Power Generation: Product Operation and Maintenance

[egrant9]

Hi there. I am going to be doing an alignment on a 3616 genset (post overhaul). I have everything all figured out including thermal growth. What I am trying to figure out is crankshaft liftoff. Cat states a bearing clearance of 0.010 to 0.015. One can reasonably assume that the crank will never run in the dead center of the bearings. How much can I expect the crank to come up when running at operating temp with proper oil pressure ? Anyone out there have any info or experience with this?

Any help is much appreciated.

Thanks.

[mikel · Accepted solution]

I've done a lot of 3600 series alignments, gas compression, power generation and marine.  In all cases we never worried about the oil film lift.  CAT did some investigation into it when the 3816 fast ferry engines were having issues, as I recall crank lift was found to be minimal and not worried about in our procedures.

CAT's published specs assume a Vulkan Rato or DG coupling style.  Geislinger type couplings in some applications had different (usually tighter) tolerences, but in every case I did the CAT published spec worked fine.

I usually tried for a vertical offset (engine low) of 0.012" cold, less than 0.004 horizontal offset, and angularity in both planes less than 1 mil/inch.  I always used a laser tool, usually a CSI UltraSpec, or a Optalign.

Hope that helps,

Mike L.

[egrant9]

Thanks very much mikel, that answers our question perfectly. We are running the Geislinger couplers on all our engines and have found them to be very good. I believe our tool is a Rotolign laser tool and have been told it is very nice but I have not seen it yet.

Again, thanks for the great info.

[meka1605]

hi mikel

let me introduce my self, i am an engineer of gas processing facility, we are new with cat engine, we bought G3616 for driving compressor for 55 MMSCFD compressing gas from 250 psig to 1000 psig

Question: what is good number for cold alignment, is still same using 0.012" vertical driver lower, 0.004" horizontal and angular 1 mil/inch for our engine (for compressing purpose).

thanks for your kindness

[mikel]

The information I provided above has worked for most units I have worked on.  All assume a fairly low temperature rise across the structure of the driven equipment.  Ideally you would do a thermal growth calculation for both the prime mover and the driven equipment on every alignment job.  The best way to assure the cold alignment offset is correct is by using vibration analysis, the vibration will be worse cold and should improve as the units warmup.  Monitoring the drive end bearing temperature (if available) is also a good indicator, typically on a properly done alignment the driven end bearing temp will be within 10 degrees C of the non-drive end when everything gets up to temp and stable.  Hot alignment checks are useful if they can be done quickly enough after a unit shutdown, if it takes too long to get the guards off, walkways out of the way, tooling setup etc, you don't get a good "hot" reading.  Same thing if the equipment is outdoors with a nice cold wind blowing.

Also, the standard CAT specs assume the couplings ability to operate with a +/- .004" horizontal and vertical offset.  Some couplings are more forgiving, and there are some with a tighter requirement.

Another QA method is to take coupling temperatue readings with a non contact thermometer.  For oil filled coupling like the Gieslinger, the coupling temp should be slightly lower than the engine oil temperature (if the coupling is engine oil fed).  Rubber element coupling like the Vulkan Rato are a little harder to find a "spec" for, but the rubber temp should not exceed the driven equipment shaft temp as a general rule, and is usually slightly cooler.  Also note that excess torsional activity (like severe misfire) can also cause the torsional element to generate excessive heat, so don't use it as a qualifier all by itself.

If you're looking for a good technical reference, the second edition of the Shaft Alignment Handbook is an excellent reference, ISBN 0-8247-9666-7

Hope that helps, Mike L.