Hitch Pin Recovery Discussion

Ryan M wrote:For what it's worth, I'd never allow someone to be "recovered" via a tow-hitch pin.  Not trying to be rude, just being honest.  That will never be an acceptable form of recovery on an NSJ sponsored run.

Ideally, picking up a trailer hitch shackle receiver would be your best option for a cheap and quick method of reliable recovery.

A little off-topic, but there's some science to your concern, Ryan. If I may...

The significant force involved with towing a trailer is typically the tongue weight, which is not taken up by the hitch pin, but rather the receiver. This is a downward load, normal to the ground. The pin is put in shear during acceleration and deceleration. Technically, it's double shear. This is when the hitch wants to pull into or out of the receiver. Giggity. I'd venture a guess to say that braking produces a greater shear force in the pin, because generally speaking, you're taking it somewhat easy on the loud pedal when hauling a trailer. These are the loads that the hitch pin is designed for. It's actually a very simple calculation, when given input loads, because of the tight-fitting geometry of the hitch and receiver. HOWEVER, when you take the hitch out of the equation, and put a strap around the pin, the loading is entirely different, because now your hitch pin is in bending and you have to account for the shear strength of the cotter pin and the width of the strap, the angle at which you're pulling and a slew of other things. In short, the pin will be much weaker in this scenario as opposed to in double shear when the hitch is used properly.

It's scary stuff when you're working with such significant, dynamic and unpredictable loads. I just don't wanna see anyone getting hurt out there!

Ryan M wrote:For what it's worth, I'd never allow someone to be "recovered" via a tow-hitch pin.  Not trying to be rude, just being honest.  That will never be an acceptable form of recovery on an NSJ sponsored run.

Ideally, picking up a trailer hitch shackle receiver would be your best option for a cheap and quick method of reliable recovery.

Doesn't the hitch receiver utilize the same hitch pin?? It might have a d-ring to loop the strap through, but it's still connected to the tow hitch via a hitch pin.

 

Rob Cote wrote:

Ryan M wrote:For what it's worth, I'd never allow someone to be "recovered" via a tow-hitch pin.  Not trying to be rude, just being honest.  That will never be an acceptable form of recovery on an NSJ sponsored run.

Ideally, picking up a trailer hitch shackle receiver would be your best option for a cheap and quick method of reliable recovery.

A little off-topic, but there's some science to your concern, Ryan. If I may...

The significant force involved with towing a trailer is typically the tongue weight, which is not taken up by the hitch pin, but rather the receiver. This is a downward load, normal to the ground. The pin is put in shear during acceleration and deceleration. Technically, it's double shear. This is when the hitch wants to pull into or out of the receiver. Giggity. I'd venture a guess to say that braking produces a greater shear force in the pin, because generally speaking, you're taking it somewhat easy on the loud pedal when hauling a trailer. These are the loads that the hitch pin is designed for. It's actually a very simple calculation, when given input loads, because of the tight-fitting geometry of the hitch and receiver. HOWEVER, when you take the hitch out of the equation, and put a strap around the pin, the loading is entirely different, because now your hitch pin is in bending and you have to account for the shear strength of the cotter pin and the width of the strap, the angle at which you're pulling and a slew of other things. In short, the pin will be much weaker in this scenario as opposed to in double shear when the hitch is used properly.

It's scary stuff when you're working with such significant, dynamic and unpredictable loads. I just don't wanna see anyone getting hurt out there!

Yea, I'd agree with that, the main thing is that you have the pin in bending rather than in shear when using a strap around the pin. The pin is way stronger in shear than in bending. Make it double shear and its twice as strongerer.

Because I'm bored at work I'm going to calculate the bending load that the pin could handle as well as the load in double shear. Will update in a tad.

The loops on the straps are typically ~1", even for a 2" wide strap.

[quote= HOWEVER, when you take the hitch out of the equation, and put a strap around the pin, the loading is entirely different, because now your hitch pin is in bending and you have to account for the shear strength of the cotter pin and the width of the strap, the angle at which you're pulling and a slew of other things. In short, the pin will be much weaker in this scenario as opposed to in double shear when the hitch is used properly.
[/quote]
Why is the hitch pin bending with the strap on it? Why does the cotter pin come into play with a strap but not a receiver?  I understand pulling at an angle that is definitely different because you can't do that with a receiver but the other pieces are all equal

Also my 2" recovery strap is definitely 2" at the loop


[quote=Because I'm bored at work I'm going to calculate the bending load that the pin could handle as well as the load in double shear. Will update in a tad.[/quote]
Very interested in this

Its much easier for the pin the fail in bending, thus even with the load distributed by the eyelet of the strap, the pin is able to bend because you're loading it with a compliant material. It WILL bend before failing in shear when loaded by a strap. I think what Rob is getting after with the cotter pin is that in order for catastrophic failure to occur the pin would first have to bend to the point where the cotter pin was the only thing holding the pin onto the receiver. Thus the cotter pin gets loaded in shear and either prevents the pin from pulling out, or the cotter pin fails in shear allowing the pin to pull out and then badness happens.

Bending calculations are harder, and I haven't done one in a while so bear with me, but in double shear a 5/8" steel pin can handle around 50,000 lbs.

