Torque Wrenches - my answer...

[Ameer Antar]

I thought about this for a while and I think I've figured out the discrepancy. I'm sure whatever recommendations for the proper torque are good, but I can see that some listers are actually concerned about figuring out the right number. Anyway, I think actually the 2 camps on this issue both have it right, it just depends on the situation. The difference lies in where you place the extension. An extension used w/ a torque wrench will always increase torque on the bolt and reduce the necessary force to be applied at the other end. In essence it makes it easier (less force needed) for the person to turn the bolt. The position of the extension is what determines where the torque multiplication begins. If you have the extension at the front of t-wrench, then the extension makes it easier for the wrench to turn the bolt. That makes the reading on the torque wrench less than what is actually being transmitted to the bolt. Of course this also lessens the effort of the mechanic turning it b/c of the increased total length. There is no way that using the 2079 as indicated would result in the same torque at the bolt as at the torque wrench head. This is reinforced by the caution in the Bentley which states, "Tightening torque applies only when extension tool 2079 is used...". So that means the use of the tool definitely affects the torque reading and the torque therefore will be different at the head of the wrench and at the bolt.

The other method used is to add a bar to the end of the wrench. The head of the wrench directly drives the bolt head, but the extension allows the user to use less force at the further distance. So instead of the extension making it easier for the torque wrench to turn the bolt as in the above paragraph, this method makes it easier for the person to get to the proper torque. The reading of the torque wrench will be accurate in this case, b/c it directly drives the bolt. Most likely, the reason torque wrenches have warnings about using extensions on the end is that they can be easily damaged if over-torqued and also they loose accuracy b/c of the play usually found in that setup. If you're going to bother to torque a bolt, you might as well use the right wrench for that range. The only way to accurately increase (aka multiply) the torque of these wrenches is to use an extension at the front, so that the torque wrench will still be in it's designed range yet be able to apply more torque to the bolt.

In essence, both camps are right, it just depends on what setup you're using. Also the person who mentioned the fact that the wrench and extension should be in a straight line is correct. In any Physics book you'll find that a force which is not exerted at right angle to the torque arm, will not exhibit the maximum torque at the pivot. You have to use trigonometry to figure the exact numbers. I know somebody who's a Physics grad student, so hopefully I'll have him straighten this out for us even further.

-Ameer

---Original Message---
Date: Tue, 02 Dec 2003 18:50:44 -0500
From: Huw Powell <audi at humanspeakers.com>
Subject: Re: Torque Wrenches - my answer...
To: Bernard Littau <bernardl at acumenassociates.com>
Cc: quattro at audifans.com
Message-ID: <3FCD2554.6030001 at humanspeakers.com>
Content-Type: text/plain; charset=us-ascii; format=flowed


> This is the fundamental problem right here.  One camp sees this as a 
> simple lever arm lengthening, whilst noting that there is a potential 
> pivot point that has no bearing on the problem.  The other camp is 
> trying to complicate the problem by thinking too hard about the pivot 
> point.

Well put...

>> Picture the microcosm of the meeting point between the two tools, if 
>> you  will.  At that junction, there is a square hunk of metal, the 
>> driving nub thing of the torque wrench, that is imparting a twisting 
>> action to the 2079.  Each of the four faces of the "nub" are pushing 
>> in different directions, 90 degrees to each other, but all 
>> perpendicular to a radius drawn from the center of the nub.
> 
> True but pointless to the problem.

Please, why is that?  Is it because at the moment in time that 
everything matters, there is no motion about that point?

A good reason to ignore the junction is all that I need, not just a 
desire to ignore it.

Having not used the tool in question (has anyone discussing this besides 
Phil even *seen* one?), I haven't had the opportunity to observe whether 
the pivot acts "rigid" at the point of interest, the 258 ft-lb "click" 
of the wrench.  Whatever I suspect or think is as irrelevant as anyone 
else's thoughts on the matter.

I can see that if the two tools were moving in unison at that time, ie, 
staying in a nice straight line, then all the lever arm length math 
would apply quite simply.

>> In terms of its internal stresses, the 2079 is more like a chain drive 
>> with a pair of equal diameter gears at each end, moving the torque 
>> from its source to the fixed point at its other end.
> 
> This is fantasy.

And that is bordering on rude.

>> Another mental picture that is useful...

> No, torque can always be translated back to a force on a lever.  The air 
> impact wrench example is specious, as is much of the discussion on this 
> matter.

Well, please accept my apology for errant speciousness.

>> So, does anyone up here in snowy (!) New England have a 2079 kicking 
>> around so I can play at some experiments and prove myself right or 
>> wrong? (at least to my satisfaction...)
>>
> Please someone :-)

Don't act so exasperated.  This is a problem that can be easily tested 
on the bench, if and when I have done so, I will report fully without 
prejudice.  I would prefer to apply my torque to a spring of some sort, 
so the results can be photographed, but I doubt if I can rig that up.

If these wandering threads bother you, by the way, the easiest way to 
take them is to delete and ignore them...

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/



------------------------------