Adding a support for flip-up shelf

[Bas]

I built a cabinet (32" high, 22" wide, 18" deep). On either side, it will have a shelf that flips up. Each shelf is attached to the cabinet via a piano hinge, and is 24" long and 10" wide.

Although quite different from what I built, the following picture will give you an idea of the wing:

Obviously, I will need a support for the shelf. I plan to use a simple "arm" of hardwood (3/4" x 3x4") that will attach to the cabinet via a small section of piano hinge. On the shelf, I will put a bracket that the arm will catch on.

Got all that? Good.

I'm trying to work out the optimal length for the arm. See the image below.

Here, ac is the cabinet, ae is the fold-up shelf, and bd is the arm. Normally, the shelf will hang down. I want the arm to be concealed by the shelf in this position.

The pivot point for the arm is located at b. Since the shelf is 24" long, ab + bc is also 24". The longer arm bd is, the stronger the support will be. But, the longer I make bd, the more obtuse the angle abd must be to compensate (as bd increases, ab must decrease to keep the ab + bc = 24). The more obtuse the angle, the weaker the support. There has to be an optimum.

My gut tells me 45 degrees for abd would be best. Applying basic trigonometry functions means arm bd is a little over 14" (14.06 to be exact). That seems reasonable.

For all you structural engineers out there - is 45 degrees the best option? Should I increase the angle to 60 degrees (effectively moving the pivot point up) and increase the arm length to 16". Better, worse?

 

[jeff...]

what's the bucket for?

 

[Outa Square]

If it was me i would focus on the strength of the top and that of the arm, The angle isn't as important. I would just move the connection as close to point e as you are comfortable with.

But if you do want a angle i would use 42:widea:

 

[skysharks]

Bas
You could do exactly like you want to do with the arm.
However you mentioned that you wanted the arm to be hidden when the shelf is at it's downward folded position.
1. You could make the arm removable and stow it away inside the cabinet.
Or hang it on a hook.
2. Or there are locking hinges that you could use, where as the arm is divided into 2 pieces. these parts being connected by this locking hinge when rotated up inot a straight line(shelf up) it locks into place. To lower the shelf you break down the arm at the hinge point and with another small hinge at the attachment point to the cabinet it (the arm) would fold up and under the shelf.
Then with a skirt around the perimeter of the shelf, in the folded down position nothing would be visible.


Hope that you can see what I am talking about.

 

[Bas]

2. Or there are locking hinges that you could use, where as the arm is divided into 2 pieces. these parts being connected by this locking hinge when rotated up into a straight line (shelf up) it locks into place. To lower the shelf you break down the arm at the hinge point and with another small hinge at the attachment point to the cabinet it (the arm) would fold up and under the shelf.

Mac, I like it, I think I've seen something like that before. That is a much better solution, since the support can span the full length of the shelf. Also, I can attach both sides of the arm to the shelf with screws, so there is no danger of the arm ever slipping out.

The $64,292.53 question (adjusted for inflation): Where might one acquire such a contraption? I looked at McMcaster-Carr, and they have a very sophisticated locking hinge, at a very sophisticated price. I want something sturdy, but not industrial.

But if you do want a angle i would use 42:widea:

Of course! That's the (ultimate) answer! Knowing my woodworking skills, I'd probably end up at 42 anyway, if I tried to make it 45 degrees

 

[woodArtz]

Something like this???

http://woodworker.com/cgi-bin/FULLPRES.exe?PARTNUM=928-035&LARGEVIEW=ON

 

[Bas]

Bob, that would work, but it's not what I had envisioned. What Mac described is two straight arm pieces that fold up when the shelf is down. As you lift up the shelf, the two arms start to form a triangle, until they are in one line when the shelf is fully raised. The hinge point moves "away" from the cabinet during this operation.

The ratcheting support should be able to do the job, but doesn't offer any advantages over a normal, hinged short arm. Plus, $50 (two shelves) is a bit more than I bargained for. But thanks for the suggestion!

 

[PurpleThumb]

Bas, let me throw another possible solution. How about two slide out (pull out) supports that would be behind your rails. If you wanted a shelf on both ends you could offset the supports, similar to a dining table pull out.

 

[Vanilla Gorilla]

Purplethumb's suggestion is what I immediately envisioned as well. When it comes to hinged supports, I always worry about the screws on the hinge eventually pulling out, or developing enough play that the shelf doesn't sit level anymore. at least with a slide out bar support underneath,. if it ever developed play you could wedge it. Plus, it seems like the hinged support might be over engineering it. the less moving parts, the less that can go wrong!!