Tonellin wrote:Why is the hitch pin bending with the strap on it? Why does the cotter pin come into play with a strap but not a receiver?  I understand pulling at an angle that is definitely different because you can't do that with a receiver but the other pieces are all equal

Also my 2" recovery strap is definitely 2" at the loop

I didn't mean that the pin WILL bend, but the analysis is different because the loading is different. The cotter pin will be a factor because it will be put in shear as the hitch pin deflects along the direction of the load applied through the strap. The simplest approximation of the load being applied through a strap is a point load at the middle. It's very different than a shear load. Short of going through the equations step by step, I'm not sure of a better way to explain it, and I'm not sure this is the proper venue for it hahaha. I don't want to bore everyone else.

My strap loops are folded over. I assumed that was typical.

hobhayward wrote:Its much easier for the pin the fail in bending, thus even with the load distributed by the eyelet of the strap, the pin is able to bend because you're loading it with a compliant material. It WILL bend before failing in shear when loaded by a strap. I think what Rob is getting after with the cotter pin is that in order for catastrophic failure to occur the pin would first have to bend to the point where the cotter pin was the only thing holding the pin onto the receiver. Thus the cotter pin gets loaded in shear and either prevents the pin from pulling out, or the cotter pin fails in shear allowing the pin to pull out and then badness happens.

Bending calculations are harder, and I haven't done one in a while so bear with me, but in double shear a 5/8" steel pin can handle around 50,000 lbs.

ahhhhh that makes sense.  I wasn't thinking about the pin bending and then the cotter pin coming into play.  Yes in the case I'd imagine the cotter pin would be as useless as a paperclip haha

@Rob I get what you mean now..this dynamic load/double shear/bending calculations conversation is over my head for hte most part..guess that's why I'm a finance nerd haha

Rob Cote wrote:

Tonellin wrote:Why is the hitch pin bending with the strap on it? Why does the cotter pin come into play with a strap but not a receiver?  I understand pulling at an angle that is definitely different because you can't do that with a receiver but the other pieces are all equal

Also my 2" recovery strap is definitely 2" at the loop

I didn't mean that the pin WILL bend, but the analysis is different because the loading is different. The cotter pin will be a factor because it will be put in shear as the hitch pin deflects along the direction of the load applied through the strap. The simplest approximation of the load being applied through a strap is a point load at the middle. It's very different than a shear load. Short of going through the equations step by step, I'm not sure of a better way to explain it, and I'm not sure this is the proper venue for it hahaha. I don't want to bore everyone else.

My strap loops are folded over. I assumed that was typical.

This ^, we're thinking along the same lines, I was going to assume 3 point bending as a worst case, but the load distribution by the strap as well as the compliance of the strap makes it a little more complicated. Going to stop talking in this thread now and maybe continue elsewhere.. Btw, you an engineer Rob?

Sorry eldomi, I'm too new to Jeeps to be of much help.

hobhayward wrote:Btw, you an engineer Rob?

Haha. Is it obvious?

My degree says I am. My job is not. It's fun to exercise the brain muscle like this now and then.

[quote="Rob Cote"]

Tonellin wrote:Why is the hitch pin bending with the strap on it? Why does the cotter pin come into play with a strap but not a receiver?  I understand pulling at an angle that is definitely different because you can't do that with a receiver but the other pieces are all equal

Also my 2" recovery strap is definitely 2" at the loop

I didn't mean that the pin WILL bend, but the analysis is different because the loading is different. The cotter pin will be a factor because it will be put in shear as the hitch pin deflects along the direction of the load applied through the strap. The simplest approximation of the load being applied through a strap is a point load at the middle. It's very different than a shear load. Short of going through the equations step by step, I'm not sure of a better way to explain it, and I'm not sure this is the proper venue for it hahaha. I don't want to bore everyone else.

My strap loops are folded over. I assumed that was typical.[/quote

You must also take into account the tensile strength of the pin itself. Without that your calculations are at best a rough estimate of the shear and bending force applied to it. The cotter pin is the weak point unless it's tensile strength is equal to that of the pins which I doubt it will be. I have 2 options for my receiver recovery. first is the shackle the second is a locking pintel hook. The pintel hook is much stronger since it's anywhere from 5 ton to 20 ton pull.

Guys, I moved this to a thread of it's own so that we don't clutter the Club Run thread if people are still looking to sign up.

Carry on, this is a lot of good information.

Thanks Ryan! Seems much more appropriate here.

I'm not an engineer, but tend to think of everything in analogies.

If you have a 2x4 resting on cement blocks a few feet apart, would it be easier to break it stomping on the board right next to the blocks, or kicking it in half from the middle?

CrawlingForward wrote:I'm not an engineer, but tend to think of everything in analogies.

If you have a 2x4 resting on cement blocks a few feet apart, would it be easier to break it stomping on the board right next to the blocks, or kicking it in half from the middle?

Essentially that's what it comes down to. Got busy and forgot to post my results, but iirc my bending calcs came up with ~12,500 lbf to cause the pin the fail when loaded such that the strap was effectively applying a point load in the middle of the pin. Still a lot of force, but its a long shot from 50,000 lbf (aprox what the pins can handle in double shear).

Now whether the load distribution provided by the width of the strap takes away from that point load is debatable.. Regardless, using a shackle receiver is a safer way to recover.

Based on the ASME code the shear force on material is tested in single shear (elasticity) and compression. Double shear comes into play after those tests are performed. If your calculations say it's 12,500 in shear and 50,000 in double shear then it should be ok. Try to calculate th compression force since this is what the pin will likely fail from the bending force. Use the tensile strength of the metal and the thickness or diameter of the pin. Without those any calculations are best guess. It's been over 20 years since I played with that one. 

12500 was in 3 point bending. 50000 was the equivalent of shearing the pin in two locations simultaneously. Only been a year since I graduated but Im pretty rusty already lol