Another Rod bolt question-why new

[stevef]

So I reassembled my 2.2 crank and rods over the weekend, did everything by "the book". Can't wait the start putting the case together. I'm standing there with the old bolts in my hand. Then I started thinking. If the bolts are stretch bolts the only way they can maintain the clamp up required for the application is if they remain stretched within there elastic limit. If they stetch beyond the elastic limit they yield and gain permanent stretch and lose there clamping force. So if they are within there elastic limit why can't they be reused. Don't think for one minute I'd reuse mine but I just got thinking. What wrong with my logic?

[350HP930]

Hysteresis.

Basically the stress/strain relationship of the material degrades each time it is fully stress cycled.

[Zeke]

Originally posted by stevef
If they stetch beyond the elastic limit they yield and gain permanent stretch and lose there clamping force. So if they are within there elastic limit why can't they be reused

Isn't that the answer right there? Do you know if and when the bolts have reached any lmit? It's not a question of logic as much as it is of testing the parts. Isn't it just cheaper to buy the bolts?

[Jim Sims]

When the stretch bolts (also known as torque to yield fasteners)yield they do not lose their clamping force but rather due to the shape of the metal's stress/strain curve the clamping force stays rather constant after yielding even as the bolt is turned a further half turn or so. This means that the required clamping force will always be achieved despite the variability of thread friction. The variability of thread friction causes the amount of bolt tension for a given torque to be a variable; the greater the friction the more torque is used to overcome it and therefore there is less to stretch (elastically and plastically) and tension the bolt. If the bolt is taken to yield then the clamping force needed to preload the bolt and rod end cap joint to prevent fatigue failure is guaranteed without regard to friction. Torque to yield fasteners should not be reused as the torque to yield procedure causes the bolt or screw to "neck" or reduce it's cross-section (at the cross-section of a thread root usually). Therefore the second time the bolt will not achieve the same clamping load as it necks even further during the second tightening. Torque to yield fasteners are typically made from less expensive metal (compare price with Raceware and ARP), are not as thread friction sensitive and may be correctly tightened with a simple torque wrench. This may be compared to a more elaborate bolt stretch measuring rig or the repetive tightening (five cycles of tightening and loosening to reach a constant and stable thread friction) sometimes required to achieve the correct tension in the more exotic superalloy rod bolts. Cheers, Jim.

[stevef]

Wow, Jim thanks. The flaw in my logic was that I didn't appreciate these bolts are torque to yield bolts. Cool, just goes to show, a little knowledge can be dangerous. Like I said I had no intention of not using new bolts, Thanks