handfulz28

I haven't seen much from Nick (CigDaze) lately, but I'd really like to see a stress analysis of a solid mount with 2 5/16" ball (1 1/4" shaft?) versus a pintle that's bolted on with 4 bolts. Not looking for a debate or argument, more for the science behind one or the other. Are bolts stronger in shear or tension, especially the caps? Is it that loads below say 15k lbs don't really apply as great a force on these parts as one would think?

Chris Sunkin

You're going to have to narrow it down. Pintles are availble in a range starting at 16K capacity and going up to 90K, and that's just highway use.

To somewhat answer your question, the pintle mount fastens where the bolts would be in tension. The 16K pintle uses 4 grade 8 bolts in 1/2" dameter. Tension yield on these bolts is around 22,600# each so you're going to have to exert 100K# of force in tension to pull them off. Fasteners are only about 80% as capable in shear vs. tension.

There's one important thing to keep in mind- there's a difference between static load and dynamic load. If you have a 10,000 lb weight hanging motionless, it presents a significantly reduced load than a 10,000 lb weight that's dropping and the chain is there to stop the fall. Although not as dramatic, pulling a trailer along at highway speeds represents a significantly different load than a trailer in the middle of a high-force maneuver- like an accident. Tough to model without alot of data.

Chris Sunkin

Thinking about your question further, I think what you really might be asking is where are the prospective failure points. The 2" solid drawbar certainly isn't. Neither is the ball itself. Most likely it's going to be the coupler pawl or the coupler body. The pawl and body are the most vulnerable in out-of-the-box loadings such as torsional twisting in an accident situation. The most vulnerable point and the most common place of failure on a hitch is where the draw tube mounts to the cross-bar. Even on the highest-rated hitched, this is the place they typically break- even in normal usage. Obviously, if you plug a pintle drawbar into a receiver, you solve nothing, either in increasing capacity or reducing hitch failures. They're less vulnerable to failures though.

handfulz28

Nah, just really wanted to understand how four bolts carrying a load compares to what is essentially one big bolt, with the further variable being that the four bolts carry the load differently than the one bolt. So actually, I was thinking about reducing all the possible failure modes to just "the bolts".

The 22.6k lb number on the pintle bolts helps explain some things. I had no idea it would take that much force to pull a bolt head off. I'm guessing your typical trailer ball isn't anything more than a chunk of mild steel, so it uses brute material quantity to develop its load rating. And yes, your typical loading on these parts, outside of an accident, isn't nearly as high as one might think.