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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: 8 years ago.Category: Car
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Answered in 3 minutes by:
11/2/2009
Auto Mechanic: brownjeff, ASE Certified Technician replied 8 years ago
brownjeff
brownjeff, ASE Certified Technician
Category: Car
Satisfied Customers: 13,886
Experience: 17 years experience, ASE certified, dealership certified, Service Manager for 15 years,
Verified
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.
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Customer reply replied 8 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
Auto Mechanic: brownjeff, ASE Certified Technician replied 8 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.
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Customer reply replied 8 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.
Auto Mechanic: SUPRYZE, Technician/Shop Foreman replied 8 years ago
SUPRYZE
SUPRYZE, Technician/Shop Foreman
Category: Car
Satisfied Customers: 2,158
Experience: GM & Saab Certified, ASE Master Technician
Verified

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.

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Customer reply replied 8 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?
Auto Mechanic: SUPRYZE, Technician/Shop Foreman replied 8 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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