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  #281  
Old 02-19-2012, 08:26 AM
gtmule gtmule is offline
 
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Quote:
Originally Posted by WAM120RV View Post
Chris

All this discussion reminds me of a TV show many years ago called Tomorrows World. The topic was tightening nuts and bolts and the elastic limit of bolts. From memory ( which is getting a bit dodgy), the best way of ensuring the bolt was tightened to its maximum without exceeding the elastic limit was a very accurate measurement of the bolt length/stretch. The second best was a skilled craftsmans hands, and they showed that some people were almost as accurate by feel. The torque wrench came in third.
Torque wrenches,used properly, especially dial and beam ones, are perfectly good at setting bolt/nut torque. What they're not good at is setting bolt pre-load. If you have a fastener that needs to be at a certain pre-load, you'll want to measure the stretch in the bolt with either a micrometer, a strain gage or a piezeoelectric crystal crystal (think time of flight of a sound wave).
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  #282  
Old 02-19-2012, 08:35 AM
rocketman1988 rocketman1988 is offline
 
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Wow, with all this discussion, I'm almost afraid to fly my primary airplane because it's got so many bolts that were probably torqued incorrectly...

Guess I'll have to call Boeing to make sure...
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  #283  
Old 02-19-2012, 08:54 AM
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Av8torTom Av8torTom is offline
 
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Default Well....

Quote:
Originally Posted by RVnoob View Post
.


Torque wrench is a measuring instrument,
it must be used at the handle.
You cannot grab the torque wrench at the middle of the shaft, or with a cheater bar.


.
I've been following this thread for a couple of days now and have gone back and forth on a few concepts, but this I KNOW is COMPLETELY wrong...
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  #284  
Old 02-19-2012, 10:23 AM
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Brantel Brantel is offline
 
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Tom,

Check out this post and video.

Quote:
Originally Posted by CMW View Post
Here's a video I just made answering one of the questions that I think Larry was asking and Steve clarified. Does it matter where you hold a clicker type torque wrench. Intuitively you would think it shouldn't matter much. But as you can see using a Snap-On 100 ft-lbs clicker torque wrench set to 25 ft-lbs (300 in-lbs) and pushing against a Snap-On 600 in-lbs dial torque wrench, there is a significant error when pushing in the middle of the wrench instead of the handle. I don't know exactly how the mechanism works but I know it's calibrated using the handle.

http://youtu.be/_1LZk0jnQrA

By the way, this clicker wrench just happens to have an effective length of 12 inches so it should work perfectly with Allan's wrench and a .8 multiplier.
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  #285  
Old 02-19-2012, 11:31 AM
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Av8torTom Av8torTom is offline
 
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Default Strange

Well now that is just strange. It must be a characteristic (flaw) of the clicker-style torque wrench because mathematically that makes NO sense. All the more reason to use a beam-style wrench.

Tom
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  #286  
Old 02-19-2012, 11:48 AM
gasman gasman is offline
 
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Quote:
Originally Posted by Av8torTom View Post
Well now that is just strange. It must be a characteristic (flaw) of the clicker-style torque wrench because mathematically that makes NO sense. All the more reason to use a beam-style wrench.

Tom
The beam style wrench also needs to be held by the handle and it must be neutral.
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  #287  
Old 02-19-2012, 12:03 PM
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Av8torTom Av8torTom is offline
 
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Default Roll over Archimedes...

Quote:
Originally Posted by gasman View Post
The beam style wrench also needs to be held by the handle and it must be neutral.
This is bizarre. At the risk of beating this to death, when a beam-style wrench says 25 inch lbs, it's 25 inch lbs - no more no less, and it makes NO difference where you hold the wrench. The only difference is that holding the wrench halfway down the handle will require exactly twice the force BY THE OPERATOR as when held at the end of the handle to achieve the same torque. The formula is simple Torque = Force X Distance X sine of the angle of the force on the handle (the sine of 90 deg is 1 so we usually disregard this). If the operator needs to exert 2 lbs of force at the end of the handle to achieve a given torque setting, then s/he will get the exact same torque when 4 lbs is applied half-way down the wrench handle...(2XL = 4Xhalf L) but the torque is WHATEVER the wrench reads. i.e., it has no idea where it's being held.


