• posted
Sorry if this is the wrong group. I'm wondering about how the use of a
crow's foot should be calculated for a desired torque setting using a
torquing screwdriver?
I fully understand how to calculate the torque setting for a torque
wrench when using an adapter, but doing the same for a torque screwdriver
is a bit more challenging.
For instance, the maintenance manual for an aircraft engine states a
torque value of 12 in.lb on a fuel injector B-nut that is difficult to
reach, requiring the use of a crow's foot that has a 1 in. reach. My
smallest torque wrench has a range of 15-100 in.lb. so it can't be used,
but my torque screwdriver is 5-36 in.lb.
How do I calculate the tool torque value that will let me deliver 12
in.lb. using a 1 in crow's foot with a torquing screwdriver, please?
• posted
Dear MJ:
Torque propagates without change in this circumstance.
Even were this a torque wrench, the torque setting occurs at the pivot in the wrench, which is about where the crow's foot attaches.
I welcome corrections here...
David A. Smith
• posted
On Fri, 25 May 2012 19:54:28 -0700, dlzc wrote in snipped-for-privacy@googlegroups.com : ...
Well Mr Smith, corrections are certainly in order.
If one uses a torque wrench with a 10 in. handle and a 1 in. crow's foot inline with the handle to achieve 25 in.lb. of torque, there is a greater leveraged arm at the fastener, so the torque value would be set at (10/(10 +1))*25 = 22.7in.lb., so a torque setting of 23 will deliver the required 25 in.lb. at the fastener.
If one set the torque wrench at the desired 25 in.lb, the torque actually delivered at the fastener due to the longer leveraged arm would be ((10 +1)/10)*25 = 27.5 in.lb., a 10% over-torque which may cause a critical failure in a sensitive application.
In general usage with short torque adapters, it is considered a negligible difference if a short crow's foot is at 90 deg. to the handle, as the length of the hypotenuse formed between the handle and the fastener isn't considerably longer than the handle length, meaning that the leveraged arm is about the same.
What I'm searching for is a mathematical justification of the use of a crow's foot with a torquing screwdriver. This is my first post to this group, and I assume the sci.engr.mech name to be indicative of some science and math, at least in the discussion of things mechanical.
Forgive me if I erred in my choice of Usenet newsgroups, please.
• posted
I will admit to no direct experience with this, but it seems to me that the reading at the torquing screwdriver needs no correction.
The correction for the crowsfoot on a torque *wrench* is based on an increased lever arm, as illustrated here:
There is no net tangential force when a torquing screw*driver* is used. The torque at the business end of a torquing screwdriver is due to the application of a couple:
Not due to the application of a net tangential force.
I agree with Mr Smith: "Torque propagates without change in this circumstance."
• posted
On Sat, 26 May 2012 11:26:18 -0400, Wally W. wrote in snipped-for-privacy@4ax.com :
...
"in this circumstance" being the important point here, I don't feel that either of you have understood the "circumstance" of the usage of a crow's foot.
Yes, I have studied the URL you posted and regard it as inadequate and unconvincing for the topic at hand; torque setting on a screwdriver using a crow's foot. The article specifically differentiates between torque and moment, then offers as "proof" the equation of moment "M = r1F1 + r2F2 + ... rnFn"
So when no crow's foot is involved at "the business end" as you say, there is no moment since there is only a single term with a radius of 0? There is only the torque as specified on the tool's scale?
If, as you both suggest, "torque propagates without change in this circumstance", then there will be no difference in the torque delivered to a fastener whether one uses a 1", 2", 4" or no crow's foot?
Empirically I can assure you that is not the case, as using a 1" crow's foot on a torquing screwdriver set to 25 in.lb. results in a leak on an aircraft fuel injector nozzle B-nut at 300psi fuel pressure, while the use of no crow's foot results in no leakage.
Some of the nozzle B-nuts require the use of a crow's foot, and I'm hoping to discover the corrective relationship that will let me deliver the required torque.
It seems like an interesting problem, thank you both.
• posted
Dear MJ:
No, we both clearly do. But you have a "common sense" idea, which is keepi= ng you from seeing the facts. A torque is the same value if it is applied = "1 inch" or "1000 yards" from a pivot point. The torque already has the le= ngth factored in.
So you found similar links before, and simply chose to bring your attitude = here. That is going to limit how much we can answer your question. You ha= ve decided that you "common sense" trumps the advice you sought.
No, if there is only the single hand-applied moment. There is no *addition= al* moment.
Correct.
Correct.
Then you are displacing other things with you hand / arm, and are placing a= sidewards force on the body of the screwdriver. The sidewards force *can*= (does not have to, depending on the angle between the foot and the force) = apply a boosting or retarding torque on the moment arm of the crow's foot
No correction required, if you apply only a torque, and are not displacing = structure or forcing your hand into position.
David A. Smith
• posted
On Sat, 26 May 2012 12:23:41 -0700, dlzc wrote in snipped-for-privacy@googlegroups.com :
No sir, not at all ... it is you ho have used the "arm-waving" to support your position, especially after stating no experience with such things and stating that no correction is needed even when using an adapter with a torque wrench which is clearly wrong, as any aviation mechanic will tell you.
I feel that we're not communicating well, so I wish you all a g'day and I'm out of here, as there is neither mathematics or mechanics involved.
• posted
Dear MJ:
I said no such thing. When analyzing structures, torques simply translate = to any point, doesn't matter where they are applied.
... yet only *your* arms are waving. Can yo provide a link of the correcti= ons you seek?
Rather than continue attitudes, can we talk about the crow's foot? Are the= jaws equally spaced around the center of rotation, or are they offset like= a good open-ended wrench. If they are offset, the the force you apply to = keep the wrench on, can add or subtract.
Tubing wrenches are closed-end wrenches that have been milled to allow the = wrench to clear the tubing. Such might not require force be applied to kee= p the wrench in place, and not contaminate the intended torque setting. Wo= uld such be required when tightening a fuel injector B-nut? If so, are the= y available as crow's feet?
There is both, if you want to continue. The answer lies between theory and = application, and we can all learn, I think.
David A. Smith

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