Posted: Sun Nov 15, 2009 8:01 pm
Remember those chewey sweets? Blackjacks, Fruit Salads etc etc?
Bolts are a bit like them. If you held one at each end a pulled they would withstand a certain amount of force and shrink back when released. Too much force and they start to 'thin down' (or neck, in tech-speak) which then means that the force is applied over a thinner area, thus leading to it getting rapidly worse.
With bolts, if you tighten them to the right torque they will clamp the 2 bits together and absorb any shock quite well. Too loose and they will allow the 2 pieces to wobble, inducing nasty shearing loads (Like an MFI wardrobe door hinge when the screws loosen). Too tight and they'll be permanently deformed by what is called plastic deformation where they start to permanently stretch, instead of elastic deformation where they return to their former shape. Elastic=good, plastic=bad. Once a bolt has been tightened beyond the designed torque (eg 38ftlbs) and also gone beyond the plastic limit then it is a matter of WHEN, not IF it is going to snap.
http://www.zerofast.com/torque.htm
If you look at the chart for 3/8-24 (3/8th inch dia, 24 teeth per inch) in a grade 5 bolt, plain, it comes out at 38 ftlbs. Guess what?
If you go to 7/16-24 (Vauxhall Viva etc) then it is 60 ftlbs.
I bet if you found the metric conversion page then it would show massive metric bolts for a 150mph BMW would be 110Nm.
OK, so I've oversimplified it a bit but the basic theory holds true. Those 110Nm'd Morris wheel studs (and nuts) are going to die, eventually, even if you re-set them at the correct figure. After all, the side loads on wheel studs are horrific! Bolts are designed to clamp things together, not to stop them sliding sideways. We have locating dowels for that.
Cheap bolts are a nightmare. When proper bolts are made they put the threads on with roller machines that squash the threads onto a piece of smooth bolt. This keeps the stresses safe. If you cut a thread (Like Champion do with their sparkplugs) then the sharp teeth of the cutter will slice nasty great gouges out of the metal and make a prime piece of realestate for a crack to develop. It also makes the cut thread on the bolt into a nice way of cutting a notch into the nut thread!
Tip for the day? Quality bolts. Spin them once down the thread with a nut to ensure no binding, use decent rust prevention to avoid corrosion causing cracks, never add to a thread by using a tap-n-die set unless absolutely necessary, ummmmmm.....anything else, guys?
Bolts are a bit like them. If you held one at each end a pulled they would withstand a certain amount of force and shrink back when released. Too much force and they start to 'thin down' (or neck, in tech-speak) which then means that the force is applied over a thinner area, thus leading to it getting rapidly worse.
With bolts, if you tighten them to the right torque they will clamp the 2 bits together and absorb any shock quite well. Too loose and they will allow the 2 pieces to wobble, inducing nasty shearing loads (Like an MFI wardrobe door hinge when the screws loosen). Too tight and they'll be permanently deformed by what is called plastic deformation where they start to permanently stretch, instead of elastic deformation where they return to their former shape. Elastic=good, plastic=bad. Once a bolt has been tightened beyond the designed torque (eg 38ftlbs) and also gone beyond the plastic limit then it is a matter of WHEN, not IF it is going to snap.
http://www.zerofast.com/torque.htm
If you look at the chart for 3/8-24 (3/8th inch dia, 24 teeth per inch) in a grade 5 bolt, plain, it comes out at 38 ftlbs. Guess what?
If you go to 7/16-24 (Vauxhall Viva etc) then it is 60 ftlbs.
I bet if you found the metric conversion page then it would show massive metric bolts for a 150mph BMW would be 110Nm.
OK, so I've oversimplified it a bit but the basic theory holds true. Those 110Nm'd Morris wheel studs (and nuts) are going to die, eventually, even if you re-set them at the correct figure. After all, the side loads on wheel studs are horrific! Bolts are designed to clamp things together, not to stop them sliding sideways. We have locating dowels for that.
Cheap bolts are a nightmare. When proper bolts are made they put the threads on with roller machines that squash the threads onto a piece of smooth bolt. This keeps the stresses safe. If you cut a thread (Like Champion do with their sparkplugs) then the sharp teeth of the cutter will slice nasty great gouges out of the metal and make a prime piece of realestate for a crack to develop. It also makes the cut thread on the bolt into a nice way of cutting a notch into the nut thread!
Tip for the day? Quality bolts. Spin them once down the thread with a nut to ensure no binding, use decent rust prevention to avoid corrosion causing cracks, never add to a thread by using a tap-n-die set unless absolutely necessary, ummmmmm.....anything else, guys?