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-   -   Stuffing Box Flax - Theory vs Practice (http://www.sailnet.com/forums/gear-maintenance/53702-stuffing-box-flax-theory-vs-practice.html)

MC1 04-21-2009 09:49 AM

Stuffing Box Flax - Theory vs Practice
 
OK, so I've read many of the very informative posts regarding replacement of stuffing box flax, and also there's a chapter on the topic in the book BoatWorks from the editors of Sail magazine. The general consensus is to cut the new flax up into (in my case) 3 pieces (cuts at a 45 deg. angle) so they lay flat against each other with the cuts offset from each other inside the stuffing box, and also not to overtighten the lock nut when initially installing it since the flax won't "bounce back". Yes?

Because I haven't done this before and despite all my reading on the topic I was still a little concerned that I might mess it up resulting in problems upon launching the boat, I paid my local yard for a 1/2 of labor to demo the installation for me. A mechanic with many years experience in the yard demo'd this for me, and what did he do? . . .

The mechanic dug out the old flax, spiral wound the new flax as one continuous piece to match the shaft size (1"), and overtightened the locking nut initially so (his words), "it would compress the flax tightly together to form a good initial seal". Per their standard process at this yard, we are to back off the locking nut as necessary when we lauch the boat to achieve the required drip rate.

I was surprised at the mismatch between much of standard wisdom on this common maintance task and the approach an experienced mechanic used when doing it himself in practice. I asked him about the two approaches and he indicated it was not necessary to cut the flax, and this wasn't done in practice (at least for this boatyard - this boat yard is a fairly high volume working yard on the Great Lakes) since it takes too much time and is not actually necessary.

Comments? Should I consider redoing his work, or leave as is and evaluate the drip rate, etc.. Does it really matter that much or is this more a matter of fine tuning the approach if you're doing it yourself and can spend the timing doing the cutting, etc.?

FYI, I just went with the basic 3/16" waxed flax as specified by the boat manufacturer.

sailingdog 04-21-2009 10:21 AM

I would re-do it... if it is one continuous piece, it is also a single point of failure. Three or four separate rings are less likely to have a single problem take them out... also, the flax isn't going to be compressed evenly—since the beginning of the spiral will have a gap on one side...where three separate rings would be evenly compressed all around.

scottyt 04-21-2009 10:25 AM

i agree with SD, what he did might work, or might not. do you want to trust a several hundred dollar shaft and a several thousand dollar boat to a might.

c40eb 04-21-2009 10:48 AM

My boat's manual states that when replacing the flax packing, cut one continuous piece at 10 1/4 inches and wrap that around the shaft.

I recently pulled the shaft and in doing so, looked into the nut...the old packing was one piece, and sure enough there is a gap at the beginning of the wrap as SD noted. Not sure if this would be a problem, but shows the point. Maybe, maybe not.

IF I was going to replace the packing, I would use the 3-4 piece method.

But, I just ordered a dripless shaft seal. Whoo hoo!

JimsCAL 04-21-2009 11:58 AM

The single piece method may work, but I have always cut separate pieces by wrapping the packing around the shaft to get the correct length. I cut each ring at 90 degrees (don't think the 45 degree cuts are necessary), and as you said, stagger the joints when inserting them. I don't like the idea of overtightening and then backing off. I prefer to initially lightly hand tighten, then tighten as required when the boat is in the water.

Maine Sail 04-21-2009 11:58 AM

Your "mechanics"
 
Your "mechanic" has installed your packing incorrectly.

There are very good reasons for installing it in separate cut rings and I will try to explain.

1) The first ring on the male end of the stuffing box must sit flat against the male end with no overlap. This creates a much better seal at that end of the box so minimal water can get up into the female threads of the stuffing nut.

2) The idea behind cut rings is to allow water to pass between the shaft and the packing and not between the packing and the female threads of the stuffing nut. With overlapping rings it can pass on the outside of the packing MUCH easier.

By having the two overlaps, one at each end of the nut, allows water to enter through the male end of the box, pass up through the overlap, and into the female threads. The water then travels around the outside of the flax using the female thread valleys as a conduit and it then comes out the transmission end of the nut where the other overlap is.

This tracking of water outside the packing leads one to believe that they have a proper drip rate when in fact they may only be getting marginal water flow between the flax and shaft.

When the nut is overtightened tightened the flax is forced into the threads there by slowing the "drip rate" and giving the appearance to the "mechanic" that what he has done is correct, when it is not..

3) By installing flax as one continuous piece it can automatically "kock" (can't use c ock or the spell checker removes it) itself inside the nut when tightened and is very, very tough to adjust properly.

4) By overlapping you can not get as many true rings of flax inside the nut. Instead of three rings you may be relying on 1.5 rings of surface mating area as opposed to three.




Some yards employ good mechanics some employ mechanics who are to lazy to do the job right. I'd say you learned your lesson about the quality of the mechanics in this yard too bad it cost you a 1/2 hours labor to be show how to do it the wrong way.....

Remember boats are NOT the only pieces of machinery to use packing. Industrial pump applications also use packing and also use cut rings. This set of industrial directions specifically states "Never wind a coil packing into a stuffing box"

How To Install Packings

badsanta 04-21-2009 12:07 PM

Are you not supposed to grease the flax?

JHJensen 04-21-2009 01:40 PM

Using grease with the flax......
 
Quote:

Originally Posted by badsanta (Post 478140)
Are you not supposed to grease the flax?

Badsanta,

Introducing another lubricant into the flax is not needed as the wax in the flax acts as a lubricant and adding another element to the mix which may not be compatable with the elements already present.

John

sailingdog 04-21-2009 02:06 PM

Also, greasing the flax may prevent water from getting in and cooling/lubricating the propshaft as it turns.
Quote:

Originally Posted by JHJensen (Post 478176)
Badsanta,

Introducing another lubricant into the flax is not needed as the wax in the flax acts as a lubricant and adding another element to the mix which may not be compatable with the elements already present.

John


badsanta 04-21-2009 02:32 PM

good thing I did not use flax!


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