View Full Version : Using a torque wrench with a crows foot

JeepsterSC

11-24-2013, 10:07 AM

I need to torque a fan clutch with a crows foot. I've found a formula that tells me how much to adjust the torque value with the added length of the crows foot here

http://www.engineersedge.com/manufacturing_spec/torque_wrench_1.htm

The problem I'm having is that this formula assumes that the angle of the torque wrench to the crows foot is 180*. I need the angle of the torque wrench to be offset by 30 to 45* in order to clear the handle of the tool that holds the water pump pulley from spinning

Does this change the torque setting of the torque wrench by any significant amount?

TbirdSCFan

11-24-2013, 02:19 PM

OK.. IF you hold the torque axis perpendicular to the bolt, the it doesn't matter that the crows foot might have an angular offset. It does matter that is has a side offset. You will need to increase the torque reading on the wrench. But here's the thing.. just look at what youre fastening.. why does it need a torque value at all? If its a head bolt, needs to be rather precise. If its a bracket bolt, then sufficiently tight is usually good enough. Very rarely do you need to have torque to within 10% of whats recommended. And the lower the number, the less precision you usually need. For example, I use a hand wrench on all my transmission fasteners.. haven't had any problems.

JeepsterSC

11-24-2013, 08:18 PM

The instructions say that if you don't get the clutch torqued down to the right spec the clutch can unthread when you shut off your engine so I think its pretty important to get it right.

I did talk to an acquaintance of mine who is an engineer. He said that you have to think of the wrench and the crows foot like 2 sides of a triangle. then imagine a line from the handle of the torque wrench to the head of the bolt. The length of the imaginary side would be the new length that you would plug into the formula to get the proper torque.

I just went to the high end of the range that I adjusted with the formula. that should do it

gr8ghost

11-24-2013, 11:21 PM

The instructions say that if you don't get the clutch torqued down to the right spec the clutch can unthread when you shut off your engine so I think its pretty important to get it right.

I highly doubt it. Every time you start the motor or hit the gas it'l tighten it more than shutting it off will loosen it.:cool:

I just went to the high end of the range that I adjusted with the formula. that should do it

Now that makes sense.:)

bowez

11-25-2013, 06:02 PM

The issue is you are applying torque to the Crow foot and not the bolt. The Crow foot multiples and applies torque to the fastener--the angle the Crow foot has in relation to handle is moot. The moment where the torque is applied is constant.

RalphP

11-26-2013, 01:04 AM

The issue is you are applying torque to the Crow foot and not the bolt. The Crow foot multiples and applies torque to the fastener--the angle the Crow foot has in relation to handle is moot. The moment where the torque is applied is constant.

No, it's not.

See, if the end of the fastener is 1 foot from the center of the fastener, and you apply 10 pounds on the end of it, you're applying 10 pound/feet.

Now if that is a 13 inch bar instead, it's 10.83 pound/feet.

If that crow's foot is 1", and you're straight it, it's that same 10.83.

If, however, you're at a 45 degree angle, then it's a bit hinkier due to the trig required.

I'm of the "Go to the bottom of the range, plus whatever it takes to square it up properly, and leave her" when you have to do something like that.

Do remember, you're actually applying torque to the nut or bolt (hopefully!) because if you were right, the darn nut/bolt wouldn't be turning!

RwP

Tony8470

11-26-2013, 03:33 AM

We use all sorts of torque wrenches and drivers extensively where I work.

Some of the old school guys still use this slide rule calculator. Here's an image of the back of it. Try using your "b" measurement as the crowfoot. And check with the snap on online torque calculator.

<a href="http://s99.photobucket.com/user/tony8470/media/18444FEF-EBDC-4D12-BBF4-4B8D6001852B.jpg.html" target="_blank"><img src="http://i99.photobucket.com/albums/l317/tony8470/18444FEF-EBDC-4D12-BBF4-4B8D6001852B.jpg" border="0" alt=" photo 18444FEF-EBDC-4D12-BBF4-4B8D6001852B.jpg"/></a>

Tony8470

11-26-2013, 03:43 AM

I should add that the base of the "a" measurement is where the center of your hand is (between your ring and middle finger) this is important. If you place your hand uncentered from the handle, you will have an incorrect torque application. Most torque wrenches have a line on the handle that represents the center.

In addition, the bottom and top %15 of the torque range shouldn't be used. The deviation from accuracy is much larger in this range.

bowez

11-26-2013, 07:41 AM

So what your saying is the only way to properly apply the torque is to take the the Integral because it changes as you apply the torque.

RalphP

11-26-2013, 08:40 AM

So what your saying is the only way to properly apply the torque is to take the the Integral because it changes as you apply the torque.

No.

Where would you get that?

The length from the PIVOT (in this case the center of the bolt) is constant.

Well, unless you ratchet the torque wrench, which changes the angle and the effective length and the multiplication factor that the crow's foot's putting in due to the extra length.

Which is also fixable - return to the original angle, and it's the same multiplier the original angle had.

But it's the angle off the PIVOT point - again, in this case, the bolt and/or nut centerpoint.

The actual multiplier for a non-straight run is easy enough to compute, though, as long as you know the angle.

You can take the cosine of the total length and the angle it's off of straight to compute the actual torque multiplication, but for smaller angles, it's easier to just use the bottom of the torque range (especially when the crow's foot is something like 5% or less of the overall length).

RwP

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