Adding a third center hinge for strength

How much strength would a third center hinge add to a door?
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None. The door strength is not affected by hinges.
Do you mean how much more load can it bear? 50%. Unless you have unusual circumstances, a third hinge will not make a difference to an existing setup. The hinge size is a big factor though. Larger hinges can carry more weight, of course, and it is easier to install.
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Also some hinges are mortised in but some, even on expensive Pella doors are surface mounted which is not nearly as strong.
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<snip>
I'm curious. I have never heard that. Just how does mortising increase the strength??
Harry K
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<< how does mortising increase the strength?? >>
Thinking ,logically, the mortised hinge plate cannot twist, hence a part of the weight load transfers directly to the wood of the jamb. Unmortised, the entire load is is carried by the mounting screws. It's like 100 divided by 3 (screws, for example) is a bigger number (load) than 100 divided by 4, so lighter load should equate to longer service life, right?
Joe
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snipped-for-privacy@aol.comtosspam (Joe Bobst) wrote in message

Sorry but I don't buy it. In order to twist the screws have to be slightly loose to begin with. Also, a 1/16" or even 1/8" sliver of wood is not going to prevent twisting.
I don't get the point of your last bit 3 vs 4. If you are referring to number of screws, you are correct but what does that have to do with mortising?
My belief is that mortising is used for better appearance.
Harry K
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That sliver of wood under each hinge will support the entire weight of the door while the screws funtion will be to hold the whole works in position on that sliver. Often when removing doors with the hinge pin intact, the door will stay in place on the jamb after the screws are removed. Mortising does improve the appearance of the job but mostly because it shows the job was done properly.
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snipped-for-privacy@aol.com (PAROADHOG) wrote in message

Yep, they will do that -as long as the screws are tight-. How long do you think that sliver of wood would hold with loose screws??
Harry K
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I'm not sure why you'd ask that, since loose screws are not all that common on door hinges . But I suppose the edge of the mortise would continue to support the door until the screws became loose enough to allow the hinge to move out past the edge of the mortise. But screws that loose would be evident long before all that occurs.
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snipped-for-privacy@aol.com (PAROADHOG) wrote in message

Screws loose -at all- will result in the mortise being ratted out soon. Again in case you missed it, the mortise will come into play only after the screws loosen by some amount and whatever holding power the mortise has will be very short lived. You are correct that loose screws are rare but I have seen them on very old construction and doors that are on their last legs. Case in point the shed door I just rehinged and remortised due to that very thing: loose screws and mortises ratted out.
Harry K
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On Wed, 11 Aug 2004 20:15:45 -0400, "New & Improved - N/F John"

Back when I was selling crack cocaine, I had people always trying to bust down my door (especially the police). I finally bought 22 hinges and 16 dead bolt locks. After reinforcing the door with 1/4" diamond plate steel, and 2 layers of 3/4" plywood, I put hinges all the way up the edge of the door. On the other side I put all the locks. One cop ended up in the hospital trying to open the door with his shoulder. After that, they just broke a window to enter, so I was forced to put cinder block in all the windows. The next time they came, they brought a bulldozer and removed the whole front wall of my house. Nothing is safe, I will repeat NOTHING IS SAFE !!!!
PS. The one problem with having 16 locks, is that it takes a half hour to figure out which key goes in which lock while sober, and I slept under the bushes in the yard when I was high.
Those bushes were really prickly too. I think they were rose bushes. But anything named bush is bad anyhow.
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New & Improved - N/F John wrote:

This exchange reminds me of yet another "door" thing Uncle Schlomo tought me, which was interesting to learn, albeit somewhat impractical.
He pointed out that the theoretically proper place to mount a wall mounted door stop is half way up the height of the door and 2/3 of the door's width out from the hinges. (For a door of uniform density)
That puts it at the "center of percussion" so that when the door crashes into the door stop it's swing is halted without putting any reaction forces on the hinges, minimizing loosening of the hinge mounting screws.
I think somewhat the same principle applies to the "sweet spot" on a baseball bat.
Just my .02,
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
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