Glue weaker than wood?

I was reading about the grading of steel bolts

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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

Reply to
Michael Press
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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.

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Reply to
Lawrence L'Hote

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.
Reply to
Joseph Smith

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.

Reply to
Howard Ruttan

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

Reply to
Frank Ketchum

Mon, Jun 28, 2004, 5:41pm (EDT+4) snipped-for-privacy@mchsi.com (Lawrence=A0L'Hote) says: However, this individual has an opinion worth noting.

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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

Reply to
J T

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.

Reply to
Michael Mcneil

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

Reply to
Jay Pique

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

JP

Reply to
Jay Pique

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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Reply to
TeamCasa

It's pretty much a non issue. All modern wood glues are stronger then the wood, as is some not so modern ones so, from that point you would have to hunt around for some glue that is weaker then the wood but strong enough to withstand the forces it will encounter and I don't think fish or vegetable glue will cut it.

To look at it in another way, unless you are talking balsa wood models, wood it's self is pretty sturdy stuff and with properly made joints, if someone is going too, in the course of normal use, bang it around badly enough to break it either at the joint or the grain maybe it probably shouldn't have been made of wood to start with or they shouldn't have been allowed near it.

Lastly, we aren't talking, say a crankshaft breaking under transmitted stress we're talking wood that can be glued back up again.

I've had to repair glue joint failures and wood grain failures. I prefer the wood grain failures, the wood goes back together easily and seamlessly where as a failed glue line is usually a raggedy ass affair that always seem to end up as an obvious repair.

In short, it's horse pucky.

.
Reply to
Mike G

On Mon, 28 Jun 2004 13:25:45 -0400, Michael Press vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

No. It's crap. Glues are stronger than wood. Most joints involve the wood of one part intruding into the other part, specifically so that the wood has to break to break the joint.

Reply to
Old Nick

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

but for an end table, i agree. it doesnt apply.

randy

Reply to
xrongor

Snip

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.

Reply to
Leon

"Breakaways" are generally made without glue and with rather loose joinery all 'round.

Take a gander at WWE's chairs: They're a design that holds together when weight is applied (they can be sat upon) but fall apart fairly easily.

Casework is often made of styrene foam.

Reply to
U-CDK_CHARLES\Charles

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

Reply to
xrongor

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.

Reply to
Lawrence Wasserman

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.

Reply to
Leon

This is correct. But they are referred to as freeze plugs by GM at least in the 70's and 80's when I was Service manager of a General Motors dealer.

Reply to
Leon

Tue, Jun 29, 2004, 1:34pm (EDT+4) snipped-for-privacy@swbell.net (Leon) says: 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.

Not quite. "Freeze plug" holes in the engine block are actually used to empty out sand, used during casting.

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Reply to
J T

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