Deaths from towballs

[PeeBee]

I am not advocating use of a towball as a hitch point where the towball is anchored with a thread into a receiver. This type of setup puts rise to potential failure of the treaded element from shear and/or shear and a bending moment as the line of action is raised to the thinner section of the towball. Most towballs have an undercut at the base of the thread where it matches the 50mm head, and this becomes the stress riser, especially as depth of undercut, radius size and surface roughness are unknown.

Additionally we have no idea what the towballs are manufactured from, could be recycled chinese bicycle seats - my towball is from a HT 4140 steel, but I would think a lot are made from a far lower grade.

What I would say is that his towball setup looks like its got 3 different sized balls, welded to a solid receiver bar, and fixed to the front of the towbar hitch using a standard 16mm pin. I would say, his weakest point in all this is either the 16mm pin in a double shear loading or the strap itself. The welded towballs are essentially 50mm pegs and the stress is going thru 3 circumferential welds into the solid hitch. The loading diagram for his hitch is not straight forward to determine actual stresses, however it is possible to get this analyzed using a Strand Stress Analysis package for certainty as it takes away the interpretation of the engineer. We don't know the materials, weld detail, weld procedure or welder certification, NDT taken place etc - there is a lot missing in this picture.

So, is the setup any weaker than using a slot in hitch on our standard towball set-up - I doubt it as ultimately the pin is the weakest element. Even if the receiver was a tube, worst case a thin walled tube, its 'probably' still going to fail on the tow strap rather than the hitch - thats my analysis anyway. On the surface it probably looks worse than it is. Whilst the guy using it claims to assess the loading conditions of the recovery - had to laugh at the h/d ear ring he pulled out for use though - and he does execute his recovery operations in seemingly the worst way, I think he is no worse off than what we do when we use a standard hitch and 16mm pin.

Final parting comment, Don't use a snatch strap on a towball for recovery.