3rd hinge needed?

[Joel OF]

I have just been given the thumbs up from my client for a gate design, 3rd hinge discussion ongoing. Picure of sample piece & designs attached. (Hex head bolts to be re-forged to square & more fixing points on round hinge plates).

Steels to be used: 30 x 20mm back stile, 30 x 10mm curves, 2mm sheet panels, 20 x 3mm riveted edge softening strips.

Total weight approx 75kg. Back stile weight approx 10kg, remaining gate weight approx 65kg.

Think the 3rd central hinge is needed to prevent the back stile bowing? No young children, infrequent grandchildren visits to use it as a swing.

[Glenn]

If in doubt, make it stout.

The client will not object if it does not fail, but you will surely hear from them if it does.

[gote]

Generally speaking:

Three hinges need a very good alignment or something must flex. It is difficult enough to get two hinges aligning well. One hinge tends to take all the vertical load and the other is just keeping the door/gate vertical. If you want to take loads on the gate you should space the hinges as much as possible to get a longer arm of momentum. A middle hinge will not contribute much. Even if the alignment is perfect it will only contribute half a much as the other two.

The momentum from the load is = Child times the width of the gate. Assuming the vertical support is on the bottom hinge, the momentum is taken up as the horizontal load times the distance between the hinges. Your distance between hinges is twice the with of the gate so the horizontal load on the upper hinge is = Child/2. A central hinge has only half the distance so it contributes only half as much or = Child/3. (1+2=3) However , if the top one is slightly loose it is stressed twice as much as the one on top would have been = Child /1.

 If in doubt, it is much better to make the hinges, and the pieces they are fixed to, sturdier.

Specifically:

In your design the middle hinge sits on a vertical bar that is only connected at top and bottom. Any force that is in that hinge will only make the vertical bar flex so the contribution of that hinge is practically nil. The reasoning above refers to a solid gate. I would leave it out since it A: contributes next to nothing B: induces a risk of binding if the three hinges are not perfectly aligned. Even if your work is perfect, the post may be out of line.

Nice design by the way. It will look very well when finished I think.

[Joel OF]

Thanks very much for the comprehensive reply. I'll read it again a few times later when this hangover has worn off.

I always set both hinge pintles facing up so the weight of the gate is spread across both hinges. I put a security pin through the bottom him pintle so the gate can't be lifted off.

The chance of a 3rd hinge binding had crossed my mind. To be clear my concerns about the back stile bowing are as per the bottom 2 images, not the top. Pic taken from Peter Parkinson's brilliant book on Forged Architectural Metalwork. I'd really recommend his books to everyone, full of great info.

[gote]

It is very difficult to spread the vertical weight evenly unless the pintels are sitting in a soft material so they can flex up and down. The requirement on exactness is overwhelming. This is the reason why a machinery shaft (including crankshafts) is always designed so that one bearing only takes the axial load. If you have a steel gate sitting in a stone post, even the temperature variations between day and night will shift the vertical load between the upper and lower pintel. Since we do not put strain gauges in the pintels we never notice how the load is distributed.

The picture of the flexing gate makes no sense to me. If the squareness of the bottom inner corner bends the inner upright outwards, why does not the upper corner bend it inwards so the bend becomes S-shaped. The other corners and bends are just as improbable. It might be possible to design a gate in such a way that the flexing is as depicted but it would be an engineering design feat - I cannot figure out how it could be done. Make yourself a weak frame of wire and try it for yourself and you will see a different pattern of bending. If there is a load on the outer upright, both horizontals will cause a bending moment in the same direction on the inner corners causing a rotation in the same direction (Clockwise in the pic) on both hinges and since one sits above the center of the inner upright and the other below the bending will be S-shaped and there will be no horizontal movement in the center of the upright.

Your design is not braced diagonally over the whole gate but it has a fair bracing in each of the three sub-frames so that any bending at all of the inner upright is not very likely and besides "so what?" The design of three sub-frames will cause a similar bending pattern in all three and you do not brace the other uprights against this spurious bending so why brace the inner with a third hinge? 

[Judson Yaggy]

Love the design of that top element.  Just going by eye and gut I'd skip the center hinge.

[arftist]

Hi.

Just went through this again with a very anal architect. I yielded to his demands on all but the hinges.  

Yes you can use three but it is much more difficult and two propperly sized hings work very well.

On wood doors with strap hinges I alway use three but on steel doors or gates two is much better.

 

 

[arkie]

Would it help if the two hinges were each longer?  That might spread the load some to quieten your architect.  Or, do they have to fit a prescribed architectural plan?

[Joel OF]

On 01/06/2016 at 1:05 AM, arkie said:

do they have to fit a prescribed architectural plan?

Not sure if that was aimed at me or Arftist but the gate is my design, not an architects. The 3rd hinge discussion came about because the client was at one point concerned about the weight on the brick pier. He's satisfied it's not a problem now & would be happy with 2 hinges, but I had drawn in 3 hinges for the sake of the gate & keeping the back stile from bowing in the middle.

The consensus seems to be that 2 is enough & 3 causes grief so I'll probably just run with 2. Cheers

[gote]

The problem area on a brick pier is pull out on he top hinge. A central third hinge will not help there at all because the minute movement needed to make the pintel loose its grip on the brick will not transfer to the middle pintel because of flex in the stile. You will need to make sure the bolts to the upper pintel are quite long. But surely you already know this.

By the way, there is a way to distribute the vertical force between pintels and that is to use properly calculated disk springs but it is unlikely to be worth the effort..

I hope that you will post a pic of the finished gate in its proper place. It looks good already.