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SUPRYZE, Technician/Shop Foreman
Category: Car
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Experience:  GM & Saab Certified, ASE Master Technician
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# How do you convert deg of rotation into ft lbs of torque

### Customer Question

how do you convert deg of rotation into ft lbs of torque
Submitted: 6 years ago.
Category: Car
Expert:  brownjeff replied 6 years ago.
Unfortunately you can't. The make a special tool that measures degrees that hook on to the ratchet or you just take an educated guess and for example 90 degrees would be a quarter turn. It is not an exact torque and as long as you are close it will work....of course the special tool is the best option.
Customer: replied 6 years ago.
shure you can otherwise repair manuals wouldnt know how much is being put on a bolt! I have a deg wheel, thats not my question
Expert:  brownjeff replied 6 years ago.
Well then I will pass your question back on to another expert as you wont accept mine. The reason so you know you can't convert it is the bolt size and thread changes the torque so a 3/8 bolt turned 90 degrees will be a different torque than a 1/4 bolt turned 90 degrees. Also fine and course thread will make a difference.
Customer: replied 6 years ago.
there is still a mathematical equation to do this otherwise how would you know what final torque is? shure a 1/4 bolt bolt will be different than a 3/8 bolt the area covered by the head of the bolt is different also.
Expert:  SUPRYZE replied 6 years ago.

Hello! your expert is 100% correct. There isn't an equation to convert degrees of rotation to a torque valve.

when a torque spec is given, for example, 90 ft/lbs, that value of "ft/lbs" is the resistance of rotation. Take three bolts: a rusty bolt, a new bolt, and a new bolt with oil on the threads. The rusty bolt will have the greatest resistance to rotation of the three, and therefore, will hit "90 ft/lbs" before the other two. The new bolt with oil on the threads will offer the least resistance, and therefore will require much more turning, to acheive the 90 ft/lbs. All this extra turning causes excessive clamping force, since each turn of the bolt brings the bolt head down lower. So if you torqued all three bolts to 90 ft/lbs, the rusty bolt won't offer enough clamping force, the new bolt will probably be just right and the new bolt with oil will be too tight.

When manufacturers give a rotation spec, they give an inititial torque valve (for example 10 ft/lbs) just to make sure the bolt is seated, usually very low. Then they give the rotation (180 degrees for example) because they know, that no matter if the bolt is rusty, clean or oily, that 180 degrees will give the same clamping force, no matter what.

If you don't beleive me, thats OK, its a hard concept to grasp. But google "torque to yeild bolts" and you will see a ton of information on the theory and benefits of using such, and maybe then you will understand the difference and why it can't be converterd.

I hope I helped and if you have additional questions, please let us know, we are happy to assist.

Customer: replied 6 years ago.
that is my question what or how do they come up with the final tourqe my question isnt about new or rusty bolts or resistance of oil vs nonoiled. if manfufactures can know that a bolt rotation of 90 degrees will always give the same clamping force then how?
Expert:  SUPRYZE replied 6 years ago.

The whole point of that explanation, was for you to see that because there are so many variables in the field and at the assembly plant, there isn't a direct connection between TTY specs and Torque specs.

When manufacturers decide how much clamping force a gasket needs to seal and survive, they have two methods available to them: Bolt stretch & Direct Tension Indicators (DTI's). DTI's give a direct reading of how much tension is under a bolt head. Bolt stretch can be used, because manufacturers know the strength of the bolt and knowing that, they can figure that if a bolt stretches 0.025", that it has XXXX amount of tension under the bolt head.

So they use either of these methods to decide how much rotation will give them the clamping force needed, and as long as the bolts meet the specifications, it will give the same amount of clamping force each and every time, regardless of other variables that throw off the old "torque" method.

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