How safe is this towbar

[hatjohan]

That towbar looks to me to be professionally designed and made. Certainly no backyard job.
I won't hesitate to use it

[Jonatan]

That towbar looks to me to be professionally designed and made. Certainly no backyard job.
I won't hesitate to use it

It obviously passed the standard requirements for towing. But somebody may have abused it with a recovery or an accident. A notched pin would bear the brunt of the abuse.

[hatjohan]

It obviously passed the standard requirements for towing. But somebody may have abused it with a recovery or an accident. A notched pin would bear the brunt of the abuse.

But so could any other towbar have been abused - and created a latent problem
But that wasn't the original question

[Jonatan]

But so could any other towbar have been abused - and created a latent problem
But that wasn't the original question

The original post said "im worried if the half pin gets worn it will be able to pull out."

[Heinz Modricky]

This is a copy of a very well used "rolling key" design developed in the 19th century - so in use at least about 140 years.

The rolling key was fitted to the crankshaft of mechanical eccentric presses, while a flywheel rotated around the crankshaft. The key would have two positions, one with the flat half of the pin rotated so that the flywheel would pass, and then when rotated, the key would engage the flywheel, transferring the motion to the crankshaft and activate the press. Only half of the key radius would engage the flywheel.

These presses were manufactured with typical punching loads from 5 ton to around 200 ton for pressing and blanking metal parts. That key would activate and take all the transfer load from the flywheel to the crankshaft every time a part was punched. A typical operator could punch a few thousand parts per day (and at least several hundred depending on how fast he could feed the steel into the machine. So that key worked really hard.

The keys did break. I probably replaced about 5 or 6 in my life, but know that the loads in the keys in eccentric presses were substantially more than what your car will ever pull.

[Heinz Modricky]

I tried to find a better video, this only shows the flywheel and the operator engaging the key that then "joins" the flywheel to the breech that punches through the steel. There is no tool in this press - it makes a huge noise as it bangs the parts out. This is typically how often this key works in a day. Many steel factories would run shift work, resulting in millions of cycles in a year.

https://www.youtube.com/shorts/x0YYsedd3iE

[hatjohan]

The original post said "im worried if the half pin gets worn it will be able to pull out."

How is that related to a recovery or an accident?

[Peter Connan]

The clue was there all along, "round pin with flat end". How much, and where, this flat is located is important. I think the design allows the square bar to be inserted without pulling the pin, merely rotating pin's flat face to the top. Personally I would replace the pin with decent steel (not a hardware store bolt) and not have a flattened section.

This is not personal, I could have quoted three others as well.

Having looked at it carefully, I think this design is significantly better than the conventional through-pin design in virtually every respect except cost to manufacture.
It is effectively the Rolls Royce of removable tow hitches.

I would use it just as it is.

[Andrew V9X]

As others have said. That is an extremely well engineered design used on hard working machines. The load stress on the actual key is spread over the whole key and not just two points as with a round pin.

[Jonatan]

Here is a better video of a rotary key clutch

https://youtu.be/9NF81PZSQSE?si=fXitg-3t8i28KhKU

We have some of that in this towbar, but a lot is missing. . . . .

If it was a true rolling key, the pin would disappear into the bottom surface when looking into the receiving hole, and there would have been no need to relief cut the step on square bar to ride over the pin. The tube only supports the pin at the vertical tube walls, not along the full length of the pin at the bottom of the tube.

[Heinz Modricky]

Here is a better video of a rotary key clutch

https://youtu.be/9NF81PZSQSE?si=fXitg-3t8i28KhKU

We have some of that in this towbar, but a lot is missing. . . . .

If it was a true rolling key, the pin would disappear into the bottom surface when looking into the receiving hole, and there would have been no need to relief cut the step on square bar to ride over the pin. The tube only supports the pin at the vertical tube walls, not along the full length of the pin at the bottom of the tube.

Yes correct, it's not exactly the same. My intention was more to point out the reason why the back half of the solid insert is cutaway, and show that this works like a rolling key in this that in one position, the rolling key locks the insert, and in another rotated position, the key unlocks the insert without the need for removing the pin. The one side of the rolling key works like it does in a press by loading the entire horizontal length of the pin, while the other "half" is in a double shear application against the vertical sides of the tube wall.

To answer the OP, where may this fail?

