Evening all,
Following on from a productive weekend of spannering and a recent project at work(product designer) got me thinking about the whole scenario of tightening up bolts on your car to certain torque values.
Car manufacturers always give you the correct tightening value for certain bolts around the cars but I wondered, what if you don't have the correct torque setting for the bolt in hand and can't locate it, but know the specific bolt specification and can assume that the bolt is either dry or lubed for friction purposes(such as cleaning the thread and spraying copper grease for a lubed scenario), can you work out the tightening torque for your required bolt to ensure you wheels don't fall off?
For anyone who isn't familiar with why we have to tighten certain bolts up to a specified torque, in laymans terms its to ensure the bolt won't start to unscrew itself or "fail" under constant loading and unloading. When you tighten a bolt up a specified torque value, you are basically stretching the bolt to within its plastic limit which is usually 75% of its ultimate tensile strength (ie it will stretch when you tighten it but wont be permanently be damaged when you release the loading) which creates enough friction between the thread of the bolt and nut to ensure that it won't start to unscrew, of course this is relative to the type of bolt used and the scenario it is placed into , higher friction = greater amount of load the bolt can take.
Most bolts on today's cars are made from carbon steel and usually come in three classes, being 8.8, 10.9 and 12.9, with ultimate tensile strengths(the amount of force required to permanently damage the bolt, ie shearing or snapping/cracking) of 800, 1000 and 1200Nmm^2( or MPa Megapascals) respectively. You can usually determine the bolt type simply by inspecting the head of the bolt, and then by measuring the pitch of the threads(the distance between the triangular peaks of the thread) which in most instances is going to be a standard pitch for the type of M bolt used, eg M8 bolt uses a 1.25 pitch (1.2mm between each point in the thread). When I say 75% pre-load i mean that the bolt will have 75% of its ultimate tensile strength applied, otherwise known as the yield strength, after ~75% the bolt starts to plastically deform but won't break until you breach the tensile limit, ie the bolt will snap and you'll be ***** off now that you have a bolt stuck in your caliper because you tightened it too much!
So going on this logic (i hope its logic!) I will take my strut bolt on my 182 as an example, this basically holds the shock absorber assembly to the swivel hub and needs to be torque up to a certain value being 140Nm (newton meters). The bolt type is an M14 and I believe it to have a standard pitch of 2.0 and its class is 8.8 (stamped on the head of the bolt). I'm going to clean the bolt and put anti seize copper grease on the threads to prevent seizing (hopefully!) so I will assume the bolt has lubrication applied and isn't dry. Using this site as a good reference
http://www.trfastenings.com/pages/P...s+for+Steel+Fasteners+Standard+Metric+Threads
We can see that the tightening torque for an M14 bolt is somewhere between 114Nm to 140Nm. This somewhat lines up to the renault recommended 140Nm, which going by that chart would assume that we have quite a large coefficient of friction between the bolt and nut, close enough.
Can I apply this logic to other bolts, at best, assuming the worst case friction scenario for tightening my bolts to their correct pre load? I know there's so many variables when trying to achieve pre-load, inaccuracy on your torque wrench, differing friction values because of dirt grit etc... Is this why they recommend locking compound to overcome all these inaccuracies, airing on the safe side?
I hope this might come as a usual explanation of the principles of why we need to tighten up bolts to the correct torque settings, other than to stop it falling off. Bolt science is a hugely complicated art, and the more i read about it the more it almost confuses me, but as long as you can understand the basics, you should hopefully be able to tighten a bolt to within its 75% pre load value or just below with the aid of some threadlock to ensure your airing on the side of caution!
Please correct any of the above below
Following on from a productive weekend of spannering and a recent project at work(product designer) got me thinking about the whole scenario of tightening up bolts on your car to certain torque values.
Car manufacturers always give you the correct tightening value for certain bolts around the cars but I wondered, what if you don't have the correct torque setting for the bolt in hand and can't locate it, but know the specific bolt specification and can assume that the bolt is either dry or lubed for friction purposes(such as cleaning the thread and spraying copper grease for a lubed scenario), can you work out the tightening torque for your required bolt to ensure you wheels don't fall off?
For anyone who isn't familiar with why we have to tighten certain bolts up to a specified torque, in laymans terms its to ensure the bolt won't start to unscrew itself or "fail" under constant loading and unloading. When you tighten a bolt up a specified torque value, you are basically stretching the bolt to within its plastic limit which is usually 75% of its ultimate tensile strength (ie it will stretch when you tighten it but wont be permanently be damaged when you release the loading) which creates enough friction between the thread of the bolt and nut to ensure that it won't start to unscrew, of course this is relative to the type of bolt used and the scenario it is placed into , higher friction = greater amount of load the bolt can take.
Most bolts on today's cars are made from carbon steel and usually come in three classes, being 8.8, 10.9 and 12.9, with ultimate tensile strengths(the amount of force required to permanently damage the bolt, ie shearing or snapping/cracking) of 800, 1000 and 1200Nmm^2( or MPa Megapascals) respectively. You can usually determine the bolt type simply by inspecting the head of the bolt, and then by measuring the pitch of the threads(the distance between the triangular peaks of the thread) which in most instances is going to be a standard pitch for the type of M bolt used, eg M8 bolt uses a 1.25 pitch (1.2mm between each point in the thread). When I say 75% pre-load i mean that the bolt will have 75% of its ultimate tensile strength applied, otherwise known as the yield strength, after ~75% the bolt starts to plastically deform but won't break until you breach the tensile limit, ie the bolt will snap and you'll be ***** off now that you have a bolt stuck in your caliper because you tightened it too much!
So going on this logic (i hope its logic!) I will take my strut bolt on my 182 as an example, this basically holds the shock absorber assembly to the swivel hub and needs to be torque up to a certain value being 140Nm (newton meters). The bolt type is an M14 and I believe it to have a standard pitch of 2.0 and its class is 8.8 (stamped on the head of the bolt). I'm going to clean the bolt and put anti seize copper grease on the threads to prevent seizing (hopefully!) so I will assume the bolt has lubrication applied and isn't dry. Using this site as a good reference
http://www.trfastenings.com/pages/P...s+for+Steel+Fasteners+Standard+Metric+Threads
We can see that the tightening torque for an M14 bolt is somewhere between 114Nm to 140Nm. This somewhat lines up to the renault recommended 140Nm, which going by that chart would assume that we have quite a large coefficient of friction between the bolt and nut, close enough.
Can I apply this logic to other bolts, at best, assuming the worst case friction scenario for tightening my bolts to their correct pre load? I know there's so many variables when trying to achieve pre-load, inaccuracy on your torque wrench, differing friction values because of dirt grit etc... Is this why they recommend locking compound to overcome all these inaccuracies, airing on the safe side?
I hope this might come as a usual explanation of the principles of why we need to tighten up bolts to the correct torque settings, other than to stop it falling off. Bolt science is a hugely complicated art, and the more i read about it the more it almost confuses me, but as long as you can understand the basics, you should hopefully be able to tighten a bolt to within its 75% pre load value or just below with the aid of some threadlock to ensure your airing on the side of caution!
Please correct any of the above below
