Torque wrench extensions

[Karl H.]

Torque is force times leverage. That's simple physics! The clicker mechanism indicates that at the pivot point of the clicker mechanism a preset torque (T1) is achieved. The torque at the clicker pivot is force exerted (F) times leverage between grip position and clicker pivot (L). The torque at the screw (end of the extension) (T2) is force times leverage too, but leverage is extended (L+A). Therefore the torque at the end of the extension is always higher than the torque at the clicker pivot. The relationship between the torque at the clicker pivot and the torque at the end of the extension is expressed by the well known formula which can easily be deduced from the basic leverage rules.

Karl


[attach=0]

T1 = F x L
T2 = F x (L+A)
=>
T1/L = T2/(L+A)
=> T1 = T2 x L / (L+A)

[Heatnbeat]

Karl, if what you say is correct then lengthening the lever arm with a cheater would increase the torque. But it doesn't.
Basic leverage rules do not apply to clicker wrenches.
Look at the design of this wrench, it's simple to under stand and it makes it obvious that handle length plays no part in the applied force.

http://www.appliedtorque.co.uk/p/handtorquewrenches/torquewrenches11/slippingwrenches/default.htm

It took a whole lot of looking but I finally found a formula that I believe works because it does not reference the length of the wrench.

I can't type it out here because I'm not good enough on a keyboard to know hoe but here it is:

Indicated torque ( wrench reading)= required torque / (1+ extension length).
Divide the extension length by 12 to find what fraction of a ft. the extension is (e.g. 3"= .250 ft)

Here's a link to a whole discussion of this problem. If you don't want to read it all
look at Oct/3/2008  - Mar/21/2011 - Dec/30/2011

[1FSTRK]

That formula does the same thing.
You are right about the applied force but you need to understand that in all the previous formulas L is a constant in the formula for the wrench being used and has the same formula value as 1 in your formula.

[Heatnbeat]

1FSTRK

My math skills are weak so I'd like to believe that.

But since I may be screwing up the formula I use this calculator from Belknap.

https://www.belknaptools.com/support-library/extensions-calculator/

Using my formula I get an actual torque of 100 with an indicated of 80 with a 3 in adapter
With the calculator I get
100 actual with indicated 72 for an 8 in wrench
100 actual with indicated 80 for a 12 in wrench
100 actual with indicated 89 for a 24 in wrench

What am I doing wrong? If the formula is normalized ( correct term?)  for wrench length why the difference?

[1FSTRK]

I stand corrected, happens when I post quickly based on others math.
This will require some additional investigation on my part to find the source of the discrepancy.

It appears at first glance that the original formulas will be accurate to calculate handle force changes but as you say not corrected wrench settings or readings because the wrench readings are taking place at the wrench head and not on the handle where the force is applied.

[Karl H.]

Karl, if what you say is correct then lengthening the lever arm with a cheater would increase the torque. But it doesn't.
Basic leverage rules do not apply to clicker wrenches.
Look at the design of this wrench, it's simple to under stand and it makes it obvious that handle length plays no part in the applied force...

You are right and wrong! Lengthening the lever arm will not increase or change the "click" torque T1 (which is preset and unchanged) but will decrease the "click" force F and slightly decrease T2 as well. T2 remains greater than T1. All that is correctly expressed by the equation.

Karl

[1FSTRK]

Look at the clicker wrench design in reply #6. I think the problem is with a clicker wrench L is the distance from the center of item #18 in the diagram to the center of item #12 and T1 is the center of item #12. The measured force is being applied at item #18, the length of the handle from #18 back has no effect on the force at #18 because of the spring/clicker will be overcome at the set point.

[Dan89flstc]

Lets say you have 2 torque wrenches, 1- 12 in long and 1- 24in long. Torque 2 bolts to 50lb/ft and they will be equally tight. Correct?

Yes

Now put your 2"  torque adapter on both wrenches, tighten both bolts to the original setting. They will both still be equally tight. Correct? 

No, not correct...

If you torque both wrenches to a 50 pound setting, with a 2 inch torque adapter installed, the bolts will not be torqued to an equal value.

