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Author Topic: Mechanical name for elbow hinge?  (Read 1336 times)

AZRedhawk44

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Mechanical name for elbow hinge?
« on: March 05, 2020, 07:46:50 PM »
My google-fu is weak, please help me.

My trailer I've built has two fold out bed platforms.  Currently they're suspended by chains, much like the hypotenuse on a triangle.  I want to eliminate the chains in favor of a length of metal, cut in half with a hinge at the midpoint.  The length when fully extended will be the hypotenuse and will support the platform and sleepers inside, and will allow for the bed platform to fold 90 degrees to the wall when stowed.

The closest I've found for what I'm looking for is an elbow hinge, but I seem to only be able to find medical prosthesis oriented products with that term. 



I figure something like this would handle the stress involved, and I need the total width of the metal, hinges, and anchor hardware to be 1/2" or less.  This vertical orientation of this type of hinge is perfect for this, but every time I search "elbow hinge" all I ever get is prosthetics.  I can't find commercial equivalents.

Looking for the best type of hinge that can support the stress of the joint at full extension.  What's going on with that center pin in something like this?  Is it a chicago screw/sex bolt through a bearing, with the other hinged arm pressed onto the bearing outer surface?  Or am I overcomplicating it?
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Fly320s

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Re: Mechanical name for elbow hinge?
« Reply #1 on: March 05, 2020, 07:58:09 PM »
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #2 on: March 05, 2020, 08:06:44 PM »
No, those are all 90 degrees off on how I want the metal to rotate/hinge.  I want them to rotate like the hinge on a pair of scissors.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #3 on: March 05, 2020, 08:12:23 PM »
More like this:

https://www.pelsupply.com/product/2625

https://www.diytrade.com/china/pd/7131960/sofa_hinge_sofa_arm_headrest_adjuster_reclinerC18.html

Neither of those specifically are suitable though.  The first has some sort of ratchet mechanism in it I don't want to deal with, and the other is pig iron chinesium.

One is a "flail arm hinge" and one is a "sofa hinge."  Still not sure what I'm supposed to call this orientation of hinge.  Elbow hinge, arm hinge, etc.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #4 on: March 05, 2020, 08:15:40 PM »
Knife hinge?  Another name for the orientation.
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Fly320s

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Re: Mechanical name for elbow hinge?
« Reply #5 on: March 05, 2020, 09:13:46 PM »
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RoadKingLarry

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Re: Mechanical name for elbow hinge?
« Reply #6 on: March 05, 2020, 10:15:33 PM »
On my boat I have a table that is hinged at the bulkhead (wall) and folds down and is supported by metal folding legs that latch into position. When the table is stowed the legs fold flat against the bottom of the table.

Something like this is what it uses
https://www.tablelegsonline.com/click-folding-leg-single-click-sngl.html

this was just a quick search and these are a bit pricey but there has to be similar for less available.

I know the ones on my sailboat salon table wouldn't be that expensive.

ETA:
These are closer to what is on my boat
https://www.amazon.com/Top-Line-FL3400-305-30-1-Locking-Folding/dp/B0016N6K90/ref=sr_1_12?keywords=folding+legs+for+table&qid=1583464648&sr=8-12

A maybe less convenient option wold be a pipe flange attached to the bottom of the bunk with the appropriate sized pipe nipple.
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dogmush

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Re: Mechanical name for elbow hinge?
« Reply #7 on: March 05, 2020, 10:23:48 PM »
How big?

That's like an old truck tailgate support.

Item 14 on this:
https://www.lmctruck.com/1967-72-chevy-gmc/bed-tailgate/cb-1967-72-fleetside-tailgate-and-components

They were all on pretty much every pickup until the late 80's.  It should fit your width requirements, but length choices might be limited.

Do you have a way to weld?  For one of these long enough for a bunk, I'd buy an existing hinge and weld extensions on the legs.

Or just take two 1/4"X 2 flat bars, screw them together with a 1/2" chicago screw, and put a teflon washer between them as a bearing.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #8 on: March 06, 2020, 10:40:53 AM »
About as big as this attachment.  There's a left and right fold out bed platform.

Currently chains form the hypotenuse to support the platform, about 4-5 feet of chain length (the platforms aren't the same dimensions).

Yep, I can weld.  I built the trailer from the ground up from steel beams.

I'm not happy with the fabric and I'm going to remove it in favor of a more waterproof and rigid and better insulated roof and walls, and the hypotenuse support needs to be relocated to the outside of the platform, and fit in the gap between the bed platform and the vertical post which supports it.  I also am a bit paranoid about how I transport it with the hinges for the bed platform being located on the bottom and nothing outside to prevent it from falling open (It's secured from the inside, but it'd be a disaster if the platforms did open while traveling).  So I'm re-jiggering so a hard roof folds from the top, down over the sleeping platform, preventing that from being possible.

