Hayman Reece Towbars

Good question for HR, ask them what the weight loading on the pin is?? Also ask about the legality of drilling another hole and fitting bolts, should be interesting answers......Ask for them in writing

One hole is enough! The hole needs to be at least 50 mm from the end of the reciever hitch and mild steel is ok for the pin. High tensile, if used, should not be of to high a grade as the higher the grade the more brittle they are and can not take a large shock load. Remember that the pin is in shear loading at two points so the load is shared (like having two tow hooks and a bridle strap). I would not use a pin on its own, use a reciever with a forged steel tow hook (NB FORGED AND NOT CAST!!!). And if you are wondering about the strength of tow hitches, how about pintle hooks? I am a member of the safety committee of a 4wd club and we have some reservations about using them for a snatch recovery. We will be testing a pintle hook to destruction soon by hooking it up in a cage at a commercial test lab and loading it up till it breaks (both straight and angle pulls). We have already done several original equipment hooks on several 4wd's and some failed by breaking with chunks of metal flying through the air! Our club now demands ONLY forged and not cast tow hooks.

A carpenter mate of mine once told me that the shear strength of a TRUE 6 inch nail was around 3 tonnes. A TRUE six inch nail is about 5 mm thick. A hitch tow pin is about 12 mm.

You work it out.

Bilbo

hi axle

know af a fellow who done some tests on the pin it took 12 tonnes of force for the pin to plastic. ( ithink thats the term he used). which means it did not retun to its normal straight state.

the test was done on arb and hayman pins.

cheers gra

Hi Axle, When I said I would prefer not to use just the pin I was meaning that I use a square section tube with a tow hook mounted on it instead of a tow ball. It is common practice in Australia to just feed a loop end of a tow / snatch strap up into the reciever and slide the hitch pin through the hole,strap,hole to anchor the strap. I am not saying that this is not a good idea just that I prefer using a tow hook.

Hi Kiwi,

You are spot on with your advice. The thing to remeber with high tensile bolts is they are designed for tensile loads. This does not make them suitable for all types of loadngs, and as you say their brittleness makes them unsuitable for shear loads.

Ken

Gents,

Let me pull out my engineering degree for a minute and add some information.

Stress = force/area so Force = stress x area, which Bonz has used.

Using Bonz's shear stress limit of 55MPa, the equation for force is:

55 * pi * 12 * 12/4 = 6220 Newtons. As the stress is applied in two places you can double this to 12440 Newtons.

Divide by 9.81 to get kilograms. So 12440 1268kg or 1.268 tonnes. Sorry Bonz you are out by a factor of 1000.

In any case the shear strength of mild steel is about 144MPa so a 12mm pin should be able to take about 3.3 tonnes of load. Keep in mind that the pin holding your towbar pin is is probably somewhat stronger than mild steel so the actual load rating is probably higher. For a 16mm pin this load capacity becomes 5.9 tonnes.

Also need to remember that the tow rating of 3500 kg is not the actual load that is placed on the towbar, it is the weight of the trailer that can be towed.

Wille, If you are watching this thread then here is some of what we found regarding original equipment tow hooks;

Big Horn - LOOK OUT they broke!

Nissan Saffari - not bolted, the side welds crack and the hooks tear off!

Surf - off set hook allows the strain to shear the rear bolt leaving the hook attached by only one bolt!

I have not seen any original equipment recovery hooks on 100 series vehicles but I have seen the factory tie-down points. I think that they are prob ok for towing but I would certainly not do any repetitive serious snatching using them. Consider double plating and using spreaders in the chassis rails and fitting an aftermarket forged recovery hook, use grade 8 bolts only.