Under a wooden plug in a counterbored hole in the mating part.Originally Posted by Derek Cohen
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Under a wooden plug in a counterbored hole in the mating part.
"Anything seems possible when you don't know what you're doing."
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If the bed does rack, and it will, you may have issues down the road.
In your application, the M&T joint is mainly for alignment, the strength of the connection is from the threaded connection, you need to make it as strong as possible. IMO M8 is too small/weak for a side rail/end connection, but that's just me. This joint takes quite a lot of force.
This contact point it very small in proportion to the width of the joint. All of the force from racking will be focused on that one point. What usually happens is that the nut starts to bury itself into the side rail, with a potential to split and/or bend the threaded rod.
This is why half moon washers are used, and the shaft hole is usually slightly oversized.
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The traditional bed bolts work well. Many of the covers look better than most ladies jewelry. Most tall post fine beds of today don’t have
any hangings and that greatly reduces the beauty. Before screened windows or air conditioning the tall post beds had “mosquito netting “.
We have a friend who found an old bed with hangings , and it is spectacular! Besides the netting, it has curtains that slide open. People who
had such beds usually had servants, so they needed privacy.
I forgot the warmth of the hangings ….and the sound proofing !
Last edited by Mel Fulks; 11-30-2022 at 9:12 AM.
[OP]
Contributor
Bed bolts come in M6 (6mm) or M8 (8mm). The equivalent Imperial is 1/4" and 3/8". I have used the larger size.
The shaft hole used is 9mm.
The reason, as I see it, for the half moon washer is to use a flat bolt surface against a curved edge (created by a hole). The flat I chiseled is as strong, keeping in mind the wood is Jarrah (very hard). I did consider a large, single hole for the bolt end, but that would have removed more wood. I was trying to minimise what was removed.
Regards from Perth
Derek
Member
The wood under the nut/washer may compress a bit, but that can be addressed by simply tightening the nut.
The weak spot is the threaded insert. It will fail over time due to the racking forces and significant leverage advantage of the legs. Bed bolts traditionally pass through the leg for some hard won ‘engineering’ reasons.
What is the size of the insert thread engaged to the leg? 12mm? …15mm? If you are thinking it will hold, then you could replace the 8mm threaded rod with a 12 or 15mm hanger bolt and get the same result. I would hazard a guess that no one would do this for two reasons: there is virtually no way to repair a failure of the wood thread; and perhaps too obviously folks think it will fail.
JMHO. (And the wood working is your typical top-notch.)
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I believe Malcolm summed it up pretty well.
The solution would be to use a central tenon and two bolts per connection, flanking the M&T joint. A single bolted connection creates a pivot point for racking. It's obviously too late for this project, but something to keep in mind.
"Bed bolts come in M6 (6mm) or M8 (8mm). The equivalent Imperial is 1/4" and 3/8". I have used the larger size"
The equivalent Imperial is 1/4" and 5/16".
A 3/8" bolt, like many older style beds use, would be 9.5mm.
One thing to keep in mind is that when you're using a dense, hard species like Jarrah, there isn't any "give". This means, any flex or movement will be in the connections, i.e., the hardware. The hardware must be as strong as the wood, so as not to create a weak link in the chain.
Hope it works out for you
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yes, and they go back to at least early 19th century. I bought a child’s crib made in a shop that was out of business by 1820.
[OP]
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What is still possible is to add a narrow mortice in the post behind the threaded insert and drop in a threaded steel plate.
I made these for my bench many years ago ...
I should add that my expectation/understanding of racking is that it would be minimal anyway. The joint is not simply via a bolt. There are two shallow but tight fitting mortice and tenon joints. These alone lock the post and rail together without any vertical movement. The bolt is to keep them together. Your thoughts?
Regards from Perth
Derek
Last edited by Derek Cohen; 11-30-2022 at 10:59 AM.
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Your expectation of racking is that it will be minimal, what often happens in the real world can be different.
If the M&T is a tight fit, this will help keep the joint from experiencing any unnecessary movement but not eliminate it totally
How these beds are used and by whom has a direct effect on the longevity of the joints remaining tight.
If someone gently sits on a bed, there is little racking force applied. If I flop down on the bed after a long day, my 200+ lbs will have more of an effect. If children are jumping up and down, etc.
Beds take a lot of abuse and must be made as strong as possible. This is why hook plates are often used as an invisible fastening system. Their design keeps them tight when weight is on the bed frame.
The main point is to make them as sturdy as possible, even beyond what you think is heavy duty. Just google wobbly bed frame and see the results.
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“Minimal” can mean a lot of things, and racking (perceived motion) vs applied force are different.
Humans activity, whether pushing it closer to the wall, or uhm… interacting, with the bed frame, will cause racking and apply a force in the long axis of the bed frame. I haven’t tried to scale your photos, or read back for dimensions, but a quick and dirty stab at a static torque diagram would lead me to believe you have built a lever with somewhere between 4:1 to 5:1 advantage. White-board calc, with typical bed dimensions, says a 5lb belly flop would pull on the insert with ~23lbs; 200lb dive pulls on it with 920lbs (417kg), not counting inertia. …my thoughts. Hope it helps.
(ETA - Reality says such a load would be shared by all 4 joints. I have assumed 1 joint carries it all as a safety factor. I tend to over-build.)
Last edited by Malcolm McLeod; 11-30-2022 at 2:47 PM. Reason: Forces
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Malcolm, that's what I was trying to say, just not as technically, thanks
Not trying to beat up on Derek, but many people don't understand the forces these joints are subject to.
A bed rail is essentially a 6" lever attached to your joint.
[OP]
Contributor
Edward, I am always ready to learn more, in this case about the stresses in bed ends. These would only be the 4th and 5th I have done, and the first with bed bolts.
It strikes me that there are two ways to beef up the existing bolt. The first is the plate I described above. This simply adds strength to the existing bolt, but does nil for racking. The second is to add small angle plates in the corners above- and below the bolt. That would resist any racking.
Regards from Perth
Derek
Contributor
No doubt the joint would be stronger with two bolts, but one would search far and wide before finding a picture of a traditional bed frame with more than one bolt through the posts.
If you are concerned about the threaded insert pulling out and have the depth to drive it deeper, then a larger cover plate let into the post would beef it up. I don't think you need be worried about using a single bolt, especially on a single bed that will see less action than one with more occupants.
Last edited by Kevin Jenness; 12-01-2022 at 11:51 PM.
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Agree with Kevin. The bolts replaced the “rope beds “ ,and that was a giant leap forward. Some people wanted the Empire State Building
“stronger” but good engineering worked okay.
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OP's bolt is a single 8mm (5/16") bolt, a larger 9.5mm (3/8") bolt is about 30% larger which adds strength.
Two 8mm bolts would be even stronger and would have somewhat more resistance to racking, as there is no single pivot point, depending on how far apart they are spaced.
Most beds bolted with a single bolt (3/8"-9.5mm) are through the posts, not threaded into an insert. This, along with their larger shaft, allows them to be tightened more and withstand more forces that act upon them.
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