I've ordered some 1/4-20 threaded chicago screws and pfte washers to mess with, see if I trust them to bear the weight.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #9 on: March 06, 2020, 10:41:56 AM »
Another pic.
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dogmush

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Re: Mechanical name for elbow hinge?
« Reply #10 on: March 06, 2020, 02:09:36 PM »
I would weld something out of flat stock for that.

Another idea:

Due to space constrictions, weld a 1/2" Diameter, 1/4 inch piece of round stock to a 1/4 flat bar.  Drill and tap for 1/4-20.  Blow a 9/16 (or 17/32) hole in the other piece of flat stock, sammich PTFE washer between them and then bolt them together with a button head bolt and washer.  Total width would probably be about 7/8" and the weight would be held by the welded 1/2" boss.  For more strength blow a 1/2" hole in the first piece and weld the boss on both sides.

More than strong enough, and cheap and easy.  Adjust dimensions as needed to fit the space.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #11 on: March 06, 2020, 03:17:14 PM »
Yup, that was the other tactic I was stewing on if I couldn't find a decent sex bolt or retail hinge in the orientation I wanted.

What I don't like about the rod protruding from the flat solution, is the possibility of not having it perfectly perpendicular/square.  And also the weld area build-up on the inboard side of the pivot and a sloped growth in diameter of the rod section causing binding when actuating the hinge.

Still stewing on a custom flanged collet or nut.  Flanged on the outboard side so I have more contact surface for my weld, thick walled for strength, threaded in the center to accept a bolt, and an oversized washer under the bolt that threads into the collet retains the other arm of the pivot.  Basically a super burly oversized sex bolt capable of being welded to one of the two articulated pieces.

But, I don't have a mill or lathe.
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Hawkmoon

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Re: Mechanical name for elbow hinge?
« Reply #12 on: March 06, 2020, 03:53:15 PM »
I think the truck tailgate hinges Dogmush suggested should do nicely.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #13 on: March 06, 2020, 03:54:45 PM »
Ooh, McMaster-Carr has just what I need for that collet/bushing.

https://www.mcmaster.com/8492a163

I tap the inside of that for 1/4-20 and I'm set.
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Doggy Daddy

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Re: Mechanical name for elbow hinge?
« Reply #14 on: March 07, 2020, 07:16:45 PM »
Quote from: AZRedhawk44 on March 06, 2020, 03:54:45 PM
Ooh, McMaster-Carr has just what I need for that collet/bushing.

https://www.mcmaster.com/8492a163

I tap the inside of that for 1/4-20 and I'm set.

Hyuh, hyuh, hyuh...  He said he'd tap that.   >:D
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tokugawa

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Re: Mechanical name for elbow hinge?
« Reply #15 on: March 08, 2020, 06:29:03 PM »
Is it a hinge you  need, or a folding support stay?

 Usually this sort of thing is done with a standard hinge type, and a stay (2) that folds up when the bed is retracted, and  extends to the outer edge  of the bed platform, for maximum support?
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #16 on: March 08, 2020, 09:05:08 PM »
There we go!

Vocabulary to the rescue.

Yes.  A folding support.

Any idea where I can get any pre-made capable of supporting ~250 pounds each (paired for 500 pounds total), and somewhere around 48" long?
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tokugawa

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Re: Mechanical name for elbow hinge?
« Reply #17 on: March 08, 2020, 11:04:07 PM »
These will be a low frequency use, maybe a couple times a day, yes?  So no worries about wear, for all practical purposes.
The unusual part is the length.

I think making them out of flat bar, say 1" or 1 1/4" wide by 1/8" thick,  with a short bolt to act as the hinge, and a hole in each end to fasten to bed and trailer frame, would be pretty easy. It might be necessary to use an angle bracket on the ends, with a hole for another pivot, depending on whether the connection point is parallel or perpendicular to the stay.
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AZRedhawk44

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Re: Mechanical name for elbow hinge?
« Reply #18 on: March 09, 2020, 01:22:39 PM »
I ended up ordering the McMaster bushing I linked earlier.  I'm going to mount that through a couple of 1/8" thick pieces of bar, probably 1" tall.  Then tap the center hole for a 1/4" bolt that will retain the two folding arms together.  Plenty stronk.  Stronker than the chain currently doing the job.  Grade 8 bolts, probably 3/8 or 7/16, will anchor the endpoints to the bed platform and main trailer frame.
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