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How to measure bolt torque without a torque wrenchHelpful Member!(10) 

Andy22 (Mechanical) (OP)
9 Nov 04 10:53
We need to torque bolts holding teeth in a conveyor drive sprocket, but there is not enough room to fit the head of the torque wrench on the nut (the bolt head is head by the tooth casting).  I have heard of a method to estimate torque by counting the number of turns after finger tight.  How would this work for an M20 bolt that needs to be torqued to 300 ft lb?
zekeman (Mechanical)
9 Nov 04 11:20
First of all, I don't think you mean 300 ft lb on a bolt.
Second, in order to torque up an inaccessuble bolt,I would use a standard ratchet wrench and measure the perpendicular distance of the  force vector to the bolt, necessary  to snug up the bolt and simply take
F= force in lbs
L= perpendicular distance of force vector (very important) to centerline of bolt.
Any rule of thumb to estimate this value like counting threads seems hopelessly wrong in my view.
MintJulep (Mechanical)
9 Nov 04 11:50
The goal when tightening a fastener is to achieve the desired level of prelaod.

Using a torque wrench is one method of estimating the preload.  In fact, it is one of the least consistent and reliable methods.

The turn-of-the-nut method is generally recognized as a better method of preload control then torque.  So, no need to involve torque in the process at all.

If the application is sufficiently important, you should test the actual configuration to obtain the relationship between turns from snug and achieved preload.

Otherwise, see Bickford's "An Introduction to the Design and Behavior of Bolted Joints".
TheTick (Mechanical)
9 Nov 04 12:10
Why not just experiment?  Set up a mock assembly, tighten the bolt, and count turns.
byrdj (Mechanical)
9 Nov 04 12:52
Is an M20 bolt about 3/4" diameter?

300 ft-lb sounds like a lot, ie near yield.  But tighting to yield is an acceptable practice if using new bolts each tightening.  With this method, one should be able to feel yeild and stop pulling on the wrench when proper torque is reached

A method used with big studs, (where torque is considered too inaccurate)is to apply a "pulled-up wrench tight" intial torque(about 30 ft-lbs in your case, just needs to be consistant), then using the thread pitch and the modulas  of elasticity of the bolt calculate the degrees rotation to stretch the stud.

For bolts I'm familar with, 30Kpsi loading is desired, which is achived with 0.0015"/inch strectch.  Then with an 8-TPI nut, one will get about 0.090/revolution (not 0.125 due to thread destortion)

from a wrench tight, the rotation will be less than a revolution Unless the thread is very fine and the bolt very long.  Measuring flats is common instead of degrees.

Helpful Member!(4)  JStephen (Mechanical)
9 Nov 04 13:29
A long cheater pipe and a fish scale should do the job.
CoryPad (Materials)
9 Nov 04 16:53
Assuming 22 mm hole size, 28 mm screw head outer contact diameter, 0.1 minimum friction coefficient, a 300 lbf-ft torque (~ 400 N m) would produce 147 kN, which is the proof stress for property class 8.8.

To achieve 147 kN via an angle tightening specification, you need to know the compliances of the joint and screw.  Then, you can calculate the required axial deflection to produce the desired preload.  Thus, you need dimensions and materials properties for all joint members.



Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

Helpful Member!  btrueblood (Mechanical)
9 Nov 04 17:53

I knew what you meant, but the image that came to mind was somebody patiently scraping the skin of a fish, and applying the results to a cheater pipe...:)
byrdj (Mechanical)
9 Nov 04 18:00
why not just goudentite?
APH (Mechanical)
9 Nov 04 18:46
Why can't you just get an extension for your torque wrench??

Or...use force gage and standard wrench to apply the force.

Helpful Member!(2)  Ron (Structural)
10 Nov 04 5:54
300 ft.-lb is not excessive for a 3/4" bolt...we do it all the time in structural applications for high strength bolted connections (generally about 375 ft-lb for 3/4" dia. bolt).  Having said that, the Turn-of-Nut method is reliable and repeatable.  

Take the bolt to a "snug tight" condition, meaning full face contact of all surfaced.  Then mark the nut or bolt (nut turning is preferred), then turn an additional 1/2 to  2/3 turn from the mark, depending on the length of the bolt.
TheTick (Mechanical)
10 Nov 04 8:12
Wouldn't the "turn of nut" method also be dependent on the length of the fastener?  As the fastener length grows (length through componenets), it would take more turns to produce an equal amount of strain.

"An object at rest can not be stopped."

byrdj (Mechanical)
10 Nov 04 8:55

Yes, the longer the stud, the more turn to achieve the same "torque".  For loading studs to only 30 to 40 Kpsi, the stretch required would be 0.0015" per lenght of active stud (for specific bolting material). With 8TPI you get 0.090" per revolution.  Thus for a 10" stud, you would want 0.015" stretch or 1 flat (60degrees) turn of the nut.  for a 20" stud, that would be 0.030" strectch, or 2 flats.  Both studs would have the same 30 to 40K loading and both would click out the same with a torque wrench (for 3/4", between 110 and 160 ft-lbs: specific prelube required, where dry thread torque would be much greater)
Note: I'm not discribing or considering bolt twist.
my torque card starts at 1"-8 and these values are only used for referance since my industry standards are first measured elongation and then turn of the nut.  torque is only used to make initial "hand tight" consistant.

again, why not just Goudentite.
Ron (Structural)
10 Nov 04 13:44
TheTick...yes, it does make a difference.  In structural applications (AISC Manual of Practice), there is a table given for various bolt length conditions.  In general though, 1/2 to 2/3 of a turn after snug tight should result in the necessary 70 percent of yield for pretensioning.
Helpful Member!(3)  chipmeyer (Mechanical)
11 Nov 04 11:36
FYI - according to my machinery's handbook:

Accuracy of bolt preload application methods:

By feel +/-35%
Torque Wrench +/-25%
Turn-of-nut +/-15%
Preload indicating washer +/-10%
Strain Gages +/-1%
Bolt Elongation +/-3 to 5%
Ultrasonic Sensing +/-1%
mburgess (Mechanical)
11 Nov 04 18:00
Can you access the nut & end of the bolt. If you so you can easily measure the stretch of the bolt & ensure it has deformed still within elastic yield & also calculate the tension on the bolt.
Ron (Structural)
11 Nov 04 22:02
mburgess...only if you have a stress-strain curve for that particular material.
tomaspin (Mechanical)
17 Nov 04 11:09
I know I'm slightly late on this one, but I stongly suggest that you look at a load indicating bolt, sold under the trade name 'Rotabolt'. As far as I know, this is about as accurate as you can get.

I hope this helps.


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