Re: Used Bolts
Joe, Just some clarifications:
1. As stated above, bolts in well-designed joints do not see any appreciable fatigue during their service life, and this includes highly cyclically loaded joints such as those you mention. Reusing bolts from such applications should not be any worse that any others.
2. ONLY fasteners tightened beyond their yield point will plasticly (permanently) deform. Fasteners which have been tightened slightly into yield are still very capable of "providing the design clamp-load for the application". All that is necessary is to reduce the load on the bolt to the range required by the application and it will work fine. Note that fasteners plastically "stretch" a lot before they reach their ultimate strength and are in danger of rupture, the early stages of this deformation are really not a problem, many fasteners are deliberately tightened into the yield range.
3. "Torque-to-yield" is not a class of fastener, it is a tightening strategy that involves tightening a given bolt to a point beyond its yield point then stopping. The resulting load thusly applied is somewhat below yield because the torsional component of the tightening load disappears when torquing is stopped. Note that virtually any fastener can be tightened using this strategy and the proper monitoring equipment, it is not reserved for a "special" class of parts.
4. It is important to understand that torque has no direct relationship to clamp-load in the joint, it is only a means of estimating clamp-load and can be significantly misleading, if lubricity is changed by cleanliness, or use of any of the lubricants you mention.
5. I don't disagree with your recommendation not to re-use the parts you listed, but only because new fasteners are fairly cheap and the components you mention are very difficult, expensive, and time consuming to repair, should a fastener failure occur. Additionally, nice fresh bolts have correct lubricity, and are clean, so some of the concerns with used bolts are eliminated.
5. The "black oxide" you mention is really usually zinc phosphate. Removal of this finish prior to assembly is not advisable, for all the reasons mentioned above.
Mike
Joe, Just some clarifications:
1. As stated above, bolts in well-designed joints do not see any appreciable fatigue during their service life, and this includes highly cyclically loaded joints such as those you mention. Reusing bolts from such applications should not be any worse that any others.
2. ONLY fasteners tightened beyond their yield point will plasticly (permanently) deform. Fasteners which have been tightened slightly into yield are still very capable of "providing the design clamp-load for the application". All that is necessary is to reduce the load on the bolt to the range required by the application and it will work fine. Note that fasteners plastically "stretch" a lot before they reach their ultimate strength and are in danger of rupture, the early stages of this deformation are really not a problem, many fasteners are deliberately tightened into the yield range.
3. "Torque-to-yield" is not a class of fastener, it is a tightening strategy that involves tightening a given bolt to a point beyond its yield point then stopping. The resulting load thusly applied is somewhat below yield because the torsional component of the tightening load disappears when torquing is stopped. Note that virtually any fastener can be tightened using this strategy and the proper monitoring equipment, it is not reserved for a "special" class of parts.
4. It is important to understand that torque has no direct relationship to clamp-load in the joint, it is only a means of estimating clamp-load and can be significantly misleading, if lubricity is changed by cleanliness, or use of any of the lubricants you mention.
5. I don't disagree with your recommendation not to re-use the parts you listed, but only because new fasteners are fairly cheap and the components you mention are very difficult, expensive, and time consuming to repair, should a fastener failure occur. Additionally, nice fresh bolts have correct lubricity, and are clean, so some of the concerns with used bolts are eliminated.
5. The "black oxide" you mention is really usually zinc phosphate. Removal of this finish prior to assembly is not advisable, for all the reasons mentioned above.
Mike
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