Torque Wrenches - my answer...

[Bernard Littau]

Yikes, I hate doing this.

Huw is a bit of an institution around here on the quattro list, so much 
of what he says is taken as gospel.  Most of it is.

Huw's post below, however, is dead wrong.  Do not use only 258 foot 
pounds on the crank bolt.  The correct value is 330 foot pounds at the bolt.

The 2079 tool does in fact act as a force multiplier, in addition to 
being a little better in the use of space.  Note the use of the term 
force.  You can't solve this math problem using only torque, you must 
convert the 258 foot pounds of torque to force at the handle end of the 
torque wrench.  The length of the extension, the length of the torque 
wrench, and the length of the combined torque wrench and extension are 
all pertinent to the problem to be solved.  The force at the handle end 
of the torque wrench over the combined length of torque wrench and 2079 
is the torque at the crank bolt. 

Best,

Bernard Littau
Woodinville, WA
'88 5ktq
I use this torque wrench:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=3293



Huw Powell wrote:

>
>> im afraid im having great difficulty with that concept. After all,
>> torque is a force about an axis at a given distance. As long as you
>> are rotating that wrench about the axis of the square drive the
>> mechanism will always react at the same point.
>
>
>> I think the point here is that torque wrenches don't actually measure
>> torque.  Somehow (and after all this thread I have a killer urge to
>> take a clicker torque wrench apart) what it actually measures is the
>> applied force.  The torque scale etched on the wrench is just the
>> measured force multiplied by the assumed constant length of the wrench.
>
>
> I think there are some glimmers of useful truth contained in these two 
> paragraphs.
>
> I also think we are wrestling with two or three separate concepts that 
> make getting lost very easy.
>
> One applies to the internal workings of the typical "click" type 
> torque wrench.  I do not think that they measure torque at the drive 
> square - I think, as it says above, they are more complicated, which 
> is why you have to hold them correctly, etc.
>
> That said, we move along to our original question - what is the 
> resultant torque on the crank bolt when using a torque wrench set at 
> 258 ft-lb with the extension tool?
>
> I say... 258 ft-lb.
>
> Keep in mind two things, the first of which Phil mentioned.  The 
> reason for the tool is not to multiply torque, it is to make the car 
> "easier" to work on, ie it moves the point of applied force out from 
> behind the bumper of the car.
>
> The second is more abstract.  I will ignore all internal mechanisms of 
> torque wrenches, since the wrench can be considered to be a "black 
> box" that simply creates a *rotational force,* or torque, in a 
> circular fashion about a small square drive nubbin thing.
>
> As far as the 2079 "knows," all that is happening is that one end is 
> stuck on the crank bolt, and the other is being *twisted* at up to 258 
> ft-lbs.
>
> I don't think it even matters what angle is made between the torque 
> wrench and the 2079, the effect is the same.
>
> This twisting force is present at one end of the 2079, and since the 
> other end is fixed in space (it can only rotate about the crank axis), 
> the torque is transferred from one end to the other.
>
> For all the lever arm math to apply, the initial torque has to be 
> turned back into a lever arm with force on it, and then connected to 
> another lever arm, like the way gears are used to trade off rpms for 
> more or less torque output.  The first gear turns the input torque 
> back into a lever arm (the radius of the gear) and the second gear is 
> the second (usually different length) lever arm.
>
> It is important to rememeber that torque, expressed as foot-pounds (or 
> any other "distance-force" configuration) cannot be turned back into a 
> lever arm and force from raw, twisting torque for the purpose of 
> calculation.
>
> It would make no difference whether the 258 ft-lb were applied to the 
> 2079 by a typical 2 foot torque wrench and a fair amount of grunt, or 
> a four foot torque wrench and less operator effort.  It would still be 
> twisted at one end with 258 ft-lb of torque.  And thence, transfer 
> that same torque to its other end, less any tool bending.  And I bet 
> the 2079 is made not to bend at 258 ft-lbs.
>
> We need two things to "verify" what I am saying - one would be someone 
> who is comfortable with the simple physics involved (where the heck 
> are you, Larry?), and the other would be a few simple experiments with 
> dry fasteners, torque wrenches, and 2079-like extenders.
>
> Well, that's what my brain churned out after three days of thinking 
> about this problem.
>