Quote:
Originally Posted by IamFODI
First time I'm hearing this. Everything I've read on Al/Sn bearings says that the Al/Sn sliding layer is as thick as the Pb and Cu layers in a trimetal bearing combined because it essentially replaces both of them, and the only thing underneath it is a very thin bonding layer of Al and then the steel backing. I've also never seen that Al/Sn bearings have babbit layers.
If the Al/Sn part were worn through, wouldn't that mean instant catastrophe? And what part of that has a darker/browner color than Al/Sn?
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I'm using the word Babbit loosely to define a soft top surface layer of a plain bearing, it may also be defined as an overlay (like some may say Coke and mean soda)...
From wikepedia:
"The original Babbitt alloy was invented in 1839 by Isaac Babbitt[1] in Taunton, Massachusetts, United States. He disclosed one of his alloy recipes but kept others as trade secrets.[2] Other formulations were developed later.[3] Like other terms whose eponymous origin is long since deemphasized (such as diesel engine or eustachian tube), the term babbitt metal is frequently styled in lowercase.[3][4] It is preferred over the term "white metal", because the latter term may refer to various bearing alloys, lead- or tin-based alloys, or zinc die-casting metal"
The 702/703 Glyco bearings still have a (thin) overlay, lining, and steel backing (shell). 702/703 bearings have an intermediate layer of
some type of copper alloy as seen in the below closeup of a 702/703 bearing I attached for illustration. The copper may be mixed with tin or aluminum. We don't know for sure.
I'm not aware of any publication outlining the exact alloy or composition of these BMW bearings, hence the lack of discussion and education on the subject. There are dozens and dozens of ways manufacturers compose bearings. There is so much more to identify beyond simply bi-metal or tri-metal composition. That might be like identifying diesel and gasoline engines and saying every engine in each category is the same. There's a lot more to define. Here are some helpful links:
https://en.wikipedia.org/wiki/Babbitt_(alloy)
http://www.kingbearings.com/material-type/lead-free/
As with any plain bearing, once the overlay is worn through and no longer able to embed debris and help lubricate the journal in the absence of oil, accelerated wear to the crankshaft journal will be observed. At some point when the oil wedge is no longer operating at peak efficiency (due to deterioration of the bearing bore shape), and the lining has enough physical contact with the journal, the bearing will gall and ultimately seize.