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Old 26-01-2011, 20:01   #16
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this is a great suggestion... imbedding nuts or tabs into the epoxy will not have much in the way of torsion strength, meaning it would be easy to strip out the nut, not that the nut or the threads will strip but the nut will spin in the epoxy, or the nut and the epoxy will spin in the foam...

creating a tab, and creating more area and creating a rectangular tab will make it damn near impossible to spin that inside the foam and or fiberglass.. not to mention pull strength...
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Old 26-01-2011, 22:27   #17
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Um what sort of cross section will this new bow roller have? Specifically at the point back from the roller at which you can finally get bolts through and into backing plates? If it is of sufficient size you may just be able to ignore the whole forward part, as it would be a mini bow sprit. While normally those all have stays on them at the very forward end, if you treat this problem as a cantilever beam, then you might just be able to bolt it solidly at the rear position and leave the forward part free floating.

It works in theory, but I have to admit its been better than 35 years since I last worked stress calculations of cantilever beams. (I used to design home built aircraft in my younger days, and spent some time looking at unsupported wing structures).

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Old 27-01-2011, 07:14   #18
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I've had great success with stainless "T" nuts embedded in West System



Here is a diagram of how I pot in the epoxy in the hull

The gray represents core material removed, the red would be holes bored through the top layer of glass to accommodate the diameter of the nuts. This diagram uses "regular" hex nuts, but I have found "T" nuts to have superior holding once potted in. The gray "void" is filled with thickened epoxy. Use caution if your ambient temperature is high, as the epoxy will "boil" if too fast of a hardener is used.
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Old 27-01-2011, 07:57   #19
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I've used SS Keenserts for my jib T-track with good success.

Drill out a larger hole and tap for the Keensert and epoxy the Keenserts in place. I use 2 together, threaded on a lubricated bolt for the insertion. I remove the locking tangs, they're not needed with epoxy.

Once it's cured the only way to get them out is by drilling.
McMaster-Carr or WW Grainger is a good source.
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Old 27-01-2011, 09:44   #20
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Originally Posted by capngeo View Post
I've had great success with stainless "T" nuts embedded in West System



Here is a diagram of how I pot in the epoxy in the hull

The gray represents core material removed, the red would be holes bored through the top layer of glass to accommodate the diameter of the nuts. This diagram uses "regular" hex nuts, but I have found "T" nuts to have superior holding once potted in. The gray "void" is filled with thickened epoxy. Use caution if your ambient temperature is high, as the epoxy will "boil" if too fast of a hardener is used.
i like these nuts, they are found on cheaper pressed wood furniture as that particle board has the same strength as foam.. heheheh

but, i dont understand the drawing/sketch... how is the core material removed and why? seems you remove it then glass it back in, then drill/core out soem holes for these T nuts?
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Old 27-01-2011, 09:46   #21
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I've used SS Keenserts for my jib T-track with good success.

Drill out a larger hole and tap for the Keensert and epoxy the Keenserts in place. I use 2 together, threaded on a lubricated bolt for the insertion. I remove the locking tangs, they're not needed with epoxy.

Once it's cured the only way to get them out is by drilling.
McMaster-Carr or WW Grainger is a good source.
I am sure there is a way to pull them out... it's called applying more force then they and the 'bonding material' is rated for...

that force will be a factor of teh area that the nut/sleeve has with the bonding material, and the strength of that bonding material to itself and to the material it is bonding to...

the bigger the area the more material that would need to be failed for it to be pulled thru
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Old 27-01-2011, 09:58   #22
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I didn't know that they were called "T nuts", but that is what I used in the photos on post # 15. I drilled the hole for the barrell as well as the little spikes, then with the thread hole filled with a temporary machine screw, I bedded the "T nut" into possition with silica thickened epoxy. As a final step, I faired in and then glassed over the base. This way it is totally encapsulated, except for the threads, and the metal threads would strip, LONG before the nut would spin. Especially in my case where I could access the back side, these work great! Mark
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Old 27-01-2011, 10:31   #23
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AGAIN, this is the best way i can think of...unless you can imbed wood or metal, butthe stongest point will be the weakest link
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Old 27-01-2011, 10:41   #24
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BTW... regarding epoxy bedded machine screws threaded into a solid epoxy plug... IF it has a correct size hole that the epoxy is poured into, and the surounding material is strong like wood VS foam, AND there is enough threaded leg in the epoxy plug, you can so VASTLY exceed the strength of the fastener left sticking out, that no nut in the epoxy is needed, and the chance of this fastening meathod being the weak link is "0"!

