bolts are graded. How did they come up with the markings such as 5 bars grade 8 and 3 bars for grade 5?

Answer 1

i own a repair shop,and the grading system was invented over 65 years ago,and it was done so that the installers wouldn't use a softer bolt, they are graded on how hard the metal is that there made of,a grade 5 bolt is softer and will stretch out before it breaks,and a grade 8 bolt will take more heat and stretching before it will break or stretch,but it will break off fast and hard if out under the right conditions,and there is always a grade 5 bolt this is a very soft metal bolt ,and will hold good as long as it don't get in a heat situation,the reason all this was done was because in the early days of the automobile the bolts they would use to put the together with would break off real easy,so they developed a better bolt,and they had to rate it,and this is how they did it,,good luck i hope this help,s

Answer 2

It's actually 6 bars for a grade 8 bolt. I wondered the same thing. All I could find is that the markings were just a quick easy way to standardize bolt strength identification.

Answer 3

all metals are having some hardness. these hardness are measured in terms of hardnes numbers.

similarly steel have a hardness. when we use steel in fasteners it has a stregth in terms of numbers ranging from 4.0 till 8.8 for normal industrial use.
tightening these fasteners have a limited applicable force that can be applied during tightening which is called as torque. these torques are measued in Foot pounds (or Neuton meters).
this torque applicator is called torque wrench.

the torque is standardised based on the diameter and strength of each bolt and available form net itself , you can download and set your torque applicable for the size of bolt.
www.imperialsupplies.com