Fork Extension reinforcement, engineering question

[AlanB]

I built some 8' long fork extensions out of 8" channel iron last weekend.

I need them to move some buildings and roofing and such that is oversize.

I unloaded the roofing yesterday and slightly bent one of the extensions downward.

I was debating if it would be stronger to add vertical rib (s) under the extension, or to plate the entire bottom of the channel and turn it into a tube?

Anyone with some engineering knowledge care to try and explain it or show me a table or some math to figure it out?

TIA

Pictures to follow

 

[mwb]

Flat plate on the bottom will help a lot. I am guessing that the extensions bent right at the end of you existing forks; if you add plate 24" long centered at the tips of your real forks that would probably solve the problem. 3/16 - 1/4 flat stock should be plenty heavy (I don't know what your FEL specs are though).

 

[easygo]

Another way to add strength is to weld a short 10-14" vertical piece of channel or other strong stuff onto the extension right over the area where the original fork ends. Then weld a long piece of flat stock on top and bed it down and weld it to the extension as well. Weld one side near the tractor end and the other about 3-4' from the working end. This will only work if you don't need the entire length of the extension, but it will make it a lot stronger. Just like some boom poles.

 

[Danno1]

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Boxing it in will be stronger than adding a vert rib in the middle of the channel. Went through that situation awhile back in work.

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[crashz]

I'm trying to picture these: The C channel is laid on top of the forks, with some metal bands welded to the flanges at the fork ends to make a box for the forks to slide into. The flat side of the channel is facing up to contact the load and the flanges are pointing down towards the ground. Correct?

When a load is placed at the tip of the extensions, the flat top (the web of the C channel) is placed into tension. The flanges are placed into compression, with the greatest compressive forces at the very edge of the flange (the channels weakest points). The flanges are also unstable and able to flex left or right under load. The flexing will cause the extension to bend and buckle.

C channel is designed to resist loading best when the web is vertical and the flanges are aligned directly above each other. By flipping this arrangment 90 degrees the design of channel is severly weaken. Think of a 2" x 8". It can only resist a slight load, when laid on it side like a plank, with out bending. But laid on its 2" end, like a floor joist, it resists a much greater load.

By plating the bottom of the extension, you are essentially boxing in the entire piece and will transfer most of the compressive load to the plating material. The plating will also stabilize the flanges and help prevent buckling. The downside to this is that the welds will be the weak link. Flexing of the extension under load could cause the welds to break as they are transfering the compressive loads through internal shear.

 

[EddieWalker]

Alan,

Are these for your 555?

I built some forks for mine out of 2x4 tubing that I bolted to the lip of my bucket. They were to help moving brush and trees. My dad put dozens and dozens of hours on them, but they just didn't hold up. The tubing just wasn't strong enough for the force of the hydraulics, and the metal twisted and broke. It actually broke in place like glass!!!! or at least that's the best I can describe the way it looked to me.

I welded on more steel and kept adding gussets, but either the metal was too stressed, or it just wasn't strong enough for the abuse my dad put it through.

It's a tangles mess right now and a total loss. I was going to build another ,but this time with the metal on edge so the linger side is the sides and the top and bottoms are the smaller sides.

You can try to make them stronger, but my guess is that it's a write off and you should probably start all over again.

Good luck,
Eddie

 

[AlanB]

Crash, yes, what you described is accurate, sorry I did not get a picture last night, it was raining and just not a good day.

Eddie, I was thinking of yours, when I built mine, and was trying to remember the discussion about which way to make the angle. I did mine like you did in the picture after thinking it through both ways. As I guessed I would, I guessed backwards

I think you could easily destroy these things with the Hoe or with Kabukie Bob, especially doing tree's etc.

The idea on these are to move very bulky but relatively light items around. This roofing I did, or the next one is I have to move some metal sheds.

Pics to follow Monday.

 

[AlanB]

So here are the pictures,

Will probably try and just plate the bottom area where the forks end (the bent place) and see how that works out.

I also need to work on my "hold on" chains. They are dropping too far down and allowing the fork to slide out when backing out under load.

 

[crashz]

Thanks for the pics Alan.

See post below.

 

[crashz]

Well I can't seem to edit my previous post.

A second look revealed a slight bend in the right side fork extension. Did you place the right fork on a tooth and the left on the cutting edge for visual perspective?

As stated above, boxing the sections will help with rigidity. I'd box in 6 ft from the forks outward, as the last 2 feet at the tip will see very little bending moment. Also keep in mind that the extension sections will not be as strong as structural box sections. The welds will be the weak point, not only because of the welding process and the uncertainty of full penetration, but also the welds will be transferring a large amount of the internal shear forces when loaded. Inspect those welds every time you use the fork extensions.

When using the extensions, you may want to slide a palet all the way onto the forks. this will help steady the load. As they are, there isn't much keeping sideways movement in check. Because of the instability of a load at the end of the extensions, keep people far away.

Good luck!