REAR RECOVERY HOOK

[muzila]

Yeah, i asked an engineer about the bolt, he suggested Cap bolts @12.9, but i know what you saying. Also the only reason I got it hardened was .. well because the guy asked me if i wanted to.. lol... seemed a good idea at the time

I just read a CCVC spec sheet on tow hooks. It is dated 29/06/05 but they state NO CAP BOLTS. Think u might want to look into that as you have cap bolts in your photo.

Is there anywhere we can put things on the board like these pdf files i have on CCVC standards??

[Demonic]

Okay, so i go back to my bolt rep and get the same grade bolt in a hex head form. Remember there are 4 bolts, not just one or 2 holding that plate on, the chances of it breaking all 4 I thought would be slim to non.
Also the bracket is hardened, but the washers I made are not, so there is some give in it.

[muzila]

Yea I found that a bit strange when I read it.
I would of thought cap bolts would be fine. But There must be a reason for that.
I would like to know why that is.

[geeves]

This is an interesting discussion. It is true that the bolts should hold everything together and the clamping force does the work but the reality is that we are useing forces towards the design breaking load rather than a safe working limit. Also we frequently pull sideways and up or down. An up pull effectively reduces the clamping force so the bolts do the work A side pull puts all the load on one bolt.
The 2 numbers have different meanings The first is the tensile strength which is the force it can take. The second is its elasticity ie ability to bend. The perfect bolt on this basis for us is a 12.4 but as far as I know it doesnt exist. I have bent the 2 12mm 8.8s in one of my hooks. The response was the replacement of all the bolts in all 4 of my hooks and retorqued to the recommended 52 foot pound. This torque is important to get the clamping force.
To my knowledge virtually all clubs specify grade 8.8 or imperial grade 5 for tow hook bolts with the exception of Nelson club which specifies 12.8.

[Demonic]

To my knowledge virtually all clubs specify grade 8.8 or imperial grade 5 for tow hook bolts with the exception of Nelson club which specifies 12.8.

I will be using the standard 8.8 bolt that comes with the hook itself, what about the bolts to mount the bracket to the chassis?.
I thought that the better the bracket was held on the better, and the breaking point would be either 1.) Snatch hook bolts, 2.) Snatch hook itself, 3.) Snatching rope or cable.
Am I looking at this the wrong way?

[daisy]

Demonic id let to snatch me out of a hole with that set up
looks similar to mine (and Holdens) ive got 8.8 bolts holding that bracket on and like you said theres 4 of them so they aint going any where especially those 12.9 but ive used 10.9 bolts for my hook.
yes your thinking right you want that snatch strap to break first or the hook to staighten

[Demonic]

Cheers Daisy, I must give you the credit tho, as it was modeled off your bracket and what you had mentioned about using crush-tubes(i wouldnt have thought about them) and backing plates.

Btw, Did you just say tug-of-war?

[mulover]

Geeves, you say the bolts bent, I feel that if these bolts where a higher tensile strength they might have broke. I would much rather have a stretched/bent bolt and my hook still attached than the hook go flying because the bolt was too hard and broke.

As you also said, its because of all the different forces we put in all the different directions that you need a bolt with a bit of "give" in them.

Demonic,
If you are using grade 8.8 for the hook, then use either the eqivalent or the next grade up (10.4?) to hold the mount onto the chassis.
You are on the right track. You absolutely don't want a hook to break, thats why the right hooks bend rather than break. If anything is to break, you want it to be the lightest/softest item, as it will have the least amount of energy and hence can't do as much damage, ie snatch strap, winch rope with a dampener. You should always try to have the least amount if items in a recovery as possible - ie have hooks so you can hook the snatch strap straight onto the vehicles, if possible hook a winch hook straight onto the vechile if the hooks are big enough.

[jezza]

To my knowledge virtually all clubs specify grade 8.8 or imperial grade 5 for tow hook bolts with the exception of Nelson club which specifies 12.8.

I will be using the standard 8.8 bolt that comes with the hook itself, what about the bolts to mount the bracket to the chassis?.
I thought that the better the bracket was held on the better, and the breaking point would be either 1.) Snatch hook bolts, 2.) Snatch hook itself, 3.) Snatching rope or cable.
Am I looking at this the wrong way?

The tow hook is designed to straighten before any of that happens.

Yeah, i asked an engineer about the bolt, he suggested Cap bolts @12.9, but i know what you saying. Also the only reason I got it hardened was .. well because the guy asked me if i wanted to.. lol... seemed a good idea at the time

I just read a CCVC spec sheet on tow hooks. It is dated 29/06/05 but they state NO CAP BOLTS. Think u might want to look into that as you have cap bolts in your photo.