The 12 inch wrench set at 50 lbs, using a 2 inch adapter will torque the fastener to 58 lbs.
The adapter is adding 16.66% to the overall length, the wrench cant see this, that is why we need to calculate using the formula.

The 24 inch wrench set at 50 lbs, using a 2 inch adapter will torque the fastener to 54 lbs.
The adapter is adding 8.33% to the overall length.

The 12 inch wrench must be set to read 42 lb.
The 24 inch wrench must be set to read 46 lb.

[Bikerscum]

Lets say you have 2 torque wrenches, 1- 12 in long and 1- 24in long. Torque 2 bolts to 50lb/ft and they will be equally tight. Correct?

Yes

Now put your 2"  torque adapter on both wrenches, tighten both bolts to the original setting. They will both still be equally tight. Correct? 

No, not correct...

If you torque both wrenches to a 50 pound setting, with a 2 inch torque adapter installed, the bolts will not be torqued to an equal value.

The 12 inch wrench set at 50 lbs, using a 2 inch adapter will torque the fastener to 58 lbs.
The adapter is adding 16.66% to the overall length, the wrench cant see this, that is why we need to calculate using the formula.

The 24 inch wrench set at 50 lbs, using a 2 inch adapter will torque the fastener to 54 lbs.
The adapter is adding 8.33% to the overall length.

The 12 inch wrench must be set to read 42 lb.
The 24 inch wrench must be set to read 46 lb.

In the example, the torque will be different because the percentage of added length is different.

[Heatnbeat]

Sorry guys but I don't believe it.


But I'll let it rest until I have a chance to set up some sort of test rig. If only I hadn't retired I'd have the stuff to do it.

[Karl H.]

Good luck! I'm out...

Karl

[build it]

i have this formula somewhere, if anyone is interested I can try and dig it up. Assuming a 3 foot extension and perpendicularity, don't worry about it.

[convert1967]

When adding an extension it is easy to not be 100% perpendicular, and you will lose torque.

[hattitude]

Karl, if what you say is correct then lengthening the lever arm with a cheater would increase the torque. But it doesn't.
Basic leverage rules do not apply to clicker wrenches.
Look at the design of this wrench, it's simple to under stand and it makes it obvious that handle length plays no part in the applied force.

I'll give this a shot, but it's been years and I'm not the world's best teacher....

With a clicker torque wrench, used without an extension, the fulcrum is on the end of the lever (on the pivot point, the bolt). This is a class 2 lever.  A longer handle makes it easier to apply the same torque value at the end of the lever (pivot point) AND, if desired, it will also allow for more torque to be applied at the end of the fulcrum.

When you add an extension, whether it is 2", 3", or 10"  and you keep the torque wrench inline with the extension... you are moving the fulcrum from the end of the lever and putting it in the middle of the lever (in the case of a torque wrench, "inside" the pivot point). It is now a class 1 lever. To measure the torque at the fulcrum, which is now in the middle area of the lever, the length of the lever becomes pertinent. Since it is at the fulcrum (where the torque wrench mates with the extension) that the torque measurement is taken, adjustments must be made to get the desired torque value at the end (pivot point) of the lever.

It's been awhile since I've was taught the physics of levers, but in my head this clear as can be.... my explanation, however, maybe lacking, if so, I apologize....

[Heatnbeat]

hattitude, I really appreciate your explanation!!

I could use some more teaching; would you mind if I emailed you?

[hattitude]

hattitude, I really appreciate your explanation!!

I could use some more teaching; would you mind if I emailed you?

You PM me, but that's about all I remember on the lever stuff.....

Sadly, I took some great physics courses back in college in the late 70's. No calculators or computers... all old school math or a slide rule (which I never really got the hang of).... but then I went into a career where physics wasn't needed on a regular basis and I've gotten rusty (meaning all but forgot) most of what I learned.

I was researching torque wrenches about 5 years ago and did a bunch of internet reading. It led me to extensions and formulas, that stimulated a few old memories. It got me reading even more.... I hate when I have to relearn something I used to know...

I'm pretty sure I remembered it correctly and more glad I was able to explain it .....