1. The tube side walls want to shear the pin - that's very unlikely that a 5mm cross section of mild steel will shear a 20 mm pin. With enough load it will distort the walls outwards and no doubt start tearing from the hole.
2. The pin wants to rip a slot in the side walls of the tube - that's equally impossible and is the same scenario you have with a solid pin through a more traditional setup.
3. The pin wants to shear off the entire backend of the solid bar insert - not happening, and ditto the back end of the bar wants to shear the pin along it's length - also not happening.

As iffy as this may look, the loads required to make this fail are just too big to make this a concern. Not even wear and tear in towing application is going to wear out the original pin which would be from a tougher steel than mild steel. The wear should show on the tube holes and the notch at the back of the solid bar.

[Jonatan]

. . . .
1. The tube side walls want to shear the pin - that's very unlikely that a 5mm cross section of mild steel will shear a 20 mm pin. . .

That pin is 16mm at most, quite possibly only 12mm. Half of it has been cut away, so the cross-section for shearing is D-shaped, only half the original cross-section. But the notch corner from the milling may be sharp, this stress-raiser could weaken it by a further factor of 2. This is a really iffy design, easily cured with a solid pin.

Also bear in mind that the towball has forces backwards and forwards (drawing and overrunning). I suspect that the Mitsubishi designer started with the pin buried into the bottom face of the tube, making a true rotating pin clutch. Beautiful. . . . . but then the pin got messed up during braking/overrunning. So there was a hurried design change

The devil is in the details. The pin material and the sharpness of the notch. Can be debated at length, and I don't have the energy for that. If it were mine, it would get a solid pin.

[Searcher]

A potential failure in waiting.

A few wrong assumptions made here wrt it's safety and robustness.

Suggest you replace.

[Prof]

A potential failure in waiting.

A few wrong assumptions made here wrt it's safety and robustness.

Suggest you replace.

Everybody went through their arguments on the yah & neah front.

Interested to hear your argument

[SeanHammon]

Yes it has a pin thats on a lock with a key that prevents it from turning.

A pin and lock with key... in the dirt and dust and wet? When that decides to stop working you're going to have a kuk time removing it!

[Searcher]

Everybody went through their arguments on the yah & neah front.

Interested to hear your argument

The pin has a flat side machined, this substantianly reduces it's cross sectional area as opposed to an equivalent diameter round pin, thus making it weaker. It will thus shear easier and at a lower load than an equivalent round pin

The fact that the retainer pin only needs to be partially rotated to release is also a weakness. Maybe ok for tar and gravel road applications, but on 4x4 tracks there are many obstacles that can cause a potential unwanted release failure, like rocks, stumps, branches or comming off a hard/rock base with a steep departure angle. etc.

As has already been mentioned, at minimum I would replace the flat machined pin with an equivalent diameter round pin.

[Peter Connan]

The pin has a flat side machined, this substantially reduces it's cross sectional area as opposed to an equivalent diameter round pin, thus making it weaker. It will thus shear easier and at a lower load than an equivalent round pin

Because the drawbar itself is solid, the shear area between those two components is not just the cross-sectional area of half the pin on both sides, it is that plus the entire width of the coupler. Thus vastly stronger in that particular aspect.

The fact that the retainer pin only needs to be partially rotated to release is also a weakness. Maybe ok for tar and gravel road applications, but on 4x4 tracks there are many obstacles that can cause a potential unwanted release failure, like rocks, stumps, branches or coming off a hard/rock base with a steep departure angle. etc.

It would be very easy to add a spring-loaded lockout pin to prevent this, or a padlock which would then also reduce the risk of theft.

[Jonatan]

Peter, the pin is not supported along the entire width of the coupler.

[Searcher]

Because the drawbar itself is solid, the shear area between those two components is not just the cross-sectional area of half the pin on both sides, it is that plus the entire width of the coupler. Thus vastly stronger in that particular aspect.



It would be very easy to add a spring-loaded lockout pin to prevent this, or a padlock which would then also reduce the risk of theft.

Peter, there are two different shear scenarios for the pin.

1) It can shear along the vertical plane cross sectional areas of the half pin equivalent diameters.
or
2) It can shear along the horizontal cross sectional area created along the half pins width.

Theses two areas are independent and not additive, so whichever has the smallest area will shear first
I suspect it will be case #1.

I will make a rough sketch to illustrate my point.


Edit: Sketch added.

Scenario for round pin with flat side.

[Searcher]

Peter, the pin is not supported along the entire width of the coupler.

Agree, that is why I say there are two independent shear scenarios.