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Old 05-22-2012, 05:43 AM   #19
Forkliftt
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City: Diberville Mississippi
Vessel Name: KnotDoneYet
Vessel Model: 1983 42' Present Sundeck
Join Date: Oct 2007
Posts: 2,450
Quote:
Originally Posted by RickB View Post
Yes, the blue lines are the hydraulic supply to the jacks. If there are multiple bolts it is normal practice to tighten or loosen all of them at the same time to ensure equal forces on the parts being fastened.

Pages 7 and 8 of the document in this link show a cutaway of how a jack works:

http://mec.novomor.com/bw.files/BWSe...ic%20tools.pdf

There is no torque applied to the nut other than hand tightening to put the nut in its proper location.

When you torque a nut with a wrench you are actually stretching the bolt and that is what provides the holding force. The threads are an inclined ramp and when the nut is tightened it acts like a wedge to pull the bolt. The elasticity that makes the bolt want to return to its normal length provides the compression that holds the parts together. A jack stretches the bolt without all the friction of highly loaded threads and the surface of the nut on a washer or the part itself. You can apply tons of holding force without the drama and risks of long bars and torque multipliers or sledge hammers.

In case the drawing isn't all that clear, the jack screws onto the threads at the top of a bolt. A spacer between the jack and the part being bolted fits over the nut. If you look at the picture of the big end in a previous post you will see that the nut is smooth and round but has a couple of small diameter holes around it. The jack threads onto the exposed threads and the spacer fits over the nut but has a slot that will expose at least two of those holes. When the jack is pressurized to the correct level, the bolt will be stretched and a little "tommy bar" about 8 inches long by 1/4 diameter will fit in one of the holes and can loosen the nut.

The opposite procedure is used to tighten the bolt. The nut is spun on by hand and the jack installed over it. When the bolt is stretched the tommy bar is used to snug up the nut and after checking for complete contact under the nut with a feeler gauge, the job is done. We make a mark on the part and on the nut before loosening and confirm that those marks line up within a specified angle after retightening as a check that the bolt has not stretched beyond its ability to return to the correct length.

We use hydraulic wrenches on large steam flanges though. These actually rotate the nut a few degrees with each stroke of a hydraulic cylinder. They are fairly unsophisticated and only tighten on bolt at a time so the usual tightening pattern and judgement applies but they are more versatile than the dedicated jacks used on large diesels.
I see what you are talking about from the picture. As the jack is screwed onto the "pulling threads", then pressure is applied and pushes against the bearing journal, the slots in the spacer allow you to hand tighten the nut. Once the "torque" is achieved and the position of the nut marked, pressure is reapplied. The nut is loosened, the pressure applied and the nut is agin spun into place by hand and the job is complete. Provided of course it doesn't spin any further than the first time. What a creative and safe way to address this. How about the bearing inserts? Are the journals ever repaired/ reground on these bigger engines? Is the lube oil supplied through the crankshaft?
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