TJ Steering

shawn said:
Let’s say a connection must be designed for a load of X, with the fasteners sized to resist a load of 1.5x. Rather than being torqued to 150ft-lbs, the fasteners are only torqued to 75. Now they can only support a load of 3/4X. Since X represents the maximum value that might ever be seen, you might go a long time without ever coming close to it. Sooner or later, you hit a value of .8X, the fasteners allow the connection to stretch or shift (thousandths of an inch being all that’s required), the fastener is subjected to a shear load that is close to the capacity of the bolt at its minor diameter (it’s conceptually like a hitch pin at this point), and a small crack forms at the root of a thread.
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You're neglecting how fatigue actually works in a tensioned bolt, which is mostly what this is while in single shear while tensioned properly (besides the bending loads, which also act in tension). The entire point of torquing a bolt is to generate a preload inside the bolted joint, like you said. Your dynamic loads are some approximation of a sine wave (cyclic load where the sine wave crosses zero, etc), and the concept is that you always want the magnitude of that sine wave to be within the preload range of the bolt, so the load never crosses zero. So you're offsetting the entire sign wave to be positive (alternating between more positive and less positive), instead of positive and negative.

Fatigue happens when that preload is not high enough for the cyclic loads that the bolted joint sees, so now your load crosses zero. That's a fatigue cycle. Loads that are above the rating can also put the bolt above the yield point and into the plastic range, but that's not the same mechanism as cyclic fatigue when the bolt has improper preload.
You're saying loosely the same thing, just in a really roundabout way and for only part of the sine wave of that cyclic load. Loosing tension and being subjected to shear is not what kills a bolt like that, it's a actual act of having a load that crosses zero, so now you have alternating positive and negative loads.


I'm doing a shit job of explaining this properly. I'm just trying to flesh everything out so it's a more complete picture.
 
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:popcorn:
 
There's some dudes talking wayyyyyy over my head in this thread right now.
I'm here for an education as long as I can actually grasp some of the technical info being discussed
 
So what I'm hearing is the bolt broke because it decided too?

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Also, if keeping the same single shear design, it would be a good idea to add a security washer between the misalignment spacer and the bolt head. There is nothing prevent the Heim body from coming right down off the bolt if the spherical pops loose from loads or wear.

If the bolt can slide around in the bracket like that (in the video), it's def not tensioned properly.
 
Fabrik8 said:
Also, if keeping the same single shear design, it would be a good idea to add a security washer between the misalignment spacer and the bolt head. There is nothing prevent the Heim body from coming right down off the bolt if the spherical pops loose from loads or wear.
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There's not a misalignment spacer. It's a Johnny joint and the stock frame bracket. There's a fat washer that's supposed to go under the nut in the OEM configuration, but it might be there to take up bare shaft so the nut doesn't bottom the threads. I can't remember for sure.
 
On Shane's I'm pretty sure he's using a 3/4" heim, which would have misalignment spacers. The Currie style usually uses a 9/16" bolt not 5/8"
 
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