Tommy4949 said:
I am pretty sure I tightened them quite a bit more than what the instructions said. The instructions said 30-35 ftlbs and I cranked them down really tight, which I have a habit of tightening bolts more than they need to be tightened in the first place. I might have compromised the strength of the bolts doing that. I will just buy 4 new grade 5 bolts and see how they hold up next time. Thanks guys.:thumbup:
"Too tight" can be just as bad as "too loose" - there are issues either way.
The idea of installation torque is a simple way to state "installed preload," meaning a certain amount of tension is upon the fastener in a static condition (overseas, "torque wrenches" are called "tension wrenches" - the jury is out on which phrase is more technically accurate.) Metal is "elastic" and "plastic" - in an engineering sense. It is "elastic" because it can withstand a certain amount of force (resulting in deformation) and recover when the force is removed. This is where "preload" comes in.
It is "plastic" in the sense that a force above a certain level can result in a permanent deformation ("plastic deformation") - this threshhold of force is known as the "elastic limit."
Preload in a specific application is selected and calculated to allow a comfortable margin of additional force without exceeding the elastic limit of the fastener. This is why
overtightening fasteners can be as bad as
undertightening them - if you undertighten, you don't have the clamping force that has been designed and accounted for. If you overtighten, you have less room for force between the static preload level and the elastic limit - the point at which your fastener deforms. Permanently.
And, why do winches come with SAE5 hardware? Because the failure mode for SAE5 is less drastic than for SAE8. If you were to look at a stress/strain plot for two fasteners - both new, both the same size, but one SAE5 and the other SAE8 - you'll see that there is a much more pronounced "peak" before ultimate failure for the SAE8 than for the SAE5. This means that the SAE8 will stretch less before it ruptures - the stronger fasteners tend to "snap" like twigs, while the weaker ones will stretch and fail more like a bit of taffy (the comparison isn't quite valid, but it serves for our purposes.)
This is why I'd tend to put mostly SAE8 fasteners in, but have two SAE5s in opposite corners. The SAE5 fasteners would also be torqued about 10-15%
less than the SAE8 - the 8's will fail, but the 5's have enough preload on them to retain the winch and prevent it from becoming a very heavy projectile in the case of drastic failure. Ouch.
Zinc chromate plating may be more resistant to environmental attack than plain "white zinc", but that is a side effect. The primary purpose of the chromate plating on SAE8 is for visual identification.
