Navara Bosal Towbar Recovery

[Nick83]

Hi all,

Just a quick question....

The towbar is bolted to the chassis rails with 10 x M12 8.8 Bolts.
The highlighted areas are 10mm thick steel.

If used with a equalizer strap, can these be used as a recovery point and possible for snatch with kinetic rope?

[Searcher]

Looking at it, I would say yes.

Can you maybe post a side view of the one side of the bracket with a few dimensions, just to get a feeling of the amount of the metal around the eye and bolt holes.

[Nick83]

Looking at it, I would say yes.

Can you maybe post a side view of the one side of the bracket with a few dimensions, just to get a feeling of the amount of the metal around the eye and bolt holes.

Meat around the hole is 17mm

[Searcher]

Quick calculation, based on mild steel.
Calculations based on the loading of one bracket.

The bracket will begin to yield at a loading of about < 8500 kg.
At 4500 kg it has a stress of about 132 MPa, which is still under it's yield strength of about 250 Mpa.

If you consider an UTS (Ultimate Tensile Strength) of 400 Mpa before it breaks, I would say you are good to go.




Disclaimer. This is not my professional engineering opinion and is only a rough "back of cigarette box" calculation. Use at own risk.

[Nick83]

Quick calculation, based on mild steel.
Calculations based on the loading of one bracket.

The bracket will begin to yield at a loading of about < 8500 kg.
At 4500 kg it has a stress of about 132 MPa, which is still under it's yield strength of about 250 Mpa.

If you consider an UTS (Ultimate Tensile Strength) of 400 Mpa before it breaks, I would say you are good to go.




Disclaimer. This is not my professional engineering opinion and is only a rough "back of cigarette box" calculation. Use at own risk.

Thus the weakest point on the setup is the eyelids (and not the bolts) but still strong enough to recover or snatch with kinetic. Using a equalizer strap will half the load making it safer and reduce the stress on each eyelid.
Thanks for the calcs. I appreciate it.

[Nick83]

Now I just need to get recovery points on the front of the vehicle.
Was leaning towards homemade with 10mmx100mm flatbar but I am not able to bend them and I am concerned with the strength of welding.

[Searcher]

Some calculations on the bolts, at 5 per side in pure shear and without considering the moment or mounting friction.
(Note: Seeing as the eye is below the bolt holes, there is a moment being created here, that will put more stress on the bolts.)

I would still expect the eye to fail before the bolts.



Disclaimer. This is not my professional engineering opinion and is only a rough "back of cigarette box" calculation. Use at own risk.

[Nick83]

Some calculations on the bolts, at 5 per side in pure shear and without considering the moment or mounting friction.
(Note: Seeing as the eye is below the bolt holes, there is a moment being created here, that will put more stress on the bolts.)

I would still expect the eye to fail before the bolts.



Disclaimer. This is not my professional engineering opinion and is only a rough "back of cigarette box" calculation. Use at own risk.

Thanks for all the calcs

[Searcher]

I bought my front and rear recovery point form Big Country in JHB. They were selling them a while back at R165 per set of two. They were made of 8 mm steel. I see they are out of stock now, but may be worth giving them a call and seeing when new stock is due.

I would stay far away from the welding of recovery points.

Brackets are mounted with 2 x M14 8.8 bolts.

(Brackets originally came with three bolt hole mounts, but were cut down and modified)




My eldest is doing his PhD in Mech Eng, on Finite Element Analysis. As a favour I got him to run a quick basic FEA analysis on my recovery brackets.
As can be seen from the results below at a 4000 kg load per bracket they are more than strong enough. Quite a high stress concentration near the contact area inside of the eye, but still all good.

[Nick83]

I bought my front and rear recovery point form Big Country in JHB. They were selling them a while back at R165 per set of two. They were made of 8 mm steel. I see they are out of stock now, but may be worth giving them a call and seeing when new stock is due.

I would stay far away from the welding of recovery points.

Brackets are mounted with 2 x M14 8.8 bolts.

(Brackets originally came with three bolt hole mounts, but were cut down and modified)




My eldest is doing his PhD in Mech Eng, on Finite Element Analysis. As a favour I got him to run a quick basic FEA analysis on my recovery brackets.
As can be seen from the results below at a 4000 kg load per bracket they are more than strong enough. Quite a high stress concentration near the contact area inside of the eye, but still all good.

Thanks.
Gave them a call just now, correct not stock but they took my number and will call me as soon as they get stock

[Peter Connan]

540mPa is not in my opinion more than fine. These things are probably either mild steel (200MPa yield) or at best S355JR (350MPa yield)...

Just my opinion.

[Searcher]

540mPa is not in my opinion more than fine. These things are probably either mild steel (200MPa yield) or at best S355JR (350MPa yield)...

Just my opinion.

No need to be to concerned. The previous image was of a course FE mesh and the 540 MPa is not a true representation of the actual exact stress within the region of interest, but a localised stress concentration, which was exaggerated by the course mesh. With the use of a finer mesh we can see that the actual stresses in this region are lower, due to stress concentration becoming more localised.

The original analysis and course mesh was initially chosen to help illustrate and highlight the area of main concern.

Here are some more images to help illustrate the stresses, with somewhat exaggerated deformations, although the colour chart in the regions still represent the estimated values.


The figure below show the same loading conditions for a finer mesh. (Image deformation exaggerated for illustrative purposes)



This figure shows the displacement field. (Image deformation exaggerated for illustrative purposes)

[Peter Connan]

Well, I don't have a PHD. Only a technikon diploma and 20 years experience.

That experience indicates to me that in areas with such a high localised stress concentration (about half-way between yield and UTS for the most likely materials), small localised cracks are likely to start forming. That of course reduces the amount of material available to resist the force for the next recovery, leading to the failure accelerating.

I suspect those points may look un-damaged after one or two applications of the load as calculated, but will eventually fail after repeated loading.

Of course, there are a number of other factors at play. Firstly, for the vast majority of recoveries the load will be much lower (especially if you diligently use bridals and thus recover from both points). On the other hand, if the vehicle is really badly stuck in deep mud and only a single point is used, the loads are likely to be much higher. Secondly, the points may be made from a much stronger material than I have assumed. In which case, it might be fine even for the calculated load.

The elephant in the room: the FEA as shown only considers the area around the pin-hole, and only in a straight recovery. The most likely point of failure in my opinion is the chassis, which is probably about half the thickness.

I would be interested to know what load Richard rates these points for.

[Searcher]

Thanks Peter for your feedback and experience.

What I also found really interesting was that the FEA identified the most likely region of the bracket to fail, which I would not have correctly guessed. (I really don't think one must read to much into the actual stress values indicated)

Just looking at the bracket and with a load in the 9 O'Clock direction, I would have definitely guessed that the bracket would fail at the narrowest part being in the 7-8 O' Clock region or otherwise in the 9 0' Clock region - that being in the direction of the load itself.

However the FEA indicates that the most probable area of failure is more likely to be in the 10-11 O' Clock region, this I did not expect. There is a lot of meat there and the failure has a long way to go.

I spoke briefly to my Son yesterday and he mentioned that at the time a very basic quick linear FEA was done. He reckons that the loaded bracket scenario is actually a non-linear problem, since it involves different deforming surfaces in contact with each other. For a more accurate analyses of the loaded bracket and shackle, he would need to run a non-linear FEA simulation, preferably including the bolt mountings.

Maybe I can convince him to run such an analysis for me sometime.