Is there anywhere we can put things on the board like these pdf files i have on CCVC standards??

I wouldn't worry about the cap bolts as there is nothing wrong with them, I hear they don't like them because dirt can build up in them and cause them to rust then break.

[geeves]

The reason they dont like cap bolts is that there is an identical mild steel bolt around that you cant tell from the real thing.
Unfortunately we cant link to or pirate the ccvc document Its jealously guarded and is meant to be in the members section of there web site. They have sold it to other clubs. It is modeled on the page thats linked to in my faq on tow hooks although theres a little more engineering data on the ccvc page.

[Yarno]

Just to stir it up more. Yes lower graded bolts have a greater sheer strength than high grade bolts but the bolts are there to provide clamp load, all the shear forces are taken by friction where the two parts meet. So there isn't a large amount of shear load, if they are torqued up correctly.

Higher tensile bolts have more sheer strength then.

15. How is shear strength of fasteners determined? Why don't industry fastener standards include such values?

Common practice for steel fasteners is to assume shear strength will approximate 60% of minimum tensile strength. Published data in commercial (non-aerospace fields) does not offer much guidance on shear strengths for bolts, screws, or studs. The first reason is that the number of components loaded in shear is considerably less than for tension, compression, bending, or torsion. The primary reason, however, is the difficulty in obtaining accurate test data. Shear testing inherently involves a number of variables. Therefore, tests are less reproducible than testing for such properties as tensile or yield strength. Most shear testing has been by arbitrary procedures that provide empirical results. The greatest need for shear test data is in structures that are riveted, pinned, or bolted, and also where service stresses are actually in shear. Notable examples are found in the aerospace industry. (A recommended shear test method is given in ASTM B565.)

from http://www.industrial-fasteners.org/faqs.html

And if the bolt doesn't have any strength on it I would bother using it at all.

[Yarno]

Here is some more good info
http://www.rockcrawler.com/techreports/fasteners/index.asp

[geeves]

This discussion on bolts is fascinating and will make brilliant reading in the future but should it be moved to either the bar or general 4x4 discussion and leave this thread just for the hook.

[mulover]

Probably not a bad idea geeves, this could go on for ever, but i think it comes down to personal preference. I don't think it's wise to use anything over grade 10.4(not sure on the exact figure), but other people have their reasons why. As every situation is different, no one could really say "if that was a _ grade bolt, that wouldn't have happened.
I for one will only ever use grade 8.8 bolts when i'm bolting on hooks or anything that gets subjected to shock loadings.

[Demonic]

This discussion on bolts is fascinating and will make brilliant reading in the future but should it be moved to either the bar or general 4x4 discussion and leave this thread just for the hook.

Hard to say, The bolt strength is very related to recovery hooks/mounts, but it would also be useful for other project, maybe the bolt section of this thread should be in a new thread.

[Yarno]

I spoke to my engineer from work yesterday and he said that the higher rated bolts like the 12.9 will out perform the lower grade bolts in every way, they are an alloy steel unlike the lower 8 grade.

[Demonic]

I read that today while looking for torque specs. Made from "Heat treated steel"...

[mulover]

Grade 12.9 will out perform the lower grade bolts in almost every way, Have you found any stats on shock loading anywhere?? A higher grade bolt won't give you any warning when its going to break, at least a lower tensile will deform a little, so you can notice if your hook has been over loaded. Also shear strength of a higher grade bolt is more than a lower grade bolt, but you try to cut a higher grade tensile bolt with a cold chisel compaired to a lower grade bolt, I think the higher grade one will break off after fewer blows than the lower grade one.

[jezza]

A higher grade bolt won't give you any warning when its going to break, at least a lower tensile will deform a little, so you can notice if your hook has been over loaded. Also shear strength of a higher grade bolt is more than a lower grade bolt, but you try to cut a higher grade tensile bolt with a cold chisel compaired to a lower grade bolt, I think the higher grade one will break off after fewer blows than the lower grade one.

The hook is designed to straighten when it's been overloaded, so if it's bending bolts they must not be tight enough. Has anyone ever seen a hook come off because a bolt was to brittle?

[mulover]

Could have worded that better, the bolts will deform if they have been overloaded, I have only ever seen one hook come off, it was off the front of a mu, i was pulling it to, doing a demo at a club safety day!!!, that was because the guy drilled out the holes where the factory one bolts on and tapped them M12. I have never had my bolts stretch either, but every time I take my bar off I replace the bolts anyways.

The bolts will still bend even if they are tight enough, if a bolt is over torqued it makes it weaker anyways. Bolts that are put under loads like these should always be torqued to the correct amount.

I have never seen a hook straighten out either though!!