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Friday, October 18, 2013

Lesson from the Real World - That's not Supposed to Look Like That!

“Those who do not read are no better off than those who cannot.”

As part of my engine maintenance class at Mount San Antonio College, we had to disassemble and measure and reassemble a Lycoming O-320 4 cylinder engine.

The assembled engine

 It was an interesting, and educational exercise.  Taking the engine apart, labeling components to make sure that they all could be easily returned to their locations when reassembling.  

The ultimate experience for a geeky engineer type.  Taking mechanical things apart!

The engine disassembled

The fascinating and frustrating portion was the inspection.  We checked parts with micrometers and feeler gauges, compared tolerances, and recorded everything.

One of the most interesting things we found was one of the pushrods. 

We removed it, and the conversation between myself and my lab mates went something like this.

Me (Holding up pushrod): What the......!

Lab Partner #1: Dude!  That's getting replaced.

Lab Partner #2 (standing about 5 feet away):  I can see that from here!  If you throw it, it'll come back to you!

The pushrod, which should be arrow straight, was visibly bent.  It was so badly bent, that it was rubbing on the inside of it's shroud, and had polished itself in places!

The pushrod laying on top of a cubical wall. The daylight can be seen underneath!
Polish marks on the pushrod
How did it happen?  I'm not exactly sure.  Perhaps the push rod was too long.  Maybe the valve stuck, and the push rod had nowhere else to go but to bend. 

But what was my lesson?  The care that had to go into our checks.  The labeling of part, the measurements. It was painstaking, it was meticulous. 

It was necessary!

There was table upon tables of values listing the acceptable limits that our parts had to be within.  Anything outside of that should be replaced.

Example from the overhaul manual.  The "Table of Limits"

There were gaps measured in ten thousandths of an inch.  That's right. .0015 inches was a gap I had to look for!

I did realize that sometimes, in the sanctity of my 3D modeler, I sometimes don't think about things beyond "net fit".  It can be easy to forget about manufacturing tolerances, what happens to a part when it's heated to operating temperature. 

These are all factors that have to be considered. 

One miscalculation, or even a little bad luck, can result in a bent, or broken part. 

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