Optimal angle for a gate support - simple engineering

[Bill Edwards]

I'm fabricating a cow gate out of 40mm steel box section. It's going to be hung from a shed with a piece of box section hinged much higher up to take some of the weight, as the gate is just over 27' (8.2m) long it'll just bend under its own weight.

The shed is a good height so the support can be as high as is needed. My question is how high should it be (at what height from the top of the gate) should point A be?
Equal to the distance it goes to along the gate?



It's a simple thing, but I'm not the type who works out stuff such as this!

 

[Sidewinder]

I would not make the gate like that, almost but not quite!
Put a diagonal from the top unhung corner to the bottom hung corner to cope with some of the overhung loading.
Then, to be honest the other calculation is a bit like the traditional salesman's problem, being the shortest + quickest + cheapest distance for him to travel on his daily rounds.
Personally I would minimise the bending moment in the vertical & put the point A as high as physically possible, that way the load on the column is nearer the vertical, that is the way it is designed to be load bearing, plus the gate gets a nearer vertical suspension.
That is if you can get it 25m+ off the ground then do it.
It's too late to do the maths now, and I've had a few beers, so that would not be a good plan perhaps!

HTH

 

[Bill Edwards]

It's copying some other gates in use that are the same style, just a lot shorter. They've stood the test of a lot of time. I've made one that's just a couple of feet shorter with a brace that runs underneath as one on top wasn't practical and I think it'll work fine as most of the time the gate is supported at the other end by the catch.
However this gate has a little extra weight and certainly will need slightly better support, which I'm happy will be enough - I'm just not sure what the optimal place would be.
Sadly the ground isn't level or it'd just be running on a wheel!
The issue I see is mainly the gate bending where the hinges are - I've see others that weren't strong enough and the box section where the hinges are turns into an S shape.
I've made my end piece with thicker wall box and it's reinforced with flat bar, but something to take a decent chunk of the weight further on is also called for. Even if the gate didn't bend, the hinges would soon wear anyway!

When I say the shed is a decent height, it won't be 25m or anything like it!
The gate itself is all welded up now so changing that is a little late, I've just got to make some holes to hang it with and do this top support.

 

[wyn]

Could you use cable with a turnbuckle for the diagonal support to allow for adjustments.

 

[blenkie1]

Would it not be better with wire rope rather than the box? Any sag can be adjusted out with an adjuster? Are the gates sheeted? Its a long gate is there any reason you cannot use two?

 

[blenkie1]

Beat me too it Wyn!

 

[Bill Edwards]

My thinking was cable, however box section was easier for the boss to get hold of, so box it is - unless there is a very good reason not to. I'll tadge something up to allow it to be adjusted to take it's fair share of the weight.

One gate is how it's being done as there is a large gap to fill for when cows are being sent through the yard. This gate swings to join onto others (which are already in place) depending on where you want them to go. There isn't any way of doing it with smaller ones, hence this monster.

 

[blenkie1]

All the gates I've made are nearly all 50x50x3 and plated where the hinges are. Gate that size I take it you are using 25mm hinges

 

[blenkie1]

Is the shed keeping cows in?

 

[Bill Edwards]

It's not actually swinging to block off the shed, just from the back of the shed to send them in a certain direction. It's not going to be subject to a herd of 800kg cows leaning against it for food (thankfully!).
The hinges are pretty thick agricultural ones - a size up from the 'normal' farm gate size which I doubt would be strong enough. Probably something like 25mm.

40mm box section was chosen as it is lighter, and the farm has several other (shorter) gates made from the stuff and it seems quite decent. The concern was that 50mm would obviously be stronger but also adding much more weight.
The gate is being left open, no cladding.

 

[blenkie1]

I make a lot of gates for farmers and they seem to like them made from 50mm box, sick of them IAE gates bending/falling apart when a cow looks at them. Farmers call them elephant gates! If it was me it would be a case of if it looks right it should do, but I would rather have a cable and be able to adjust it easily.

 

[Sidewinder]

Add a diagonal to the frame before you hang it.
Fully welded it will transfer a lot of the mass load, & bending moment.

 

[dan.taylor.1]

Can you not make a gate thats movable? have seen two 15" normal yard gates bolted through the hinge holes with an axle at the centre and a couple of barrow wheels on so it can be put where ever its needed.

Dan

 

[dannyp]

weld a bit of flat in diagonaly corner to corner or over your span id be tempted to put a pair of X's to help hold it square

 

[Justme]

As you seem to be stuck with using box for the top strut I would add in a manual adjustable tractor toplink.

Its basically a H/D turn buckle. You can then easily adjust the support it provides.

 

[pedrobedro]

That's not how the gates are usually made, the support bar should be part of the gate not just loose on the top. Extend the hinge side and run the support from that and add an extra hinge if needed and a support wheel at the underside near the end of the gate if possible.

 

[gordon stephenson]

As per Wyn's suggestion for an adjustable cable arrangement, but have two cables.One to support the outer end and another in the centre,that will be a pretty long gate for 40mm box section.

 

[Bill Edwards]

Thanks for the ideas and suggestions. I've still got the original question though, no matter how I go about it - how high should the support ideally go?

 

[KemppiFrog]

OK, It can't be too high, so the limitation is the bit it is attached to. Based on your drawing, as close to 45 degree would be good. Bit less, not a problem. One of the shortcomings of 40 box is that it is MUCH less torsionally stable than 50. So your gate will wobble. Def put some diagonal in the gate. If the hinges were strong enough and right on the top and bottom rails (avoiding the 'S' stress) and used bearings (like needle rollers) You would not need the top support. But it is easier/cheaper and twice the height halves the load on each hinge. Personally I wouls use 16mm round bar with some form of adjuster, say a stirrup on the top with some thread on the end of the bar and a nut. The top hinge on the gate will only be there to stop it twisting, as the load will be shared between the bottom (in compression) and the brace in tension. Think of that when making the bits.
KF

 

[brightspark]

http://www.chegg.com/homework-help/...wooden-frame-supported-wire-assume-p-q1501588