Thread: Rear recovery hook

Originally Posted by spudboy

Interesting that a 3mm increase in size gives almost twice the strength.

Should I be remembering something about Pi x R squared from high school maths as an explanation for this.....?

It's basically adding up the area of every bit of metal and the distance it is from the centre.
The result is called the second moment of area (SMOA) and usually is called the "I" value. Mathematically it's how stiff a section is in bending.

For round sections, I = pi*d^4/64 and it's the same in every direction.
For rectangles it's I = bd^3/12 and they have a smaller I value in the weaker direction.

In bending, the material is stressed more the further it is from the centreline (neutral axis). So while bigger parts are stronger, they have to be a lot stronger because they're more rigid. The strength is the I value divided by the distance from the neutral axis. So the strength is the depth ^3 while the stiffness is depth ^4.

This stuff does make sense if you can get someone to explain it well. I have a farming (orcharding) background too and I'll be back into it one day.

If you make a rod twice the diameter (or flat twice the thickness), it's (2^4 =) 16 times stiffer but only (2^3 =) 8 times as strong.
Which is why your average leaf spring is made up of several thinner leaves instead of one really thick one (yes I know there are monoleafs out there).

Bending and breaking mars bars shows this quite well.

thanks for hanging in there Dougal ...see what l mean about your invaluable info ...and the fact that people are interested and the importance IMHO of playing the devils advocate

cheers mate

Originally Posted by AndyRevill

I wasn't talking about anything specifically and certainly not a cliff face, merely suggesting that it seemed more likely you could control the alignment of the two vehicles horizontally but that was harder to do vertically - say on a beach where someone has tried to get off, gets bogged and someone on hard sand pulls them out, there could be a reasonable height difference - but when does this become significant? or does this only become an issue at greater angles? just interested to know if I should re-drill mine. It seems a lot of these are sold with the shackle pin horizontal?

Andy

Usually most winching (4WD recovery) operations are done with both vehicles on the same plane so any variation (height) over the length of a snatch strap would have to be very steep to cause a problem with alignment of the shackle (with the pin in the vertical axis).
Easy rule to follow, if when under load the shackle pin is at an angle to the hole in the recovery point (jammed to one side or the other) and the eye of the snatch strap is to one side (off centre) of the "D" or "BOW" shackle body, then you are using the shackle for a purpose it was not designed for. In most cases (that I have seen) the shackle pin tends to strip the thread, either in the shackle bolt or the pin itself when the body of the shackle is pulled sideways against the thread.
I was going to buy one of these tow hithces (that slide into tow bar) but it was a hollow tube with the recovery eye welded on, I prefer the solid billet one's, you can buy them with the recovery hole either vertical or horizontal. I asked the bloke selling them why they don't just sell them with the vertical pin hole and he said, " most people like the look of their shackle hanging down for some reason" fair enough.
If you use a rated shackle, (BTW a "rated shackle should have a pin diameter larger than the diameter of the shackle body and should be stamped (cast) with the SWL or WLL which has a Safety Factor (SF) of 5 or 6 (depending on use) built in", the formula for SWL or WLL is SWL X SF = Guaranteed Breaking Strain (GBS) ), of 4.5T and use it correctly there is NO WAY the shackle will break when using an 8T (GBS or MBS) snatch strap. The weak point remains the actual Recovery point. In my case I have a tow bar that is attached by 3 Hi-Tensile bolts, each side of the chassis with a spreader bar supporting the chassis and 2 extra plates with 2 more H/T bolts in each one, I am confident that this setup will not fail under any circumstances. I think more effort should be directed to having unbreakable, safe recovery points, as they say, "a chain is no stronger than it's weakest link", Regards Frank.
P.S. I wouldn't drill an extra hole in either a solid or tube bar, for an extra $50 you can buy another, with the desired pin hole orientation, cheap insurance.

Originally Posted by Tank

P.S. I wouldn't drill an extra hole in either a solid or tube bar, for an extra $50 you can buy another, with the desired pin hole orientation, cheap insurance.

Hey Frank - I can understand why you wouldn't want to drill a second hole in the same plane as the original, but if the second hole was in a different plane ie further forward, would there be a strength reduction over the original?
Not sure whether its possible to do anyway as there may not be enough room in the receiver to push the extra length through.
Those solid inserts are pretty heavy - I'd prefer to drill another hole if practical and safe rather than carry two inserts.

Steve

Originally Posted by steveG

Hey Frank - I can understand why you wouldn't want to drill a second hole in the same plane as the original, but if the second hole was in a different plane ie further forward, would there be a strength reduction over the original?
Not sure whether its possible to do anyway as there may not be enough room in the receiver to push the extra length through.
Those solid inserts are pretty heavy - I'd prefer to drill another hole if practical and safe rather than carry two inserts.

Steve

Since we all like pictures soo much, here's another:



As you can see, the second hole is not a problem for strength, the stress levels around that are around 90MPa, RHS starts to permanently deform around 350 MPa. However, if you're making a recovery receiver, you'll want to use thicker wall than the 3mm shown in the picture. Solid is un-necessary though.

That's a neat analysis package you've got there Dougal. What is it?

Thanks
David

As stated before what do you guys think of using a pintle hook for snatch recoveries.

Originally Posted by spudboy

That's a neat analysis package you've got there Dougal. What is it?

Thanks
David

It's part of Solidworks which is the 3D CAD package I use. They used to call it Cosmosworks (which sounded impressive), but they recently changed it to the lame sounding "solidworks simulation".

It's only a mid-level package that's not that impressive for analysis, but it's great for cranking out impressive looking pictures. My clients love impressive looking pictures