Estimate of Strain on This Bolt Shown Below

How much drag does it have? How big is the area?

Bolt looks too small from here. What grade?

The answer is going to depend on the position of the shackle relative to the black plates. If it would fit between the two of them, that would be optimum, else, could you use short pieces of pipe to hold the shackle eyes tight against the plates. I'll run some numbers in the morning if I remember to come back to it.

The position of the shackle in the picture, eyes midway between plates, is the worst for bending of the bolt.

from personal experience with similar emergency setups you should be able to get at least 1 - 2 tons of pulling force before the bolt bends far enough to slip through the plates.

Since it's a 3/8" shackle, why not drill a 3/8" hole through all four plates and use a 3/8" bolt? The whole rig-up would then be more than twice as strong.

I do not have a long enough bit that can do this. since its just the bottom plate that is off , I might just rout those holes.

I was just curious tonight to see what a 1/4" bolt would hold.

At the very least you should put a nut and washer on the bolt as it will certainly bend and get drawn up and through. A carriage bolt is one of the weakest types of bolt, it was invented for putting wooden carriages together, the square neck under the head pulls into the wood and keeps it from rotating as you tighten it. They only need to be stronger than wood, so they bend easily. You must have a very short drill bit, a standard jobbers length would easily get through if you drill from both sides. Never stand under a load.

There will be no more than 1500 lbs. of bollard pull on this shackle as measured years ago with a "S" load cell.

I can rout out the bottom plate holes as the Bottom plate is what is off.

5/16" common bolts from the hardware, is what we have used for years now.

I just wanted to know what 1/4" would hold if used in this position and under these circumstances.

You guys know I am always curious.

Thank you for any answers.

Drew, I thank you for the formula and the calculations.

I have 10 sets ( 2 per set) complete and 8 of the 10 are not properly aligned to the other three plates...or better yet, the Bottom plate is integrated to the Trawl door as ONE piece. Its the other 3 plates that are off by 1/8" to 3/16".

* It would be too time consuming to ship them back as they weigh 75lbs. each and I need some of them and the nets heading for Anchorage, AK very, very soon.

Given that 1/4" will work but is very marginal, and that we have been successful in the past with 5/16"...I think I will just ream out the Bottom holes and go back with 5/16".

I do not want to fight with all 8 sets ( 16 doors in all) with a hand drill. I can't get the doors in a vice properly and my left hand is very weak. Not to mention I would have to go into town and hunt down a longer bit .

Thank you for the calculations as the next size up DOES require larger bolts and now I have an idea of what bends or shears at what point.

Thank you for your time in this.

I think given the design, the best would be to fit a bracket that utilizes all of the contact points. It shouldn't be too hard to have (or make) something that fits into the black metal you showed. You want a tight fit so there is little wearing.

If you upgraded to a stainless steel bolt you would increase the strength significantly and eliminate the rusting problem too.

A 1/4 stainless carriage bolt would yeild at 3549 lbf and a 3/8 would yeild at 7985.3 lbf.

For calculation I used the yeild strength of stainless is 72,300 psi.

Drew K

Yes! A very good way of explaining what I was trying to say. As long as the pipe used is thick and strong enough to transfer the load to the edges better it turns it into a shear problem and increases your possible loading greatly.

Drew K

Try drilling them out with a 1/2 inch step drill (sometimes called a unibit) if you have one. Any good hardware store, Home depot, or Amazon will get you one. They require less torque to drill as they have no lead and only one flute. I have used them on 3/4 inch plate and they will drill through quite quickly, especially if you have a pilot hole as you do. Some of them have a hex shank and you can use a long drywall bit holder to extend the reach. A little cutting oil helps. I wouldn't put more than a couple hundred pounds on the setup you showed if it is a live load. A 5/16 bolt will have 50% more cross section. A 3/8 inch bolt will have double the cross section and will thread through your shackle. A grade 5 bolt will have twice the tinsel strength.

More like 1000 x the strength of tinsel....

Thank you all for the input. These things are getting reamed out and 5/16 SS bolts are going in.

Thank all. Question answered.

I still learn so much here....its amazing.

I have to agree with, "Netguy, If you use a short piece of pipe as a spacer to keep the eyes close to the black plates, then Drew's answer takes over, it is controlled by shear."

Good luck.

Looking at the assembly, the bolt is variously equally loaded (approximately) and cantilevered in the plates on the right but simply supported in the single plate under the bolt head on the left.

I merely note the point. It's always best not to put long bolts in bending, They're intended for tension and shear (although the friction between surfaces of a properly tightened bolt should relieve a shear load; shear is better carries through a fitted pin).

By the way, it's stress in which we're all interested. not strain. Strain is stretch or distortion.

If you were to cut two pieces of pipe that is 1/4" ID and 3/8" OD that will fit between the plates then the pipe will give added strength to the assembly. This can be done without drilling any of the holes out.

The shackle has to be able to be moved up or down , forward or backward to accommodate the desired effect of the Pelagic trawl.

Forward up is for bringing the net towards the surface and a light spread.... rear back is maximum dive to 1500' Plus depths and maximum opening.

My fear is that the small pipes will be lost by the techs that work with these biologists.

We have let the shackle have some 'play' in the past as it makes for corrections in the stretch and sway of the long bridles that these rigs have.

however, I fully understand the process and will consider pre-rigging the doors with multiple shackles and the pipe add ins, on the 3 most popular settings. These guys are NOT commercial fishermen. They are scientists and are looking for samples ( under 25 kg of catch effort). The less they tinker with the gear the better I like it!

D and E can be 90- 120' long. The tilt " Forward or Backward" and the sway "Out or In" gives the trawl door its direction ....down hard, down easy, surface hard, medium lay, hard spread, light spread, medium spread, etc.

THIS is the reason I let it have some slack.... so far we have never sheared a 5/16" bolt but have bent one or two when the user allowed the net to be deployed in the wrong areas and smashed the entire rig into a rising sea mount with 4,000 feet of tow wire out, doing about 4 knots....the door damage was insignificant to what happened when that Dyneema netting in the Aluette trawl grabbed a hold of a bunch of rock and mud! * Pelagic means in the middle of the water....NOT on the BOTTOM.

Thanks for the explanation...it is always nice to learn more about the projects : )

Drew K

2000 lb