Safe to use recovery strap on hitch pin instead of shackle?

[ntsqd]

I haven't had one out-right fail, but I have had to use a hammer and a drift pin to remove one that tried really hard to fail. Once out it was obvious that it had partly shear-deformed at both holes.

Some time ago the bending in the pin or bolt incurred by the strap at the middle of the receiver came up (as I'm sure that it will again) and I did a simple Shear-Moment diagram on the system. The results are probably posted around here somewhere, I don't recall the specific results but I do recall that the bending load could be significant. I also recall that the material properties of the hitch pins was not info that I could find at the time, so I had no idea how they might compare to a bolt.

SAE graded bolts (& a lot of others) are really intended to be loaded in tension only. They frequently are loaded in shear and abused in all sorts of manners, but they are properly referred to as "Tension Bolts." That is why their shear strength is not published and has to be calculated.
Simply running a nut down hand tight on the side of the receiver tube will convert some of the bending load into tension loading since the bolt is no longer free to slide through the hole as it bends.

I built my own shackle slider and it is part of my normal recovery gear bag. Were I to consider abandoning it for just using the receiver & pin I would make a sleeve for the pin to slide through and that the strap loop fit over. That would act to spread out the bending load on the pin.

 

[michaelgroves]

I don't visualize it that way. To me the crossbar seems rigid enough that the pulling force is straight back on the unibody, or whatever direction you're pulling

If the crossbar is rigid enough, then my point doesn't apply. But bear in mind that the bar is designed for towing - snatch straps impart forces that are way in excess of towing limits. If the bar does bend and shorten even slightly, it starts to pull the frame inwards very strongly (the same way that a very, very short harness would).

 

[DaveInDenver]

That is why their shear strength is not published and has to be calculated.

Rule of thumb is 60% of tensile, although the relationship isn't perfectly linear. A490 ASTM bolts are rated at 75 ksi for shear but are 150 ksi tensile rated fasteners. They are only allowed 40 ksi because of the factor of safety in structures, though.

 

[winkosmosis]

The hitch pin is 5/8" in diameter and so has an area of about 0.30 sq-inches. Grade 5 fasteners under 1" have a minimum proof of 85 ksi and grade 8 120 ksi. So a 5/8" grade 5 rod (or bolt with minor diameter of 0.625", so make sure to use a shouldered bolt long enough that the unthreaded part spans both receiver holes) will minimally tolerate about 25,500 lbf and grade 8 about 36,000 lbf without deformation.

Their yields are 92 ksi and 130 ksi, so a hitch pin would be 27,600 lbf and 39,000 lbf before leaving their elastic region. Where the difference in grade 5 and 8 comes in is that grade 5 has min tensile yield of 120 ksi and grade 8 150 ksi, so each could potentially fail at 40,000 lbf or 50,000 lbf respectively, so obviously there is a broader region where a grade 5 bolt would bend (from 92 to 120 ksi) compared to a grade 8 (from 130 to 150 ksi). But notice that the grade 5 bolt fails before the grade 8 even hits its elastic limit.

Also the bolt through a hitch receiver is in double shear, so unless you are driving a M35 and pulling out an M1A1 it's unlikely that you will stress a hitch pin or bolt to failure yanking out Hiluxes and Wranglers. The only disadvantage IMO to using the loop-and-pin method is convenience, stuffing the strap in can be a pain and sharp edges do need to be smoothed to keep abrasion minimized. But it's a perfectly legitimate and safe way to connect to a recovery point and does eliminate additional connections, which is good.

Good to know that grade 8 is stronger, not just harder. Makes that an easty decision.


Protecting the strap seems like a pretty good idea. You could even cut a sock and slip it over the strap. Once the loop is in, you can slide it down into the receiver.

 

[R_Lefebvre]

I found some unrelated discussion on hitch pins. http://grassrootsmotorsports.com/forum/grm/towing-hitch-pin-thingamabob/526/page1/

I'm sorry, but this author has no idea what he's talking about, and is just continuing a common internet myth. The only advantage to a Grade 5 over Grade 8 is the 5 will show that it is failing more than an 8 will since it probably has more elongation. But it will always fail before an 8, period.

 

[ntsqd]

Rule of thumb is 60% of tensile, although the relationship isn't perfectly linear. A490 ASTM bolts are rated at 75 ksi for shear but are 150 ksi tensile rated fasteners. They are only allowed 40 ksi because of the factor of safety in structures, though.

My refs and Strengths of Materials class text book put it at 57% for an average, depending on what formula you use. Not sure that the 3% is significant, but just in case......

 

[rickc]

A buddy of mine used his hitch for an extraction and although he got out OK the pin bent just enough that it could not be removed so he had to drive home with the pin and strap still attached at the back. The strap end was squished in tight providing no room for a cutting tool so he ended up cutting the strap end to be able to cut the pin out. He has since got a hitch bar with a shackle end.

 

[R_Lefebvre]

Is the assumption of 60% (+/- 2% ) based on single shear loading, or double shear? The bolt will be loaded in double shear in this usage.

 

[winkosmosis]

Wait.. it's not shear or tension, but a bending load. Can you calculate the amount of bending force a bolt can take?

 

[Antichrist]

The shear force is acting on the bolt/pin where it meets the tube on each side.

 

[ntsqd]

Per each shear zone.

 

[R_Lefebvre]

Wait.. it's not shear or tension, but a bending load. Can you calculate the amount of bending force a bolt can take?

Not really. It's not a linear problem since the load on the bolt is not evenly distributed. The only way to do it properly would be a finite element analysis.

Per each shear zone.

Ah, so in double shear a bolt will hold more than it would in tension.

 

[winkosmosis]

The shear force is acting on the bolt/pin where it meets the tube on each side.

But the concern here is the pin bending or breaking at the center, since if the strap is exerting its force there, that is the maximum stress on the pin with the biggest risk of failure.

 

[BIGdaddy]

this is a great discussion, keep it up. I'm using my hitch as main rear recovery point, and would like to know which method:

pin,
ball-mount d-ring,
or 2" tube steel mounted D-rings...

I should use. I'm leaning towards a receiver mounted d-ring cuz I can take it off, when I know I'm not going offroad, and can resort to a pin/strap method in a pinch. The permamount 2" shackles seems like they would add weight ALL the time and seem to be less favorable than a center pull/"engaging the whole assembly" yank.

 

[Antichrist]

this is a great discussion, keep it up. I'm using my hitch as main rear recovery point, and would like to know which method:

pin,
ball-mount d-ring,
or 2" tube steel mounted D-rings...

IMO, the best would be recovery points attached to the frame rail, inline with the frame rail and using a bridle to attach to them.
Class III hitches are typically rated for 10,000lb.