How bad is really bad?

[Jack]

I'm into a D2 5U engine which has an abundance of things wrong with it. I gauged the crankshaft throws, found about 0.002 flat, about 0.008 clearance (blue plastigauge). The throws are worn 0.002-0.003 except in the middle where the oil clearance is, so there is a high middle area between two worn areas. In addition, there is rust pitting on two of the throws but it in this middle area where the bearings don't run. Acording to the shop book this crank is usable, is well short of the maximum allowable wear.

Can I rely on the judgment in the book? Or is this crank barely marginal? Or can I expect a reasonably long life from this engine just bolted back together? A car engine with a crank like that wouldn't make it out the driveway.

Please put forward some experience on this if you will. Old magnet? SJ? Anybody else? Thanks in advance,

Jack

[Old Magnet]

Hi Jack,
Those dimensions are not all that bad. What do you have for clearances on the mains? Also thrust?
Just for reference max allowable rod clearance is .013
Max allowable on the mains is .015
Max allowable thrust .030
Max allowable out of round .006
Max allowable journal wear is .007

I would recommend actually checking with mike and only use the plastigauge as reference.

Is this engine going to be run flat out on a gen set? and be critical power or occasional use with utility co. back up.

If everything checks out at about 50% of spec I think you could clean it up and run it.

[Jack]

Hi O.M.

It's the ridge in the middle of the throw that worries me. Worst average taper is 0.0017 in #1. Worst ave eccentric is 0.0010 in #2. Worst undersize is 0.0036 in #1. It's cold out in my shop. I had to correct the micrometer by +0.0006 but I believe I'm within about 0.0002 of accuracy.

Should I maybe put new bearings on that shaft? Should tighten it right up.

End travel was 0.014 measured by dial. I havn't gotten into the mains yet, want to pull the sleeves before I roll it over.

engine is going on an induction generator that has a name plate RPM of about 1780. I plan to belt drive so I can run between 1450 and 1550 and shoot for about 80% power output and change the belt sheaves around until I get there. I don't believe even a Cat should be run flat out full time.

This is the same engine that had the wierd manifold modification--holes drilled over the intake ports--and they sure enough dusted it good. It needs all new pistons and liners. I measured 0.041" ring clearance on top ring of best piston.

Thanks for advice,

Jack

[Old Magnet]

Hi Jack,
I didn't realize you were in to that much of a rebuild.
Replacing the rod bearings would gain a lot as most of the wear seems to be on the inserts, not the journals. The .002 - .003 journal wear amounting to a .001 to .0015 ridge at the oil channel is not enough to worry about. Lets see what you get on the mains before concluding what to do bearing wise. Rest of the readings look good. Agree with your thinking on engine loading.

[ccjersey]

Jack, figure on rpm OVER 1800 for an induction GEN, at 1740 it's an induction MOTOR.

[Jack]

Right, CCJersey, that's why I opted for the belt drive. I can set it where ever it needs to be. I have a 25 HP motor. I assume the 1780 tag means no-load RPM. If it takes up to 50-60 turns to achieve full load as a motor, it should take about that many over driven to make it generate at 25 HP, as best I can guess. If you have some experience with this, I'd sure like to hear about it. Thanks for input.

Jack

[ccjersey]

I think you have it right.

The thing that makes it pull the load as a motor (or even rotate at all) is the "slip" or difference between the rotation of the armature and the rotation of the magnetic field in the stator. Running as a motor, the armature cannot ever catch up with the field, because if it gets too close, slip, torque and amperage drop and it slows back down, so it stays in a balance at some point slower than 1800 depending on the load that's on it.

the rated RPM is the FULL load rpm and if you overdrive the motor that much above synchronous rpm (in this case above 1800), you should see it generate full load amps and take rated hp to do it.

Governing an induction generator should be relatively easy, it should almost govern itself since if it tries to go too fast, the engine will lug down and then the load will drop a little and allow it to speed back up a little.

[Jack]

ccjersey,

So 1800 RPM would be a normal no-load as it is a function of 60 cycles power? That would make sense. If 1780 is full load as a motor, then about 1820 should generate 25 HP. Am I correct, or at least close?

[ccjersey]

ccjersey,

So 1800 RPM would be a normal no-load as it is a function of 60 cycles power? That would make sense. If 1780 is full load as a motor, then about 1820 should generate 25 HP. Am I correct, or at least close?

You got it, except that an induction motor cannot ever quite keep up with the magnetic field because torque drops toward zero as slip decreases, so even at no load, it runs a little less than synchronous rpm. Synchronous rpm for 2 pole motors running on 60hz electricity is 3600 rpm, 4 pole 1800, 6 pole 1200, 8 pole 900, etc.

[SJ]

Normally the Cat diesel electric set has a different governor spring so it has a 3% droop from high idle to full load instead of the 10% that tractor engines has. I looked in the Cat fuel inection spec. book & it shows about a 2% droop but the books always called it the 3% governor setup. The engines with electronic governor controls has a 0% droop & we had two high schools here that ran the total energy setup with Cat engines & they had the electronic governors on them to keep the cycles up to specs.