Thread: Rear recovery hook

Shear force against tension.
Take a structural bolt of say M16 size.
The shear force is always higher than the tension the bolt can take.
In structural steel(buildings) most connections are designed to have the bolts in shear.
See the thread about Scorpion Racing swivel recovery points.
In the end its all about shear.

Originally Posted by Bytemrk

Sprint... one hiccup in your theory... SWL as in 4.5Tonne SWL... means Safe Working Load...

That's not the load it will break at... it's load that is considered safe , taking into account the required safety factor.

Others here will know more accurately than me.. but the difference between SWL and MBL (minimum breaking load) if usually a factor of about 5. So your 4.5 tonne SWL shackle will probably break at around the 22 tonne mark.

The ratings on snatch straps are not swl.. Thats why your snatch will break before the shackle and it's crucial if you have to use a shackle.. that it is connected to something strong and secure enough...


Mark

which is why i put (breaking strain of?)

i couldnt remember the formula for calculating the SWL from the breaking strain for a shackle

So lets take the snatch from the rear tow bar to the front of the vehicle being recovered.

What are the size of the bolts attaching the front recovery points to the vehicle?

From memory they are a smaller diameter than the tow pin, yet we do not hear about lots of those failing.

Originally Posted by black betty

So lets take the snatch from the rear tow bar to the front of the vehicle being recovered.

What are the size of the bolts attaching the front recovery points to the vehicle?

From memory they are a smaller diameter than the tow pin, yet we do not hear about lots of those failing.

Completely different loading.
Bolts are in shear, so is the receiver pin if used with the right attachments.
2 x 12mm bolts on your usual chinese two hook give 226mm^2 of section area.
1 x 16mm pint gives 201mm^2 of section area, with the right attachment it's in double shear giving 402mm^2 of section area.

If you load both in shear the pin is the winner by a long shot. If you put the strap directly on the pin, the receiver pin is the weakest link by a very big margin.

Originally Posted by BigJon

Good thing you are in another country then.
Does some "knowledge" imparted by an "expert" over the net change the forces in a recovery? How is the recovery worse?

As I mentioned I have never seen a bent pin, much less bent one myself. Now all of a sudden the pin is goint to fracture?? Show me the pics of all these bent and fractured pins you have seen and maybe I will change my tune.

When I can come up with a better (cost effective) solution I will use it.
Incidentally in the last four years I have done about 10 or less snatch recoveries. All have been "gentle" (walking pace or less), because as I mentioned if you are pulling hard enough to do damage then you are using the wrong recovery technique.

People always react badly when told what they're doing is unsafe or wrong, typical reaction is to attack the credibility of the people who show them the problem.
Also typical is banding together with people who do the same thing to pick up some support for their actions.

I suggest you guys find another engineer and get them to run the numbers on a receiver pin loaded the way you are using it. It may cost you to get those results but they will reinforce what I am saying.
Recovering using a strap directly on the receiver pin is a bad and unsafe practise. No matter how safe you think you are.

Know your method is unsafe and trying to use it gently is not an excuse?

Originally Posted by Dougal

Completely different loading.
Bolts are in shear, so is the receiver pin if used with the right attachments.
2 x 12mm bolts on your usual chinese two hook give 226mm^2 of section area.
1 x 16mm pint gives 201mm^2 of section area, with the right attachment it's in double shear giving 402mm^2 of section area.

If you load both in shear the pin is the winner by a long shot. If you put the strap directly on the pin, the receiver pin is the weakest link by a very big margin.



People always react badly when told what they're doing is unsafe or wrong, typical reaction is to attack the credibility of the people who show them the problem.
Also typical is banding together with people who do the same thing to pick up some support for their actions.

I suggest you guys find another engineer and get them to run the numbers on a receiver pin loaded the way you are using it. It may cost you to get those results but they will reinforce what I am saying.
Recovering using a strap directly on the receiver pin is a bad and unsafe practise. No matter how safe you think you are.

Know your method is unsafe and trying to use it gently is not an excuse?

mate i don't think anyone is doubting your credibility ...( if anything I'm very interested in what you saying ) ...i think most people that think there doing the wrong thing are only victims of there own environment, as in there told buy someone they trust that what they are doing is kosher and thus need to play devils advocate to justify there mistakes they have been making ...and in playing the devils advocate most people , including myself would be hard pressed to accepting that building something the right way would be anymore less affective than the thing there buying at ten times the price

Caloundra 4x4 Accessories - Ironman Suspension

Google Image Result for http://www.4b.com.au/BushHook.gif

Landrover Tuning :: Accessories - Default Store View


the one's have pictured is one on the market as we speak and it not the only on ether so as you can see this is where i think the devils advocate come into play...if you get my meaning

cheers

Originally Posted by cucinadio

mate i don't think anyone is doubting your credibility ...( if anything I'm very interested in what you saying ) ...i think most people that think there doing the wrong thing are only victims of there own environment, as in there told buy someone they trust that what they are doing is kosher and thus need to play devils advocate to justify there mistakes they have been making ...and in playing the devils advocate most people , including myself would be hard pressed to accepting that building something the right way would be anymore less affective than the thing there buying at ten times the price

Caloundra 4x4 Accessories - Ironman Suspension

Google Image Result for http://www.4b.com.au/BushHook.gif

Landrover Tuning :: Accessories - Default Store View


the one's have pictured is one on the market as we speak and it not the only on ether so as you can see this is where i think the devils advocate come into play...if you get my meaning

cheers

There's nothing wrong with any of the attachments that have been shown in this thread. But the type which use a shackle are a much better design IMO than the type which use a hook.
The only question then becomes: "Is your towbar and it's mounts strong enough?"

Some towbars are strong enough, some certainly aren't. I'm certainly interested in analysing your HR towbar too see what it should take. A large chunk of that interest is because I've been planning to build my own square receiver towbar for years now. Just haven't got there yet.

Originally Posted by Dougal

I'm certainly interested in analysing your HR towbar too see what it should take. A large chunk of that interest is because I've been planning to build my own square receiver towbar for years now. Just haven't got there yet.

hope this helps mate







cheers

Originally Posted by Dougal

Completely different loading.
Bolts are in shear, so is the receiver pin if used with the right attachments.
2 x 12mm bolts on your usual chinese two hook give 226mm^2 of section area.
1 x 16mm pint gives 201mm^2 of section area, with the right attachment it's in double shear giving 402mm^2 of section area.

If you load both in shear the pin is the winner by a long shot. If you put the strap directly on the pin, the receiver pin is the weakest link by a very big margin.



People always react badly when told what they're doing is unsafe or wrong, typical reaction is to attack the credibility of the people who show them the problem.
Also typical is banding together with people who do the same thing to pick up some support for their actions.

I suggest you guys find another engineer and get them to run the numbers on a receiver pin loaded the way you are using it. It may cost you to get those results but they will reinforce what I am saying.
Recovering using a strap directly on the receiver pin is a bad and unsafe practise. No matter how safe you think you are.

Know your method is unsafe and trying to use it gently is not an excuse?

Righto. I faithfully promise to never effect another snatch recovery ever again unless I use a pin loaded in shear rather than bending.

How does that grab you?