I can understand the hesatation. When I was building my previous trimaran, and got to the part of the plans where one installs the rigs shroud attatchment "chainplates"... Two, 1/2" thick, "U" bolts, who's 6" legs were simply bedded in liquid epoxy, with NO fasteners, I was VERY sceptical.

Now, I have been building or repairing boats for over 40 years, but only finished one year of mechanical engineering, so I called the designer, John Marples. (He REALLY knows what he is talking about)! He had a clear answer in about two minutes flat! It was something like this: You figure the mahogany wood has a sheer strength of about X... The 1.5" wide pour hole is X cubic inches of surface... the tensile strength of the epoxy is X... The amount of thread contact is X... the load on the "U" bolt is devided by the two legs... if perfectly fastened, the breaking strength of the "U" bolt is X... ETC ETC, this went on for a couple of minutes. Then allowing for even as much as a 20% error factor, he could assure me that the fastening meathod was in the neighborhood of 15 times the strength of the hardware itself! This set my mind at ease.

I don't have John's enginerring and mathmatical skills, so I fall back on experiments instead. The technique I mentioned of bedding 3" of a 5/16" machine screw, and it exceeding the strength of the screw's head, was not guessing, it was done with destruction samples. (I have done hundreds of such experiments) Since the load on the machine screw should not even approach that which would pull the head off, I know that this attachment meathod is sufficient.

My only failed SS parts, ever, were stanion base bolts, that WERE backed with nuts and washers... it was from crevice corosion under the heads.

Moral being... If done right, epoxied fastenings can be vastly stronger than the hardware itself... IF it is done right. If you are in doubt, make a real scale mockup, and pull on the hardware with a crowbar and extended pipe handle. PULL IT TO DESTRUCTION! If the fastenings fail... go bigger / deeper, and try again. Or consult an engineer that specializes in the field.
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Old 27-01-2011, 13:26   #25
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i agree, if and this is a big ifffff...

if it is the correct type of epoxy for creating a 'mechancial' bond tot he substrate, and it is of sufficienent strength, and or fiber reinforced to increase the assorted forces, torsions, lateral, etc, and is of sufficient depth to allow sufficent thread contact, then yes... epoxy or other material would be sufficient.

we use epoxy for bonding of rebar into existing concrete to new concrete section... and also the attachement of imbeds similar to the end use here...

but, typically the thread / depth count is more then two to three times the length..that is, a nut may have 1/4 inch, or 3/8, in a typical nut so the epoxy would have to be at least 1/2-1" of threads/ then you would have epoxy all around it and beyond the screw, etc.

It can be done, and is probablyis done in may locations... and purposes...

btw: my numbers and estimates are not trained or engineered either, just from observations and my own brand of common sense, whihc is noth really common... sorry..
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Old 27-01-2011, 15:13   #26
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True.. In my case I am referring to a high modulus epoxy, like WEST brand. (I mostly make wood/epoxy structures)... Its bond is to a good gluing high to medium density wood like mahogany, and the oversized hole at least twice to three times the size of the fastener. In the comparison of a bolt's sheer strength, without a doubt a totally sideways, almost "breaking load" on a fastener, held in solely with a nut and fender washer, would crush and elongate the "wood only" bolt hole. This would bend the fastener, cause a leak, and be ruined.

With the exact same sheer load, (but no more), applied to a fastener like I described, it would likely have NO damage. IF, after drilling a 3/4" or 1" hole... (A 5/16" machine screw with 3" of its all threaded leg, bedded all the way down in West epoxy, mixed with "High D", "Silica", and "finely chopped glass fiber".

This would be because the epoxy/filler/fiberglass plug, that the fastener is bonded in with, its MUCH more crush resistant that just a wood hole, and the load is now spread out much more because we are comparing the sideways crush strength of a wood hole just over 5/16 in dia., to the sideways crush strength of a wood hole 3/4" in dia., with what is essentially a reinforced plastic bearing bonded in, using a glue that is stronger that the bond of the wood to itself!

