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Author Topic: Inline-four crankshaft  (Read 99078 times)
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Location: Ashfield, in the Territories of Western Massachusetts
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« Reply #360 on: August 13, 2017, 05:36:09 AM »

     Great news!  All the best for everything to continue to  go well now.

     Great chat with you at Loring, looking forward to another one when I'm not quite so busy.

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Location: backwoods Oregon
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« Reply #361 on: August 13, 2017, 09:45:26 AM »

This was explained to me while I was in my early 20's so some things are sorta vague now.  Picture in your mind two moving smooth plates with one laying on top of the other and an oil film between them.  One plate is moving to the right at the same speed the other plate is moving to the left.  The oil at the boundary where it touches the plate is moving at the speed of the plate in the direction the plate is going.  The average speed of the oil halfway between the plates is zero.

The relationships between distances and velocities are "shear."  The shear increases when the plate velocities increase.  It also increases when the plates are closer together.  Lubricants have a limit of shear they can withstand.  The clearances between the moving objects need to be wide enough to keep the shear within acceptable limits.

Flexure can cause clearances to decrease between a shaft and a journal during operation beyond their values when the mechanism is static.  This can lead to lubrication failure.  The specific instance where I learned this was the outside crank journal bushing on a BSA A-65 twin.  There is no center journal on these cranks and they flex.  The clearances at the ends of the journals would be tight in some places due to this flexure.  Sometimes it would occur on the edges of the rod bearings.

What I did was to build the engine, break it in fairly gently, run it hard for a short time, and then tear it down.  Then, I would look for signs of overheating or galled spots on the journals and bushings, scrape them, and put the engine back together.  What I was trying to do was to keep the oil film shear within the limits the lubricant could withstand.   

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