Posted by Northern Calif Dave on Wednesday, December 28, 2005 at 3:18PM :
In Reply to: Stainless bolts posted by Northern Calif Dave on Tuesday, December 27, 2005 at 9:46PM :
First, thank you all for your opinions and facts. This is what I believe is a summary, but please add or correct anything you believe not to be correct.
Stainless Steel is expensive but can still achieve the same strength properties as a Grade 8 steel bolt (assuming similar grade Stainless Bolts), however it has a higher friction coefficient so if used where a specific torque is required one must be aware that there is torque preload. In other words when the bolt reaches full torque on the wrench, it might only have reached it by preload caused by friction.
Second, the stainless steel bolt is more brittle and hard. When placed in a situation where bending moment occurs (winch, bumper, tow hook), the bolt may fail. It will break before it bends.
Course thread versus fine thread. Fine threads are better in high tension areas because there is more surface to bear on (more threads), however the thickness of the threads is smaller so they strip easier. For the same size bolt, the fine threaded bolt actually has better shear properties because the actual bolt shaft size is larger (since the threads are smaller), page 11 Fastenal fastener guide. Fine threads yield about 10% additional strength over course threads. However the course thread allows for thicker corrosion resistance coatings, and tends not to cross-thread or strip as easily as fine threads. In addition, course threaded bolts can be easier to remove when dirty as there is slightly more room to allow the debri to pass between the nut and bolt.
Grade 8 vs Grade 5. As I understand it, any automotive application requires a minimum of a grade 5 bolt. There are applications where grade 5 may be beneficial over grade 8 where bolt failure maybe desired before the vehicle is ripped apart. For instance, grade 5 maybe better for attaching the winch where it may be desirable to allow the bolts to fail prior to the winch ripping the frame apart. Of course with a properly operating PTO winch with properly sized shear pins (1/4 or 5/16”) attaching the driveshaft to the winch, the shear pins fail before the winch exerts enough pressure to rip the frame apart, thus negating the need for a lower grade bolt for designed failure (which could be very dangerous if someone is standing in front, which should never happen anyway).
Use of stainless bolts into mild steel or castings. Using a stainless bolt into an area such as attaching the manifolds to the engine should not be done. Since the stainless is so much harder than carbon steel that it can easily strip the threads in the carbon steel ruining the entire application. In this case a steel bolt should be used.
So in summary, there is no one specific fastener to be used for all cases. Stainless is more expensive but the cost maybe beneficial in some areas. Stainless should probably be used in high moisture areas where bending is not an issue such as bolting the fenders on with course thread, except in high vibration areas a product like loctite or nylock nuts should be used for vibration resistance. Stainless could also be used for simple bolting as well such as brackets to the frame, ect. Care needs to be taken to clean the threads prior to removal so as not to try to back the stainless nut over dirt because of the higher friction already produced with stainless fasteners. If greater tightening precision is needed, then fine thread should be used. Corrosion resistant coated steel bolts should be used where the bolt may experience greater bending loads. Again loctite or nylock nuts should be used in high vibration areas (such as the bumper, winch, towhooks).
Regardless, steel bolts should be coated with zinc, cadmium or galvanized for corrosion resistance. When corrosion coatings are used with a steel bolt, to minimize hydrogen enbrittlement, these bolts need to be heat treated such as grade 5 or 8. With coatings available now such as Anti-seize and Loc tite, these should be used to aid in future maintenance and they provide a certain amount of corrosion resistance too. It should be noted that using loctite will require slightly more force to break the bond, however it is less than the force required to break a rust bond. One last note, with older steel bolts that are sufficiently long, a tap can be run up the threads to clean them prior to the nut being backed off. This won’t help break the rust bond, but it will reduce the effort needed to back the nut off over the entire shaft length (specifically thinking of the long bolts for the headlight buckets and turn signals if tap can be slipped over the wiring) and will aid in the reuse of old non-strength critical bolts.
References:
http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf
http://www.tapmatic.com/tech_manual/coarse_fine.html
Suppliers of stainless and steel bolts
http://www.mcmaster.com/
http://www.fastenal.com/web/home.ex
http://www.baybolt.com/
http://www.boltdepot.com/
Follow Ups: