Glue weaker than wood?

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I was reading about the grading of steel bolts (http://www.sizes.com/tools/bolts_SAEtork.htm .) At the end it made this interesting statement:
"It isn't always a good idea to replace a bolt with a stronger one. Some bolts are deliberately chosen so that they are weak enough to fail before the stress or strain damages some more expensive or critical part of the equipment. For the same reason, in making furniture cabinetmakers use glues that are weaker than wood. That way, if the furniture is overloaded, the joints break. It is much easier to reglue a broken joint than to replace a piece of broken wood."
Is this true? I recall reading on this NG that regular wood glue forms a bond stronger than sold hardwood and when tested the wood breaks near the joint before the joint separates.
Thanks, Michael
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---snip-----
I have seen pictures of tests done with glue and various joints on this NG but don't have any reference to that study handy. However, this individual has an opinion worth noting. http://atos.stirlingprop.com/kbase/HOTHIDEGLUE.htm
Larry
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Mon, Jun 28, 2004, 5:41pm (EDT+4) snipped-for-privacy@mchsi.com (Lawrence L'Hote) says: <snip> However, this individual has an opinion worth noting. http://atos.stirlingprop.com/kbase/HOTHIDEGLUE.htm
I think my favorite part is where he goes on about the glue being weaker then the wood, even tho some wood will go with the glue when a joint fails, then says that means nothing. Amazing. Absolutely amazing. I stopped reading about then.
JOAT That the peope have a right to bear arms for the defense of themselves and the state. - Pennsylvania Constitution of 1776
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On Mon, 28 Jun 2004 17:10:36 -0400, snipped-for-privacy@webtv.net (J T) wrote:

I thought he brought up some interesting points. I haven't yet used hide glue, but I believe I'll dig out that bag that's been gathering dust on the back shelf for a little research.
JP
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The July/August Handy magazine just did an article on glues. They did a review on everything from plain old elmer's, Tightbonds, Epoxies, hot-melts, Eurathanes, etc.... They stated that most glues (other than hobby glues) have a rated shear resistance of 2000-4000 psi, which is greater than most wood species and that some epoxies go 8000 psi or higher.
Joey in Chesapeake

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"Michael Press" wrote ...

There is no way to really quantify what "Stronger than the wood itself" or "weaker..." means. I have seen several articles that try to explain it but you can't really put a number on it. No furniture maker will consciously make a joint that has a weak spot in case of later breakage. You try to make the joint as strong as possible so it will never break. To do that you need a properly executed joint and a properly applied adhesive. Any study you can find will show pictures of joints (one in FWW several years back comes to mind) that failed where the wood around the joint broke instead of the glueline. It seems to me that common sense would indicate that the glue was not the weakest point in the equation. Of course it also means that once you have reached the point where the surrounding wood fails before the glueline, that requirement for additional strength is debatable.
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Howard
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I don't think this is accurate. I can't think of an instance where something is designed in woodworking to fail. The idea is that something should be built so that it will never fail. It is probably a valid point when dealing with metal mechanical things such as engines, transmissions, etc. I don't believe the same philosophy has any place in the construction of an endtable.
Frank
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Quite right.
PVA is a release agent when mixed with almost anything else except water. I believe meths is used with it in fibreglass molds. Any glue joint will fail too where one or both sides is rubbed down with a damp cloth and the pores are full of water when the glue is applied. These are not engineered devices but poor practice.
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On Mon, 28 Jun 2004 21:20:48 +0000 (UTC), "Michael Mcneil"

Shoji screens are made with rice glue so that they can be easily knocked apart, repaired and reassembled.
JP

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construction
many things have 'sacrificial' parts or points of known failure. electronic circuits have a fuse which is essentially a wire that burns up. car engines have plugs in them so if they overheat the plugs pop before the block cracks. boat owners toss zinc over the side with a wire attached so it corrodes and not the boat. the company that makes my electric toothbrush thoughtfully makes the plastic on the replacable brushes much softer than the expensive handle and will in fact repeatadly crack in the same place if over tightened, trust me on this one <g>
but for an end table, i agree. it doesnt apply.
randy
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Snip

the block

They pop out to prevent damage but not because of overheating. Those plugs are known as FREEZE plugs. If the cooling system water in the engine block freezes the expanding ice will pop out the plugs to help prevent damage to the engine. The radiator cap is the safety valve for an overheating cooling system, not the freeze plugs.
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not
ya freeze. it was late.
nonetheless, what good is the radiator cap if all the coolant is missing and the temp sensor doesnt work.... the plugs will help with that too. my dad grossly overheated a slant 6 in that condition and plugs popped.
randy
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and
dad
Modern cars, built since the mid 70's, and I realize the old Dodge slant 6 and been around longer than that, have coolant recovery tanks. As the temperature of the coolant cools inside the cooling system the coolant is sucked back into the radiator through the radiator cap and back into the radiator. The radiator cap is a 2 way valve. When pressure builds up high enough because of temperature the radiator cap lets water bypass it and into the hose going into the coolant recovery tank. When the coolant cools and contracts, the coolant passes back through the cap and into the radiator. If the freeze plugs indeed pop out because of overheating, you probably had a number of problems with that engine. #1, you temp sensor did not work properly, probably too much corrosion built up for it to get a correct reading or, there was an air pocket and the coolant was not submerged in the coolant. #2, the radiator cap did not work properly or was one with too high of a pressure setting for that car. #3, since the freeze plugs popped out, the engine over heated and the temp gauge did not work, I suspect a rusted out cooling system and the freeze plugs were probably about rusted out anyway.
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I was going to let this OT pass, but, though they are commonly referred to as freeze plugs, the technical term IIRC is welch plugs. They are NOT designed to protect the block or other casting from freezing, they are simply required as part of the casting process.
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Larry Wasserman Baltimore, Maryland
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Tue, Jun 29, 2004, 1:34pm (EDT+4) snipped-for-privacy@swbell.net (Leon) says: <snip>Those plugs are known as FREEZE plugs. If the cooling system water in the engine block freezes the expanding ice will pop out the plugs to help prevent damage to the engine. <snip>
Not quite. "Freeze plug" holes in the engine block are actually used to empty out sand, used during casting.
JOAT When you're up to your ass in alligators, today is the first day of the rest of your life. - Archchancellor Mustrum Ridcully, UU
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Really a dual purpose part. As you indicated, to close an opening that is intended to facilitate the manufacture of the block and,
Definition: An expansion plug located in the side of an engine block that is supposed to protect the block against freeze damage. Water expands when it turns to ice, and if the coolant doesn't have enough antifreeze protection it can freeze and crack the engine block. The freeze plugs (there are several) are supposed to pop out under such conditions to relieve the pressure on the block. Freeze plugs can often be a source of troublesome leaks as a result of internal cooling system corrosion. Ease of replacement depends on accessibility.
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On Tue, 29 Jun 2004 18:57:11 -0400, snipped-for-privacy@webtv.net (J T) vaguely proposed a theory ......and in reply I say!:
remove ns from my header address to reply via email

I would have said "last day"....
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Wed, Jun 30, 2004, 8:29am (EDT+12) snipped-for-privacy@dodo.com.au (Old Nick) says: I would have said "last day"....
But, if you believe in reincarnation...
JOAT When you're up to your ass in alligators, today is the first day of the rest of your life. - Archchancellor Mustrum Ridcully, UU
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The key part of this information is "expensive or critical part of the equipment". Furniture does not generally fall into these categories. I can't think of a reason save movie break-apart furniture, where a joint would be designed to fail. Well maybe some IKEA stuff!
Dave

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