Scott Fisher made a couple of interesting observations on this particular
topic.To cover the first,one must realize that lubrication and bearing
technology are, of course, very complex subjects and we as users take a lot
for granted when "improving" a particular engine.
Many tests have shown that one of the most important properties of a good
bearing material is the "embeddability",by which is meant the ability to
engulf minute particles of foreign matter by local flow of the bearing
metal, without causing local high spots that lead to overheating and
failure. Naturally the softer the metal, the greater the "embeddability ".
The other desirable property of the bearing is "conformability",defined as
the ability to yield under high loads and therefore redistribute the load
to a wider bearing surface without cracking.These properties,together with
a low coefficient of friction are not exclusive to a single metal or alloy.
Therefore a lot of development has been carried out on this field.Starting
in the thirties,the thin-wall bearing strived to combine the properties of
embeddability and conformability,by supporting a very thin layer of white
metal (babbit) on a steel or bronze shell.Later, companies such as Moraine
in USA and Glacier in England replaced the white metal by other tin based
alloys.The trimetal bearing that we now use is a result of this development.
The very thin antifriction material is supported by a layer of lead bronze
(the "copper" we see on worn bearings) atop the steel shell.The
intermediate layer is there to prevent catastrophic failure (crankshaft/
shell contact)if the top layer fails.
Because of the thinness of the antifriction material,the
greatest danger to the bearing remains the foreign matter carried by the
circulating oil.In particular when the engine has just been rebuilt, and
the clearances are small.The observation of Scott Fisher is entirely
correct, the older splash lubricated engines used soft white metal bearings
with a high degree of embeddability, and therefore were quite immune to
dirty oil.(May be that was the reason for the durability of such
primitive engines as the Ford Model A ). But of course that may be the
reason for failures attributed to "lack of pressure", as modern bearings
are vastly more sensitive to dirt,compensating however this inability with
a greater load capacity and better thermal conduction to sustain high rpm.
Scott made some comments on the ancestry of the BMC B type engine.I
believe that attributing the inspiration of this design to the inline Chevy
six is a bit far fetched. Take for example the design of the cylinder head.
The combustion chamber design in the BMC has the peculiar heart shape,
certainly not found in the Chevrolet.At least in the initial design the
Chevy engine had inclined valves versus the vertical valves of the the BMC.
The drive to the distributor ( I am looking at the Austin A70/A90 engine)
is also different. The explanation may be much simpler.When BMC was formed,
Austin merged with Nuffield ( Morris,MG,Wolseley). If one looks at the ohv
Wolseley engines of 1939-1940, it is apparent they have many
features that were later carried into the BMC B-type;possibly these
engines were designed by the same team.
Lastly it is intriguing to speculate what caused the demise of the vertical
(downdraft) SU carb found in quite a few British cars of the late thirties,
including the Wolseley mentioned above, the Lagonda V-12 and
others.The standard argument, that the piston wore excessively on the lower
side seems not very convincing.
Sergio Montes Department of Civil and Mechanical Engineering
University of Tasmania
Box 252C,Hobart 7000,Tasmania,Australia
Ph. 56-02-202113 (Int) 002-202113 (Australia)
Fax 56-02-234611
e-mail montes@cmech.utas.edu.au
"Lo bueno,si breve,dos veces bueno" Gracian
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