Re: Want to Talk to Experienced Engine Rebuilder

Posted by Allan on November 24, 2003 at 00:46:43 from (148.63.132.164):

In Reply to: Want to Talk to Experienced Engine Rebuilder posted by moonlite on November 23, 2003 at 16:18:40:

Hi,

I'm no expert by any means, am not a metallurgist nor a machinist, but I can float you a theory or two. I would think that it is a question of both stress and balance.

I don't like the idea of cutting down a journal at all; but rather, would opt for the idea of first building it back up and then cutting it back to the original size if replacing the complete crank is just not an option.

The reason I say this is because in the whole of the engine, the rod has the most load placed upon it. Both at the wrist pin as well as at that journal.

The journal is where the power flow is converted into rotary motion. At both extreme ends of the rod's travel, for any given speed, the velocity of this moving piece of iron is at it's absolute greatest and then it is required to immediately & completely stop and then abruptly reverse directions; twice, once at each end of the travel. So, even at a conservative idle speed of say, 600 Rpm, we are changing this direction what, 20 times per second?

Now, let’s drill a couple of angled holes in the center of this journal for the pressure lube and we’ve taken away material at the very point of the most stress.

So then, if we were to cut down the diameter of the journal, the flank of that journal is now heavier and the whole of the throw is out of balance with itself. Even this small amount of weight, when compounded by speed, is magnified tremendously.

Add to this the fact that the overall load on the crank is divided fairly evenly between all cylinders; cutting one journal down in size compared to the others is opening the door for a failure, IMHO. Cutting all the journals down would simply lower the overall reliability of the complete unit.

Under normal conditions, that crank is in a state of constant flex, both lengthwise and sideways and is the reason it is constructed from a relatively soft material, witnessed by the fact that the journal can easily be damaged merely by the act of bumping it with a rod bolt during assembly.

Also, this ‘flexing’ is why the stronger engines have a main bearing placed between each rod throw in an attempt to control this unwanted movement, which is constantly going on.

It is no wonder then, that an engine shakes when it is missing; or if run with a bad or missing harmonic balancer, it will set up vibrations, which are literally trying to break the crankshaft in two. When an engine is running without a balancer, you can actually see the end of the crank “wobble” at about 1200 Rpm.

There is a lot of stress going on with the crankshaft even under the best of conditions, as there is a lot of heavy iron flying about. This is why I think a line bore is so very important at the time of an overhaul.

Just my take on it, anyway,

Allan

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