The torque in the illustration is the same at position A and B = 20 ft lbs.

I have NO idea what's going on with that clicker wrench in the video - that's just weird... but important to know.
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Last edited by Av8torTom : 02-19-2012 at 12:09 PM.
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  #288  
Old 02-19-2012, 12:05 PM
CMW CMW is offline
 
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Quote:
Originally Posted by gtmule View Post
Torque wrenches,used properly, especially dial and beam ones, are perfectly good at setting bolt/nut torque. What they're not good at is setting bolt pre-load. If you have a fastener that needs to be at a certain pre-load, you'll want to measure the stretch in the bolt with either a micrometer, a strain gage or a piezeoelectric crystal crystal (think time of flight of a sound wave).
You are absolutely correct. Torque coarsely measures the preload of a bolt thru the relationship: Torque = .2*Preload*Bolt Diameter. This relationship has many variables such as lubrication, surface finish of the threads, etc. In a situation where preload is critical, such as connecting rod bolts, bolt stretch or strain should be measured.

Here's a couple of pictures of an extensometer that I designed for work that measures the strain in a 20 foot long 3" diameter tie rod used in a nuclear reactor. The total strain over the three foot length of the tool was only .003" at the preload that was required. The tool also had to be installed remotely 60 feet underwater. It's a fun job sometimes.





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  #289  
Old 02-19-2012, 12:31 PM
BillL BillL is offline
 
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Default Click Wrench Innards

http://www.tegger.com/hondafaq/torque_wrench/index.html

Here is an exposed view of the insides of a click wrench. It may help to see how it works.

Cool discussion on bolt stretch. Caterpillar large engines for locomotives use hydraulic pullers to pull all 4 head studs to a fixed force/extension/strain, then a cylindrical "nut" is moved in place with a small dowel rod and simply snugged by hand force. Connecting rod bolts use stretch to "set" pre-load.

Nearly all our critical fasteners that are torqued, are lubricated to ensure known thread friction, and use relatively hard washers as well for the same reason. It is interesting that we don't do this in aero, I can only think that class 5 aero fasteners are not stressed that high to avoid fatigue failures that result in fastener separation. It is more desirable to just come loose while remaining intact.

Hope this is not too much off topic.
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  #290  
Old 02-19-2012, 12:55 PM
CMW CMW is offline
 
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Quote:
Originally Posted by Av8torTom View Post
This is bizarre. At the risk of beating this to death, when a beam-style wrench says 25 inch lbs, it's 25 inch lbs - no more no less, and it makes NO difference where you hold the wrench. The only difference is that holding the wrench halfway down the handle will require exactly twice the force BY THE OPERATOR as when held at the end of the handle to achieve the same torque. The formula is simple Torque = Force X Distance X sine of the angle of the force on the handle (the sine of 90 deg is 1 so we usually disregard this). If the operator needs to exert 2 lbs of force at the end of the handle to achieve a given torque setting, then s/he will get the exact same torque when 4 lbs is applied half-way down the wrench handle...(2XL = 4Xhalf L) but the torque is WHATEVER the wrench reads. i.e., it has no idea where it's being held.


The torque in the illustration is the same at position A and B = 20 ft lbs.

I have NO idea what's going on with that clicker wrench in the video - that's just weird... but important to know.
Tom,

A beam style wrench is measuring the torque by indicating the deflection in the beam. Therefore where you apply the load is critical.

For the clicker, intuitively you would think it doesn't matter where you hold the wrench because of the internal spring and pawl mechanism. But there are internal frictional forces (like the point where the wrench "breaks") that increase if you move your hand closer to the head of the tool because you have to apply more force for the same torque. I believe this is why there is a relatively large error shown in the video. If I add an extension to the handle the wrench breaks at a smaller torque than it is set to but the error is much less. On the request of BillL, I reran the test in the video holding the dial torque wrench at exactly the area on the handle marked as the active area by holding the wrench in a collet on my lathe. This was very stable and I got the exact same results as shown in the video.
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