It is a moot point however, as there should never be that much sheer load on a single fastener. If it is marginal, then more fasteners of a larger size is just fine, and WAY less work. Especially on high load applications like my chainplates, that are going through something relatively thin, like my 1" cabin side, I prefer nuts and bolts with stacked fender washers, (or epoxy bedded backing plates). I do line the plywood holes with enough epoxy to increase crush strength and add back up water proofing to the plywood, in the event of caulk failure.

When I was suggesting, and then explaining, epoxy bonding in fasteners, I was talking about situations where EITHER the back side is unavailable to put on a nut, or the material being mounted to is SOO thick, that an appropriate fastener might be hard to find, or install. Epoxy bonding is great when the depth of the hole is like... 15 times the diameter of the fastener going into the hole, and the oversized hole drilled first is at least twice the dia. Then there is no guess work, and no engineer necessary, "Nothing succeeds like excess"!

REFERRING TO OUTWARD HOLDING POWER...

In the example, where you are using say... an 8" long X 1/2" dia. 304 SS bolt, with a 1/2" tall SS nut and large fender washer, going through an all wood 1/2" hole... as the conventional approach.

Compare that to the same fastener size, but this time the nut is not 1/2" tall SS, but 7.5" tall and made of highly reinforced hard plastic. Also, this time there is 100% thread contact, a bit of adhesive bond, and the bolt hole that the fastener runs through is about twice the strength of the previous scenario's bolt hole!

Like I said, I normally use NUTS N BOLTS like everybody else, but there are applications all over my boat where I have used epoxy bedded hardware, or with the right materials, no fasteners at all!

Mark
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Old 27-01-2011, 15:21   #27
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I've had good luck with stainless togglers

As well as running helicoils into epoxy thickened with silica. I installed the head with helicoils tapped into epoxy that fills holes drilled into plywood. If it's good enough for the head.......it's good enough for an anchor
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Old 27-01-2011, 16:22   #28
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yes, i agree with the loads and forces we are talking about i too agree that it likly wont be a problem at design levels...
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Old 28-01-2011, 03:19   #29
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Wow! Thank you for all your replies.

Many great suggestions in there!

In terms of tapping/threading epoxy - I might be able to do this for the first 4" or so to a depth of 2.5" or so, but thereafter, the maximum depth that I could sink the fastener would be about 1"... so not really possible to get to 10X the fastener diameter elsewhere..

As for tee-nuts from McMaster - I would kill to have a McMaster in Australia - but unfortunately all we have is something like a Home Depot and that's about it.. nothing more exotic than nylocs sold there. Actually I was desperately in need of tee-nuts for another project a while back and could barely find mild steel ones (which I painted etc and used in a totally dry area inside the boat) - but can't find any stainless ones (well actually there are two places which sell 1/4" and 3/8" (only!) stainless tee-nuts, but with no specs as to which grade/dimensions etc...)

I do have heaps of zinc plated/mild steel tee-nuts from the other project, but woudln't feel comfortable using them "outside", even if they will be totally potted in epoxy.

Having said this - I could borrow from the rod stock idea given above (thanks!) and probably cut a small square of flat bar off and weld a normal nut onto it, thereby making a super-heavy-duty tee nut.. hmmm..

As for the dimensions of the cross section - the entire thing is 6mm (1/4" plate) - the base is 3" wide and the sides are 4" high - except for the cantilevered part and about 5" fwd and back from that point which is about 6" high.

The entire assembly will weigh around 40-45 lbs (20kg), and I believe that the strength of it in an up/down plane should be more than adequate. However side-to-side, I'm not 100% sure.

I've used beam-boy (software) to calculate deflection etc based on the cross section and there's almost no deflection expected even with a ridiculous load straight down... but again, no idea about side loads.

Welding is TIG, all seam welded with filler, except the inner joints which are just fuse-welded (no filler) - as the entire thing will turn into a pretzel if any more heat is applied over an almost 4 ft straight line (!)

...

So I think I will use a combination of the advice here - drill/tap/thread where I have heaps of depth and weld up some home-made gorilla tee-nuts where I have deck thickness depth only.

Thanks again!
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Old 28-01-2011, 06:14   #30
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Is this a multihull? Is there a crash tank? Even in the skinniest cats I've owned, if there wasn't a crash tank a long-armed youth or a child could always get there. Sometimes I would extend the handle of a wrench. Make a large threaded backing plate and all they have to do is hold it in place while you catch a thread. Much simpler.

You said something about hex heads; I wouldn't. I would use plentiful antisieze and NO dissimilar metals. But I understand your point.
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