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Antique Gas Engine Discussion

De La Vergne crank troubles


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  #1  
Old 06-04-2005, 11:00:19 PM
Bill Hazzard Bill Hazzard is offline
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Default De La Vergne crank troubles

We have a 100 Hp De La Vergne SI with a slight problem with the crankshaft. If anyone knows of a spare crank or a shop that can fix it please let me know.
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  #2  
Old 06-04-2005, 11:22:38 PM
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Tanner Remillard Tanner Remillard is offline
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Default Re: De La Vergne crank troubles

WOW, how in the hell did that happen? That is one big piece of iron to snap
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Old 06-05-2005, 12:21:37 AM
Andrew Mackey Andrew Mackey is offline
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Lightbulb Re: De La Vergne crank troubles

Possible stress fracture - from running the engine too slow? Woodpecker 2 HP engines snap their cranks in virtually the same spot! The reason - torsional stress! When the engine is started, maximum stress is made at the crank throw, after spark occures, during the power stroke. The force of combustion pressures attempts to turn the crank, delivering maxumum pressure at the crank throw. As this occurs, the crank, in turn, tries to accelerate the flywheels. As per Newtons law - a mass that is in motion (the crank) tends to remain in motion, and a mass that is still (the flywheels) tends to remain still. What happens is the crank torques up - it twists a little bit, storing energy until the flywheels accelerate to match the speed at which the crank is rotating. The twist them imparts its energy to the acceleration and the flywheels. As the flywheels gain momentum, the amount of torsional stress and the amount of twist imported to the crank diminishes as the speed of the rotation increases, and the engine does not have to work so hard to keep the flywheels in motion. With the woodpecker engines (and I suspect the poor De Laverne) when the engine is slowed to a certain point, the torsional stress is greater and of longer duration. This maximized twist cannot be endured forever. Not only does the crank have to accelerate both flywheels at the same time, but the wheels accelerate at different rates because they do not weigh exactly the same. The lighter wheel will accelerate faster than the heavy one, Besides the initial power pulse, the crank now also has to contend with a resonant vibration as the combination of torsional forces work against each other. As with any metal, you can only bend it so far and for so long before it gets brittle, and eventually won't bend anymore. This is called metal fatigue. (try twisting a metal coat hanger back and forth for a while - built up stresses make the wire hot, and eventually the stored energy eventually will cause the metal to undergo catastrophic failure - The same thing happens with the crankshafts, the metal gets fatigued and The cranks break! Hopefully somewhere out there there is another crank for you. Other wise, someone is in for one hell of a weld job! Your other alternative is to have another one made of cast steel and have it machined. BIG $ any way you look at it. Good luck in your search.
Andrew
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Old 06-05-2005, 08:23:30 AM
Orrin Orrin is offline
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Default Re: De La Vergne crank troubles

Craig Prucha seems to specialize in medium to large sized engines. What with his considerable amount of restoration experience, he could probably point you in the right direction.

http://www.antique-engine.com/

His e-mail is linked at the bottom of the page.

Good luck! Keep us informed.

Regards,

Orrin
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Old 06-05-2005, 08:53:06 AM
Bill Hazzard Bill Hazzard is offline
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Default Re: De La Vergne crank troubles

The engine ran an amonia compressor for an ice plant. The engine was running unattended over night when the engineer came to work the engine was pounding but still running. He shut it down and found the crank had broken some time in the night. The crank most likely broke because of the bearings being out of alignment. There are two main bearings on the bed plate and one bearing on the compressor which acts as an outboard bearing. The center bearing probably wore down more than the others which caused the crank to flex in the throw area at every revolution. The constant flexing caused the steel to crystalize which makes the steel britle and then in breaks under a normal load.
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