part has been repaired or refurbished.

The lower half of the crankpin journal bearing has ten holes drilled in a groove and, they are the communicating path of the oil going from the crankshaft through the connecting rod to the wrist pin or piston pin.

 

[satchmo]

Cavitation has more to do with suction pressure. When the suction pressure is at the saturation pressure of a fluid for a particular temperature, the fluid will flash to steam, and then the vapor bubbles will collapse on the impeller, causing all kinds of problems.

You can check your suction pressure, but I doubt that cavitation is your problem.

 

[romke]

are we talking about two stroke trunk piston engines here? in that case cavitation of the lower bearing shell would be possible, since most of the time there is no load at all at the lower half of the bearing.

 

[21121956]

Hello everybody:

romke, these are 18V engines, four stroke trunk piston delivering 17.000 kW.

The eroded (pitted) area found in at least 6 engines, is in the lower bearing shell of the B8 cylinder. In a few cases, the same damage was found in the bearing of B7 cylinder as well.

I think it is important to say that, in other engines inspected, the same pattern of damage is just beginning.

Thanks

 

[Tmoose]

Is the groove cut in the bearing insert so the bore in the con rod is a continuous surface under the insert, that is, no grooves or holes in it?

Is the groove wider than the hole diameter? Like this?

http://www.acarbidemanufacturer.com/images/Oil Grove Logo.jpg

Is the erosion in a surface or edge that touches the rod cap?
Like this, if the groove was on the bearing OD instead of the bore.

http://suzuki.sanchez-family.org/images/DSCN3808_small.JPG

Or, is the eroded surface some distance away from the rod bore?

 

[21121956]

Hello everybody:

Here I am sending a couple of pictures of two damaged bearing shells. Please note that the eroded area is on the back side (opposite side) of the running surface.

http://files.engineering.com/getfil...49b4-a84e-7659ddad5e5a&file=Bearing_shell.doc

 

[Tmoose]

There is clean area outlining the hole pattern.
Is that the groove cut in the rod cap?

What does the manufacturer say? If the repair parts were bought OEM I think they should be willing to share their experiences.

 

[21121956]

Hello everybody:

Here I have uploaded more pictures. The manufacturer has said nothing valuable (I think so).

http://files.engineering.com/getfil...783-b4fd-5a99ceb91ebf&file=Bearing_damage.doc

Thanks